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Sample records for subduction zone implications

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

    NASA Astrophysics Data System (ADS)

    Mookherjee, M.; Mainprice, D.

    2014-12-01

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

  2. The Fate of Eclogitized Oceanic Crust During Subduction: Implications for Subduction Zone Dynamics

    NASA Astrophysics Data System (ADS)

    Angiboust, S.; Agard, P.; Langdon, R.; Waters, D.; Raimbourg, H.; Yamato, P.; Chopin, C.

    2011-12-01

    The Monviso ophiolite is composed of two main tectonic slices: the Monviso Unit (MU) to the west, which overlies the Lago Superiore Unit (LSU). Our PT estimates show that the MU has been subducted down to 480°C-23kbar (~70km) during Alpine subduction while the LSU reached slightly deeper conditions (540°C-26kbar, i.e. ~80km). This ophiolite, which comprises large (10-20km long) ophiolite fragments therefore does not correspond to a serpentinite mélange, and may be the southern extension of the Zermatt-Saas ophiolite (Angiboust et al., 2009; 2011). The well-preserved LSU constitutes an almost continuous upper fragment of oceanic lithosphere subducted between 50 and 40 Ma and later exhumed along the subduction interface. It therefore provides a unique opportunity to study strain partitioning and deep mechanical behaviour of the subducting lithosphere. The LSU comprises (i) several hundred meters of eclogitized basaltic crust (+ minor calcschist lenses) overlying a 100-400m thick metagabbroic body and (ii) a serpentinite sole (c. 1km thick). We herein focus on eclogite-facies shear zones, which are found at the boundary between basalts and gabbros, and between gabbros and serpentinites, i.e. between material with marked rheological contrasts. Eclogite facies blocks within the shear zones display intense fracturation, fragment rotation and dispersion in the serpentinite schists which line up the shear zones. We also report the first finding of eclogite-facies breccias, constituted of rotated eclogite mylonitic clasts cemented within an eclogite-facies matrix. Local fracturation of garnet within these breccias is attested by the presence of numerous fracture networks within garnet, generally healed by a Mg-enriched composition. The shear zones also preserve clear evidence of pervasive and channelized fluid flow (of variable duration) leading to alteration of bulk rock composition, weakening of the rock and widespread crystallization of lawsonite. Our results provide

  3. Subduction of fracture zones

    NASA Astrophysics Data System (ADS)

    Constantin Manea, Vlad; Gerya, Taras; Manea, Marina; Zhu, Guizhi; Leeman, William

    2013-04-01

    Since Wilson proposed in 1965 the existence of a new class of faults on the ocean floor, namely transform faults, the geodynamic effects and importance of fracture zone subduction is still little studied. It is known that oceanic plates are characterized by numerous fracture zones, and some of them have the potential to transport into subduction zones large volumes of water-rich serpentinite, providing a fertile water source for magma generated in subduction-related arc volcanoes. In most previous geodynamic studies, subducting plates are considered to be homogeneous, and there is no clear indication how the subduction of a fracture zone influences the melting pattern in the mantle wedge and the slab-derived fluids distribution in the subarc mantle. Here we show that subduction of serpentinized fracture zones plays a significant role in distribution of melt and fluids in the mantle wedge above the slab. Using high-resolution tree-dimensional coupled petrological-termomechanical simulations of subduction, we show that fluids, including melts and water, vary dramatically in the region where a serpentinized fracture zone enters into subduction. Our models show that substantial hydration and partial melting tend to concentrate where fracture zones are being subducted, creating favorable conditions for partially molten hydrous plumes to develop. These results are consistent with the along-arc variability in magma source compositions and processes in several regions, as the Aleutian Arc, the Cascades, the Southern Mexican Volcanic Arc, and the Andean Southern Volcanic Zone.

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

    NASA Astrophysics Data System (ADS)

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

    2003-12-01

    With ca. forty thousand kilometers of subduction zones and convergence rates from 30 km Ma-1 to 180 km Ma-1, subduction carries massive amounts of material into seafloor trenches, and beyond. Most of the subducting plate is made of mantle material returning to the depths from which it originated. The hydrated and altered upper oceanic section and the overlying sediments, however, carry a record of low-temperature interaction with the ocean, atmosphere, and continents. Subduction and recycling of these components into the mantle has the potential to change mantle composition in terms of volatile contents, heat-producing elements, radiogenic isotope systematics, and trace element abundances. Enrichments in volatile and potassium, uranium, and thorium contents could change the rheological, thermal, and geodynamical behavior of portions of the mantle. Changing isotope and trace-element systematics provide a means for tracking mantle mixing and the possible subduction modification of the deep mantle. A large number of studies point to possible contributions of subducted sediments and altered oceanic crust (AOC) to the mantle-source region for enriched mantle II (EMII) and high mu (HiMU) enriched oceanic island basalts. Transit through the subduction zone, however, changes the composition of the subducting sediment and AOC from that measured outboard of trenches.This chapter focuses on subduction zone processes and their implications for mantle composition. It examines subduction contributions to the shallow mantle that may be left behind in the wedge following arc magma genesis, as well as the changing composition of the slab as it is processed beneath the fore-arc, volcanic front and rear arc on its way to the deep mantle. Much of this chapter uses boron and the beryllium isotopes as index tracers: boron, because it appears to be completely recycled in volcanic arcs with little to none subducted into the deep mantle, and cosmogenic 10Be, with a 1.5 Ma half

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

    NASA Astrophysics Data System (ADS)

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

    1996-05-01

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

  6. Modeling mantle circulation and density distributions in subduction zones: Implications for seismic studies

    NASA Astrophysics Data System (ADS)

    Kincaid, C. R.; Druken, K. A.; Griffiths, R. W.; Long, M. D.; Behn, M. D.; Hirth, G.

    2009-12-01

    Subduction of ocean lithosphere drives plate tectonics, large-scale mantle circulation and thermal-chemical recycling processes through arcs. Seismologists have made important advances in our ability to map circulation patterns in subduction zones though anisotropy data/methods and in providing detailed images of mantle density fields. Increasingly, seismic and geodynamic disciplines are combining to extend our understanding of time varying subduction processes and associated vertical mass and energy fluxes. We use laboratory experiments to characterize three-dimensional flow fields in convergent margins for a range in plate forcing conditions and background, buoyancy-driven flow scenarios. Results reveal basic patterns in circulation, buoyant flow morphologies and density distributions that have implications for reconciling seismic data with mantle convection models. Models utilize a glucose working fluid with a temperature dependent viscosity to represent the upper 2000km of the mantle. Subducting lithosphere is modeled with a Phenolic plate and back-arc extension is produced using Mylar sheets. We recreate basic subduction styles observed in previous dynamic subduction models using simplified, kinematic forcing. Slab plate segments, driven by hydraulic pistons, move with various combinations of downdip, rollback and steepening motion. Neutral density finite strain markers are distributed throughout the fluid and used as proxies for tracking the evolution of olivine alignment through space and time in the evolving flow fields. Particle image velocimetry methods are also used to track time varying 3D velocity fields for use in directly calculating anisotropy patterns. Results show that complex plate motions (rollback, steepening, back-arc extension) in convergent margins produce relatively simple anisotropy patterns (e.g., trench-normal alignments) and underscore the importance of initial strain marker orientations on alignment patterns in the wedge. Results also

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

    NASA Astrophysics Data System (ADS)

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

    2011-12-01

    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

  8. Cascadia Subduction Zone

    USGS Publications Warehouse

    Frankel, Arthur D.; Petersen, Mark D.

    2008-01-01

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

  9. Field-based evidence for devolatilization in subduction zones: implications for arc magmatism.

    PubMed

    Bebout, G E

    1991-01-25

    Metamorphic rocks on Santa Catalina Island, California, afford examination of fluid-related processes at depths of 15 to 45 kilometers in an Early Cretaceous subduction zone. A combination of field, stable isotope, and volatile content data for the Catalina Schist indicates kilometer-scale transport of large amounts of water-rich fluid with uniform oxygen and hydrogen isotope compositions. The fluids were liberated in devolatilizing, relatively low-temperature (400 degrees to 600 degrees C) parts of the subduction zone, primarily by chlorite-breakdown reactions. An evaluation of pertinent phase equilibria indicates that chlorite in mafic and sedimentary rocks and melange may stabilize a large volatile component to great depths (perhaps >100 kilometers), depending on the thermal structure of the subduction zone. This evidence for deep volatile subduction and large-scale flow of slab-derived, water-rich fluids lends credence to models that invoke fluid addition to sites of arc magma genesis.

  10. Elastic thickness structure of the Andaman subduction zone: Implications for convergence of the Ninetyeast Ridge

    NASA Astrophysics Data System (ADS)

    Ratheesh Kumar, R. T.; Windley, B. F.; Rajesh, V. J.; Santosh, M.

    2013-12-01

    We use the Bouguer coherence (Morlet isostatic response function) technique to compute the spatial variation of effective elastic thickness (Te) of the Andaman subduction zone. The recovered Te map resolves regional-scale features that correlate well with known surface structures of the subducting Indian plate and the overriding Burma plate. The major structure on the India plate, the Ninetyeast Ridge (NER), exhibits a weak mechanical strength, which is consistent with the expected signature of an oceanic ridge of hotspot origin. However, a markedly low strength (0 < Te < 3 km) in that region, where the NER is close to the Andaman trench (north of 10°N), receives our main attention in this study. The subduction geometry derived from the Bouguer gravity forward modeling suggests that the NER has indented beneath the Andaman arc. We infer that the bending stresses of the viscous plate, which were reinforced within the subducting oceanic plate as a result of the partial subduction of the NER buoyant load, have reduced the lithospheric strength. The correlation, Te < Ts (seismogenic thickness) reveals that the upper crust is actively deforming beneath the frontal arc Andaman region. The occurrence of normal-fault earthquakes in the frontal arc, low Te zone, is indicative of structural heterogeneities within the subducting plate. The fact that the NER along with its buoyant root is subducting under the Andaman region is inhibiting the subduction processes, as suggested by the changes in trench line, interrupted back-arc volcanism, variation in seismicity mechanism, slow subduction, etc. The low Te and thinned crustal structure of the Andaman back-arc basin are attributed to a thermomechanically weakened lithosphere. The present study reveals that the ongoing back-arc spreading and strike-slip motion along the West Andaman Fault coupled with the ridge subduction exerts an important control on the frequency and magnitude of seismicity in the Andaman region.

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

    NASA Astrophysics Data System (ADS)

    Nishikawa, T.; Ide, S.

    2013-12-01

    Since the pioneering study of Uyeda and Kanamori (1979), it has been thought that world's subduction zones can be classified into two types: Chile and Mariana types. Ruff and Kanamori (1980) suggested that the maximum earthquake size within each subduction zone correlates with convergence rate and age of subducting lithosphere. Subduction zones with younger lithosphere and larger convergence rates are associated with great earthquakes (Chile), while subduction zones with older lithosphere and smaller convergence rates have low seismicity (Mariana). However, these correlations are obscured after the 2004 Sumatra earthquake and the 2009 Tohoku earthquake. Furthermore, McCaffrey (2008) pointed out that the history of observation is much shorter than the recurrence times of very large earthquakes, suggesting a possibility that any subduction zone may produce earthquakes larger than magnitude 9. In the present study, we compare world's subduction zones in terms of b-values in the Gutenberg-Richer relation. We divided world's subduction zones into 146 regions, each of which is bordered by a trench section of about 500 km and extends for 200 km from the trench section in the direction of relative plate motion. In each region, earthquakes equal to or larger than M4.5 occurring during 1988-2009 were extracted from ISC catalog. We find a positive correlation between b-values and ages of subducting lithosphere, which is one of the two important variables discussed in Ruff and Kanamori (1980). Subduction zones with younger lithosphere are associated with high b-values and vice versa, while we cannot find a correlation between b-values and convergence rates. We used the ages determined by Müller et al. (2008) and convergence rate calculated using PB2002 (Bird, 2003) for convergence rate. We also found a negative correlation between b-values and the estimates of seismic coupling, which is defined as the ratio of the observed seismic moment release rate to the rate calculated

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

    NASA Astrophysics Data System (ADS)

    Emry, Erica L.; Wiens, Douglas A.

    2015-03-01

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

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

    NASA Astrophysics Data System (ADS)

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

    1995-06-01

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

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

    USGS Publications Warehouse

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

    2005-01-01

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

  15. Water supplement by silica diagenesis in cold subduction zone: an implication for the Japan Trench

    NASA Astrophysics Data System (ADS)

    Hina, S.; Hamada, Y.; Kameda, J.; Yamaguchi, A.; Kimura, G.

    2011-12-01

    The fluid existing at plate interfaces in subduction zone makes a strong effect on seismicity and fault slip of the plate boundary megathrusts. As a source of the fluid, pore fluid included in subducting sedimentsand dehydration reaction of clay minerals have been discussed in detail, however, dewatering from siliceous sediments such as diatom and radiolarian ooze are poorly investigated in spite of their major occurrence in old oceanic plate. Silica in the siliceous sediment is transformed from amorphous silica into quartz via cristobalite phase (Opal A → Opal CT → Quartz) releasing structured water. In this study, we evaluate the amount of dehydration from siliceous sediment in subducting plate. Silica diagenesis and dehydration are calculated quantitatively introducing reaction kinetics (Mizutani, 1970) and temperature profile models of the Japan Trench, a cold type subduction zone where the siliceous sediments subduct in (Peacock and Wang, 1999; Wang and Suyehiro, 1999; Wada and Wang, 2009; Kimura et al., submitted). As a result, through this diagenetic conversion, structured water of silica minerals is released as much as 140g/m^2/year at shallow plate boundary (~13km depth below the sea floor), where the temperature is about ~100 -~120°C. This water should generate an excess pore pressure which drops effective stress and rock strength along the décollement. Dehydration of silica can play an important role in slip propagation to shallow portion of plate boundary as the Grate Tohoku Earthquake (9 March 2011).

  16. Subduction zone locking, strain partitioning, intraplate deformation and their implications to Seismic Hazards in South America

    NASA Astrophysics Data System (ADS)

    Galgana, G. A.; Mahdyiar, M.; Shen-Tu, B.; Pontbriand, C. W.; Klein, E.; Wang, F.; Shabestari, K.; Yang, W.

    2014-12-01

    We analyze active crustal deformation in South America (SA) using published GPS observations and historic seismicity along the Nazca Trench and the active Ecuador-Colombia-Venezuela Plate boundary Zone. GPS-constrained kinematisc models that incorporate block and continuum techniques are used to assess patterns of regional tectonic deformation and its implications to seismic potential. We determine interplate coupling distributions, fault slip-rates, and intraplate crustal strain rates in combination with historic earthquakes within 40 seismic zones crust to provide moment rate constraints. Along the Nazca subduction zone, we resolve a series of highly coupled patches, interpreted as high-friction producing "asperities" beneath the coasts of Ecuador, Peru and Chile. These include areas responsible for the 2010 Mw 8.8 Maule Earthquake and the 2014 Mw 8.2 Iquique Earthquake. Predicted tectonic block motions and fault slip rates reveal that the northern part of South America deforms rapidly, with crustal fault slip rates as much as ~20 mm/a. Fault slip and locking patterns reveal that the Oca Ancón-Pilar-Boconó fault system plays a key role in absorbing most of the complex eastward and southward convergence patterns in northeastern Colombia and Venezuela, while the near-parallel system of faults in eastern Colombia and Ecuador absorb part of the transpressional motion due to the ~55 mm/a Nazca-SA plate convergence. These kinematic models, in combination with historic seismicity rates, provide moment deficit rates that reveal regions with high seismic potential, such as coastal Ecuador, Bucaramanga, Arica and Antofagasta. We eventually use the combined information from moment rates and fault coupling patterns to further constrain stochastic seismic hazard models of the region by implementing realistic trench rupture scenarios (see Mahdyiar et al., this volume).

  17. Massive barite deposits in the convergent margin off Peru: Implications for fluid circulation within subduction zones

    NASA Astrophysics Data System (ADS)

    Aquilina, L.; Dia, A. N.; Boulègue, J.; Bourgois, J.; Fouillac, A. M.

    1997-03-01

    The convergent margin of Peru, characterized by an extensional tectonic regime and the lack of a well-developed accretionary prism, has been investigated by a deep-sea submersible during the Nautiperc cruise (March-April, 1991). This allowed the collection of fluid samples, soft sediments, and barite concretions in the vicinity of biological communities associated with fluid steps. Major and trace element contents as well as strontium, oxygen, hydrogen, and sulfur isotopic compositions have been measured on fluid and/or solid samples to constrain the nature and origin of fluid circulating in this extensional tectonic context. Chemical variations with respect to bottom seawater composition have been recorded in the fluid samples and suggest the presence of a nonlocal component in the fluid expelled at the seafloor. The major variations correspond to elevations of the Cl, Na, and the Ba contents as well as the 87Sr /86Sr ratios. This is interpreted as the expulsion of a radiogenic, continent-related (basinal brine and/or meteoric water) fluid. Massive barite concretions have been collected at the seafloor in two areas of major fluid venting. The radiogenic signature (strontium isotopic composition) of the barite concretions implies that they are related to the nonlocal deep fluid component identified in the fluid samples. Furthermore, it is shown that these barite deposits testify to a hot, short, and intensive fluid circulation event. Compared to subduction zones that exhibit venting fluid with a strong oceanic water signature, the nature and origin of venting fluid along the subduction zone of Peru are different. The extensional tectonic regime of the Peru continental margin, locally associated with a dense E-W trending fault network, is an agent which may help to drain continent-related fluid as deep as the subduction scarp at the trench-slope boundary.

  18. Fluid processes in subduction zones.

    PubMed

    Peacock, S A

    1990-04-20

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

  19. Fluid processes in subduction zones.

    PubMed

    Peacock, S A

    1990-04-20

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

  20. Crustal growth in subduction zones

    NASA Astrophysics Data System (ADS)

    Vogt, Katharina; Castro, Antonio; Gerya, Taras

    2015-04-01

    crustal units may collide with continental margins to form collisional orogens and accreted terranes in places where oceanic lithosphere is recycled back into the mantle. We use thermomechanical-petrological models of an oceanic-continental subduction zone to analyse the dynamics of terrane accretion and its implications to arc magmatisim. It is shown that terrane accretion may result in the rapid growth of continental crust, which is in accordance with geological data on some major segments of the continental crust. Direct consequences of terrane accretion may include slab break off, subduction zone transference, structural reworking, formation of high-pressure terranes and partial melting (Vogt and Gerya., 2014), forming complex suture zones of accreted and partially molten units. Castro, A., Vogt, K., Gerya, T., 2013. Generation of new continental crust by sublithospheric silicic-magma relamination in arcs: A test of Taylor's andesite model. Gondwana Research, 23, 1554-1566. Vogt, K., Castro, A., Gerya, T., 2013. Numerical modeling of geochemical variations caused by crustal relamination. Geochemistry, Geophysics, Geosystems, 14, 470-487. Vogt, K., Gerya, T., 2014. From oceanic plateaus to allochthonous terranes: Numerical Modelling. Gondwana Research, 25, 494-508

  1. Evolution of a Subduction Zone

    NASA Astrophysics Data System (ADS)

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

    2014-05-01

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

  2. Immiscible Hydrocarbon and Aqueous Fluids Under Subduction Zone Conditions and Implications for the Deep Carbon Cycle

    NASA Astrophysics Data System (ADS)

    Huang, F.; Daniel, I.; Cardon, H.; Montagnac, G.; Sverjensky, D. A.

    2015-12-01

    Subducting slabs recycle rocks into the deep Earth releasing fluids which may cause partial melting and possible oxidation of the mantle wedge. Recent theoretical studies1 indicate that at pressures greater than about 3.0 GPa these fluids could contain high concentrations of organic and inorganic C-species with a wide range of C-oxidation states at equilibrium. If so, such fluids could play an important role in the deep carbon cycle, including the formation of diamond. However, direct experimental observations of the speciation in the fluids are needed. We studied 1.0 M aqueous Na-formate and 1.0 M Na-acetate solutions in the diamond anvil cell using Raman spectroscopy at 300 ºC and 3.0 GPa for up to 60 hours. Our preliminary results indicate that formate rapidly decomposed to bicarbonate/carbonate species and methane, with no detectable H2. Acetate decomposed much more slowly. Within the first two hours of heating, crystals of Na2CO3 precipitated in the fluid, and kept growing while immiscible droplets of hydrocarbon appeared and persisted throughout the experiments at elevated temperature and pressure. In the aqueous fluid, acetate and HCO3- were present during the first 6 hours, and then CO32- and acetate after 20 hours of heating. The final HCO3- /CO32- ratio was constant indicating a constant pH. This is the first in situ observation of persistent immiscible fluid hydrocarbons formed from an aqueous precursor at upper mantle pressures. Our results suggest that Earth's subduction zone fluids at high pressures might involve fluid hydrocarbon species as well as inorganic and organic aqueous C-species, which considerably broadens the picture of deep carbon sources, cycles and sinks. [1] Sverjensky et at. (2014), Nat. Geosci. 7, 909-913.

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

    USGS Publications Warehouse

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

    2012-01-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2010-12-01

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

  6. GPS geodesy in the northern Lesser Antilles: implications for arc kinematics and subduction zone dynamics

    NASA Astrophysics Data System (ADS)

    Matson, S. E.; Rodriguez, H.; Jansma, P. E.; Mattioli, G. S.

    2002-12-01

    The Lesser Antilles island arc extends from Grenada in the south to the island of Sombrero in the north and formed from the westward subduction of Atlantic lithosphere below the Caribbean plate. A break in the Wadati-Benioff zone occurs between Martinique and St. Lucia effectively splitting the island arc into a more seismically active north and lesser seismically active south. GPS geodesy has been conducted in the northern Lesser Antilles since 1998 when sites (number in parentheses) were established on Saba (4), St. Eustatius (4), St. Kitts (4), Nevis (4) and Antigua (2). Sites established in 2001 and 2002 include: St. Martin (2); Anguilla (2); Barbuda (1); and one additional site each in St. Kitts and Nevis. Islands with potentially active volcanoes (Saba, St. Eustatius, St. Kitts, and Nevis) have both tectonic and volcanic sites. Data from occupations in 1998, 2000, 2001, and 2002 have been processed with GIPSY-OASIS II with final orbit and clock parameters from JPL and recast into the Caribbean reference frame to assess 1) velocities of individual island sites relative to the stable Caribbean, 2) internal island deformation and 3) seismic coupling along the plate interface. Results from individual quiescent volcanoes demonstrate that they are deforming at several millimeters per year with respect to the stable Caribbean. The cause of the observed deformation is likely a combination of edifice failure and coupling along the plate interface. Simple 2D locking models have been investigated and will be presented.

  7. He isotope ratios in the Nankai Trough and Costa Rica subduction zones - implications for volatile cycling

    NASA Astrophysics Data System (ADS)

    Kastner, M.; Hilton, D. R.; Jenkins, W. J.; Solomon, E. A.; Spivack, A. J.

    2013-12-01

    The noble gas 3He is a clear indicator of primordial volatile flux from the mantle, thus providing important insights on the interaction between Earth's interior and exterior reservoirs. Volatile cycling at ridge-crests and its impact on the evolution of seawater chemistry is rather well known as constrained by the 3He flux, whereas the impact of volatile cycling at subduction zones (SZs) on seawater chemistry is as yet poorly known. Constraining chemical and isotopic cycling at SZs is important for understanding the evolution of the mantle-crust and ocean-atmosphere systems. To gain insights on volatile cycling in SZs, pore fluids were sampled for He concentration and isotopic analyses at two tectonically contrasting SZs, Nankai Trough (offshore Japan, Muroto and Kumano transects), an accretionary SZ, and Costa Rica (Offshore Osa Peninsula), an erosional SZ. Sampling for He was achieved by rapidly subsampling core sediments, cleaning and transferring these samples into Ti squeezers in a glove bag, and storing the squeezed pore fluids in crimped Cu tubes for shore-based He concentration and isotope ratio analyses. At the Nankai Trough SZ there is a remarkable range of He isotopic values. The 3He/4He ratios relative to atmospheric ratio (RA) range from mostly crustal 0.47 RA to 4.30 RA which is ~55% of the MORB value of 8 RA. Whereas at the Costa Rica SZ, offshore Osa Peninsula, the ratios range from 0.86 to 1.14 RA, indicating the dominance of crustal radiogenic 4He that is from U and Th decay. The distribution of the He isotope values at Nankai Trough is most interesting, fluids that contain significant mantle 3He components (3He/4He >1) were sampled along and adjacent to fluid conduits that were identified by several chemical and isotopic data (i.e. Cl, B, and Li), including the presence of thermogenic hydrocarbons. Whereas the fluids dominated by 4He (3He/4He ≤1) were obtained from sediment sections that were between the fluid conduits. At Costa Rica, however

  8. Building a Subduction Zone Observatory

    USGS Publications Warehouse

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

    2016-01-01

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

  9. Characterizing Seismic Anisotropy across the Peruvian Flat-Slab Subduction Zone: Implications for the Dynamics of Flat-Slabs

    NASA Astrophysics Data System (ADS)

    Eakin, Caroline; Long, Maureen; Beck, Susan; Wagner, Lara; Tavera, Hernando

    2014-05-01

    Although 10% of subduction zones worldwide today exhibit shallow or flat subduction, we are yet to fully understand how and why these slabs go flat. An excellent study location for such a problem is in Peru, where the largest region of flat-subduction currently exists, extending ~1500 km in length (from 3 °S to 15 °S) and ~300 km in width. Across this region we investigate the pattern of seismic anisotropy, an indicator for past and/or ongoing deformation in the upper mantle. To achieve this we conduct shear wave splitting analyzes at 40 broadband stations from the PULSE project (PerU Lithosphere and Slab Experiment). These stations were deployed for 2+ years across the southern half of the Peruvian flat-slab region. We present detailed shear wave splitting results for both teleseismic events (such as SKS, SKKS, PKS, sSKS) that sample the upper mantle column beneath the stations as well as direct S from local events that constrain anisotropy in the upper portion of the subduction zone. We analyze the variability of our results with respect to initial polarizations, ray paths, and frequency content as well as spatial variability between stations as the underlying slab morphology changes. Teleseismic results show predominately NW-SE fast polarizations (trench oblique to sub-parallel) over the flat-slab region east of Lima. These results are consistent with observations of more complex multi-layered anisotropy beneath a nearby permanent station (NNA) that suggests a trench-perpendicular fast direction in the lowest layer in the sub-slab mantle. Further south, towards the transition to steeper subduction, the splitting pattern becomes increasingly dominated by null measurements. Over to the east however, beyond Cuzco, where the mantle wedge might begin to play a role, we record fast polarizations quasi-parallel to the local slab contours. Local S results indicate the presence of weak (delay times typically less than 0.5 seconds) and heterogeneous supra

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

    USGS Publications Warehouse

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

    2012-01-01

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

  11. Permeability control on transient slip weakening during gypsum dehydration: Implications for earthquakes in subduction zones

    NASA Astrophysics Data System (ADS)

    Leclère, Henri; Faulkner, Daniel; Wheeler, John; Mariani, Elisabetta

    2016-05-01

    A conflict has emerged from recent laboratory experiments regarding the question of whether or not dehydration reactions can promote unstable slip in subduction zones leading to earthquakes. Although reactions produce mechanical weakening due to pore-fluid pressure increase, this weakening has been associated with both stable and unstable slip. Here, new results monitoring strength, permeability, pore-fluid pressure, reaction progress and microstructural evolution during dehydration reactions are presented to identify the conditions necessary for mechanical instability. Triaxial experiments are conducted using gypsum and a direct shear sample assembly with constant normal stress that allows the measurement of permeability during sliding. Tests are conducted with temperature ramp from 70 to 150 °C and with different effective confining pressures (50, 100 and 150 MPa) and velocities (0.1 and 0.4 μm s-1). Results show that gypsum dehydration to bassanite induces transient stable-slip weakening that is controlled by pore-fluid pressure and permeability evolution. At the onset of dehydration, the low permeability promoted by pore compaction induces pore-fluid pressure build-up and stable slip weakening. The increase of bassanite content during the reaction shows clear evidence of dehydration related with the development of R1 Riedel shears and P foliation planes where bassanite is preferentially localized along these structures. The continued production of bassanite, which is stronger than gypsum, provides a supporting framework for newly formed pores, thus resulting in permeability increase, pore-fluid pressure drop and fault strength increase. After dehydration reaction, deformation is characterized by unstable slip on the fully dehydrated reaction product, controlled by the transition from velocity-strengthening to velocity-weakening behaviour of bassanite at temperature above ∼140 °C and the localization of deformation along narrow Y-shear planes. This study

  12. Recurrence, Rates, and Paleogeodetic Implications: Southern Cascadia Subduction Zone, Northern California

    NASA Astrophysics Data System (ADS)

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

    2010-12-01

    Earthquake and tsunami hazard for northern California and southern Oregon is dominated by estimates of recurrence for earthquakes on the Cascadia subduction zone (CSZ) and upper plate thrust faults. Recurrence interval (RI)estimates derived from site based terrestrial data (270-1,370 years) are inconsistent with the regional marine record of great earthquakes (RI = ~240 years). Reconciling these differences reveals information regarding different sources or magnitudes of coseismic or interseismic deformation in the southern CSZ. Early paleoseismic investigations utilized bulk peat for 14C age determinations and lack vertical elevation control. All terrestrial data sets are compiled, evaluated, ranked, and excluded according to their paleoseismic relevance. We construct an OxCal age model to evaluate the discriminated 14C based space-time relations graphically and statistically. We interpret a regional timing of tectonic deformation that is consistent with the timing of the marine record. Not all events are observed in each region and not all events have age control. Some regions lack cores representing the complete modern tidal elevation range (biasing the paleoseismic record). For example, when individual sites in the same region are combined, a more complete record of coseismic subsidence can be assumed, reducing the terrestrial RI to 360+-40, yet still longer than the marine RI. We consider relative sea-level (RSL), as the relation between land-level and sea-level and we find that chronologically distinct buried soils are found in settings segregated by elevation. Subsidence in southern Humboldt Bay occurred in positions of higher RSL at ~1,500, 2,200, and ~3,500 cal yrs BP. We pose that the RSL position does not relate to the time preceding the earthquake, but may relate to the accumulated strain at the time of the earthquake. RSL with a higher position would correspond with more accumulated strain in the upper plate. We evaluate the various factors that may

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

    NASA Astrophysics Data System (ADS)

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

    2016-07-01

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

  14. Carbonation by fluid-rock interactions at High-Pressure conditions: implications for Carbon cycling in subduction zones

    NASA Astrophysics Data System (ADS)

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

    2016-04-01

    Carbonate-bearing lithologies are the main carbon carrier into subduction zones. Their evolution during metamorphism largely controls the fate of carbon regulating its fluxes between shallow and deep reservoirs. In subduction zones, most works have focused on subtractive processes responsible for carbon release from subducting slabs. As an example, several recent works have stressed on the importance of carbonate dissolution as a mean to mobilize large amounts of carbon in subduction zones. By contrast, little is known on additive processes such as rock carbonation at high-pressure (HP) conditions. At shallow depths (e.g. ocean floor and shallow subduction zones, i.e. <40 km), carbonation of mafic and ultramafic rocks deeply contributes to the regulation of carbon fluxes between the geo-biosphere and the atmosphere. We report the occurrence of eclogite-facies marbles associated with metasomatic systems in HP metamorphic unit in Alpine Corsica (France). We performed a field-based study on metasomatic marbles. We will present the petrology and geochemistry that characterize carbonate metasomatism together with fluid inclusions study and pseudosection modeling. Altogether, we bring strong evidences for the precipitation of these carbonate-rich assemblages from carbonic fluids during HP metamorphism. We propose that rock carbonation can occur at HP conditions by either vein-injection or chemical replacement mechanisms. Rock carbonation indicates that carbonic fluids produced by decarbonation reactions and carbonate dissolution may not be directly transferred to the mantle wedge, but may have a preferential and complex pathway within the slab and along slab/mantle interface. Rock carbonation by fluid-rock interactions has a potentially great impact on the residence time of carbon and oxygen and on carbonates isotopic signature in subduction zones. Lastly, carbonation may modulate the emission of CO2 at volcanic arcs over geological time scales.

  15. Phlogopite and K-amphibole in the upper mantle: Implication for magma genesis in subduction zones

    SciTech Connect

    Sudo, Akira; Tatsumi, Yoshiyuki )

    1990-01-01

    High-pressure phase relations have been examined for phlogopite + diopside with and without enstatite under vapor absent conditions in the pressure range of 5 to 13 GPa and in the temperature range of 1,000 to 1,300C. Phlogopite in these systems can be stable up to 6-7 GPa and decomposes through pressure-dependent reactions to crystallize phases including potassic amphibole. The experimental results suggest that phlogopite, which is one of main hydrous phases in the downdragged hydrated peridotite at the base of mantle wedge, plays an important role in the formation of magmas at the backarc side of a volcanic arc. The existence of potassic amphibole at higher pressure regions may imply the involvement of subduction component in magma generation in the region far away from the trench axis.

  16. A new view into the Cascadia subduction zone and volcanic arc: Implications for earthquake hazards along the Washington margin

    USGS Publications Warehouse

    Parsons, T.; Trehu, A.M.; Luetgert, J.H.; Miller, K.; Kilbride, F.; Wells, R.E.; Fisher, M.A.; Flueh, E.; ten Brink, U.S.; Christensen, N.I.

    1998-01-01

    In light of suggestions that the Cascadia subduction margin may pose a significant seismic hazard for the highly populated Pacific Northwest region of the United States, the U.S. Geological Survey (USGS), the Research Center for Marine Geosciences (GEOMAR), and university collaborators collected and interpreted a 530-km-long wide-angle onshore-offshore seismic transect across the subduction zone and volcanic arc to study the major structures that contribute to seismogenic deformation. We observed (1) an increase in the dip of the Juan de Fuca slab from 2??-7?? to 12?? where it encounters a 20-km-thick block of the Siletz terrane or other accreted oceanic crust, (2) a distinct transition from Siletz crust into Cascade arc crust that coincides with the Mount St. Helens seismic zone, supporting the idea that the mafic Siletz block focuses seismic deformation at its edges, and (3) a crustal root (35-45 km deep) beneath the Cascade Range, with thinner crust (30-35 km) east of the volcanic arc beneath the Columbia Plateau flood basalt province. From the measured crustal structure and subduction geometry, we identify two zones that may concentrate future seismic activity: (1) a broad (because of the shallow dip), possibly locked part of the interplate contact that extends from ???25 km depth beneath the coastline to perhaps as far west as the deformation front ???120 km offshore and (2) a crustal zone at the eastern boundary between the Siletz terrane and the Cascade Range.

  17. High-resolution transition zone structures of the Gorda Slab beneath the western United States: Implication for deep water subduction

    NASA Astrophysics Data System (ADS)

    Cao, Aimin; Levander, Alan

    2010-07-01

    The current data set from USArray provides an unprecedented opportunity to investigate mantle transition zone structures beneath the western United States. We have made transition zone images with the Common Converted Point (CCP) stacking method. More than 9600 high quality receiver functions were stacked with reference to two different three-dimensional tomography models and a one-dimensional velocity model. Where the Gorda plate passes through the transition zone, the 410 discontinuity has been elevated ˜25 km and the 660 discontinuity has been depressed ˜35 km. We interpret the transition zone topography in terms of mineral physics results in several different ways, noting in particular that recent measurements on the Clapeyron slope for the ringwoodite-to-perovskite phase transition under dry conditions give a phase boundary slope of ˜-1.3 to -0.4 MPa/K. The ˜35 km deflection of the 660 discontinuity observed in the receiver functions seems to be the evidence that the subducted slab can carry abundant water from the surface to the transition zone, and in the transition zone the water in the slab may be fully saturated (e.g. the water content is ˜2.0 wt%). Analyses of the velocity perturbations in the tomography models and the transition zone thickness indicate that the deep water is likely well confined within the subducted slab. We infer that the presence of water in the subducted Gorda slab might have contributed ˜15 km and the thermal anomaly in the slab might have contributed ˜20 km to the depression of the 660 discontinuity.

  18. Carbonation of subduction-zone serpentinite (high-pressure ophicarbonate; Ligurian Western Alps) and implications for the deep carbon cycling

    NASA Astrophysics Data System (ADS)

    Scambelluri, Marco; Bebout, Gray E.; Belmonte, Donato; Gilio, Mattia; Campomenosi, Nicola; Collins, Nathan; Crispini, Laura

    2016-05-01

    Much of the long-term carbon cycle in solid earth occurs in subduction zones, where processes of devolatilization, partial melting of carbonated rocks, and dissolution of carbonate minerals lead to the return of CO2 to the atmosphere via volcanic degassing. Release of COH fluids from hydrous and carbonate minerals influences C recycling and magmatism at subduction zones. Contradictory interpretations exist regarding the retention/storage of C in subducting plates and in the forearc to subarc mantle. Several lines of evidence indicate mobility of C, of uncertain magnitude, in forearcs. A poorly constrained fraction of the 40-115 Mt/yr of C initially subducted is released into fluids (by decarbonation and/or carbonate dissolution) and 18-43 Mt/yr is returned at arc volcanoes. Current estimates suggest the amount of C released into subduction fluids is greater than that degassed at arc volcanoes: the imbalance could reflect C subduction into the deeper mantle, beyond subarc regions, or storage of C in forearc/subarc reservoirs. We examine the fate of C in plate-interface ultramafic rocks, and by analogy serpentinized mantle wedge, via study of fluid-rock evolution of marble and variably carbonated serpentinite in the Ligurian Alps. Based on petrography, major and trace element concentrations, and carbonate C and O isotope compositions, we demonstrate that serpentinite dehydration at 2-2.5 GPa, 550 °C released aqueous fluids triggering breakdown of dolomite in nearby marbles, thus releasing C into fluids. Carbonate + olivine veins document flow of COH fluids and that the interaction of these COH fluids with serpentinite led to the formation of high-P carbonated ultramafic-rock domains (high-P ophicarbonates). We estimate that this could result in the retention of ∼0.5-2.0 Mt C/yr in such rocks along subduction interfaces. As another means of C storage, 1 to 3 km-thick layers of serpentinized forearc mantle wedge containing 50 modal % dolomite could sequester 1.62 to

  19. Fast rates of subduction erosion along the Costa Rica Pacific margin: Implications for nonsteady rates of crustal recycling at subduction zones

    USGS Publications Warehouse

    Vannucchi, P.; Ranero, C.R.; Galeotti, S.; Straub, S.M.; Scholl, D. W.; McDougall-Ried, K.

    2003-01-01

    At least since the middle Miocene (???16 Ma), subduction erosion has been the dominant process controlling the tectonic evolution of the Pacific margin of Costa Rica. Ocean Drilling Program Site 1042 recovered 16.5 Ma nearshore sediment at ???3.9 km depth, ???7 km landward of the trench axis. The overlying Miocene to Quaternary sediment contains benthic foraminifera documenting margin subsidence from upper bathyal (???200 m) to abyssal (???2000 m) depth. The rate of subsidence was low during the early to middle Miocene but increased sharply in the late Miocene-early Pliocene (5-6.5 Ma) and at the Pliocene-Pleistocene boundary (2.4 Ma). Foraminifera data, bedding dip, and the geometry of slope sediment indicate that tilting of the forearc occurred coincident with the onset of rapid late Miocene subsidence. Seismic images show that normal faulting is widespread across the continental slope; however, extension by faulting only accounts for a minor amount of the post-6.5 Ma subsidence. Basal tectonic erosion is invoked to explain the subsidence. The short-term rate of removal of rock from the forearc is about 107-123 km3 Myr-1 km-1. Mass removal is a nonsteady state process affecting the chemical balance of the arc: the ocean sediment input, with the short-term erosion rate, is a factor of 10 smaller than the eroded mass input. The low 10Be concentration in the volcanic arc of Costa Rica could be explained by dilution with eroded material. The late Miocene onset of rapid subsidence is coeval with the arrival of the Cocos Ridge at the subduction zone. The underthrusting of thick and thermally younger ocean crust decreased the subduction angle of the slab along a large segment of the margin and changed the dynamic equilibrium of the margin taper. This process may have induced the increase in the rate of subduction erosion and thus the recycling of crustal material to the mantle. Copyright 2003 by the American Geophysical Union.

  20. Density model of the Cascadia subduction zone

    USGS Publications Warehouse

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

    2001-01-01

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

  1. Consolidation state of incoming sediments to the Nankai Trough subduction zone: Implications for sediment deformation and properties

    NASA Astrophysics Data System (ADS)

    Kitajima, Hiroko; Saffer, Demian M.

    2014-07-01

    hydromechanical properties of accreted and underthrust sediments are key parameters controlling the mechanics of earthquakes and the development of fluid pressure in subduction zones. We conducted consolidation tests on sediments from the Philippine Sea Plate (PSP) in the Nankai Trough to understand the consolidation state and hydraulic properties of the incoming sediment section before its incorporation into the subduction zone. We used mudstone and sandstone cores sampled from the Integrated Ocean Drilling Program Nankai Trough Seismogenic Zone Experiment at two reference sites (Site C0011 located on a basement low; and Site C0012 located on a basement high). Our experimental results indicate that most of the mudstone samples are normally consolidated or overconsolidated, with overconsolidation ratios (OCR) ranging from 0.89 to 2.52 at Site C0011 and 0.86 to 3.85 at Site C0012. Higher OCR values at Site C0012, at least at shallow depths, are likely caused by erosional unloading. This implies that Site C0011 may serve as a better geotechnical reference site. We also find that mudstones accreted along the frontal thrust are severely overconsolidated relative to coeval mudstones at Site C0011, which likely reflects enhanced consolidation due to increased horizontal tectonic stress. Sandstones in the incoming section on the PSP exhibit 2-3 orders of magnitude higher in situ permeability than the mudstones, and the siliciclastic sandstone we tested maintains a high permeability at stresses up to at least 70 MPa, suggesting that the sandstones may act as important pathways for drainage or pore pressure translation from depths of several kilometers.

  2. A long-term rock uplift rate for eastern Crete and geodynamic implications for the Hellenic subduction zone

    NASA Astrophysics Data System (ADS)

    Strobl, M.; Hetzel, R.; Fassoulas, C.; Kubik, P. W.

    2014-08-01

    The island of Crete in the forearc of the Hellenic subduction zone has a rugged topography with local relief exceeding 2 km. Based on the elevation of marine shorelines, rates of rock uplift during the Late Holocene were previously estimated to range between 1 and 4 mm/a in different parts of the island. These rates may, however, not be representative for longer timescales, because subduction earthquakes with up to 9 m of vertical coseismic displacement have affected Crete in the Late Holocene. Here we use a well preserved sequence of marine terraces near Kato Zakros in eastern Crete to determine the rate of rock uplift over the last ∼600 ka. Field investigations and topographic profiles document a flight of more than 13 marine bedrock terraces that were carved into limestones of the Tripolitza unit. Preliminary age constraints for the terraces were obtained by 10Be exposure dating of rare quartz-bearing sandstone clasts, which are present on some terraces. The 10Be ages of these samples, which have been corrected for an inherited nuclide component, yielded exposure ages between ∼100 ka and zero. Combined with geomorphologic evidence the two oldest 10Be ages suggest that the terraces T4 and T5, with shoreline angles at an elevation of ∼68 and ∼76 m above sea level, respectively, formed during the marine isotope stage 5e about 120 ka ago. The correlation of the higher terraces (T6 to T13) with regional sea-level highstands indicates sustained rock uplift at a rate of ∼0.5 m/ka since at least ∼600 ka. As normal faulting has dominated the tectonics of Crete during the last several million years, upper crustal shortening can be ruled out as a cause for rock uplift. We argue that the sustained uplift of the island results from the continuous underplating of sediments, which are transferred from the subducting African plate to the base of the crust beneath Crete.

  3. Supra-subduction zone extensional magmatism in Vermont and adjacent Quebec: Implications for early Paleozoic Appalachian tectonics

    USGS Publications Warehouse

    Kim, J.; Coish, R.; Evans, M.; Dick, G.

    2003-01-01

    Metadiabasic intrusions of the Mount Norris Intrusive Suite occur in fault-bounded lithotectonic packages containing Stowe, Moretown, and Cram Hill Formation lithologies in the northern Vermont Rowe-Hawley belt, a proposed Ordovician arc-trench gap above an east-dipping subduction zone. Rocks of the Mount Norris Intrusive Suite are characteristically massive and weakly foliated, have chilled margins, contain xenoliths, and have sharp contacts that both crosscut and are parallel to early structural fabrics in the host metasedimentary rocks. Although the mineral assemblage of the Mount Norris Intrusive Suite is albite + actinolite + epidote + chlorite + calcite + quartz, intergrowths of albite + actinolite are probably pseudomorphs after plagioclase + clinopyroxene. The metadiabases are subalkaline, tholeiitic, hypabyssal basalts with preserved ophitic texture. A backarc-basin tectonic setting for the intrusive suite is suggested by its LREE (light rare earth element) enrichment, negative Nb-Ta anomalies, and Ta/Yb vs. Th/Yb trends. Although no direct isotopic age data are available, the intrusions are broadly Ordovician because their contacts are clearly folded by the earliest Acadian (Silurian-Devonian) folds. Field evidence and geochemical data suggest compelling along-strike correlations with the Coburn Hill Volcanics of northern Vermont and the Bolton Igneous Group of southern Quebec. Isotopic and stratigraphic age constraints for the Bolton Igneous Group bracket these backarc magmas to the 477-458 Ma interval. A tectonic model that begins with east-dipping subduction and progresses to outboard west-dipping subduction after a syncollisional polarity reversal best explains the intrusion of deformed metamorphosed metasedimentary rocks by backarc magmas.

  4. Nature of Subduction Megathrust Faults at the Ryukyu Subduction Zone

    NASA Astrophysics Data System (ADS)

    Arai, R.; Kaiho, Y.; Takahashi, T.; Nakanishi, A.; Fujie, G.; Nakamura, Y.; Miura, S.; Kodaira, S.; Kaneda, Y.

    2015-12-01

    The Ryukyu subduction zone (RSZ) has been intensively examined in terms of seismic coupling along the plate boundary and tsunami potentials. On the contrary to other subduction zones nearby, such as the Nankai Trough and the Japan Trench, the RSZ has lacked clear evidence of great interplate earthquakes (M>8) for the last few hundred years and thus the overall interplate coupling is thought to be weak (Peterson and Seno, 1984). Correspondingly, geodetic observation implies that a possible coupled zone is narrow and limited to a shallow portion of the plate boundary near the trench (Ando et al., 2009). Recent seismic studies show that very low frequency earthquakes (VLFEs) are ubiquitously distributed in the forearc region of the Ryukyu arc, implying a variety of slip behaviors along the subduction faults related to fluid distribution (Nakamura and Sunagawa, 2015). However, these findings were derived from land station network and did not have sufficient resolution near the trench to determine spatial relationship of megathrust faults to the seismic activity. Since 2013 we JAMSTEC have been carrying out marine active- and passive-source seismic experiments in the Ryukyu subduction zone to reveal the fine-scale geometry and nature of subduction faults. Here we present integrated seismological evidence for megathrust fault structure and its relation to VLFEs at the southern Ryukyu Trench. Active-source seismic data consistently reveal that the plate boundary and backstop interface form a 40-km-wide frontal prism where low-velocity sedimentary rocks fill in. We find VLFEs occur around the low-velocity wedge where fluids are distributed as suggested by negative polarity in the reflection data. This forearc structure is also coincident with the source region of Yaeyama earthquake tsunami in 1771 (Nakamura, 2009), the most devastating disaster known in this region. Slow ruptures enhanced by the fluid-rich condition at the plate boundary and/or surrounding faults may be a

  5. Partitioning of halogens between mantle minerals and aqueous fluids: implications for the fluid flow regime in subduction zones

    NASA Astrophysics Data System (ADS)

    Bernini, Diego; Wiedenbeck, Michael; Dolejš, David; Keppler, Hans

    2013-01-01

    We have performed phase equilibrium experiments in the system forsterite-enstatite-pyrope-H2O with MgCl2 or MgF2 at 1,100 °C and 2.6 GPa to constrain the solubility of halogens in the peridotite mineral assemblage and the fluid-mineral partition coefficients. The chlorine solubility in forsterite, enstatite and in pyrope is very low, 2.1-3.9 and 4.0-11.4 ppm, respectively, and it is independent of the fluid salinity (0.3-30 wt% Cl), suggesting that some intrinsic saturation limit in the crystal is reached already at very low chlorine concentrations. Chlorine is therefore exceedingly incompatible in upper-mantle minerals. The fluorine solubility is 170-336 ppm in enstatite and 510-1,110 ppm in pyrope, again independent of fluid salinity. Forsterite dissolves 1,750-1,900 ppm up to a fluid salinity of 1.6 wt% F. At higher fluorine contents in the system, forsterite is replaced by the minerals of the humite group. The lower solubility of chlorine by three orders of magnitude when compared to fluorine is consistent with increasing lattice strain. Fluid-mineral partition coefficients are 100-102 for fluorine and 103-105 for chlorine. Since the latter values are orders of magnitude higher than those for hydroxyl partitioning, fluid flow from the subducting slab through the mantle wedge will lead to an efficient sequestration of H2O into the nominally anhydrous minerals in the wedge, whereas chlorine becomes enriched in the residual fluid. Simple mass balance calculations reveal that rock-fluid ratios of up to >3,000 are required to produce the elevated Cl/H2O ratios observed in some primitive arc magmas. Accordingly, fluid flow from the subducted slab into the zone of melting in the mantle wedge does not only occur rapidly in narrow channels, but at least in some subduction zones, fluid pervasively infiltrates the mantle peridotite and interacts with a large volume of the mantle wedge. Together with the Cl/H2O ratios of primitive arc magmas, our data therefore constrain

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

    USGS Publications Warehouse

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

    2015-01-01

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

  7. Osmium Recycling in Subduction Zones

    PubMed

    Brandon; Creaser; Shirey; Carlson

    1996-05-10

    Peridotite xenoliths from the Cascade arc in the United States and in the Japan arc have neodymium and osmium isotopic compositions that are consistent with addition of 5 to 15 percent of subducted material to the present-day depleted mantle. These observations suggest that osmium can be partitioned into oxidized and chlorine-rich slab-derived fluids or melts. These results place new constraints on the behavior of osmium (and possibly other platinum group elements) during subduction of oceanic crust by showing that osmium can be transported into the mantle wedge. PMID:8662577

  8. Osmium Recycling in Subduction Zones

    PubMed

    Brandon; Creaser; Shirey; Carlson

    1996-05-10

    Peridotite xenoliths from the Cascade arc in the United States and in the Japan arc have neodymium and osmium isotopic compositions that are consistent with addition of 5 to 15 percent of subducted material to the present-day depleted mantle. These observations suggest that osmium can be partitioned into oxidized and chlorine-rich slab-derived fluids or melts. These results place new constraints on the behavior of osmium (and possibly other platinum group elements) during subduction of oceanic crust by showing that osmium can be transported into the mantle wedge.

