Science.gov

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. Rheology of magnesite and implications for subduction zone dynamics

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

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

  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. Shear deformation of lawsonite blueschist at high pressures and implications for earthquakes in the subduction zones

    NASA Astrophysics Data System (ADS)

    Jung, Haemyeong; Choi, Seungsoon; Jung, Sejin

    2017-04-01

    Recent seismological observations indicate that many earthquakes occur at the top of subducting slabs where oceanic crust is transformed to blueschist facies rocks under high pressure and temperature conditions. Episodic slip and tremor (ETS) events and low frequency earthquakes (LFEs) and intermediate-depth earthquakes in cold subduction zones often occur where lawsonite blueschist is stable at the top of the subducting slab, but the mechanism of these earthquakes is still poorly constrained because of a lack of laboratory measurements of rock properties (i.e., lawsonite blueschist) in shear experiments at various conditions reflecting the source region of these earthquakes. Here we report the results of experimental deformation of lawsonite blueschist under high pressure and temperature conditions consistent with the stability field of lawsonite blueschist. Our data show that lawsonite blueschist deforms cataclastically at high pressures (1-2 GPa), producing faults and slip weakening through the formation of nanoparticles and amorphous phases along highly localized faults. Our results have important implications for the understanding of seismogenesis and the mechanism behind ETS/LFEs and intermediate-depth earthquakes in cold subduction zones.

  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. State of stress and age offsets at oceanic fracture zones and implications for subduction initiation

    NASA Astrophysics Data System (ADS)

    Johnston, M. D.; Long, M. D.; Silver, P. G.

    2009-12-01

    The recycling of oceanic lithosphere back into the Earth’s interior through subduction is a central component of plate tectonics. The process by which new subduction zones initiate, however, remains poorly understood. A variety of mechanisms has been proposed by which a new subduction zone might initiate, including passive margin collapse (aided by sediment loading and/or rheological weakening due to the presence of volatiles) and forced convergence across a zone of preexisting lithospheric weakness. In this paper we focus on the latter model, which identifies three conditions necessary for subduction initiation: a zone of weakness such as a fracture zone, an age (and therefore density) offset along the fracture zone, and significant normal compressive stress which leads to shortening. We identify regions on the present-day Earth that meet these conditions and that may correspond to regions of relatively likely subduction initiation in the near future. Using a digital seafloor age model, we have created a database of oceanic fractures and quantified the associated age offsets. We have evaluated the state of stress on these lithospheric weak zones using two different global stress models. We find that the conditions needed to initiate subduction via the forced convergence model are relatively rare on the present-day Earth, and that there is no indication of incipient subduction at regions identified as relatively likely for subduction initiation. Using the same technique, we have evaluated the state of stress and seafloor age offset at regions of inferred present-day incipient subduction, and find that most of these regions are not associated with both high far-field compressive stresses and large age (and thus density) offsets. Subduction has likely initiated via forced convergence across preexisting zones of lithospheric weakness in the past, but our results indicate that the conditions needed for this type of subduction initiation are rare on the present

  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. Implications for metal and volatile cycles from the pH of subduction zone fluids

    NASA Astrophysics Data System (ADS)

    Galvez, Matthieu E.; Connolly, James A. D.; Manning, Craig E.

    2016-11-01

    The chemistry of aqueous fluids controls the transport and exchange—the cycles—of metals and volatile elements on Earth. Subduction zones, where oceanic plates sink into the Earth’s interior, are the most important geodynamic setting for this fluid-mediated chemical exchange. Characterizing the ionic speciation and pH of fluids equilibrated with rocks at subduction zone conditions has long been a major challenge in Earth science. Here we report thermodynamic predictions of fluid-rock equilibria that tie together models of the thermal structure, mineralogy and fluid speciation of subduction zones. We find that the pH of fluids in subducted crustal lithologies is confined to a mildly alkaline range, modulated by rock volatile and chlorine contents. Cold subduction typical of the Phanerozoic eon favours the preservation of oxidized carbon in subducting slabs. In contrast, the pH of mantle wedge fluids is very sensitive to minor variations in rock composition. These variations may be caused by intramantle differentiation, or by infiltration of fluids enriched in alkali components extracted from the subducted crust. The sensitivity of pH to soluble elements in low abundance in the host rocks, such as carbon, alkali metals and halogens, illustrates a feedback between the chemistry of the Earth’s atmosphere-ocean system and the speciation of subduction zone fluids via the composition of the seawater-altered oceanic lithosphere. Our findings provide a perspective on the controlling reactions that have coupled metal and volatile cycles in subduction zones for more than 3 billion years7.

  10. Implications for metal and volatile cycles from the pH of subduction zone fluids.

    PubMed

    Galvez, Matthieu E; Connolly, James A D; Manning, Craig E

    2016-11-17

    The chemistry of aqueous fluids controls the transport and exchange-the cycles-of metals and volatile elements on Earth. Subduction zones, where oceanic plates sink into the Earth's interior, are the most important geodynamic setting for this fluid-mediated chemical exchange. Characterizing the ionic speciation and pH of fluids equilibrated with rocks at subduction zone conditions has long been a major challenge in Earth science. Here we report thermodynamic predictions of fluid-rock equilibria that tie together models of the thermal structure, mineralogy and fluid speciation of subduction zones. We find that the pH of fluids in subducted crustal lithologies is confined to a mildly alkaline range, modulated by rock volatile and chlorine contents. Cold subduction typical of the Phanerozoic eon favours the preservation of oxidized carbon in subducting slabs. In contrast, the pH of mantle wedge fluids is very sensitive to minor variations in rock composition. These variations may be caused by intramantle differentiation, or by infiltration of fluids enriched in alkali components extracted from the subducted crust. The sensitivity of pH to soluble elements in low abundance in the host rocks, such as carbon, alkali metals and halogens, illustrates a feedback between the chemistry of the Earth's atmosphere-ocean system and the speciation of subduction zone fluids via the composition of the seawater-altered oceanic lithosphere. Our findings provide a perspective on the controlling reactions that have coupled metal and volatile cycles in subduction zones for more than 3 billion years(7).

  11. Field-based evidence for devolatilization in subduction zones: Implications for Arc magmatism

    SciTech Connect

    Bebout, G.E. )

    1991-01-25

    Metamorphic rocks on Santa California Island, 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 sediment 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.

  12. 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.

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

    NASA Astrophysics Data System (ADS)

    Nishikawa, T.; Ide, S.

    2013-12-01

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

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

    NASA Astrophysics Data System (ADS)

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

    1995-06-01

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

  15. The Behavior of Li in Subduction Zones with Implications for Fluid Cycling

    NASA Astrophysics Data System (ADS)

    Kastner, M.

    2008-12-01

    The chemical and isotopic compositions of pore fluids provide important insights on fluid-rock diagenetic or metamorphic reactions, hence, on the subsurface hydrology. Li is one of the most prominent tracers used for these objectives. Like the other alkali elements it strongly partitions into the fluid-phase, in particular at moderate to elevated temperatures. The magnitude of the partition is strongly temperature dependent. Lui Chan who was a world leader on Li and its isotopes for tracing fluid reactions and cycling focused on processes at plate boundaries. In subduction zones she recorded the behavior of Li from the incoming plate to the arc volcanics, and concluded that the variability from incoming plate to arc reflects the nature of the subducted material. Data from two subduction zones, Costa Rica, and Nankai Trough, will be presented. Recent hydrothermal experiments by Wei Wei on MORB-seawater and smectite-seawater, 35-350°C at 25°C steps, and 600 bars, greatly expanded the data-base, thus, insight, on the behavior of Li. The results indicate that Li is released into the fluid-phase throughout the temperature range of the experiments, with a strong threshold of significant release at ~250°C; indeed, Li concentrations increase in fluids with depth in subduction zones. Accordingly, because clay-rich sediments and altered oceanic crust are enriched in Li and the Li isotope values are lower than the seawater value, the fluids that migrate up-dip from a deeper source into the ocean should have a lower isotope signature, eventually approaching the source material, as observed in the pore fluids of the décollement zones at the Costa Rica and Nankai Trough subduction zones. The recent recovery of formation fluids at two sites at the Costa Rica subduction zone provide for the first time two year records on temporal variations (1) on the chemistry of the incoming plate upper basement formation fluid, (2) on the décollement fluid at 0.6 km arcward of the

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

    USGS Publications Warehouse

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

    2005-01-01

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

  17. Global systematics of formation conditions of subduction zone magmas and their tectonic implications

    NASA Astrophysics Data System (ADS)

    Ogitsu, I.; Ozawa, K.

    2009-12-01

    Subduction zone magmatism plays an important role in material recycling of the earth’s interior, and it is imperative to understand melt generation mechanisms in the wedge mantle. The subduction zone is more complicated and diverse as a magma generation site than that of mid-ocean ridge or hot spot mostly because of the more significant and variable contribution of H2O-rich fluid expelled from the subducting slab, and because of the complexity of thermal and flow structures in the wedge mantle. The spatial variations of compositions of primary magmas in subduction zones have been recognized (e.g., Kuno, 1966; Sakuyama and Nesbitt, 1986), and are attributed to the diversity of melt segregation depth, degree of melting, or involvement of hydrous fluids with peculiar geochemical signature. It is particularly critical if melting at subduction zone is controlled mostly by the addition of H2O-rich fluid (Tatsumi, 1986; Iwamori, 1998) or by thermal and flow structure in the wedge mantle (Plank and Langmuir, 1993; Schmidt and Poli, 1998). In order to address this issue, quantitative estimation of melting conditions with clarification of the critical tectonic factors controlling magma generation is requisite and has been attempted in many studies with limited success. In this study, melting conditions of frontal volcanoes of world subduction zones are quantitatively estimated on the basis of major element composition of volcanic rocks. For quantitative estimation of melting conditions, we adopted appropriate models not only for mantle melting but also for fractional crystallization in the crust, simultaneous determination of both of which ensures the consistency of estimation procedure (Ogitsu and Ozawa, in prep.). Unknown parameters are optimized by least-squares method with input from a major element data of volcanic rocks. This approach is applicable to more differentiated rocks, which are not suitable for olivine addition methods widely used in estimation of primary

  18. 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).

  19. Splay Fault Branching from the Hikurangi Subduction Shear Zone: Implications for Slow Slip and Fluid Flow

    NASA Astrophysics Data System (ADS)

    Henrys, S. A.; Plaza-Faverola, A. A.; Pecher, I. A.; Klaeschen, D.; Wallace, L.

    2016-12-01

    Seismic reflection data along the East Coast of the New Zealand North Island are used to map the offshore character and geometry of the central Hikurangi subduction thrust and outer wedge in a region of short term ( 2-3 weeks duration) geodetically determined slow-slip events (SSEs). Pre-stack depth migration of line 05CM-38 was used to derive subducting slab geometry and upper crustal structure together with a Vp image of the crust that is resolved to 14 km depth. The subduction interface is a shallow dipping thrust at < 7 km deep near the trench and steps down to 14 km depth along an approximately 18 km long ramp, beneath Porangahau Ridge. This bend in the subducted plate is associated with splay fault branching and coincides with the zone of maximum slip (90 mm) inferred on the subduction interface during slow slip events in June and July 2011. We infer that the step down in the décollement transfers slip on the plate interface from the top of subducting sediments to the oceanic crust and drives underplating beneath the inner margin of central Hikurangi margin. Low-velocity subducting sediments (LVZ) beneath the plate interface, updip of the plate interface ramp, are interpreted as being capped with a low permeability condensed layer of chalk and interbedded mudstones. We interpret this LVZ as fluid-rich overpressured sediments that have been displaced and later imbricated by splay faults in a region that may mark the up-dip transition from seismic to aseismic behavior. Further, we hypothesize that fluids derived from the overpressured sediment are channeled along splay faults to the shallow sub-seafloor near Porangahau Ridge where seafloor seepage and an upwarping of the gas hydrate Bottom-Simulating Reflector have been documented.

  20. Splay fault branching from the Hikurangi subduction shear zone: Implications for slow slip and fluid flow

    NASA Astrophysics Data System (ADS)

    Plaza-Faverola, A.; Henrys, S.; Pecher, I.; Wallace, L.; Klaeschen, D.

    2016-12-01

    Prestack depth migration data across the Hikurangi margin, East Coast of the North Island, New Zealand, are used to derive subducting slab geometry, upper crustal structure, and seismic velocities resolved to ˜14 km depth. We investigate the potential relationship between the crustal architecture, fluid migration, and short-term geodetically determined slow slip events. The subduction interface is a shallow dipping thrust at <7 km depth near the trench and steps down to 14 km depth along an ˜18 km long ramp, beneath Porangahau Ridge. This apparent step in the décollement is associated with splay fault branching and coincides with a zone of maximum slip (90 mm) inferred on the subduction interface during slow slip events in June and July 2011. A low-velocity zone beneath the plate interface, updip of the plate interface ramp, is interpreted as fluid-rich overpressured sediments capped with a low permeability condensed layer of chalk and interbedded mudstones. Fluid-rich sediments have been imbricated by splay faults in a region that coincides with the step down in the décollement from the top of subducting sediments to the oceanic crust and contribute to spatial variation in frictional properties of the plate interface that may promote slow slip behavior in the region. Further, transient fluid migration along splay faults at Porangahau Ridge may signify stress changes during slow slip.

  1. P-wave tomography of subduction zones around the central Philippines and its geodynamic implications

    NASA Astrophysics Data System (ADS)

    Fan, Jianke; Zhao, Dapeng; Dong, Dongdong; Zhang, Guangxu

    2017-09-01

    High-resolution tomographic images are obtained by inverting a large number of arrival-time data of local earthquakes and teleseismic events to depict the 3-D crustal and upper mantle structure beneath the central Philippines. Our tomographic results show that the subducted South China Sea slab beneath the southern segment of the Manila Trench steepens and tears, resulting in migration of the locus of active volcanism in the Macolod Corridor, due to the collision between the Palawan microcontinental block and the Philippine Mobile Belt. The subduction of the Philippine Sea Plate along the Philippine Trench started at 10-12°N or south of 12°N, the central part of the trench, from at least ∼10 Ma estimated from our tomographic images. Our results reveal clearly a high-velocity anomaly in and around the mantle transition zone, which is interpreted as the subducted Proto South China Sea slab that sinks deeper southeastward, being well consistent with geological results that the age of collision between the Palawan microcontinental block and the Philippine Mobile Belt becomes younger from the south to the north. This collision zone can be divided into northern and southern segments, demarcated by the salient point of the collision zone, which is probably the boundary between the South China Sea slab and the Proto South China Sea slab, and may be ascribed to the complete consumption of the two slabs.

  2. Crystallographic preferred orientation (CPO) of antigorite from the Motagua fault zone, Guatemala: Implications for subduction zone seismic anisotropy

    NASA Astrophysics Data System (ADS)

    Brownlee, S. J.; Seward, G.; Hacker, B. R.; Harlow, G. E.

    2010-12-01

    Serpentinites play an important role in subduction zones. They are a key factor for many important subduction zone processes including fluid transport, deformation localization, and seismicity (e.g. Bezacier et al. 2010, Hilairet et al. 2007). Antigorite, the stable serpentine mineral at high temperature, has single crystal seismic anisotropy of 46% in Vp, and 66% in Vs (Bezacier et al. 2010). Because of the high single crystal anisotropy, and the likelihood that antigorite will have a CPO in deformed subduction zone rocks, the seismic anisotropy resulting from the presence of antigorite is a property worth exploring. Antigorite CPOs have been measured by X-ray diffraction (e.g. Kern et al. 1997), and most recently by electron backscatter diffraction (EBSD) (Van de Moortele et al. 2010). The previously reported EBSD results are limited to one sample of antigorite from the high-pressure Escambray massif in central Cuba (Van de Moortele et al. 2010). It is important to understand the range of CPOs exhibited in naturally deformed antigorite to properly interpret seismic data from subduction zones. This study reports CPO data collected using the EBSD technique from a suite of samples from the Motagua fault zone in Guatemala. The samples are from the north and south sides of the Motagua fault, which have experienced different subduction and exhumation histories. They are almost pure antigorite with very little protolith remaining, so the influence of other phases on antigorite CPO, and seismic properties will be minimal. CPO results are combined with single crystal elastic properties to calculate the resulting seismic velocities and anisotropies.

  3. Lattice preferred orientation of talc and implications for seismic anisotropy in subduction zones

    NASA Astrophysics Data System (ADS)

    Lee, Jungjin; Jung, Haemyeong; Klemd, Reiner

    2017-04-01

    Since hydrous phases such as talc and serpentine are elastically very anisotropic, the lattice preferred orientation (LPO) of both minerals when formed in the mantle wedge or the subducting slab can cause large seismic anisotropies in subduction zones. Although, fabric studies of talc-associated phases (e.g., serpentine, amphibole) have been reported, up to now no quantitative measurements of the talc LPO have been conducted. In order to examine the LPO of talc, SEM/EBSD analyses were performed on highly deformed garnet-chloritoid-talc schists from the Makbal UHP terrane in the Tianshan orogen (Kazakhstan). These rocks underwent subduction-related eclogite-facies metamorphism corresponding to a burial depth of ca. 92 km (P ≅ 2.9 GPa). The samples contain between 20 and 40 vol. % talc. The LPO results showed that talc has a strong alignment of [001] axes subnormal to the foliation and, in addition, the [100] and [010] axes display a weak concentration with a girdle subparallel to the foliation. The seismic anisotropy of the polycrystalline talc was calculated using the obtained LPO and the pressure-dependent elastic constants of single-crystal talc. The magnitude of the seismic anisotropy of talc due to its LPO was 68‒69 % for P-waves and 21‒23 % for S-waves under the ambient pressure. The seismic anisotropies of talc decreased to 36‒37 % for P-waves and 13‒17 % for S-waves under high pressure (2.9 GPa), however they still remained high. The polarization direction of vertically propagating fast S-waves of the talc was trench-parallel and it was influenced by the strength of talc LPO of both [100] and [010] axes, pressure, and the dipping angle of the subducting slab. Our results indicate that the presence of strong LPO of talc in the hydrated mantle can contribute significantly to the trench-parallel seismic anisotropy and long delay time of S-waves observed in many subduction zones.

  4. 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.

  5. 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

  6. 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

  7. Electrical conductivity of the Cascadia subduction zone and implications for the plate interface

    NASA Astrophysics Data System (ADS)

    Livelybrooks, D.; Bedrosian, P.; Egbert, G. D.; Key, K.; Schultz, A.; Parris, B. A.; Yang, B.; Bowles-martinez, E.

    2016-12-01

    The Magnetotelluric Observations of Cascadia using a Huge Array (MOCHA) experiment resulted in the collection of 146 amphibious, long-period magnetotelluric stations acquired between 2012 and 2014. These data, supplemented with the previously-acquired CAFÉ, EMSLAB, SWORMT and EarthScope (MT) Transportable Array stations, have been interpreted to provide electrical conductivity models of Cascadia spanning from the trench eastward through the Cascades, and extending to about 150km depth. We have a particular interest in understanding the roles electrically-conductive, aqueous fluids play in Cascadia subduction processes at or near the plate interface, thus inversions of data are predisposed to accommodate an initially-resistive (McCrory et al. 2014) slab. Beginning at the mantle wedge corner, 3-D inversions reveal significant, latitudinal variation in the conductivity, with enhanced conductivity at 47oN and south at 42oN. Two-dimensional inversions at 44.5oN allowing for a step discontinuity at the Moho give two distinct zones of conductance, one at the MWC tip (c.f. Furukowa, 2009) and another further down-dip, with a conductivity `plume' directed eastwards. At depths of between 20-25km we image a latitudinally-discontinuous resistive lower crust immediately overlying resistive subducted slab. This implies a lack of free fluids near the plate interface. Krogstad et al. (2016) have analyzed historic uplift data and can model the presence of an inboard `secondary locked zone' near 44.5oN. One explanation for both observations—a down-dip, `pinned interface' that is shielding the traditionally-modeled off-shore locked zone from stress accumulation, would explain the paucity of seismicity observed off the north-central Oregon coast during the four-year Cascadia Initiative. At coastal longitudes a narrow, supra-slab conductive zone is imaged at 22km depth with a southern termination at 45oN. It is notable that some researchers place the inboard boundary of the (mostly

  8. Thermal considerations in inferring frictional heating from vitrinite reflectance and implications for shallow coseismic slip within the Nankai Subduction Zone

    NASA Astrophysics Data System (ADS)

    Fulton, P. M.; Harris, R. N.

    2011-12-01

    Frictional properties within the upper few kilometers of subduction zones are generally thought to inhibit rupture propagation. Understanding whether rupture propagates to the surface during large megathrust earthquakes is important for characterizing tsunami hazard. Recent vitrinite reflectance analysis by Sakaguchi et al. [2011] on IODP cores from the megasplay and frontal thrust of the Nankai Subduction Zone have been interpreted to suggest that these faults reached temperatures ~ 380 oC, considerably larger than background values, implying they hosted coseismic slip at shallow depths. Analysis of other temperature proxies on the megasplay by Hirono et al. [2009], however, suggest temperatures have not exceeded 300 oC and are inconclusive as to whether the fault slipped coseismically. Here we evaluate the effects of frictional heat generation on the spatial distribution of vitrinite reflectance and the sensitivity to slip zone thickness, slip duration, and the cumulative effects of numerous events. Using this analysis we show how the vitrinite reflectance data from Nankai can be reconciled with the inorganic chemistry data. We also integrate our resulting heat generation estimates with friction data from these faults to place quantitative constraints on the characteristic amount of slip each has supported during large earthquakes. These results suggest both faults experienced large coseismic slip, with the megasplay hosting considerably more than the frontal thrust. Our results provide insight into the shallow slip behavior of large megathrust earthquakes and their associated geologic signature, and have important implications for earthquake and tsunami hazard. Hirono, T. et al. (2009), Estimation of temperature rise in a shallow slip zone of the megasplay fault in the Nankai Trough, Tectonophysics, 478, 215-220, doi: 10.1016/j.tecto.2009.08.001. Sakaguchi, A. et al. (2011), Seismic slip propagation to the updip end of plate boundary subduction interface faults

  9. Mantle transition zone discontinuities beneath the Indochina Peninsula: Implications for slab subduction and mantle upwelling

    NASA Astrophysics Data System (ADS)

    Yu, Youqiang; Gao, Stephen S.; Liu, Kelly H.; Yang, Ting; Xue, Mei; Le, Khanh Phon

    2017-07-01

    While the northward indentation of the Indian into Eurasian plates has been intensively investigated, its oblique subduction beneath the Indochina Peninsula (ICP) and the role it played on mantle structure and dynamics remain enigmatic. In this first regional-scale receiver function study of the mantle transition zone (MTZ) discontinuities beneath the ICP and its surrounding areas, we stack ˜12,000 receiver functions recorded at 33 stations using a non-plane wave common-conversion-point stacking technique. Systematic spatial variations of MTZ thickness with departures between -21 and +24 km from the globally averaged value are revealed, providing independent evidence for the presence of slab segments in the MTZ beneath the central and a slab window beneath the western ICP. The results also support the existence of broad mantle upwelling adjacent to the eastern edge of the slab segments, which might be responsible for the widespread Cenozoic volcanisms and pervasively observed upper mantle low velocities in the area.

  10. State of stress and age offsets at oceanic fracture zones and implications for the initiation of subduction

    NASA Astrophysics Data System (ADS)

    Johnston, Mignon D.; Long, Maureen D.; Silver, Paul G.

    2011-11-01

    The recycling of oceanic lithosphere back into the Earth's interior through subduction is a central component of plate tectonics. The process by which new subduction zones initiate, however, remains poorly understood. Several different mechanisms for subduction initiation have been proposed, including passive margin collapse (aided by sediment loading and/or rheological weakening due to the presence of volatiles) and forced convergence across a zone of preexisting lithospheric weakness. In this paper we focus on the latter type of model, which identifies three conditions necessary for subduction initiation: a zone of weakness such as a fracture zone, an age (and therefore density) offset along the fracture zone, and significant normal compressive stress which leads to shortening. We identify regions on the present-day Earth which meet these conditions and which may correspond to regions of relatively likely subduction initiation in the near future. Using a digital seafloor age model, we have created a database of oceanic fractures and quantified the associated age offsets. We have evaluated the state of stress on these lithospheric weak zones using two different global stress models. We find that the conditions needed to initiate subduction via the forced convergence model are relatively rare on the present-day Earth, and that there is little indication of incipient subduction at regions identified as relatively likely for subduction initiation. Using the same technique, we have evaluated the state of stress and seafloor age offset at regions of inferred present-day incipient subduction, and find that most of these regions are not associated with both high far-field compressive stresses and large age (and thus density) offsets. Subduction has likely initiated via forced convergence across preexisting zones of lithospheric weakness in the past, but our results indicate that the conditions needed for this type of subduction initiation are rare on the present

  11. Low-Stress Upper Plate Near Subduction Zones and Implications for Temporal Changes in Loading Forces

    NASA Astrophysics Data System (ADS)

    Wang, K.; Hu, Y.; Yoshida, K.

    2016-12-01

    Subduction megathrusts are weak, often with effective friction coefficients as low as 0.03. Consequently, differential stress (S1 - S3) in the nearby upper plate is low. Compression due to plate coupling and tension due to gravity are in a subtle balance that can be tipped by small perturbations. For example, the 2011 M=9 Tohoku-oki earthquake, which has a rupture-zone-average stress drop of only a few MPa, switched offshore margin-normal stress from compression to tension and affected seismicity pattern and stress directions of various parts of the land area. The low differential stress is also reflected in spatial variations of stresses, such as with changes in topography. In the Andes, crustal earthquake focal mechanisms change from thrust-faulting in low-elevation areas to normal-faulting in high-elevation areas. Given the lack of evidence for a pervasively weak crust, the low differential stress may indicate that in general the crust near subduction zones is not critically stressed. If so, crustal earthquakes do not represent pervasive failure but only local failure due to stress, material, and fluid pressure heterogeneity. If distributed permanent deformation that creates topography is not the norm, it either happens in brief episodes or took place in the past. The outer wedge may enter a compressively or extensionally critical state due to coseismic strengthening or weakening, respectively, of the shallow megathrust in largest interplate earthquakes. Temporal changes in loading forces must occur also at much larger temporal and spatial scales in response to changes in the nature of the subducting plate and other tectonic conditions. We propose that submarine wedges and high topography in the upper plate attain their geometry in geologically brief episodes of high differential stress. They normally stay in a low-stress stable state, but their geometry often reflects high-stress episodes of critical states in the past. In other words, rocks have a sustained

  12. Regional variations in the nature of the incoming plate and its implication to the subduction zone

    NASA Astrophysics Data System (ADS)

    Fujie, Gou; Kodaira, Shuichi; Obana, Koichiro; Kaiho, Yuka; Sato, Takeshi; Yamamoto, Yojiro; Takahashi, Tsutomu; Miura, Seiichi; Yamada, Tomoaki

    2017-04-01

    The megathrust earthquakes like the 2011 Tohoku earthquake are caused by the interaction between the overlying plate and the subducting oceanic plate, indicating that the properties of the subducing oceanic plate, such as their geometry, thermal state, lithology, and water content, have a potential to controll the megathrust earthquakes. Of these properties, water content (degree of hydration) is highly influential because water transported by the incoming plate lowers the temperature of the subduction zone, promotes forearc metamorphism. Moreover, the presence of water and hydrated materials like serpentine can affect interplate seismic coupling on the plate interface. Accordingly, the regional variations in the degree of the hydration within the incoming plate might have strong influences on the regional variations in the interplate earthquakes. To reveal the regional variations in the nature of the incoming oceanic plate and its evolution owing to bending-related faulting near the trench axis, we conducted extensive controlled-source seismic surveys in the trench-outer rise region off northeastern Japan arc. We confirmed the systematic changes in seismic velocities owing to the bending-realated faulting, suggesting the water content within the incoming oceanic plate increases toward the trench accompanied by the development of bending-related fractures. In addition, we found along-trench variations in the seismic structure of the incoming oceanic plate; lower seismic velocities and higher Vp/Vs ration around the ancient fracture zones associated with ridge propagation. This observation suggests that the ancient scar on the oceanic plate influences along-trench variations in the current water amount transported by the oceanic plate. If we extend the ancient fracture zone toward the forearc region, it corresponds to an area of weak interplate coupling, characterized by low Vp and high Vp/Vs ratio around the depth of the plate interface. Our observations suggest

  13. 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.

  14. 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

  15. Frictional properties of JFAST core samples and implications for slow earthquakes at the Tohoku subduction zone

    NASA Astrophysics Data System (ADS)

    Sawai, Michiyo; Niemeijer, André R.; Hirose, Takehiro; Spiers, Christopher J.

    2017-09-01

    Slow earthquakes occur in the shallow (<20 km deep) part of the Tohoku subduction zone. To understand how frictional properties of the plate boundary fault affect the generation of these slow earthquakes, we conducted friction experiments using borehole samples retrieved from the plate boundary décollement and footwall during Integrated Ocean Drilling Program Expedition 343 (Japan Trench Fast Drilling Project (JFAST)). The plate boundary fault material exhibits a transition from velocity-strengthening to velocity-weakening behavior at temperatures between 50 and 150°C, which, in the framework of rate-and-state friction laws, is a necessary condition for the generation of slow earthquakes, whereas the footwall sample mainly shows velocity-strengthening behavior except at temperatures of <50°C. The downdip temperature limit of slow earthquakes in the Japan Trench also lies between 100 and 150°C. Our results suggest that the frictional properties of the plate boundary fault may play a key role in controlling the locations of observed slow earthquakes.

  16. 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.

  17. 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.