  9. A long-term rock uplift rate for eastern Crete and geodynamic implications for the Hellenic subduction zone

    NASA Astrophysics Data System (ADS)

    Strobl, M.; Hetzel, R.; Fassoulas, C.; Kubik, P.

    2014-12-01

    The island of Crete in the forearc of the Hellenic subduction zone has a rugged topography with a local relief exceeding 2 km. Based on the elevation of marine shorelines, rates of rock uplift during the Late Holocene were previously estimated to range between 1 and 4 mm/a in different parts of the island (e.g. Lambeck, 1995). These rates may, however, not be representative for longer timescales, because subduction earthquakes with up to 9 m of vertical coseismic displacement have recently affected Crete (Stiros, 2001). Here we use a well preserved sequence of marine terraces near Kato Zakros in eastern Crete to determine the rate of rock uplift over the last ˜600 ka. Field investigations and topographic profiles document a flight of more than 13 marine bedrock terraces that were carved into limestones of the Tripolitza unit. Preliminary age constraints for the terraces were obtained by 10Be exposure dating of rare quartz-bearing sandstone clasts, which are present on some terraces. The 10Be ages of these samples, which have been corrected for an inherited nuclide component, yielded exposure ages between ˜100 ka and zero. Combined with geomorphologic evidence the two oldest 10Be ages suggest that the terraces T4 and T5, with shoreline angles at an elevation of ca. 68 and ca. 76 m above sea level, respectively, formed during the marine isotope stage 5e about 120 ka ago. The correlation of the higher terraces (T6 to T13) with regional sea-level high-stands (Siddall et al., 2003) indicates sustained rock uplift at a rate of ˜0.5 m/ka since at least ˜600 ka. As normal faulting has dominated the tectonics of Crete during the last several million years, upper crustal shortening can be ruled out as a cause for rock uplift. We argue that the sustained uplift of the island results from the continuous underplating of sediments, which are transferred from the subducting African plate to the base of the crust beneath Crete. Lambeck, 1995, Geophys. J. Int. 122, 1022

  10. Mid mantle seismic anisotropy around subduction zones

    NASA Astrophysics Data System (ADS)

    Faccenda, M.

    2014-02-01

    There is increasing evidence for mid mantle seismic anisotropy around subduction zones whose interpretation remains elusive. In this study I estimate the strain-induced mid mantle fabric and associated seismic anisotropy developing in 3D petrological-thermo-mechanical subduction models where the slab is either stagnating over the 660 km discontinuity or penetrating into the lower mantle. The modelling of synthetic lattice-preferred-orientation (LPO) development of wadsleyite and perovskite has been calibrated with results from deformational experiments and ab-initio atomic scale models, and the single crystal elastic tensor of the different mineral phases is scaled by local P-T conditions. The lower transition zone (ringwoodite + garnet) is assumed to be isotropic. Mid mantle fabric develops in proximity of the subducting slab where deformation and stresses are high, except at depths where upwelling or downwelling material undergoes phase transformations, yielding to LPO reset. The upper transition zone (wadsleyite + garnet) is characterized by weak transverse isotropy (2-3%) with symmetry axes oriented and fast S wave polarized dip-normal. A slightly stronger transverse isotropy develops in the lower mantle (perovskite + periclase), where the symmetry axes, the polarization of the fast S wave and the maximum Vp and dVs are parallel to the slab dip and subduction direction. For stagnating slab models this translates into negative and positive radial anisotropy in the upper transition zone and lower mantle back-arc, respectively, minimum delay times for vertically travelling shear waves and large shear wave splitting for waves propagating horizontally in the lower mantle. These results may help in reconciling the seismic anisotropy patterns observed in some subduction zones with subduction-induced deformation, such as those measured in the mid mantle between the Australian plate and the New Hebrides-Tonga-Kermadec trenches that I interpret as related to stagnating

  11. The Sulfur Cycle at Subduction Zones

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

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

  12. Metamorphic Perspectives of Subduction Zone Volatiles Cycling

    NASA Astrophysics Data System (ADS)

    Bebout, G. E.

    2008-12-01

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

  13. Buoyant subduction on Venus: Implications for subduction around coronae

    NASA Astrophysics Data System (ADS)

    Burt, J. D.; Head, J. W.

    1993-03-01

    Potentially low lithospheric densities, caused by high Venus surface and perhaps mantle temperatures, could inhibit the development of negative buoyancy-driven subduction and a global system of plate tectonics/crustal recycling on that planet. No evidence for a global plate tectonic system was found so far, however, specific features strongly resembling terrestrial subduction zones in planform and topographic cross-section were described, including trenches around large coronae and chasmata in eastern Aphrodite Terra. The cause for the absence, or an altered expression, of plate tectonics on Venus remains to be found. Slab buoyancy may play a role in this difference, with higher lithospheric temperatures and a tendency toward positive buoyancy acting to oppose the descent of slabs and favoring under thrusting instead. The effect of slab buoyancy on subduction was explored and the conditions which would lead to under thrusting versus those allowing the formation of trenches and self-perpetuating subduction were defined. Applying a finite element code to assess the effects of buoyant forces on slabs subducting into a viscous mantle, it was found that mantle flow induced by horizontal motion of the convergent lithosphere greatly influences subduction angle, while buoyancy forces produce a lesser effect. Induced mantle flow tends to decrease subduction angle to near an under thrusting position when the subducting lithosphere converges on a stationary overriding lithosphere. When the overriding lithosphere is in motion, as in the case of an expanding corona, subduction angles are expected to increase. An initial stage involved estimating the changes in slab buoyancy due to slab healing and pressurization over the course of subduction. Modeling a slab, descending at a fixed angle and heated by conduction, radioactivity, and the heat released in phase changes, slab material density changes due to changing temperature, phase, and pressure were derived.

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

    NASA Astrophysics Data System (ADS)

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

    2016-04-01

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

  15. The Seismic Coupling of Subduction Zones Revisited

    NASA Astrophysics Data System (ADS)

    Scholz, C.; Campos, J.

    2012-04-01

    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.

  16. The seismic coupling of subduction zones revisited

    NASA Astrophysics Data System (ADS)

    Scholz, Christopher H.; Campos, Jaime

    2012-05-01

    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.

  17. Three-dimensional velocity structure of the outer fore arc of the Colombia-Ecuador subduction zone and implications for the 1958 megathrust earthquake rupture zone

    NASA Astrophysics Data System (ADS)

    García Cano, Lina Constanza; Galve, Audrey; Charvis, Philippe; Marcaillou, Boris

    2014-02-01

    In 2005, an onshore, offshore 3-D refraction and wide-angle reflection seismic experiment was conducted along the convergent margin at the border between Colombia and Ecuador, over the rupture zone of the 1958, Mw 7.6 subduction earthquake. A well-defined Vp velocity model of the plate boundary and upper and lower plates was constructed, down to 25 km depth, using first arrival traveltimes inversion. The model reveals a several kilometers thick, low-velocity zone in the upper plate, located immediately above the interplate contact. This low-velocity zone might be related to alteration and fracturing of the mafic and ultramafic rocks, which composed the upper plate in this area by fluids released by the lower plate with possible contributions from sediment underplating. Near the toe of the margin, the model shows a low-velocity gradient in the outer wedge, which is interpreted as highly faulted and fractured rocks. This low-velocity/low-gradient region appears to limit the oceanward extension of the rupture zones of the 1958 and 1979 earthquakes, possibly because coseismic deformation and uplift of the outer margin wedge dissipates most of the seismic energy.

  18. Molybdenum isotope systematics in subduction zones

    NASA Astrophysics Data System (ADS)

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

    2016-08-01

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

  19. Segmentation of the Lesser Antilles subduction zone

    NASA Astrophysics Data System (ADS)

    Wadge, G.; Shepherd, J. B.

    1984-12-01

    Precise hypocentral locations of earthquakes recorded on local seismic networks from 1978 to 1984 have established the configuration of the Benioff zone beneath the Lesser Antilles island arc. There are two distinct segments to the zone: one to the north of Martinique which trends NNW and one to the south of St. Lucia which trends NNE. The northern zone dips at 60-50°. The southern zone has a dip of 50-45° in the north but is vertical in the south. Pleistocene volcanoes are aligned in distinct segments which closely mirror the kink in the Benioff zone. Mid-Miocene swarms of dykes in Martinique and St. Lucia have NW and NE azimuthal modes respectively. We interpret this to mean that this kink in the subduction zone existed as long ago as the Miocene. The kink may represent the deformation of a single subducting plate or two separate North and South American plates subducting beneath the Caribbean plate with a triple junction in the region of Martinique-St. Lucia.

  20. Zr complexation in high pressure fluids and silicate melts and implications for the mobilization of HFSE in subduction zones

    NASA Astrophysics Data System (ADS)

    Louvel, Marion; Sanchez-Valle, Carmen; Malfait, Wim J.; Testemale, Denis; Hazemann, Jean-Louis

    2013-03-01

    mobilization of HFSE in subduction zones. Fluid-rock interactions and/or P/T variations as fluids migrate through the mantle wedge could affect the stability of these complexes, triggering the precipitation of HFSE-bearing accessory phases that are eventually recycled into the mantle, contributing to the dispersion of HFSE. These processes provide a possible explanation for the characteristic HFSE depletion recorded in arc magmas.

  1. 3D velocity structure of the outer forearc of the Colombia-Ecuador subduction zone; implications for the 1958 megathrust earthquake rupture zone

    NASA Astrophysics Data System (ADS)

    Galve, A.; Charvis, P.; Garcia Cano, L.; Marcaillou, B.

    2013-12-01

    In 2005, we conducted an onshore-offshore 3D refraction and wide-angle reflection seismic experiment over the rupture zone of the 1958 subduction earthquake that occurred near the border between Colombia and Ecuador. This earthquake was part of a sequence of 3 large ruptures (1942, Mw=7.8; 1958, Mw=7.7; 1979, Mw=8.2), which successively broke from south to north the segments of the megathrust that had been ruptured in 1906 by a single, very large magnitude (8.8) earthquake. Using first arrival traveltime inversion, we constructed a well-defined Vp velocity model of the plate boundary and of the upper and lower plates, down to 25 km depth. The model reveals a 5-km thick, low velocity zone in the upper plate, located immediately above the interplate contact. Because similar low-velocity zones are commonly observed along margins made of oceanic or island-arc accreted terranes, we suggest that the low-velocity zone might result from the alteration and hydration of mafic and ultramafic rocks in the upper plate basement, rather than from hydrofracturing alone. Sediments underplated beneath the inner wedge might contribute to the low-velocity zone but it is unlikely that they are several kilometers thick. Nevertheless, fluids expelled by the compaction and dehydration of those underplated sediments possibly favor the alteration of the overlying rocks. The low-velocity zone is spatially coincident with the 1958 rupture area. Near the toe of the margin, the model shows a low velocity gradient in the outer wedge that we interpret as a zone of highly faulted and fractured rocks or of poorly consolidated sediments. This low velocity/low gradient region forms the oceanward limit of the rupture zones of both the 1958 and the 1979 earthquakes. We suggest that the two earthquake ruptures were arrested by the low velocity zone because its low rigidity contributed to dissipate most of the seismic energy and of the coseismic strain/stress. This might be the reason why the 1958

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

    NASA Astrophysics Data System (ADS)

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

    2016-09-01

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

  3. Geochemical and isotopic constraints on the age and origin of the Nidar Ophiolitic Complex, Ladakh, India: Implications for the Neo-Tethyan subduction along the Indus suture zone

    NASA Astrophysics Data System (ADS)

    Ahmad, T.; Tanaka, T.; Sachan, H. K.; Asahara, Y.; Islam, R.; Khanna, P. P.

    2008-04-01

    .P., 2001. Early Cretaceous radiolarians from the Indus suture zone, Ladakh, northern India. In: News of Osaka Micropaleontologists (NOM), Spec. Vol., 12, 257-270.) and cooling ages of 110-130 Ma based on 39Ar/ 40Ar for Nidar-Spontang ophiolitic rocks (Mahéo, G., Berttrand, H., Guillot, S., Villa, I. M., Keller, F., Capiez, P., 2004. The South Ladakh Ophiolites (NW Himalaya, India): an intra-oceanic tholeiitic arc origin with implications for the closure of the Neo-Tethys. Chem. Geol., 203, 273-303.). As these gabbroic and volcanic rocks are interpreted to be arc related, the new Sm-Nd age data may indicate that intra-ocean subduction in the Neo-Tethyan ocean may have started much before ˜ 140 ± 32 Ma as this date is interpreted as the age of crystallization of the arc magma. Present and published age data on the arc magmatic rocks from the Indus suture zone may collectively indicate episodic magmatism with increasing maturity of the arc from more basic (during ~ 140 ± 32 Ma) when the arc was immature through intermediate (andesitic/granodioritic) at ~ 100 Ma to more felsic (rhyolitic/dioritic) magmatism at ~ 50-45 Ma, when the Indian and the Asian plates collided.

  4. Temperature Models for the Mexican Subduction Zone

    NASA Astrophysics Data System (ADS)

    Manea, V. C.; Kostoglodov, V.; Currie, C.; Manea, M.; Wang, K.

    2002-12-01

    It is well known that the temperature is one of the major factors which controls the seismogenic zone. The Mexican subduction zone is characterized by a very shallow flat subducting interplate in its central part (Acapulco, Oaxaca), and deeper subduction slabs northern (Jalisco) and southern (Chiapas). It has been proposed that the seismogenic zone is controlled, among other factors, by a temperature. Therefore, we have developed four two-dimensional steady state thermal models for Jalisco, Guerrero, Oaxaca and Chiapas. The updip limit of the seismogenic zone is taken between 100 §C and 150 §C, while the downdip limit is thought to be at 350 §C because of the transition from stick-slip to stable-sliding. The shape of the subducting plate is inferred from gravity and seismicity. The convergence velocity between oceanic and continental lithospheric plates is taken as the following: 5 cm/yr for Jalisco profile, 5.5 for Guerrero profile, 5.8 for Oaxaca profile, and 7.8 for Chiapas profile. The age of the subducting plates, since they are young, and provides the primary control on the forearc thermal structure, are as the following: 11 My for Jalisco profile, 14.5 My for Guerrero profile, 15 My for Oaxaca profile, and 28 My for Chiapas profile. We also introduced in the models a small quantity of frictional heating (pore pressure ratio 0.98). The value of 0.98 for pore pressure ratio was obtained for the Guerrero profile, in order to fit the intersection between the 350 §C isotherm and the subducting plate at 200 Km from trench. The value of 200 km coupling zone from trench is inferred from GPS data for the steady interseismic period and also for the last slow aseismic slip that occurred in Guerrero in 2002. We have used this value of pore pressure ratio (0.98) for all the other profiles. For the others three profiles we obtained the following coupling extents: Jalisco - 100 km, Oaxaca - 170 km and Chiapas - 125 km (from the trench). Independent constrains of the

  5. Geochemistry of primary-carbonate bearing K-rich igneous rocks in the Awulale Mountains, western Tianshan: Implications for carbon-recycling in subduction zone

    NASA Astrophysics Data System (ADS)

    Yang, Wu-Bin; Niu, He-Cai; Shan, Qiang; Chen, Hua-Yong; Hollings, Pete; Li, Ning-Bo; Yan, Shuang; Zartman, Robert E.

    2014-10-01

    Arc magmatism plays an important role in the recycling of subducted carbon and returning it to the surface. However, the transfer mechanisms of carbon are poorly understood. In this study, the contribution of subducted carbonate-rich sediments to the genesis of the carbonate-bearing K-rich igneous rocks from western Tianshan was investigated. Four key triggers are involved, including sediments subduction, slab decarbonation, partial melting and magma segregation. The globular carbonate ocelli show C-O isotope signatures intermediate between oceanic sediments and mantle, suggesting that the carbon of the primary carbonate ocelli was derived from recycled subducted sediments in the mantle. Decarbonation of the subducted slab is regarded as the primary agent to carbonize the mantle wedge. Geochemical features indicate that the carbonate ocelli are primary, and that the parental K- and carbon-rich mafic alkaline magma was derived from partial melting of carbonated mantle wedge veined with phlogopite. Major and trace element compositions indicate that globular carbonate ocelli hosted in the Bugula K-rich igneous rocks are calcio-carbonate and formed primarily by segregation of the differentiated CO2-rich alkaline magma after crystallization fractionation. The K-rich alkaline magma, which formed from partial melting of metasomatized (i.e., phlogopite bearing) mantle wedge in the sub-arc region, is a favorable agent to transport subducted carbon back to the Earth's surface during carbon recycling in subduction zones, because of the high CO2 solubility in alkaline mafic magma. We therefore propose a model for the petrogenesis of the carbonate-bearing K-rich igneous rocks in western Tianshan, which are significant for revealing the mechanism of carbon recycling in subduction zones.

  6. Modeled temperatures and fluid source distributions for the Mexican subduction zone: Effects of hydrothermal circulation and implications for plate boundary seismic processes

    NASA Astrophysics Data System (ADS)

    Perry, Matthew; Spinelli, Glenn A.; Wada, Ikuko; He, Jiangheng

    2016-02-01

    In subduction zones, spatial variations in pore fluid pressure are hypothesized to control the sliding behavior of the plate boundary fault. The pressure-temperature paths for subducting material control the distributions of dehydration reactions, a primary control on the pore fluid pressure distribution. Thus, constraining subduction zone temperatures are required to understand the seismic processes along the plate interface. We present thermal models for three margin-perpendicular transects in the Mexican subduction zone. We examine the potential thermal effects of vigorous fluid circulation in a high-permeability aquifer within the basaltic basement of the oceanic crust and compare the results with models that invoke extremely high pore fluid pressures to reduce frictional heating along the megathrust. We combine thermal model results with petrological models to determine the spatial distribution of fluid release from the subducting slab and compare dewatering locations with the locations of seismicity, nonvolcanic tremor, slow-slip events, and low-frequency earthquakes. Simulations including hydrothermal circulation are most consistent with surface heat flux measurements. Hydrothermal circulation has a maximum cooling effect of 180°C. Hydrothermally cooled crust carries water deeper into the subduction zone; fluid release distributions in these models are most consistent with existing geophysical data. Our models predict focused fluid release, which could generate overpressures, coincident with an observed ultraslow layer (USL) and a region of nonvolcanic tremor. Landward of USLs, a downdip decrease in fluid source magnitude could result in the dissipation in overpressure in the oceanic crust without requiring a downdip increase in fault zone permeability, as posited in previous studies.

  7. Tomography and Dynamics of Western-Pacific Subduction Zones

    NASA Astrophysics Data System (ADS)

    Zhao, D.

    2012-01-01

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

  8. Strain accumulation along the Cascadia subduction zone

    USGS Publications Warehouse

    Murray, M.H.; Lisowski, M.

    2000-01-01

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

  9. Numerical Modelling of Subduction Zones: a New Beginning

    NASA Astrophysics Data System (ADS)

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

    2016-04-01

    , if any (e.g., Doglioni et al., 2007; Afonso et al., 2008). Therefore we attempt to generate a different model setup in which are included both a decoupling at the lithosphere base and the "westward" drift of the lithosphere that implies a relative "eastward" mantle flow. The method used for this task is an implementation of I2VIS code, a 2D thermomechanical code incorporating both a characteristics based marker-in-cell method and conservative finite-difference (FD) schemes (Gerya and Yuen, 2003). The implementation involves both the integration of the LVZ and the application of an incoming and outgoing mantle flow through the lateral boundaries of the rectangular box (that represent the basic setup of the models). This new insight in numerical modelling of subduction zones could help to have a more accurate comprehension on what is actually influencing subduction zones dynamics in order to successively explain what are the causes of this fundamental process and what are its implications on plate tectonics dynamics.

  10. Seismicity of the eastern Hellenic Subduction Zone

    NASA Astrophysics Data System (ADS)

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

    2012-04-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

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

  12. Subduction zone guided waves in Northern Chile

    NASA Astrophysics Data System (ADS)

    Garth, Thomas; Rietbrock, Andreas

    2016-04-01

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

  13. The radiated seismic energy and apparent stress of interplate and intraplate earthquakes at subduction zone environments; implications for seismic hazard estimation

    USGS Publications Warehouse

    Choy, George L.; Boatwright, John L.; Kirby, Stephen H.

    2001-01-01

    The radiated seismic energies (ES) of 980 shallow subduction-zone earthquakes with magnitudes ? 5.8 are used to examine global patterns of energy release and apparent stress. In contrast to traditional methods which have relied upon empirical formulas, these energies are computed through direct spectral analysis of broadband seismic waveforms. Energy gives a physically different measure of earthquake size than moment. Moment, being derived from the low-frequency asymptote of the displacement spectra, is related to the final static displacement. Thus, moment is crucial to the long-term tectonic implication of an earthquake. In contrast, energy, being derived from the velocity power spectra, is more a measure of seismic potential for damage to anthropogenic structures. There is considerable scatter in the plot of ES-M0 for worldwide earthquakes. For any given M0, the ES can vary by as much as an order of magnitude about the mean regression line. The global variation between ES and M0, while large, is not random. When subsets of ES-M0 are plotted as a function of seismic region, tectonic setting and faulting type, the scatter in data is often substantially reduced. There are two profound implications for the estimation of seismic and tsunamic hazard. First, it is now feasible to characterize the apparent stress for particular regions. Second, a given M0 does not have a unique ES. This means that M0 alone is not sufficient to describe all aspects of an earthquake. In particular, we have found examples of interplate thrust-faulting earthquakes and intraslab normal-faulting earthquakes occurring in the same epicentral region with vastly different macroseismic effects. Despite the gross macroseismic disparities, the MW?s in these examples were identical. However, the Me?s (energy magnitudes) successfully distinguished the earthquakes that were more damaging.

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

    USGS Publications Warehouse

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

    2009-01-01

    Arc magmatism at subduction zones (SZs) most voluminously supplies juvenile igneous material to build rafts of continental and intra-oceanic or island arc (CIA) crust. Return or recycling of accumulated CIA material to the mantle is also most vigorous at SZs. Recycling is effected by the processes of sediment subduction, subduction erosion, and detachment and sinking of deeply underthrust sectors of CIA crust. Long-term (>10-20 Ma) rates of additions and losses can be estimated from observational data gathered where oceanic crust underruns modern, long-running (Cenozoic to mid-Mesozoic) ocean-margin subduction zones (OMSZs, e.g. Aleutian and South America SZs). Long-term rates can also be observationally assessed at Mesozoic and older crust-suturing subduction zone (CSSZs) where thick bodies of CIA crust collided in tectonic contact (e.g. Wopmay and Appalachian orogens, India and SE Asia). At modern OMSZs arc magmatic additions at intra-oceanic arcs and at continental margins are globally estimated at c. 1.5 AU and c. 1.0 AU, respectively (1 AU, or Armstrong Unit,= 1 km3 a-1 of solid material). During collisional suturing at fossil CSSZs, global arc magmatic addition is estimated at 0.2 AU. This assessment presumes that in the past the global length of crustal collision zones averaged c. 6000 km, which is one-half that under way since the early Tertiary. The average long-term rate of arc magmatic additions extracted from modern OMSZs and older CSSZs is thus evaluated at 2.7 AU. Crustal recycling at Mesozoic and younger OMSZs is assessed at c. 60 km3 Ma-1 km-1 (c. 60% by subduction erosion). The corresponding global recycling rate is c. 2.5 AU. At CSSZs of Mesozoic, Palaeozoic and Proterozoic age, the combined upper and lower plate losses of CIA crust via subduction erosion, sediment subduction, and lower plate crustal detachment and sinking are assessed far less securely at c. 115 km3 Ma-1 km-1. At a global length of 6000 km, recycling at CSSZs is accordingly c. 0

  15. Acceleration spectra for subduction zone earthquakes

    USGS Publications Warehouse

    Boatwright, J.; Choy, G.L.

    1989-01-01

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

  16. Imaging the Subduction Decollement, Hikurangi Subduction Zone, New Zealand

    NASA Astrophysics Data System (ADS)

    Bannister, S.; Toulmin, S.; Henrys, S.; Reyners, M.; Barker, D.; Pecher, I.; Sutherland, R.; Uruski, C.; Maslen, G.

    2006-12-01

    Beneath the eastern coastline of North Island, New Zealand, the subducted Pacific plate dips at less than 3 degrees to the northwest and the subduction decollement is at a depth of less than 15-km. The active-source NIGHT and passive-source CNIPSE experiments carried out in 2001 image the shallow dipping decollement down to more than 6 s (twt) at which point it steepens landward, 120 km from the Hikurangi trench. This change in dip appears to be closely associated with the onset of seismogenesis in the subducted plate. Velocity inversion of CNIPSE earthquake times reveals the forearc to be a relatively low Vp (<5.5 km/s), high Vp/Vs (>1.85), high Poisson's ratio (>0.29) region overlying the 12-15 km thick subducted crust. In March-May 2005 a new industry-seismic survey, 05CM, was carried out offshore the east coast, seismically imaging the subducted plate. More than 278000 airgun shots were also recorded by temporary seismometer stations placed along the coastline, out to offsets of more than 100 km. The combined marine and offshore- onshore seismic data highlight an area of more than 400 sq.kms. with higher reflectivity on the subduction decollement up-dip of the up-dip limit of seismogenesis, which we further examine using AVO and finite- difference modelling.

  17. A mega shear zone in the Central Range of Taiwan and it's implication for the Late Mesozoic subduction of the paleo-Pacific plate

    NASA Astrophysics Data System (ADS)

    Yi, D. C.; Lin, C. W.

    2012-04-01

    The metamorphic basement "Tananao Complex" exposed in the eastern flank of the Central Range of Taiwan. The ancient Asian continental margin deposited a thick sequence of sandstone, shale, limestone and volcanic rocks that was the protolith of Tananao Complex. In Late Mesozoic Era, the thick sequence of rocks was subjected to several phases of metamorphism and deformation to form the pair metamorphic belts which were the western Tailuko Belt and the eastern Yuli Belt. The Tailuko belt is composed of phyllite, quartzite, quartz-mica schist, meta-conglomerate, gneiss, meta-basite, amphibolite, serpentinite, marble and meta-chert, etc. The Yuli belt is composed of a monotonous assemblage of quartz-mica schist, subordinate meta-basite and serpentinite, etc. It is believed that the boundary of the Tailuko belt and the Yuli belt is a large fault, but the field evidence of the fault has never been found. In this study, meso-scale field investigation of the lithologies and rock fabrics indicate that a mega shear zone, called "The Daguan shear zone", separated the Tailuko belt from the Yuli belt. The Daguan shear zone is a NNE trending and west dipping mega shear zone which is mainly composed of mylonitic dark gray quartz-mica schist and mica schist, intercalated with 1 to 2 centimeters thick of elongated meta-conglomerate band. The shear zone is composed of numerous meso-scale ductile shear zones. Additionally, the shaer zone is characterized by abundant varied quartz veins that have been refolded to lenticular or pod shape and nearly parallel to S2 cleavage. Compaed to the existing geological information of the Central Range, we believe that the Daguan shear zone played a role as the boundary of the subduction zone which the paleo-Pacific Plate subducted into the Eurasian Plate in Late Mesozoic Era.

  18. Subduction zone evolution and low viscosity wedges and channels

    NASA Astrophysics Data System (ADS)

    Manea, Vlad; Gurnis, Michael

    2007-12-01

    Dehydration of subducting lithosphere likely transports fluid into the mantle wedge where the viscosity is decreased. Such a decrease in viscosity could form a low viscosity wedge (LVW) or a low viscosity channel (LVC) on top of the subducting slab. Using numerical models, we investigate the influence of low viscosity wedges and channels on subduction zone structure. Slab dip changes substantially with the viscosity reduction within the LVWs and LVCs. For models with or without trench rollback, overthickening of slabs is greatly reduced by LVWs or LVCs. Two divergent evolutionary pathways have been found depending on the maximum depth extent of the LVW and wedge viscosity. Assuming a viscosity contrast of 0.1 with background asthenosphere, models with a LVW that extends down to 400 km depth show a steeply dipping slab, while models with an LVW that extends to much shallower depth, such as 200 km, can produce slabs that are flat lying beneath the overriding plate. There is a narrow range of mantle viscosities that produces flat slabs (5 to10 × 10 19 Pa s) and the slab flattening process is enhanced by trench rollback. Slab can be decoupled from the overriding plate with a LVC if the thickness is at least a few 10 s of km, the viscosity reduction is at least a factor of two and the depth extent of the LVC is several hundred km. These models have important implications for the geochemical and spatial evolution of volcanic arcs and the state of stress within the overriding plate. The models explain the poor correlation between traditional geodynamic controls, subducting plate age and convergence rates, on slab dip. We predict that when volcanic arcs change their distance from the trench, they could be preceded by changes in arc chemistry. We predict that there could be a larger volatile input into the wedge when arcs migrate toward the trench and visa-versa. The transition of a subduction zone into the flat-lying regime could be preceded by changes in the volatile

  19. Dynamics of intraoceanic subduction initiation: 1. Oceanic detachment fault inversion and the formation of supra-subduction zone ophiolites

    NASA Astrophysics Data System (ADS)

    Maffione, Marco; Thieulot, Cedric; van Hinsbergen, Douwe J. J.; Morris, Antony; Plümper, Oliver; Spakman, Wim

    2015-06-01

    Subduction initiation is a critical link in the plate tectonic cycle. Intraoceanic subduction zones can form along transform faults and fracture zones, but how subduction nucleates parallel to mid-ocean ridges, as in e.g., the Neotethys Ocean during the Jurassic, remains a matter of debate. In recent years, extensional detachment faults have been widely documented adjacent to slow-spreading and ultraslow-spreading ridges where they cut across the oceanic lithosphere. These structures are extremely weak due to widespread occurrence of serpentine and talc resulting from hydrothermal alteration, and can therefore effectively localize deformation. Here, we show geochemical, tectonic, and paleomagnetic evidence from the Jurassic ophiolites of Albania and Greece for a subduction zone formed in the western Neotethys parallel to a spreading ridge along an oceanic detachment fault. With 2-D numerical modeling exploring the evolution of a detachment-ridge system experiencing compression, we show that serpentinized detachments are always weaker than spreading ridges. We conclude that, owing to their extreme weakness, oceanic detachments can effectively localize deformation under perpendicular far-field forcing, providing ideal conditions to nucleate new subduction zones parallel and close to (or at) spreading ridges. Direct implication of this, is that resumed magmatic activity in the forearc during subduction initiation can yield widespread accretion of suprasubduction zone ophiolites at or close to the paleoridge. Our new model casts the enigmatic origin of regionally extensive ophiolite belts in a novel geodynamic context, and calls for future research on three-dimensional modeling of subduction initiation and how upper plate extension is associated with that.

  20. Sublithospheric Triggers for Episodic Silicic Magmatism in Subduction Zones

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

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

  1. P-and S-wave velocities of exhumed metasediments from the Aleutian subduction megathrust: Implications for the interpretation of low velocity zones and fault reflectivity

    NASA Astrophysics Data System (ADS)

    Miller, P.; Bate, C.; Saffer, D. M.; Abers, G. A.; Shillington, D. J.; Keranen, K. M.; Becel, A.; Li, J.

    2015-12-01

    Subduction zones are host to the planet's largest recorded earthquakes and tsunamis. The strength and slip behavior of the megathrust plate interface is tightly linked to the in situ conditions and physical state of sediments that are entrained along the plate interface. The mechanical properties of these sediments change as they are heated, consolidated, and metamorphosed with progressive subduction and burial beneath the forearc; understanding this evolution of rock properties is essential toward quantitative interpretation of geophysical signatures in the context of stress state, pore fluid pressure, and composition down-dip along the megathrust. In particular, observed zones of high-amplitude seismic reflectivity, low P- and S-wave velocities (Vp and Vs), and high Vp/Vs along subduction megathrusts have been widely interpreted to indicate near-lithostatic pore fluid pressure, yet the role of low-velocity and highly anisotropic metasediments along and beneath the plate interface remain weakly constrained. To address this question, we conducted triaxial deformation experiments on a suite of argillaceous metasedimentary fault zone rocks exhumed from ~12-15 depth in the Aleutian subduction zone, in order to define Vp and Vs at mean effective stresses up to ~100 MPa. In our experiments we observed a rapid increase in Vp with increasing effective stress that we interpret to be microcrack closure. We also observed significant velocity anisotropy in samples tested perpendicular and parallel to foliation with respective Vp of 5.0 km/s and 5.9 km/s (17% anisotropy) at peak stress. Velocity predictions based on whole rock composition and simple assumptions about pore structure and volume tend to over predict observed velocities, but this discrepancy may be reconciled if pores are highly elongate. Synthetic waveform modeling shows that these wavespeeds are broadly consistent with observed reflectivity and low-frequency low velocity zones at 20-40 km depth (10-30 km from

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

    USGS Publications Warehouse

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

    1997-01-01

    Geochemical differences between island arc basalts (LAB) and ocean-floor basalts (mid-ocean ridge basalts; MORB) suggest that the large-ion lithophile elements (LILE) K, Ba, Rb and Cs are probably mobilized in subduction zone fluids and melts. This study documents LILE enrichment of eclogite, amphibolite, and epidote ?? garnet blueschist tectonic blocks and related rocks from melanges of two subduction complexes. The samples are from six localities of the Franciscan Complex, California, and related terranes of Oregon and Baja California, and from the Samana Metamorphic Complex, Samana Peninsula, Dominican Republic. Most Franciscan blocks are MORB-like in their contents of rare earth elements (REE) and high field strength elements (HFSE); in contrast, most Samana blocks show an LAB signature of these elements. The whole-rock K2O contents of both groups range from 1 to 3 wt %; K, Ba, Rb, and Cs are all strongly intercorrelated. Many blocks display K/Ba similar to melasomatized transition zones and rinds at their outer margins. Some transition zones and rinds are enriched in LILE compared with host blocks; others are relatively depleted in these elements. Some LILE-rich blocks contain 'early' coarse-grained muscovite that is aligned in the foliation defined by coarse-grained omphacite or amphibole grains. Others display 'late' muscovite in veins and as a partial replacement of garnet; many contain both textural types. The muscovite is phengite that contains ???3??25-3??55 Si per 11 oxygens, and ???0??25-0??50 Mgper 11 oxygens. Lower-Si phengite has a significant paragonite component: Na per 11 oxygens ranges to ???0??12. Ba contents of phengite range to over 1 wt % (0??027 per 11 oxygens). Ba in phengite does not covary strongly with either Na or K. Ba contents of phengite increase from some blocks to their transition zones or rinds, or from blocks to their veins. Averaged KlBa ratios for phengite and host samples define an array which describes other subsamples of

  3. Geology of the Eel River basin and adjacent region: Implications for late Cenozoic tectonics of the southern Cascadia subduction zone and Mendocino triple junction

    SciTech Connect

    Clarke, S.H. Jr. )

    1992-02-01

    Two upper Cenozoic depositional sequences of principally marine strata about 4,000 m thick overlie accreted basement terranes of the Central and Coastal belts of the Franciscan Complex in the onshore-offshore Eel River basin of northwestern California. The older depositional sequence is early to middle Miocene in age and represents slope basin and slope-blanket deposition, whereas the younger sequence, later Miocene to middle Pleistocene in age, consists largely of forearc basin deposits. Youthful tectonic activity related to Gorda-North American plate convergence indicates an active Cascadia subduction zone and strong partial coupling between these plates. Structures of the northeastern margin of the Eel River basin are principally north-northwest-trending, east-northeast-dipping thrust and reverse faults that form imbricate thrust fans. The Coastal belt fault, the early Tertiary accretionary suture between the Franciscan Central and Coastal belts, can be traced from Arcata Bay northward offshore to the southern Oregon border. It is tentatively extended farther northward based on aeromagnetic data to an offshore position west of Cape Blanco. Thereafter, it may coincide with the offshore Fulmar fault. The Cascadia subduction zone (CSZ) does not join the Mendocino transform fault at the commonly depicted offshore location of the Mendocino triple junction (MTJ). Instead, the CSZ extends southeastward around the southern Eel River basin and shoreward along Mendocino Canyon to join the Petrolia shear zone. Similarly, the Mendocino fault may extend shoreward via Mattole Canyon and join the Cooskie shear zone. These two shear zones intersect onshore north of the King Range, and the area of their intersection is the probable location of the MTJ.

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

    USGS Publications Warehouse

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

    2005-01-01

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

  5. Geochemistry of tholeiitic to alkaline lavas from the east of Lake Van (Turkey): Implications for a late Cretaceous mature supra-subduction zone environment

    NASA Astrophysics Data System (ADS)

    Özdemir, Yavuz

    2016-08-01

    Arc-related rocks of the Yüksekova Complex extend from Kahramanmaraş to Hakkari throughout the Southeast Anatolia representing the remnants of the Southern Branch of Neotethys. The volcanic members of this zone from the eastern parts of Lake Van suggest three different types of rock chemistry; tholeiitic (type I), calc-alkaline (type II) and alkaline (type III). Tholeiitic and calc-alkaline members suggest a subduction-related environment with their HFS and LIL element distributions. RE and trace element systematics and modelings indicate that i) the intermediate and the felsic calc-alkaline rocks are the result of fractional crystallization from a basic endmember, ii) alkaline members have originated from enriched mantle source relative to the tholeiitic and calc-alkaline lavas. Overall data from Yüksekova Complex suggest a mature supra-subduction zone environment within the southern Neotethyan Ocean during Upper Cretaceous time. The existence of Lutetian OIB like asthenospheric lavas at the upper parts of the ophiolitic assemblage in the eastern parts of Lake Van proposes the end of the normal ophiolite formation and the possible continuation of the magmatism with OIB like lavas during Middle Eocene.

  6. Mass transfer in subduction zones: an elemental and isotopic perspective

    NASA Astrophysics Data System (ADS)

    Turner, S.; George, R.

    2005-12-01

    Little doubt remains that subduction zone lavas contain elements recycled from the subducting slab. However, whether the key agents of this mass transfer are fluids, supercritical fluids or melts has major implications for the thermal structure of the mantle wedge. The evidence for contributions from both subducted sediment and altered oceanic crust are compelling and in most arcs their relative proportions vary inversely. Thus, so-called "fluid-rich" lavas with high Ba/Th and Sr.Th ratios have low Sr and Be isotopes etc and the converse is true for the so-called "sediment-rich"lavas with elevated La/Sm. A complicating factor is that many individual arcs tend to be dominated by one end-member. Nevertheless, experimental partition coefficient data are consistent with the differences between the fluid and sediment components being formed in the presence of different residual mineralogies. Sediment fluids appear to be poor in incompatible elements, relative to those derived from altered oceanic crust and cannot easily replicate the sediment end-member. We suggest that subducted sediments dehydrate at relatively shallow levels and that these fluids are not strongly sampled by arc lavas. Altered oceanic crust may dehydrate more extensively and to greater depths and may be buffered against melting. Model melts of dehydrated sediment residues provide a much better simulation of the inferred sediment end-member but may require ~800 C at ~ 2GPa, consistent with recent temperature-dependant viscosity models. These general inferences are strongly supported by Be and U-series isotope data which suggest that the sediment (melt) end-member is added 100's kyr to several Myr prior to eruption whereas addition of fluid components continues until a few 1000 yrs prior to eruption. Thus, the fluid and sediment end-member contributions are separate in composition, space and time. These data argue strongly against the involvement of any single supercritical fluid.

  7. GPS Monitoring of Subduction Zone Deformation in Costa Rica

    NASA Technical Reports Server (NTRS)

    Lundgren, Paul

    1997-01-01

    The subduction of the Cocos plate beneath Costa Rica is among the highest convergence rates in the world. The high subduction rate and nearness of the Nicoya Peninsula, Costa Rica to the Middle America Trench (MAT) provide a unique opportunity to map variations in interseismic strain of the crust above the seismogenic zone in response to variations in seismic coupling.

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

    NASA Astrophysics Data System (ADS)

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

    2010-12-01

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

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

    USGS Publications Warehouse

    Hardebeck, Jeanne L.

    2015-01-01

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

  10. Ambient noise surface wave tomography of the Makran subduction zone, south-east Iran: Implications for crustal and uppermost mantle structures

    NASA Astrophysics Data System (ADS)

    Abdetedal, Mahsa; Shomali, Zaher Hossein; Gheitanchi, Mohammad Reza

    2015-08-01

    Seismic ambient noise of surface wave tomography was applied to estimate Rayleigh wave empirical Green's functions (EGFs) and then to study crust and uppermost mantle structure beneath the Makran region in south-east Iran. 12 months of continuous data from January 2009 through January 2010, recorded at broadband seismic stations, were analyzed. Group velocities of the fundamental mode Rayleigh wave dispersion curves were obtained from the empirical Green's functions. Multiple-filter analysis was used to plot group velocity variations at periods from 10 to 50 s. Using group velocity dispersion curves, 1-D v S velocity models were calculated between several station pairs. The final results demonstrate significant agreement to known geological and tectonic features. Our tomography maps display low-velocity anomaly with SW-NE trend, comparable with volcanic arc settings of the Makran region which may be attributable to the geometry of Arabian Plate subducting beneath the overriding the Lut block. The northward subducting Arabian Plate is determined by high-velocity anomaly along the Straits of Hormuz. At short periods (<20 s), there is a sharp transition boundary between low- and high-velocity transition zone with the NW trending at the western edge of Makran which is attributable to the Minab fault system.

  11. Subduction obliquity as a prime indicator for geotherm in subduction zone

    NASA Astrophysics Data System (ADS)

    Plunder, Alexis; Thieulot, Cédric; van Hinsbergen, Douwe

    2016-04-01

    The geotherm of a subduction zone is thought to vary as a function of subduction rate and the age of the subducting lithosphere. Along a single subduction zone the rate of subduction can strongly vary due to changes in the angle between the trench and the plate convergence vector, namely the subduction obliquity. This phenomenon is observed all around the Pacific (i.e., Marianna, South America, Aleutian…). However due to observed differences in subducting lithosphere age or lateral convergence rate in nature, the quantification of temperature variation due to obliquity is not obvious. In order to investigate this effect, 3D generic numerical models were carried out using the finite element code ELEFANT. We designed a simplified setup to avoid interaction with other parameters. An ocean/ocean subduction setting was chosen and the domain is represented by a 800 × 300 × 200 km Cartesian box. The trench geometry is prescribed by means of a simple arc-tangent function. Velocity of the subducting lithosphere is prescribed using the analytical solution for corner flow and only the energy conservation equation is solved in the domain. Results are analysed after steady state is reached. First results show that the effect of the trench curvature on the geotherm with respect to the convergence direction is not negligible. A small obliquity yields isotherms which are very slightly deflected upwards where the obliquity is maximum. With an angle of ˜30°, the isotherms are deflected upwards of about 10 kilometres. Strong obliquity (i.e., angles from 60° to almost 90°) reveal extreme effects of the position of the isotherms. Further model will include other parameter as the dip of the slab and convergence rate to highlight their relative influence on the geotherm of subduction zone.

  12. Multi-stage origin of the Coast Range ophiolite, California: Implications for the life cycle of supra-subduction zone ophiolites

    USGS Publications Warehouse

    Shervais, J.W.; Kimbrough, D.L.; Renne, P.; Hanan, B.B.; Murchey, B.; Snow, C.A.; Zoglman, Schuman M.M.; Beaman, J.

    2004-01-01

    The Coast Range ophiolite of California is one of the most extensive ophiolite terranes in North America, extending over 700 km from the northernmost Sacramento Valley to the southern Transverse Ranges in central California. This ophiolite, and other ophiolite remnants with similar mid-Jurassic ages, represent a major but short-lived episode of oceanic crust formation that affected much of western North America. The history of this ophiolite is important for models of the tectonic evolution of western North America during the Mesozoic, and a range of conflicting interpretations have arisen. Current petrologic, geochemical, stratigraphic, and radiometric age data all favor the interpretation that the Coast Range ophiolite formed to a large extent by rapid extension in the forearc region of a nascent subduction zone. Closer inspection of these data, however, along with detailed studies of field relationships at several locales, show that formation of the ophiolite was more complex, and requires several stages of formation. Our work shows that exposures of the Coast Range ophiolite preserve evidence for four stages of magmatic development. The first three stages represent formation of the ophiolite above a nascent subduction zone. Rocks associated with the first stage include ophiolite layered gabbros, a sheeted complex, and volcanic rocks vith arc tholeiitic or (roore rarely) low-K calc-alkaline affinities. The second stage is characterized by intrusive wehrlite-clinopyroxenite complexes, intrusive gabbros, Cr-rich diorites, and volcanic rocks with high-Ca boninitic or tholeiitic ankaramite affinities. The third stage includes diorite and quartz diorite plutons, felsic dike and sill complexes, and calc-alkaline volcanic rocks. The first three stages of ophiolite formation were terminated by the intrusion of mid-ocean ridge basalt dikes, and the eruption of mid-ocean ridge basalt or ocean-island basalt volcanic suites. We interpret this final magmatic event (MORB

  13. Gravity anomalies, crustal structure, and seismicity at subduction zones: 1. Seafloor roughness and subducting relief

    NASA Astrophysics Data System (ADS)

    Bassett, Dan; Watts, Anthony B.

    2015-05-01

    An ensemble averaging technique is used to remove the long-wavelength topography and gravity field from subduction zones. >200 residual bathymetric and gravimetric anomalies are interpreted within fore arcs, many of which are attributed to the tectonic structure of the subducting plate. The residual-gravimetric expression of subducting fracture zones extends >200 km landward of the trench axis. The bathymetric expression of subducting seamounts with height ≥1 km and area ≥500 km2 (N=36), and aseismic ridges (N>10), is largest near the trench (within 70 km) and above shallow subducting slab depths (SLAB1.0 <17 km). Subducting seamounts are similar in wavelength, amplitude, and morphology to unsubducted seamounts. Morphology, spatial distributions, and reduced levels of seismicity are considered inconsistent with mechanical models proposing wholesale decapitation, and the association of subducting seamounts with large-earthquakes. Subducting aseismic ridges are associated with uplift and steepening of the outer fore arc, a gradual reduction in residual bathymetric expression across the inner fore arc, and a local increase in the width and elevation of the volcanic-arc/orogen. These contrasting expressions reflect the influence of margin-normal variations in rigidity on where and how the upper plate deforms, both to accommodate subducting relief and in response to stresses transmitted across the plate interface. The outer fore arc and arc have lower rigidity due to fracturing and thermal weakening, respectively. Similar associations with complex earthquakes and fault creep suggest aseismic ridge subduction may also be accommodated by the development and evolution of a broad fracture network, the geometrical strength of which may exceed the locking strength of a smooth fault.

  14. Seismic Characterization of the Transition from Continental to Oceanic Subduction along the western Hellenic Subduction Zone

    NASA Astrophysics Data System (ADS)

    Pearce, F. D.; Rondenay, S.; Zhang, H.; Sachpazi, M.; Charalampakis, M.; Royden, L.

    2010-12-01

    The Hellenic subduction zone is located in the east-central Mediterranean region and exhibits large variations in convergence rate along its western edge. Differences in the lithosphere entering the subduction zone are believed to drive the different rates of convergence. While seismic reflection data has shown a transition from continental to oceanic lithosphere along the foreland, no detailed images of the mantle-wedge structure have been available to test this hypothesis. Here, we use high-resolution seismic images across northern and southern Greece to investigate differences in the subducted crust along the western Hellenic subduction zone. We deployed 40 broadband seismometers from the IRIS PASSCAL pool across Greece in a northern line (NL, across Northern Greece) and southern line (SL, across Peloponnesus, Attica, and Evia), each roughly perpendicular to the trench axis. We recorded over 50 high-quality teleseismic events with good azimuthal coverage from each line. We processed them using a 2D teleseismic migration algorithm based on the Generalized Radon Transform and a 3D receiver function algorithm that includes dipping interfaces. In addition, we constructed a 3D velocity model by applying double-difference tomography to ~5000 local earthquakes. The 3D velocity model was used to construct an optimal background model for the teleseismic imaging. Migration and RF images reveal N60E dipping low-velocity layers beneath both NL and SL. From high-resolution migration images, we interpret an ~8 km thick low-velocity layer beneath SL as subducted oceanic crust and a ~20 km thick low-velocity layer beneath NL as subducted continental crust. Relocated earthquakes show that the NL subducted crust is seismically active near the foreland down to 50 km depth presumably as a result of slab flexure. Beyond this region, the subducted crust is aseismic until its signal disappears at ~70 km depth. In contrast, the SL subducted crust is marked by seismicity that extends

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

    NASA Astrophysics Data System (ADS)

    Meyer, Melanie; Klemd, Reiner; Konopelko, Dmitry

    2013-09-01

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

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

    USGS Publications Warehouse

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

    1998-01-01

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

  17. Effects of "Hot Fingers" on the Thermal Structure and Mantle Wedge Flow Pattern in Subduction Zones: Implications for Seismic Anisotropy and Volcanic Spacing in Northeast Japan

    NASA Astrophysics Data System (ADS)

    Lee, C.; Wada, I.

    2015-12-01

    Geophysical observations in Northeast Japan indicate its complex mantle wedge dynamics: in particular, 1) narrow low-seismic-velocity regions extending from the back-arc into the sub-arc mantle, so called "hot fingers" [Tamura et al., 2002], and 2) abrupt rotation of the seismically fast polarization direction from trench-normal to trench-parallel beneath the arc [Nakajima and Hasegawa, 2004]. Although the origin of hot fingers is not well understood, its strong spatial correlation with volcanic clustering at surface indicates that they play an important role in controlling arc volcanism. The cause of trench-parallel fast direction in the fore-arc mantle is also unclear; it has been attributed to a range of mechanisms, such as trench-normal mantle wedge flow, the presence of B-type olivine and trench-parallel alignment of melt pockets in the mantle wedge, and hydration along trench-parallel deep-cutting faults in the subducting slab. In this study, we examine the effects of hot fingers on the mantle wedge flow pattern and thermal structure beneath the arc and forearc through three-dimensional numerical experiments and investigate its implications for volcanic spacing and seismic anisotropy in Northeast Japan. In the model, the effects of hot fingers are implemented by imposing thermal anomalies on the back-arc-side vertical boundary. We found that the presence of hot fingers results in 3-D dynamic pressure gradients, which induce local along-arc mantle flow between hot fingers beneath the fore-arc, consistent with the observed pattern of seismic anisotropy in the mantle wedge. Between hot fingers, the along-arc mantle flow in the fore-arc suppresses corner flow, causing lower temperatures in the mantle wedge and in the upper portion of the subducting slab. The cooler condition is likely to hinder the dehydration of the subducting slab and flux melting in the mantle wedge, and this can explain the paucity of Quaternary arc volcanism between hot fingers in Northeast

  18. Lithification facilitates frictional instability in argillaceous subduction zone sediments

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

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

  19. Downgoing plate controls on overriding plate deformation in subduction zones

    NASA Astrophysics Data System (ADS)

    Garel, Fanny; Davies, Rhodri; Goes, Saskia; Davies, Huw; Kramer, Stephan; Wilson, Cian

    2014-05-01

    Although subduction zones are convergent margins, deformation in the upper plate can be extensional or compressional and tends to change through time, sometimes in repeated episodes of strong deformation, e.g, phases of back-arc extension. It is not well understood what factors control this upper plate deformation. We use the code Fluidity, which uses an adaptive mesh and a free-surface formulation, to model a two-plate subduction system in 2-D. The model includes a composite temperature- and stress-dependent rheology, and plates are decoupled by a weak layer, which allows for free trench motion. We investigate the evolution of the state of stress and topography of the overriding plate during the different phases of the subduction process: onset of subduction, free-fall sinking in the upper mantle and interaction of the slab with the transition zone, here represented by a viscosity contrast between upper and lower mantle. We focus on (i) how overriding plate deformation varies with subducting plate age; (ii) how spontaneous and episodic back-arc spreading develops for some subduction settings; (iii) the correlation between overriding plate deformation and slab interaction with the transition zone; (iv) whether these trends resemble observations on Earth.

  20. Physical characteristics of subduction-type seismogenic zones revisited

    NASA Astrophysics Data System (ADS)

    Heuret, A.; Lallemand, S.; Piromallo, C.; Funiciello, F.

    2009-12-01

    Based on both the Centennial earthquake catalog, the revised 1964-2007 EHB hypocenters and the 1976-2007 CMT Harvard catalog, we have extracted the hypocenters, nodal planes and seismic moments of worldwide subduction earthquakes for the period 1900-2007. For the period 1976-2007, we use the focal solutions provided by Harvard and the revised hypocenters from Engdahl et al. (1998). Older events are extracted from the Centennial catalogue (Engdahl and Villasenor, 2002) and they are used for the estimate of the cumulated seismic moment only. The criteria used to select the subduction earthquakes are similar to those used by Mc Caffrey (1994), i.e., we test if the focal mechanisms are consistent with 1/ shallow thrust events (positive slips, at least one nodal plane get dip < 45° and depth > 70 km), and, 2/ the plate interface local geometry and orientation (one nodal plane is oriented toward the volcanic arc, the azimut of this nodal plane is ± 45° with respect to the trench one, its dip is ± 20° with respect to the slab one and the epicenter is located seaward of the volcanic arc). Our study concerns segments of subduction zones that fit with estimated paleoruptures associated with major events (M > 8). We assume that the seismogenic zone coincides with the distribution of 5.5 < M < 7 subduction earthquakes. We then provide a map of the seismogenic zone for 36% of the oceanic subduction plates boundaries including dip, length, downdip and updip limits. The remnant 64% correspond to either weakly coupled oceanic subduction zones, slow subduction rates, or long recurrence period between earthquakes. We then revisit the statistical study done by Pacheco et al. (1993) and tested some empirical laws obtained for example by Kanamori (1986) in light of a more complete, more detailed, more accurate and more uniform description of the subduction interplate seismogenic zone. Since the subduction earthquakes result from stress accumulation along the interplate and that

  1. Revisiting the physical characterisitics of the subduction interplate seismogenic zones

    NASA Astrophysics Data System (ADS)

    Heuret, Arnauld; Lallemand, Serge; Funiciello, Francesca; Piromallo, Claudia

    2010-05-01

    Based on the Centennial earthquake catalog, the revised 1964-2007 EHB hypocenters catalog and the 1976-2007 CMT Harvard catalog, we have extracted the hypocenters, nodal planes and seismic moments of worldwide subduction earthquakes for the 1900-2007 period. For the 1976-2007 period, we combine the focal solutions provided by Harvard and the revised hypocenters from Engdahl et al. (1998). Older events are extracted from the Centennial catalogue (Engdahl and Villasenor, 2002) and they are used to estimate the cumulated seismic moment only. The selection criteria for the subduction earthquakes are similar to those used by Mc Caffrey (1994), i.e., we test if the focal mechanisms are consistent with 1/ shallow thrust events (depth > 70 km, positive slips, and at least one nodal plane gets dip < 45°), and, 2/ the plate interface local geometry and orientation (one nodal plane is oriented toward the volcanic arc, the azimuth of this nodal plane ranges between ± 45° with respect to the trench one, its dip ranges between ± 20° with respect to the slab one and the epicentre is located seaward of the volcanic arc). Our study concerns segments of subduction zones that fit with estimated paleoruptures associated with major events (M > 8). We assume that the seismogenic zone coincides with the distribution of 5.5 < M < 7 subduction earthquakes. We provide a map of the interplate seismogenic zones for 80% of the trench systems including dip, length, downdip and updip limits, we revisit the statistical study done by Pacheco et al. (1993) and test some empirical laws obtained for example by Ruff and Kanamori (1980) in light of a more complete, detailed, accurate and uniform description of the subduction interplate seismogenic zone. Since subduction earthquakes result from stress accumulation along the interplate and stress depends on plates kinematics, subduction zone geometry, thermal state and seismic coupling, we aim to isolate some correlations between parameters. The

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

    NASA Astrophysics Data System (ADS)

    Cassidy, John F.

    2015-03-01

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

  3. Dynamic modelling of the subduction zone of central Mexico

    NASA Astrophysics Data System (ADS)

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

    2000-12-01

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

  4. Subduction zone earthquakes and stress in slabs

    NASA Technical Reports Server (NTRS)

    Vassiliou, M. S.; Hager, B. H.

    1988-01-01

    Simple viscous fluid models of subducting slabs are used to explain observations of the distribution of earthquakes as a function of depth and the orientation of stress axes of deep (greater than 300 km) and intermediate (70-300 km) earthquakes. Results suggest the following features in the distribution of earthquakes with depth: (1) an exponential decrease from shallow depths down to 250 to 300 km, (2) a minimum near 250 to 300 km, and (3) a deep peak below 300 km. Many shallow subducting slabs show only the first characteristic, while deeper extending regions tend to show all three features, with the deep peak varying in position and intensity. These data, combined with the results on the stress orientations of various-depth earthquakes, are consistent with the existence of a barrier of some sort at 670-km depth and a uniform viscosity mantle above this barrier.

  5. Influence of the Amlia fracture zone on the evolution of the Aleutian Terrace forearc basin, central Aleutian subduction zone

    USGS Publications Warehouse

    Ryan, Holly F.; Draut, Amy E.; Keranen, Katie M.; Scholl, David W.

    2012-01-01

    During Pliocene to Quaternary time, the central Aleutian forearc basin evolved in response to a combination of tectonic and climatic factors. Initially, along-trench transport of sediment and accretion of a frontal prism created the accommodation space to allow forearc basin deposition. Transport of sufficient sediment to overtop the bathymetrically high Amlia fracture zone and reach the central Aleutian arc began with glaciation of continental Alaska in the Pliocene. As the obliquely subducting Amlia fracture zone swept along the central Aleutian arc, it further affected the structural evolution of the forearc basins. The subduction of the Amlia fracture zone resulted in basin inversion and loss of accommodation space east of the migrating fracture zone. Conversely, west of Amlia fracture zone, accommodation space increased arcward of a large outer-arc high that formed, in part, by a thickening of arc basement. This difference in deformation is interpreted to be the result of a variation in interplate coupling across the Amlia fracture zone that was facilitated by increasing subduction obliquity, a change in orientation of the subducting Amlia fracture zone, and late Quaternary intensification of glaciation. The change in coupling is manifested by a possible tear in the subducting slab along the Amlia fracture zone. Differences in coupling across the Amlia fracture zone have important implications for the location of maximum slip during future great earthquakes. In addition, shaking during a great earthquake could trigger large mass failures of the summit platform, as evidenced by the presence of thick mass transport deposits of primarily Quaternary age that are found in the forearc basin west of the Amlia fracture zone.

  6. Role of pore fluid pressure on transient strength changes and fabric development during serpentine dehydration at mantle conditions: Implications for subduction-zone seismicity

    NASA Astrophysics Data System (ADS)

    Proctor, Brooks; Hirth, Greg

    2015-07-01

    To further investigate the dehydration embrittlement hypothesis and its possible link to subduction-zone seismicity, we conducted deformation experiments on antigorite serpentinite in a Griggs-type apparatus at conditions below and above antigorite stability. Temperature ramps (crossing the antigorite thermal stability) were used in conjunction with a new experimental method that allows fluid produced during dehydration reactions to be drained, partially drained or undrained. During temperature ramps, weakening coupled with transient slip initiated at ˜ 650 °C, coincident with the predicted phase transition of antigorite to olivine and talc at ˜ 1 GPa. The weakening-rate and steady-state strength were dependent on drainage conditions; undrained samples weakened over a few minutes and supported the lowest shear stress (˜ 50 MPa), while drained samples weakened over a few hours and supported the highest shear stress (˜ 210 MPa). The coefficient of friction (shear stress over normal stress) in drained samples decreased from ˜0.4 to ˜0.16 after the temperature ramp. The strengths of samples that were first annealed at 700 °C for ˜ 12 h, then deformed, were similar to those observed in the temperature ramp experiments. Strain localization along fractures occurred in all samples during temperature ramping, regardless of the drainage conditions. However, microstructural observations indicate deformation by ductile mechanisms at higher strain under both undrained and drained conditions. The rheology and microstructures suggest dehydrating serpentinite deforms via semibrittle flow with grain-scale ductile deformation more active at high pore fluid pressures. Our results suggest that earthquakes in serpentinized mantle do not nucleate as a direct result of unstable frictional sliding along fractures generated at the onset of dehydration reactions.

  7. CO2 solubility and speciation in rhyolitic sediment partial melts at 1.5-3.0 GPa - Implications for carbon flux in subduction zones

    NASA Astrophysics Data System (ADS)

    Duncan, Megan S.; Dasgupta, Rajdeep

    2014-01-01

    much as 2.6-5.5 wt.% CO2 to the sub-arc mantle source regions. At saturation, 1.6-3.3 wt.% sediment partial melt relative to the mantle wedge is therefore sufficient to bring up the carbon budget of the mantle wedge to produce primary arc basalts with 0.3 wt.% CO2. Sediment plumes in mantle wedge: Sediment plumes or diapirs may form from the downgoing slab because the sediment layer atop the slab is buoyant relative to the overlying, hanging wall mantle (Currie et al., 2007; Behn et al., 2011). Via this process, sediment layers with carbonates would carry CO2 to the arc source region. Owing to the higher temperature in the mantle wedge, carbonate can breakdown. Behn et al. (2011) suggested that sediment layers as thin as 100 m, appropriate for modern arcs, could form sediment diapirs. They predicted that diapirs would form from the slab in the sub-arc region for most subduction zones today without requiring hydrous melting. H2O-rich fluid driven carbonate breakdown: Hydrous fluid flushing of the slab owing to the breakdown of hydrous minerals could drive carbonate breakdown (Kerrick and Connolly, 2001b; Grove et al., 2002; Gorman et al., 2006). The addition of water would cause decarbonation creating an H2O-CO2-rich fluid that would then flux through the overlying sediment layer, lower the solidus temperature, and trigger melting. Recent geochemical (Cooper et al., 2012) and geodynamic (van Keken, 2003; Syracuse et al., 2010) constraints suggest that the sub-arc slab top temperatures are above the hydrous fluid-present sediment solidus, thus in the presence of excess fluid, both infiltration induced decarbonation and sediment melting may occur. Hot subduction: This is relevant for subduction zones such as Cascadia and Mexico, where slab-surface temperatures are estimated to be higher (Syracuse et al., 2010). A higher temperature could cause carbonate breakdown and sediment partial melting without requiring a hydrous fluid flux. In this case a relatively dry silicate

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

    NASA Astrophysics Data System (ADS)

    Frohling, E.; Szeliga, W.

    2016-04-01

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

  9. CO2 Solubility in Rhyolitic Melts as a Function of P, T, and fO2 - Implications for Carbon Flux in Subduction Zones

    NASA Astrophysics Data System (ADS)

    Duncan, M. S.; Dasgupta, R.

    2013-12-01

    Understanding the balance between subduction inputs vs. arc output of carbon is critical for constraining the global carbon cycle. However, the agent of carbon transfer from slab to sub-arc mantle is not constrained [1]. Partial melt of ocean-floor sediments is thought to be a key agent of mass transfer in subduction zones, accounting for the trace element characteristics of arc magmas [2]. Yet the carbon carrying capacity of rhyolitic partial melts of sediments remains unknown at sub-arc depths. In our previous work [3], we constrained CO2 solubility of natural rhyolite from 1.5-3.0 GPa, 1300 °C and logfO2 at FMQ×1.0. However, the effects of T and fO2 on CO2 solubility remain unconstrained. In particular, for sediments with organic carbon, graphite stability is expected and the fO2 of C-dissolution can be lower, which may affect the solubility. Thus it is critical to constrain the CO2 solubility of sediment partial melts under graphite-saturated conditions. We determined CO2 solubility of a model rhyolite composition, similar to partial melt composition of natural metapelite [4], at graphite saturation, using Pt/Gr capsules and a piston cylinder device. Experiments were conducted at 1.5-3.0 GPa and 1100-1400 °C. FTIR was employed to measure the concentrations of CO2 and H2O in doubly polished experimental glasses. Raman and SIMS were used to determine the presence of reduced carbon species and total carbon, respectively. FTIR spectra reveal that CO2 is dissolved as both molecular CO2 (CO2mol.) and carbonates (CO32-). For graphite-saturated, hydrous melts with measured H2O ~2.0 wt.%, CO2tot. (CO2mol.+CO32-) values increase with increasing P from ~0.6 to 1.2 wt.% from 1.5 to 3.0 GPa at 1300 °C. These values are lower than more oxidized melts with the same water content, which were 0.85 to 1.99 wt.% CO2 as P increased. At 3 GPa, graphite-saturated experiments from 1100 to 1300 °C yield CO2tot. value of 1.18-1.20 wt.%, suggesting minor effect of temperature in

  10. CO2 Solubility in Natural Rhyolitic Melts at High Pressures - Implications for Carbon Flux in Subduction Zones by Sediment Partial Melts

    NASA Astrophysics Data System (ADS)

    Duncan, M. S.; Dasgupta, R.

    2011-12-01

    Partial melts of subducting sediments is thought to be a critical agent in carrying trace elements and water to arc basalt source regions. For subduction zones that contain significant amount of carbonates in ocean-floor sediments, sediment melts likely also act as a carrier of CO2. However, the CO2 carrying capacity of natural rhyolitic melts at sub-arc depths remains unconstrained. We conducted experiments on a synthetic composition, similar to average, low-degree experimental partial melt of pelitic sediments. The composition was constructed with reagent grade oxides and carbonates, the source of excess CO2. Experiments were conducted between 1 and 3 GPa at 1200 °C in Au80Pd20 capsules using a piston cylinder apparatus with a half-inch BaCO3 assembly at Rice University. Quench products showed glasses with bubbles, the latter suggesting saturation of the melt with a CO2-rich vapor phase. Oxygen fugacity during the experiments was not strictly controlled but the presence of CO2 bubbles and absence of graphite indicates fO2 above the CCO buffer. Major element concentrations of glasses were measured using EPMA. The CO2 and H2O contents of experimental doubly polished (50-110 μm), bubble-free portions of the glass chips were determined using a Thermo Nicolet Fourier Transform Infrared Spectrometer. Spectra were recorded with a resolution of 4 cm-1, 512 scans, from 650 to 4000 cm-1, under a nitrogen purge to eliminate atmospheric gases. Dissolved volatile concentrations were quantified using the Beer-Lambert law and linear molar absorption coefficients from previous studies [1, 2]. Total dissolved carbon dioxide of experimental glasses was determined from the intensity of the ν3 antisymmetric stretch bands of CO32- at 1430 cm-1 and CO2mol at 2348 cm-1. Dissolved water content of experimental glasses was determined from the intensity of O-H stretching at 3520 cm-1. Estimated total CO2 concentrations at 3 GPa are in the range of 1-2 wt%, for melts with H2O contents

  11. Viscosity of Fluids in Subduction Zones

    NASA Astrophysics Data System (ADS)

    Audétat, Andreas; Keppler, Hans

    2004-01-01

    The viscosities of aqueous fluids with 10 to 80 weight percent dissolved silicates have been measured at 600° to 950°C and 1.0 to 2.0 gigapascals by in situ observation of falling spheres in the diamond anvil cell. The viscosities at 800°C range from 10-4 to 100.5 pascal seconds. The combination of low viscosities with a favorable wetting angle makes silicate-rich fluid an efficient agent for material transport at low-volume fractions. Our results therefore suggest that there may be a direct relationship between the position of the volcanic front and the onset of complete miscibility between water and silicate melt in the subducting slab.

  12. Viscosity of fluids in subduction zones.

    PubMed

    Audétat, Andreas; Keppler, Hans

    2004-01-23

    The viscosities of aqueous fluids with 10 to 80 weight percent dissolved silicates have been measured at 600 degrees to 950 degrees C and 1.0 to 2.0 gigapascals by in situ observation of falling spheres in the diamond anvil cell. The viscosities at 800 degrees C range from 10(-4) to 10(0.5) pascal seconds. The combination of low viscosities with a favorable wetting angle makes silicate-rich fluid an efficient agent for material transport at low-volume fractions. Our results therefore suggest that there may be a direct relationship between the position of the volcanic front and the onset of complete miscibility between water and silicate melt in the subducting slab. PMID:14739456

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

    NASA Astrophysics Data System (ADS)

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

    2012-12-01

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

  14. Stress distribution and subduction of aseismic ridges in the Middle America Subduction Zone

    NASA Astrophysics Data System (ADS)

    Lefevre, L. Victoria; McNally, Karen C.

    1985-05-01

    The regional distribution of stresses associated with the subduction of the Cocos plate is inferred from a synthesis of 190 earthquake focal mechanisms, body and surface wave analyses of large earthquakes, and seismicity distributions. Broad patterns of consistent behavior are found across the region, from the Rivera Plate boundary in the northwest to the Guatemala/El Salvador border in the southeast, and are used as a framework to evaluate evidence for variations in local stresses due to the subduction of two aseismic ridges, the Tehuantepec Ridge and the Orozco Fracture Zone. Information which bears on the seismic potential at locations of aseismic ridge subduction is particularly important in that no large (Ms ≥ 7.5) earthquakes have occurred historically. We identify three major zones with consistent patterns in focal mechanisms and hypocentral distributions of seismicity. The first, closest to the trench and reflecting the mechanical interaction of the converging plates, is a zone of shallow thrust earthquakes extending 100-150 km inland from the trench. The second is a zone of normal faulting, beginning at about 200 km inland from the trench, h ≥ 60 km, which extends continuously along the entire length of the descending plate throughout the region. The third distinct zone exhibits a relatively low level of activity and separates the zones of thrust and normal faulting at about 150-200 km inland from the trench. This zone extends from the Rivera plate boundary in the northwest to the Guatamala region in the southeast. At this point, the quiet region pinches out, and the thrust and normal faulting zones abut and overlap. Superimposed on this overall pattern, we find locally only minor changes in areas of aseismic ridge subduction, aside from the prominent seismic slip gaps. Furthermore, on October 25, 1981, the Playa Azul earthquake (Ms = 7.3) occurred in the midregion of the Orozco Fracture Zone. Body and surface wave analyses of this event show a simple

  15. Seismological detection of "730-km" discontinuity beneath Japan subduction zone

    NASA Astrophysics Data System (ADS)

    Liu, Z.; Park, J. J.; Karato, S. I.

    2015-12-01

    Because the mantle transition zone likely contains a large amount of water (Karato, 2011; Pearson et al., 2014), vertical material transport across the transition would cause partial melting that may produce seismic signals above and/or below the transition zone. Schmandt et al. (2014) observed a seismic low-velocity zone (LVZ) at the top of the lower mantle (~730 km) beneath the southwestern US, arguing for dehydration melting due to downward flow across the 670-km discontinuity (670) from the transition zone. These authors further proposed a correlation between seismic velocity reductions and the direction of water transport, in which LVZ at ~730 km indicates materials moving downward from the transition zone, while the lack of LVZ at this depth would suggest an upward flow of mantle materials. Other regions also need to be investigated to confirm the correlation between this seismic feature and mantle water transport. We test their model by detecting "730-km" discontinuity beneath the Japan subduction zone using frequency-dependent receiver functions. In addition, water transport above the 410-km discontinuity (410) also plays an important role in global water circulation (Bercovici and Karato, 2003). Seismological studies (e.g. Courtier and Revenaugh, 2007; Schaeffer and Bostock, 2010) have observed LVZs above the 410, which might be caused by dehydration melting due to the upwelling of hydrated materials across the 410-km discontinuity from the transition zone. In this study, we also detect potential LVZs above 410 to establish a correlation between seismic velocity drop and flow direction. Around the Japan subduction zone, our preliminary results show evidence of low velocity zones below 670 in regions where stagnant slab is present for a substantial amount of time but not in other regions suggesting a variety of vertical mass transport in this region. Key words: transition zone, water transport, subduction zone, melting, receiver functions

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

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

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

  17. Subduction Zone Diversity and Nature of the Plate Contact

    NASA Astrophysics Data System (ADS)

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

    2008-12-01

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

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

    NASA Astrophysics Data System (ADS)

    Garth, T.; Rietbrock, A.

    2013-12-01

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

  19. Numerical modeling of fluid migration in subduction zones

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

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

  20. Modelling guided waves in the Alaskan-Aleutian subduction zone

    NASA Astrophysics Data System (ADS)

    Coulson, Sophie; Garth, Thomas; Reitbrock, Andreas

    2016-04-01

    Subduction zone guided wave arrivals from intermediate depth earthquakes (70-300 km depth) have a huge potential to tell us about the velocity structure of the subducting oceanic crust as it dehydrates at these depths. We see guided waves as the oceanic crust has a slower seismic velocity than the surrounding material, and so high frequency energy is retained and delayed in the crustal material. Lower frequency energy is not retained in this crustal waveguide and so travels at faster velocities of the surrounding material. This gives a unique observation at the surface with low frequency energy arriving before the higher frequencies. We constrain this guided wave dispersion by comparing the waveforms recorded in real subduction zones with simulated waveforms, produced using finite difference full waveform modelling techniques. This method has been used to show that hydrated minerals in the oceanic crust persist to much greater depths than accepted thermal petrological subduction zone models would suggest in Northern Japan (Garth & Rietbrock, 2014a), and South America (Garth & Rietbrock, in prep). These observations also suggest that the subducting oceanic mantle may be highly hydrated at intermediate depth by dipping normal faults (Garth & Rietbrock 2014b). We use this guided wave analysis technique to constrain the velocity structure of the down going ~45 Ma Pacific plate beneath Alaska. Dispersion analysis is primarily carried out on guided wave arrivals recorded on the Alaskan regional seismic network. Earthquake locations from global earthquake catalogues (ISC and PDE) and regional earthquake locations from the AEIC (Alaskan Earthquake Information Centre) catalogue are used to constrain the slab geometry and to identify potentially dispersive events. Dispersed arrivals are seen at stations close to the trench, with high frequency (>2 Hz) arrivals delayed by 2 - 4 seconds. This dispersion is analysed to constrain the velocity and width of the proposed waveguide

  1. Deep view of the Subduction-Transform Edge Propagator (STEP) fault in the Calabrian Subduction Zone

    NASA Astrophysics Data System (ADS)

    Maesano, Francesco Emanuele; Tiberti, Mara Monica; Basili, Roberto

    2016-04-01

    The Calabrian Subduction Zone plays a key role in the evolution of the central Mediterranean in the framework of the convergence between Africa and Europe. Here, the remnants of the World's oldest oceanic crust form a narrow NW-dipping slab passively subducting beneath the Calabrian Arc. Recently published high-resolution seismic profiles and bathymetric data of the western Ionian Sea highlight the presence of a NNW-SSE faulting system connected with a series of Plio-Pleistocene syn-tectonic basins. These features are correlated with the recent activity of a major NNW-SSE deformation zone confining the active subduction to the SW and interpreted as a Subduction-Transform Edge Propagator (STEP) fault. The goal of this work is to jointly reconstruct the geometry of the STEP fault and the subduction interface in its surroundings. We use multichannel seismic profiles acquired in the southwestern part of the Calabrian accretionary wedge to focus on the STEP fault geometry at depth and to analyse its relationships with shallow deformation features. The quantitative analysis and enhancement of seismic data provided an accurate image of the internal structure of the accretionary wedge at various depths, showing growth strata in the Plio-Pleistocene succession and major discontinuities in the lower crust. Our results depict a main subvertical, slightly east-dipping, lithospheric fault cutting the oceanic crust down to the Moho, and a rich set of associated secondary synthetic and antithetic faults. This picture also provides new insights on the STEP fault propagation mechanism. In addition, the tridimensional correlation of the STEP fault occurrences in various seismic profiles provides a preliminary scheme of its segmentation and highlights the relationships of this master fault with other main structural elements of the Calabrian Arc and Eastern Sicily, including some of the faults deemed to be responsible for major historical earthquakes in the area.

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

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

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

  3. Earthquakes, fluid pressures and rapid subduction zone metamorphism

    NASA Astrophysics Data System (ADS)

    Viete, D. R.

    2013-12-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

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

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

    SciTech Connect

    Hilde, T.W.C.

    1984-08-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2010-12-01

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

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

    NASA Astrophysics Data System (ADS)

    Zhao, D.

    2011-12-01

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

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

    PubMed

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

    2014-01-01

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

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

    PubMed Central

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

    2014-01-01

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

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

    PubMed Central

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

    2011-01-01

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

  12. Slow earthquakes linked along dip in the Nankai subduction zone.

    PubMed

    Hirose, Hitoshi; Asano, Youichi; Obara, Kazushige; Kimura, Takeshi; Matsuzawa, Takanori; Tanaka, Sachiko; Maeda, Takuto

    2010-12-10

    We identified a strong temporal correlation between three distinct types of slow earthquakes distributed over 100 kilometers along the dip of the subducting oceanic plate at the western margin of the Nankai megathrust rupture zone, southwest Japan. In 2003 and 2010, shallow very-low-frequency earthquakes near the Nankai trough as well as nonvolcanic tremor at depths of 30 to 40 kilometers were triggered by the acceleration of a long-term slow slip event in between. This correlation suggests that the slow slip might extend along-dip between the source areas of deeper and shallower slow earthquakes and thus could modulate the stress buildup on the adjacent megathrust rupture zone.

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

    SciTech Connect

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

    1995-11-10

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

  14. Unraveling topography around subduction zones from laboratory models

    NASA Astrophysics Data System (ADS)

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

    2012-03-01

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

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

    NASA Astrophysics Data System (ADS)

    Bajolet, Flora; Faccenna, Claudio; Funiciello, Francesca

    2014-05-01

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

  16. Opening and closing slab windows in congested subduction zones

    NASA Astrophysics Data System (ADS)

    Moresi, Louis

    2013-04-01

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

  17. Water and the Oxidation State of Subduction Zone Magmas

    SciTech Connect

    Kelley, K.; Cottrell, E

    2009-01-01

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

  18. Evolving subduction zones in the Western United States, as interpreted from igneous rocks

    USGS Publications Warehouse

    Lipman, P.W.; Prostka, H.J.; Christiansen, R.L.

    1971-01-01

    Variations in the ratio of K2O to SiO4 in andesitic rocks suggest early and middle Cenozoic subduction beneath the western United States along two subparallel imbricate zones dipping about 20 degrees eastward. The western zone emerged at the continental margin, but the eastern zone was entirely beneath the continental plate. Mesozoic subduction apparently occurred along a single steeper zone.

  19. Evolving subduction zones in the Western United States, as interpreted from igneous rocks.

    PubMed

    Lipman, P W; Prostka, H J; Christiansen, R L

    1971-11-19

    Variations in the ratio of K(2)O to SiO(2) in andesitic rocks suggest early and middle Cenozoic subduction beneath the western United States along two subparallel imbricate zones dipping about 20 degrees eastward. The western zone emerged at the continental margin, but the eastern zone was entirely beneath the continental plate. Mesozoic subduction apparently occurred along a single steeper zone.

  20. Oceanic asthenosphere subduction and its geological implications

    NASA Astrophysics Data System (ADS)

    Zhou, Q.; Liu, L.

    2014-12-01

    We investigate the evolution of oceanic asthenosphere during subduction by exploring various scenarios including plate kinematics and plausible values of asthenosphere viscosity and density. We find that the oceanic asthenosphere will always subduct with the down-going slab as long as its average viscosity value is no smaller than 1×1018 Pa s. In order to allow slabs to subduct into the deep upper mantle, a maximum oceanic asthenosphere density reduction relative to the underlying mantle should be no larger than 0.7%, assuming a 200-km-thick asthenosphere channel. We find that a significant portion of the asthenosphere buoyancy should result from its excess temperature from the long-term thermal evolution of mantle convection. Our results are in contrast to an earlier suggestion that negligible amount (<30 km thick) of asthenosphere could get subducted, which is likely due to over-simplicity of subduction geometry and model boundary conditions. The recycling of a weak and hot asthenosphere provides a novel mechanism for the formation of slow seismic anomalies within the deep mantle. This, in turn, questions the commonly believed deep mantle plume origin of intra-plate volcanism, with a typical example being the Yellowstone volcanic system. Our current results suggest that a buoyant asthenosphere can be dragged down into the lower mantle and then moves upward due to its buoyancy when the overlying slab barrier is removed. To further test our hypothesis, we construct a 4D subduction model for western North America during the Cenozoic. We use data assimilation techniques to incorporate plate kinematics and sea floor ages as boundary conditions, and seismic anomalies converted density structure as internal buoyancy source. The subduction history is calibrated through a hybrid of forward and adjoint simulations satisfying multiple observational constraints. Some preliminary results will be presented.

  1. Plate detachment, asthenosphere upwelling, and topography across subduction zones

    NASA Astrophysics Data System (ADS)

    Gvirtzman, Zohar; Nur, Amos

    1999-06-01

    This study analyzes the topography across subduction zones, considering the separate contributions of the crust and the mantle lithosphere to the observed surface elevation. We have found a transition from a region where the overriding plate is coupled to the descending slab and pulled down along with it to a region where the overriding plate floats freely on the asthenosphere. When the subducting slab retreats oceanward rapidly this transition is abrupt, and the edge of the overriding plate is uplifted. We propose that at some point during rapid slab rollback the overriding plate detaches and rebounds like a boat released from its keel. This event is associated with suction of asthenospheric material into the gap that is opened between the plates up to the base of the crust. As a result, the forearc uplifts, and magmatism in the arc increases.

  2. Numerical modeling of fluid migration in subduction zones

    NASA Astrophysics Data System (ADS)

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

    2015-04-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2015-04-01

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

  4. Inside the subduction factory: Modeling fluid mobile element enrichment in the mantle wedge above a subduction zone

    NASA Astrophysics Data System (ADS)

    Shervais, John W.; Jean, Marlon M.

    2012-10-01

    Enrichment of the mantle wedge above subduction zones with fluid mobile elements is thought to represent a fundamental process in the origin of arc magmas. This "subduction factory" is typically modeled as a mass balance of inputs (from the subducted slab) and outputs (arc volcanics). We present here a new method to model fluid mobile elements, based on the composition of peridotites associated with supra-subduction ophiolites, which form by melt extraction and fluid enrichment in the mantle wedge above nascent subduction zones. The Coast Range ophiolite (CRO), California, is a Jurassic supra-subduction zone ophiolite that preserves mantle lithologies formed in response to hydrous melting. We use high-precision laser ablation ICP-MS analyses of relic pyroxenes from these peridotites to document fluid-mobile element (FME) concentrations, along with a suite of non-fluid mobile elements that includes rare earth and high-field strength elements. In the CRO, fluid-mobile elements are enriched by factors of up to 100× DMM, whereas fluid immobile elements are progressively depleted by melt extraction. The high concentrations of fluid mobile elements in supra-subduction peridotite pyroxene can be attributed to a flux of aqueous fluid or fluid-rich melt phase derived from the subducting slab. To model this enrichment, we derive a new algorithm that calculates the concentration of fluid mobile elements added to the source: C=[C/[[D/(D-PF)]∗[1-(PF/D)

  5. A possible source of water in seismogenic subduction zones

    NASA Astrophysics Data System (ADS)

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

    2010-12-01

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

  6. Cyclic stressing and seismicity at strongly coupled subduction zones

    USGS Publications Warehouse

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

    1996-01-01

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

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

    PubMed

    Calvert, Andrew J

    2004-03-11

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

  8. Constraints on Subduction Zone Temperatures and Chemical Fluxes from Accessory Phase Saturation in Subducted Sediments (Invited)

    NASA Astrophysics Data System (ADS)

    Blundy, J.; Skora, S.

    2009-12-01

    . For the case of 7 wt% added H2O, monazite is exhausted at 825 °C; it disappears at 780 °C with 15 wt% added H2O. In our experiments monazite fractionates LREE from Th, such that fluids preserving the original sedimentary ratio must be generated at or above monazite-out temperatures in subducted slabs where red clay is the dominant sedimentary lithology. We propose that the subducted sedimentary signature is imparted by fluxing of H2O derived from hydrated (e.g. serpentinised) portions of the deeper subducting slab, triggering copious “flash melting” of the sediment at the point were its temperature exceeds ~800 °C. Without the addition of H2O sediment melting is too restricted to allow any appreciable fluid release into the overlying mantle. Moreover, under such conditions the Th/LREE ratio is strongly fractionated due to the abundance of residual monazite. The availability of H2O in hydrated portions of subducted slabs, e.g. in the vicinity of fracture zones, may exercise an important control on the spatial distribution of subduction zone magmatism.

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

    USGS Publications Warehouse

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

    2006-01-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

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

  11. Shallow subduction zone earthquakes and their tsunamigenic potential

    NASA Astrophysics Data System (ADS)

    Polet, J.; Kanamori, H.

    2000-09-01

    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

  12. Stability and dynamics of serpentinite layer in subduction zone

    NASA Astrophysics Data System (ADS)

    Hilairet, Nadege; Reynard, Bruno

    2009-02-01

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

  13. Crustal contamination versus subduction zone enrichment: Examples from the Lesser Antilles and implications for mantle source compositions of island arc volcanic rocks

    NASA Astrophysics Data System (ADS)

    Davidson, Jon P.

    1987-08-01

    Isotopic and geochemical data are presented from the northern and central Lesser Antilles arc. Island arc tholeiites from the northern islands are restricted in 143Nd /144Nd and 87Sr /86Sr and show large enrichments in LILE relative to other trace elements, as exemplified particularly in high Ba/La ratios. These volcanic rocks represent typical island arc compositions, similar to those described from the Aleutians, Marianas and South Sandwich Islands. In contrast, samples from the central islands (Martinique and St. Lucia) are highly variable in isotopic and trace element ratios, and reflect overprinting of primitive island arc magma chemistry by subsequent crustal contamination. Such magmas therefore do not represent island arc mantle source characteristics. Northern Lesser Antilles arc mantle sources are displaced from MORB Pb isotopic compositions towards the higher 207Pb /204Pb ratios characteristic of sediments. Also, Sr and Nd isotopic compositions are highly restricted and distinct from those of MORB. As a result, mixing models between MORB source and subducted sediment are tightly constrained in terms of the allowable proportion of sediment. The characteristically high LILE/REE and LILE/HFSE ratios of island arc volcanics cannot be easily reconciled with sediment + MORB source mixing. Buffering of isotopic compositions together with considerable relative LILE enrichment might be achieved by mixing between mantle and a hydrous fluid which has equilibrated with not only sediment but also a large proportion of variably altered oceanic crust. The fluid has isotopic compositions closer to those of MORB, thereby permitting a greater range of mixing proportions. At the same time, the LILE characteristics of the modified mantle source are dominated by the very high concentrations of these elements in the fluid. High degrees of partial melting of mantle modified by a hydrous fluid reproduce the trace element pattern of the source in the derivative volcanic rocks and

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

    NASA Astrophysics Data System (ADS)

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

    2013-03-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

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

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

    PubMed

    Chaves, Esteban J; Schwartz, Susan Y

    2016-01-01

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

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

    PubMed Central

    Chaves, Esteban J.; Schwartz, Susan Y.

    2016-01-01

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

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

    PubMed

    Chaves, Esteban J; Schwartz, Susan Y

    2016-01-01

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

  19. Experimentally determined distribution of fluorine and chlorine upon hydrous slab melting, and implications for F-Cl cycling through subduction zones

    NASA Astrophysics Data System (ADS)

    Van den Bleeken, Greg; Koga, Kenneth T.

    2015-12-01

    Fluorine and chlorine are volatile elements known to be enriched in primitive arc magmas, and variations of F/Cl ratios can carry information about slab devolatilization processes. Recent experiments on the fractionations of these elements suggest that aqueous fluid has limited capacity to enrich the magma source region in F. Hence, it is difficult to explain observations of primitive arc magmas particularly rich in F. To complement previous experimental studies, we examined the fractionation of fluorine and chlorine during hydrous partial melting of subducting slab. Element-doped phase equilibria experiments were carried out in a complex chemical system at conditions equivalent to potential slab melting temperatures (750-1000 °C) across the amphibolite to eclogite facies transition (1.3-3 GPa). Partition coefficients of F and Cl between hydrous silicic melts and minerals were determined by electron microprobe and/or ion probe. Fluorine is compatible in amphibole (DFamp/glass = 1.18-1.85), and incompatible in garnet (0.034-0.140), clinopyroxene (0.059-0.505), and allanite (0.205-0.504). Hence, amphibole is an important F host, and can retain significant quantities of F in the solid residue of partial melting. On the contrary, Cl is incompatible, with DClmineral/glass generally decreasing from amphibole (0.079-0.625; one outlier at 1.87) to allanite (0.163), clinopyroxene (0.066-0.158), and garnet (0.031-0.153; outlier at 0.492). As a result, Cl is easily mobilized during partial melting. Fluorine and chlorine release during slab melting have been quantified by applying our partition coefficients to a non-modal batch melting model. The model shows that amphibole plays a key role in F/Cl fractionation during partial melting, while F/Cl is close to that of source for the melting of amphibole free eclogite. Moreover, the results from a flux-melting model employing several source compositions are compared to F and Cl abundances in primitive arc magmas. The observed

  20. Effect of Bulk Water Concentration on Mantle Wedge Hybridization by Rhyolitic Sediment Melt - Implications for Generation of K-rich Basalts to Andesites in Subduction Zones

    NASA Astrophysics Data System (ADS)

    Mallik, A.; Dasgupta, R.; Nelson, J. M.; Tsuno, K.

    2014-12-01

    Similarities in trace element geochemistry between ocean-floor sediments and arc lavas suggest the involvement of subducted sediments in the mantle source of arc volcanoes. Siliciclastic sediments produce rhyo-dacitic, hydrous partial melts at sub-arc depths, which must react with wedge peridotite during their ascent. In addition to fluids, these sediment melts can be a major carrier of water to the arc source. Here we investigate the effects of bulk water concentration on the phase equilibria of reaction between sediment partial melt and peridotite. Piston-cylinder experiments were performed using Au-Pd capsules, at 2 and 3 GPa, 1050 - 1350 °C with mixtures of 25% rhyolite + 75% lherzolite, bearing bulk water content of 2 (low-water) and 4 wt.% (high-water). Melting degree is higher in high-water experiments at both 2 and 3 GPa with a sharp increase in melt mode from 31 to 53 wt.% at 1250-1300 °C, 2 GPa and 21 to 49 wt.% at 1225-1250 °C, 3 GPa. This sharp increase in melt mode is accompanied by a corresponding abrupt increase in residual olivine to opx ratio at both pressures (0.11 to 0.53 at 1250-1300 °C, 2 GPa and 0 to 0.71 at 1225-1250 °C, 3 GPa). The stability field of phlogopite, clinopyroxene, and garnet are reduced in high-water experiments due to higher degrees of partial melting. Low-water experiments produce basalts with SiO2, on a volatile-free basis, increasing from 49 to 51 wt.% at 2 GPa and 46 to 48 wt.% at 3 GPa. For high-water experiments, melt SiO2 contents at 2 GPa are slightly higher than those in low-water experiments for a given temperature, varying from 51 to 52 wt.%, and, at 3 GPa, the melts trend towards andesitic compositions with SiO2 ~54 wt.%. These compositional characteristics of the melts can be attributed to the effect of increased olivine to opx ratios in the residue as a function of increasing bulk water concentration. Our study shows that a spectrum of ultra-potassic, high-Mg arc lavas (MgO varying from 10-16 wt.%) from

  1. Isotopic Characteristics of Thermal Fluids from Mexican Subduction Zone

    NASA Astrophysics Data System (ADS)

    Taran, Y.; Inguaggiato, S.

    2007-05-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2004-10-01

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

  3. Foundering lithosphere triggers transient basins and backarc magmatism at subduction zones?

    NASA Astrophysics Data System (ADS)

    Wang, H.; Currie, C. A.; DeCelles, P. G.

    2015-12-01

    Many upper-plate processes at subduction zones cannot be directly explained by traditional subduction mechanisms. In the Central Andes, the crust is shortened and thickened by the subduction of Nazca plate, but the lower lithosphere is anomalously thin at present. Within the plateau, localized, transient basins have formed since the Miocene. These basins have experienced subsidence, internal shortening, and then inversion. One hypothesis is these basins are related to the formation and foundering of dense eclogite rocks in the lithosphere. Along the eastern plateau, there are sites of basaltic magmatism which show a gradual westward migration. Geochemistry studies suggest that these magmas are mainly caused by upwelling asthenosphere, indicating lithosphere thinning beneath this area. However, the magmas are landward of the basins, and therefore the formation and removal of the dense anomaly is spatially and temporally offset from the region of lithosphere thinning. In this study, 2D numerical models are used to investigate lithosphere removal within a subduction zone. A dense root is placed in lower crust of the upper plate to simulate the eclogitization process and initiate gravitational removal. The model evolves in three phases: 1) As the root becomes denser, the overlying surface subsides and a basin forms; 2) once the root is denser than mantle, it sinks and decouples from the upper plate. During this period, the basin inverts and uplifts. 3) Meanwhile, the mantle lithosphere landward of the root is sheared by the corner flow in the mantle wedge. As the lithosphere is carried trenchward, a gap forms at the landside of plateau which widens over time. Hot asthenosphere upwells to fill the gap and undergoes decompression melting. The model results are consistent with observations from the Central Andes and could have implications for other subduction regions with enigmatic transient basins and backarc magmatism, such as those in North America and Eastern China.

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

    USGS Publications Warehouse

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

    1992-01-01

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

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

    PubMed

    Clarke, S H; Carver, G A

    1992-01-10

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

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

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2011-12-01

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

  8. Subduction zone structures and slip behavior in megathrust

    NASA Astrophysics Data System (ADS)

    Kodaira, S.; Nakanishi, A.; Nakamura, Y.

    2014-12-01

    Earthquake, tsunami and geodetic data show that co-seismic slips of a large megathrust earthquakes do not uniformly propagate along a plate boundary. For example, a clear segmentation of slip zones of magnitude-8 class megathrust earthquakes are well recognized in the Nankai Trough. Moreover, a lateral variation of the slips are revealed even in one segment. In the Japan Trench, the most characteristic slip behavior of the 2011 Tohoku-oki earthquake is an extremely large slip reaching to the trench axis, but geodetic, tsunami or teleseismic show the slip was heterogeneous near the trench. In order to examine whether those complex slip distributions are attributed by any distinct structural factor, we have been carried out active-source seismic surveys in the subduction seismogenic. In the Nankai Trough, large-scale subducted seamounts, ridges and doming structure intruded in an overriding accretion wedge are imaged. Comparing co-seismic slip distribution of the 1944 Tonankai and the 1946 Nankai earthquakes with the seismic images, we concluded that those structures are key factors to control the slip distributions. In the central part of the Japan Trench area, we fund a rough basement geometry is overprinted on the horst-and-graben structure. Those complex geometry of basement cause a strong lateral variation of the thickness of subducting pelagic/hemi-pelagic sediment. Many geological studies suggest that properties of the plate-boundary sediment attribute the large slip near the trench. We therefore plan to acquire additional high-resolution seismic data in the entire Japan Trench in order to examine a role of incoming sediment on the large slip to the trench axis. In this presentation we present an overview of the structural factors controlling slips in megathrust earthquakes, including new data acquired in the Nankai Trough and Japan Trench.

  9. Implications of the 26 December 2004 Sumatra-Andaman earthquake on tsunami forecast and assessment models for great subduction-zone earthquakes

    USGS Publications Warehouse

    Geist, Eric L.; Titov, Vasily V.; Arcas, Diego; Pollitz, Fred F.; Bilek, Susan L.

    2007-01-01

    Results from different tsunami forecasting and hazard assessment models are compared with observed tsunami wave heights from the 26 December 2004 Indian Ocean tsunami. Forecast models are based on initial earthquake information and are used to estimate tsunami wave heights during propagation. An empirical forecast relationship based only on seismic moment provides a close estimate to the observed mean regional and maximum local tsunami runup heights for the 2004 Indian Ocean tsunami but underestimates mean regional tsunami heights at azimuths in line with the tsunami beaming pattern (e.g., Sri Lanka, Thailand). Standard forecast models developed from subfault discretization of earthquake rupture, in which deep- ocean sea level observations are used to constrain slip, are also tested. Forecast models of this type use tsunami time-series measurements at points in the deep ocean. As a proxy for the 2004 Indian Ocean tsunami, a transect of deep-ocean tsunami amplitudes recorded by satellite altimetry is used to constrain slip along four subfaults of the M >9 Sumatra–Andaman earthquake. This proxy model performs well in comparison to observed tsunami wave heights, travel times, and inundation patterns at Banda Aceh. Hypothetical tsunami hazard assessments models based on end- member estimates for average slip and rupture length (Mw 9.0–9.3) are compared with tsunami observations. Using average slip (low end member) and rupture length (high end member) (Mw 9.14) consistent with many seismic, geodetic, and tsunami inversions adequately estimates tsunami runup in most regions, except the extreme runup in the western Aceh province. The high slip that occurred in the southern part of the rupture zone linked to runup in this location is a larger fluctuation than expected from standard stochastic slip models. In addition, excess moment release (∼9%) deduced from geodetic studies in comparison to seismic moment estimates may generate additional tsunami energy, if the

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

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

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

  11. Complex rupture processes of the Solomon Islands subduction zone earthquake and subduction controlled upper mantle structure beneath Anatolia

    NASA Astrophysics Data System (ADS)

    Biryol, Cemal Berk

    This dissertation explores subduction zone-related deformation both on short time scales in the form of subduction zone earthquakes and over larger time and geographical scales in the form of subduction rollback or detachment of the subducting lithosphere. The study presented here is composed of two parts. First, we analyzed the source-rupture processes of the April 1, 2007 Solomon Islands Earthquake (Mw=8.1) using a body-wave inversion technique. Our analysis indicated that the earthquake ruptured approximately 240 km of the southeast Pacific subduction zone in two sub-events. In the second part of this study, we used shear-wave splitting analysis to investigate the effects of the subducting African lithosphere on the upper-mantle flow field beneath the Anatolian Plate in the Eastern Mediterranean region. Our shear-wave splitting results are consistent with relatively uniform southwest-directed flow towards the actively southwestward-retreating Aegean slab. Based on spatial variations in observed delay times we identified varying flow speeds beneath Anatolia and we attribute this variation to the differential retreat rates of the Aegean and the Cyprean trenches. Finally, we used teleseismic P-wave travel-time tomography to image the geometry of the subducting African lithosphere beneath the Anatolia region. Our tomograms show that the subducting African lithosphere is partitioned into at least two segments along the Cyprean and the Aegean trenches. We observed a gap between the two segments through which hot asthenosphere ascends beneath the volcanic fields of western Anatolia. Our results show that the Cyprean slab is steeper than the Aegean slab. We inferred that this steep geometry, in part, controls the flow regime of asthenosphere beneath Anatolia causing variations in flow speeds inferred from shear-wave splitting analysis.

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

    USGS Publications Warehouse

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

    2010-01-01

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

  13. Does subduction zone magmatism produce average continental crust

    NASA Technical Reports Server (NTRS)

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

    1988-01-01

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

  14. Earthquake rupture stalled by a subducting fracture zone.

    PubMed

    Robinson, D P; Das, S; Watts, A B

    2006-05-26

    We showed that the rupture produced by the great Peru earthquake (moment magnitude 8.4) on 23 June 2001 propagated for approximately 70 kilometers before encountering a 6000-square-kilometer area of fault that acted as a barrier. The rupture continued around this barrier, which remained unbroken for approximately 30 seconds and then began to break when the main rupture front was approximately 200 kilometers from the epicenter. The barrier had relatively low rupture speed, slip, and aftershock density as compared to its surroundings, and the time of the main energy release in the earthquake coincided with the barrier's rupture. We associate this barrier with a fracture zone feature on the subducting oceanic plate.