  18. 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

  19. 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

  20. Fluid pathways in subduction zones

    NASA Astrophysics Data System (ADS)

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

    2009-12-01

    A large amount of water captured in the oceanic crust and mantle is recycled in subduction zones. Upon compaction and heating most fluids are expelled, but a significant amount of water can be carried in hydrated mineral phases and point defects. While the qualitative role of volatiles and dehydration reactions is well appreciated in the mechanisms for intermediate depth seismicity, mantle wedge melting and arc volcanism, the quantitative details of the metamorphic reactions and the pathways of fluids and melts in the slab are poorly understood. We provide finite element models, combined with thermodynamic and mineralogical constraints, to estimate the water release and migration from the subducting slab to overlying arc. We use models from a selection of warm (e.g., Cascadia), cold (Central Honshu) and intermediate (Nicaragua) subduction zones, using slab geometries constrained from seismological observations. The fluid release is predicted from the breakdown of hydrated phases in sediments, oceanic crust and slab mantle. We use newly developed high resolution models for the flow of these released fluids that take into account permeability and compaction pressures. While the detailed structure depends on the chosen rheology and permeability, we find that for reasonable assumptions of permeability, a significant amount of fluids can travel through the wedge along nearly vertical pathways at rates and paths, consistent with geochronological and geochemical constraints. For models considered to date, we find that the principal source of fluids that feed the wedge come from the hydrated oceanic crust and particularly the hydrated slab mantle. Fluids released from the sediments and shallow crust, tend to travel along high permeability zones in the subducting slab before being released to hydrate the cold corner of subduction zones, suggesting that the cold and hydrated forearc region that is imaged in many subduction zones is maintained by an active hydrological cycle

  1. 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

  2. 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

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

    NASA Astrophysics Data System (ADS)

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

    2016-07-01

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

  4. The Moho in subduction zones

    NASA Astrophysics Data System (ADS)

    Bostock, M. G.

    2013-12-01

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

  5. 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.

  6. 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.

  7. Earthquake hazards on the cascadia subduction zone

    SciTech Connect

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

    1987-04-10

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

  8. Earthquake hazards on the cascadia subduction zone.

    PubMed

    Heaton, T H; Hartzell, S H

    1987-04-10

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

  9. Thermal state of the Explorer segment of the Cascadia subduction zone: Implications for seismic and tsunami hazards

    NASA Astrophysics Data System (ADS)

    Gao, Dawei; Wang, Kelin; Davis, Earl E.; Jiang, Yan; Insua, Tania L.; He, Jiangheng

    2017-04-01

    The Explorer segment of northernmost Cascadia is an end-member "warm" subduction zone with very young incoming plate and slow-convergence rate. Understanding the megathrust earthquake potential of this type of subduction zone is of both geodynamic and societal importance. Available geodetic observations indicate that the subduction megathrust of the Explorer segment is currently locked to some degree, but the downdip extent of the fault area that is potentially seismogenic is not known. Here we construct finite-element models to estimate the thermally allowed megathrust seismogenic zone, using available knowledge of regional plate kinematics, structural data, and heat flow observations as constraints. Despite ambiguities in plate interface geometry constrained by hypocenter locations of low-frequency earthquakes beneath Vancouver Island, the thermal models suggest a potential rupture zone of ˜60 km downdip width located fully offshore. Using dislocation modeling, we further illustrate that a rupture zone of this size, even with a conservative assumption of ˜100 km strike length, can cause significant tsunami-genic deformation. Future seismic and tsunami hazard assessment in northern Cascadia must take the Explorer segment into account.

  10. Boron desorption and fractionation in Subduction Zone Fore Arcs: Implications for the sources and transport of deep fluids

    NASA Astrophysics Data System (ADS)

    Saffer, Demian M.; Kopf, Achim J.

    2016-12-01

    At many subduction zones, pore water geochemical anomalies at seafloor seeps and in shallow boreholes indicate fluid flow and chemical transport from depths of several kilometers. Identifying the source regions for these fluids is essential toward quantifying flow pathways and volatile fluxes through fore arcs, and in understanding their connection to the loci of excess pore pressure at depth. Here we develop a model to track the coupled effects of boron desorption, smectite dehydration, and progressive consolidation within sediment at the top of the subducting slab, where such deep fluid signals likely originate. Our analysis demonstrates that the relative timing of heating and consolidation is a dominant control on pore water composition. For cold slabs, pore water freshening is maximized because dehydration releases bound water into low porosity sediment, whereas boron concentrations and isotopic signatures are modest because desorption is strongly sensitive to temperature and is only partially complete. For warmer slabs, freshening is smaller, because dehydration occurs earlier and into larger porosities, but the boron signatures are larger. The former scenario is typical of nonaccretionary margins where insulating sediment on the subducting plate is commonly thin. This result provides a quantitative explanation for the global observation that signatures of deeply sourced fluids are generally strongest at nonaccretionary margins. Application of our multitracer approach to the Costa Rica, N. Japan, N. Barbados, and Mediterranean Ridge subduction zones illustrates that desorption and dehydration are viable explanations for observed geochemical signals, and suggest updip fluid migration from these source regions over tens of km.

  11. 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

  12. Interplay between deformation, fluid release and migration across a nascent subduction interface: evidence from Oman-UAE and implications for warm subduction zones

    NASA Astrophysics Data System (ADS)

    Agard, Philippe; Prigent, Cécile; Soret, Mathieu; Guillot, Stéphane; Dubacq, Benoît

    2017-04-01

    Frozen-in subduction plate interfaces preserving the first 1-2 My of the subduction history are found beneath ophiolites. These contacts are a key target to study the inception of mantle wedge metasomatism and the mechanical coupling between the upper plate and the top part of the sinking slab shortly after subduction initiation. Combining structural field and EBSD data, detailed petrology, thermodynamic modelling and geochemistry on both sides, i.e. the base of the mantle wedge (Oman-UAE basal peridotites) and the underlying accreted crustal fragments from the subducting slab (metamorphic soles), this study documents the continuous evolution of the plate contact from 1 GPa 900-750°C to 0.6 GPa 750-600°C, with emphasis on strain localization and feedbacks between deformation and fluid migration. In the mantle wedge, the (de)formation of proto-ultramylonitic peridotites is coeval with mantle metasomatism by focused hydrous fluid migration. Peridotite metasomatism results in the precipitation of new minerals (clinopyroxene, amphibole and spinel ± olivine and orthopyroxene) and their enrichment in FMEs (particularly B, Li and Cs, with concentrations up to 40 times that of the PM). Boron concentrations and isotopes (δ11B of metasomatized peridotites up to +25‰) suggest that these fluids with a "subduction signature" are probably sourced from the dehydrating amphibolitic metamorphic sole. Concomitantly, deformation in the lower plate results in the stepwise formation, detachment and accretion to the mylonitic s.l. mantle of successive slices of HT metabasalts from the downgoing slab, equilibrated at amphibolite/granulite conditions (900-750°C). Two major stages may be outlined: - between 900 and 750°C, the garnet-clinopyroxene-amphibole bearing sinking crust (with melting < 6 vol%) gets juxtaposed and mechanically coupled to the mantle, leading to the transfer of subduction fluids and metasomatism (possibly into the arc zone ultimately). Deformation is

  13. Link between heterogeneous dehydration and deformation during experiments on gypsum: implications for subduction zones

    NASA Astrophysics Data System (ADS)

    Leclere, Henri; Faulkner, Dan; Wheeler, John; Mariani, Elisabetta

    2017-04-01

    The understanding of the influence of pore-fluid pressure and friction on stability of fault zones is of first order importance to unravel earthquake triggering. Here, the effects of dehydration reactions on hydraulic and mechanical properties of rock are analysed to better understand the conditions required to trigger earthquakes. Triaxial experiments are conducted using gypsum and a direct shear sample assembly allowing to maintain the normal stress constant and to measure permeability during sliding. The evolutions of shear stress, pore-fluid pressure and permeability are continuously measured throughout the experiment until dehydration reaction reached completion. 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 induces transient stable slip weakening that is controlled by pore-fluid pressure and permeability evolutions followed by unstable slip on fully dehydrated product. Microstructural analysis shows clear evidence of dehydrated product preferentially localized along Riedel shear structures. A conceptual model is then proposed to explain transient slip weakening during dehydration reactions incorporating the key role played by permeability, and to provide a framework to define the conditions required to trigger unstable events during dehydration reactions. Is clear that the reaction product forms preferentially on shear bands: whether that relates to permeability heterogeneities or to a more fundamental link between stress and reaction is a point to be discussed.

  14. 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.

  15. 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.

  16. Influence of dehydration on the electrical conductivity of epidote and implications for high-conductivity anomalies in subduction zones

    NASA Astrophysics Data System (ADS)

    Hu, Haiying; Dai, Lidong; Li, Heping; Hui, Keshi; Sun, Wenqing

    2017-04-01

    The anomalously high electrical conductivities ( 0.1 to 1 S/m) in deep mantle wedge regions extensively detected by magnetotelluric studies are often associated with the presence of fluids released from the progressive dehydration of subducting slabs. Epidote minerals are the Ca-Al-rich hydrous silicates with huge stability fields exceeding those of amphibole (>70-80 km) in subducting oceanic crust, and they may therefore be transported to greater depth than amphibole and release water to the mantle wedge. In this study, the electrical conductivities of epidote were measured at 0.5-1.5 GPa and 573-1273 K by using a Solartron-1260 Impedance/Gain-Phase Analyzer in a YJ-3000t multianvil pressure within the frequency range of 0.1-106 Hz. The results demonstrate that the influence of pressure on electrical conductivity of epidote is relatively small compared to that of temperature. The dehydration reaction of epidote is observed through the variation of electrical conductivity around 1073 K, and electrical conductivity reaches up to 1 S/m at 1273 K, which can be attributed to aqueous fluid released from epidote dehydration. After sample dehydration, electrical conductivity noticeably decreases by as much as nearly a log unit compared with that before dehydration, presumably due to a combination of the presence of coexisting mineral phases and aqueous fluid derived from the residual epidote. Taking into account the petrological and geothermal structures of subduction zones, it is suggested that the aqueous fluid produced by epidote dehydration could be responsible for the anomalously high conductivities in deep mantle wedges at depths of 70-120 km, particularly in hot subduction zones.

  17. The systematics of chlorine, fluorine, and water in Izu arc front volcanic rocks: Implications for volatile recycling in subduction zones

    NASA Astrophysics Data System (ADS)

    Straub, Susanne M.; Layne, Graham D.

    2003-11-01

    Cl and H 2O recycled at arc would be lower. Extrapolating the Izu data to the total length of global arcs (˜37,000 km), the global arc outflux of fluid-recycled Cl and H 2O at subduction zones amounts to Cl ˜2.9-3.8 × 10 12 g/yr and H 2O ˜0.7-1.0 × 10 14 g/yr, respectively—comparable to previous estimates. Further, we obtain a first estimate of global arc outflux of fluid-recycled F of ˜0.3-0.4 × 10 12g/yr. Despite the inherent uncertainties, our results support models suggesting that the slab becomes strongly depleted in Cl and H 2O in subduction zones. In contrast, much of the subducted F appears to be returned to the deep mantle, implying efficient fractionation of Cl and H 2O from F during the subduction process. However, if slab devolatilization produces slab fluids with high Cl/F (˜9.5), slab melting will still produce components with low Cl/F ratios (˜0.9), similar to those characteristic of the upper continental crust (Cl/F ˜0.3-0.9).

  18. 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.

  19. Seismic coupling and uncoupling at subduction zones

    NASA Technical Reports Server (NTRS)

    Ruff, L.; Kanamori, H.

    1983-01-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

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

  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. 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.

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

    NASA Astrophysics Data System (ADS)

    Meijers, M. J. M.; Vrouwe, B.; van Hinsbergen, D. J. J.; Kuiper, K. F.; Wijbrans, J.; Davies, G. R.; Stephenson, R. A.; Kaymakcı, N.; Matenco, L.; Saintot, A.

    2010-10-01

    The early Cretaceous and younger opening of the Black Sea has obliterated much of the older record of Tethyan subduction below southeastern Europe. The earlier Mesozoic evolution was dominated by opening and closure of Tethyan oceans between Gondwana and Laurasia with their consumption, at least in part, accommodated along the southern Eurasian margin. Crimea (Ukraine), a peninsula in the northern Black Sea, represents the northernmost region of southeastern Europe that exposes a record of a pre-Cretaceous Tethyan active margin. To shed new light on the paleosubduction zone configuration of the southeastern European margin in the Jurassic, we report 40Ar/ 39Ar isotope dating on 10 samples and whole rock geochemistry on 31 samples from supposedly Jurassic magmatic rocks from the Crimean peninsula. The samples can be subdivided into two age groups: middle Jurassic (~ 172-158 Ma) and uppermost Jurassic to lowermost Cretaceous (~ 151-142 Ma), that both have a subduction-related geochemical signature. The ages of the younger group are in conflict with previously assigned biostratigraphic ages of the units under- and overlying the volcanic complex. This might suggest a scenario where the latter were juxtaposed by faulting. We argue that the Crimean volcanics represent a fragment of a volcanic arc overlying the southeastern European continental margin. These data therefore provide evidence for Jurassic northwards subduction below the Eurasian margin, preceding the opening of the Black Sea as a back-arc basin. We argue that the corresponding Jurassic trench was already positioned south of the Turkish Pontides and the Caucasus belt, implying a very shallow slab angle in the Jurassic.

  4. Experimental Constraints on CO2 Solubility in Rhyolitic Slab Melts - Implications for Carbon Flux in Subduction Zone

    NASA Astrophysics Data System (ADS)

    Duncan, M. S.; Dasgupta, R.

    2014-12-01

    Understanding the fate of carbon in subduction zones is critical to understand carbon cycle on a global scale. The amount of carbonate and reduced (organic) carbon that is subducted and the amount of CO2 that is released from arc volcanoes vary for subduction zones around the globe. If the agent of carbon transfer from slab to sub-arc mantle is a partial melt of either ocean-floor sediments [1] or hydrous basalt [2], we need to know the solubility of CO2 in rhyolitic slab melt to constrain the flux of carbon in subduction zones. Our previous experiments have constrained CO2 content in silicic slab melts as a function of P (1.5-3.0 GPa) and melt H2O content (0.5-3.0 wt.%) [3]. Here we extend our experiments to constrain the effect of temperature (1100-1400 °C) and fO2 (CO2 vapor-saturated [3] and graphite-saturated) on CO2 solubility and speciation in natural rhyolitic melts. From our data, we constructed empirical and thermodynamic models to calculate CO2 content in slab melts at P and T appropriate for the sub-arc region of the subducting slab at variable fO2 [4]. These experiments and models show that CO2 solubility increases with increasing P, fO2, and melt H2O contents to ~3.5 wt.%, while there is a only slight increase in CO2 solubility with increasing T though the effect is much smaller. Our study constrains the extent of C-cycling to the deep interior and to the arc source for graphite-saturated domains of the downgoing crust. Further, there is a general correspondence between CO2 solubility in slab-derived, rhyolitic melts at sub-arc depth with measured CO2 outflux at arcs [5]. For hotter slabs (T>800 °C) the calculated CO2 contents using our thermodynamic model, for example, are 1.5-3.4 wt.% for a low-H2O melt generated near the FMQ buffer and correspond to arc fluxes of 50-500 × 109 mol/yr. For colder slabs (T<800 °C) the calculated CO2 contents are 0.9-1.6 wt.% for a low-H2O melt generated near the FMQ buffer and correspond to arc fluxes of 0.1-15

  5. Downdip variations in seismic reflection character: Implications for fault structure and seismogenic behavior in the Alaska subduction zone

    NASA Astrophysics Data System (ADS)

    Li, Jiyao; Shillington, Donna J.; Bécel, Anne; Nedimović, Mladen R.; Webb, Spahr C.; Saffer, Demian M.; Keranen, Katie M.; Kuehn, Harold

    2015-11-01

    Seismic reflection data collected offshore of Alaska Peninsula across the western edge of the Semidi segment show distinctive variations in reflection characteristics of the megathrust fault with depth, suggesting changes in structure that may relate to seismic behavior. From the trench to ~40 km landward, two parallel reflections are observed, which we interpret as the top and bottom of the subducted sediment section. From ~50 to 95 km from the trench, the plate interface appears as a thin (<400 ms) reflection band. Deeper and farther landward, the plate interface transitions to a thicker (1-1.5 s) package of reflections, where it appears to intersect the fore-arc mantle wedge based on our preferred interpretation of the continental Moho. Synthetic waveform modeling suggests that the thin reflection band is best explained by a single ~100 to 250 m thick low-velocity zone, whereas the thick reflection band requires a 3 to 5 km thick zone of thin layers. The thin reflection band is located at the center of the 1938 Mw 8.2 Semidi earthquake rupture zone that now experiences little interplate seismicity. The thick reflection band starts at the downdip edge of the rupture zone, correlates with a dipping band of seismicity, and projects to the location of tremor at greater depth. We interpret the thin reflection band as a compacted sediment layer and/or localized shear zone. The thick reflection band could be caused by a wide deformation zone with branching faults and/or fluid-rich layers, representing a broad transition from stick-slip sliding to slow slip and tremor.

  6. 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.

  7. 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.

  8. Thermal considerations in inferring frictional heating from vitrinite reflectance and implications for shallow coseismic slip within the Nankai Subduction Zone

    NASA Astrophysics Data System (ADS)

    Fulton, Patrick M.; Harris, Robert N.

    2012-06-01

    Frictional properties within the upper few kilometers of subduction zones are generally thought to inhibit rupture propagation. Understanding whether large rapid slip propagates to the surface during megathrust earthquakes is important for characterizing tsunami hazard. Recent vitrinite reflectance analysis by Sakaguchi et al. (2011) on cores from the NanTroSEIZE drilling transect at the Nankai Trough, Japan, has been interpreted to suggest that these faults reached temperatures ˜380 °C, considerably larger than background temperature values, implying they hosted coseismic slip at shallow depths. Analysis of other temperature proxies on the megasplay by Hirono et al. (2009), however, suggests temperatures have not exceeded 300 °C and is inconclusive as to whether the fault slipped at high velocity. We evaluate the effects of frictional heat generation on the spatial distribution of vitrinite reflectance, its sensitivity to slip zone thickness and slip duration, and the cumulative effects of numerous events. We build on the analysis of Sakaguchi et al. (2011) by estimating frictional heating scenarios that are consistent with both the peak and spatial extent of anomalous vitrinite reflectance data. Our results imply coseismic slip magnitudes of several 10s of meters. Peak temperature estimates from the vitrinite reflectance data can be reconciled with the other geochemical constraints only by assuming the vitrinite reflectance results from the cumulative effects of multiple earthquakes. However, this results in unrealistically large estimates of total displacement. Our results imply that current understanding of how vitrinite reflectance is affected by fault slip is incomplete.

  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. S wave attenuation structure on the western side of the Nankai subduction zone: Implications for fluid distribution and dynamics

    NASA Astrophysics Data System (ADS)

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

    2014-10-01

    We estimated the S wave attenuation structure in southwestern Japan and the western Nankai Trough by analyzing maximum S wave amplitudes at 4-8, 8-16, and 16-32 Hz with a correction term for apparent amplitude attenuation due to multiple forward scattering. Because the estimated attenuation (Q-1) in our tomographic study was much larger than Q-1 due to wide-angle scattering, our estimated Q-1 was composed mainly of intrinsic attenuation. High-attenuation areas (Q-1 > 1/300 at 4-8 Hz) were imaged beneath Quaternary volcanoes and south off Shikoku. Low (<1/1500 at 4-8 Hz) or moderate Q-1 (1/500-1/1000 at 4-8 Hz) was imaged beneath Shikoku and nonvolcanic areas of Chugoku. High and moderate Q-1 in and around Shikoku are located near the top of subducting Philippine Sea Plate. This correspondence implies that these high and moderate Q-1 reflect fluid in the subducting slab. By applying a theoretical model of attenuation in water-saturated porous random media, we examined wave-induced fluid flow induced by lower frequency (<1 Hz) seismic waves that may be related with triggering of nonvolcanic tremor by surface waves. Even though Q-1 structure in this study cannot fully explain the tremor triggering by wave-induced fluid flow, large uncertainties of Q-1 in tremor zone suggest that high resolution imaging of Q-1 and random inhomogeneities would give some constraints for the spatial variation of permeability and other medium properties.

  11. Dissociation of aqueous silica at high P-T and implications for metal complexes in subduction-zone fluids

    NASA Astrophysics Data System (ADS)

    Huang, F.; Sverjensky, D. A.

    2016-12-01

    Upper mantle fluids are important in the deep carbon cycle because they serve as the link in the transport of materials from subducting plates to the initiation of magmatic activities beneath volcanic arcs, and because they are involved in diamond formation and mantle metasomatism, all of which record the growth and evolution of continental crust and lithospheric mantle. However, no quantitative models exist for the speciation and reactivity of these fluids at the elevated temperatures and pressures in the upper mantle, which severely hampers the development of models for planetary volatile evolution. Preliminary speciation solubility models predict concentrations of major elements that are too low compared to experimental measurements of peridotite and eclogite solubilities1,2. The discrepancies are likely due to the lack of some important complexes. In this study, we used the Deep Earth Water model3 to predict equilibrium constants of aqueous silica dissociation over a large P-T range (up to 1000 ºC and 5 GPa). Results show that increasing pressure strongly favors the dissociation of aqueous silica into H+ and HSiO3-, which could bond with metal ions to form complexes. These complexes are suggested to be stable at upper mantle conditions and can substantially enhance the solubility of major elements in aqueous fluids. Eclogite solubility data (garnet, clinopyroxene, and coesite)1 were used to calibrate the thermodynamic properties of HSiO3- complexes of major elements such as Fe, Mg, Al, Ca. By including these complexes, predictions of the concentrations of the major elements in fluid inclusions in diamond are consistent with measured data. This agreement in turn supports the potential importance of metal silicate complexes in upper mantle fluids, which can increase the mobility of many elements in subduction zones. [1] Kessel et al. EPSL (2005), Dvir et al. Contrib Mineral Petrol (2011); [2] Tomlinson et al. EPSL (2006); [3] Sverjensky et al. GCA (2014)

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

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

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

  13. The earthquake cycle in subduction zones

    NASA Technical Reports Server (NTRS)

    Melosh, H. J.; Fleitout, L.

    1982-01-01

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

  14. 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

  15. 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

  16. 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

  17. Metamorphic zirconology of continental subduction zones

    NASA Astrophysics Data System (ADS)

    Chen, Ren-Xu; Zheng, Yong-Fei

    2017-09-01

    Zircon is widely used to date geological events and trace geochemical sources in high-pressure (HP) to ultrahigh-pressure (UHP) metamorphic rocks of continental subduction zones. However, protolith zircons may be modified by three different types of metamorphic recrystallization via mechanisms of solid-state transformation, metasomatic alteration and dissolution reprecipitation; new zircon growth may be induced by dehydration reactions below the wet solidus of crustal rocks (metamorphic zircon) or peritectic reactions above the wet solidus (peritectic zircon). As a consequence, there are different origins of zircon domains in high-grade metamorphic rocks from collisional orogens. Thus, determining the nature of individual zircon domains is substantial to correct interpretation of their origin in studies of isotopic geochronology and geochemical tracing. We advocate an integrated study of zircon mineragraphy (internal structure and external morphology), U-Pb ages, mineral inclusions, trace elements, and Lu-Hf and O isotope compositions. Only in this way we are in a position to advance the simple zircon applications to metamorphic zirconology, enabling discrimination between the different origins of zircon and providing constraints on the property of fluid activity at subduction-zone conditions. The metamorphic recrystallization of protolith zircons and the new growth of metamorphic and peritectic zircons are prominent in HP to UHP metamorphic rocks of collisional orogens. These different types of recrystallized and grown zircons can be distinguished by their differences in element and isotope compositions. While the protolith nature of metamorphosed rocks dictates water availability, the P-T conditions of subduction zones dictate the property of subduction-zone fluids. The fluids of different properties may be produced at different positions of subducting and exhuming crustal slices, and they may physically and chemically mix with each other in continental

  18. Halogen Behavior during Subduction-Zone Processes

    NASA Astrophysics Data System (ADS)

    Barnes, J.; Manning, C. E.; Scambelluri, M.; Selverstone, J.

    2016-12-01

    Halogens (Cl, F, I, Br) are enriched in surface reservoirs compared to the mantle. The subduction of these reservoirs in the form of sedimentary pore fluids, sediments, altered oceanic crust (AOC), and serpentinites returns halogens to the mantle and to regions of arc magma genesis. Pore fluids are particularly enriched in iodine, yet shallow pore fluid loss due to compaction makes pore fluids a negligible halogen source at depths > 5 km. Sediments can host large quantities of halogens, particularly I. However, serpentinites ± AOC deliver the largest amount of halogens to depths of magma genesis. Due to their hydrophilic nature, halogens are lost to aqueous slab-derived fluids during prograde metamorphic reactions. The addition of halogens, particularly Cl, increases the ability of subduction-zone fluids to transport metals and trace elements. The amount of Cl in solution is a function of the P-T conditions of the subduction zone, such that higher temperatures at a given depth and lower pressures at a given temperature favor ion pair formation (NaClaq, KClaq). Therefore, ion pairing will be more important in subduction zones with warmer geotherms compared to those with cooler geotherms. High halogen concentrations in melt inclusions and volcanic gas emissions from the arc front support the efficiency of fluid loss and transport from the slab to the region of magma genesis. Despite this high efficiency, mass balance calculations and halogen concentrations in back-arc basalts and ocean island basalts show that more halogens are subducted than returned to the Earth's surface through volcanic arc fronts, implying transport of halogens into the upper mantle. We estimate that 1-80% of Cl, 70-99% of I, 80-95% of Br, and 95-98% of F, subduct past the arc. The wide range for Cl is hampered by large uncertainties in Cl outputs. Shallow loss of I and Br are not accounted for in outputs, thus overestimating the recycling of these elements to the mantle.

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

    NASA Technical Reports Server (NTRS)

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

    1993-01-01

    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.

  20. 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.

  1. 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.

  2. Compression-extension transition of continental crust in a subduction zone: A parametric numerical modeling study with implications on Mesozoic-Cenozoic tectonic evolution of the Cathaysia Block

    PubMed Central

    Chan, Lung Sang; Gao, Jian-Feng

    2017-01-01

    The Cathaysia Block is located in southeastern part of South China, which situates in the west Pacific subduction zone. It is thought to have undergone a compression-extension transition of the continental crust during Mesozoic-Cenozoic during the subduction of Pacific Plate beneath Eurasia-Pacific Plate, resulting in extensive magmatism, extensional basins and reactivation of fault systems. Although some mechanisms such as the trench roll-back have been generally proposed for the compression-extension transition, the timing and progress of the transition under a convergence setting remain ambiguous due to lack of suitable geological records and overprinting by later tectonic events. In this study, a numerical thermo-dynamical program was employed to evaluate how variable slab angles, thermal gradients of the lithospheres and convergence velocities would give rise to the change of crustal stress in a convergent subduction zone. Model results show that higher slab dip angle, lower convergence velocity and higher lithospheric thermal gradient facilitate the subduction process. The modeling results reveal the continental crust stress is dominated by horizontal compression during the early stage of the subduction, which could revert to a horizontal extension in the back-arc region, combing with the roll-back of the subducting slab and development of mantle upwelling. The parameters facilitating the subduction process also favor the compression-extension transition in the upper plate of the subduction zone. Such results corroborate the geology of the Cathaysia Block: the initiation of the extensional regime in the Cathaysia Block occurring was probably triggered by roll-back of the slowly subducting slab. PMID:28182640

  3. Compression-extension transition of continental crust in a subduction zone: A parametric numerical modeling study with implications on Mesozoic-Cenozoic tectonic evolution of the Cathaysia Block.

    PubMed

    Zuo, Xuran; Chan, Lung Sang; Gao, Jian-Feng

    2017-01-01

    The Cathaysia Block is located in southeastern part of South China, which situates in the west Pacific subduction zone. It is thought to have undergone a compression-extension transition of the continental crust during Mesozoic-Cenozoic during the subduction of Pacific Plate beneath Eurasia-Pacific Plate, resulting in extensive magmatism, extensional basins and reactivation of fault systems. Although some mechanisms such as the trench roll-back have been generally proposed for the compression-extension transition, the timing and progress of the transition under a convergence setting remain ambiguous due to lack of suitable geological records and overprinting by later tectonic events. In this study, a numerical thermo-dynamical program was employed to evaluate how variable slab angles, thermal gradients of the lithospheres and convergence velocities would give rise to the change of crustal stress in a convergent subduction zone. Model results show that higher slab dip angle, lower convergence velocity and higher lithospheric thermal gradient facilitate the subduction process. The modeling results reveal the continental crust stress is dominated by horizontal compression during the early stage of the subduction, which could revert to a horizontal extension in the back-arc region, combing with the roll-back of the subducting slab and development of mantle upwelling. The parameters facilitating the subduction process also favor the compression-extension transition in the upper plate of the subduction zone. Such results corroborate the geology of the Cathaysia Block: the initiation of the extensional regime in the Cathaysia Block occurring was probably triggered by roll-back of the slowly subducting slab.

  4. Decoupling of Pacific subduction zone guided waves

    NASA Astrophysics Data System (ADS)

    Garth, T.; Rietbrock, A.

    2010-12-01

    Subduction zone guided wave arrivals have been observed in many circum Pacific subduction zones and have been attributed to the presence of a low velocity layer (LVL) in the subducting slab. This LVL acts as a waveguide for the high frequency energy, while lower frequency energy is not retained and travels in the higher velocity surrounding mantle. This leads to the characteristic dispersion of seismic waves observed. The commonly accepted model for the LVL is the persistence of basaltic oceanic crust to a depth of greater than 150 km. This basaltic oceanic crust has not yet undergone phase transformation to eclogite due to kinetic hindering, and so still has a distinguishably lower velocity than the surrounding mantle. It has been shown that guided waves are only seen from events that occur in or near to the low velocity layer. Similarly it would be expected that guided waves are only seen when the receiver is on the wave guide. However in a subduction zone setting it has been shown that guided wave energy is decoupled from the waveguide, due to the bend of the slab (Martin et al., 2003). Therefore high frequency guided wave energy escapes the waveguide and so can be observed at receivers placed in specific positions on the overriding plate. This decoupling mechanism allows guided waves from intermediate and deep Wadati-Benioff zone earthquakes to be observed. We use a two dimensional finite difference model to investigate the decoupling of wave guide energy due to the geometry of various Pacific subduction zones in order to predict the occurrence of guided wave arrivals along up-dip and along-strike propagation paths. The slab geometry is inferred from the USGS slab contour model slab 1.0. An explosive source is used so that frequency effects of the source do not complicate the results. The thickness of the LVL is inferred from published observations of Pacific subduction zone guided waves. For the along-strike profile we concentrate on the observations of guided

  5. Comparative Study of Subduction Zone Thermal Structure: Implications for Slab Dehydration and Fluid Supply for Mantle Wedge Serpentinization and Arc Volcanism

    NASA Astrophysics Data System (ADS)

    Wada, I.; Wang, K.; He, J.; Hyndman, R. D.