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

    NASA Astrophysics Data System (ADS)

    Shimizu, N.; Mandeville, C. W.

    2011-12-01

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

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

    NASA Astrophysics Data System (ADS)

    Padron-Navarta, J.

    2013-12-01

    Large-scale deep water cycling takes place through subduction zones in the Earth, making our planet unique in the solar system. This idiosyncrasy is the result of a precise but unknown balance between in-gassing and out-gassing fluxes of volatiles. Water is incorporated into hydrous minerals during seafloor alteration of the oceanic lithosphere. The cycling of volatiles is triggered by dehydration of these minerals that release fluids from the subducting slab to the mantle wedge and eventually to the crust or to the deep mantle. Whereas the loci of such reactions are reasonably well established, the mechanisms of fluid migration during dehydration reactions are still barely known. One of the challenges is that dehydration reactions are dynamic features evolving in time and space. Experimental data on low-temperature dehydration reactions (i.e. gypsum) and numerical models applied to middle-crust conditions point to a complex spatiotemporal evolution of the dehydration process. The extrapolation of these inferences to subduction settings has not yet been explored but it is essential to understand the dynamism of these settings. Here I propose an alternative approach to tackle this problem through the textural study of high-pressure terrains that experienced dehydration reactions. Spatiotemporal evolution of dehydration reactions should be recorded during mineral nucleation and growth through variations in time and space of the reaction rate. Insights on the fluid migration mechanism could be inferred therefore by noting changes in the texture of prograde assemblages. The dehydration of antigorite in serpentinite is a perfect candidate to test this approach as it releases a significant amount of fluid and produces a concomitant porosity. Unusual alternation of equilibrium and disequilibrium textures observed in Cerro del Almirez (Betic Cordillera, S Spain)[1, 2] attest for a complex fluid migration pattern for one of the most relevant reactions in subduction zones

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

    USGS Publications Warehouse

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

    2006-01-01

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

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

    PubMed

    Kelemen, Peter B; Manning, Craig E

    2015-07-28

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

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

    NASA Astrophysics Data System (ADS)

    Kelemen, Peter B.; Manning, Craig E.

    2015-07-01

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

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

    PubMed Central

    Kelemen, Peter B.; Manning, Craig E.

    2015-01-01

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

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

    PubMed

    Kelemen, Peter B; Manning, Craig E

    2015-07-28

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

  2. Modeling the Migration of Fluids in Subduction Zones

    NASA Astrophysics Data System (ADS)

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

    2011-12-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2014-05-01

    Seepage of methane-dominated hydrocarbons is heterogeneous in space and time, and trigger mechanisms of episodic seep events are not well constrained. It is generally found that free hydrocarbon gas entering the local gas hydrate stability field in marine sediments is sequestered in gas hydrates. In this manner, gas hydrates can act as a buffer for carbon transport from the sediment into the ocean. However, the efficiency of gas hydrate-bearing sediments for retaining hydrocarbons may be corrupted: Hypothesized mechanisms include critical gas/fluid pressures beneath gas hydrate-bearing sediments, implying that these are susceptible to mechanical failure and subsequent gas release. Although gas hydrates often occur in seismically active regions, e.g., subduction zones, the role of earthquakes as potential triggers of hydrocarbon transport through gas hydrate-bearing sediments has hardly been explored. Based on a recent publication (Fischer et al., 2013), we present geochemical and transport/reaction-modelling data suggesting a substantial increase in upward gas flux and hydrocarbon emission into the water column following a major earthquake that occurred near the study sites in 1945. Calculating the formation time of authigenic barite enrichments identified in two sediment cores obtained from an anticlinal structure called "Nascent Ridge", we find they formed 38-91 years before sampling, which corresponds well to the time elapsed since the earthquake (62 years). Furthermore, applying a numerical model, we show that the local sulfate/methane transition zone shifted upward by several meters due to the increased methane flux and simulated sulfate profiles very closely match measured ones in a comparable time frame of 50-70 years. We thus propose a causal relation between the earthquake and the amplified gas flux and present reflection seismic data supporting our hypothesis that co-seismic ground shaking induced mechanical fracturing of gas hydrate-bearing sediments

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

    NASA Astrophysics Data System (ADS)

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

    2007-12-01

    Accretive and erosive subduction zones differ both in their forearc structure and seismic release character. For instance the greatest historical megathrust earthquakes concentrated along accretive margins, tectonically characterized by forearc shortening (e.g. Sumatra, Southern Chile, Alaska), whereas erosive margins, tectonically characterized by forearc extension (e.g. Peru, Kuriles), have often been the locus of tsunami earthquakes (i.e. slow and shallow events). Here we investigate the implied link between internal forearc deformation and megathrust seismogenesis and its implications for seismic hazard in subduction zones. We interpret quasi two- dimensional plastoelastic (allowing deformation to localize, permanent shortening dominates) and elastoplastic (elastic deformation dominates, minor internal deformation) granular wedge models as analogs of accretive and erosive subduction forearcs, respectively, overlying a rate-state frictional plate interface which represents a seismogenic megathrust. Experimental observations support current hypotheses that internal forearc deformation is controlled by stress changes associated with the megathrust seismic cycle: Consistent with the theory of dynamic Coulomb wedges, coseismic compression at the updip limit of great earthquakes triggers shallow postseismic forearc deformation. Plastic shortening of the outer forearc wedge and shallow afterslip both are interpreted as transient postseismic relaxation mechanisms with the first being dominant in plastoelastic/accretive settings and the second being dominant in elastoplastic/erosive settings. Interseismically, permanent crustal shortening localizes in both settings above the downdip limit of great earthquakes and may lead to uplift of a coastal cordillera. Longterm coastal uplift rates at elastoplastic/erosive margins are about one order of magnitude lower than in plastoelastic/accretive settings, and associated with permanent crustal extension above the seismogenic

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

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

    The mechanism of slab-to-mantle volatile transfer is strongly related to the fluid speciation, which in turn is a function of oxygen fugacity, in a system buffered by equilibria involving redox-sensitive elements. However, the redox processes taking place in the portion of mantle wedge on top of the subducting slab are poorly investigated and the oxidising power of fluids is still unknown. Information on such fluid/melt related processes can be gained by the study of orogenic metasomatised ultramafic rocks associated with deeply subducted crust. We present two case studies of mantle-derived garnet peridotites from Sulu (China) and the Western Gneiss Region (Norway), unique examples of metasomatised mantle wedge that interacted with COH fluids subducted up to 200 km depth. Sulu peridotites record a multistage metasomatism by alkali-rich silicate melt, and a subsequent influx of a slab-derived incompatible element and silicate-rich fluid during the Triassic UHP metamorphism. We performed Fe3+/ΣFe flank method and electron energy loss spectroscopy measurements on garnet and pyroxenes, to quantify the Fe3+ distribution among the peridotite phases and estimate the bulk oxidation state of the peridotite. The results indicate that garnets are zoned, with Fe2O3 increasing from ~0.8 to ~2.5 wt.%, and clinopyroxenes contain high Fe3+/ΣFe ratios (0.48 to 0.51) and Na contents. Peridotites from Norway preserve remnants of crust-derived fluids which precipitated daughter Cr- spinel + phlogopite/K-amphibole + dolomite/magnesite + graphite/diamond in polyphase inclusions hosted by majoritic garnet. They witness COH fluid/mineral interaction responsible for diamond formation. We determined the fO2 of the peak mineral assemblage starting from Fe3+ analyses in majoritic garnet. The fO2 values are up FMQ-2 along a trend from arc lavas (FMQ+1.5 - FMQ+3) to mantle wedge garnet peridotites from Sulu (FMQ - FMQ+2). The fO2 determination together with Fe3+ distribution among the hydrate

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

    NASA Astrophysics Data System (ADS)

    Ritter, O.; Araya, J.

    2014-12-01

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

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

    USGS Publications Warehouse

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

    1991-01-01

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

  8. Seismo-thermo-mechanical modeling of subduction zone seismicity

    NASA Astrophysics Data System (ADS)

    van Dinther, Y.; Gerya, T.; Dalguer, L. A.; Mai, P. M.

    2013-12-01

    spontaneous unstable rupturing of off-megathrust events. Resulting outerrise normal, compressional splay, and listric antithetic normal events geometrically resemble natural observations. They are triggered by megathrust-induced static stress changes and agree with analytical predictions of dynamic Coulomb wedge theory. Furthermore, the importance of these off-megathrust events for seismic hazard assessment is demonstrated. Megathrust cycling is distinctly affected by extended updip locking and premature, updip triggering of megathrust events. While tsunami hazards increase due to outer wedge yielding and steeply dipping off-megathrust fault planes. References: van Dinther, Y., Gerya, T., Dalguer, L., Corbi, F., Funiciello, F., and Mai, P. (2013). The seismic cycle at subduction thrusts: 2. Dynamic implications of geodynamic simulations validated with laboratory models, JGR, 118(4), 1502-1525, doi:10.1029/2012JB009479.

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

    NASA Astrophysics Data System (ADS)

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

    2008-12-01

    and 5 GPa, and T between 350 and 700C, at strain rates between 10-4 and 10-5 s-1. Dehydration of serpentine was conducted around 4 GPa and 10-5 s-1. The results will be presented together with their implications, in particular the influence of the talc-like phase within a dehydrating serpentine, and more generally for subduction zones dynamics. 1 Ulmer and Trommsdorff GSA spec. publ. (1999). 2 Perrillat et al EPSL (2005) 3 Hilairet et al Science (2007) 4 Escartin et al EPSL (2008) 5 Moore and Rymer Nature (2007) 6 Wang et al RSI (2003)

  10. Subduction of Fracture Zones control mantle melting and geochemical signature above slabs

    NASA Astrophysics Data System (ADS)

    Constantin Manea, Vlad; Leeman, William; Gerya, Taras; Manea, Marina; Zhu, Guizhi

    2014-05-01

    The geochemistry of arc volcanics proximal to oceanic fracture zones (FZs) is consistent with higher than normal fluid inputs to arc magma sources. Here, enrichment of boron (B/Zr) in volcanic arc lavas is used to evaluate relative along-strike inputs of slab-derived fluids in the Aleutian, Andean, Cascades, and Trans-Mexican arcs. Significant B/Zr spikes coincide with subduction of prominent FZs in the relatively cool Aleutian and Andean subduction zones, but not in the relatively warm Cascadia and Mexican subduction zones, suggesting that FZ subduction locally enhances fluid introduction beneath volcanic arcs, and retention of fluids to sub-arc depths diminishes with subduction zone thermal gradient. Geodynamic treatments of lateral inhomogeneities in subducting plates have not previously considered how FZs may influence the melt and fluid distribution above the slab. Using high-resolution three-dimensional coupled petrological-thermomechanical numerical simulations of subduction, we show that fluids, including melts and water, concentrate in areas where fracture zones are subducted, resulting in along-arc variability in magma source compositions and processes.

  11. Numerical Models of Subduction Beneath Non-Uniform Overriding Plates. Implications for Subduction Velocity and Seismic Anisotropy

    NASA Astrophysics Data System (ADS)

    Rodríguez-González, J.; Billen, M. I.; Negredo, A. M.

    2015-12-01

    Subduction zones show significant along-strike variations of different subduction features. In particular there is a good correlation between variations of overriding plate thickness and variations of slab dip, seismic anisotropy and subduction velocity. Previous numerical models have shown that overriding plate thermal state influences the slab dip and variations in slab dip can cause trench-parallel flow above the slab. This suggests a causal link between overriding plate structure, slab geometry and mantle flow in subduction zones. Models also show that interplate coupling is stronger for colder overriding plates, which might lead to lower subduction velocities. We implement generic numerical models to study the effect of the overriding plate structure on the evolution of slab geometry, induced mantle flow and subduction velocity. We solve the 3D, time dependent thermo-mechanical equations with a non-linear rheology to simulate buoyancy driven subduction processes. We find that along-strike variations in thermal thickness of the overriding plate cause increased hydrodynamic suction and shallower slab dip beneath the colder portion of the overriding plate; the variation in slab geometry drives strong trench-parallel flow beneath the slab and a complex flow pattern above the slab. Induced mantle flow varies significantly with time, suggesting that the global variability in seismic anisotropy observations in subduction zones is in part due to the non-steady-state behavior of these systems. This new mechanism for driving trench-parallel flow provides a good explanation for seismic anisotropy observations from the Middle and South America subduction zones. Our results also show that increased interplate coupling beneath a colder portion of the overriding plate leads to a reduction of the subduction velocity in the region, leading to along-strike variations subduction velocity. These results provide a good explanation of the observed present-day variations in

  12. Automated Tremor Analysis From the Cascadia Subduction Zone

    NASA Astrophysics Data System (ADS)

    Aguiar, A. C.; Melbourne, T. I.; Scrivner, C. W.

    2006-12-01

    Reanalysis of geodetic GPS time series from the Cascadia subduction zone have revealed at least 30 resolvable slow slip events along the megathrust since 1997, ranging from northern California to southern British Columbia. Many of the smaller and more recent events are barely detectable with GPS, but stand out clearly as tremor sequences on band-pass filtered record sections of surface seismic data. With the intent of assessing whether a fixed frequency-magnitude relationship applies to subduction zone slow slip events, and assuming that tremor and GPS transients are different manifestations of the same faulting process at depth, we have implemented automated processes to identify the frequency-magnitude relationship of slow slip events that are not resolvable with GPS. Our methodology is as follows. We first consolidate daily seismic files from the northern Puget Basin of Washington State and SW British Columbia, where GPS density is highest. Seismic traces are included from the Pacific Northwest Seismic Network, the Plate Boundary Observatory borehole seismic network, the Earthscope-funded CAFÉ experiment, and locally deployed seismometers around long-baseline tiltmeters, all running at 100 sps. We then band-pass the data between 1-6 Hz and run it through several tremor-detection algorithms. The envelopes of rectified traces are then regionally averaged to provide a single metric indicative of tremor activity. We also compute spectrograms of daily files and use power in the 1-6 Hz range to assess tremor onset and duration. To validate that this automated approach works as expected, we first manually inspected time periods of slow slip events, particularly the Sept 6-30 of 2005, when GPS transients are known. Secondly, hours per week of tremor identified on at least three stations separated by 25 Km were summed manually and show clear spikes during the GPS-inferred timing, indicating that envelope averaging does accurately identify the largest known events. We

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

    NASA Astrophysics Data System (ADS)

    Sisson, T. W.

    2002-12-01

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

  14. Imaging the Seismic Cycle in the Central Andean Subduction Zone from Geodetic Observations

    NASA Astrophysics Data System (ADS)

    Ortega-Culaciati, F.; Becerra-Carreño, V. C.; Socquet, A.; Jara, J.; Carrizo, D.; Norabuena, E. O.; Simons, M.; Vigny, C.; Bataille, K. D.; Moreno, M.; Baez, J. C.; Comte, D.; Contreras-Reyes, E.; Delorme, A.; Genrich, J. F.; Klein, E.; Ortega, I.; Valderas, M. C.

    2015-12-01

    We aim to quantify spatial and temporal evolution of fault slip behavior during all stages of the seismic cycle in subduction megathrusts, with the eventual goal of improving our understanding of the mechanical behavior of the subduction system and its implications for earthquake and tsunami hazards. In this work, we analyze the portion of the Nazca-SouthAmerican plates subduction zone affected by the 1868 southern Peru and 1877 northern Chile mega-earthquakes. The 1868 and 1878 events defined a seismic gap that did not experience a large earthquake for over 124 years. Only recently, the 1995 Mw 8.1 Antofagasta, 2001 Mw 8.4 Arequipa, 2007 Mw 7.7 Tocopilla, and 2014 Mw 8.2 Pisagua earthquakes released only a small fraction of the potential slip budget, thereby raising concerns about continued seismic and tsunami hazard. We use over a decade of observations from continuous and campaign GPS networks to analyze inter-seismic strain accumulation, as well as co-seimic deformation associated to the more recent earthquakes in the in the Central Andean region. We obtain inferences of slip (and back-slip) behavior using a consistent and robust inversion framework that accounts for the spatial variability of the constraint provided by the observations on slip across the subduction megathrust. We present an updated inter-seismic coupling model and estimates of pre-, co- and post- seismic slip behavior associated with the most recent 2014 Mw 8.2 Pisagua earthquake. We analyze our results, along with published information on the recent and historical large earthquakes, to characterize the regions of the megathrust that tend to behave aseismically, and those that are capable to accumulate a slip budget (ultimately leading to the generation of large earthquakes), to what extent such regions may overlap, and discuss the potential for large earthquakes in the region.

  15. The 2014 Mw6.2 Eketahuna earthquake, Hikurangi subduction zone - normal faulting in the subducted Pacific Plate crust

    NASA Astrophysics Data System (ADS)

    Abercrombie, R. E.; Bannister, S. C.; Francois-Holden, C.; Hamling, I. J.; Ristau, J. P.

    2014-12-01

    The 2014 January 20th M6.2 Eketahuna earthquake occurred in the subducted crust of the Pacific plate at the Hikurangi subduction zone, beneath North Island, New Zealand. Moment tensor analysis together with aftershock relocations show that this event was an oblique-normal faulting intraplate event, with hypocentre depth ca.30 km, and with rupture on a northwest-dipping fault extending through the subducted crust up to the subduction megathrust at ca.18-20 km depth. More than 3500 aftershocks were subsequently recorded by the New Zealand GeoNet network, with only minor migration of the aftershocks away from the inferred mainshock rupture, and with very few aftershocks within +/- 1 km of the subduction megathrust. The megathrust in this particular region is inferred to be interseismically locked with no seismic or aseismic slip, although slow slip is occurring ca.15-30 km down-dip (Wallace et al, 2013). Similar oblique-normal faulting events have previously occurred along the Hikurangi subduction margin, including in 1985 (ML5.7) and 1990 (Mw6.2). Earlier earthquakes in 1942 (Mw6.8) and 1921 (Mw6.8) are also inferred to have occurred at a similar depth within the subducted crust. The 1990 earthquake sequence occurred ~40 km along-strike from the 2014 Eketahuna event, and involved a Mw6.2 oblique-normal faulting event in the subducted crust, which was quickly followed by a Mw6.4 event in the overlying crust, with both thrust and dextral strike-slip components, possibly responding to deeper aseismic slip. Deeper earthquakes of similar type at other subduction margins are thought to be high stress drop. We calculate the stress drops of the mainshock and larger aftershocks, using a direct wave, empirical Green's function (EGF) approach that includes measurement uncertainties and objective criteria for assessing the quality of each spectral ratio (Abercrombie, 2013). We compare the results to those for earthquakes in other tectonic regions of New Zealand, calculated using

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

    SciTech Connect

    Scholz, C.H.; Campos, J.

    1995-11-10

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

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

    ERIC Educational Resources Information Center

    Shea, James Herbert

    1991-01-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2010-12-01

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

  19. A model for the termination of the Ryukyu subduction zone against Taiwan: A junction of collision, subduction/separation, and subduction boundaries

    USGS Publications Warehouse

    Wu, F.T.; Liang, W.-T.; Lee, J.-C.; Benz, H.; Villasenor, A.

    2009-01-01

    The NW moving Philippine Sea plate (PSP) collides with the Eurasian plate (EUP) in the vicinity of Taiwan, and at the same time, it subducts toward the north along SW Ryukyu. The Ryukyu subduction zone terminates against eastern Taiwan. While the Ryukyu Trench is a linear bathym??trie low about 100 km east of Taiwan, closer to Taiwan, it cannot be clearly identified bathymetrically owing to the deformation related to the collision, making the location of the intersection of the Ryukyu with Taiwan difficult to decipher. We propose a model for this complex of boundaries on the basis of seismicity and 3-D velocity structures. In this model the intersection is placed at the latitude of about 23.7??N, placing the northern part of the Coastal Range on EUP. As PSP gets deeper along the subduction zone it collides with EUP on the Taiwan side only where they are in direct contact. Thus, the Eurasian plate on the Taiwan side is being pushed and compressed by the NW moving Philippine Sea plate, at increasing depth toward the north. Offshore of northeastern Taiwan the wedge-shaped EUP on top of the Ryukyu subducting plate is connected to the EUP on the Ryukyu side and coupled to the NW moving PSP by friction at the plate interface. The two sides of the EUP above the western end of the subduction zone are not subjected to the same forces, and a difference in motions can be expected. The deformation of Taiwan as revealed by continuous GPS measurements, geodetic movement along the east coast of Taiwan, and the formation of the Hoping Basin can be understood in terms of the proposed model. Copyright 2009 by the American Geophysical Union.

  20. First finding of microdiamond, coesite and other UHP phases in felsic granulites in the Moldanubian Zone: Implications for deep subduction and a revised geodynamic model for Variscan Orogeny in the Bohemian Massif

    NASA Astrophysics Data System (ADS)

    Perraki, Maria; Faryad, Shah Wali

    2014-08-01

    Heavy mineral fractions and polished thin sections from felsic granulites from the Moldanubian Zone of the Bohemian Massif were thoroughly studied by means of Raman microspectroscopy combined with optical microscopy and scanning electron microscopy. The following phases were identified, among others, as inclusions in robust minerals such as garnet and zircon: Diamond, characterized by an intense narrow peak at 1332 cm- 1, was found in two inclusions in zircon. They have a size of ~ 5 μm. Coesite, identified by its very characteristic peak at ~ 520 cm- 1, was found in an inclusion in garnet together with quartz. Coesite has been almost completely transformed into quartz; only minor coesite nano-domains remain. Kumdykolite, the orthorhombic polymorph of NaAlSi3O8, characterized by strong peaks at 220, 456 and 492 cm- 1, occurs either as single crystals or as a part of multiphase inclusions in garnet and in zircon along with other mineral phases such as K-feldspar, phengite, rutile. Moissanite, SiC, exhibiting the characteristic Raman bands at ~ 767, 788 and 969 cm- 1, occurs as inclusions in garnet. Diamond and coesite are considered to have formed at the peak ultrahigh-pressure metamorphic (UHPM) conditions. Kumdykolite has been proposed to be a metastable phase formed during rapid cooling from high temperature. Moissanite points to extremely reduced conditions during subduction to great depths. The finding of UHP phases in felsic granulites in the Moldanubian Zone is clear evidence for subduction of crustal materials to mantle depths. The garnet hosting the UHP phase inclusions usually preserves prograde compositional zoning; this in combination with the UHPM mineral inclusions suggests that the felsic material should have passed UHP metamorphism at a low-temperature gradient. Isothermal decompression (the commonly accepted model) at temperatures of 850-950 °C would have substantially modified and homogenized the garnet composition eliminating any compositional

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

    NASA Astrophysics Data System (ADS)

    Wang, Hongliang; Huismans, Ritske; Rondenay, Stéphane

    2016-04-01

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

  2. Permeability-porosity relationships of subduction zone sediments

    USGS Publications Warehouse

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

    2011-01-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

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

  4. Accretion, underplating and exhumation along a subduction interface: From subduction initiation to continental subduction (Tavşanlı zone, W. Turkey)

    NASA Astrophysics Data System (ADS)

    Plunder, Alexis; Agard, Philippe; Chopin, Christian; Pourteau, Amaury; Okay, Aral I.

    2015-06-01

    We herein reappraise the pressure-temperature (PT) evolution of the high-pressure and low-temperature (HP-LT) Tavşanlı zone (western Turkey) in order to (i) better characterize rock units exhumed along a cooling subduction interface, from birth to steady state and (ii) constrain exhumation and detachment dynamics, as well as mechanical coupling between plates. Based on PT estimates and field observations three oceanic complexes are recognized between the HP-LT continental margin and the obducted ophiolite, with PT estimates ranging from incipient metamorphism to blueschist-facies conditions. PT conditions for the continental unit are reappraised to 24 kbar and ~ 500 °C on the basis of pseudosection modelling and Raman spectroscopy on carbonaceous material. A tentative reconstruction of the subduction zone evolution is proposed using available radiometric and palaeogeographic data and recent thermomechanical modelling. Both PT conditions and field observations point out to the slicing of km-sized units at different preferred depths along the subduction interface, thus providing constraints on the dynamics of accretion and underplating. In particular, the comparison of PT estimates for the Tavşanlı zone and for other broadly similar fossil subduction settings (i.e., Oman, Corsica, New Caledonia, Franciscan, Schistes Lustrés) suggests that units are detached preferentially from the slab at specific depths of 30-40 km (i.e., downdip of the seismogenic zone) and ~ 80 km. We propose that these depths are controlled by major changes in mechanical coupling along the plate interface, whereas exhumation through time would rather be controlled by large-scale geodynamic boundary conditions.

  5. Fluid Overpressure Distribution and Permeability Structure in the Cascadia Subduction Zone Under Southern Vancouver Island

    NASA Astrophysics Data System (ADS)

    Spinelli, G. A.; Wada, I.

    2012-12-01

    We develop hydrogeologic models to examine the fluid overpressure distribution in the northern Cascadia subduction zone resulting from dewatering of the subducting Juan de Fuca slab. Anomalous seismic velocities indicative of relatively high Poisson's ratios observed in the subducting crust at subduction zones, such as Cascadia and Nankai, have been interpreted to indicate fluid overpressure (Shelly et al., 2006; Audet et al., 2009; Peacock et al., 2011). In northern Cascadia, the inferred fluid overpressure beneath Vancouver Island disappears farther landward. One of the proposed mechanisms for the distribution of fluid overpressure is the down-dip change in the permeability of the plate boundary fault. In this scenario, permeability is low under Vancouver Island, limiting fluid escape from the slab; permeability increases farther landward, allowing more efficient fluid migration out of the subducting slab (Audet et al., 2009). We test this conceptual hydrogeologic model with numerical models of fluid transport. Our models include fluid sources from porosity loss and mineral dehydration reactions. The volume of dehydration-derived fluid release from the subducting crust is calculated using a thermal model for Cascadia and the thermodynamic calculation code Perple_X. Modeled fluid source magnitudes are highest in a ~50 km wide region of upper oceanic crust under Vancouver Island. The cessation of these fluid sources in the subducting slab further landward combined with fluid flow from the slab contribute to the landward dissipation of fluid overpressure, even in the absence of enhanced fault zone permeability landward of Vancouver Island.

  6. 3D Finite-Difference Modeling of Scattered Teleseismic Wavefields in a Subduction Zone

    NASA Astrophysics Data System (ADS)

    Morozov, I. B.; Zheng, H.

    2005-12-01

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

  7. Mantle transition zone thickness in the Central South-American Subduction Zone

    NASA Astrophysics Data System (ADS)

    Braunmiller, Jochen; van der Lee, Suzan; Doermann, Lindsey

    We used receiver functions to determine lateral variations in mantle transition zone thickness and sharpness of the 410- and 660-km discontinuities in the presence of subducting lithosphere. The mantle beneath the central Andes of South America provides an ideal study site owing to its long-lived subduction history and the availability of broadband seismic data from the dense BANJO/SEDA temporary networks and the permanent station LPAZ. For LPAZ, we analyzed 26 earthquakes between 1993-2003 and stacked the depth-migrated receiver functions. For temporary stations operating for only about one year (1994-1995), station stacks were not robust. We thus stacked receiver functions for close-by stations forming five groups that span the subduction zone from west to east, each containing 12 to 25 events. We found signal significant at the 2σ level for several station groups from P to S conversions that originate near 520- and 850-900 km depth, but most prominently from the 410- and 660-km discontinuities. For the latter, the P to S converted signal is clear in stacks for western groups and LPAZ, lack of coherent signal for two eastern groups is possibly due to incoherent stacking and does not necessitate the absence of converted energy. The thickness of the mantle transition zone increases progressively from a near-normal 255 km at the Pacific coast to about 295 km beneath station LPAZ in the Eastern Cordillera. Beneath LPAZ, the 410-km discontinuity appears elevated by nearly 40 km, thus thickening the transition zone. We compared signal amplitudes from receiver function stacks calculated at different low-pass frequencies to study frequency dependence and possibly associated discontinuity sharpness of the P to S converted signals. We found that both the 410- and 660-km discontinuities exhibit amplitude increase with decreasing frequency. Synthetic receiver function calculations for discontinuity topography mimicking observed topography show that the observed steep

  8. Intra-Panthalassa Ocean subduction zones revealed by fossil arcs and mantle structure

    NASA Astrophysics Data System (ADS)

    van der Meer, D. G.; Torsvik, T. H.; Spakman, W.; van Hinsbergen, D. J. J.; Amaru, M. L.

    2012-03-01

    The vast Panthalassa Ocean once surrounded the supercontinent Pangaea. Subduction has since consumed most of the oceanic plates that formed the ocean floor, so classic plate reconstructions based on magnetic anomalies can be used only to constrain the ocean's history since the Cretaceous period, and the Triassic-Jurassic plate tectonic evolution of the Panthalassa Ocean remains largely unresolved. Geological clues come from extinct intra-oceanic volcanic arcs that formed above ancient subduction zones, but have since been accreted to the North American and Asian continental margins. Here we compile data on the composition, the timing of formation and accretion, and the present-day locations of these volcanic arcs and show that intra-oceanic subduction zones must have once been situated in a central Panthalassa location in our plate tectonic reconstructions. To constrain the palaeoposition of the extinct arcs, we correlate them with remnants of subducted slabs that have been identified in the mantle using seismic-wave tomographic models. We suggest that a series of subduction zones, together called Telkhinia, may have defined two separate palaeo-oceanic plate systems--the Pontus and Thalassa oceans. Our reconstruction provides constraints on the palaeolongitude and tectonic evolution of the Telkhinia subduction zones and Panthalassa Ocean that are crucial for global plate tectonic reconstructions and models of mantle dynamics.

  9. Preseismic, Postseismic and Slow Faulting in Subduction Zones

    NASA Astrophysics Data System (ADS)

    Melbourne, T. I.; Webb, F. H.; Miller, M. M.

    2002-12-01

    The last several years have witnessed a broad reappraisal of our understanding of the energy budgets of subduction zones. Due primarily to the deployment of continuous geodetic instrumentation along convergent margins worldwide, we now recognize that fault rupture commonly occurs over rates ranging from kilometers per second to millimeters per day. Along with transient postseismic slip, both isolated and episodic slow slip events have now been recorded along convergent margins offshore Japan, Alaska, Mexico, Cascadia and Peru, and thus would appear to constitute a fundamental mode of strain release only observable through geodetic methods. In many instances, postseismic creep along the deeper plate interface is triggered by seismogenic rupture up-dip. Continuous GPS measurements from three earthquakes in México (Mw=8.0,1995), Peru (Mw=8.4,2001) and Japan (Mw=7.7, 1994) show that deep postseismic creep was triggered by local Coulomb stress increases of the order of one half bar produced by their mainshock ruptures. For these three events, afterslip along their primary coseismic asperities is significantly less important than triggered deep creep. Deeper slow faulting does not have to be triggered by adjacent seismogenic rupture. In Cascadia, eight episodic slow slip events since 1991 have been recognized to have an astonishingly regular 14.5-month onset period, the most recent of which began in February of 2002. For these events, time dependent inversion of GPS data map the propagation of creep fronts and show they released moment with magnitudes in excess of Mw=6.5. If they occur throughout the Cascadia interseismic period, then cumulatively they rival the moment release of the infrequent Mw=9.0 megathrust events. Most recently, an 18-hour precursor to an Mw=7.6 aftershock of the 2001 Mw=8.4 Peru earthquake was detected at Arequipa, Peru. This precursor appears as a ~3 cm departure from a continuous time series broken only by the coseismic displacements of the

  10. The fate of salt in the Cyprus subduction zone

    NASA Astrophysics Data System (ADS)

    Reiche, Sönke; Hübscher, Christian; Ehrhardt, Axel; Klimke, Jennifer

    2014-05-01

    The area between Cyprus and Hecataeus Rise to the north and Eratosthenes Seamount (ESM) further south is presently accommodating plate tectonic convergence between Africa and Anatolia. A number of studies have focused on the convergence history, especially after drilling close to the plate boundary in the course of ODP Leg 160. Even though drilling at Site 968 has reached late Messinian Lago Mare deposits, little information on deeper trench strata exist, owing to limited penetration of previously published seismic data. Here we show results from bathymetric data and a dense grid of seismic lines collected during research cruises MSM14/2 and MSM14/3 in 2010 across the Cyprus trench, shedding new light on the tectonostratigraphic evolution of the plate boundary. Evaporites of locally more than 1.5 km thickness occupy the northern trench area. Between Cyprus and ESM evaporites are heavily deformed and appear to be thrust southward over Pliocene-Quaternary strata. Thus significant post-Messinian tectonic shortening at the plate boundary was accommodated by allochthonous salt advance towards the ESM which is currently being thrust beneath the island of Cyprus. Such observations may provide an example of how salt efficiently escapes the subduction cycle. In contrast, evaporites between ESM and Hecataeus Rise have not experienced sufficient shortening for initiating allochthonous salt advance, even though compression-related thickening is clearly evident. The observed pattern of intensively deformed salt between Cyprus and ESM and moderately deformed evaporites in the eastern trench area is believed to reflect a predominately N-S-oriented post-Messinian convergence direction. Such implications raise the question about a fairly recent coupling between the motion of Cyprus and Anatolia. Along the entire study area, the southward salt limit coincides with the seafloor stepping down towards the ESM, suggesting thickening and ESM-directed advance of the evaporites to have

  11. Pore pressure evolution at the plate interface along the Cascadia subduction zone from the trench to the ETS transition zone

    NASA Astrophysics Data System (ADS)

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

    2010-12-01

    Pore fluid pressures in subduction zones are a primary control on fault strength and slip dynamics. Numerous studies document elevated pore pressures in the outer wedge along several margins. Seismic observations and the occurrence of non-volcanic tremor provide additional evidence for the presence of near-lithostatic pore pressures at the plate interface far down-dip from the trench (~35 km depth). Here we use numerical models in one and two dimensions to evaluate the pore pressure and compaction state of sediments on the subducting Juan de Fuca plate in Cascadia from the trench to the ETS zone. 2-D models allow pressure to diffuse vertically and also laterally normal to strike of the megathrust; 1-D models simulate only vertical diffusion. Model parameters are chosen with reference to two strike-normal profiles: one through central Oregon and one through the Olympic Peninsula of Washington. We examine temporal variations in sediment input to the trench and consider implications for fault strength and permeability as well as the down-dip extent to which compactive dewatering can be considered a significant fluid source. In 1-D, we use a general and fully nonlinear model of sediment compaction derived without making any assumptions regarding stress-strain or porosity-permeability relations and allowing finite strains. In contrast, most previous models of fluid flow in subduction zones have used linear models of diffusion that rely on assumptions of constant sediment permeability and infinitesimal strains for their formulation. Our nonlinear finite-strain model remains valid at greater depths, where stresses and strains are large. Boundary conditions in 1-D are constrained by pore pressure estimates along the megathrust fault that are based on seismic velocities (e.g. Tobin and Saffer, 2010) and data from consolidation tests conducted on sediments gathered during ODP Leg 204 (Tan, 2001). Initial conditions rely on input sediment thickness; while sediment thickness

  12. Overriding plate deformation and variability of fore-arc deformation during subduction: Insight from geodynamic models and application to the Calabria subduction zone

    NASA Astrophysics Data System (ADS)

    Chen, Zhihao; Schellart, Wouter P.; Duarte, João. C.

    2015-10-01

    In nature, subducting slabs and overriding plate segments bordering subduction zones are generally embedded within larger plates. Such large plates can impose far-field boundary conditions that influence the style of subduction and overriding plate deformation. Here we present dynamic laboratory models of progressive subduction in three-dimensional space, in which the far-field boundary conditions at the trailing edges of the subducting plate (SP) and overriding plate (OP) are varied. Four configurations are presented: Free (both plates free), SP-Fixed, OP-Fixed, and SP-OP-Fixed. We investigate their impact on the kinematics and dynamics of subduction, particularly focusing on overriding plate deformation. The results indicate that the variation in far-field boundary conditions has an influence on the slab geometry, subduction partitioning, and trench migration partitioning. Our models also indicate that in natural (narrow) subduction zones, assuming a homogeneous overriding plate, the formation of back-arc basins (e.g., Tyrrhenian Sea, Aegean Sea, and Scotia Sea) is generally expected to occur at a comparable location (250-700 km from the trench), irrespective of the boundary condition. In addition, our models indicate that the style of fore-arc deformation (shortening or extension) is influenced by the mobility of the overriding plate through controlling the force normal to the subduction zone interface (trench suction). Our geodynamic model that uses the SP-OP-Fixed setup is comparable to the Calabria subduction zone with respect to subduction kinematics, slab geometry, trench curvature, and accretionary configuration. Furthermore, the model can explain back-arc and fore-arc extension at the Calabria subduction zone since the latest middle Miocene as a consequence of subduction of the narrow Calabrian slab and the immobility of the subducting African plate and overriding Eurasian plate. This setting induced strong trench suction, driving fore-arc extension, and

  13. Aseismic Slip on the Northern Cascadia Subduction Zone: Impacts on Seismic Hazard Estimates

    NASA Astrophysics Data System (ADS)

    Dragert, H.; Mazzotti, S.; Wang, K.

    2002-12-01

    Based on data from the few longer operating continuous GPS sites in southwestern British Columbia and northwestern Washington State, aseismic slip appears to occur repeatedly on the deeper interface of the Cascadia Subduction Zone (CSZ) underlying the eastern Olympics and southern Vancouver Is. The spatial and temporal character of the slip events observed so far have implications for regional seismic hazard estimates. During the period between slips, stress accumulates over an interface region wider than the normal locked/transition zone of the CSZ. However, it appears that there is little long-term stress accumulation on the deeper interface and the potential rupture zone for the next megathrust earthquake remains predominantly offshore. Because of its location downdip of the locked plate interface, a deep aseismic slip produces a small, discrete Coulomb stress increment of the order of 0.01 MPa which moves the locked zone closer to rupture, potentially acting as a trigger for a future great thrust earthquake. The spatial correlation of the boundary of the slip zone with the location of large in-slab earthquakes also suggests a common structural cause or a possible stress interaction. The silent slips and in-slab earthquakes may both be related to a common process of slab dehydration. However, the observed slip events create a Coulomb shadow for normal earthquakes on steeply dipping faults within the descending slab downdip from the slip zone. The region of slip also underlies the areas of high crustal seismicity of Puget Sound and Georgia Strait, but estimates of Coulomb stress changes on crustal faults due to deep slip are extremely sensitive to geometry, thus obscuring patterns of possible crustal stress interactions. To date, no temporal correlations have been found between rates of crustal seismicity and the occurrence of aseismic slip.

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

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

    Mantle convection drives plate tectonics and the size, number and thermotectonic age of plates codetermines the convection pattern. However, the degree of coupling of surface deformation and mantle flow is unclear. The use of SKS waves seismic anisotropy shows a coherence of mantle and surface deformation, but significant examples depart from this scenario. We review geological observations and present kinematic reconstructions of the Aegean and Middle East and 3D numerical models to discuss the role of asthenospheric flow in crustal deformation. At the scale of the Mediterranean backarcs, lithosphere-mantle coupling is effective below the most extended regions as shown by the alignment of SKS fast orientations and stretching directions in MCCs. In the Aegean, the directions of mantle flow, crustal stretching and GPS velocities are almost parallel, while, below the main part of the Anatolian plate, SKS fast orientations are oblique to GPS velocities. When considering the long-term geological history of the Tethyan convergent, one can conclude that asthenospheric flow has been an important player. The case of Himalaya and Tibet strongly supports a major contribution of a northward asthenospheric push, with no persistent slab that could drive India after collision, large thrust planes being then decoupling zones between deep convection and surface tectonics. The African plate repeatedly fragmented during its migration, with rifting of large pieces of continents that had then been moving northward faster than Africa (Apulia, Arabia). This also suggests a dominant role of an underlying flow at large scale, dragging and mechanically eroding plates and breaking them into fragments, then passively carried. Mantle flow thus seems to be able to carry plates toward subduction zones, break-away pieces of plates, and deform backarc upper crust where the lithosphere is the thinnest. Most numerical models of lithospheric deformation are designed such that strain is a consequence

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

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

    Sediment hydraulic properties, consolidation state, and ambient pore pressure development are key parameters that affect fluid migration, deformation, and the slip behavior and mechanical strength of subduction zone megathrusts. In order to better understand the dynamics and mechanisms of large subduction earthquakes, Integrated Oceanic Drilling Program (IODP) Expedition 343, drilled into the toe of the Japan Trench subduction zone in a region of large shallow slip in the M 9.0 Tohoku earthquake, as part of the Japan Trench Fast Drilling Project (J-FAST). Here, we report on two constant rate of strain (CRS) uniaxial consolidation experiments and two triaxial deformation experiments on bedded claystone and clayey mudstone core samples collected from the frontal prism and subducted sediment section cored at Site C0019, 2.5 km landward of the Japan Trench, from depths of 697.18 and 831.45 mbsf. The goals of our experiments were: (1) to define the hydraulic and acoustic properties of sediments that host the subduction megathrust fault that slipped in the M 9.0 Tohoku earthquake; and (2) to constrain in-situ consolidation state and its implications for in-situ stress. The permeability-porosity trends are similar for the two samples, and both exhibit permeability that decreases systematically with increasing effective stress and decreasing porosity, and which varies log-linearly with porosity. Permeabilities of material from the frontal prism decrease from 5×10-18 m2 at 5 MPa effective stress, to 3.0×10-19 m2 at 70 MPa, and porosities decrease from 51% to 29%, while permeabilities of the subducted sediment sample decrease from 5×10-18 m2 at 5 MPa to 3.6×10-19 m2 at 90 MPa, and porosities decrease from 49% to 36%. In-situ permeabilities for the prism and underthrust sediment samples, estimated using laboratory defined permeability-porosity relationships, are 4.9×10-18 m2 and 3.7×10-18 m2, respectively. Elastic wavespeeds increase systematically with increasing

  16. Basin-centered asperities in great subduction zone earthquakes: A link between slip, subsidence, and subduction erosion?

    USGS Publications Warehouse

    Wells, R.E.; Blakely, R.J.; Sugiyama, Y.; Scholl, D. W.; Dinterman, P.A.

    2003-01-01

    Published areas of high coseismic slip, or asperities, for 29 of the largest Circum-Pacific megathrust earthquakes are compared to forearc structure revealed by satellite free-air gravity, bathymetry, and seismic profiling. On average, 71% of an earthquake's seismic moment and 79% of its asperity area occur beneath the prominent gravity low outlining the deep-sea terrace; 57% of an earthquake's asperity area, on average, occurs beneath the forearc basins that lie within the deep-sea terrace. In SW Japan, slip in the 1923, 1944, 1946, and 1968 earthquakes was largely centered beneath five forearc basins whose landward edge overlies the 350??C isotherm on the plate boundary, the inferred downdip limit of the locked zone. Basin-centered coseismic slip also occurred along the Aleutian, Mexico, Peru, and Chile subduction zones but was ambiguous for the great 1964 Alaska earthquake. Beneath intrabasin structural highs, seismic slip tends to be lower, possibly due to higher temperatures and fluid pressures. Kilometers of late Cenozoic subsidence and crustal thinning above some of the source zones are indicated by seismic profiling and drilling and are thought to be caused by basal subduction erosion. The deep-sea terraces and basins may evolve not just by growth of the outer arc high but also by interseismic subsidence not recovered during earthquakes. Basin-centered asperities could indicate a link between subsidence, subduction erosion, and seismogenesis. Whatever the cause, forearc basins may be useful indicators of long-term seismic moment release. The source zone for Cascadia's 1700 A.D. earthquake contains five large, basin-centered gravity lows that may indicate potential asperities at depth. The gravity gradient marking the inferred downdip limit to large coseismic slip lies offshore, except in northwestern Washington, where the low extends landward beneath the coast. Transverse gravity highs between the basins suggest that the margin is seismically segmented and

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

    USGS Publications Warehouse

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

    2012-01-01

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

  18. Dynamic Linkages Between the Transition Zone & Surface Plate Motions in 2D Models of Subduction

    NASA Astrophysics Data System (ADS)

    Arredondo, K.; Billen, M. I.

    2013-12-01

    While slab pull is considered the dominant force controlling plate motion and speed, its magnitude is controlled by slab behavior in the mantle, where tomographic studies show a wide range of possibilities from direct penetration to folding, or stagnation directly above the lower mantle (e.g. Fukao et al., 2009). Geodynamic studies have investigated various parameters, such as plate age and two phase transitions, to recreate observed behavior (e.g. Běhounková and Cízková, 2008). However, past geodynamic models have left out known slab characteristics that may have a large impact on slab behavior and our understanding of subduction processes. Mineral experiments and seismic observations have indicated the existence of additional phase transitions in the mantle transition zone that may produce buoyancy forces large enough to affect the descent of a subducting slab (e.g. Ricard et al., 2005). The current study systematically tests different common assumptions used in geodynamic models: kinematic versus free-slip boundary conditions, the effects of adiabatic heating, viscous dissipation and latent heat, compositional layering and a more complete suite of phase transitions. Final models have a complete energy equation, with eclogite, harzburgite and pyrolite lithosphere compositional layers, and seven composition-dependent phase transitions within the olivine, pyroxene and garnet polymorph minerals. Results show important feedback loops between different assumptions and new behavior from the most complete models. Kinematic models show slab weakening or breaking above the 660 km boundary and between compositional layers. The behavior in dynamic models with a free-moving trench and overriding plate is compared to the more commonly found kinematic models. The new behavior may have important implications for the depth distribution of deep earthquakes within the slab. Though the thermodynamic parameters of certain phase transitions may be uncertain, their presence and

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

    NASA Astrophysics Data System (ADS)

    Zhang, H.; Thurber, C. H.

    2005-12-01

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

  20. Thermal structure, coupling and metamorphism in the Mexican subduction zone beneath Guerrero

    NASA Astrophysics Data System (ADS)

    Manea, V. C.; Manea, M.; Kostoglodov, V.; Currie, C. A.; Sewell, G.

    2004-08-01

    Temperature is one of the most important factors that controls the extent and location of the seismogenic coupled and transition, partially coupled segments of the subduction interplate fault. The width of the coupled fault inferred from the continuous GPS observations for the steady interseismic period and the transient width of the last slow aseismic slip event (Mw~ 7.5) that occurred in the Guerrero subduction zone in 2001-2002 extends up to 180-220 km from the trench. Previous thermal models do not consider this extremely wide coupled interface in Guerrero subduction zone that is characterized by shallow subhorizontal plate contact. In this study, a finite element model is applied to examine the temperature constraints on the width of the coupled area. The numerical scheme solves a system of 2-D Stokes equation and 2-D steady-state heat transfer equations. The updip limit of the coupling zone is taken between 100 and 150 °C, while the downdip limit is accepted at 450 °C as the transition from partial coupling to stable sliding. From the entire coupled zone, the seismogenic zone extends only up to ~82 km from the trench (inferred from the rupture width of large subduction thrust earthquakes), corresponding to the 250 °C isotherm. Only a small amount of frictional heating is needed to fit the intersection of the 450 °C isotherm and the subducting plate surface at 180-205 km from the trench. The calculated geotherms in the subducting slab and the phase diagram for MORB are used to estimate the metamorphic sequences within the oceanic subducting crust. A certain correlation exists between the metamorphic sequences and the variation of the coupling along the interplate fault.

  1. Mid-mantle seismic anisotropy patterns around subduction zones predicted by numerical modelling

    NASA Astrophysics Data System (ADS)

    Faccenda, Manuele

    2014-05-01

    There is increasing evidence for mid mantle seismic anisotropy around subduction zones whose interpretation remains elusive. In this study I estimate the strain-induced mid mantle fabric and associated seismic anisotropy developing in 3D petrological-thermo-mechanical subduction models where the slab is either stagnating over the 660 km discontinuity or penetrating into the lower mantle. The modeling of synthetic lattice-preferred-orientation (LPO) development of wadsleyite and perovskite has been calibrated with results from deformational experiments and ab-initio atomic scale models, and the single crystal elastic tensor of the different mineral phases is scaled by local P-T conditions. The lower transition zone (ringwoodite + garnet) is assumed to be isotropic. Mid mantle fabric develops in proximity of the subducting slab where deformation and stresses are high, except at depths where upwelling or downwelling material undergoes phase transformations, yielding to LPO reset. The upper transition zone (wadsleyite + garnet) is characterized by weak transverse isotropy (2-3%) with symmetry axes oriented and fast S wave polarized dip-normal. A slightly stronger transverse isotropy develops in the lower mantle (perovskite + periclase), where the symmetry axes, the polarization of the fast S wave and the maximum Vp and dVs are parallel to the slab dip and subduction direction. For stagnating slab models this translates into negative and positive radial anisotropy in the upper transition zone and lower mantle back-arc, respectively, minimum delay times for vertically travelling shear waves and large shear wave splitting for waves propagating horizontally in the lower mantle. These results may help in reconciling the seismic anisotropy patterns observed in some subduction zones with subduction-induced deformation, such as those measured in the mid mantle between the Australian plate and the New Hebrides-Tonga-Kermadec trenches that I interpret as related to stagnating

  2. Uncertainty in turbidite correlations along the Cascadia Subduction Zone

    NASA Astrophysics Data System (ADS)

    Atwater, B. F.

    2012-12-01

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

  3. Revisiting the subduction zone carbon cycle: What goes down, mostly comes up

    NASA Astrophysics Data System (ADS)

    Kelemen, Peter; Manning, Craig

    2016-04-01

    As we reported (PNAS 2015), carbon fluxes in subduction zones can be better constrained by including new estimates of carbon concentration in subducting mantle peridotites, consideration of carbonate solubility in aqueous fluid along subduction geotherms, and diapirism of carbon-bearing metasediments. Whereas previous studies concluded that about half the subducting carbon is returned to the convecting mantle, we find it is likely that relatively little carbon is recycled. If so, input from subduction zones into the overlying plate is larger than output from arc volcanoes plus diffuse venting, and substantial quantities of carbon are stored in the mantle lithosphere and crust. Also, if the subduction zone carbon cycle is nearly closed on time scales of 5-10 Ma, then the carbon content of the mantle lithosphere + crust + ocean + atmosphere must be increasing. This is consistent with inferences from noble gas data and crustal carbon inventories (review in Hayes & Waldbauer PTRSL 2006). Carbon in diamonds, which may have been recycled into the convecting mantle, is a small fraction of the global carbon inventory. Increasing NaCl and decreasing pH and fO2 in aqueous fluids all increase carbon solubility at HP to UHP conditions, strengthening the prediction of wt% solubility (Manning & Kelemen, Fall AGU 2015), while hydrous carbonatite formed on high T subduction geotherms (Poli, Nat Geosci 2015) has still higher concentrations. Fractures heal rapidly at UHP conditions, so fluid transport is mainly via porous flow, with increasing downstream solubility and porosity due to heating in the subducting plate and base of the mantle wedge. Depending on flow and reaction rates vs diffusivity (Damkohler number), this could yield diffuse or channelized flow. High, increasing solubilities and reaction rates, with slow diffusion, can produce diffuse, pervasive porous flow (e.g., Hoefner & Fogler, AIChEJ 1988; Spiegelman et al, JGR 2001) and efficient recycling of carbon.

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

    NASA Astrophysics Data System (ADS)

    Rosner, Martin Siegfried

    2003-10-01

    Late Miocene to Quaternary volcanic rocks from the frontal arc to the back-arc region of the Central Volcanic Zone in the Andes show a wide range of delta 11B values (+4 to -7 ‰) and boron concentrations (6 to 60 ppm). Positive delta 11B values of samples from the volcanic front indicate involvement of a 11B-enriched slab component, most likely derived from altered oceanic crust, despite the thick Andean continental lithosphere, and rule out a pure crust-mantle origin for these lavas. The delta 11B values and B concentrations in the lavas decrease systematically with increasing depth of the Wadati-Benioff Zone. This across-arc variation in delta 11B values and decreasing B/Nb ratios from the arc to the back-arc samples are attributed to the combined effects of B-isotope fractionation during progressive dehydration in the slab and a steady decrease in slab-fluid flux towards the back arc, coupled with a relatively constant degree of crustal contamination as indicated by similar Sr, Nd and Pb isotope ratios in all samples. Modelling of fluid-mineral B-isotope fractionation as a function of temperature fits the across-arc variation in delta 11B and we conclude that the B-isotope composition of arc volcanics is dominated by changing delta 11B composition of B-rich slab-fluids during progressive dehydration. Crustal contamination becomes more important towards the back-arc due to the decrease in slab-derived fluid flux. Because of this isotope fractionation effect, high delta 11B signatures in volcanic arcs need not necessarily reflect differences in the initial composition of the subducting slab. Three-component mixing calculations for slab-derived fluid, the mantle wedge and the continental crust based on B, Sr and Nd isotope data indicate that the slab-fluid component dominates the B composition of the fertile mantle and that the primary arc magmas were contaminated by an average addition of 15 to 30 % crustal material. Spät-miozäne bis quartäre Vulkanite

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

    NASA Astrophysics Data System (ADS)

    Rosner, Martin Siegfried

    2003-10-01

    Late Miocene to Quaternary volcanic rocks from the frontal arc to the back-arc region of the Central Volcanic Zone in the Andes show a wide range of delta 11B values (+4 to -7 ‰) and boron concentrations (6 to 60 ppm). Positive delta 11B values of samples from the volcanic front indicate involvement of a 11B-enriched slab component, most likely derived from altered oceanic crust, despite the thick Andean continental lithosphere, and rule out a pure crust-mantle origin for these lavas. The delta 11B values and B concentrations in the lavas decrease systematically with increasing depth of the Wadati-Benioff Zone. This across-arc variation in delta 11B values and decreasing B/Nb ratios from the arc to the back-arc samples are attributed to the combined effects of B-isotope fractionation during progressive dehydration in the slab and a steady decrease in slab-fluid flux towards the back arc, coupled with a relatively constant degree of crustal contamination as indicated by similar Sr, Nd and Pb isotope ratios in all samples. Modelling of fluid-mineral B-isotope fractionation as a function of temperature fits the across-arc variation in delta 11B and we conclude that the B-isotope composition of arc volcanics is dominated by changing delta 11B composition of B-rich slab-fluids during progressive dehydration. Crustal contamination becomes more important towards the back-arc due to the decrease in slab-derived fluid flux. Because of this isotope fractionation effect, high delta 11B signatures in volcanic arcs need not necessarily reflect differences in the initial composition of the subducting slab. Three-component mixing calculations for slab-derived fluid, the mantle wedge and the continental crust based on B, Sr and Nd isotope data indicate that the slab-fluid component dominates the B composition of the fertile mantle and that the primary arc magmas were contaminated by an average addition of 15 to 30 % crustal material. Spät-miozäne bis quartäre Vulkanite

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

    NASA Astrophysics Data System (ADS)

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

    2016-06-01

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

  7. Seismicity and Geometry Properties of the Hellenic Subduction Zone

    NASA Astrophysics Data System (ADS)

    Papadimitriou, E. E.; Karakostas, V. G.; Vallianatos, F.; Makropoulos, C.; Drakatos, G.

    2014-12-01

    Recent seismicity and fault plane solutions of earthquakes that occurred along the Hellenic Arc-Trench system are engaged for approximating the geometry of the subducted oceanic plate. Seismicity and focal mechanisms confirm the gentle subduction (~15o-20o) of the oceanic crust reaching a depth of 20 km at a distance of 100 km from the trench. The slab is then bending at larger angles, and in particular at ~45o up to the depth of 80 km and at ~65o up to the depth of 180 km, when seismicity ceased. This geometry of the slab is shown in a bunch of cross sections normal to the convergence strike, up to ~25o (east Crete Island). To the east the sparse inslab seismicity reveals an almost vertical dipping of the lower part (from 80 km downdip) of the descending slab. The slab interface that accommodates hazardous earthquakes is clearly nonplanar with the main seismic moment release taking place on its up-dip side. The fore-arc, upper plate seismicity, is remarkably low in comparison with both subduction and back arc seismicity, and confined inside a seismogenic layer having a width not exceeding the 20km. Offshore seismicity is spatially variable forming distinctive streaks thus revealing that parts of the oceanic crust are probably slipped aseismically. This observation along with the fact that coupling in the Hellenic arc is only about a tenth of the plate motion, imply the presence of areas of lower and higher coupling across the subduction interface. Areas of high coupling imply areas of the slab interface subjected to high normal forces and correlate with earthquake asperities. Although asperity distributions vary substantially through time, identification of such characteristics in the seismogenesis can have a significant impact in the seismic hazard assessment. This research has been co-funded by the European Union (European Social Fund) and Greek national resources under the framework of the "THALES Program: SEISMO FEAR HELLARC" project.

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

    NASA Astrophysics Data System (ADS)

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

    2009-01-01

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

  9. Long-term seismogenic process for major earthquakes in subduction zones

    NASA Astrophysics Data System (ADS)

    Evison, F. F.; Rhoades, D. A.

    A qualitative physical process for the long-term seismogenesis of major earthquakes in subduction zones is proposed on the basis of quantitative empirical evidence that swarms, mainshocks and aftershocks are closely related phenomena. The relations, which have been identified in the comprehensive, long-term catalogues of New Zealand and Japan, represent swarms as predictors of mainshocks with respect to location, time and magnitude. Clustering of swarms and of mainshock/aftershock events is allowed for. With a database of 15 sequences of swarms, mainshocks and aftershocks, tests are being conducted with the object of refining the relations and evaluating them as a possible means of practical synoptic forecasting. Three sequences have culminated in major earthquakes since the tests began, and the systematic study now relates a total of 36 swarms with 29 mainshock/aftershock events. These empirical results strengthen and quantify the connection between swarms and major earthquakes, which several authors have demonstrated by means of numerical/physical modelling. The proposed seismogenic process includes swarms, mainshocks and aftershocks as separate event stages which are related by predictability. Interevent conditions are specified according to the Mogi criteria for the medium; cracks at which fractures subsequently occur constitute nonuniformity in the Mogi sense, and post-earthquake healing restores uniformity. Where the Gutenberg-Richter relation occurs, it is accepted as possible evidence of deterministic chaos and unpredictability; as a corollary, the process is noncyclical. The principle of scaling is held to apply except when modified by large-scale boundaries in the medium. Subduction zones and some other localities where water is abundant are indicated by the main empirical studies as favourable to the occurrence of swarms. Fluid overpressuring is therefore proposed as a mechanism for the self-triggering of swarms, and this is supported by additional

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

    NASA Astrophysics Data System (ADS)

    Lin, Shu-Chuan; Kuo, Ban-Yuan

    2016-03-01

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

  11. A Bayesian Approach for Apparent Inter-plate Coupling in the Central Andes Subduction Zone

    NASA Astrophysics Data System (ADS)

    Ortega Culaciati, F. H.; Simons, M.; Genrich, J. F.; Galetzka, J.; Comte, D.; Glass, B.; Leiva, C.; Gonzalez, G.; Norabuena, E. O.

    2010-12-01

    We aim to characterize the extent of apparent plate coupling on the subduction zone megathrust with the eventual goal of understanding spatial variations of fault zone rheology, inferring relationships between apparent coupling and the rupture zone of big earthquakes, as well as the implications for earthquake and tsunami hazard. Unlike previous studies, we approach the problem from a Bayesian perspective, allowing us to completely characterize the model parameter space by searching a posteriori estimates of the range of allowable models instead of seeking a single optimum model. Two important features of the Bayesian approach are the possibility to easily implement any kind of physically plausible a priori information and to perform the inversion without regularization, other than that imposed by the way in which we parameterize the forward model. Adopting a simple kinematic back-slip model and a 3D geometry of the inter-plate contact zone, we can estimate the probability of apparent coupling (Pc) along the plate interface that is consistent with a priori information (e.g., approximate rake of back-slip) and available geodetic measurements. More generally, the Bayesian approach adopted here is applicable to any region and eventually would allow one to evaluate the spatial relationship between various inferred distributions of fault behavior (e.g., seismic rupture, postseismic creep, and apparent interseismic coupling) in a quantifiable manner. We apply this methodology to evaluate the state of apparent inter-seismic coupling in the Chilean-Peruvian subduction margin (12 S - 25 S). As observational constraints, we use previously published horizontal velocities from campaign GPS [Kendrick et al., 2001, 2006] as well as 3 component velocities from a recently established continuous GPS network in the region (CAnTO). We compare results from both joint and independent use of these data sets. We obtain patch like features for Pc with higher values located above 60 km

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

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

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

  13. Mantle plumes in the vicinity of subduction zones

    NASA Astrophysics Data System (ADS)

    Mériaux, C. A.; Mériaux, A.-S.; Schellart, W. P.; Duarte, J. C.; Duarte, S. S.; Chen, Z.

    2016-11-01

    We present three-dimensional deep-mantle laboratory models of a compositional plume within the vicinity of a buoyancy-driven subducting plate with a fixed trailing edge. We modelled front plumes (in the mantle wedge), rear plumes (beneath the subducting plate) and side plumes with slab/plume systems of buoyancy flux ratio spanning a range from 2 to 100 that overlaps the ratios in nature of 0.2-100. This study shows that 1) rising side and front plumes can be dragged over thousands of kilometres into the mantle wedge, 2) flattening of rear plumes in the trench-normal direction can be initiated 700 km away from the trench, and a plume material layer of lesser density and viscosity can ultimately almost entirely underlay a retreating slab after slab/plume impact, 3) while side and rear plumes are not tilted until they reach ∼600 km depth, front plumes can be tilted at increasing depths as their plume buoyancy is lessened, and rise at a slower rate when subjected to a slab-induced downwelling, 4) rear plumes whose buoyancy flux is close to that of a slab, can retard subduction until the slab is 600 km long, and 5) slab-plume interaction can lead to a diversity of spatial plume material distributions into the mantle wedge. We discuss natural slab/plume systems of the Cascadia/Bowie-Cobb, and Nazca/San Felix-Juan Fernandez systems on the basis of our experiments and each geodynamic context and assess the influence of slab downwelling at depths for the starting plumes of Java, Coral Sea and East Solomon. Overall, this study shows how slab/plume interactions can result in a variety of geological, geophysical and geochemical signatures.

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

    SciTech Connect

    Cloos, M. )

    1992-07-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

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

  16. Osmium isotope constraints on ore metal recycling in subduction zones

    PubMed

    McInnes; McBride; Evans; Lambert; Andrew

    1999-10-15

    Veined peridotite xenoliths from the mantle beneath the giant Ladolam gold deposit on Lihir Island, Papua New Guinea, are 2 to 800 times more enriched in copper, gold, platinum, and palladium than surrounding depleted arc mantle. Gold ores have osmium isotope compositions similar to those of the underlying subduction-modified mantle peridotite source region, indicating that the primary origin of the metals was the mantle. Because the mantle is relatively depleted in gold, copper, and palladium, tectonic processes that enhance the advective transport and concentration of these fluid soluble metals may be a prerequisite for generating porphyry-epithermal copper-gold deposits. PMID:10521343

  17. Osmium isotope constraints on ore metal recycling in subduction zones

    PubMed

    McInnes; McBride; Evans; Lambert; Andrew

    1999-10-15

    Veined peridotite xenoliths from the mantle beneath the giant Ladolam gold deposit on Lihir Island, Papua New Guinea, are 2 to 800 times more enriched in copper, gold, platinum, and palladium than surrounding depleted arc mantle. Gold ores have osmium isotope compositions similar to those of the underlying subduction-modified mantle peridotite source region, indicating that the primary origin of the metals was the mantle. Because the mantle is relatively depleted in gold, copper, and palladium, tectonic processes that enhance the advective transport and concentration of these fluid soluble metals may be a prerequisite for generating porphyry-epithermal copper-gold deposits.

  18. Dynamics of intra-oceanic subduction initiation, part 2: supra-subduction zone ophiolite formation and metamorphic sole exhumation in context of absolute plate motions

    NASA Astrophysics Data System (ADS)

    Maffione, M.; Van Hinsbergen, D. J. J.; Peters, K.; Spakman, W.; Guilmette, C.; Thieulot, C.; Plumper, O.; Guerer, D.; Brouwer, F. M.; Aldanmaz, E.; Kaymakci, N.

    2015-12-01

    Analyzing subduction initiation is key for understanding the coupling between plate tectonics and underlying mantle. Here we focus on supra-subduction zone (SSZ) ophiolites and how their formation links to intra-oceanic subduction initiation in an absolute plate motion frame. SSZ ophiolites form the majority of exposed oceanic lithosphere fragments and are widely recognized to have formed during intra-oceanic subduction initiation. Structural, petrological, geochemical, and plate kinematic constraints on their kinematic evolution show that SSZ crust forms at forearc spreading centers at the expense of a mantle wedge, thereby flattening the nascent slab. This leads to the typical inverted pressure gradients found in metamorphic soles that form at the subduction plate contact below and during SSZ crust crystallization. Former spreading centers are preserved in forearcs when subduction initiates along transform faults or off-ridge oceanic detachments. We show how these are reactivated when subduction initiates in the absolute plate motion direction of the inverting weakness zone. Upon inception of slab-pull due to e.g. eclogitization, the sole is separated from the slab, remains welded to the thinned overriding plate lithosphere and can become intruded by mafic dikes upon asthenospheric influx into the mantle wedge. We propound that most ophiolites thus formed under special geodynamic circumstances and may not be representative of normal oceanic crust. Our study highlights how far-field geodynamic processes and absolute plate motions may force intra-oceanic subduction initiation as key towards advancing our understanding of the entire plate tectonic cycle.

  19. Precollisional, multistage exhumation of subducted continental crust: The Sesia Zone, western Alps

    NASA Astrophysics Data System (ADS)

    Babist, J.; Handy, M. R.; Konrad-Schmolke, M.; Hammerschmidt, K.

    2006-12-01

    The Sesia Zone within the Tertiary arc of the western Alps is a relic of the subducted part of the Adriatic continental margin along the SE border of the Tethyan ocean. The Sesia Zone comprises three basement nappes which individuated during Late Cretaceous (65-80 Ma) subduction to different depths at high-pressure (HP, blueschist, eclogite facies) conditions (peak pressures of 1.0-1.2, 1.0-1.5, and 1.5-2.0 GPa). The thrusts bounding these nappes developed where the crust was previously thinned during Jurassic rifting. Crustal-scale shear zones partly overprinted these early thrusts and exhumed coherent slices of crust containing HP rocks. Initial exhumation of the internal part of the accreted margin involved thrusting (D1) and transpressional shearing (D2) along a subvertical, E-W trending mylonitic shear zone under retrograde blueschist- to greenschist-facies conditions. This exhumation was nearly isothermal to a depth of about 25 km, where the basement nappes were juxtaposed. Subsequent exhumation of these nappes to a common depth of about 15-20 km occurred in the footwall of a greenschist-facies, top-SE extensional shear zone (D3) preserved in some of the highest mountain peaks of the Sesia Zone. New Rb-Sr mineral ages constrain D2 to have occurred at about 60-65 Ma and D3 at about 45-55 Ma. Thus top-SE extensional exhumation was broadly coeval with Eocene, SE directed subduction of the Liguro-Piemont oceanic lithosphere beneath the Adriatic margin. Slow cooling and erosional denudation of the Sesia Zone from 45 to 30 Ma occurred in the hanging wall of the Gressoney extensional shear zone (D4), which itself contributed to the exhumation of Eocene HP and ultra-HP oceanic rocks in its footwall. By 30 Ma, HP rocks of the Sesia Zone were intruded by shallow granitic plutons which were eroded and redeposited within volcanoclastic sediments. Oligo-Miocene Insubric backfolding and thrusting (D5) only exhumed northeastern parts of the Sesia Zone, where HP metamorphism

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

    NASA Astrophysics Data System (ADS)

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

    2008-05-01

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

  1. Louisville seamount subduction and its implication on mantle flow beneath the central Tonga-Kermadec arc.

    PubMed

    Timm, Christian; Bassett, Daniel; Graham, Ian J; Leybourne, Matthew I; de Ronde, Cornel E J; Woodhead, Jon; Layton-Matthews, Daniel; Watts, Anthony B

    2013-01-01

    Subduction of intraplate seamounts beneath a geochemically depleted mantle wedge provides a seldom opportunity to trace element recycling and mantle flow in subduction zones. Here we present trace element and Sr, Nd and Pb isotopic compositions of lavas from the central Tonga-Kermadec arc, west of the contemporary Louisville-Tonga trench intersection, to provide new insights into the effects of Louisville seamount subduction. Elevated (206)Pb/(204)Pb, (208)Pb/(204)Pb, (86)Sr/(87)Sr in lavas from the central Tonga-Kermadec arc front are consistent with localized input of subducted alkaline Louisville material (lavas and volcaniclastics) into sub-arc partial melts. Furthermore, absolute Pacific Plate motion models indicate an anticlockwise rotation in the subducted Louisville seamount chain that, combined with estimates of the timing of fluid release from the subducting slab, suggests primarily trench-normal mantle flow beneath the central Tonga-Kermadec arc system. PMID:23591887

  2. Louisville seamount subduction and its implication on mantle flow beneath the central Tonga-Kermadec arc.

    PubMed

    Timm, Christian; Bassett, Daniel; Graham, Ian J; Leybourne, Matthew I; de Ronde, Cornel E J; Woodhead, Jon; Layton-Matthews, Daniel; Watts, Anthony B

    2013-01-01

    Subduction of intraplate seamounts beneath a geochemically depleted mantle wedge provides a seldom opportunity to trace element recycling and mantle flow in subduction zones. Here we present trace element and Sr, Nd and Pb isotopic compositions of lavas from the central Tonga-Kermadec arc, west of the contemporary Louisville-Tonga trench intersection, to provide new insights into the effects of Louisville seamount subduction. Elevated (206)Pb/(204)Pb, (208)Pb/(204)Pb, (86)Sr/(87)Sr in lavas from the central Tonga-Kermadec arc front are consistent with localized input of subducted alkaline Louisville material (lavas and volcaniclastics) into sub-arc partial melts. Furthermore, absolute Pacific Plate motion models indicate an anticlockwise rotation in the subducted Louisville seamount chain that, combined with estimates of the timing of fluid release from the subducting slab, suggests primarily trench-normal mantle flow beneath the central Tonga-Kermadec arc system.

  3. Investigation of complex slow slip behavior along the Hikurangi subduction zone with earthquake simulator RSQSim

    NASA Astrophysics Data System (ADS)

    Colella, H.; Ellis, S. M.; Williams, C. A.

    2015-12-01

    The Hikurangi subduction zone (New Zealand) is one of many subudction zones that exhibit slow slip behavior. Geodetic observations along the Hikurangi subduction zone are unusual in that not only does the subduction zone exhibit periodic slow slip events at "typical" subduction-zone depths of 25-50 km along the southern part of the margin, but also much shallower depths of 8-15 km along the northern part of the margin. Furthermore, there is evidence for interplay between slow slip events at these different depth ranges (between the deep and shallow events) along the central part of the margin, and some of the slow slip behavior is observed along regions of the interface that were previously considered locked, which raises questions about the slip behavior of this region. This study employs the earthquake simulator, RSQSim, to explore variations in the effective normal stress (i.e., stress after the addition of pore fluid pressures) and the frictional instability necessary to generate the complex slow slip events observed along the Hikurangi margin. Preliminary results suggest that to generate slow slip events with similar recurrence intervals to those observed the effective normal stress (MPa) is 3x higher in the south than the north, 6-9MPa versus 2-3MPa, respectively. Results also suggest that, at a minimum, that some overlap along the central margin must exist between the slow slip sections in the north and south to reproduce the types of slip events observed along the Hikurangi subduction zone. To further validate the results from the simulations, Okada solutions for surface displacements will be compared to geodetic solution to more accurately constrain the areas in which slip behavior varies and the cause(s) for the variation(s).

  4. Tsunamigenic potential of Mediterranean fault systems and active subduction zones

    NASA Astrophysics Data System (ADS)

    Petricca, Patrizio; Babeyko, Andrey

    2016-04-01

    Since the North East Atlantic and Mediterranean Tsunami Warning System (NEAMTWS) is under development by the European scientific community, it becomes necessary to define guidelines for the characterization of the numerous parameters must be taken into account in a fair assessment of the risk. Definition of possible tectonic sources and evaluation of their potential is one of the principal issues. In this study we systematically evaluate tsunamigenic potential of up-to-now known real fault systems and active subduction interfaces in the NEAMTWS region. The task is accomplished by means of numerical modeling of tsunami generation and propagation. We have simulated all possible uniform-slip ruptures populating fault and subduction interfaces with magnitudes ranging from 6.5 up to expected Mmax. A total of 15810 individual ruptures were processed. For each rupture, a tsunami propagation scenario was computed in linear shallow-water approximation on 1-arc minute bathymetric grid (Gebco_08) implying normal reflection boundary conditions. Maximum wave heights at coastal positions (totally - 23236 points of interest) were recorded for four hours of simulation and then classified according to currently adopted warning level thresholds. The resulting dataset allowed us to classify the sources in terms of their tsunamigenic potential as well as to estimate their minimum tsunamigenic magnitude. Our analysis shows that almost every source in the Mediterranean Sea is capable to produce local tsunami at the advisory level (i.e., wave height > 20 cm) starting from magnitude values of Mw=6.6. In respect to the watch level (wave height > 50 cm), the picture is less homogeneous: crustal sources in south-west Mediterranean as well as East-Hellenic arc need larger magnitudes (around Mw=7.0) to trigger watch levels even at the nearby coasts. In the context of the regional warning (i.e., source-to-coast distance > 100 km) faults also behave more heterogeneously in respect to the minimum

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

    NASA Astrophysics Data System (ADS)

    Ague, Jay J.; Nicolescu, Stefan

    2014-05-01

    The balance between the subduction of carbonate mineral-bearing rocks into Earth's mantle and the return of CO2 to the atmosphere by volcanic and metamorphic degassing is critical to the carbon cycle. Carbon is thought to be released from subducted rocks mostly by simple devolatilization reactions. However, these reactions will also retain large amounts of carbon within the subducting slab and have difficulty in accounting for the mass of CO2 emitted from volcanic arcs. Carbon release may therefore occur via fluid-induced dissolution of calcium carbonate. Here we use carbonate δ18O and δ13C systematics, combined with analyses of rock and fluid inclusion mineralogy and geochemistry, to investigate the alteration of the exhumed Eocene Cycladic subduction complex on the Syros and Tinos islands, Greece. We find that in marble rocks adjacent to two fluid conduits that were active during subduction, the abundance of calcium carbonate drastically decreases approaching the conduits, whereas silicate minerals increase. Up to 60-90% of the CO2 was released from the rocks--far greater than expected via simple devolatilization reactions. The δ18O of the carbonate minerals is 5-10 lighter than is typical for metamorphosed carbonate rocks, implying that isotopically light oxygen was transported by fluid infiltration from the surroundings. We suggest that fluid-mediated carbonate mineral removal, accompanied by silicate mineral precipitation, provides a mechanism for the release of enormous amounts of CO2 from subduction zones.

  6. Predicting the permeability of sediments entering subduction zones

    NASA Astrophysics Data System (ADS)

    Daigle, Hugh; Screaton, Elizabeth J.

    2015-07-01

    Using end-member permeabilities defined by a worldwide compilation of sediment permeabilities at convergent margins, we compare permeability predictions using a geometric mean and a two-component effective medium theory (EMT). Our implementation of EMT includes a threshold fraction of the high-permeability component that determines whether flow occurs dominantly in the high- or low-permeability component. We find that this threshold fraction in most cases is equal to the silt + sand-sized fraction of the sediment. This suggests that sediments undergoing primary consolidation tend to exhibit flow equally distributed between the high- and low-permeability components. We show that the EMT method predicts permeability better than the weighted geometric mean of the end-member values for clay fractions <0.6. This work provides insight into the microstructural controls on permeability in subducting sediments and valuable guidance for locations which lack site-specific permeability results but have available grain-size information.

  7. The Neo-Tethyan subduction zone(s,?) in Azerbaijan, NW Iran: preliminary results

    NASA Astrophysics Data System (ADS)

    Lechmann, Anna; Burg, Jean-Pierre; Faridi, Mohammad

    2015-04-01

    Azerbaijan in NW Iran, and in particular the Khoy ophiolitic complex, require more detailed documentation to integrate them as elements of the Alpine-Himalayan orogenic belt. They are attributed to multiple accretion and collision after subduction and closure of the Tethys Ocean and related seaways. We are interested in the pre- to syn-collisional relationships between the ophiolitic, arc and other magmatic units. This work investigates to what extent single or multiple collisions and orogeny have shaped the NW Iranian Plateau. In particular, we want to understand the changes in deformation style within the collision zone and the effects of several possibly coeval events such as closure of two suture zones separated by an arc and possibly followed by slab break-off(s). Fieldwork focused on sampling the different magmatic rock units to specify the structural record and the structural relationships between the various lithological units. Cretaceous to Quaternary, regionally distributed magmatic rocks were collected to have good resolution of their changes in space and time. Petrological, geochemical and isotope studies will characterize magmatic rocks and their sources. Major and trace element geochemistry of mantle and crustal suites of the Khoy ophiolitic complex help to constrain the tectonic setting. Two complexes were defined on the basis of K-Ar dating (Khalatbari-Jafari et al., 2004). An older, probably subducted ophiolite of Triassic-Jurassic age and a younger non-metamorphic ophiolite of Late Cretaceous age. Fossil-bearing sediments provide stratigraphic ages of important contacts. Preliminary results are present in form of bulk rock and trace element chemistry of ultramafic and mafic rocks of the Khoy ophiolite(s, ?) and offer a first possibility to compare the data with already existing publications. Additionally, petrological studies of various magmatic rocks present first products for a starting discussion on the geodynamic evolution of the NW part of

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

    NASA Astrophysics Data System (ADS)

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

    2009-04-01

    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 that the asthenospheric viscosity is varied from 3 × 1018 Pa s for a thin (140 km) asthenosphere to 4 × 1019 Pa s for a thick (380 km) asthenosphere. Present-day vertical crustal motion predicted by the GIA models shows rates of a few tenths of a millimeter per year, consistent with previous analyses. The model viscosities largely pertain to the viscosity of the oceanic mantle beneath the subducting Juan de Fuca slab but include a contribution from the mantle wedge above the slab. For comparison, effective viscosities for the upper mantle due to the tectonic regime (subduction) were computed using the strain rates and temperatures of an independent geodynamic model of the CSZ with a wet olivine power law rheology. The effective viscosities agree well with GIA model viscosities of 1019 Pa s or less, corresponding to an asthenosphere of 100 or 200 km thickness. The agreement suggests a significant role for power law flow in the GIA response. Regardless of the microphysical mechanisms responsible for the GIA response, the viscosity values inferred from GIA can be applied to studies of the megathrust earthquake cycle because both processes take place on comparable time scales.

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

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

    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.

  10. Seismic structure in central Mexico: Implications for fragmentation of the subducted Cocos plate

    NASA Astrophysics Data System (ADS)

    Dougherty, Sara L.; Clayton, Robert W.; Helmberger, Don V.

    2012-09-01

    The fine-scale seismic structure of the central Mexico subduction zone is studied using moderate-sized (M4-6) intraslab earthquakes. Regional waveforms from the Mapping the Rivera Subduction Zone (MARS) seismic array are complicated and contain detailed information about the subduction zone structure, including evidence of lateral heterogeneity. This waveform information is used to model the structure of the subducted plates, particularly along the transition from flat to normal subduction, where recent studies have shown evidence for possible slab tearing along the eastern projection of the Orozco Fracture Zone (OFZ). The lateral extent of a thin ultra-slow velocity layer (USL) imaged atop the Cocos slab in recent studies along the Meso America Subduction Experiment array is examined here using MARS waveforms. We find an edge to this USL which is coincident with the western boundary of the projected OFZ region. Forward modeling of the 2D structure of the subducted Rivera and Cocos plates using a finite difference algorithm provides constraints on the velocity and geometry of each slab's seismic structure in this region and confirms the location of the USL edge. We propose that the Cocos slab is currently fragmenting into a North Cocos plate and a South Cocos plate along the projection of the OFZ, in agreement with observations of variable Cocos plate motion on either side of the OFZ. This tearing event may be a young analogy to the 10 Ma Rivera-Cocos plate boundary, and may be related to the slab rollback in central Mexico.

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

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

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

  12. Why Ignoring Anisotropy When Imaging Subduction Zones Could be a Bad Idea

    NASA Astrophysics Data System (ADS)

    Bezada, M. J.; Faccenda, M.; Toomey, D. R.; Humphreys, E.

    2014-12-01

    Mantle flow patterns around subduction zones and the consequent seismic anisotropy have been the subject of many studies across different disciplines. However, even though upper mantle anisotropy is not controversial, our primary means of imaging subduction zones in the upper mantle -teleseismic tomography- commonly assumes that the mantle is isotropic. We investigate the possible effects of unaccounted-for anisotropy in seismic imaging of the upper mantle in a subduction setting by carrying out a synthetic test in three steps: (1) We build an anisotropic velocity model of a subduction zone. The model was built from self-consistent estimates of mantle velocity structure and strain-induced anisotropy that are derived from thermo-mechanical and microstructural modeling. (2) We generate P-wave travel-time delay data for this model using an event distribution that is representative of what is typically recorded by a temporary seismic array. The anisotropic travel times are calculated through the prescribed model using a graph-theory ray tracer. (3) We invert the anisotropic synthetic delays under the assumption of isotropy, as is common practice. The tomographic inversion of the synthetic data recovers the input velocity structure fairly well, but delays caused solely by anisotropy result in very significant additional isotropic velocity anomalies that are artificial. Some of these artifacts are nonetheless attractive targets for (mis)interpretation. For example, one of the most notable artifacts is a low velocity zone in the mantle wedge. Our initial results suggest that significant artifacts may be common in isotropic velocity models of subduction zones and stress the need for mantle imaging that properly handles anisotropy.

  13. Segmentation of Makran Subduction Zone and its consequences on tsunami hazard estimations

    NASA Astrophysics Data System (ADS)

    Mokhtari, M.

    2009-04-01

    In a plate tectonic setting like that of the Makran Accretionary Complex of Oman Sea, a fairly high earthquake activity would be expected, as in many of the other major Accretionary complexes/subduction zones around the world. But this region which is located between the Zendan-Minab Fault System and Oranch Fault Zone shows relatively low seismicity in comparison with the surrounding region. Better documented tsunami events in the Makran subduction zones are 3, including two events of seismic origin, and one of unknown origin. The latest event is the major earthquake generated tsunami of 1945 in eastern Makran that ruptured approximately one-fifth the length of the subduction zone. It is important to note that, the epicenter of this event is also close to the Sonne Fault which has created segments on the Makran Subduction Zone. The crossing points between Makran Subduction Zone and these oblique fault zones can be a location for occurrence of major earthquake activities. However, more studies are required for further clarification. In contrast to the east, the plate boundary in western Makran has no clear record of historically as well as instrumental great events. The large changes in seismicity between eastern and western Makran suggest segmentation of the subduction zone. This is being supported by Kukowski et al., (2000) where they introduce a new boundary coinciding very well with the Sonne strike-slip fault. As mentioned the western part is characterized by the absence of events. East of the Sonne fault and west of long 64°E is the only region with a clustering of events within the submarine and southernmost onshore part of the Accretionary Wedge, also including the Mw 8.1 event of 1945 (Byrne et al., 1992). Most events in the wedge appear to be pure-thrust earthquakes and are interpreted as plate boundary events (Quittmeyer and Kafka, 1984; Byrne et al., 1992). The earthquake of August 12, 1963, a few tens of kilometers east of the Sonne fault, had a large

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

    NASA Astrophysics Data System (ADS)

    Reynard, Bruno

    2013-04-01

    Seismological and magneto-telluric tomographies are potential tools for imaging fluid circulation when combined with petrophysical models. Recent measurements of the physical properties of serpentine allow refining hydration of the mantle and fluid circulation in the mantle wedge from geophysical data. In the slab lithospheric mantle, serpentinization caused by bending at the trench is limited to a few kilometers below the oceanic crust (<5 km). Double Wadati-Benioff zones, 20-30 km below the crust, are explained by deformation of dry peridotites, not by serpentine dehydration. It reduces the required amount of water stored in solid phases in the slab (Reynard et al., 2010). In the cold (<700°C) fore-arc mantle wedge above the subducting slab, serpentinization is caused by the release of large amounts of hydrous fluids in the cold mantle above the dehydrating subducted plate. Low seismic velocities in the wedge give a time-integrated estimate of hydration and serpentinization. Serpentinization reaches 50-100% in hot subduction, while it is below 10% in cold subduction (Bezacier et al., 2010; Reynard, 2012). Electromagnetic profiles of the mantle wedge reveal high electrical-conductivity bodies. In hot areas of the mantle wedge (> 700°C), water released by dehydration of the slab induces melting of the mantle under volcanic arcs, explaining the observed high conductivities. In the cold melt-free wedge (< 700°C), high conductivities in electromagnetic profiles provide "instantaneous" images of fluid circulation because the measured electrical conductivity of serpentine is below 0.1 mS/m (Reynard et al., 2011). A small fraction (ca. 1% in volume) of connective high-salinity fluids accounts for the highest observed conductivities. Low-salinity fluids (≤ 0.1 m) released by slab dehydration evolve towards high-salinity (≥ 1 m) fluids during progressive serpentinization in the wedge. These fluids can mix with arc magmas at depths and account for high-chlorine melt

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

    NASA Astrophysics Data System (ADS)

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

    2016-10-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2015-10-01

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

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

    USGS Publications Warehouse

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

    1992-01-01

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

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

    USGS Publications Warehouse

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

    2004-01-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2011-12-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2012-04-01

    The Hellenic subduction zone is the most seismically active region in Europe [Becker & Meier, 2010]. The spatial and temporal distribution of seismicity as well as the analysis of the magnitude distribution of earthquakes concerning the Hellenic subduction zone, has been studied using the concept of Non-Extensive Statistical Physics (NESP) [Tsallis, 1988 ; Tsallis, 2009]. Non-Extensive Statistical Physics, which is a generalization of Boltzmann-Gibbs statistical physics, seems a suitable framework for studying complex systems (Vallianatos, 2011). Using this concept, Abe & Suzuki (2003;2005) investigated the spatial and temporal properties of the seismicity in California and Japan and recently Darooneh & Dadashinia (2008) in Iran. Furthermore, Telesca (2011) calculated the thermodynamic parameter q of the magnitude distribution of earthquakes of the southern California earthquake catalogue. Using the external seismic zones of 36 seismic sources of shallow earthquakes in the Aegean and the surrounding area [Papazachos, 1990], we formed a dataset concerning the seismicity of shallow earthquakes (focal depth ≤ 60km) of the subduction zone, which is based on the instrumental data of the Geodynamic Institute of the National Observatory of Athens (http://www.gein.noa.gr/, period 1990-2011). The catalogue consists of 12800 seismic events which correspond to 15 polygons of the aforementioned external seismic zones. These polygons define the subduction zone, as they are associated with the compressional stress field which characterizes a subducting regime. For each event, moment magnitude was calculated from ML according to the suggestions of Papazachos et al. (1997). The cumulative distribution functions of the inter-event times and the inter-event distances as well as the magnitude distribution for each seismic zone have been estimated, presenting a variation in the q-triplet along the Hellenic subduction zone. The models used, fit rather well to the observed

  1. Seismic evidence for flow in the hydrated mantle wedge of the Ryukyu subduction zone

    NASA Astrophysics Data System (ADS)

    Nagaya, Takayoshi; Walker, Andrew M.; Wookey, James; Wallis, Simon R.; Ishii, Kazuhiko; Kendall, J.-Michael

    2016-07-01

    It is widely accepted that water-rich serpentinite domains are commonly present in the mantle above shallow subducting slabs and play key roles in controlling the geochemical cycling and physical properties of subduction zones. Thermal and petrological models show the dominant serpentine mineral is antigorite. However, there is no good consensus on the amount, distribution and alignment of this mineral. Seismic velocities are commonly used to identify antigorite-rich domains, but antigorite is highly-anisotropic and depending on the seismic ray path, its properties can be very difficult to distinguish from non-hydrated olivine-rich mantle. Here, we utilize this anisotropy and show how an analysis of seismic anisotropy that incorporates measured ray path geometries in the Ryukyu arc can constrain the distribution, orientation and amount of antigorite. We find more than 54% of the wedge must consist of antigorite and the alignment must change from vertically aligned to parallel to the slab. This orientation change suggests convective flow in the hydrated forearc mantle. Shear wave splitting analysis in other subduction zones indicates large-scale serpentinization and forearc mantle convection are likely to be more widespread than generally recognized. The view that the forearc mantle of cold subduction zones is dry needs to be reassessed.

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

    NASA Astrophysics Data System (ADS)

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

    2014-06-01

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

  3. Seismic evidence for flow in the hydrated mantle wedge of the Ryukyu subduction zone.

    PubMed

    Nagaya, Takayoshi; Walker, Andrew M; Wookey, James; Wallis, Simon R; Ishii, Kazuhiko; Kendall, J-Michael

    2016-01-01

    It is widely accepted that water-rich serpentinite domains are commonly present in the mantle above shallow subducting slabs and play key roles in controlling the geochemical cycling and physical properties of subduction zones. Thermal and petrological models show the dominant serpentine mineral is antigorite. However, there is no good consensus on the amount, distribution and alignment of this mineral. Seismic velocities are commonly used to identify antigorite-rich domains, but antigorite is highly-anisotropic and depending on the seismic ray path, its properties can be very difficult to distinguish from non-hydrated olivine-rich mantle. Here, we utilize this anisotropy and show how an analysis of seismic anisotropy that incorporates measured ray path geometries in the Ryukyu arc can constrain the distribution, orientation and amount of antigorite. We find more than 54% of the wedge must consist of antigorite and the alignment must change from vertically aligned to parallel to the slab. This orientation change suggests convective flow in the hydrated forearc mantle. Shear wave splitting analysis in other subduction zones indicates large-scale serpentinization and forearc mantle convection are likely to be more widespread than generally recognized. The view that the forearc mantle of cold subduction zones is dry needs to be reassessed. PMID:27436676

  4. Scenarios of tsunamigenic earthquakes generated along the Hellenic subduction zone and impact along the French coastlines

    NASA Astrophysics Data System (ADS)

    Gailler, Audrey; Hébert, Hélène; Schindelé, François

    2016-04-01

    The Hellenic subduction is an active deformation zone characterized by a sustained day-to-day seismicity (magnitude < 4.5) among the strongest in Europe. The last significant earthquake along the Hellenic subduction zone detected and characterized by the French tsunami warning center (CENALT) occurred on 16th April 2015 (Mw = 6.0) along the southeastern coasts of Crete, without any tsunami risk for the French coastlines. Even if great subduction earthquakes (magnitude > 7.5) are less frequent than in Chile or Japan, the Hellenic area experienced several strong events by the past, the biggest being associated with major tsunamis (e.g., in 551, in 1303). The last known sequence dates the end of the 19th beginning of the 20th century with a seismic gap located along the South Peloponnese - West Crete segment. The legendary 365 AD great earthquake (magnitude 8 to 8.5) is assumed to have ruptured along a major inverse fault parallel to the trench in this area, generating a large tsunami observed up to the Adriatic. In this work we investigate the tsunami potential of earthquakes localized along the Hellenic subduction zone, especially the minimum magnitude required to generate a tsunami that would be able to cross from Eastern to Western Mediterranean. The impact along Corsica coastlines is discussed through the modeling of a set of tsunami scenarios (magnitude ranging from 8.0 to 8.5) established from historical events parameters.

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

    NASA Astrophysics Data System (ADS)

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

    2015-10-01

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

  6. Seismic evidence for flow in the hydrated mantle wedge of the Ryukyu subduction zone

    PubMed Central

    Nagaya, Takayoshi; Walker, Andrew M.; Wookey, James; Wallis, Simon R.; Ishii, Kazuhiko; Kendall, J. -Michael

    2016-01-01

    It is widely accepted that water-rich serpentinite domains are commonly present in the mantle above shallow subducting slabs and play key roles in controlling the geochemical cycling and physical properties of subduction zones. Thermal and petrological models show the dominant serpentine mineral is antigorite. However, there is no good consensus on the amount, distribution and alignment of this mineral. Seismic velocities are commonly used to identify antigorite-rich domains, but antigorite is highly-anisotropic and depending on the seismic ray path, its properties can be very difficult to distinguish from non-hydrated olivine-rich mantle. Here, we utilize this anisotropy and show how an analysis of seismic anisotropy that incorporates measured ray path geometries in the Ryukyu arc can constrain the distribution, orientation and amount of antigorite. We find more than 54% of the wedge must consist of antigorite and the alignment must change from vertically aligned to parallel to the slab. This orientation change suggests convective flow in the hydrated forearc mantle. Shear wave splitting analysis in other subduction zones indicates large-scale serpentinization and forearc mantle convection are likely to be more widespread than generally recognized. The view that the forearc mantle of cold subduction zones is dry needs to be reassessed. PMID:27436676

  7. Seismic evidence for flow in the hydrated mantle wedge of the Ryukyu subduction zone.

    PubMed

    Nagaya, Takayoshi; Walker, Andrew M; Wookey, James; Wallis, Simon R; Ishii, Kazuhiko; Kendall, J-Michael

    2016-07-20

    It is widely accepted that water-rich serpentinite domains are commonly present in the mantle above shallow subducting slabs and play key roles in controlling the geochemical cycling and physical properties of subduction zones. Thermal and petrological models show the dominant serpentine mineral is antigorite. However, there is no good consensus on the amount, distribution and alignment of this mineral. Seismic velocities are commonly used to identify antigorite-rich domains, but antigorite is highly-anisotropic and depending on the seismic ray path, its properties can be very difficult to distinguish from non-hydrated olivine-rich mantle. Here, we utilize this anisotropy and show how an analysis of seismic anisotropy that incorporates measured ray path geometries in the Ryukyu arc can constrain the distribution, orientation and amount of antigorite. We find more than 54% of the wedge must consist of antigorite and the alignment must change from vertically aligned to parallel to the slab. This orientation change suggests convective flow in the hydrated forearc mantle. Shear wave splitting analysis in other subduction zones indicates large-scale serpentinization and forearc mantle convection are likely to be more widespread than generally recognized. The view that the forearc mantle of cold subduction zones is dry needs to be reassessed.

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

    NASA Astrophysics Data System (ADS)

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

    2016-04-01

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

  9. Slab seismicity in the Western Hellenic Subduction Zone: Constraints from tomography and double-difference relocation

    NASA Astrophysics Data System (ADS)

    Halpaap, Felix; Rondenay, Stéphane; Ottemöller, Lars

    2016-04-01

    The Western Hellenic subduction zone is characterized by a transition from oceanic to continental subduction. In the southern oceanic portion of the system, abundant seismicity reaches intermediate depths of 100-120 km, while the northern continental portion rarely exhibits deep earthquakes. Our study aims to investigate how this oceanic-continental transition affects fluid release and related seismicity along strike, by focusing on the distribution of intermediate depth earthquakes. To obtain a detailed image of the seismicity, we carry out a tomographic inversion for P- and S-velocities and double-difference earthquake relocation using a dataset of unprecedented spatial coverage in this area. Here we present results of these analyses in conjunction with high-resolution profiles from migrated receiver function images obtained from the MEDUSA experiment. We generate tomographic models by inverting data from 237 manually picked, well locatable events recorded at up to 130 stations. Stations from the permanent Greek network and the EGELADOS experiment supplement the 3-D coverage of the modeled domain, which covers a large part of mainland Greece and surrounding offshore areas. Corrections for the sphericity of the Earth and our update to the SIMULR16 package, which now allows S-inversion, help improve our previous models. Flexible gridding focusses the inversion on the domains of highest gradient around the slab, and we evaluate the resolution with checker board tests. We use the resulting velocity model to relocate earthquakes via the Double-Difference method, using a large dataset of differential traveltimes obtained by crosscorrelation of seismograms. Tens of earthquakes align along two planes forming a double seismic zone in the southern, oceanic portion of the subduction zone. With increasing subduction depth, the earthquakes appear closer to the center of the slab, outlining probable deserpentinization of the slab and concomitant eclogitization of dry crustal

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

    NASA Astrophysics Data System (ADS)

    Dragert, H.; Wang, K.; Rogers, G.

    2004-12-01

    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 locked, seismogenic portion of the subduction zone, and, for the geographic region around Victoria, British Columbia, repeat at 13 to 16 month intervals. These episodes of slip are accompanied by distinct, low-frequency tremors, similar to those reported in the forearc region of southern Japan. Although the processes which generate this phenomenon of episodic tremor and slip (ETS) are not well understood, it is possible that the ETS zone may constrain the landward extent of megathrust rupture, and conceivable that an ETS event could precede the next great thrust earthquake.

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

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2004-04-01

    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 Fuca Plate (~N70°E). Delay times of 1.0 to 1.5 s indicate a mantle source for the anisotropy, most likely strain-induced lattice-preferred orientation of anisotropic mantle minerals. The anisotropy may be related to present-day subduction-induced deformation of the mantle beneath the subducting plate. The delay times show an increase with distance from the deformation front (trench), which may be indicative of 3-5 per cent anisotropy within the forearc mantle wedge, with a fast direction parallel to the subduction direction. Above the Explorer Plate at the northern end of the CSZ, the fast directions are N25°E. This may reflect either the more northerly subduction direction of that plate, or a transition from subduction-related deformation to along-margin flow parallel to the transcurrent Queen Charlotte Fault to the north. At four stations in the central backarc of the CSZ, fast directions are parallel to the Juan de Fuca-North America convergence direction, consistent with mantle deformation due to subduction-induced mantle wedge flow, as well as deformation of the uppermost backarc mantle associated with motion of the overriding plate. No clear splitting was observed at the two most northern backarc stations, indicating either little horizontal anisotropy or highly complex anisotropy beneath these stations, possibly associated with complex mantle flow around the northern edge of the subducted plate. The hot, thin backarc lithosphere of the Cascadia subduction zone extends to the Rocky Mountain Trench, the western boundary of the cold, stable North America craton. At two stations on the North

  14. Modeling the role of back-arc spreading in controlling 3-D circulation and temperature patterns in subduction zones

    NASA Astrophysics Data System (ADS)

    Kincaid, C.

    2005-12-01

    are nearly uniform across the plate. Results have implications for geochemical and seismic models of 3-D flow in subduction zones influenced by back-arc spreading, such as the Marianas.

  15. An Examination of Seismicity Linking the Solomon Islands and Vanuatu Subduction Zones

    NASA Astrophysics Data System (ADS)

    Neely, J. S.; Furlong, K. P.

    2015-12-01

    The Solomon Islands-Vanuatu composite subduction zone represents a tectonically complex region along the Pacific-Australia plate boundary in the southwest Pacific Ocean. Here the Australia plate subducts under the Pacific plate in two segments: the South Solomon Trench and the Vanuatu Trench. The two subducting sections are offset by a 200 km long, transform fault - the San Cristobal Trough (SCT) - which acts as a Subduction-Transform Edge Propagator (STEP) fault. The subducting segments have experienced much more frequent and larger seismic events than the STEP fault. The northern Vanuatu trench hosted a M8.0 earthquake in 2013. In 2014, at the juncture of the western terminus of the SCT and the southern South Solomon Trench, two earthquakes (M7.4 and M7.6) occurred with disparate mechanisms (dominantly thrust and strike-slip respectively), which we interpret to indicate the tearing of the Australia plate as its northern section subducts and southern section translates along the SCT. During the 2013-2014 timeframe, little seismic activity occurred along the STEP fault. However, in May 2015, three M6.8-6.9 strike-slip events occurred in rapid succession as the STEP fault ruptured east to west. These recent events share similarities with a 1993 strike-slip STEP sequence on the SCT. Analysis of the 1993 and 2015 STEP earthquake sequences provides constraints on the plate boundary geometry of this major transform fault. Preliminary research suggests that plate motion along the STEP fault is partitioned between larger east-west oriented strike-slip events and smaller north-south thrust earthquakes. Additionally, the differences in seismic activity between the subducting slabs and the STEP fault can provide insights into how stress is transferred along the plate boundary and the mechanisms by which that stress is released.