    2007-12-01

    Aqueous fluid from the dehydrating slab is critical to the processes of forearc mantle wedge serpentinization and arc volcanism. Its availability depends mainly on the thermal structure of the subducting slab, which is strongly controlled by the age of the slab and mantle wedge flow. In this study, we develop 2-D steady state numerical thermal models for a number of subduction zones to investigate how the thermal structure affects the fluid supply. Subduction zones investigated in this comparative study include Cascadia, Chile, Colombia-Ecuador, Costa Rica, Hikurangi, Kermadec, Mariana, Mexico, Nankai, NE Japan, and Sumatra. Geophysical and geological observations indicate that the shallow part of the forearc mantle wedge is decoupled from the subducting slab and does not participate in the wedge flow. The maximum depth of the slab-mantle wedge decoupling is one of the most important parameters controlling the subduction zone thermal structure. In our models, the depth of downdip transition from decoupling to coupling is constrained by surface heat flow and the location of the arc, beneath which the mantle wedge temperature is required to be greater than 1200°C. We find that the optimal transition depth for most subduction zones is in the range of 70 to 90 km; too shallow a transition will over-predict the forearc heat flow, and too deep a transition will under-predict the mantle temperature beneath the arc. The model results show that, for all subduction zones, the stagnant part of the forearc mantle wedge is sufficiently cold to allow serpentine to be stable, but the actual degree of its serpentinization should differ between different subduction zones depending on the availability of fluids. For subduction zones with a young and warm slab such as Cascadia and Nankai, dehydration of the subducting crust peaks at depths shallower than the decoupling-coupling transition depth and therefore provides ample fluid to serpentinize the overlying stagnant mantle wedge

  6. Comparative Study of Subduction Zone Thermal Structure: Implications for Slab Dehydration and Fluid Supply for Mantle Wedge Serpentinization and Arc Volcanism

    NASA Astrophysics Data System (ADS)

    Wada, I.; Wang, K.; He, J.; Hyndman, R. D.

    2004-12-01

    Aqueous fluid from the dehydrating slab is critical to the processes of forearc mantle wedge serpentinization and arc volcanism. Its availability depends mainly on the thermal structure of the subducting slab, which is strongly controlled by the age of the slab and mantle wedge flow. In this study, we develop 2-D steady state numerical thermal models for a number of subduction zones to investigate how the thermal structure affects the fluid supply. Subduction zones investigated in this comparative study include Cascadia, Chile, Colombia-Ecuador, Costa Rica, Hikurangi, Kermadec, Mariana, Mexico, Nankai, NE Japan, and Sumatra. Geophysical and geological observations indicate that the shallow part of the forearc mantle wedge is decoupled from the subducting slab and does not participate in the wedge flow. The maximum depth of the slab-mantle wedge decoupling is one of the most important parameters controlling the subduction zone thermal structure. In our models, the depth of downdip transition from decoupling to coupling is constrained by surface heat flow and the location of the arc, beneath which the mantle wedge temperature is required to be greater than 1200°C. We find that the optimal transition depth for most subduction zones is in the range of 70 to 90 km; too shallow a transition will over-predict the forearc heat flow, and too deep a transition will under-predict the mantle temperature beneath the arc. The model results show that, for all subduction zones, the stagnant part of the forearc mantle wedge is sufficiently cold to allow serpentine to be stable, but the actual degree of its serpentinization should differ between different subduction zones depending on the availability of fluids. For subduction zones with a young and warm slab such as Cascadia and Nankai, dehydration of the subducting crust peaks at depths shallower than the decoupling-coupling transition depth and therefore provides ample fluid to serpentinize the overlying stagnant mantle wedge

  7. Alteration and dehydration of subducting oceanic crust within subduction zones: implications for décollement step-down and plate-boundary seismogenesis

    NASA Astrophysics Data System (ADS)

    Kameda, Jun; Inoue, Sayako; Tanikawa, Wataru; Yamaguchi, Asuka; Hamada, Yohei; Hashimoto, Yoshitaka; Kimura, Gaku

    2017-04-01

    The alteration and dehydration of predominantly basaltic subducting oceanic crustal material are thought to be important controls on the mechanical and hydrological properties of the seismogenic plate interface below accretionary prisms. This study focuses on pillow basalts exposed in an ancient accretionary complex within the Shimanto Belt of southwest Japan and provides new quantitative data that provide insight into clay mineral reactions and the associated dehydration of underthrust basalts. Whole-rock and clay-fraction X-ray diffraction analyses indicate that the progressive conversion of saponite to chlorite proceeds under an almost constant bulk-rock mineral assemblage. These clay mineral reactions may persist to deep crustal levels ( 320 °C), possibly contributing to the bulk dehydration of the basalt and supplying fluid to plate-boundary fault systems. This dehydration can also cause fluid pressurization at certain horizons within hydrous basalt sequences, eventually leading to fracturing and subsequent underplating of upper basement rock into the overriding accretionary prism. This dehydration-induced breakage of the basalt can explain variations in the thickness of accreted basalt fragments within accretionary prisms as well as the reported geochemical compositions of mineralized veins associated with exposed basalts in onland locations. This fracturing of intact basalt can also nucleate seismic rupturing that would subsequently propagate along seismogenic plate interfaces.[Figure not available: see fulltext.

  8. 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

  9. Nonvolcanic tremors in the Mexican subduction zone

    NASA Astrophysics Data System (ADS)

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

    2007-05-01

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

  10. 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

  11. 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.

  12. 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

  13. Rutile Solubility in Supercritical Albite-H2O fluids: Implications for Element Mobility in Subduction Zones

    NASA Astrophysics Data System (ADS)

    Antignano, A.; Manning, C. E.

    2006-12-01

    Supercritical fluids with compositions intermediate between H2O and silicate are widely invoked as important transport agents in subduction zones. This proposal is in part motivated by the expectation that such fluids might have greater ability to dissolve and transport key trace elements at high P and T. As a test of this hypothesis, we measured the solubility of rutile (TiO2) in supercritical albite (ab, NaAlSi3O8)-H2O at 900°C, 1.5 GPa, from Xab = 0 to 0.3. At this P and T, rutile has very low solubility in H2O and there is full miscibility between H2O and ab melt. Experiments were conducted in a piston-cylinder apparatus with NaCl-graphite furnaces. In each, a 1.6 mm OD Pt inner capsule with a synthetic rutile crystal was lightly crimped and placed in a 3.5 OD Pt capsule with ultra pure H2O and powdered Amelia albite. Equilibrium was achieved after 4 hrs. Solubility was determined by the weight loss of the rutile grain. Quench textures consistent with supercritical behavior were observed in all runs. Residual corundum is present in the H2O-rich runs, but it decreases with increasing ab concentration. Results show that rutile solubility initially rises sharply with increasing ab concentration from 38 ppm in pure H2O to 739 ppm at Xab =0.05 (44 wt%). With further increase in ab, rutile solubility increases only slightly, to 922 ppm at Xab =0.25 (83 wt%). No significant solubility increase was noted near the critical compositon (~50 wt% ab). Our results show that intermediate fluids do not significantly enhance Ti solubility above dilute silicate-bearing solutions. The presence of residual Al2O3 and the sharp initial rise in rutile solubility at low Xab imply that, by analogy with silicate melts, Ti is present in solution as Na-Ti-O complexes (e.g., Dickenson and Hess, 1985, GCA, 49, 2289). However, the lack of residual corundum at high Xab suggests a transiton to different Ti species, perhaps aqueous NaAlSi3O8-like complexes. Our results give insight into rutile

  14. Seismic probing of continental subduction zones

    NASA Astrophysics Data System (ADS)

    Zhao, Liang; Xu, Xiaobing; Malusà, Marco G.

    2017-09-01

    High-resolution images of Earth's interior provide pivotal information for the understanding of a range of geodynamic processes, including continental subduction and exhumation of ultrahigh-pressure (UHP) metamorphic rocks. Here we present a synthesis of available global seismic observations on continental subduction zones, and selected examples of seismic probing from the European Alps, the Himalaya-Tibet and the Qinling-Dabie orogenic belts. Our synthesis and examples show that slabs recognized beneath exhumed continental UHP terranes generally have shallow dip angles (<45°) at depths <100 km, to become much steeper at depths >100 km. Slabs underlined by a clear high velocity anomaly from Earth's surface to the mantle are generally Cenozoic in age. Some of these slabs are continuous, whereas other continental subduction zones are located above discontinuous high velocity anomalies possibly suggesting slab breakoff. The density of seismic stations and the quality of recordings are of primary importance to get high-resolution images of the upper mantle to be used as a starting point to provide reliable geodynamic interpretations. In some cases, areas previously indicated as possible site of slab breakoff, such as the European Alps, have been later proven to be located above a continuous slab by using higher quality travel time data from denser seismic arrays. Discriminating between oceanic and continental slabs can be challenging, but valuable information can be provided by combining teleseismic tomography and receiver function analysis. The upper mantle beneath most continental UHP terranes generally shows complex seismic anisotropy patterns that are potentially preserved even in pre-Cenozoic subduction zones. These patterns can be used to provide information on continental slabs that are no longer highlighted by a clear high-velocity anomaly.

  15. 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.

  16. 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.

  17. Controls on Earthquake Rupture and Triggering Mechanisms in Subduction Zones

    DTIC Science & Technology

    2010-06-01

    322. Hyndman, R. D., and K. Wang (1993), Thermal constraints on the zone of major thrust earthquake failure: The Cascadia subduction zone , J. Geo- phys...2010-08 DOCTORAL DISSERTATION by Andrea Lesley Llenos June 2010 Controls on Earthquake Rupture and Triggering Mechanisms in Subduction Zones MIT...cited as: Andrea Lesley Llenos, 2010. Controls on Earthquake Rupture and Triggering Mechanisms in Subduction Zones . Ph.D. Thesis. MIT/ WHOI , 2010·08

  18. 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.

  19. Compositional diversity of Late Cenozoic basalts in a transect across the southern Washington Cascades: Implications for subduction zone magmatism

    SciTech Connect

    Leeman, W.P. ); Smith, D.R. ); Hildreth, W. ); Palacz, Z.; Rogers, N. )

    1990-11-10

    Major volcanoes of the Southern Washington Cascades (SWC) include the large quaternary stratovolcanoes of Mount St. Helens (MSH) and Mount Adams (MA) and the Indian Heaven (IH) and Simcoe Mountain (SIM) volcanic fields. There are significant differences among these volcanic centers in terms of their composition and evolutionary history. The authors conclude that subducted fluids and sediments do not play an essential role in producing these magmas. Rather, they infer that they formed by variable degree melting of a mixed mantle source consisting mainly of heterogeneously distributed OIB and mid-ocean ridge basalt source domains. Relatively minor occurrences of high field strength element (HFSE) depleted arclike basalts may reflect the presence of a small proportion of slab-metasomatized subarc mantle. The juxtaposition of such different mantle domains within the lithospheric mantle is viewed as a consequence of (1) tectonic mixing associated with accretion of oceanic and island arc terranes along the Pacific margin of North America prior to Neogene time, and possibly (2) a seaward jump in the locus of subduction at about 40 Ma. The Cascades arc is unusual in that the subducting oceanic plate is very young and hot. They suggest that slab dehydration outboard of the volcanic front resulted in a diminished role of aqueous fluids in generating or subsequently modifying SWC magmas compared to the situation at most convergent margins. Furthermore, with low fluid flux conditions, basalt generation is presumably triggered by other processes that increase the temperature of the mantle wedge (e.g., convective mantle flow, shear heating, etc.).

  20. The implications of revised Quaternary palaeoshoreline chronologies for the rates of active extension and uplift in the upper plate of subduction zones

    NASA Astrophysics Data System (ADS)

    Roberts, G. P.; Meschis, M.; Houghton, S.; Underwood, C.; Briant, R. M.

    2013-10-01

    fault are due to interaction between “regional” uplift and subsidence associated with the local active normal faulting. We discuss (a) how our synchronous correlation technique should trigger a re-appraisal of palaeoshoreline chronologies worldwide, and (b) the implications for the tectonics and seismic hazard of Calabria, suggesting that perturbations in the uplift-rate field are a key criterion to map the locations of active faults, their deformation rates, and hence seismic hazard above subduction zones.

  1. Seismicity of the eastern Hellenic Subduction Zone

    NASA Astrophysics Data System (ADS)

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

    2012-04-01

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

  2. Development of Forearcs of Intraoceanic Subduction Zones

    NASA Astrophysics Data System (ADS)

    Lundberg, Neil

    1983-02-01

    The uplifted Costa Rican forearc landward of the Middle America Trench and the Mariana forearc drilled on IPOD leg 60 both lack the thick clastic sequences, complex deformation, and abundant evidence of accretion which characterize more widely known forearcs that border continents. Both regions contain significant in situ accumulations of pelagic and hemipelagic sediments in place of thick trench and trench slope basin sequences composed of terrigenous turbidites. The Nicoya Peninsula of Costa Rica contains no significant melange terranes. Deformation of the mafic igneous basement and its thin cover of pelagic, hemipelagic, and first-cycle volcanogenic material is mild overall, with discrete zones of intense deformation disrupting otherwise well-preserved stratigraphic sections. Intraoceanic subduction zones lacking longitudinal trench feed are sites of little or no accretion of sediments, and recently suggested experimental and theoretical models of subduction zone processes involving flow melanges are inappropriate for intraoceanic forearcs. Intraoceanic forearcs generally lack high-grade exotic components such as blueschist and eclogite tectonically incorporated as blocks in lower-grade matrix, although uplift and erosion of the forearc basement may provide detritus of amphibolite and ultramafic rock to the trench and trench slope.