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

    NASA Astrophysics Data System (ADS)

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

    2016-02-01

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

  17. Lattice-Preferred orientations of olivine in subducting oceanic lithosphere derived from the observed seismic anisotropies in double seismic zones

    NASA Astrophysics Data System (ADS)

    Han, Peng; Wei, Dongping; Zhang, Keliang; Sun, Zhentian; Zhou, Xiaoya

    2016-08-01

    Subduction zones can generally be classified into Mariana type and Chilean type depending on plate ages, plate thicknesses, subduction angles, back-arc deformation patterns, etc. The double seismic zones (DSZs) in subduction zones are mainly divided into type I and type II which, respectively, correspond to the Mariana type and Chilean type in most cases. Seismic anisotropy is an important parameter characterizing the geophysical features of the lithosphere, including the subduction zones, and can be described by the two parameters of delay time δt and fast wave polarization direction ϕ. We totally collected 524 seismic anisotropy data records from 24 DSZs and analyzed the statistical correlations between seismic anisotropy and the related physical parameters of DSZs. Our statistical analysis demonstrated that the fast wave polarization directions are parallel to the trench strike with no more than 30° for most type I DSZs, while being nearly perpendicular to the trench strike for type II DSZs. We also calculated roughly linear correlations that the delay time δt increases with dip angles but decreases with subduction rates. A linear equation was summarized to describe the strong correlation between DSZ's subduction angle α DSZ and seismic anisotropy in subduction zones. These results suggest that the anisotropic structure of the subducting lithosphere can be described as a possible equivalent crystal similar to the olivine crystal with three mutually orthogonal polarization axes, of which the longest and the second axes are nearly along the trench-perpendicular and trench-parallel directions, respectively.

  18. Geometry of the Philippine Sea plate subducting beneath the southwestern Nankai seismogenic zone

    NASA Astrophysics Data System (ADS)

    Nakanishi, A.; Kodaira, S.; Fujie, G.; Obana, K.; Takahashi, T.; Yamamoto, Y.; Sato, T.; Kashiwase, K.; Fujimori, H.; Kaneda, Y.

    2010-12-01

    In the Nankai Trough subduction seismogenic zone, the Nankai and Tonankai earthquakes had often occurred simultaneously, and caused a great event. Possibility of a megathrust earthquake along the Nankai Trough from Tokai to the Hyuga-nada, east off the Kyushu Island, Japan, is recently pointed out. To understand rupture synchronization and segmentation of the Nankai megathrust earthquake, it is important to know the geometry of the subducting Philippine Sea plate along the Nankai Trough. To obtain the deep subduction structure from the Hyuga-nada (off the Kyushu) to off the Shikoku area, the large-scale high-resolution wide-angle seismic study was conducted in 2008 and 2009. In this study, 160 and 200 ocean bottom seismographs were deployed, and a tuned airgun system (7800 cu. in.) shot every 200m along 11 profiles. Long-term observation was conducted for ~9 months by 21 OBSs off the Shikoku area. This research is part of ‘Research concerning Interaction Between the Tokai, Tonankai and Nankai Earthquakes’ funded by Ministry of Education, Culture, Sports, Science and Technology, Japan. Structural images beneath the Hyuga-nada clearly indicate the structural variation of the subducting Philippine Sea Plate from the subducting oceanic crust of Nankai Trough to the thick crust of the Kyushu Palau Ridge. The structural boundary between the oceanic crust and the Kyushu Palau Ridge is considered to lead to the southwestern rim of the coseismic slip zone of the 1968 Hyuga-nada earthquake (Mw7.5) (Yagi et al., 1999). This structural boundary may control the southwestern end of the megathrust earthquake of the Nankai Trough from the Tokai to Hyuga-nada. Moreover, geometry of the Philippine Sea plate estimated based on the structural images is shown in this presentation. Previously obtained seismic data is also used to make precise and detailed geometry of the subducting plate.

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

    USGS Publications Warehouse

    Sherrod, Brian; Gomberg, Joan

    2014-01-01

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

  20. Using the Vertical Component of the Surface Velocity Field to Map the Locked Zone at Cascadia Subduction Zone

    NASA Astrophysics Data System (ADS)

    Moulas, E.; Brandon, M. T.; Podladchikov, Y.; Bennett, R. A.

    2014-12-01

    At present, our understanding of the locked zone at Cascadia subduction zone is based on thermal modeling and elastic modeling of horizontal GPS velocities. The thermal model by Hyndman and Wang (1995) provided a first-order assessment of where the subduction thrust might be cold enough for stick-slip behavior. The alternative approach by McCaffrey et al. (2007) is to use a Green's function that relates horizontal surface velocities, as recorded by GPS, to interseismic elastic deformation. The thermal modeling approach is limited by a lack of information about the amount of frictional heating occurring on the thrust (Molnar and England, 1990). The GPS approach is limited in that the horizontal velocity component is fairly insensitive to the structure of the locked zone. The vertical velocity component is much more useful for this purpose. We are fortunate in that vertical velocities can now be measured by GPS to a precision of about 0.2 mm/a. The dislocation model predicts that vertical velocities should range up to about 20 percent of the subduction velocity, which means maximum values of ~7 mm/a. The locked zone is generally entirely offshore at Cascadia, except for the Olympic Peninsula region, where the underlying Juan De Fuca plate has an anomalously low dip. Previous thermal and GPS modeling, as well as tide gauge data and episodic tremors indicate the locked zone there extends about 50 to 75 km onland. This situation provides an opportunity to directly study the locked zone. With that objective in mind, we have constructed a full 3D geodynamic model of the Cascadia subduction zone. At present, the model provides a full representation of the interseismic elastic deformation due to variations of slip on the subduction thrust. The model has been benchmarked against the Savage (2D) and Okada (3D) analytical solutions. This model has an important advantage over traditional dislocation modeling in that we include temperature-sensitive viscosity for the upper and

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

    NASA Astrophysics Data System (ADS)

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

    2016-10-01

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

  2. Tectonic tremor and slow slip along the northwestern section of the Mexico subduction zone

    NASA Astrophysics Data System (ADS)

    Brudzinski, Michael R.; Schlanser, Kristen M.; Kelly, Nicholas J.; DeMets, Charles; Grand, Stephen P.; Márquez-Azúa, Bertha; Cabral-Cano, Enrique

    2016-11-01

    The southwestern coast of Mexico is marked by active subduction of the Rivera and Cocos plates, producing megathrust earthquakes that tend to recur every 50-100 yr. Herein, we use seismic and GPS data from this region to investigate the potential relationship between earthquakes, tectonic (non-volcanic) tremor, and transient slip along the westernmost 200 km of the Mexico subduction zone. Visual examination of seismograms and spectrograms throughout the 18-month-long MARS seismic experiment reveals clear evidence for frequent small episodes of tremor along the Rivera and Cocos subduction zones beneath the states of Jalisco, Colima, and Michoacán. Using a semi-automated process that identifies prominent energy bursts in envelope waveforms of this new data, analyst-refined relative arrival times are inverted for source locations using a 1-D velocity model. The resulting northwest-southeast trending linear band of tremor is located downdip from the rupture zones of the 1995 Mw 8.0 Colima-Jalisco and 2003 Mw 7.2 Tecoman subduction-thrust earthquakes and just below the regions of afterslip triggered by these earthquakes. Despite the close proximity between tremor and megathrust events, there is no evidence that the time since the last great earthquake influences the spatial or temporal pattern of tremor. A well-defined gap in the tremor beneath the western Colima Graben appears to mark a separation along the subducted Rivera-Cocos plate boundary. From the position time series of 19 continuous GPS sites in western Mexico, we present the first evidence that slow slip events occur on the Rivera plate subduction interface. Unlike the widely-recorded, large-amplitude, slow slip events on the nearly horizontal Cocos plate subduction interface below southern Mexico, slow slip events below western Mexico have small amplitudes and are recorded at relatively few, mostly coastal stations. The smaller slow slip beneath western Mexico may be due to the steeper dip, causing a

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

    NASA Astrophysics Data System (ADS)

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

    2009-12-01

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

  4. The effects of plate-bending-related aquifer thickening on temperatures in the Japan Trench subduction zone

    NASA Astrophysics Data System (ADS)

    Spinelli, G. A.

    2015-12-01

    Accurate subduction zone thermal models are necessary to understand a wide range of geophysical and geochemical processes, including: metamorphic reaction progress, mantle wedge hydration, and melt generation. For decades, plate convergence rate and subducting plate age and dip have been recognized as basic factors affecting subduction zone temperatures. Recent discoveries highlight the important effects of fluid circulation in oceanic lithosphere on subduction zone temperatures. However, there are contrasting hypotheses for the distribution of the regions within the oceanic lithosphere that host vigorous fluid circulation: one with a constant thickness aquifer extending both seaward and landward of the trench, one with an aquifer that thickens as it approaches the trench (due to bend-related faulting) but assumes vigorous fluid circulation only occurs prior to subduction, and a hybrid that considers both aquifer thickening seaward of the trench and continued circulation in the subducting plate. I examine the effects of bend-related aquifer thickening on temperatures within the Japan Trench subduction zone with a suite of thermal models. Kawada et al. [2014] hypothesize that plate-bending faults offshore northern Japan increase the thickness of the oceanic crustal aquifer, and therefore influence heat redistribution in the system. Existing models explore the effects of aquifer thickening on heat flux seaward of the trench, but they do not examine the effects of this process on temperatures within the subduction zone; additionally, they treat the aquifer seaward of the trench as completely isolated from the aquifer in the subducted plate. Here, I exploit the fact that aquifer thickening from the outer rise to the trench and continued fluid circulation in subducting crust are expected to produce distinct surface heat flux anomalies, in order to constrain the thermal effects of each process. I find the combinations of the amount of aquifer thickening and the degree of

  5. The impact of thick subduction zone sediment input sections on earthquake and tsunami potential

    NASA Astrophysics Data System (ADS)

    McNeill, L. C.; Smith, G. L.; Henstock, T.

    2012-12-01

    the trench, generating a wide, partly offshore seismogenic zone beneath the accretionary prism. Integrating an estimated rupture width from thermal structure of the plate boundary with likely rupture length and slip scenarios, generates potential M8.5-9 earthquakes on this margin. Similar thermal modeling of the North Sumatra margin places the updip limit ~30-40 km landward of the trench, broadly in agreement with slip models for the 2004 earthquake and placing a significant part of the seismogenic zone beneath the accretionary prism. This position coincides with structural and morphological changes thought to relate to changes in internal prism and basal fault properties. For subduction plate boundary rupture, the seaward extent of the seismogenic zone has implications for tsunami generation with potential for an increased submarine rupture area and rupture in deeper water. These examples support the idea that the morphology and structure of the forearc can be used to assess the position of the updip seismogenic limit and suggest that other margins with very thick sediment input should be carefully examined for their seismic potential.

  6. Altimetry data and the elastic stress tensor of subduction zones

    NASA Technical Reports Server (NTRS)

    Caputo, Michele

    1987-01-01

    The maximum shear stress (mss) field due to mass anomalies is estimated in the Apennines, the Kermadec-Tonga Trench, and the Rio Grande Rift areas and the results for each area are compared to observed seismicity. A maximum mss of 420 bar was calculated in the Kermadec-Tonga Trench region at a depth of 28 km. Two additional zones with more than 300 bar mss were also observed in the Kermadec-Tonga Trench study. Comparison of the calculated mss field with the observed seismicity in the Kermadec-Tonga showed two zones of well correlated activity. The Rio Grande Rift results showed a maximum mss of 700 bar occurring east of the rift and at a depth of 6 km. Recorded seismicity in the region was primarily constrained to a depth of approximately 5 km, correlating well to the results of the stress calculations. Two areas of high mss are found in the Apennine region: 120 bar at a depth of 55 km, and 149 bar at the surface. Seismic events observed in the Apennine area compare favorably with the mss field calculated, exhibiting two zones of activity. The case of loading by seamounts and icecaps are also simulated. Results for this study show that the mss reaches a maximum of about 1/3 that of the applied surface stress for both cases, and is located at a depth related to the diameter of the surface mass anomaly.

  7. Multiscale Finite-frequency Seismic Imaging of the Southern Alaska Subduction Zone

    NASA Astrophysics Data System (ADS)

    Song, X.; Hung, S. H.; Tong, P.; Liu, Q.

    2015-12-01

    Southern Alaska is one of the most seismically active regions in north America as the Pacific plate subducts northward beneath North America plate along the Aleutian trench. In this study, we determine 3-D variations of P- and S-wave speed and Possion's ratio (Vp/Vs) perturbations of the southern Alaska subduction zone based on broadband tele-seismic data recorded by 198 seismic stations for about 2000 events with magnitudes greater than 5.5 during the period from June 2000 to December 2014. Relative arrival times of P and S phases bwtween stations are accurately measured by adapting the efficient multi-channel cross-correlation (MCCC) technique. The obtained arrival-time data are then used to tomographically image the Vp and Vs structures beneath the stations based on 3-D finite-frequency sensitivity kernels and a wavelet-based multi-scale model parameterization. Our results show strong positive velocity anomalies in the crust and upper mantle starting at a depth of about 50km and extending to northwestward down to a depth of 200 km and covering about 350 km in horizontal distance. The high velocity feature interpreted as a cold slab has a thickness of about 50km and a subducting angle of about 45o, consistent with some previous studies of southern Alaska. We also plan to further obtain high-resolution seismic imaging of southern Alaska subduction zone by utilizing the converted and coda waves of tele-seismic main phases (e.g., P and S) based on a hybrid tomographic technique combining spectral-element method (SEM) and frequency-wavenumber (FK) method. The 3D Vp and Vs models obtained from finite-frequency traveltime tomography thus can serve as a proper starting velocity model for the hybrid SEM-FK imaging to further reveal high-resolution details of the subduction zone.

  8. Forearc deformation and megasplay fault system of the Ryukyu subduction zone

    NASA Astrophysics Data System (ADS)

    Hsu, S.; Yeh, Y.; Sibuet, J.; Tsai, C.; Doo, W.

    2011-12-01

    A great tsunami caused by a subduction earthquake had struck south Ryukyu islands and killed ~12000 people in 1771. Here we report the existence of a megasplay fault system along the south Ryukyu forearc. Analyses of deep multi-channel seismic reflection profiles indicate that the megasplay fault system is rising from the summit of a ~1 km high mount sitting on a ~5° landward dipping subducted plate interface. The fault system has accumulated large strain as evidenced by the active and widespread normal faults in the inner wedge. The along-trench length of the megasplay fault system is estimated to be ~450 km. The origin of this south Ryukyu megasplay fault system is linked to the subduction of elevated ridges parallel to the fracture zones. In contrast, no similar splay fault system is found in the west of 125. 5°E where the oblique subduction has produced shear zones along the south Ryukyu forearc. We infer that the megasplay fault system is responsible for the 1771 south Ryukyu tsunami. Likewise, after a quiescence of ~240 years, a near-future great earthquake and tsunami is anticipated as the extensional feature is strongly widespread over the south Ryukyu forearc.

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

    NASA Astrophysics Data System (ADS)

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

    2014-05-01

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

  10. Zircon solubility and of Zr species in subduction zone fluids

    NASA Astrophysics Data System (ADS)

    Wilke, M.; Schmidt, C.; Rickers, K.; Pascarelli, S.; Manning, C. E.; Stechern, A.

    2009-12-01

    The geochemical signature of igneous rocks at convergent plate margins is thought to result from complex melt formation processes involving aqueous solutions derived from dehydration of the subducted slab. In these processes, the depletion of high-field-strength elements (HFSE) may be controlled by the presence of accessory phases such as zircon and rutile, which can strongly fractionate these elements; however, the stability and solubility of these phases depends strongly on the fluid composition, including concentration and stoichiometry of Na-Al silicate components. Here we present new data on the influence of the fluid composition on zircon solubility as well as data on the Zr complexation in these fluids at P&T. Experiments were conducted using a modified hydrothermal diamond-anvil cell (HDAC). Zr contents at P&T were determined using SR-µXRF spectra. Zr K-edge X-ray Absorption Fine Structure (XAFS) spectra were acquired to investigate the Zr complexation in-situ at P&T. A grain of synthetic crystalline zircon was equilibrated with an aqueous fluid containing Na2Si2O5 or Na2Si2O5 + Al2O3 components. XAFS and SR-µXRF spectra were taken at the dispersive beamline ID24 of the ESRF, Grenoble, France. Some additional SR-µXRF spectra were taken at HASYLAB, Hamburg, beamline L. The observed Zr concentrations in fluids containing 7-33 wt% Na2Si2O5 and variable Al contents were between 75 and 720 ppm at 500 to 750°C and ~300 MPa to ~700 MPa. These values match expected solubilities calculated from linear interpolation of the maximum solubility in pure H2O (from the detection limit) and the solubility in the most alkaline high-silica melts reported by Ellison and Hess (1986, CMP, 94, 343). The high Zr solubility in sodium silicate-bearing solutions signifies that aqueous fluids with alkali silicates offer an efficient mechanism for HFSE transport. This can be explained by complexation of HFSE with Si, Na, and perhaps also Al, via formation of polymerized solutes

  11. Smectite reactions and slip instabilities in subduction zones

    NASA Astrophysics Data System (ADS)

    Gadenne, Leslie; Raimbourg, Hugues; Champallier, Remi; Yamamoto, Yuzuru

    2015-04-01

    -to-illite reaction may be responsible for triggering the instabilities. This catastrophic dehydration is potentially a major control on the genesis of instabilities in natural conditions, as (1) smectite is a major component of subducted sediments and (2) its transformation into illite may occur over a large depth range.

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

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

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

  13. Building a Framework Earthquake Cycle Deformational Model for Subduction Megathrust Zones: Integrating Observations with Numerical Models

    NASA Astrophysics Data System (ADS)

    Furlong, Kevin P.; Govers, Rob; Herman, Matthew

    2016-04-01

    Subduction zone megathrusts host the largest and deadliest earthquakes on the planet. Over the past decades (primarily since the 2004 Sumatra event) our abilities to observe the build-up in slip deficit along these plate boundary zones has improved substantially with the development of relatively dense observing systems along major subduction zones. One, perhaps unexpected, result from these observations is a range of present-day behavior along the boundaries. Some regions show displacements (almost always observed on the upper plate along the boundary) that are consistent with elastic deformation driven by a fully locked plate interface, while other plate boundary segments (oftentimes along the same plate boundary system) show little or no plate motion directed displacements. This latter case is often interpreted as reflecting little to no coupling along the plate boundary interface. What is unclear is whether this spatial variation in apparent plate boundary interface behavior reflects true spatial differences in plate interface properties and mechanics, or may rather reflect temporal behavior of the plate boundary during the earthquake cycle. In our integrated observational and modeling analyses, we have come to the conclusion that much of what is seen as diverse behavior along subduction margins represents different time in the earthquake cycle (relative to recurrence rate and material properties) rather than fundamental differences between subduction zone mechanics. Our model-constrained conceptual model accounts for the following generalized observations: 1. Coseismic displacements are enhanced in "near-trench" region 2. Post-seismic relaxation varies with time and position landward - i.e. there is a propagation of the transition point from "post" (i.e. trenchward) to "inter" (i.e. landward) seismic displacement behavior. 3. Displacements immediately post-EQ (interpreted to be associated with "after slip" on megathrust?). 4. The post-EQ transient response can

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

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

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

  15. Water, oceanic fracture zones and the lubrication of subducting plate boundaries—insights from seismicity

    NASA Astrophysics Data System (ADS)

    Schlaphorst, David; Kendall, J.-Michael; Collier, Jenny S.; Verdon, James P.; Blundy, Jon; Baptie, Brian; Latchman, Joan L.; Massin, Frederic; Bouin, Marie-Paule

    2016-03-01

    We investigate the relationship between subduction processes and related seismicity for the Lesser Antilles Arc using the Gutenberg-Richter law. This power law describes the earthquake-magnitude distribution, with the gradient of the cumulative magnitude distribution being commonly known as the b-value. The Lesser Antilles Arc was chosen because of its along-strike variability in sediment subduction and the transition from subduction to strike-slip movement towards its northern and southern ends. The data are derived from the seismicity catalogues from the Seismic Research Centre of The University of the West Indies and the Observatoires Volcanologiques et Sismologiques of the Institut de Physique du Globe de Paris and consist of subcrustal events primarily from the slab interface. The b-value is found using a Kolmogorov-Smirnov test for a maximum-likelihood straight line-fitting routine. We investigate spatial variations in b-values using a grid-search with circular cells as well as an along-arc projection. Tests with different algorithms and the two independent earthquake cataloges provide confidence in the robustness of our results. We observe a strong spatial variability of the b-value that cannot be explained by the uncertainties. Rather than obtaining a simple north-south b-value distribution suggestive of the dominant control on earthquake triggering being water released from the sedimentary cover on the incoming American Plates, or a b-value distribution that correlates with on the obliquity of subduction, we obtain a series of discrete, high b-value `bull's-eyes' along strike. These bull's-eyes, which indicate stress release through a higher fraction of small earthquakes, coincide with the locations of known incoming oceanic fracture zones on the American Plates. We interpret the results in terms of water being delivered to the Lesser Antilles subduction zone in the vicinity of fracture zones providing lubrication and thus changing the character of the

  16. A permanent record of subduction zone earthquake cycle deformation in the northern Chilean forearc

    NASA Astrophysics Data System (ADS)

    Loveless, J. P.; Allmendinger, R. W.; Pritchard, M. E.; González, G.

    2006-12-01

    Patterns of faulting in the northern Chilean forearc are consistent with modeled stress fields resulting from the subduction zone earthquake cycle. We define positive Coulomb stress change as encouraging normal faulting motion on steeply-dipping planes striking approximately parallel to the plate boundary, as shown by fault kinematic data collected in the field. Simulations show that coastal regions experience positive Coulomb stress changes due to interseismic strain accumulation on the subduction interface. This is compatible with the structural character of the forearc, typified by 100 m-scale scarps constructed by normal faulting. Conversely, the best-constrained models of interplate slip associated with the 1995 Mw 8.0 Antofagasta earthquake indicate that near-surface coastal areas experienced either zero or negative coseismic stress change, implying that subduction zone earthquakes may be capable of driving reverse motion on these structures if the absolute stress level is sufficiently low. Field exposures show minor amounts of reverse reactivation of some normal faults, expressed both through bedrock exposure and scarp morphology. The consistency between deformation fields related to the seismic cycle and permanent strain demonstrated by observable structures argues for the long-term influence of the earthquake cycle on the structural evolution of the forearc. The distribution of normal and reverse faulting as well as open cracks can thus be used to gain insight into the plate boundary processes that drive the evolution of structures. The change in strike and eastward step of the Atacama Fault System around the latitude of the Mejillones Peninsula (23°S) coincides with a change in subduction zone locking depth from ~35 km south of the peninsula to ~50 km to the north as determined through analyses of teleseismic, local seismic, and GPS data. Dense arrays of open cracks in several forearc localities show mean strikes consistent with static extension axes

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

    SciTech Connect

    Ben-Avraham, Z.; Nur, A.

    1980-08-10

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

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

    NASA Astrophysics Data System (ADS)

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

    2016-06-01

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

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

    NASA Astrophysics Data System (ADS)

    von Huene, Roland

    2010-05-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2016-04-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2011-12-01

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

  2. Geodetic observations of interseismic strain segmentation at the Sumatra Subduction Zone

    NASA Astrophysics Data System (ADS)

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

    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 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 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.

  3. Cycling of Li, K, Rb, and Cs at Subduction Zones and Ridge Crests With Implications for Ocean Chemistry: Hydrothermal Experiments at 35-350° C and 600 bars

    NASA Astrophysics Data System (ADS)

    Wei, W.; Kastner, M.; Rosenbauer, R.; Weinstein, Y.; Chan, L.

    2004-12-01

    clearly identified. Rb/K, in contrast to Li/K and Cs/K, do not have a maximum, but steadily increase from 35-350° C in the MORB experiments. In the smectite experiments, the fluid Rb/K ratios are considerably higher, hence, are as well indicators of sediment contribution. At 350° C the enrichment factors (ER) of Li/Cl and Cs/Cl are higher in the smectite than in the MORB experiments ( ˜70 & 150 vs. ˜20 & 30, respectively). The ER of Rb/Cl is ˜10 in both experiments, but that of K/Cl is higher in the MORB vs. smectite experiments (3.0 & 1.7, respectively). Like Sr, the fluid Li totally exchanges its isotopes with MORB or with the smectite. In the MORB experiments, the fluid δ 7Li fall on a SW-MORB mixing line, shifting from 30.95‰ , the SW value, to17.4‰ at 150° C, and 7.33‰ at 350° C. Hence, δ 7Li values together with Li/K, Li/Rb and Li/Cs ratios constrain the reaction temperature (±20-30° C) and the nature of the rock involved. Applications of the experimental data (1) to pore fluids from two subduction zones: the Middle America Trench offshore Costa Rica, and the Nankai Trough, and to an 'average' ridge crest hydrothermal fluid composition, and (2) to revised Rb and Cs ocean budgets, will be presented.

  4. Overview of Recent Coastal Tectonic Deformation in the Mexican Subduction Zone

    NASA Astrophysics Data System (ADS)

    Ramírez-Herrera, M. Teresa; Kostoglodov, Vladimir; Urrutia-Fucugauchi, Jaime

    2011-08-01

    Holocene and Pleistocene tectonic deformation of the coast in the Mexico subudction margin is recorded by geomorphic and stratigraphic markers. We document the spatial and temporal variability of active deformation on the coastal Mexican subduction margin. Pleistocene uplift rates are estimated using wave-cut platforms at ca. 0.7-0.9 m/ka on the Jalisco block coast, Rivera-North America tectonic plate boundary. We examine reported measurements from marine notches and shoreline angle elevations in conjunction with their radiocarbon ages that indicate surface uplift rates increasing during the Holocene up to ca. 3 ± 0.5 m/ka. In contrast, steady rates of uplift (ca. 0.5-1.0 m/ka) in the Pleistocene and Holocene characterize the Michoacan coastal sector, south of El Gordo graben and north of the Orozco Fracture Zone (OFZ), incorporated within the Cocos-North America plate boundary. Significantly higher rates of surface uplift (ca. 7 m/ka) across the OFZ subduction may reflect the roughness of subducting plate. Absence of preserved marine terraces on the coastal sector across El Gordo graben likely reflects slow uplift or coastal subsidence. Stratigraphic markers and their radiocarbon ages show late Holocene (ca. last 6 ka bp) coastal subsidence on the Guerrero gap sector in agreement with a landscape barren of marine terraces and with archeological evidence of coastal subsidence. Temporal and spatial variability in recent deformation rates on the Mexican Pacific coast may be due to differences in tectonic regimes and to localized processes related to subduction, such as crustal faults, subduction erosion and underplating of subducted materials under the southern Mexico continental margin.

  5. 15 Years Of Ecuadorian-French Research Along The Ecuadorian Subduction Zone

    NASA Astrophysics Data System (ADS)

    Charvis, P.

    2015-12-01

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

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

    NASA Astrophysics Data System (ADS)

    McGuire, J. J.; Llenos, A.

    2007-05-01

    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 directivity and termination, we estimated the rupture areas of a set of Mw 7.5-8.7 earthquakes that occurred in circum-Pacific subduction zones. We compare synthetic and observed seismograms by measuring frequency- dependent amplitude and arrival time differences of the first orbit Rayleigh waves. At low frequencies, the amplitude anomalies primarily result from the spatial and temporal extent of the rupture. We then invert the amplitude and arrival time measurements to estimate the second moments of the slip distribution which describe the rupture length, width, duration and propagation velocity of each earthquake. Comparing the rupture areas to the trench-parallel gravity anomaly (TPGA, Song and Simons 2003) above each rupture, we find that in 12 of the 14 events considered in this study the TPGA increases between the centroid and the limits of the rupture. Thus, local increases in TPGA appear to be related to the physical conditions along the plate interface that favor rupture termination. Owing to the inherently long time scales required for forearc basin formation, the correlation between the TPGA field and rupture termination regions indicates that long-lived material heterogeneity rather than short time-scale stress heterogeneities are responsible for arresting most great subduction zone ruptures.

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

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

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

  8. Shallow seismicity patterns in the northwestern section of the Mexico Subduction Zone

    NASA Astrophysics Data System (ADS)

    Abbott, Elizabeth R.; Brudzinski, Michael R.

    2015-11-01

    This study characterizes subduction related seismicity with local deployments along the northwestern section of the Mexico Subduction Zone where 4 portions of the plate interface have ruptured in 1973, 1985, 1995, and 2003. It has been proposed that the subducted boundary between the Cocos and Rivera plates occurs beneath this region, as indicated by inland volcanic activity, a gap in tectonic tremor, and the Manzanillo Trough and Colima Graben, which are depressions thought to be associated with the splitting of the two plates after subduction. Data from 50 broadband stations that comprised the MARS seismic array, deployed from January 2006 to June 2007, were processed with the software program Antelope and its generalized source location algorithm, genloc, to detect and locate earthquakes within the network. Slab surface depth contours from the resulting catalog indicate a change in subduction trajectory between the Rivera and Cocos plates. The earthquake locations are spatially anti-correlated with tectonic tremor, supporting the idea that they represent different types of fault slip. Hypocentral patterns also reveal areas of more intense seismic activity (clusters) that appear to be associated with the 2003 and 1973 megathrust rupture regions. Seismicity concentrated inland of the 2003 rupture is consistent with slip on a shallowly dipping trajectory for the Rivera plate interface as opposed to crustal faulting in the overriding North American plate. A prominent cluster of seismicity within the suspected 1973 rupture zone appears to be a commonly active portion of the megathrust as it has been active during three previous deployments. We support these interpretations by determining focal mechanisms and detailed relocations of the largest events within the 1973 and inland 2003 clusters, which indicate primarily thrust mechanisms near the plate interface.

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

    NASA Astrophysics Data System (ADS)

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

    2002-12-01

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

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

    NASA Astrophysics Data System (ADS)

    Höchner, Andreas; Babeyko, Andrey; Zamora, Natalia

    2014-05-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2012-04-01

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

  12. Paleotsunamis from the central Kuril Islands segment of the Japan-Kuril-Kamchatka subduction zone

    NASA Astrophysics Data System (ADS)

    MacInnes, Breanyn; Kravchunovskaya, Ekaterina; Pinegina, Tatiana; Bourgeois, Joanne

    2016-07-01

    Paleotsunami records from the central Kuril Island segment of the Japan-Kuril-Kamchatka subduction zone indicate that the region has been frequently inundated by tsunamis. As many as 20-22 tsunami deposits are recognized on Matua Island for the past 3300 yr with an average tsunami recurrence interval of ∼150 yr, and 34-36 tsunami deposits are evident on Simushir Island for the past 2350 yr with an average recurrence of ∼65 yr. These intervals are short, but comparable to other segments of the Japan-Kuril-Kamchatka subduction zone. Results from all survey locations reveal shortening recurrence intervals toward the present, especially for the last 600 yr, indicating a possible preservation bias. On Simushir, tsunamis at least 11 m higher than the modern tsunamis in 2006 and 2007 occurred every ∼300 yr on average. On Matua, tsunamis with slightly farther inundation than the 2006 and 2007 tsunamis occurred every ∼215 yr while those with at least 100 m farther inland inundation occur every ∼750 yr. Our paleotsunami record almost certainly includes tsunamis that are not from great subduction zone earthquakes in the central Kuril segment: we expect the Matua record includes volcanic tsunamis and the Simushir record includes tsunamis from the southern Kuril segment.

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

    PubMed

    Foley, Stephen; Tiepolo, Massimo; Vannucci, Riccardo

    2002-06-20

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

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

    NASA Astrophysics Data System (ADS)

    Weston, J.; Shirzaei, M.

    2016-01-01

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

  15. Rupture process of large earthquakes in the northern Mexico subduction zone

    NASA Astrophysics Data System (ADS)

    Ruff, Larry J.; Miller, Angus D.

    1994-03-01

    The Cocos plate subducts beneath North America at the Mexico trench. The northernmost segment of this trench, between the Orozco and Rivera fracture zones, has ruptured in a sequence of five large earthquakes from 1973 to 1985; the Jan. 30, 1973 Colima event ( M s 7.5) at the northern end of the segment near Rivera fracture zone; the Mar. 14, 1979 Petatlan event ( M s 7.6) at the southern end of the segment on the Orozco fracture zone; the Oct. 25, 1981 Playa Azul event ( M s 7.3) in the middle of the Michoacan “gap”; the Sept. 19, 1985 Michoacan mainshock ( M s 8.1); and the Sept. 21, 1985 Michoacan aftershock ( M s 7.6) that reruptured part of the Petatlan zone. Body wave inversion for the rupture process of these earthquakes finds the best: earthquake depth; focal mechanism; overall source time function; and seismic moment, for each earthquake. In addition, we have determined spatial concentrations of seismic moment release for the Colima earthquake, and the Michoacan mainshock and aftershock. These spatial concentrations of slip are interpreted as asperities; and the resultant asperity distribution for Mexico is compared to other subduction zones. The body wave inversion technique also determines the Moment Tensor Rate Functions; but there is no evidence for statistically significant changes in the moment tensor during rupture for any of the five earthquakes. An appendix describes the Moment Tensor Rate Functions methodology in detail. The systematic bias between global and regional determinations of epicentral locations in Mexico must be resolved to enable plotting of asperities with aftershocks and geographic features. We have spatially “shifted” all of our results to regional determinations of epicenters. The best point source depths for the five earthquakes are all above 30 km, consistent with the idea that the down-dip edge of the seismogenic plate interface in Mexico is shallow compared to other subduction zones. Consideration of uncertainties in

  16. Subduction zone and crustal dynamics of western Washington; a tectonic model for earthquake hazards evaluation

    USGS Publications Warehouse

    Stanley, Dal; Villaseñor, Antonio; Benz, Harley

    1999-01-01

    The Cascadia subduction zone is extremely complex in the western Washington region, involving local deformation of the subducting Juan de Fuca plate and complicated block structures in the crust. It has been postulated that the Cascadia subduction zone could be the source for a large thrust earthquake, possibly as large as M9.0. Large intraplate earthquakes from within the subducting Juan de Fuca plate beneath the Puget Sound region have accounted for most of the energy release in this century and future such large earthquakes are expected. Added to these possible hazards is clear evidence for strong crustal deformation events in the Puget Sound region near faults such as the Seattle fault, which passes through the southern Seattle metropolitan area. In order to understand the nature of these individual earthquake sources and their possible interrelationship, we have conducted an extensive seismotectonic study of the region. We have employed P-wave velocity models developed using local earthquake tomography as a key tool in this research. Other information utilized includes geological, paleoseismic, gravity, magnetic, magnetotelluric, deformation, seismicity, focal mechanism and geodetic data. Neotectonic concepts were tested and augmented through use of anelastic (creep) deformation models based on thin-plate, finite-element techniques developed by Peter Bird, UCLA. These programs model anelastic strain rate, stress, and velocity fields for given rheological parameters, variable crust and lithosphere thicknesses, heat flow, and elevation. Known faults in western Washington and the main Cascadia subduction thrust were incorporated in the modeling process. Significant results from the velocity models include delineation of a previously studied arch in the subducting Juan de Fuca plate. The axis of the arch is oriented in the direction of current subduction and asymmetrically deformed due to the effects of a northern buttress mapped in the velocity models. This

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

    NASA Astrophysics Data System (ADS)

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

    2012-12-01

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

  18. Separate zones of sulfate and sulfide release from subducted mafic oceanic crust

    NASA Astrophysics Data System (ADS)

    Tomkins, Andrew G.; Evans, Katy A.

    2015-10-01

    Liberation of fluids during subduction of oceanic crust is thought to transfer sulfur into the overlying sub-arc mantle. However, despite the importance of sulfur cycling through magmatic arcs to climate change, magma oxidation and ore formation, there has been little investigation of the metamorphic reactions responsible for sulfur release from subducting slabs. Here, we investigate the relative stability of anhydrite (CaSO4) and pyrite (FeS2) in subducted basaltic oceanic crust, the largest contributor to the subducted sulfur budget, to place constraints on the processes controlling sulfur release. Our analysis of anhydrite stability at high pressures suggests that this mineral should dominantly dissolve into metamorphic fluids released across the transition from blueschist to eclogite facies (∼450-650 °C), disappearing at lower temperatures on colder geothermal trajectories. In contrast, we suggest that sulfur release via conversion of pyrite to pyrrhotite occurs at temperatures above 750 °C. This higher temperature stability is indicated by the preservation of pyrite-bornite inclusions in coesite-bearing eclogites from the Sulu Belt in China, which reached temperatures of at least 750 °C. Thus, sulfur may be released from subducting slabs in two separate pulses; (1) varying proportions of SO2, HSO4- and H2S are released via anhydrite breakdown at the blueschist-eclogite transition, promoting oxidation of remaining silicates in some domains, and (2) H2S is released via pyrite breakdown well into the eclogite facies, which may in some circumstances coincide with slab melting or supercritical liquid generation driven by influx of serpentinite-derived fluids. These results imply that the metallogenic potential in the sub-arc mantle above the subducting slab varies as a function of subduction depth, having the greatest potential above the blueschist-eclogite transition given the association between oxidised magmas and porphyry Cu(-Au-Mo) deposits. We speculate

  19. Empirical relationships between instrumental ground motions and observed intensities for two great Chilean subduction zone earthquakes

    NASA Astrophysics Data System (ADS)

    Cilia, M. G.; Baker, L. M.

    2015-12-01

    We determine empirical relationships between instrumental peak ground motions and observed intensities for two great Chilean subduction earthquakes: the 2010 Mw8.8 Maule earthquake and the 2014 Mw8.2 Iquique earthquake. Both occurred immediately offshore on the primary plate boundary interface between the Nazca and South America plates. They are among the largest earthquakes to be instrumentally recorded; the 2010 Maule event is the second largest earthquake to produce strong motion recordings. Ground motion to intensity conversion equations (GMICEs) are used to reconstruct the distribution of shaking for historical earthquakes by using intensities estimated from contemporary accounts. Most great (M>8) earthquakes, like these, occur within subduction zones, yet few GMICEs exist for subduction earthquakes. It is unclear whether GMICEs developed for active crustal regions, such as California, can be scaled up to the large M of subduction zone events, or if new data sets must be analyzed to develop separate subduction GMICEs. To address this question, we pair instrumental peak ground motions, both acceleration (PGA) and velocity (PGV), with intensities derived from onsite surveys of earthquake damage made in the weeks after the events and internet-derived felt reports. We fit a linear predictive equation between the geometric mean of the maximum PGA or PGV of the two horizontal components and intensity, using linear least squares. We use a weighting scheme to express the uncertainty of the pairings based on a station's proximity to the nearest intensity observation. The intensity data derived from the onsite surveys is a complete, high-quality investigation of the earthquake damage. We perform the computations using both the survey data and community decimal intensities (CDI) calculated from felt reports volunteered by citizens (USGS "Did You Feel It", DYFI) and compare the results. We compare the GMICEs we developed to the most widely used GMICEs from California and

  20. Subduction of the Tehuantepec oceanic fracture zone and the relationship with a seismic gap in southern Mexico

    NASA Astrophysics Data System (ADS)

    Constantin Manea, Vlad; Manea, Marina; Taras, Gerya; Valenzuela, Raul W.

    2016-04-01

    It is accepted that key constraints on the size and recurrence time of large subduction earthquakes originate from the degree of locking between the subducting and overriding plates. Since the interseismic locking degree is influenced by the rheological properties of crustal and mantle rocks, any variations along strike will result in significant changes in seismic behavior due to a change in frictional stability. Additionally, recent seismic studies show that the subduction of hydrothermally altered oceanic fracture zones induces strong pore-fluid pressure variations that control the degree of interseismic locking. The Mexico Subduction Zone (MSZ) is characterized by major along-strike changes in subduction geometry, as well as important structural variations of the incoming oceanic plate. One of the main tectonic features of the Cocos plate is the Tehuantepec fracture zone (FZ) that is currently subducting beneath southern Mexico. The analysis of seismicity revealed that the area around where Tehuantepec fracture zone is currently subducting is conspicuously quiet and considered a seismic gap. Here, no significant quake (Ms ≥ 7.0) has occurred in more than 100 years, and the origin of Tehuantepec Seismic Gap (TSG) has not been elucidated yet. Based on the dimensions of the Tehuantepec gap (125 km length and 80 km width), an earthquake of Mw = 8.0 may be possible. This study aims to shed some light on the relationship between the TSG with the subduction of Tehuantepec oceanic fracture zone. Previous studies show that the uppermost oceanic lithosphere beneath the Tehuantepec FZ is partially serpentinized due to seawater infiltrations along faults. Using high-resolution three-dimensional coupled petrological-thermomechanical numerical simulations specifically tailored for the subduction of the Tehuantepec FZ at MSZ we show that the weakened serpentinized fracture zone is partially scraped out in the forearc region because of its low strength and positive buoyancy

  1. A consistent model for fluid distribution, viscosity distribution, and flow-thermal structure in subduction zone

    NASA Astrophysics Data System (ADS)

    Horiuchi, Shun-suke; Iwamori, Hikaru

    2016-05-01

    Water plays crucial roles in the subduction zone dynamics affecting the thermal-flow structure through the fluid processes. We aim to understand what controls the dynamics and construct a model to solve consistently fluid generation, fluid transport, its reaction with the solid and resultant viscosity, and thermal-flow structure. We highlight the effect of mechanical weakening of rocks associated with hydration. The viscosity of serpentinite (ηserp) in subduction zones critically controls the flow-thermal structure via extent of mechanical coupling between the subducting slab and overlying mantle wedge. When ηserp is greater than 1021 Pa s, the thermal-flow structure reaches a steady state beneath the volcanic zone, and the melting region expands until Cin (initial water content in the subducting oceanic crust) reaches 3 wt %, and it does not expand from 3 wt %. On the other hand, when ηserp is less than 1019 Pa s, the greater water dependence of viscosity (expressed by a larger fv) confines a hot material to a narrower channel intruding into the wedge corner from a deeper part of the back-arc region. Consequently, the overall heat flux becomes less for a larger fv. When ageba (age of back-arc basin as a rifted lithosphere) = 7.5 Ma, the increase in fv weakens but shifts the melting region toward the trench side because of the narrow channel flow intruding into the wedge corner, where as it shuts down melting when ageba=20 Ma. Several model cases (particularly those with ηserp=1020 to 1021 Pa s and a relatively large fv for Cin=2 to 3 wt %) broadly account for the observations in the Northeast Japan arc (i.e., location and width of volcanic chain, extent of serpentinite, surface heat flow, and seismic tomography), although the large variability of surface heat flow and seismic tomographic images does not allow us to constrain the parameter range tightly.

  2. Carbonate-Iron Interaction: Kinetic Approach for Carbonate Subduction Down to the Mantle Transition Zone

    NASA Astrophysics Data System (ADS)

    Martirosyan, N.; Yoshino, T.; Shatskiy, A.; Litasov, K. D.; Chanyshev, A.

    2015-12-01

    One of the principal redox exchange reactions at the slab-mantle interface during subduction is CaCO3-Fe0 interaction. Using uniaxial press multianvil apparatus, we investigate this reaction at temperatures from 650 to 1400 °C, and pressures from 4 to 16 GPa. These conditions are representative for the PT path of the slab from the surface to the mantle transition zone. Based on our results, redox reaction can be written as: 3 CaCO3 (aragonite) + 13 Fe0 (metal) = Fe7C3 (carbide) + 3 CaFe2O3 (Ca-wüstite). It is a diffusion-controlled process with the reaction rate constant (k) being log-linear in 1/T. The experimental results allow to calculate the length scale over which the reaction kinetics between aragonite and metallic iron is likely effective. Using relatively simple relationship between the characteristic distance of diffusion as a function of k and duration of the process, we estimate the length scale of the reaction for time scales 4-16 Myr, which corresponds to subduction rates of 2-8 cm/year from 250 km (metal-saturation boundary) to 470 km, for different slab PT-profiles. Assuming that carbonates are in a direct contact with iron, the maximum degree of carbonate reduction can be evaluated using the results of our study and the data on CO2 distribution in altered oceanic basalts. The estimates suggest that up to 0.5, 12, and 20 vol. % of carbonates can be reduced in such case during subduction down to the mantle transition zone at the conditions of cold, medium and hot geotherms, respectively. The sluggish kinetics of established CaCO3-Fe0 interaction suggests that even over the entire history of the Earth, carbonates could survive during subduction from metal saturation boundary near 250 km depth, down to the transition zone, and presumably to the lower mantle.

  3. An International Coastline Collaboratory to Broaden Scientific Impacts of a Subduction Zone Observatory

    NASA Astrophysics Data System (ADS)

    Bodin, P.

    2015-12-01

    A global Subduction Zone Observatory (SZO) presents an exciting opportunity to broaden involvement in scientific research and to ensure multidisciplinary impact. Most subduction zones feature dynamic interactions of the seafloor, the coastline, and the onshore environments also being perturbed by global climate change. Tectonic deformation, physical environment changes (temperature and chemistry), and resulting ecological shifts (intertidal population redistribution, etc.) are all basic observables for important scientific investigation. Yet even simple baseline studies like repeated transects of intertidal biological communities are rare. A coordinated program of such studies would document the local variability across time and spatial scales, permit comparisons with other subducting coastlines, and extend the reach and importance of other SZO studies. One goal is to document the patterns, and separate the component causes of, coastal uplift and subsidence and ecological response to a subduction zone earthquake using a database of pre-event biological and surveying observations. Observations would be directed by local scientists using students and trained volunteers as observers, under the auspices of local educational entities and using standardized sampling and reporting methods. The observations would be added to the global, Internet-accessible, database for use by the entire scientific community. Data acquisition and analysis supports the educational missions of local schools and universities, forming the basis for educational programs. All local programs would be coordinated by an international panel convened by the SZO. The facility would include a web-hosted lecture series and an annual web conference to aid organization and collaboration. Small grants could support more needy areas. This SZO collaboratory advances not only scientific literacy, but also multinational collaboration and scholarship, and (most importantly) produces important scientific results.

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

    USGS Publications Warehouse

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

    1989-01-01

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

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

    USGS Publications Warehouse

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

    2006-01-01

    This paper presents the first regional three-dimensional, P wave velocity model for the Northern Cascadia Subduction. Zone (SW British Columbia and NW Washington State) constructed through tomographic inversion of first-arrival traveltime data from active source experiments together with earthquake traveltime data recorded at permanent stations. The velocity model images the structure of the subducting Juan de Fuca plate, megathrust, and the fore-arc crust and upper mantle. Beneath southern Vancouver Island the megathrust above the Juan de Fuca plate is characterized by a broad zone (25-35 km depth) having relatively low velocities of 6.4-6.6 km/s. This relative low velocity zone coincides with the location of most of the episodic tremors recently mapped beneath Vancouver Island, and its low velocity may also partially reflect the presence of trapped fluids and sheared lower crustal rocks. The rocks of the Olympic Subduction Complex are inferred to deform aseismically as evidenced by the lack of earthquakes withi the low-velocity rocks. The fore-arc upper mantle beneath the Strait of Georgia and Puget Sound is characterized by velocities of 7.2-7.6 km/s. Such low velocities represent regional serpentinization of the upper fore-arc mantle and provide evidence for slab dewatering and densification. Tertiary sedimentary basins in the Strait of Georgia and Puget Lowland imaged by the velocity model lie above the inferred region of slab dewatering and densification and may therefore partly result from a higher rate of slab sinking. In contrast, sedimentary basins in the Strait of Juan de Fuca lie in a synclinal depression in the Crescent Terrane. The correlation of in-slab earthquake hypocenters M>4 with P wave velocities greater than 7.8 km/s at the hypocenters suggests that they originate near the oceanic Moho of the subducting Juan de Fuca plate. Copyright 2006 by the American Geophysical Union.

  6. Slab2 - Providing updated subduction zone geometries and modeling tools to the community

    NASA Astrophysics Data System (ADS)

    Hayes, G. P.; Hearne, M. G.; Portner, D. E.; Borjas, C.; Moore, G.; Flamme, H.

    2015-12-01

    The U.S. Geological Survey database of global subduction zone geometries (Slab1.0) combines a variety of geophysical data sets (earthquake hypocenters, moment tensors, active source seismic survey images of the shallow subduction zone, bathymetry, trench locations, and sediment thickness information) to image the shape of subducting slabs in three dimensions, at approximately 85% of the world's convergent margins. The database is used extensively for a variety of purposes, from earthquake source imaging, to magnetotelluric modeling. Gaps in Slab1.0 exist where input data are sparse and/or where slabs are geometrically complex (and difficult to image with an automated approach). Slab1.0 also does not include information on the uncertainty in the modeled geometrical parameters, or the input data used to image them, and provides no means to reproduce the models it described. Currently underway, Slab2 will update and replace Slab1.0 by: (1) extending modeled slab geometries to all global subduction zones; (2) incorporating regional data sets that may describe slab geometry in finer detail than do previously used teleseismic data; (3) providing information on the uncertainties in each modeled slab surface; (4) modifying our modeling approach to a fully-three dimensional data interpolation, rather than following the 2-D to 3-D steps of Slab1.0; (5) migrating the slab modeling code base to a more universally distributable language, Python; and (6) providing the code base and input data we use to create our models, such that the community can both reproduce the slab geometries, and add their own data sets to ours to further improve upon those models in the future. In this presentation we describe our vision for Slab2, and the first results of this modeling process.

  7. The 1977 Sumba earthquake series: Evidence for Slab pull force acting at a subduction zone

    NASA Astrophysics Data System (ADS)

    Spence, William

    1986-06-01

    The great 1977 Sumba earthquake occurred at the eastern Sunda trench, just west of the collision of Australian continental lithosphere with the island arc. The length of the aftershock zone of this normal-faulting earthquake is about 200 km. Aftershocks are concentrated 65-115 km east of the main shock epicenter, with very few aftershocks in a 50-km-long segment that spans the main shock epicenter. Relocated hypocenters and focal mechanism data are consistent with normal faulting throughout the upper 28 km of the oceanic lithosphere. There is no evidence for thrust faulting of the deeper aftershocks. These data imply that the neutral bending surface must be at least 35-40 km deep. A second aftershock zone, about 180 km northwest of the main shock, became active immediately following the main shock, but events were concentrated during days 50-52. This zone is a 70-km-long lineation that trends toward the main shock epicenter and reflects right-lateral, strike-slip faulting within the subducted oceanic plate. Seismicity exists to a depth of about 650 km in the very old plate beneath the Sunda-Banda arc, and that plate's negative buoyancy causes very large slab pull forces. Great interface thrust earthquakes are absent at the Sumba region, and slab pull forces are inferred to have partially decoupled the subducted plate from the overriding plate. This decoupling permits slab pull stresses to be guided updip to the region of the Sumba main shock. Such shallow-acting slab pull provides a bending moment at the trench and explains the deformation and timing observed for the entire Sumba earthquake series. In this model, slab pull forces stretch the subducted plate until the increasing stresses at the shallow subduction zone lead to a subduction zone earthquake. Postseismically, the released oceanic plate undergoes a pulse of downdip strain, returning the plate to a less extended state. The moment of this downdip plate motion could exceed the seismic moment of the main

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

    NASA Astrophysics Data System (ADS)

    Loveless, J. P.; Meade, B. J.

    2015-12-01

    Spatial patterns of coupling on global subduction zones can be used to guide seismic hazard assessment, but estimates of coupling are often constrained using a limited temporal range of geodetic data. Here we analyze 19 years of observations from the GEONET network to assess time-dependent variations in the spatial distribution of coupling on the subduction zones offshore Japan. We divide the position time series into five, 3.75-year epochs each decomposed into best-fit velocity, periodic signals, coseismic offsets, and postseismic effects following five major earthquakes. Nominally interseismic velocities are interpreted in terms of combined tectonic block motions and earthquake cycle activity. Our derived velocity fields reveal significant consistency since 1996 in the spatial distribution of coupling on the Nankai subduction zone, with variation limited primarily to Tokai, where long-term slow slip events have occurred, and persistently coupled regions coincident with areas that slipped in historic great earthquakes, including estimates of the 1707 Hoei earthquake based on tsunami deposits. On the Sagami subduction zone south of Tokyo, we estimate relatively stable coupling through time, though the area of coupling is reduced by the occurrence of a slow-slip event in 2002. On the Japan Trench, we image significant coupling variations owing to effects of the 1994 Sanriku-oki, 2003 Tokachi-oki, and 2011 Tohoku-oki earthquakes. In particular, strong coupling becomes more spatially extensive following the 1994 event until 2011, and a decrease in coupling and occurrence of coseismic-sense slip precede the Tohoku-oki event. Despite occurrence of the 2003 Tokachi-oki earthquake, coupling offshore Hokkaido suggests ongoing seismic hazard, possibly similar to past Mw 9-class earthquakes interpreted from coastal paleoseismic records. This time-dependent analysis of interseismic deformation illuminates rich diversity in the distribution of subduction zone processes

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

    NASA Astrophysics Data System (ADS)

    Loveless, John P.; Meade, Brendan J.

    2016-02-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2013-02-01

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

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

    NASA Astrophysics Data System (ADS)

    Dragert, H.

    2003-12-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2011-12-01

    Using a finite-fault rupture model, we ran a finite difference code to simulate a variety of Mw 8 and larger events on the Cascadia subduction zone using a 3D regional velocity model and two different 3D velocity models for the Seattle basin. Our results reveal the magnitude and duration of shaking that should be expected in the built environment for a megathrust event with a rupture length less than the entire length of the subduction zone. In the next step we will consider events that rupture the entire length of the subduction zone, similar in scope to the 1700 event, and compare our results to those considered for the national seismic hazard maps. In order to make predictions on the strength and duration of shaking in Cascadia due to a large megathrust event, we developed a kinematic fault rupture model based on a k-2 decay in final slip spectrum that has a scale-dependent rise time. This produces a ω-2 decay in the radiated displacement spectrum above the corner frequency, which is then modified by rupture directivity. In order to produce a k-2 decay in the final slip spectrum, we modeled the final slip as the sum of asperities with various wave numbers produced by calculating normal modes for a membrane, then shifting the phase of the standing waves to randomize the pattern. In this way, slip naturally decays towards the edges of the rupture without having to use a taper, we can produce a final slip model with any spectrum we choose, and each wave number can be assigned a unique rise time. The slip on each individual asperity initiates in time according to its closest distance to the hypocenter and the rupture velocity. The Cascadia subduction zone off the coast of northwestern United States and southwestern Canada is capable of producing megathrust earthquakes with magnitudes up to Mw 9.0 for margin-wide events and magnitudes greater than Mw 8.0 if only part of the subduction zone ruptures. The average recurrence interval for margin-wide megathrust

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

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

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

  14. Empirical ground-motion relations for subduction-zone earthquakes and their application to Cascadia and other regions

    USGS Publications Warehouse

    Atkinson, G.M.; Boore, D.M.

    2003-01-01

    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 response parameters are also better determined by the data. We use the database to develop global ground-motion relations for interface and in-slab earthquakes, using a maximum likelihood regression method. We analyze regional variability of ground-motion amplitudes across the global database and find that there are significant regional differences. In particular, amplitudes in Cascadia differ by more than a factor of 2 from those in Japan for the same magnitude, distance, event type, and National Earthquake Hazards Reduction Program (NEHRP) soil class. This is believed to be due to regional differences in the depth of the soil profile, which are not captured by the NEHRP site classification scheme. Regional correction factors to account for these differences are

  15. A recent phase of accretion along the southern Costa Rican subduction zone

    NASA Astrophysics Data System (ADS)

    Bangs, Nathan L.; McIntosh, Kirk D.; Silver, Eli A.; Kluesner, Jared W.; Ranero, César R.

    2016-06-01

    In 2011 we acquired a 3D seismic reflection volume across the Costa Rica margin NW of the Osa Peninsula to investigate the complex structure and the development of the seismogenic zone within the Costa Rican subduction zone in the vicinity of recent International Ocean Drilling Program (IODP) drilling. In contrast to previous interpretations, these newly acquired seismic images show that the margin wedge is composed of a layered fabric that is consistent with clastic sediments, similar to materials recovered from IODP drilling, that have been thrust and thickened into thrust-bounded folded sequences. These structures are consistent with a balanced sequence that has been frontally accreted in the context of an accretionary model. We interpret these sequences as sediment originally deposited on the subducting crust in a trench basin created by the southward migration of the Cocos-Nazca-Caribbean triple junction, and accreted during recent margin subduction that also accelerated with passage of the triple junction. The margin is composed of relatively rapidly accreted sediment that was added to the margin during a phase of accretion within the last ∼5 Ma that was probably preceded throughout the Neogene by periods of non-accretion or erosion.

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

    USGS Publications Warehouse

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

    2003-01-01

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

  17. Characteristics of slow earthquakes in the very low frequency band: Application to the Cascadia subduction zone

    NASA Astrophysics Data System (ADS)

    Ide, Satoshi

    2016-08-01

    Deep tectonic tremors detected in many subduction zones worldwide are often accompanied by very low frequency (VLF) signals, detectable by broadband seismometers but usually hidden in large ambient noise. By stacking broadband seismograms relative to tremor hypocentral times, we can recover the VLF signals. The stacked signals are then inverted to determine a moment tensor solution, using a procedure previously applied to VLF signals in Japan, Taiwan, and Mexico. Here we apply this method to the Cascadia subduction zone, where tremors and slow slip events are clearly observed. As expected, we successfully recover VLF signals for almost the entire tremor region beneath southern Vancouver Island and northern Washington State. The moment tensors are mostly well determined as low-angle thrust type, but source depths are poorly constrained. The slip direction is slightly rotated counterclockwise with respect to the local plate motion direction, probably due to bending of the subducting plate. The seismic moment measured in VLF band is proportional to the seismic energy of tremors, with a scaled energy of about 3 × 10-9. The widespread observability of VLF signals suggests that the deformation associated with tremors and SSEs is actually a very broadband phenomenon, as suggested by stochastic models.

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

    NASA Astrophysics Data System (ADS)

    Kostoglodov, Vladimir; Husker, Allen; Shapiro, Nikolai M.; Payero, Juan S.; Campillo, Michel; Cotte, Nathalie; Clayton, Robert

    2010-12-01

    The last decade featured an explosive sequence of discoveries of slow slip events (SSE) and nonvolcanic tremor (NVT) in different subduction zones and continental faults. Many observations show that SSE is usually associated with an increased NVT activity but it is not clear yet if those events are the result of the same process or are independent expressions of a common underlying seismotectonic source. A large SSE in Central Mexico occurred in 2006 during the Meso-American Subduction Experiment (MASE) which provided continuous observations of the NVT for the years 2005-2007. GPS and abundant seismic data show that although the NVT energy increased notably during the 2006 SSE, the two phenomena were separated spatially and not completely synchronized in time. Significant NVT episodes occur during the period between SSEs, suggesting again that large slow slip events and NVT observed in the Mexican subduction zone are of different origins. The results presented here contribute to uncovering the nature of these two separate phenomena that have been indistinguishable in some other regions.

  19. Effect of hydrothermal circulation on slab dehydration for the subduction zone of Costa Rica and Nicaragua

    NASA Astrophysics Data System (ADS)

    Rosas, Juan Carlos; Currie, Claire A.; Harris, Robert N.; He, Jiangheng

    2016-06-01

    Dehydration of subducting oceanic plates is associated with mantle wedge melting, arc volcanism, intraslab earthquakes through dehydration embrittlement, and the flux of water into the mantle. In this study, we present two-dimensional thermal models of the Costa Rica-Nicaragua subduction zone to investigate dehydration reactions within the subducting Cocos plate. Seismic and geochemical observations indicate that the mantle wedge below Nicaragua is more hydrated than that below Costa Rica. These trends have been hypothesized to be due to a variation in either the thermal state or the hydration state of the subducting slab. Despite only small variations in plate age along strike, heat flow measurements near the deformation front reveal significantly lower heat flow offshore Nicaragua than offshore Costa Rica. These measurements are interpreted to reflect an along-strike change in the efficiency of hydrothermal circulation in the oceanic crust. We parameterize thermal models in terms of efficient and inefficient hydrothermal circulation and explore their impact on slab temperature in the context of dehydration models. Relative to models without fluid flow, efficient hydrothermal circulation reduces slab temperature by as much at 60 °C to depths of ∼75 km and increases the predicted depth of eclogitization by ∼15 km. Inefficient hydrothermal circulation has a commensurately smaller influence on slab temperatures and the depth of eclogitization. For both regions, the change in eclogitization depth better fits the observed intraslab crustal seismicity, but there is not a strong contrast in the slab thermal structure or location of the main dehydration reactions. Consistent with other studies, these results suggest that observed along-strike differences in mantle wedge hydration may be better explained by a northwestward increase in the hydration state of the Cocos plate before it is subducted.

  20. A complex, young subduction zone imaged by three-dimensional seismic velocity, Fiordland, New Zealand

    NASA Astrophysics Data System (ADS)

    Eberhart-Phillips, Donna; Reyners, Martin

    2001-09-01

    The Fiordland subduction zone, where subduction developed in the late Miocene, has been imaged with P and S-P arrival-time data from 311 earthquakes in a simultaneous inversion for hypocentres and 3-D VP and VP/VS models. The three-month microearthquake survey, recorded with 24 portable seismographs, provides excellent coverage, and, since earthquakes to depths of 130km are included, parts of the model are well-resolved to depths of 100km. The crustal features are generally consistent with geology. The low velocity in the upper 10km is associated with the Te Anau and Waiau basins. The Western Fiordland Orthogneiss is associated with a prominent feature from near-surface to over 40km depth, which includes the residue from the basaltic source rocks. It is defined by high VP (7.4kms-1 at 15km depth) and slightly low VP/VS, and has distinct boundaries on its southern and eastern margins. Adjacent to the deepest earthquakes, there is high-velocity Pacific mantle below 80km depth, inferred to be the mantle expression of ongoing shortening since the early Miocene. As the subducting slab moves down and northeast, it is hindered by the high-velocity body and bends to near-vertical. Bending is accommodated by distributed fracturing evidenced by high VP/VS and persistent deep earthquake activity. Buckling of the subducted plate pushes up the Western Fiordland Orthogneiss. In the transition to the Alpine fault in northern Fiordland, a prominent low-velocity crustal root is consistent with ductile thickening in combination with downwarp of the subducted plate.

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

    NASA Astrophysics Data System (ADS)

    Lee, Hsin-Hua; Lin, Jing-Yi

    2013-04-01

    Characterizing the seismogenic zone of major subduction plate boundaries provides us a possible to reduce large earthquakes hazard. In the past several decades, many scientists have analyzed various geophysical methods and datasets, such as seismic and geodetic ground motion data, historical tsunami deposits, aftershock distributions, and seafloor bathymetry, trying to understand the mechanisms behind great devastating earthquakes, and to estimate the probability of a major earthquake occurrence in the future. In this study, by using the global earthquake catalog (GCMT) from January 1, 1976 to December 31, 2011. We firstly re-examines the outer-rise earthquake model proposed by the Christensen (1988) at the subduction zones suggested to have different coupling levels. The compressive stress cumulated during the subducting processes are often reflected by the occurrence of compressional outer-rise earthquakes. Thus, in the region where the compressional outer-rise earthquakes take place without any corresponding large underthrusting earthquakes, the seismic potential is usually considered to be high. We re-examined the high seismic potential areas determined by this criteria in Christensen (1988) and confirm that the large underthrusting earthquakes did really occur in the 30 years following the appearance of compressional outer-rise events, such as in Tonga region in the vicinity of 20S, a Mw 8.3 large earthquake occurred in 2006. This result represents that the outer-rise earthquake model could be an indicator for the generation of large earthquakes along subduction zones. In addition, to have a more accurate estimation for the seismic potential, we discuss the relationship between the generation of earthquakes and the change of cumulative gravitational potential energy caused by earthquakes (ΔGPE) over time. Our result shows an acceleration of ΔGPE before large earthquakes. Our result also shows that the extensional outer-rise events for strong seismic coupling

  2. Sensitivity of the short-to-intermediate wavelength geoid to rheologic structure in subduction zones

    NASA Astrophysics Data System (ADS)

    Hines, J. M.; Billen, M. I.

    2010-12-01

    It is well established that the long wavelength geoid and dynamic topography are responsive to the radial viscosity structure of the mantle, but recent studies indicate that lateral viscosity variations affect the geoid at shorter wavelengths. These studies, however, only consider Newtonian viscosity structures, although experimental deformation studies of mantle minerals and seismic observations of lattice preferred orientation in the upper mantle provide evidence that dislocation creep is an active deformation mechanism at mantle conditions. In addition, the lithosphere is expected to yield plastically at high stresses based on laboratory measurements of yield strength. To quantify the effects of lateral viscosity variations and realistic flow laws on the short-to-intermediate wavelength dynamic topography and geoid near subduction zones, we consider a composite viscosity that accounts both for Newtonian and stress-dependent deformation mechanisms, including plastic yielding. Regional models of instantaneous stokes flow models are computed on a variable resolution mesh using CitcomS, where the resolution ranges from 25 km away from the subduction zone to 5 km in the vicinity of the subducting slab. The slab is defined as an 80 million year old lithosphere temperature anomaly smoothed above and below by half-space cooling models, and extends 100 km into the lower mantle. The buoyancy and stress fields are expanded to spherical harmonic degree 360, corresponding to a spatial resolution of about 110 km. These fields include the effects of self-gravitation and are used to solve for the surface geoid, as well as for dynamic topography at the surface and core-mantle boundary. Results of preliminary, layered mantle viscosity models are consistent with previous geoid studies, the main conclusion being that a more positive geoid at subduction zones is the product of relative viscosity increases with depth. In layered models, increased viscous support of the down

  3. Excess pore pressure generation of oceanic basalt by permeabilty evolution at Nankai subduction zone

    NASA Astrophysics Data System (ADS)

    Tanikawa, W.; Kameda, J.; Yamaguchi, A.; Hamada, Y.; Tadai, O.

    2015-12-01

    Excess pore pressure can be generated along or near interfaces of plate boundary at subduction zones, which is predicted by seismic velocities, and it is a key to understand coseismic slip behaviors during large earthquakes. Evolution of permeability within fault zones and upper sediments during burial and subduction is one of possible mechanisms that generate excess fluid pressure. This evolution process for sediment materials has been reported in previous studies, though the permeability evolution for oceanic crust, which also controls the pore pressure distribution around plate boundary, is not known well. Basalt brocks in the Cretaceous Shimanto accretionary complex of Japan preserve paleo Nankai Subduction structure, and based on vitrinite reflectance, Ro, for neighbor sedimentary rocks, basalt brocks at different areas show different degree of alteration. Therefore, the evolution of fluid transport properties for oceanic basalt at Nankai Subduction zone is estimated by comparing transport properties for various basaltic rocks from the on shore Shimanto belt, South-western Japan. We measured the rock physical properties from Okitsu-Kozurutsu, Kure, Mugi, and Makimine sites in the southeast Japan. Permeability and porosity was measured at room temperature and under confining pressure from 1 to 160 MPa. The steady state gas flow method was applied to evaluate permeability by using nitrogen gas as a pore fluid. Permeability decreases from 10-18 to 10-22 m2 with an increase in the degree of alteration from 1 to 4.5 of Ro (Maximum paleo-temperatures are 80 and 320 oC, respectively). Porosity was also decreased from 5 to 0.5 % with alteration. The relationship between permeability and porosity is described by power law. Our results suggest that based on the evolution curves of permeability and porosity and dehydration rate, porosity reduction in oceanic basalt will contribute to pore pressure generation at shallower subduction zone. On the other hand, clay

  4. Relations between plate kinematics, slab geometry and overriding plate deformation in subduction zones: insights from statistical observations and laboratory modelling

    NASA Astrophysics Data System (ADS)

    Heuret, A.; Funiciello, F.; Faccenna, C.; Lallemand, S.

    2005-12-01

    3-D laboratory models have been performed in order to investigate the way plates kinematics (subducting and overriding plate absolute motions and the resulting plate convergence rate) influences the geometry of the slab and the overriding plate deformation in subduction zones. In the experiments a viscous plate of silicone (subducting plate) is pushed beneath another plate, which is itself pushed toward or pulled away from the trench (overriding plate), and sinks into a viscous layer of glucose syrup (upper mantle). The subducting and overriding plate velocities explored the variability field of natural subduction plates kinematics. The overriding plate motion exerts a primary role in the control of slab geometries and overriding plate deformation rates. The experiments have revealed two different subduction behaviours: (Style I) the overriding plate moves toward the trench and shortens at high rates, the slab is flat and deflected when reaching the bottom of the box in a forward direction; (Style II) the overriding plates moves away from the trench and shortens at low rates the slab is steep and deflected on the box bottom in a backward direction. To a lesser extent, increasing subducting plate motion is associated to increasing slab dips and overriding plate shortening. Slab geometry and overriding plate deformation are less sensitive to the overall plate convergence rate. These laboratory models behaviours are consistent with statistical analysis performed on natural subduction zones, and enlighten the first order control exerted by the overriding plate absolute motion, on the geometry adopted by the slab and the way the overriding plate deforms.

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

    USGS Publications Warehouse

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

    2008-01-01

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

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

    SciTech Connect

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

    1990-06-10

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

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

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

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

  8. Search For Paleoseismolgical Evidences Of Subduction-zone Earthquakes Along The Northwestern (Rakhine) Coast Of Myanmar

    NASA Astrophysics Data System (ADS)

    Aung, T. T.; Okamura, Y.; Satake, K.; Swe, W.; Swe, T. L.; Saw, H.; Tun, S. T.

    2006-12-01

    The northwestern Myanmar (Rakhine) coast is located along the northern extension of the 2004 Sumatra- Andaman earthquake source. Historical data since 1809 indicate that a number of large earthquakes and tsunami have been struck off the western coast of Myanmar, but none of them was great in size as the 2004 earthquake. We started paleoseismological investigation of geological and geomorphological evidences of past subduction-zone earthquakes along the northwestern coast of Myanmar, and a reconnaissance survey was conducted in February 2006. Survey points of tidal flat, lagoon, coral exposure and marine terrace were chosen through examinations of geomorphic nature in available topographic maps and ASTER images. We visited total of 29 survey points around two major cities, Sittway and Thandwe. No clear evidence for tsunami deposits or uplifted coral microatolls that indicate tsunamis or coseismic sea level changes due to past subdcution zone earthquakes was recognized. Marine terraces with three distinct emerged steps were found on the coasts of Myengun and Tandin islands, near Sittway city. Coral samples on the upper steps of marine terraces, with the elevation of about 13 m at one location and 5 m of the other, were dated as about 3000 yr BP. Maximum elevations of marine terraces indicate 2-4 mm/yr uplift rate of the Rakhine coast. This is consistent with horizontal motion estimated from GPS measurement (~20 mm/yr) on the northwestern coast of Myanmar (Rakhine coast) by Socquet et al. (2006) Distribution and changes in elevation of these emerged terraces were interpreted as due to past subduction zone earthquakes occurred off the western coast of Myanmar. Further mapping the lateral extents and detailed field work of marine terraces are needed to ensure that these terraces were formed by tectonic activity associated with the subduction-zone earthquakes. Thus we plan to continue our survey on the western coast of Myanmar in early 2007.

  9. Subduction Zone Fluid Flow and Infiltrative Metasomatism in Franciscan Complex Exotic Ultramafic Blocks

    NASA Astrophysics Data System (ADS)

    King, R. L.; Kohn, M. J.; Eiler, J.

    2001-12-01

    Exotic ultramafic blocks within mud-matrix mélange of the Franciscan Complex, CA, preserve a series of metasomatic mineral zones due to the infiltration of SiO2-rich fluids during subduction. These blocks have experienced extremely large fluid fluxes during metamorphism within the Franciscan subduction channel and appear to have recorded the composition of the fluids present within the Franciscan subduction zone. Chemical constituents dissolved in this fluid include many elements expected to be mobile in high-T aqueous fluids (e.g. SiO2), but also include elements generally believed to be immobile in fluids liberated during the devolatilization of subducted material (e.g. TiO2). We examined the petrology, whole-rock geochemistry, and oxygen isotope composition of these mineral zones to place further constraints on the petrologic structure of the forearc mantle wedge. The ultramafic blocks preserve the relict peridotite mineralogy Ol + Opx + Cpx + Cr-Spl. Fluid flow forms serpentinite (Srp + Mgt + Chl +/- relict Cr-Spl) after peridotite. Additional infiltration formed Tlc (Tlc + Chl +/- relict Cr-Spl) and is the strongest evidence for fluid-mediated addition of SiO2. A fourth mineral zone, outside the scope of the models presented by Manning [1995, 1997], was created by metasomatism of Tlc-bearing rocks, and is composed of Tr + barroisite + Chl + Czo + Ttn + Ap + Zrc +/- relict Cr-Spl. The amphiboles appear to buffer SiO2 at higher activities, stabilizing this diverse group of minerals. Whole-rock geochemical changes occurring during metasomatism include wholesale removal of approximately 6-8 wt% of CaO by through-going fluids during serpentinization. During the production of Tlc from Srp, minor amounts of Al2O3 were added to the rock in addition to the requisite SiO2. At the final stage of metasomatism preserved, Tr-rich rocks show increases in SiO2, TiO2, Al2O3, P2O5, and especially CaO. Separates of Srp from synmetamorphic Srp + Cal veins give Srp δ 18O of 8

  10. Diversity of tsunamigenic earthquakes along the Sunda subduction zone: 2004- 2006

    NASA Astrophysics Data System (ADS)

    Geist, E. L.

    2006-12-01

    In the past three years, three major tsunamigenic earthquakes have occurred along the Sunda subduction zone that spanned a wide range of tsunami generating capacity. In this study, the efficiency of tsunami excitation is measured by average and maximum observed local runup relative to the scalar seismic moment of the earthquake. The overall severity of the tsunami from the Mw~9.2 December 26, 2004 Sumatra-Andaman earthquake closely matched that expected from an earthquake of this magnitude. In contrast, the March 28, 2005 Nias earthquake was deficient in tsunami excitation for an earthquake of Mw=8.7. The tsunami from the latest earthquake along the subduction zone, the Mw=7.7 July 17, 2006 Java event, was greater than expected and in fact, was greater in terms of average and maximum runup than the March 2005 event. Seismogenic tsunamis are primarily generated from coseismic vertical displacement of the seafloor in response to slip on a fault. The variation in tsunamis from these three earthquakes can be explained by dip-directed variations in the distribution of slip or moment density over the rupture area. Moment density concentrated up dip along the interplate thrust results in four effects that contribute to increased tsunami excitation: (1) a strong reduction in shear modulus in the shallow part of subduction zone results in greater slip for a given moment density; (2) the circumstance of seafloor rupture results in a traction free boundary condition that increases the amount of slip near the trench, in comparison to an imbedded rupture; (3) shallower fault depth below the seafloor results in greater vertical displacement for a given amount of slip; and (4) an increase in the water depth over tsunami generation regions near the trench results in greater amplification of tsunami waves during shoaling according to Green's Law. The difference, therefore, in tsunami excitation from the March 2005 and July 2006 earthquakes can be ascribed to the fact that most of

  11. Shear wave anisotropy in textured phase D and constraints on deep water recycling in subduction zones

    NASA Astrophysics Data System (ADS)

    Rosa, Angelika D.; Sanchez-Valle, Carmen; Nisr, Carole; Evans, Shaun R.; Debord, Regis; Merkel, Sébastien

    2013-09-01

    Regions of low seismic velocity and high shear anisotropies in cold subducted slabs have often been related to anisotropic fabrics in hydrous phases mainly induced by slab deformation. The interpretation of these seismic anomalies in terms of hydration thus relies on a better knowledge of the elasticity and plastic deformation mechanisms of candidate hydrous phases. Here we investigate the development of lattice preferred orientations (LPO) in phase D [MgSi2H2O6, 10-18 wt% H2O], the ultimate water carrier in hydrous subducted peridotite. The samples were deformed non-hydrostatically up to 48 GPa in a diamond anvil cell and the texture and strength were obtained from analysis of the X-ray diffraction patterns collected in radial diffraction geometry. We find that at low strains the layered structure of phase D displays strong 0001 texture, where the stacking fault axis (c-axis) preferentially align parallel to the compression axis. A subsidiary 101¯0 texture develops at higher strains. Plasticity simulations in polycrystalline aggregates using a viscoplastic self-consistent model suggest that these LPO patterns are consistent with shape preferred orientation mechanism during the first compaction steps and, with dominant easy glide on basal planes and harder first order pyramidal slip, respectively, upon further compression. We find that phase D displays the lowest strength and the highest anisotropy among phases in hydrous peridotite in the uppermost lower mantle and might thus control the shear wave anisotropy generated in subducted slabs below the transition zone. We further evaluate the effect of textured phase D on the seismic velocity structure and shear wave anisotropy of deformed hydrous peridotite and compare the results to seismic observations in Tonga subduction. We show that 16 vol% of phase D in hydrous subducted peridotite is required to explain the negative velocity anomalies of 3%, the extent of shear wave splitting (0.9±0.3%) and the shear wave ray

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

    NASA Astrophysics Data System (ADS)

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

    2009-04-01

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

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

    NASA Astrophysics Data System (ADS)

    Suarez, G.; Mortera, C.

    2013-05-01

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

  14. Defining the worst case scenario for the Makran Subduction Zone: the 1008 AD tsunami

    NASA Astrophysics Data System (ADS)

    Hoffmann, Goesta

    2016-04-01

    The Makran Subduction Zone is located within the Arabian Sea (Northern Indian Ocean) and marks the boundary between the Arabian and the Eurasian plate. The sinistral strike-slip Sonne fault separates the subduction zone in an eastern and western segment. The convergence rate is about 40 mm/yr and slightly faster in the east than in the west. The seismicity is low in general and the few documented seismic events are concentrated in the eastern segment. No seismic activity is known from the western segment in historic times. The hazard potential is enigmatic as the only documented and recorded tsunamigenic earthquake (MW 8.1) within the subduction zone occurred in Nov 1945. However, thermal modelling suggests a wide potential seismogenic zone, apparently capable of generating very significant (>MW 8.5) tsunamigenic earthquakes. Furthermore, submarine slumping is another tsunami trigger which has to be taken into account. We used the modelling results as a hypothesis and mapped extreme wave event deposits along the coastline of Oman, bordering the Arabian Sea. We were able to document extensive boulder fields along rocky parts of the coastline. These boulders are decorated with marine sessile organism such as e.g oysters or barnacles testifying for an intertidal setting of the boulder prior to dislocation. The organism remains were used for radiocarbon dating assuming that the death of the organism was related to the relocation of the boulder. Storm-induced boulder movement is possible as the coastline is subject to infrequent tropical cyclone impact. However, boulder movement was not observed during the strongest storm on record in 2007. The dating exercise revealed a cluster of dates around 1000 AD, coinciding with a potential earthquake event known from a historic Persian text dating to the year 1008 AD. Archaeological evidence, mainly pottery artefacts found along the sea shore near the capital area Muscat/Oman also indicate a catastrophic event which may be

  15. Permeability anisotropy of serpentinite and fluid pathways in a subduction zone

    NASA Astrophysics Data System (ADS)

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

    2011-12-01

    Subduction zones are the only sites where water is transported into the Earth's deep interior. Although the fluid released into the mantle wedge is generally believed to ascend under buoyancy, it is possible that fluid movement is influenced by anisotropic permeability in localized shear zones. The mantle rocks at the plate interface of a subducting slab are subjected to non-coaxial stress and commonly develop a strong foliation. Indeed, the existence of foliated serpentinite is indicated by strong seismic anisotropy in the forearc mantle wedge (e.g., Katayama et al., 2009; Bezacier et al., 2010). Therefore, fluid pathways in the mantle wedge may be controlled by the preferred orientation of highly anisotropic minerals. In this study, we measured the permeability of highly foliated natural serpentinite, in directions parallel and perpendicular to the foliation, and we discuss the influence of permeability anisotropy on fluid flow in subduction zones. The permeability was measured by an intra-vessel deformation and fluid flow apparatus housed at Hiroshima University. In the measurements, we used nitrogen gas as a pore fluid and maintained constant pore pressure during the measurements (Pp < 6 MPa). The obtained gas permeability was then converted to intrinsic permeability using the Klinkenberg effect, which is known to be insensitive to the type of pore fluid. Under low confining pressure, all the experiments show similar permeability, in the order of 10-19 m2. However, permeability anisotropy appears under high confining pressures, with the specimens oriented parallel to the foliation having higher permeability than those oriented normal to the foliation. At a confining pressure of 50 MPa, the difference in permeability between the samples with contrasting orientations reaches several orders of magnitude, possibly reflecting the pore tortuosity of the highly sheared serpentinite, as indicated by the Kozeny-Carman relation. The present experimental data show that

  16. Implications of Smectite Subduction at the Costa Rican Convergent Margin

    NASA Astrophysics Data System (ADS)

    Cardace, D.; Morris, J. D.; Underwood, M. B.; Spinelli, G.

    2003-12-01

    Legs 205/170 of the Ocean Drilling Program (ODP) drilled a reference section on the incoming plate and sites at the toe of the sedimentary prism at the Costa Rican convergent margin. Complete sediment subduction has been documented, with the prism described by Leg 205/170 shipboard scientists as a paleoslump prism. Despite sediment subduction, Costa Rican arc lava geochemistry shows little sediment signal. Though subduction erosion has been posited as a mechanism for damping the geochemical sediment signal, this abstract addresses whether the clay content and distribution in the subducting pile can (a) play a role in localizing the decollement and (b) impact subduction of sediment to depth. X-ray diffraction (XRD) analyses of bulk sediment, with biogenic silica determinations, have been carried out for samples from the prism, through the decollement, to the underthrust sediments. Clay fractions have been isolated and silica studied for a subset of these samples. XRD peak areas of bulk samples were transformed into relative abundances via matrix singular value decomposition (Fisher and Underwood, 1995, Proc. ODP, Init. Repts., 156: 29-37), and adjusted following silica determination; volcanic ash has been neglected as a sedimentary component. Average relative weight percents of dominant minerals and biogenic silica (bSiO2) for prism toe units (Site 1040) are: P1A (silty clay, 74.8 m thick) 82 wt% clay, 5 wt% quartz, 13 wt% plagioclase, 0 wt% calcite; P1B (silty clay, 296.4 m thick) 82.1 wt% clay, 6.0 wt% quartz, 10.4 wt% plagioclase, 0 wt% calcite, 1.4 wt% bSiO2. Below the decollement, underthrust abundances are: U1A (clayey diatomite, 13.2 m thick) 82.7 wt% clay, 5.2 wt% quartz, 8.9 wt% plagioclase, 0 wt% calcite, 3.2 wt% bSiO2; U1B (clayey diatomite, 38.2 m thick) 80.7 wt% clay, 4.4 wt% quartz, 6.6 wt% plagioclase, 0 wt% calcite, 8.2 wt% bSiO2; U2 (silty claystone, 57.1 m thick) 84.8 wt% clay, 4.5 wt% quartz, 6.8 wt% plagioclase, 0 wt% calcite, 3.9 wt% bSiO2; U3A

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

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2015-04-01

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

  19. Rupture Dynamics Simulations Along Subduction Zones: Bimaterial Interfaces and Free Surface Interaction

    NASA Astrophysics Data System (ADS)

    Scala, A.; Vilotte, J. P.; Festa, G.

    2015-12-01

    Largest earthquakes occur along subduction zones, where normal and tangential stress coupling drives the earthquake rupture due to the geometry of the subduction interface between dissimilar materials and the interaction with waves reflected from free surface as the rupture propagates toward the trench. We numerically investigate these effects in the context of dynamic rupture simulations. We revisit the problem of in-plane interface rupture propagation between dissimilar elastic media, in the case of slip-weakening friction, by performing a numerical study using the Spectral Element Method with a non-smooth contact formulation. For classical slip-weakening friction, the problem is ill posed due to a missing length or time scale in the response of the frictional shear stress to dynamic normal stress perturbations. We first perform a parametric study of the regularization formulation proposed by Rubin and Ampuero (2007). We show that the dynamic regularization, driven by local slip rate does not allow for a proper modeling of the asymptotic rupture propagation. We propose a new regularization approach based on the non-local length scale, associated to the actual size of the process zone. Numerical results are shown to be consistent with mathematical modeling of dynamic interface rupture propagation with a process zone ahead of the rupture front. The numerical study is extended to inclined ruptures intersecting a free surface at different angles. We investigate interaction between rupture propagation and stress changes induced by waves reflected from the free surface, in the generation of large interface slip, transient healing and opening effects. Finally, preliminary in-plane dynamic simulations of the 2011 Tohoku earthquake, incorporating the along-dip structure and geometry of the subduction interface, are presented enlightening the role of the geometry of the bi-material interface and of the free surface in the rupture propagation and radiation.

  20. Testing mechanisms of subduction zone segmentation and seismogenesis with slip distributions from recent Andean earthquakes

    NASA Astrophysics Data System (ADS)

    Loveless, J. P.; Pritchard, M. E.; Kukowski, N.

    2010-11-01

    A long-standing goal of subduction zone earthquake studies is to determine whether or not there are physical processes that control seismogenesis and the along-strike segmentation of the megathrust. Studies of individual earthquakes and global compilations of earthquakes find favorable comparison between coseismic interplate slip distributions and several different long-lived forearc characteristics, such as bathymetry, coastline morphology, crustal structure, and interplate frictional properties, but no single explanation seems to govern the location and slip distribution of all earthquakes. One possible reason for the lack of a unifying explanation is that the inferred earthquake parameters, most importantly the slip distribution, calculated in some areas were inaccurate, blurring correlation between earthquake and physical parameters. In this paper, we seek to test this possibility by comparing accurate slip distributions constrained by multiple datasets along several segments of a single subduction zone with the various physical properties that have been proposed to control or correlate with seismogenesis. We examine the rupture area and slip distribution of 6 recent and historical large ( Mw > 7) earthquakes on the Peru-northern Chile subduction zone. This analysis includes a new slip distribution of the 14 November 2007 Mw = 7.7 earthquake offshore Tocopilla, Chile constrained by teleseismic body wave and InSAR data. In studying the 6 events, we find that no single mechanism can explain the location or extent of rupture of all earthquakes, but analysis of the forearc gravity field and its gradients shows correlation with many of the observed slip patterns, as suggested by previous studies. Additionally, large-scale morphological features including the Nazca Ridge, Arica Bend, Mejillones Peninsula, and transverse crustal fault systems serve as boundaries between distinct earthquake segments.

  1. Fifteen Years of Slow Slip and Tremor Observations at the Northern Costa Rica Subduction Zone

    NASA Astrophysics Data System (ADS)

    Schwartz, S. Y.; Dixon, T. H.; Protti, M.; González, V. M.

    2015-12-01

    Coordinated long-term geophysical observations at the northern Costa Rica seismogenic zone, facilitated by NSF's MARGINS program, have greatly expanded our understanding of its megathrust behavior. Here we review fifteen years of seismic, geodetic, ocean bottom fluid flow and pressure sensor data collected on or near the Nicoya Peninsula, above the shallow thrust interface that document a variety of slow slip behaviors. These include relatively deep (~30-40 km), large slow slip events that occur about every 2 years, smaller events that locate at more intermediate depth (10-15 km) and occur more frequently (~1 per year), and very shallow events at the toe of the margin wedge that produce no discernible GPS signal on land but are detected on seafloor pressure sensors. Most of these slow slip events at the toe are accompanied by seismic tremor. Short-term, GPS only observations might have detected a few of these slow slip events; however, the longer more diverse instrument deployment was necessary to reveal their greater complexity. This demonstrates the need for a sustained, multi-instrument deployment and off-shore instrumentation at several different subduction zones, like that proposed for the Subduction Zone Observatory (SZO), to significantly advance our understanding of slow slip at convergent boundaries. Similar instrumentation to what exists in Nicoya is presently being established in the Osa-Burica region of southern Costa Rica to capture earthquake cycle deformation there. These two installations can provide a good nucleus for a larger circum-Pacific SZO effort.

  2. P wave azimuthal and radial anisotropy of the Hokkaido subduction zone

    NASA Astrophysics Data System (ADS)

    Niu, Xiongwei; Zhao, Dapeng; Li, Jiabiao; Ruan, Aiguo

    2016-04-01

    We present the first three-dimensional P wave radial anisotropy tomography of the Hokkaido subduction zone, as well as P wave azimuthal anisotropy and S wave tomography, which are determined by inverting 298,430 P wave and 233,934 S wave arrival times from 14,245 local earthquakes recorded by 344 seismic stations. Our results reveal significant velocity heterogeneity, seismic anisotropy, and upwelling flows beneath the study region. In the mantle wedge, prominent low-velocity (low-V) anomalies exhibit trench-normal fast-velocity directions (FVDs) and a negative radial anisotropy (i.e., vertical velocity > horizontal velocity), which may reflect upwelling mantle flows. Fan-shaped FVDs are found at depths of 65-90 km, and a detailed 3-D mantle flow pattern is revealed, which may be caused by a combination of oblique subduction of the Pacific plate and collision of the Kuril arc with the Honshu arc beneath southern Hokkaido. The radial anisotropy changes at ~100 km depth, which may reflect variations in temperature and fluid conditions there. The subducting Pacific slab exhibits a positive radial anisotropy (i.e., horizontal velocity > vertical velocity), which may reflect the original fossil anisotropy when the Pacific plate formed at the mid-ocean ridge.

  3. Tomographic imaging of the Cascadia subduction zone: Constraints on the Juan de Fuca slab

    NASA Astrophysics Data System (ADS)

    Chen, Chuanxu; Zhao, Dapeng; Wu, Shiguo

    2015-04-01

    We used 40,343 P-wave arrival times from 1883 local earthquakes and 105,455 P-wave arrivals from 6361 teleseismic events to study the detailed structure of the Cascadia subduction zone. We conducted tomographic inversions using a starting velocity model which includes the high-velocity subducting Juan de Fuca slab as a priori information. A number of such slab-constrained inversions are conducted by changing the slab thickness and the velocity contrast between the slab and the surrounding mantle. Our optimal 3-D velocity model fits the data much better than that determined by an inversion with a 1-D homogeneous starting model. Our results show that the subducting Juan de Fuca slab has a thickness of 30-50 km and a P-wave velocity of 1-3% higher than that of the surrounding mantle. Beneath the northern and southern parts of the Cascadia, P-wave velocity is lower in the slab and along the slab interface, which may reflect a more hydrated slab and more active slab dehydration there. The lateral velocity variations may indicate different degrees of slab dehydration and forearc mantle serpentinization. The segmentation in episodic tremor and slip (ETS) is also spatially coincident with the velocity heterogeneities, indicating that the ETS occurrence and recurrence interval are controlled by fluid activity in and around the mantle wedge corner.

  4. Detailed Seismic Velocity Structure of the Plate Boundary, Cascadia Subduction Zone, from Prestack Waveform Inversion

    NASA Astrophysics Data System (ADS)

    Fortin, W.; Holbrook, W.; Tobin, H. J.; Keranen, K. M.; Everson, E.; Mallick, S.; Padhi, A.

    2013-12-01

    Understanding the geologic makeup of the Cascadia Subduction Zone (CSZ) has great importance for understanding seismic hazards in the coastal margin of the U.S. Pacific Northwest. The Cascadia margin is a potential earthquake and tsunami threat to the many millions who live in the area, yet details of its structure and mechanics remain poorly understood. In particular, the character of the subduction interface is elusive due to the CSZ's relatively aseismic behavior and low seismic reflectivity, making imaging difficult for passive and active source methods, respectively. In July 2012 seismic data were acquired as a part of the COAST project, spanning the important transition from the Cascadia basin, across the deformation front, and into the accretionary wedge. This modern data, coupled with sophisticated pre-stack full waveform seismic inversion methods, allows us to create highly detailed velocity models. While still computationally expensive, current computing clusters can perform these inversions with enough lateral density to yield highly detailed velocity information in both the vertical and horizontal. Here we present pre-stack full waveform inversions of a seismic line from the center of the COAST survey offshore Washington state as a cross section of the velocity structure of the CSZ. This detailed velocity model is a necessary initial step toward a detailed porosity cross section to be used to determine the role of fluids in the CSZ. Using these new data we investigate the lateral variability in reflectivity of the subducting plate boundary reflection in terms of its seismic velocity.

  5. Fluid and mass transfer into the cold mantle wedge of subduction zones: budgets and seismic constraints

    NASA Astrophysics Data System (ADS)

    Abers, G. A.; Hacker, B. R.; Van Keken, P. E.; Nakajima, J.; Kita, S.

    2015-12-01

    Dehydration of subducting plates should hydrate the shallow overlying mantle wedge where mantle is cold. In the shallow mantle wedge hydrous phases, notably serpentines, chlorite, brucite and talc should be stable to form a significant reservoir for H2O. Beneath this cold nose thermal models suggest only limited slab dehydration occurs at depths less than ca. 80 km except in warm subduction zones, but fluids may flow updip from deeper within the subducting plate to hydrate the shallow mantle. We estimate the total water storage capacity in cold noses, at temperatures where hydrous phases are stable, to be roughly 2-3% the mass of the global ocean. At modern subduction flux rates its full hydration could be achieved in 50-100 Ma if all subducting water devolatilized in the upper 100 km flows into the wedge; these estimates have at least a factor of two uncertainty. To investigate the extent to which wedge hydration actually occurs we compile and generate seismic images of forearc mantle regions. The compilation includes P- and S-velocity images with good sampling below the Moho and above the downgoing slab in forearcs, from active-source imaging, local earthquake tomography and receiver functions, while avoiding areas of complex tectonics. Well-resolved images exist for Cascadia, Alaska, the Andes, Central America, North Island New Zealand, and Japan. We compare the observed velocities to those predicted from thermal-petrologic models. Among these forearcs, Cascadia stands out as having upper-mantle seismic velocities lower than overriding crust, consistent with high (>50%) hydration. Most other forearcs show Vp close to 8.0 km/s and Vp/Vs of 1.73-1.80. We compare these observations to velocities predicted from thermal-mineralogical models. Velocities are slightly slower than expected for dry peridotite and allow 10-20% hydration, but also could also be explained as relict accreted rock, or delaminated, relaminated, or offscraped crustal material mixed with mantle

  6. Evolution of the Grenada and Tobago basins and the onset of the Lesser Antilles subduction zone

    NASA Astrophysics Data System (ADS)

    Zitter, T. A. C.; Rangin, C.

    2012-04-01

    The Lesser Antilles active island arc marks the eastern boundary of the Caribbean plate, where the Atlantic oceanic crust is subducted. Geodynamic history of the Grenada and Tobago basins, accepted as both the back arc and fore arc basins respectively for this convergent zone, is the key for a better understanding of the Antilles arc subduction onset. Still, recent studies propose that these two basins formed as a single paleogene depocenter. Analysis of industrial and academical seismic profiling supports this hypothesis, and shows these basins are two half-graben filled by 15 kilometers of cenozoic sediments. The seismic profiles across these basins, and particularly the Geodinos Bolivar seismic profiles, indicate that the Antilles magmatic arc develops in the midst of the previously-extended Grenada-Tobago basin from Miocene time to present. The pre-cenozoic basement of the Grenada-Tobago basin can be traced from the Aves ridge to the Tobago Island where cretaceous meta-volcanic rocks are cropping out. Therefore, this large basin extension has been initiated in early Paleocene time during stretching or subsidence of the great cretaceous Caribbean arc and long time before the onset of the lesser Antilles volcanic arc. The question arises for the mechanism responsible of this intra-plate extension. The Tobago Ridge consists of the backstop of the Barbados prism. The innermost wedge is particularly well imaged on seismic data along the Darien Ridge, where the isopach paleogene sediments are jointly deformed in latest Oligocene. This deformation is starved with the early miocene piggy-back basin. Hence, we conclude the innermost wedge in contact with the butresss is late Oligocene in age and can be considered as the onset of the subduction along the Antilles arc. This 30 Ma subduction onset is also supported by the 750 km long Atlantic slab, imaged in tomography, indicating this subduction was active with constant velocity of 2.5 km/yr. Consequently, another

  7. Rupture and creep behaviours of subduction interface controlled by fault zone heterogeneity

    NASA Astrophysics Data System (ADS)

    Wang, Kelin; Brown, Lonn; Gao, Xiang; Bilek, Susan

    2015-04-01

    The behaviour of fault slip varies tremendously, ranging from seismic rupture to aseismic creep. We explore the role of fault zone heterogeneity in controlling large-scale (> 100 km in strike dimension) rupture and creep behaviours of subduction faults. Geometrically smooth subduction faults can (although not always) provide relatively homogeneous structural and stress conditions to allow large fault patches to be locked over prolonged periods and then rupture in great earthquakes. During the rupture, however, frictional heterogeneities arising from lithological changes, pore-fluid pressure variations, and low-amplitude geometrical irregularities always cause a very heterogeneous distribution of stress drop. Although some parts of the fault may undergo high or complete stress drop (local weakening), many other parts undergo very low stress drop or stress increase (local strengthening). The mixing of stress drop and increase in different parts of the rupture zone makes the average stress drop in each great earthquake very small, of the order of a couple of MPa, as widely observed in seismological studies. We use the 2011 Mw=9 Tohoku-oki earthquake to demonstrate this averaging effect. Geometrically extremely rough subduction faults, such as those featuring multiple subducting seamounts, provide very heterogeneous structural and stress conditions that promote creep and numerous small earthquakes. A global inspection of geodetically constrained locking and creeping states of subduction zones indicates that these extremely rough faults all tend to creep (Wang and Bilek, 2014). Depending on the degree of roughness and other geological conditions (e.g., sediment and fluid), some of the rough faults may host a mixture of seismic and aseismic patches and may exhibit a variety of creep behaviour ranging from steady creep to transient creep pulses (i.e., slow slip events) of different time scales. It can be envisioned that the heterogeneity in these rough faults is generally

  8. 3D Lithospheric Density Structure of the Central American Subduction Zone from Gravity Data

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

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

  9. A real-time cabled observatory on the Cascadia subduction zone

    NASA Astrophysics Data System (ADS)

    Vidale, J. E.; Delaney, J. R.; Toomey, D. R.; Bodin, P.; Roland, E. C.; Wilcock, W. S. D.; Houston, H.; Schmidt, D. A.; Allen, R. M.

    2015-12-01

    Subduction zones are replete with mystery and rife with hazard. Along most of the Pacific Northwest margin, the traditional methods of monitoring offshore geophysical activity use onshore sensors or involve conducting infrequent oceanographic expeditions. This results in a limited capacity for detecting and monitoring subduction processes offshore. We propose that the next step in geophysical observations of Cascadia should include real-time data delivered by a seafloor cable with seismic, geodetic, and pressure-sensing instruments. Along the Cascadia subduction zone, we need to monitor deformation, earthquakes, and fluid fluxes on short time scales. High-quality long-term time series are needed to establish baseline observations and evaluate secular changes in the subduction environment. Currently we lack a basic knowledge of the plate convergence rate, direction and its variations along strike and of how convergence is accommodated across the plate boundary. We also would like to seek cycles of microseismicity, how far locking extends up-dip, and the transient processes (i.e., fluid pulsing, tremor, and slow slip) that occur near the trench. For reducing risk to society, real-time monitoring has great benefit for immediate and accurate assessment through earthquake early warning systems. Specifically, the improvement to early warning would be in assessing the location, geometry, and progression of ongoing faulting and obtaining an accurate tsunami warning, as well as simply speeding up the early warning. It would also be valuable to detect strain transients and map the locked portion of the megathrust, and detect changes in locking over the earthquake cycle. Development of the US portion of a real-time cabled seismic and geodetic observatory should build upon the Ocean Observatories Initiative's cabled array, which was recently completed and is currently delivering continuous seismic and pressure data from the seafloor. Its implementation would require

  10. A mega-splay fault system and tsunami hazard in the southern Ryukyu subduction zone

    NASA Astrophysics Data System (ADS)

    Hsu, Shu-Kun; Yeh, Yi-Ching; Sibuet, Jean-Claude; Doo, Wen-Bin; Tsai, Ching-Hui

    2013-01-01

    In April 1771, a subduction earthquake generated a great tsunami that struck the south Ryukyu islands and killed ˜12,000 people, whereas its mechanism is still enigmatic (Nakata and Kawana, 1995; Nakamura, 2006; Matsumoto et al., 2009). In this paper, we show its probable source on a mega-splay fault system existing along the southern Ryukyu forearc. Analyses of deep multi-channel seismic reflection profiles indicate that the mega-splay fault system is rising from the summit of a ˜1 km high ridge situated at a ˜5° landward dipping plate interface. An outer ridge marks the seafloor outcrop of the splay fault system and separates the landward inner wedge and the oceanward outer wedge. The inner wedge is uplifting and exhibits widespread normal faulting while the outer wedge shows folded structures. The mega-splay fault system is parallel to the Ryukyu Trench east of 125.5°E and is estimated to be ˜450 km long. The origin of this south Ryukyu mega-splay fault system is ascribed to a resistant subduction of the elevated transverse ridges associated with the subducting portion of the trench-parallel Luzon-Okinawa Fracture Zone. In contrast, no similar splay fault is found west of 125.5°E where the oblique subduction has produced large shear zones along the south Ryukyu forearc. We infer that a thrust earthquake linked to the mega-splay fault system is responsible for the south Ryukyu tsunami. However, another possible scenario of generating a large tsunami affecting the south Ryukyu islands is that the subducted ridge in the western end of the mega-splay fault system nucleated a large earthquake and simultaneously triggered the ˜100 km long E-W trending strike-slip fault west of 125.5°E and induced a southward-dipping tsunami-genic subsidence. In any case, after a quiescence of ˜241 yr, a large earthquake and tsunami is anticipated in the south Ryukyu forearc in the near future.