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

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

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

  4. 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

  5. Introduction to the structures and processes of subduction zones

    NASA Astrophysics Data System (ADS)

    Zheng, Yong-Fei; Zhao, Zi-Fu

    2017-09-01

    Subduction zones have been the focus of many studies since the advent of plate tectonics in 1960s. Workings within subduction zones beneath volcanic arcs have been of particular interest because they prime the source of arc magmas. The results from magmatic products have been used to decipher the structures and processes of subduction zones. In doing so, many progresses have been made on modern oceanic subduction zones, but less progresses on ancient oceanic subduction zones. On the other hand, continental subduction zones have been studied since findings of coesite in metamorphic rocks of supracrustal origin in 1980s. It turns out that high-pressure to ultrahigh-pressure metamorphic rocks in collisional orogens provide a direct target to investigate the tectonism of subduction zones, whereas oceanic and continental arc volcanic rocks in accretionary orogens provide an indirect target to investigate the geochemistry of subduction zones. Nevertheless, metamorphic dehydration and partial melting at high-pressure to ultrahigh-pressure conditions are tectonically applicable to subduction zone processes at forearc to subarc depths, and crustal metasomatism is the physicochemical mechanism for geochemical transfer from the slab to the mantle in subduction channels. Taken together, these provide us with an excellent opportunity to find how the metamorphic, metasomatic and magmatic products are a function of the structures and processes in both oceanic and continental subduction zones. Because of the change in the thermal structures of subduction zones, different styles of metamorphism, metasomatism and magmatism are produced at convergent plate margins. In addition, juvenile and ancient crustal rocks have often suffered reworking in episodes independent of either accretionary or collisional orogeny, leading to continental rifting metamorphism and thus rifting orogeny for mountain building in intracontinental settings. This brings complexity to distinguish the syn-subduction

  6. Introduction to the structures and processes of subduction zones

    NASA Astrophysics Data System (ADS)

    Zheng, Yong-Fei; Zhao, Zi-Fu

    2017-09-01

    Subduction zones have been the focus of many studies since the advent of plate tectonics in 1960s. Workings within subduction zones beneath volcanic arcs have been of particular interest because they prime the source of arc magmas. The results from magmatic products have been used to decipher the structures and processes of subduction zones. In doing so, many progresses have been made on modern oceanic subduction zones, but less progresses on ancient oceanic subduction zones. On the other hand, continental subduction zones have been studied since findings of coesite in metamorphic rocks of supracrustal origin in 1980s. It turns out that high-pressure to ultrahigh-pressure metamorphic rocks in collisional orogens provide a direct target to investigate the tectonism of subduction zones, whereas oceanic and continental arc volcanic rocks in accretionary orogens provide an indirect target to investigate the geochemistry of subduction zones. Nevertheless, metamorphic dehydration and partial melting at high-pressure to ultrahigh-pressure conditions are tectonically applicable to subduction zone processes at forearc to subarc depths, and crustal metasomatism is the physicochemical mechanism for geochemical transfer from the slab to the mantle in subduction channels. Taken together, these provide us with an excellent opportunity to find how the metamorphic, metasomatic and magmatic products are a function of the structures and processes in both oceanic and continental subduction zones. Because of the change in the thermal structures of subduction zones, different styles of metamorphism, metasomatism and magmatism are produced at convergent plate margins. In addition, juvenile and ancient crustal rocks have often suffered reworking in episodes independent of either accretionary or collisional orogeny, leading to continental rifting metamorphism and thus rifting orogeny for mountain building in intracontinental settings. This brings complexity to distinguish the syn-subduction

  7. 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

  8. Imaging segmentation along the Cascadia subduction zone

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

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

  9. The Run-Up of Subduction Zones

    NASA Astrophysics Data System (ADS)

    Riquelme, S.; Bravo, F. J.; Fuentes, M.; Matias, M.; Medina, M.

    2016-12-01

    Large earthquakes in subduction zones are liable to produce tsunamis that can cause destruction and fatalities. The Run-up is a geophysical parameter that quantifies damage and if critical facilities or population are exposed to. Here we use the coupling for certain subduction regions measured by different techniques (Potency and GPS observations) to define areas where large earthquakes can occur. Taking the slab 1.0 from the United States Geological Survey (USGS), we can define the geometry of the area including its tsunamigenic potential. By using stochastic earthquakes sources for each area with its maximum tsunamigenic potential, we calculate the numerical and analytical run-up for each case. Then, we perform a statistical analysis and calculate the envelope for both methods. Furthermore, we build an index of risk using: the closest slope to the shore in a piecewise linear approach (last slopecriteria) and the outputsfrom tsunami modeling. Results show that there are areas prone to produce higher run-up than others based on the size of the earthquake, geometrical constraints of the source, tectonic setting and the coast last slope. Based on these results, there are zones that have low risk index which can define escape routes or secure coastal areas for tsunami early warning, urban and planning purposes when detailed data is available.

  10. 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.

  11. 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

  12. U-series disequilibria in Guatemalan lavas, crustal contamination, and implications for magma genesis along the Central American subduction zone

    NASA Astrophysics Data System (ADS)

    Walker, James A.; Mickelson, J. Erik; Thomas, Rebecca B.; Patino, Lina C.; Cameron, Barry; Carr, Michael J.; Feigenson, Mark D.; Edwards, R. Lawrence

    2007-06-01

    New U-series results indicate that Guatemalan volcanic rocks display both 238U and 230Th excesses. 230Th excess is restricted to volcanoes in central Guatemala, both along and behind the front. 230Th excess correlates with a number of incompatible element ratios, such as Th/Nb and Ba/Th. It also shows a negative correlation with MgO. Guatemalan volcanic rocks have (230Th/232Th) ratios that overlap those of Costa Rican volcanics and are therefore considerably lower than the unusually high ratios characterizing volcanic rocks from Nicaragua. Along-arc variations in (230Th/232Th) therefore mirror those of a number of diagnostic geochemical parameters, such as Ba/La, which are symmetrical about a peak in west central Nicaragua. The one siliceous lava analyzed, from the Cerro Quemado dome complex, has a recognizable crustal imprint, distinguished, for instance, by high Th/Nb and low Ba/Th. In mafic samples, 238U excess is attributed to addition of a U-enriched fluid component from the subducting Cocos plate. Our preferred explanation for 230Th excess in Guatemalan mafic samples, on the other hand, is crustal contamination, consistent with the relatively high Th/Nb and low Ba/Th ratios in these samples. We suspect, however, that crustal contamination only exerts a sizable control over the U-series disequilibrium of mafic magmas in Guatemala, and not elsewhere along the Central American volcanic front. This agrees with previously published trace element and isotopic evidence that throughout Central America, with the exception of Guatemala, mafic magmas are largely uncontaminated by crustal material.

  13. Discrepancies in thermal gradients from BSR depth and seafloor thermometry on the Hikurangi Margin, New Zealand - possible implications for gas hydrate formation and subduction-zone processes

    NASA Astrophysics Data System (ADS)

    Pecher, I. A.; Wood, W.; Funnell, R.; Toulmin, S. J.; Hamdan, L. J.; Coffin, R. B.; Henrys, S. A.; Kukowski, N.

    2010-12-01

    transient advective heatflow as proposed for a similar discrepancy between BSR and seafloor thermometry off Chile (Grevemeyer et al., 2006, Geophys. J. Int., 166, 461-468). We have not reached any conclusions as to which mechanism is most likely to cause the discrepancies in thermal gradients. However, the suggested processes could have profound implications. If rock properties were responsible, heatflow on this margin would have been underestimated so far, affecting e.g., the predicted limits of the seismogenic zone. Transient processes on the other hand may provide a mechanism for gas hydrate formation. For example, cooling after a fluid pulse would lead to “freezing” of gas to hydrate because of a downward moving base of gas hydrate stability.

  14. Trace element composition of rutile in eclogite from the Karakaya Complex, NW Anatolia: Implications for rutile growth during subduction zone metamorphism

    NASA Astrophysics Data System (ADS)

    Şengün, Fırat

    2017-04-01

    High-pressure/low-temperature (HP/LT) eclogite-facies terranes are widely regarded to represent exhumed fragments of subducted slabs. Therefore, the metamorphic studies of eclogites and associated high-pressure rocks yield crucial information about their P-T evolution and associated tectonometamorphic processes at depth in subduction zones. Especially rutile in eclogites record chemical history of subduction zones and also constrain metamorphic temperatures of subduction zone processes. Eclogites occur as a tectonic slice within metabasite-phyllite-marble intercalation of the Karakaya Complex. In this study, trace element geochemistry of rutiles and Zr-in-rutile thermometry have been investigated. The main mineralogical composition eclogites are composed of omphacite, garnet, glaucophane, epidote and quartz. Core-rim analyses through rutile grains yield remarkable trace element zoning with lower contents of Nb, Ta and Zr in the core than in the rim. The variations in Nb, Ta and Zr can be ascribed to the growth zoning rather than diffusion effect. The Nb/Ta and Zr/Hf ratios increase with a decrease in Ta and Hf contents, which could be ascribed to the effect of metamorphic dehydration at subduction zones on rutile Nb/Ta differentiation. The rutile grains from eclogites in the Karakaya Complex are characterized by subchondritic Nb/Ta and Zr/Hf ratios. It can be noted that the subchondritic Nb/Ta ratios may record rutile growth from local sinks of aqueous fluids from metamorphic dehydration. The Zr contents of the all rutile grains vary between 81 and 160 ppm with the average of 123 ppm. The Zr-in-rutile thermometer yielded the metamorphic temperature of 559-604 oC (average 585 oC) for eclogites occurring in the Karakaya Complex. This average temperature suggests the peak growth temperature of rutile. Moreover, Zr contents and calculated temperatures in both inclusion rutile and matrix rutile from eclogites are identical to each other, which suggests that rutiles in

  15. Metamorphic record of catastrophic pressure drops in subduction zones

    NASA Astrophysics Data System (ADS)

    Yamato, P.; Brun, J. P.

    2017-01-01

    When deeply buried in subduction zones, rocks undergo mineral transformations that record the increase of pressure and temperature. The fact that high-pressure metamorphic parageneses are found at the Earth’s surface proves that rock burial is followed by exhumation. Here we use analysis of available data sets from high-pressure metamorphic rocks worldwide to show that the peak pressure is proportional to the subsequent decompression occurring during the initial stage of retrogression. We propose, using a simple mechanical analysis, that this linear relationship can be explained by the transition from burial-related compression to extension at the onset of exhumation. This major switch in orientation and magnitude of principal tectonic stresses leads to a catastrophic pressure drop prior to actual rock ascent. Therefore, peak pressures are not necessarily, as commonly believed, directly dependent on the maximum burial depth, but can also reflect a change of tectonic regime. Our results, which are in agreement with natural data, have significant implications for rock rheology, subduction zone seismicity, and the magnitudes of tectonic pressures sustained by rocks. Current views of subduction dynamics could be reconsidered in that perspective.

  16. Effect of temperature on frictional behavior of smectite and illite: Implication for the updip limit of seismogenic zone along subduction thrust

    NASA Astrophysics Data System (ADS)

    Kubo, T.; Katayama, I.

    2014-12-01

    Along plate boundary subduction thrusts, the transformation of smectite to illite within fault gouge at temperatures of ~150C is one of the key mineralogical changes thought to control the updip limit of seismicity. Saffer and Marone (2003) reported illite shale exhibited only velocity-strengthening behavior, whereas illite is widely expected to be velocity-weakening behavior. The limitation of their experiments were temperature, in which the frictional experiments were csrried out at room temperature, although the updip limit of seismogenic zone is thermally controlled that occurs at temperature around 150C. Therefore, in this study, we determined the effect of temperature of frictional properties of smectite and illite and discuss whether the smectite-illite transition accounts for the updip limit of seismogenic zone along subduction thrust. In the frictional experiments, we determined the velocity dependence of sliding friction, which is a key parameter for stable or unstable sliding. After steady-state sliding, the loading velocity of 3 μm/s was abruptly changed to 33 μm/s in each frictional experiments to determine the velocity dependence of these clay minerals. The velocity dependence of both smectite and illite at room temperature shows always positive at normal stress higher than 40 MPa, which is similar to the results of Saffer and Marone (2003). However, at temperature of 200C, illite shows negative values of (a-b), suggesting that illite exhibits unstable velocity-weakening behavior. den Hartog et al. (2012) also showed the velocity-weakening for illite gouge by high temperature ring shear experiments, but the transition from velocity strengthening to weakening occurs at temperature around 250C. These results explain that smectite is potentially aseismic for stable sliding at the subduction thrust, whereas illite becomes seismic due to a negative velocity dependence and unstable sliding at high temperatures. Thus, the smectite-illite transition has a

  17. Thermal modeling of the SW Ryukyu forearc (Taiwan): Implications for the seismogenic zone and the age of the subducting Philippine Sea Plate (Huatung Basin)

    NASA Astrophysics Data System (ADS)

    Gutscher, M.-A.; Klingelhoefer, F.; Theunissen, T.; Spakman, W.; Berthet, T.; Wang, T. K.; Lee, C.-S.

    2016-12-01

    Subduction mega-thrust earthquakes in the SW Ryukyu trench pose a seismic and tsunami hazard. One of the objectives of this study is to estimate the downdip width of the seismogenic zone using numerical modeling to determine the temperature distribution along the plate interface. However, this approach depends strongly on the thermal parameters of the subducting slab. While the Philippine Sea plate (PSP) subducting beneath the central and eastern Ryukyu arc is of Eocene age (35-50 Ma), its age west of the Gagua Ridge is uncertain, with proposed ages ranging from Lower Cretaceous (140 Ma) to Upper Eocene (35 Ma). Since the sparse available heat flow data are insufficient to resolve this debate, both end-member hypotheses are tested as input parameters. We examined two transects at 122.5°E and 123.5°E on either side of the N-S trending, 4-km high, Gagua Ridge. The shallow forearc geometry is obtained from wide-angle seismic data. The deep slab geometry was obtained from hypocenter distribution and tomography. For an Eocene slab age, we obtain a 100 km and 110 km wide seismogenic zone (between the 150 °C and 350 °C isotherms) west and east of Gagua Ridge, respectively. This is in good agreement with the observed distribution of hypocenters. Using a Cretaceous slab west of Gagua Ridge predicts a deep seismogenic zone (25 km-60 km depth), inconsistent with observed thrust earthquakes. Tomographic images at 122.5°E and 123.5°E show a similar slab thickness of 70-80 km suggesting that the oceanic lithosphere has a young (Eocene) thermal age. The westernmost PSP (Huatung Basin) may have been thermally rejuvenated by mantle convection near the slab corner. The tectonic history since 6 Ma (transition from subduction to collision beneath Taiwan) may have also perturbed the thermal structure.

  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. 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

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

    PubMed

    Hall, P S; Kincaid, C

    2001-06-29

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

  1. 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

  2. Role of subduction obliquity in controlling mantle wedge flow and subduction zone processes

    NASA Astrophysics Data System (ADS)

    Wada, I.; He, J.; Jilek, E.

    2016-12-01

    In this study, we investigate the role of subduction obliquity and its along-arc variation in controlling the 3-D mantle wedge flow pattern and subduction zone processes, using 3-D coupled kinematic-dynamic models that are developed for regions with oblique subduction, including southern Cascadia, Northeast Japan, Hikurangi, and the Izu-Bonin-Mariana arc. In subduction zones, the motion of the subducting slab drives the overlying mantle to flow away from the mantle wedge corner. The dynamic pressure gradient induced by the mantle outflow then drives the mantle in the back-arc region to flow in towards the wedge corner, resulting in mantle wedge flow. 3-D subduction models with realistic slab geometries indicate that the obliquity of the subduction direction relative to the local strike of the subducting slab is a critical factor that contributes to along-arc dynamic pressure gradients in the mantle wedge, inducing a 3-D mantle wedge flow pattern. In general, oblique subduction causes the streamlines of mantle inflow and outflow to be approximately symmetric about the axis normal to the strike of the slab, and the angle between the two streamlines increases with subduction obliquity. In regions with large subduction obliquity, such as southern Hikurangi and northern Mariana, the angle can locally become so large that mantle flow appears nearly arc-parallel. Along-arc variation in the slab geometry adds further complexity to the 3-D mantle wedge flow pattern via changes in the strike of the slab and thus subduction obliquity. Mantle wedge flow patterns affect a number of important processes in subduction zones, such as the transport of heat and volatiles, development of crystal and shape preferred orientations, mantle wedge hydration, and slab dehydration. We test the modeled-predicted mantle wedge flow patterns for the aforementioned subduction zones against available geophysical observations, including seismic velocity and attenuation structures, mantle wedge

  3. Noble gases recycled into the mantle through cold subduction zones

    NASA Astrophysics Data System (ADS)

    Smye, Andrew J.; Jackson, Colin R. M.; Konrad-Schmolke, Matthias; Hesse, Marc A.; Parman, Steve W.; Shuster, David L.; Ballentine, Chris J.