  11. 1D crustal structure from quality seismological data for the Cyprus subduction zone

    NASA Astrophysics Data System (ADS)

    Perk, Şükran; Deǧer, Ali; Özbakır, Karabulut, Hayrullah

    2013-04-01

    The eastern Mediterranean is a tectonically complex region, where long-term subduction and accretion processes have shaped the overall evolution. Recently, many seismic tomography studies have shown subducted slabs of the Neo-Tethyan lithosphere, continuing its subduction in the Hellenic trench, stalled in the Cyprus trench and being torn near the intersection between them. Antalya bay is a key region located on the western flank of the Cyprus Subduction Zone (CSZ), close to the junction between the Hellenic and Cyprus Arcs. Here deep earthquakes are nucleated, which otherwise cannot be seen anywhere else along the CSZ. For this reason, we focus our attention specifically to the Antalya Bay area but also the remaining parts of the CSZ. Several regional studies have been carried out to define the velocity structure beneath the region but none have been able to locate the CSZ. One of the main reasons for this was the lack of incorporation of a wide seismic network in those regional studies. We compile a large catalog of seismicity and relocate earthquakes to infer 1D local crustal structure using the clusters of seismicity. We used seismic data between 2005 - 2011 which are recorded at more than 335 seismic stations operated by several agencies and portable deployments. The data-set is composed of over 10,000 events and earthquakes can be grouped in several distinct clusters. We defined five of these clusters, where the total number of events is more than 4500, among which we selected over 2000 events with the highest data quality. 1-D local P-wave velocity models are developed using this high quality data-set and the earthquakes are relocated using the local velocity models. The compiled and reanalyzed data will contribute to perform local earthquake tomography. Moreover, obtained local velocity models represent a fundamental step towards an improved seismic tomography studies in a very crucial region in the eastern Mediterranean.

  12. Towards understanding carbon recycling at subduction zones - lessons from Central America

    NASA Astrophysics Data System (ADS)

    Hilton, D. R.; Barry, P. H.; Fischer, T. P.

    2010-12-01

    Subduction zones provide the essential pathways for input of carbon from Earth’s external reservoirs (crust, sediments, oceans) to the mantle. However, carbon input to the deep interior is interrupted by outputs via the fore-arc, volcanic front, and back-arc regions. Coupled CO2 and He isotope data for geothermal fluids from throughout Central American (CA) are used to derive estimates of the output carbon flux for comparison with inputs estimated for the subducting Cocos Plate. The carbon flux carried by the incoming sediments is ~1.6 × 109 gCkm-1yr-1[1], as is the ratio of input carbon derived from pelagic limestone (L) and organic sediment (S), i.e., L/S ~10.7. Additionally, the upper 7 km of oceanic (crustal) basement supplies ~9.1 × 108 gCkm-1yr-1[2]: this flux is dominated by L-derived CO2. In terms of output, measured carbon concentrations coupled with flow rates for submarine cold seeps sites at the Costa Rica outer forearc yield CO2 and CH4 fluxes of ~ 6.1 × 103 and 8.0 × 105 (gCkm-1yr-1), respectively [3]. On the Nicoya Peninsula, the Costa Rica Pacific coastline (including the Oso Peninsula) and the Talamanca Mountain Range, coupled CO2-He studies allow recognition of a deep input (3He/4He up to 4RA) and resolution of CO2 into L- and S-components. There is an increase in the L/S ratio arc-ward with the lowest values lying close to diatomaceous ooze in the uppermost sequence of subducting sediment package. This observation is consistent with under-plating and removal of the uppermost organic-rich sediment from deeper subduction. As the input carbon fluxes of the individual sedimentary layers are well constrained [1], we can limit the potential steady-state flux of carbon loss at the subaerial fore-arc to ~ 6 × 107 gCkm-1yr-1, equivalent to ~88% of the input flux of the diatomaceous ooze, or < 4% of the total incoming sedimentary carbon. The greatest loss of slab-derived carbon occurs at the volcanic front. Estimates of the output CO2 flux along the

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

    NASA Astrophysics Data System (ADS)

    Li, Tao; Hampel, Andrea

    2013-04-01

    Tide-gauge and geodetic measurements of coseismic and interseismic displacements in the forearc of subduction zones showed that the coastal region undergoes uplift during the interseismic phase and subsidence during the coseismic phase, while opposite vertical movements are observed in the neighbouring regions (e.g., Savage & Thatcher 1992; Hyndman & Wang 1995). Horizontal displacements during the interseismic phase are typically directed landward, whereas the forearc moves seaward during the earthquake (e.g., Klotz et al. 1999). Here we use two-dimensional finite-element modelling to evaluate how the friction coefficient along the plate interface, the length and the position of the downdip end of the locked zone affect the coseismic and interseismic displacements. Our model consists of a deformable, rheologically stratified upper plate and an undeformable oceanic plate, which rotates at a prescribed angular velocity (cf. Cailleau & Oncken, 2008). The frictional plate interface is divided - from the trench to the base of the continental lithosphere - into a seismogenic zone, a transition zone and a landward free slip zone. During an initial phase, the seismogenic zone is locked, which leads to the accumulation of elastic strain in the forearc. During the subsequent coseismic phase, the strain is released and causes sudden slip of several meters on the plate interface. During the next interseismic phase, the seismogenic zone is locked again. Our model results show patterns of vertical and horizontal displacements that are in general agreement with geodetically observed patterns. A sensitivity analysis reveals that the magnitude of the vertical displacements is strongly influenced by the friction coefficients of the seismogenic zone and the transition zone. The location of the zones of maximum interseismic uplift and coseismic subsidence in the coastal regions depends on the length and position of the locked zone. Preliminary results from three-dimensional models

  14. Spatial and temporal patterns of nonvolcanic tremor along the southern Cascadia subduction zone

    NASA Astrophysics Data System (ADS)

    Boyarko, Devin C.; Brudzinski, Michael R.

    2010-08-01

    Episodic tremor and slip (ETS), the spatial and temporal correlation of slow slip events monitored via GPS surface displacements and nonvolcanic tremor (NVT) monitored via seismic signals, is a newly discovered mode of deformation thought to be occurring downdip from the seismogenic zone along several subduction zone megathrusts. To provide overall constraints on the distribution and migration behavior of NVT in southern Cascadia, we apply a semiautomated location algorithm to seismic data available during the EarthScope Transportable Array deployment to detect the most prominent pulses of NVT and invert analyst-refined relative arrival times for source locations. In the processing, we also detect distinct and isolated bursts of energy within the tremor similar to observations of low-frequency earthquakes in southwest Japan. We investigate in detail eight NVT episodes between November 2005 and August 2007 with source locations extending over a 650 km along-strike region from northern California to northern Oregon. We find complex tremor migration patterns with periods of steady migration (4-10 km/d), halting, and frequent along-strike jumps (30-400 km) in activity. The initiation and termination points of laterally continuous tremor activity appear to be repeatable features between NVT episodes which support the hypothesis of segmentation within the ETS zone. The overall distribution of NVT epicenters occur within a narrow band primarily confined by the surface projections of the 30 and 40 km contours of the subducting plate interface. We find as much as 50 km spatial offset from the updip edge of the tremor source zone to the downdip edge of the thermally and geodetically defined transition zone, which may inhibit ETS from triggering earthquakes further updip. Intriguingly, NVT activity is spatially anticorrelated with local seismicity, suggesting the two processes are mutually exclusive. We propose that the transition in frictional behavior coupled with high pore

  15. Interseismic coupling, segmentation and mechanical behavior of the Central Chile subduction zone.

    NASA Astrophysics Data System (ADS)

    Métois, M.; Socquet, A.; Vigny, C.

    2012-04-01

    Global Positioning System (GPS) measurements carried out in Chile over the last two decades showed that an entire portion of the Nazca-South America subduction zone (38°S-24°S) was locked over this period of time. The induced accumulation of elastic deformation in the upper-plate was not released until the recent Maule earthquake of 27 February 2010 (Mw 8.8) that ruptured the southern part of this section. Locking or coupling between the two plates varies both with depth and along strike. Here we use our own GPS data (an updated solution of our extended network in central Chile), combined with other published data sets, to quantify the spatial variations of the coupling that prevailed before the Maule earthquake. Using a simple elastic model based on the back-slip assumption, we show that coupling variations on the subduction plane are sufficient to explain the observed surface deformation, with no need of a sliver in central Chile. We identify four segments characterized by higher coupling and separated by narrow areas of lower coupling. This segmentation is in good agreement with historical and recent seismicity in Chile. In particular, the La Serena Bay (30°S-28°S) where the locked zone vanishes is a stable boundary where historical events stopped. The Maule region that ruptured in feb. 27th of 2010 (Mw 8.8) from 38 S to 34 S, was characterized by a well developped fully locked zone that extents far indepth and narrows where the earthquake stopped propagating (San Antonio at 34°S and south of Arauco peninsula at 38°S). These narrow zones of lower coupling are often associated with irregular bathymetric or coastal features (fracture zones or peninsulas). Finally, coseismic and early post-seismic slip distribution of the Maule earthquake, occurring either in previously highly or weakly coupled zones, map a complex distribution of velocity-weakening and velocity-strengthening patches on the subduction interface.

  16. Seismic Velocity and Attenuation Images of the Nankai Subduction Zone: New Insight into Megathrust Earthquakes

    NASA Astrophysics Data System (ADS)

    Zhao, D.; Liu, X.

    2015-12-01

    Dapeng Zhao, Xin Liu (Tohoku University, Japan) Many large interplate earthquakes (M > 7) occurred on the megathrust fault of the Nankai subduction zone, where the young Philippine Sea plate is subducting beneath the Eurasian plate along the Nankai Trough. The most significant megathrust events in this region are the 1944 Tonankai (Mw 8.1), the 1946 Nankai (Mw 8.3) and the 1968 Hyuganada (Mw 7.5) earthquakes. The landward down-dip limit of the Nankai megathrust seismogenic zone is located at a depth of ~30-40 km, marked by the occurrence of episodic tremors and slips. The seaward up-dip limit is not very distinct, being generally at a depth of ~10 km and correlated with a suite of diagenetic to low-grade metamorphic processes. To clarify the causal mechanism of the megathrust earthquakes, we studied the detailed three-dimensional P and S wave velocity (Vp and Vs), attenuation (Qp and Qs), and Poisson's ratio (σ) structures of the SW Japan forearc, using a large number of high-quality arrival time and t* data measured precisely from seismograms of local earthquakes. The suboceanic earthquakes used are relocated precisely using sP depth phase and ocean bottom seismometer data. Our results show the existence of two prominent high-V, high-Q, and low-σ patches separated by low-V, low-Q, and high-σ anomalies in the Nankai megathrust zone. Megathrust earthquakes during 1900 to 2013 nucleated in or around the high-V, high-Q, and low-σ patches, which may represent strongly coupled areas (i.e., asperities) in the megathrust zone. This feature is very similar to that of the NE Japan megathrust zone where the great Tohoku-oki earthquake (Mw 9.0) occurred on 11 March 2011 (e.g., Zhao, 2015). These results indicate that structural heterogeneities in the megathrust zone, such as the subducting seafloor topography and compositional variations, control the nucleation of megathrust earthquakes.

  17. Subduction interface processes recorded by eclogite-facies shear zones (Monviso, W. Alps)

    NASA Astrophysics Data System (ADS)

    Angiboust, S.; Agard, P.; Raimbourg, H.; Yamato, P.; Huet, B.

    2011-11-01

    The Monviso ophiolite Lago Superiore Unit constitutes a well-preserved, almost continuous upper fragment of oceanic lithosphere subducted at c. 80 km depth, thereby providing a unique opportunity to study mechanical coupling processes and meter-scale fluid-rock interactions occurring at such depths in present-day subduction zones. It is made of (i) a variably thick (50-500 m) section of eclogitized basaltic crust (associated with minor calcschist lenses) overlying a 100-400 m thick metagabbroic body and of (ii) a c. 1 km thick serpentinite sole. We herein focus on the three major eclogite-facies shear zones found at the top of the unit, at the boundary between basalts and gabbros, and between gabbros and serpentinites, respectively. Strain localization occurred at lithological interfaces, irrespective of material strength. While ductile deformation dominates along the shear zones, local brittle behavior is demonstrated by the existence of numerous eclogite breccias of Fe-Ti metagabbros and widespread garnet fractures, possibly linked with intermediate-depth eclogite-facies (micro)seismicity. These m- to hm-sized fragments of Fe-Ti metagabbros were later sheared and disseminated within serpentinite schists along the gabbro-serpentinite boundary (Lower Shear zone; LSZ). Pervasive and focused fluid flow is attested in the LSZ by significant alteration of bulk rock compositions, weakening of the rocks and widespread crystallization of hydrous parageneses. By contrast, the Intermediate Shear zone (ISZ) shows evidence for more restricted, short-range fluid flow. The activity of both the ISZ and LSZ ceased during early lawsonite eclogite-facies exhumation, when deformation localized deeper within the serpentinite sole, allowing for the detachment (and preservation) of this large ophiolitic fragment.

  18. Preliminary models of normal fault development in subduction zones: lithospheric strength and outer rise deformation

    NASA Astrophysics Data System (ADS)

    Naliboff, J. B.; Billen, M. I.

    2010-12-01

    A characteristic feature of global subduction zones is normal faulting in the outer rise region, which reflects flexure of the downgoing plate in response to the slab pull force. Variations in the patterns of outer rise normal faulting between different subduction zones likely reflects both the magnitude of flexural induced topography and the strength of the downgoing plate. In particular, the rheology of the uppermost oceanic lithosphere is likely to strongly control the faulting patterns, which have been well documented recently in both the Middle and South American trenches. These recent observations of outer rise faulting provide a unique opportunity to test different rheological models of the oceanic lithosphere using geodynamic numerical experiments. Here, we develop a new approach for modeling deformation in the outer rise and trench regions of downgoing slabs, and discuss preliminary 2-D numerical models examining the relationship between faulting patterns and the rheology of the oceanic lithosphere. To model viscous and brittle deformation within the oceanic lithosphere we use the CIG (Computational Infrastructure for Geodynamics) finite element code Gale, which is designed to solve long-term tectonic problems. In order to resolve deformation features on geologically realistic scales (< 1 km), we model only the portion of the subduction system seaward of the trench. Horizontal and vertical stress boundary conditions on the side walls drive subduction and reflect, respectively, the ridge-push and slab-pull plate-driving forces. The initial viscosity structure of the oceanic lithosphere and underlying asthenosphere follow a composite viscosity law that takes into account both Newtonian and non-Newtonian deformation. The viscosity structure is consequently governed primarily by the strain rate and thermal structure, which follows a half-space cooling model. Modification of the viscosity structure and development of discrete shear zones occurs during yielding

  19. Source Mechanisms of Destructive Tsunamigenic Earthquakes occurred along the Major Subduction Zones

    NASA Astrophysics Data System (ADS)

    Yolsal-Çevikbilen, Seda; Taymaz, Tuncay; Ulutaş, Ergin

    2016-04-01

    Subduction zones, where an oceanic plate is subducted down into the mantle by tectonic forces, are potential tsunami locations. Many big, destructive and tsunamigenic earthquakes (Mw > 7.5) and high amplitude tsunami waves are observed along the major subduction zones particularly near Indonesia, Japan, Kuril and Aleutan Islands, Gulf of Alaska, Southern America. Not all earthquakes are tsunamigenic; in order to generate a tsunami, the earthquake must occur under or near the ocean, be large, and create significant vertical movements of the seafloor. It is also known that tsunamigenic earthquakes release their energy over a couple of minutes, have long source time functions and slow-smooth ruptures. In this study, we performed point-source inversions by using teleseismic long-period P- and SH- and broad-band P-waveforms recorded by the Federation of Digital Seismograph Networks (FDSN) and the Global Digital Seismograph Network (GDSN) stations. We obtained source mechanism parameters and finite-fault slip distributions of recent destructive ten earthquakes (Mw ≥ 7.5) by comparing the shapes and amplitudes of long period P- and SH-waveforms, recorded in the distance range of 30° - 90°, with synthetic waveforms. We further obtained finite-fault rupture histories of those earthquakes to determine the faulting area (fault length and width), maximum displacement, rupture duration and stress drop. We applied a new back-projection method that uses teleseismic P-waveforms to integrate the direct P-phase with reflected phases from structural discontinuities near the source, and customized it to estimate the spatio-temporal distribution of the seismic energy release of earthquakes. Inversion results exhibit that recent tsunamigenic earthquakes show dominantly thrust faulting mechanisms with small amount of strike-slip components. Their focal depths are also relatively shallow (h < 40 km). As an example, the September 16, 2015 Illapel (Chile) earthquake (Mw: 8.3; h: 26 km

  20. P wave anisotropic tomography of the Nankai subduction zone in Southwest Japan

    NASA Astrophysics Data System (ADS)

    Wang, Jian; Zhao, Dapeng

    2012-05-01

    The active subduction of the young Philippine Sea (PHS) plate and the old Pacific plate has resulted in significant seismic heterogeneity and anisotropy in Southwest (SW) Japan. In this work we determined a detailed 3-D P wave anisotropic tomography of the crust and upper mantle beneath SW Japan using ˜540,000 P wave arrival times from 5,249 local earthquakes recorded by 1095 stations. The PHS slab is imaged clearly as a high-velocity (high-V) anomaly which exhibits considerable lateral variations. Significant low-velocity (low-V) anomalies are revealed above and below the PHS slab. The low-V anomalies above the PHS slab may reflect the upwelling flow in the mantle wedge and the PHS slab dehydration, and they form the source zone of the arc volcanoes in SW Japan. The low-V zones under the PHS slab may reflect the upwelling flow in the big mantle wedge above the Pacific slab. The anisotropy in the crust and upper mantle is complex. In Kyushu, the P wave fast velocity direction (FVD) is generally trench-normal in the mantle wedge under the back-arc, which is consistent with the corner flow driven by the PHS slab subduction. The FVD is trench-parallel in the subducting PHS slab under Kyushu. We think that the intraslab seismicity is a potential indicator to the slab anisotropy. That is, the PHS slab with seismicity has kept its original fossil anisotropy formed at the mid-ocean ridge, while the aseismic PHS slab has reproduced the anisotropy according to its current deformation.

  1. Seismic Structure of the Northernmost Ryukyu Subduction Zone, Southward Offshore of Kyushu, Japan

    NASA Astrophysics Data System (ADS)

    Nishizawa, A.; Oikawa, M.; Kaneda, K.; Kasahara, J.

    2007-12-01

    The Philippine Sea plate subducts northwest beneath the Eurasian Plate along the Nankai Trough in the north and the Nansei-Shoto (Ryukyu) Trench in the south at a rate of about 50-60 mm/year. Magnitude 8-class earthquakes occurred repeatedly along the Nankai Trough in the northeastern neighborhood, while the largest earthquake ever recorded is at most 7.6 along the northernmost Ryukyu Trench in the southern neighborhood. We carried out five ocean bottom seismographic (OBS) and multi-channel seismic (MCS) profiles in the northernmost Ryukyu subduction zone and obtained seismic velocity structural models related to the characteristics of the earthquakes in this region. For each investigation in 2005 and 2006, we shot a tuned airgun array with a volume of 8,040 cubic inches at an interval of 200 m for the wide-angle seismic profiles and at 50 m for the MCS (480 channels, 60 folds) profiles. The OBSs were deployed at an average interval of 5 km, which provided us dense data of high quality. The OBS data were modeled by a tomographic inversion, two-dimensional ray tracing and synthetic seismograms. The obtained MCS records show distinct reflections from the subducting plate boundary beneath the landward slope of the Ryukyu Trench and low velocity (Vp < 4 km/s) and thick (~ 8 km) materials above and on the decollement zone are modeled from the OBS data. Both the MCS and OBS records give clear images of very rough sea bottom configuration of the northern extension of the Kyushu-Palau Ridge and Amami Plateau below the landward slope. Such strong undulations of the plate boundary may constrain the sizes and/or natures of the asperities in this region. In addition, the estimated position of the seismic asperity of 1968 Hyuga-nada earthquake (Mw 7.5) corresponds to high velocity materials ascending in our P-wave velocity model.

  2. Identification of source lithology at south segment of Kamchatka subduction zone

    NASA Astrophysics Data System (ADS)

    Gavrilenko, M.; Herzberg, C. T.; Portnyagin, M.; Ozerov, A.

    2012-12-01

    Kamchatka peninsula (Russia) is an island-arc with a complex geological history and structure. It has three distinct volcanic fronts, the origin of which is still debated. Moreover, a junction with the Aleutian Arc (at ~56°N) complicates the understanding of geodynamics at the region. However, the south part (from ~53°N) of Kamchatka peninsula is thought to be a "textbook case" of subduction zone with relatively rapid (over 8 cm/yr) near-normal convergence and a steep (over 50°) angle of subduction. Kamchatka is unusual in the world because its volcanoes contain a significant amount of primitive high MgO lavas that are rich in olivine crystals. Furthermore, high precision contents of Ni, Ca, and Mn can help to constrain the source lithology. Straub et al. (2008) reported high Ni contents on olivines from a limited number of samples from the Mexican Volcanic Front, and concluded that pyroxenite melting was important. Portnyagin et al. (2009) reported high precision Ni, Ca, and Mn contents of olivines from a wide range of volcanoes from Kamchatka, and similarly concluded that pyroxenite melting is widespread. We have extended the work of Portnyagin et al. (2009) by analyzing olivine phenocrysts from volcanoes in the southernmost Kamchatka peninsula. Our work confirms that there are regional variations in olivine phenocryst composition that likely arises from variations in pyroxenite composition, the amount of peridotite melt that mixes with pyroxenite melts, and a variable role played by magnetite fractionation. We conclude that pyroxenite melting is likely to be important in subduction zones world-wide, but its significance has been underestimated because of the general rarity of olivine-bearing high MgO lavas.

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

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2015-02-01

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

  5. Antigorite crystallographic preferred orientation as a potential temperature probe in subduction zones

    NASA Astrophysics Data System (ADS)

    Liu, W.; Zhang, J.; Liu, C.

    2012-12-01

    Antigorite, the high P/T polytype of serpentine, is considered an important component in the mantle wedge and the subducting slab of subduction zone. Its crystallographic preferred orientation (CPO) may play an important role in trench parallel anisotropy. However, there is significant discrepancies in antigorite CPO among natural serpentinites and experimental samples. Toward a better understanding the fabric development in antigorite, we have conducted preliminary shear experiments on a hot-pressed serpentinite at 500-650 degree C and 1.0-2.5 GPa, using a 5GPa Griggs-type deformation apparatus. The serpentinite was hot-pressed from a powdered serpentinized peridotite with different proportions of antigorite and olivine and a grain size of 20-70 um. The CPOs of antigorite are characterized by the (001) nearly parallel to the foliation plane, the [010] axes dominantly parallel to the lineation at 500-600 degree C and a girdle with multiple high concentrations in the foliation plane at 600-650 degree C, respectively. In addition, Katayama et al. (2009) reported a dominant [100](001) slip system at 300-400 degree C. These results suggest a transition of dominant slip direction in antigorite from [100] to [010] and to [hk0] with increasing temperature. Increasing pressure can increase slightly the fabric transition temperature in antigorite. In contrast to antigorite, the coexisting olivine develops random to weak C-type ([001](100)) fabrics, confirming a much greater strength of olivine. We propose that antigorite CPOs might be used as a temperature probe in subduction zones.

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

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

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

  7. Seismic Fault Zone Rocks from a Subduction Megathrust (Kodiak Is., AK)

    NASA Astrophysics Data System (ADS)

    Meneghini, F.; di Toro, G.; Moore, C. J.; Rowe, C. D.

    2008-12-01

    Subduction megathrusts nucleate some of the largest earthquakes on Earth, including the 1964 Mw9.2 Alaskan earthquake. We describe the fault zone and the fault rocks from the thickest slipping zone ever described in subduction complexes. The aim is to discriminate (microstructurally and chemically) fault rocks produced during seismic slip and to reconstruct the seismic cycle in the fault zone. In the ancient analogue of the active Alaskan subduction complex, cropping out in Kodiak Island, decimeter- thick cohesive black-colored layers are at the core of 10's of meters thick foliated cataclasites. The cataclasites are part of a melange regarded as a paleo-decollement active at 12 - 14 km in depth and 230 - 260 ° C. Each black layer is traced continuously for tens of meters along a single outcrop, and, through structural correlations, across 2 km of section along strike. The black rocks features a complex layering of glass-looking and granular-looking layers. "Glassy" and "granular" layers textures are composed of sub-rounded grains (< 100 micron) of quartz and albite floating in an ultrafine matrix (< 4 micron). In the matrix of glassy-looking layers, tabular microlites of albite are common, showing an oscillatory zoning typical of magmatic rocks that is absent in the cataclasites. "Granular" layers, are more tightly packed, less sorted, enriched in crushed feldspar microlites and depleted in phyllosilicates with respect to the "glassy" layers. XRF and XRPD analyses suggest chemical fractionation between the foliated cataclasites and the black rocks (e.g. enrichment in Na in the black rocks). Crosscutting relationships between granular- and glassy-like layers occur. Alternatively, flow and intrusion structures between the two layers are observed, suggesting that they flowed and deformed in a ductile fashion. Based on these observations, we hypothesize that the black rocks (1) are the result of frictional melting (glassy-looking layers) and fluidization (granular

  8. Geodetic, teleseismic, and strong motion constraints on slip from recent southern Peru subduction zone earthquakes

    NASA Astrophysics Data System (ADS)

    Pritchard, M. E.; Norabuena, E. O.; Ji, C.; Boroschek, R.; Comte, D.; Simons, M.; Dixon, T. H.; Rosen, P. A.

    2007-03-01

    We use seismic and geodetic data both jointly and separately to constrain coseismic slip from the 12 November 1996 Mw 7.7 and 23 June 2001 Mw 8.5 southern Peru subduction zone earthquakes, as well as two large aftershocks following the 2001 earthquake on 26 June and 7 July 2001. We use all available data in our inversions: GPS, interferometric synthetic aperture radar (InSAR) from the ERS-1, ERS-2, JERS, and RADARSAT-1 satellites, and seismic data from teleseismic and strong motion stations. Our two-dimensional slip models derived from only teleseismic body waves from South American subduction zone earthquakes with Mw > 7.5 do not reliably predict available geodetic data. In particular, we find significant differences in the distribution of slip for the 2001 earthquake from models that use only seismic (teleseismic and two strong motion stations) or geodetic (InSAR and GPS) data. The differences might be related to postseismic deformation or, more likely, the different sensitivities of the teleseismic and geodetic data to coseismic rupture properties. The earthquakes studied here follow the pattern of earthquake directivity along the coast of western South America, north of 5°S, earthquakes rupture to the north; south of about 12°S, directivity is southerly; and in between, earthquakes are bilateral. The predicted deformation at the Arequipa GPS station from the seismic-only slip model for the 7 July 2001 aftershock is not consistent with significant preseismic motion.

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

    PubMed

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

    2015-01-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

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

  11. Subduction zones: Not relevant to present-day problems of waste disposal

    USGS Publications Warehouse

    Silver, E.A.

    1972-01-01

    SUBDUCTION zones are considered to be sites of disposal for vast areas of the Earth's surface1, while new surface is generated simultaneously at rise crests2. Bostrom and Sherif3 suggest that the world's industrial and domestic waste be dumped into subduction zones at deep sea trenches to allow nature to complete the recycling process at geologically rapid rates of 5 to 10 cm/yr. They also point out that trenches are often sites of rapid rates of deposition and suggest that the dumped wastes would, speaking geologically, soon be buried. Francis4 suggests that canisters of toxic chemical and radioactive wastes could be dumped onto trench sediments and be expected to sink at rates of 20 m/yr, assuming that the mass of turbidites in the trench fill often spontaneously liquefies on shaking by earthquakes. The assumption is based on the supposed lack of evidence for deformed sediment in trenches. I will argue that the suggestion of Bostrom and Sherif3 is not useful for the next few dozen generations of human populations and will point out observational evidence to show that Francis's4 assumption is incorrectly founded. ?? 1972 Nature Publishing Group.

  12. Evaluating Earth degassing in subduction zones by measuring helium fluxes from the ocean floor

    NASA Astrophysics Data System (ADS)

    Lan, Tefang F.; Sano, Yuji; Yang, Tsanyao F.; Takahata, Naoto; Shirai, Kotaro; Pinti, Daniele L.

    2010-10-01

    Volatiles are lost from the Earth's mantle to the atmosphere, hydrosphere and the crust through subaerial and submarine volcanism. Quantifying the volatile sources bears fundamental information on a number of issues in Earth sciences, from the evolution of the atmosphere and oceans to the nature of chemical heterogeneity of the Earth's mantle. The primordial noble gas isotope 3He provides an unambiguous measure of the volatile flux from the mantle, yet so far in the ocean region; it has been only measured at a mid-ocean ridge. Here, we present original measurements of the 3He flux at the Mid-Okinawa Trough back-arc basin. The 3He flux was estimated from 3He/ 20Ne vertical profiles measured in deep-sea sediment pore water. Diffusive 3He fluxes vary from 1.6 3He atoms cm -2 s -1 at the hydrothermally active Izena Cauldron to 0.57 3He atoms cm -2 s -1 at the background site, 13 km away. These values are about 20% of the 3He flux measured at the East Pacific Rise, supporting the never-proven hypothesis that 3He mantle flux from subduction zones is a quarter of that at MOR. Measured ocean-floor 4He flux ranges from 3.3 × 10 5 to 4.8 × 10 54He atoms cm -2 s -1, higher than that measured worldwide, suggesting that 4He flux at subduction zones might have been previously underestimated.

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

    PubMed

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

    2015-01-01

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

  14. Existing Instrumentation and Scientific Drivers for a Subduction Zone Observatory in Latin America

    NASA Astrophysics Data System (ADS)

    Frassetto, A.; Woodward, R.; Detrick, R. S.

    2015-12-01

    The subduction zones along the western shore of the Americas provide numerous societally relevant scientific questions that have yet to be fully explored and would make an excellent target for a comprehensive, integrated Subduction Zone Observatory (SZO). Further, recent discussions in Latin America indicate that there are a large number of existing stations that could serve as a backbone for an SZO. Such preexisting geophysical infrastructure commonly plays a vital role in new science initiatives, from small PI-led experiments to the establishment of the USArray Transportable Array, Reference Network, Cascadia Amphibious Array, and the redeployment of EarthScope Transportable Array stations to Alaska. Creating an SZO along the western coast of the Americas could strongly leverage the portfolio of existing seismic and geodetic stations across regions of interest. In this presentation, we will discuss the concept and experience of leveraging existing infrastructure in major new observational programs, outline the state of geophysical networks in the Americas (emphasizing current seismic networks but also looking back on historical temporary deployments), and provide an overview of potential scientific targets in the Americas that encompass a sampling of recently produced research results and datasets. Additionally, we will reflect on strategies for establishing meaningful collaborations across Latin America, an aspect that will be critical to the international partnerships, and associated capacity building, needed for a successful SZO initiative.

  15. Compressive sensing of frequency-dependent seismic radiation from subduction zone megathrust ruptures

    PubMed Central

    Yao, Huajian; Shearer, Peter M.; Gerstoft, Peter

    2013-01-01

    Megathrust earthquakes rupture a broad zone of the subducting plate interface in both along-strike and along-dip directions. The along-dip rupture characteristics of megathrust events, e.g., their slip and energy radiation distribution, reflect depth-varying frictional properties of the slab interface. Here, we report high-resolution frequency-dependent seismic radiation of the four largest megathrust earthquakes in the past 10 y using a compressive-sensing (sparse source recovery) technique, resolving generally low-frequency radiation closer to the trench at shallower depths and high-frequency radiation farther from the trench at greater depths. Together with coseismic slip models and early aftershock locations, our results suggest depth-varying frictional properties at the subducting plate interfaces. The shallower portion of the slab interface (above ∼15 km) is frictionally stable or conditionally stable and is the source region for tsunami earthquakes with large coseismic slip, deficient high-frequency radiation, and few early aftershocks. The slab interface at intermediate depths (∼15–35 km) is the main unstable seismogenic zone for the nucleation of megathrust quakes, typically with large coseismic slip, abundant early aftershocks, and intermediate- to high-frequency radiation. The deeper portion of the slab interface (∼35–45 km) is seismically unstable, however with small coseismic slip, dominant high-frequency radiation, and relatively fewer aftershocks.

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

    USGS Publications Warehouse

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

    2001-01-01

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

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

    PubMed

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

    2015-01-01

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

  18. Long Period Co-Seismic Gravity Modeling of Silent Slip Earthquakes Along the Cascadia Subduction Zone

    NASA Astrophysics Data System (ADS)

    Hayes, T. J.

    2004-05-01

    The Cascadia Subduction Zone (CSZ) is an area of large and potentially catastrophic seismic events which occur as large magnitude (Mm>8) events. The mitigation of such hazards within highly populated areas presents a difficult problem which is dependent upon such observations as plate motion and strain accumulation. Long period Bouguer anomalies may act as a proxy for permanent strain deformation at depth. To date there are no large scale models that successfully model the temporal gravity signal over extended spatial regions encompassing more than one fault. These deep slip events typically last for days to weeks which would generate a long period signal. The highly periodic (13--16 months) silent slip events along the Cascadia Subduction Zone (CSZ) present a ideal location for the observation of such long period signals. Models of co-seismic gravity changes based on the analytical solutions of Okada (1985) and Okubo (1992), which act as an upper limit, are in the range of 30 μ gals--800 μ gals. These amplitudes are well within the range of land based observations and potentially within the observable limits of several remote sensing satellites designed specifically for gravity data (e.g. GRACE, CHAMP, GEOS). This same technique should be applicable to any mechanism in which deformation occurs such as volcanic activity or glacial rebound.

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

    PubMed Central

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

    2015-01-01

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

  20. Shape of the subducted Rivera and Cocos plates in southern Mexico: Seismic and tectonic implications

    NASA Astrophysics Data System (ADS)

    Pardo, Mario; SuáRez, Gerardo

    1995-07-01

    The geometry of the subducted Rivera and Cocos plates beneath the North American plate in southern Mexico was determined based on the accurately located hypocenters of local and teleseismic earthquakes. The hypocenters of the teleseisms were relocated, and the focal depths of 21 events were constrained using a body wave inversion scheme. The suduction in southern Mexico may be approximated as a subhorizontal slab bounded at the edges by the steep subduction geometry of the Cocos plate beneath the Caribbean plate to the east and of the Rivera plate beneath North America to the west. The dip of the interplate contact geometry is constant to a depth of 30 km, and lateral changes in the dip of the subducted plate are only observed once it is decoupled from the overriding plate. On the basis of the seismicity, the focal mechanisms, and the geometry of the downgoing slab, southern Mexico may be segmented into four regions: (1) the Jalisco region to the west, where the Rivera plate subducts at a steep angle that resembles the geometry of the Cocos plate beneath the Caribbean plate in Central America; (2) the Michoacan region, where the dip angle of the Cocos plate decreases gradually toward the southeast, (3) the Guerrero-Oaxaca region, bounded approximately by the onshore projection of the Orozco and O'Gorman fracture zones, where the subducted slab is almost subhorizontal and underplates the upper continental plate for about 250 km, and (4) the southern Oaxaca and Chiapas region, in southeastern Mexico, where the dip of the subduction gradually increases to a steeper subduction in Central America. These drastic changes in dip do not appear to take place on tear faults, suggesting that smooth contortions accommodate these changes in geometry. The inferred 80 and 100 km depth contours of the subducted slab lie beneath the southern front of the Trans-Mexican Volcanic Belt, suggesting it is directly related to the subduction. Thus the observed nonparallelism with the Middle

  1. Decimeter Scale Ultra-Fine Fault Rocks (Possible Pseudotachylites) in an Ancient Subduction Thrust Zone

    NASA Astrophysics Data System (ADS)

    Rowe, C. D.; Moore, J. C.; Meneghini, F.; McKiernan, A. W.

    2004-12-01

    Large bodies of ultrafine fault rock (possible pseudotachylite or frictional melt) occur within cataclastic thrust zones in the Ghost Rocks Formation, Kodiak Accretionary Complex, Alaska. The Paleocene Ghost Rocks Formation includes map-scale mélange belts formed by flattening and shearing of seafloor sediments and volcanic rocks at about 250 degrees C and 325 MPa (~13 km depth) during subduction between 65-60Ma. Ten to 15-meter thick cataclastite zones crosscut the mélange fabric at a low angle, representing a stage of increasingly localized shear during subduction thrusting. Ultrafine fault rocks occur as thick (10-25cm) continuous planar beds along the boundaries of cataclastites, or in discontinuous accumulation bodies within cataclastite zones. The boundaries of the ultrafine fault rocks are intrusive, sharp but irregular and deform the cataclastite host fabric. Single pulse intrusions of the ultrafine fault rock range up to 0.5m in intrusive dimension and form complex morphologies resembling both upward and downward directed flame structures and dike-sill complexes, as well as sheath folds and disharmonic flow banding and folding. These field characteristics indicate fluidization and perhaps frictional melting of the ultrafine fault rocks. Ultrafine fault rock bodies can be traced laterally for meters to tens of meters at individual outcrops and occur for about 2 km along strike. Preliminary SEM analysis reveals that the primary matrix material is physically and chemically homogenous down to few-micron scale, consistent with the field identification of pseudotachylite. Thin sections show rounded remnant quartz aggregates, typical of pseudotachylytes. Although some thin sections show suggest melting others may represent ultracataclastite. Some ultrafine fault rock material is rebrecciated and cataclastized to a fine scale, indicating reactivation of previous fault rock generation surfaces. These ultrafine fault rock zones represent the most highly deformed

  2. Modeling Activity of Very-Low-Frequency Earthquakes in Shallow Subduction Zone Considering Splay Faults and High Pore Pressure Zones

    NASA Astrophysics Data System (ADS)

    Shibazaki, B.; Ito, Y.; Ujiie, K.

    2010-12-01

    Recent observations reveal that very-low-frequency (VLF) earthquakes occur in the shallow subduction zones in the Nankai trough, Hyuganada, and off the coast of Tokachi, Japan (Obara and Ito, 2005; Asano et al., 2008; Obana and Kodaira, 2009). The ongoing super drilling project, Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE), involves sampling the core of seismogenic faults and conducting analyses, experiments, and in-situ borehole measurements at the Nankai trough where VLF earthquakes occur. The data obtained in this project will be used to develop a model of VLF earthquakes that integrates seismological observations, laboratory experimental results, and geological observations. In the present study, first, we perform 2D quasi-dynamic modeling of VLF earthquakes in an elastic half-space on the basis of a rate- and state-dependent friction law. We set a local unstable zone in a shallow stable zone. To explain very low stress drops and short recurrence intervals of VLF earthquakes, the effective stress is assumed to be around 0.2 MPa. The results indicate that VLF earthquakes are unstable slips that occur under high pore pressure conditions. The probable causes for the high pore pressure along the faults of VLF earthquakes are the sediment compaction and dehydration that occur during smectite-to-illite transition in the shallow subduction zone. Then, we model the generation process of VLF earthquakes by considering splay faults and the occurrences of large subduction earthquakes. We set the local unstable zones with high pore pressure in the stable splay fault zones. We assume the long-term average slip velocity of the splay faults, and that the shear stress is accumulated by the delay of the fault slip from the long-term slip motion. Depending on the frictional properties of the shallow splay faults, two types of VLF earthquakes can occur. When the effective stress is low all over the splay faults, the rupture of large earthquakes propagates to the

  3. Nonvolcanic tremor along the Oaxaca segment of the Middle America subduction zone

    NASA Astrophysics Data System (ADS)

    Brudzinski, Michael R.; Hinojosa-Prieto, HéCtor R.; Schlanser, Kristen M.; Cabral-Cano, Enrique; Arciniega-Ceballos, Alejandra; Diaz-Molina, Oscar; Demets, Charles

    2010-08-01

    The Oaxaca subduction zone is an ideal area for detailed studies of plate boundary deformation as rapid convergent rates, shallow subduction, and short trench-to-coast distances bring the thermally defined seismogenic and transition zones of the plate interface over 100 km inland. Previous analysis of slow slip events in southern Mexico suggests that they may represent motion in the transition zone, defining the downdip edge of future megathrust earthquakes. A new deployment consisting of broadband seismometers distributed inland along the Oaxaca segment provide the means to examine whether nonvolcanic tremor (NVT) signals can also be used to characterize the boundary between the seismogenic and transition zones. In this study, we established that NVT exists in the Oaxaca region based on waxing and waning of seismic energy on filtered day-long seismograms that were correlated across neighboring stations and were further supported by appropriate relative time moveouts in record sections and spectrograms with narrow frequency bands. Eighteen prominent NVT episodes that lasted upwards of a week were identified during the 15 months analyzed (June 2006 to September 2007), recurring as frequently as every 2-3 months in a given region. We analyze NVT envelope waveforms with a semiautomated process for identifying prominent energy bursts, and analyst-refined relative arrival times are inverted for source locations. NVT burst epicenters primarily occur between the 40-50 km contours for depth of the plate interface, except in eastern Oaxaca where they shift toward the 30 km contour as the slab steepens. NVT hypocenters correlate well with a high conductivity zone that is interpreted to be due to slab fluids. NVT is more frequent, shorter in duration, and located further inland than GPS-detected slow slip, while the latter is associated with a zone of ultra-slow velocity interpreted to represent high pore fluid pressure. This zone of slow slip corresponds to approximately 350

  4. A self-consistent mechanism for slow dynamic deformation and large tsunami generation for earthquakes in the shallow subduction zone

    NASA Astrophysics Data System (ADS)

    Ma, Shuo

    2012-06-01

    Dynamic pore pressure changes in the overriding wedge above a shallow-dipping plate interface significantly affect the rupture dynamics of shallow subduction zone earthquakes and their tsunamigenesis. For a wedge on the verge of Coulomb failure everywhere including the basal fault, the dynamic pore pressure increase due to up-dip rupture propagation leads to widespread yielding within the wedge, which is greatly enhanced by the shallow dip of the fault. The widespread yielding reduces the stress drop, slip velocity, slip, and rupture velocity, giving rise to prolonged rupture duration, thus explaining many anomalous features of shallow subduction zone earthquakes. Significant inelastic seafloor uplift occurs in the case of a shallow fault dip, with the largest uplift located landward from the trench. Integrating this physical mechanism with existing seismic, geodetic, and tsunami observations can provide new insights into earthquake dynamics and deformation processes in shallow subduction zones.

  5. Reflected and mode-converted seismic waves within the shallow aleutian subduction zone, southern Kenai Peninsula, Alaska

    USGS Publications Warehouse

    Stephens, C.D.; Page, R.A.; Lahr, J.C.

    1990-01-01

    Pronounced secondary phases observed in local recordings of quarry shots and earthquakes on the southern Kenai Peninsula are identified as reflected P and S and converted S-to-P phases originating within four depth ranges: in the upper few kilometers of the Cook Inlet Tertiary basin, at midcrustal depths within the overthrust North American plate, at about 35 km depth near the top of the Wadati-Benioff seismic zone in proximity to the inferred interplate megathrust, and at about 5-10 km below the megathrust in the subducted Pacific plate. The positions and origins of the mid-upper plate (MUP) discontinuity and similar reflectors are discussed. It appears the the MUP discontinuity is seismically inactive and does not represent a brittle-ductile transition zone within the upper plate. The two converted S-to-P phases generated near the top of the subducted plate could indicate a low velocity zone associated with subducted oceanic crust. -after Authors

  6. Nanoscale Properties of Rocks and Subduction Zone Rheology: Inferences for the Mechanisms of Deep Earthquakes

    NASA Astrophysics Data System (ADS)

    Riedel, M. R.

    2007-12-01

    Grain boundaries are the key for the understanding of mineral reaction kinetics. More generally, nanometer scale processes involved in breaking and establishing bonds at reaction sites determine how and at which rate bulk rock properties change in response to external tectonic forcing and possibly feed back into various geodynamic processes. A particular problem is the effects of grain-boundary energy on the kinetics of the olivine-spinel phase transformation in subducting slabs. Slab rheology is affected in many ways by this (metastable) mineral phase change. Sluggish kinetics due to metastable hindrance is likely to cause particular difficulties, because of possible strong non-linear feedback loops between strain-rate and change of creep properties during transformation. In order to get these nanoscale properties included into thermo-mechanical models, reliable kinetic data is required. The measurement of grain-boundary energies is, however, a rather difficult problem. Conventional methods of grain boundary surface tension measurement include (a) equilibrium angles at triple junction (b) rotating ball method (c) thermal groove method, and others (Gottstein & Shvindlerman, 1999). Here I suggest a new method that allows for the derivation of grain-boundary energies for an isochemical phase transformation based on experimental (in-situ) kinetic data in combination with a corresponding dynamic scaling law (Riedel and Karato, 1997). The application of this method to the olivine-spinel phase transformation in subducting slabs provides a solution to the extrapolation problem of measured kinetic data: Any kinetic phase boundary measured at the laboratory time scale can be "scaled" to the correct critical isotherm at subduction zones, under experimentelly "forbidden" conditions (Liou et al., 2000). Consequences for the metastability hypothesis that relates deep seismicity with olivine metastability are derived and discussed. References: Gottstein G, Shvindlerman LS (1999

  7. Role of iron content on serpentinite dehydration depth in subduction zones: Experiments and thermodynamic modeling

    NASA Astrophysics Data System (ADS)

    Merkulova, Margarita; Muñoz, Manuel; Vidal, Olivier; Brunet, Fabrice

    2016-11-01

    A series of dehydration experiments in the piston-cylinder apparatus was carried out at 2 GPa and 550-850 °C on a natural antigorite sample mixed with 5 wt.% of magnetite. Chemical analyses of experimental products show a progressive decrease of the Mg# in antigorite and clinopyroxene between 550 and 675 °C, whereas the Mg# of olivine increases. The observed behavior of Mg# signifies Fe-Mg exchange between coexisting minerals. At higher temperatures, between 700 and 850 °C, compositions remain stable for all minerals in experimental assemblages. Thermodynamic parameters of the ferrous antigorite end-member were refined with the use of Holland and Powell (1998) data set and added to the antigorite solid solution. Good agreement between theoretical calculations performed for the studied bulk composition and experimental results confirms extrapolated thermodynamic data for Fe-antigorite. Constrained parameters allowed to calculate phase relationships for various serpentinite compositions. First, we assessed the effect of bulk iron content, from 0 to 10 wt.% FeO, on the stability field of antigorite. The results show significant decrease of the antigorite thermal stability with increasing bulk Fe content. Second, we demonstrated the influence of bulk iron content on dehydration reactions in subduction zones along typical thermal gradients. Dehydration observed in pure MSH (MgO-SiO2-H2O) systems comprised of antigorite appears as a univariant reaction, which happens at 710 °C/3.7 GPa and 640 °C/6 GPa in "hot" and "cold" subduction, respectively. In contrast, more complex in composition Fe-bearing serpentinites show spread dehydration profiles through divariant reactions from ~ 300 °C/0.8 GPa to 700 °C/3.6 GPa and from 450 °C/4 GPa to 650 °C/7.4 GPa for "hot" and "cold" thermal gradients respectively. A comparison between depths of "water-release events" and "earthquake occurrence" in the South Chile slab ("hot" subduction) highlights a clear correlation between

  8. The River Network, Active Tectonics and the Mexican Subduction Zone, Southwest Mexico

    NASA Astrophysics Data System (ADS)

    Gaidzik, K.; Ramirez-Herrera, M. T.; Kostoglodov, V.; Basili, R.

    2014-12-01

    Rivers, their profiles and network reflect the integration of multiple processes and forces that are part of the fundamental controls on the relief structure of mountain belts. The motivation of this study is to understand active tectonic processes in the forearc region of subduction zones, by distinguishing evidence of active deformation using the river network and topography. To this end, morphotectonic and structural studies have been conducted on fifteen drainage basins on the mountain front, parallel to the Mexican subduction zone, where the Cocos plate underthrusts the North American plate. The southwest - northeast Cocos plate subduction stress regime initiated ca. 20 MA. NE-SW to NNE-SSW normal faults as well as sub-latitudinal to NW-SE strike-slip faults (both dextral and sinistral) constitute the majority of mesofaults recorded in the field within the studied drainage basins. Occasionally dextral N-S strike-slip faults also occur. The stress tensor reconstruction suggests two main evolution stages of these faults: 1) the older is dominated by a NW-SE to WNW-ESE extensional regime and 2) the younger is a transcurrent regime, with NNE-SSW σ1 axis. The drainage pattern is strongly controlled by tectonic features, whereas lithology is only a subordinate factor, with only one exception (Petatlán river). Generally, major rivers flow from north to south mainly through NE-SW and NNE-SSW normal faults, and/or sub-longitudinal dextral (also locally sinistral) strike-slip faults. In the central and eastern part of the studied area, rivers also follow NW-SE structures, which are generally normal or sinistral strike-slip faults (rarely reverse). In most cases, local deflections of the river main courses are related to sub-latitudinal strike-slip faults, both dextral and sinistral. Within the current stress field related to