    2017-08-01

    Subduction of hydrous and carbonated oceanic lithosphere replenishes the mantle volatile inventory. Substantial uncertainties exist on the magnitudes of the recycled volatile fluxes and it is unclear whether Earth surface reservoirs are undergoing net-loss or net-gain of H2O and CO2. Here, we use noble gases as tracers for deep volatile cycling. Specifically, we construct and apply a kinetic model to estimate the effect of subduction zone metamorphism on the elemental composition of noble gases in amphibole - a common constituent of altered oceanic crust. We show that progressive dehydration of the slab leads to the extraction of noble gases, linking noble gas recycling to H2O. Noble gases are strongly fractionated within hot subduction zones, whereas minimal fractionation occurs along colder subduction geotherms. In the context of our modelling, this implies that the mantle heavy noble gas inventory is dominated by the injection of noble gases through cold subduction zones. For cold subduction zones, we estimate a present-day bulk recycling efficiency, past the depth of amphibole breakdown, of 5-35% and 60-80% for 36Ar and H2O bound within oceanic crust, respectively. Given that hotter subduction dominates over geologic history, this result highlights the importance of cooler subduction zones in regassing the mantle and in affecting the modern volatile budget of Earth's interior.

  4. First-principles prediction of fast migration channels of potassium ions in KAlSi3O8 hollandite: Implications for high conductivity anomalies in subduction zones

    NASA Astrophysics Data System (ADS)

    He, Yu; Sun, Yang; Lu, Xia; Gao, Jian; Li, Hong; Li, Heping

    2016-06-01

    Materials sharing the hollandite structure were widely reported as fast ionic conductors. However, the ionic conductivity of KAlSi3O8 hollandite (K-hollandite), which can be formed during the subduction process, has not been investigated so far. Here first-principles calculations are used to investigate the potassium ion (K+) transport properties in K-hollandite. The calculated K+ migration barrier energy is 0.44 eV at a pressure of 10 GPa, an energy quite small to block the K+ migration in K-hollandite channels. The calculated ionic conductivity of K-hollandite is highly anisotropic and depends on its concentration of K+ vacancies. About 6% K+ vacancies in K-hollandite can lead to a higher conductivity compared to the conductivity of hydrated wadsleyite and ringwoodite in the mantle. K+ vacancies being commonly found in many K-hollandite samples with maximum vacancies over 30%, the formation of K-hollandite during subduction of continental or alkali-rich oceanic crust can contribute to the high conductivity anomalies observed in subduction zones.

  5. Dehydration-driven topotaxy in subduction zones

    NASA Astrophysics Data System (ADS)

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

    2014-05-01

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

  6. 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.

  7. 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.

  8. Seismic probing of hydration and dehydration reactions in subduction zones

    NASA Astrophysics Data System (ADS)

    Rondenay, Stéphane; McGary, R. Shane; Halpaap, Felix; Goes, Saskia; Perrin, Alexander; Wang, Hongliang; Huismans, Ritske; Ottemöller, Lars

    2017-04-01

    Over the past decade, high-resolution images based on teleseismic scattered waves have given us new insight into the distribution and movement of water in subduction zones. In particular, these images have shown us where the subducted crust loses the bulk of its water through eclogitization and where the mantle wedge becomes hydrated via serpentinization. The first images provided adequate constraints to infer where these processes occur for uniformly hydrated/dehydrated components of the system. However, we know that this assumption of uniformity does not really apply owing to petrological evidence that prograde and retrograde metamorphic reactions do not proceed uniformly across the subducted crust or mantle wedge. Here, we expand on previous work by (i) comparing high-resolution images from a catalogue that now samples a wide range of subduction zones, and (ii) jointly interpreting these high-resolution images with results from complementary seismic/geodynamic/petrological modelling. Our goal is to generate a set of new models that can help us better constrain the variable levels of hydration within the subducted slab and mantle wedge, and to use these models to better understand how fluid transfer between the various components of the system relates to seismicity. We illustrate these concepts with examples from the Cascadia subduction zone, where we find strong evidence for a layer of metastable gabbro in the lower portion of the subducted crust, and the Western Hellenic subduction zone, where the distribution of intraslab seismicity seems indicative of variable hydration/dehydration regimes along strike.

  9. 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.

  10. 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.

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

    PubMed

    Hardebeck, Jeanne L

    2015-09-11

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

  12. 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.

  13. Silicate-Sulfide-Oxide Phase Equilibria in Subduction Zones

    NASA Astrophysics Data System (ADS)

    Brown, J. L.; Ellis, D. J.; Christy, A. G.; Arculus, R. J.

    2008-12-01

    Classical O2- and S2-dependent equilibria have been extrapolated to high P, showing that fO2 and fS2 in a wide variety of geological environments can be gauged by comparison to fluid buffering reactions. Many of these equilibria involve magnetite, which in intermediate to mafic rock types is eliminated with increasing P during the gabbro to eclogite transition (Green and Ringwood, 1967). It is not generally recognized that traditional buffers, such as QFM (quartz-fayalite-magnetite), are not appropriate for evaluating fluid conditions in subduction zones. This paper will present new fS2 and fO2 equilibria that are consistent with silicate, oxide, and sulfide petrography of the blueschist-eclogite belt in northern New Caledonia (Brown, 2007), and are therefore appropriate for evaluating fluid conditions consistent with subduction zone metamorphism. We evaluate reactions, which are supported by textural evidence in natural specimens, between silicates, oxides, and sulfides (stable at low P) to form garnet (at higher P). Several analogues that eliminate plagioclase +/- magnetite to produce garnet are proposed. A consequence of reacting Fe in sulfide to produce garnet is the concentration of chalcophile elements in the remaining sulfide phase. This is consistent with petrographic observations in the New Caledonian high P belt. Implications of new equilibiria demonstrate that phase relations proposed with garnet cannot be related to classical fS2-fO2 phase diagrams involving QFM. The only reactions involving mineral phases commonly present throughout the range of PT conditions in subducted crust involve ilmenite-rutile-sulfide equilibria. These equilibria can be widely applied to evaluating fluid conditions in the crust and mantle, not just subduction zones. References Brown, J.L. (2007) The Deep Sulfur Cycle: Insights from sulfide metamorphism in blueschist and eclogite, NE New Caledonia. PhD Thesis, The Australian National University. Green, D.H. and Ringwood, A

  14. Deformation and melt transport in a highly depleted peridotite massif from the Canadian Cordillera: Implications to seismic anisotropy above subduction zones

    NASA Astrophysics Data System (ADS)

    Tommasi, Andréa; Vauchez, Alain; Godard, Marguerite; Belley, France

    2006-12-01

    Seismic anisotropy in subduction zones results from a combination of various processes. Although it depends primarily on the orientation of olivine in response to flow, the presence of water and melt in the wedge may modify the deformation of olivine. The melt distribution also influences anisotropy. Direct observations of the deformation and melt-rock interactions in a strongly depleted spinel-harzburgite massif from the Cache Creek terrane in the Canadian Cordillera allow evaluating the relative contribution of each process. Structural mapping shows that this massif has recorded high-temperature, low-stress deformation, high degrees of partial melting, and synkinematic melt-rock interaction at shallow depths (< 70 km) in the mantle, probably above an oblique subduction. Deformation, marked by shallow-dipping lineations and steep foliations, controlled melt distribution: reactive dunites and pyroxenite dykes are dominantly parallel to the foliation. Analysis of olivine crystal preferred orientations (CPO) indicates deformation by dislocation creep with dominant [100] glide. Glide planes are however different in harzburgites and dunites, suggesting that higher melt contents may favor glide on (001) relative to (010). Seismic properties, calculated by considering explicitly the large-scale structure of the massif, the olivine and pyroxene CPO, and possible melt distributions, show that the strain-induced olivine CPO results in up to 5% P- and S-wave anisotropy with fast seismic directions parallel to the lineation. Synkinematic melt transport by diffuse porous flow leading to melt pockets or dykes aligned in the foliation may significantly enhance this anisotropy, in particular for S-waves. In contrast, focused melt flow is not recorded by seismic anisotropy, unless associated with very high instantaneous melt fractions. Orientation of pyroxenite dykes suggests that the present orientation of the structures is representative of the pre-obduction situation, implying

  15. 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.

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

    NASA Astrophysics Data System (ADS)

    Kopp, H.

    2011-12-01

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

  17. Role of H2O in Generating Subduction Zone Earthquakes

    NASA Astrophysics Data System (ADS)

    Hasegawa, A.

    2017-03-01

    A dense nationwide seismic network and high seismic activity in Japan have provided a large volume of high-quality data, enabling high-resolution imaging of the seismic structures defining the Japanese subduction zones. Here, the role of H2O in generating earthquakes in subduction zones is discussed based mainly on recent seismic studies in Japan using these high-quality data. Locations of intermediate-depth intraslab earthquakes and seismic velocity and attenuation structures within the subducted slab provide evidence that strongly supports intermediate-depth intraslab earthquakes, although the details leading to the earthquake rupture are still poorly understood. Coseismic rotations of the principal stress axes observed after great megathrust earthquakes demonstrate that the plate interface is very weak, which is probably caused by overpressured fluids. Detailed tomographic imaging of the seismic velocity structure in and around plate boundary zones suggests that interplate coupling is affected by local fluid overpressure. Seismic tomography studies also show the presence of inclined sheet-like seismic low-velocity, high-attenuation zones in the mantle wedge. These may correspond to the upwelling flow portion of subduction-induced secondary convection in the mantle wedge. The upwelling flows reach the arc Moho directly beneath the volcanic areas, suggesting a direct relationship. H2O originally liberated from the subducted slab is transported by this upwelling flow to the arc crust. The H2O that reaches the crust is overpressured above hydrostatic values, weakening the surrounding crustal rocks and decreasing the shear strength of faults, thereby inducing shallow inland earthquakes. These observations suggest that H2O expelled from the subducting slab plays an important role in generating subduction zone earthquakes both within the subduction zone itself and within the magmatic arc occupying its hanging wall.

  18. 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

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

    NASA Astrophysics Data System (ADS)

    Schellart, W. P.; Rawlinson, N.

    2013-12-01

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

  20. 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

  1. Stability of Aqueous Carbon Species in Subduction Zone Fluids

    NASA Astrophysics Data System (ADS)

    Guild, M. R.; Shock, E.

    2016-12-01

    Subduction zone fluids transfer elements from the surface to Earth's interior. Because these fluids cannot be observed at high pressures and temperatures their chemistry is enigmatic. Owing to recent advances in theoretical thermodynamic calculations the stability of aqueous species in these fluids at depth can now be modeled. This study focuses on the behavior of aqueous carbon species from 10-60 kbar and 200-1000ºC using the Deep Earth Water (DEW) model (Sverjensky et al., 2014) with implications for the deep carbon cycle. Our new estimates for C1, C2, and C3 aqueous organic compounds make it possible to test which small organic compounds are participants in the deep carbon cycle. The dominant carbon species in these fluids is commonly thought to be CO2, with minor amounts of methane. However, our calculations suggest other forms of aqueous organic carbon, such as the C1 compounds formic acid and carbon monoxide, the C2 compounds ethane, acetic acid, and ethanol, as well as the C3 compounds propane and propanoic acid are stable at 30 kbar and temperatures > 400ºC, and can account for a significant percentage of dissolved carbon in subduction fluids. These results suggest that characterizing the behavior of alkanes, alcohols, and acids at these high pressure-temperature conditions will clarify the involvement of aqueous organic carbon in the deep carbon cycle.

  2. 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.

  3. Complicated Kinematics In The Southern Cascadia Subduction Zone

    NASA Astrophysics Data System (ADS)

    McPherson, R. C.; Smith, S. W.; Williams, T. B.; Pryor, I. S.; Hemphill-Haley, M. A.

    2016-12-01

    In the southern Cascadia subduction zone, strain accumulation in the overriding North American plate is pulsed by frequent offshore earthquakes. These offshore strike-slip earthquakes, responses of the Gorda plate being forced through a tectonic die, result in triangular-shaped fragments of this plate moving under the North American plate. The integrated effect of the movement of fragments of the Gorda plate as a result of these numerous offshore quakes is to increase strain coseismically within the coupled zone from north of Humboldt Bay south to the Mendocino triple junction. The style of strain accumulation in southern Cascadia differs from the subduction zone's loading to the north, which is uncomplicated by the effects of offshore earthquakes. We propose that the southern Cascadia subduction zone is a distinct segment, pervasively sheared offshore, with a fold and thrust belt onshore, and should be considered independently in hazard models from the rest of the Cascadia subduction zone. Our findings suggest that the Cascadia subduction zone has profound "edge effects" near its southern boundary.

  4. Subduction of oceanic plate irregularities and seismicity distribution along the Mexican Subduction Zone

    NASA Astrophysics Data System (ADS)

    Manea, Marina; Constantin Manea, Vlad; Gerya, Taras; Wong, Raul-Valenzuela; Radulian, Mircea

    2017-04-01

    It is known that oceanic plates morphology is not a simple one, but rather complicated by a series of irregularities as seamounts, fracture zones and mid-ocean ridges. These features present on the oceanic floor form part of the fabric of oceanic crust, and once formed they move together with the oceanic plates until eventually enter a subduction zone. Offshore Mexico the oceanic Cocos plate seafloor is littered with relatively small but numerous seamounts and seamount chains, and also large fracture zones. In this study we investigate the relationship between these oceanic irregularities located in the vicinity of the trench in Mexico and the distribution of subduction seismicity, including the rupture history of large subduction zone earthquakes. 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. Our preliminary study shows a direct relationship between the presence of seamounts chains on the incoming oceanic plate and the subduction seismicity distribution. We also found a clear relationship between the subduction of the Tehuantepec fracture zone (TFZ) and the low seismic activity in the region where this fracture zone intersects the trench. This region is also long term conspicuously quiet and considered a seismic gap where no significant large earthquake has occurred in more than 100 years. Using high-resolution three-dimensional coupled petrological-thermomechanical numerical simulations specifically tailored for the subduction of the Cocos plate in the region of TFZ we show that the weakened serpentinized fracture zone is partially scraped out in the forearc region because of its low strength and positive buoyancy. The presence of serpentinite in the fore arc apparently lowers the degree of interseismic locking, producing a seismic gap in southern Mexico.

  5. Water sources for subduction zone volcanism: new experimental constraints.

    PubMed

    Pawley, A R; Holloway, J R

    1993-04-30

    Despite its acknowledged importance, the role of water in the genesis of subduction zone volcanism is poorly understood. Amphibole dehydration in subducting oceanic crust at a single pressure is assumed to generate the water required for melting, but experimental constraints on the reaction are limited, and little attention has been paid to reactions involving other hydrous minerals. Experiments on an oceanic basalt at pressure-temperature conditions relevant to subducting slabs demonstrate that amphibole dehydration is spread over a depth interval of at least 20 kilometers. Reactions involving other hydrous minerals, including mica, epidote, chloritoid, and lawsonite, also release water over a wide depth interval, and in some subduction zones these phases may transport water to deep levels in the mantle.

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

    NASA Astrophysics Data System (ADS)

    Mori, Yasushi; Shigeno, Miki; Nishiyama, Tadao

    2014-12-01

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

  7. Boundary conditions traps when modeling interseismic deformation at subduction zones

    NASA Astrophysics Data System (ADS)

    Contreras, Marcelo; Gerbault, Muriel; Tassara, Andres; Bataille, Klaus; Araya, Rodolfo

    2017-04-01

    In order to gain insight on the controling factors for elastic strain build-up in subduction zones, such as those triggering the Mw 8. 2010 Maule earthquake, we published a modeling study to test the influence of the subducting plate thickness, variations in the updip and downdip limit of a 100% locked interplate zone, elastic parameters, and velocity reduction at the base of the subducted slab (Contreras et al., Andean Geology 43(3), 2016). When comparing our modeled predictions with interseismic GPS observations, our results indicated little influence of the subducting plate thickness, but a necessity to reduce the velocity at the corner-base of the subducted slab below the trench region, to 10% of the far-field convergence rate. Complementary numerical models allowed us to link this velocity reduction at the base of subducting slab with a long-term high flexural stress resulting from the mechanical interaction of the slab with the underlying mantle. This study discusses that even if only a small amount of these high deviatoric stresses transfer energy towards the upper portion of the slab, it may participate in triggering large earthquakes such as the Mw8.8 Maule event. The definition of initial and boundary conditions between short-term to long-term models evidence the mechanical inconsistencies that may appear when considering pre-flexed subducting slabs and unloaded underlying asthenosphere, potentially creating mis-balanced large stress discontinuities.

  8. 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.

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

    NASA Astrophysics Data System (ADS)

    Baldwin, S.; Das, J. P.

    2013-12-01

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

  10. On subduction zone earthquakes and the Pacific Northwest seismicity

    SciTech Connect

    Chung, Dae H.

    1991-12-01

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

  11. 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

  12. 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.

  13. 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.

  14. 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.

  15. Linking giant earthquakes with the subduction of oceanic fracture zones

    NASA Astrophysics Data System (ADS)

    Landgrebe, T. C.; Müller, R. D.; EathByte Group

    2011-12-01

    Giant subduction earthquakes are known to occur in areas not previously identified as prone to high seismic risk. This highlights the need to better identify subduction zone segments potentially dominated by relatively long (up to 1000 years and more) recurrence times of giant earthquakes. Global digital data sets represent a promising source of information for a multi-dimensional earthquake hazard analysis. We combine the NGDC global Significant Earthquakes database with a global strain rate map, gridded ages of the ocean floor, and a recently produced digital data set for oceanic fracture zones, major aseismic ridges and volcanic chains to investigate the association of earthquakes as a function of magnitude with age of the downgoing slab and convergence rates. We use a so-called Top-N recommendation method, a technology originally developed to search, sort, classify, and filter very large and often statistically skewed data sets on the internet, to analyse the association of subduction earthquakes sorted by magnitude with key parameters. The Top-N analysis is used to progressively assess how strongly particular "tectonic niche" locations (e.g. locations along subduction zones intersected with aseismic ridges or volcanic chains) are associated with sets of earthquakes in sorted order in a given magnitude range. As the total number N of sorted earthquakes is increased, by progressively including smaller-magnitude events, the so-called recall is computed, defined as the number of Top-N earthquakes associated with particular target areas divided by N. The resultant statistical measure represents an intuitive description of the effectiveness of a given set of parameters to account for the location of significant earthquakes on record. We use this method to show that the occurrence of great (magnitude ≥ 8) earthquakes on overriding plate segments is strongly biased towards intersections of oceanic fracture zones with subduction zones. These intersection regions are

  16. 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.

  17. 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.

  18. Constraining the hydration of the subducting Nazca plate beneath Northern Chile using subduction zone guided waves

    NASA Astrophysics Data System (ADS)

    Garth, Tom; Rietbrock, Andreas

    2017-09-01

    Guided wave dispersion is observed from earthquakes at 180-280 km depth recorded at stations in the fore-arc of Northern Chile, where the 44 Ma Nazca plate subducts beneath South America. Characteristic P-wave dispersion is observed at several stations in the Chilean fore-arc with high frequency energy (>5 Hz) arriving up to 3 s after low frequency (<2 Hz) arrivals. This dispersion has been attributed to low velocity structure within the subducting Nazca plate which acts as a waveguide, retaining and delaying high frequency energy. Full waveform modelling shows that the single LVL proposed by previous studies does not produce the first motion dispersion observed at multiple stations, or the extended P-wave coda observed in arrivals from intermediate depth events within the Nazca plate. These signals can however be accurately accounted for if dipping low velocity fault zones are included within the subducting lithospheric mantle. A grid search over possible LVL and faults zone parameters (width, velocity contrast and separation distance) was carried out to constrain the best fitting model parameters. Our results imply that fault zone structures of 0.5-1.0 km thickness, and 5-10 km spacing, consistent with observations at the outer rise are present within the subducted slab at intermediate depths. We propose that these low velocity fault zone structures represent the hydrated structure within the lithospheric mantle. They may be formed initially by normal faults at the outer rise, which act as a pathway for fluids to penetrate the deeper slab due to the bending and unbending stresses within the subducting plate. Our observations suggest that the lithospheric mantle is 5-15% serpentinised, and therefore may transport approximately 13-42 Tg/Myr of water per meter of arc. The guided wave observations also suggest that a thin LVL (∼1 km thick) interpreted as un-eclogitised subducted oceanic crust persists to depths of at least 220 km. Comparison of the inferred seismic

  19. Stress Distribution in the Subducted Slab in the Transition Zone

    NASA Astrophysics Data System (ADS)

    Běhounková, M.; Běhounková, M.; Čížková, H.; Matyska, C.; Špičák, A.

    2006-12-01

    We present the results of numerical modelling of subduction process in a 2-D cartesian box. Our numerical code is based on the method of Gerya and Yuen 2003. We concentrate on the deformation and stress distribution within the slab in the transition zone. Our composite rheological model includes diffusion creep, dislocation creep and power-law stress limiter. The effects of phase transitions at the depths 410 km and 660 km are taken into account. The model is applied to the Tonga subduction region, where the currently subducting plate might face the remnants of the high viscosity subducted material in the transition zone. This material might possibly originate either from a previous episode of the subduction (Chen and Brudzinski, 2001) or from the buoyant detached slab broken off from the active subducting slab (Green, 2001). We prescribe the cold and relatively high viscosity piece of old slab lying above the 660 km interface. The stress distribution in the new subducting place is then investigated as the plate approaches these remnants of old slab. Stress directions and amplitudes are compared to the data available from the analyses of the earthquake mechanisms in Tonga region. Chen W.-P., Brudzinski R, 2001. Evidence for a Large-Scale Remnant of Subducted Lithosphere Beneath Fiji, Science 292, 2475--2479. Gerya T.V., Yuen D.A., 2003. Characteristics-based marker-in-cell method with conservative finite-differences schemes for modelling geological flows with strongly variable transport properties, Phys. Earth Planet. Int. 140, 293--318. Green, H.W., 2001. A graveyard for buoyant slabs?, Science 292, 2445-2446.

  20. Rutile solubility in H2O-NaAlSi3O8 fluids at High T and P: Implications form HFSE mobility in Subduction zones

    NASA Astrophysics Data System (ADS)

    Antignano, A.; Manning, C. E.

    2005-12-01

    been proposed that polymerized aqueous albite-like molecules occur in high-P fluids (Manning, 2004). Since field observations show that subduction zone fluids contain significant quantities of dissolved Na, Al and Si, then silicate complexing almost certainly controls Ti solubility in natural settings. Substitution of Ti into such complexes could explain the presence of rutile in the fluid-derived veins found in subduction zone complexes. This suggests that models of HFSE mobility which are based on rutile solubility in pure H2O may significantly underestimate the ability of water-rich fluids to transport these elements.

  1. Resolution Study of Marine CSEM Imaging of Subduction Zones

    NASA Astrophysics Data System (ADS)

    Gustafson, C.; Key, K.

    2016-12-01

    Marine controlled source electromagnetic (CSEM) data allow us to image seafloor electrical resistivity from which we can constrain the porosity and fluid content of the subsurface. In subduction zones, CSEM data can be used to constrain geologic structure, hydrogeology and fluid-tectonic processes. The scales of features we are interested in recovering with CSEM data range from large-scale features such as the incoming tectonic plate and subducting slab, to the narrow dipping plate boundary interface where slip occurs, to thin faults that cut the overriding forearc crust and shallow fluid seeps and mounds on the seafloor. Thus electrical structure is expected to vary on scales ranging from scales of meters to tens of kilometers. CSEM data collected by Scripps at the Middle America Trench in 2010 is the first and to-date the only application of the method for studying a subduction zone. The results from this pioneering data set highlight the types of new discoveries that are possible with CSEM data, such as imaging conductive bending faults and a water-rich channel of subducting sediments. In this work we explore the magnitude and scale of 2D resistivity structures that can be resolved with CSEM data through a suite of synthetic inversion studies. We build resistivity models that are representative of various known and hypothesized subduction zone plate boundary structures. We generate synthetic noisy data for these models and invert them using the freely available MARE2DEM inversion code. We compare the recovered models to the original models in order to determine which resistivity structures may be successfully identified using CSEM. We explore the potential effects of receiver spacing, frequency bandwidth and system noise levels on the ability of CSEM to recover these different subduction zone structures.

  2. Chlorine and fluorine partition coefficients and abundances in sub-arc mantle xenoliths (Kamchatka, Russia): Implications for melt generation and volatile recycling processes in subduction zones

    NASA Astrophysics Data System (ADS)

    Bénard, A.; Koga, K. T.; Shimizu, N.; Kendrick, M. A.; Ionov, D. A.; Nebel, O.; Arculus, R. J.

    2017-02-01

    We report chlorine (Cl) and fluorine (F) abundances in minerals, interstitial glasses, and melt inclusions in 12 andesite-hosted, spinel harzburgite xenoliths and crosscutting pyroxenite veins exhumed from the sub-arc lithospheric mantle beneath Avacha volcano in the Kamchatka Arc (NE Russia). The data are used to calculate equilibrium mineral-melt partition coefficients (D mineral / melt) for Cl and F relevant to subduction-zone processes and unravel the history of volatile depletion and enrichment mechanisms in an arc setting. Chlorine is ∼100 times more incompatible in pyroxenes (DClmineral/melt = 0.005-0.008 [±0.002-0.003]) than F (DFmineral/melt = 0.50-0.57 [±0.21-0.24]), which indicates that partial melting of mantle sources leads to strong depletions in Cl relative to F in the residues. The data set in this study suggests a strong control of melt composition on DCl,Fpyroxene/melt, in particular H2O contents and Al/(Al + Si), which is in line with recent experiments. Fluorine is compatible in Ca-amphibole in the 'wet' sub-arc mantle (DFamphibole/melt = 3.5-3.7 [±1.5]) but not Cl (DClamphibole/melt = 0.03-0.05 [±0.01-0.03]), indicating that amphibole may fractionate F from Cl in the mantle wedge. The inter-mineral partition coefficients for Cl and F in this study are consistent amongst different harzburgite samples, whether they contain glass or not. In particular, disseminated amphibole hosts much of the Cl and F bulk rock budgets of spinel harzburgites (DClamphibole/pyroxene up to 14 and DFamphibole/pyroxene up to 40). Chlorine and fluorine are variably enriched (up to 1500 ppm Cl and 750 ppm F) in the parental arc picrite and boninite melts of primitive pyroxenite veins (and related melt inclusions) crosscutting spinel harzburgites.

  3. Slab Rollback and Subduction Erosion Model for the North Pamir - Alai Intracontinental Subduction Zone

    NASA Astrophysics Data System (ADS)

    Sobel, E. R.; Schoenbohm, L. M.; Chen, J.; Thiede, R. C.; Stockli, D. F.; Sudo, M.

    2011-12-01

    Cenozoic convergence between the North Pamir and the Tien Shan is thought to have been primarily accommodated along the south-dipping intracontinental Alai subduction zone. The North Pamir has moved ~300 km north with respect to stable Asia along the surface trace of this subduction, the Main Pamir Thrust (MPT) system, subducting the basin now represented only by the Alai Basin and the westernmost portion of the Tarim Basin. As there is no evidence that the Tien Shan has moved southward during the late Cenozoic, we suggest that significant northward motion of the North Pamir is best explained as a consequence of slab rollback in a contractile setting. The ca. 500 km along-strike width of the North Pamir is extremely short for a subduction zone; the belt is highly concave. Published compilations show that short plate segments are characterized by strong curvature and rapid slab rollback rates. The MPT system has previously been treated as a large overthrust. The hanging wall of such a large structure should have experienced significant exhumation. However, new and previously published thermochronologic data show that the North Pamir experienced only minor late Cenozoic exhumation. If the North Pamir is viewed as the overriding plate in a subduction zone, the lack of significant exhumation may be explained by subduction erosion, which can remove material from the toe of the overriding plate without causing significant crustal thickening. Subduction erosion is common in slab rollback settings. In our model, early-middle Miocene north-south extension in the E-W trending Central Pamir domes is related to back-arc extension, while the 11 Ma Taxkorgan alkali complex and subsequent east-west extension in the N-S trending Kongur detachment are related to slab rollback edge effects. Published studies of the deformation history of the Alai Basin, westernmost Tarim, and Tien Shan suggest two main periods of Cenozoic deformation: in the Oligo-Miocene and since the mid

  4. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2010-12-01

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

  6. 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.

  7. Highly oxidising fluids generated during serpentinite breakdown in subduction zones.

    PubMed

    Debret, B; Sverjensky, D A

    2017-09-04

    Subduction zones facilitate chemical exchanges between Earth's deep interior and volcanism that affects habitability of the surface environment. Lavas erupted at subduction zones are oxidized and release volatile species. These features may reflect a modification of the oxidation state of the sub-arc mantle by hydrous, oxidizing sulfate and/or carbonate-bearing fluids derived from subducting slabs. But the reason that the fluids are oxidizing has been unclear. Here we use theoretical chemical mass transfer calculations to predict the redox state of fluids generated during serpentinite dehydration. Specifically, the breakdown of antigorite to olivine, enstatite, and chlorite generates fluids with high oxygen fugacities, close to the hematite-magnetite buffer, that can contain significant amounts of sulfate. The migration of these fluids from the slab to the mantle wedge could therefore provide the oxidized source for the genesis of primary arc magmas that release gases to the atmosphere during volcanism. Our results also show that the evolution of oxygen fugacity in serpentinite during subduction is sensitive to the amount of sulfides and potentially metal alloys in bulk rock, possibly producing redox heterogeneities in subducting slabs.

  8. 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

  9. 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

  10. 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

  11. The seismic structure of the Rivera subduction zone

    NASA Astrophysics Data System (ADS)

    Grand, S. P.; Yang, T.; Wilson, D.; Guzman Speziale, M.; Gomez Gonzalez, J.; Dominguez Reyes, T.; Ni, J.

    2007-12-01

    The subduction zone of western Mexico is a unique region on Earth where microplate capture and overriding plate disruption are occurring today. The small Rivera plate is subducting beneath western most Mexico primarily beneath Jalisco state while to the east it is the Cocos plate that is subducting. Above the Rivera plate the Jalisco block of Mexico is bounded by the north trending Colima Rift and the northwest trending Tepic-Chapala Rift and may form a microplate in its own right. Magmatism is present throughout the region and is unusual for a subduction zone in that geochemical analyses indicate an ocean island basalt component to some of the lavas. Also, Colima volcano is offset trenchward from other volcanoes in the Mexican Volcanic Belt. Little is known of the subducting Rivera plate geometry due to the paucity of seismicity within the plate yet the geometry of the Rivera and Cocos plates at depth are likely critical for understanding the tectonic evolution of western Mexico. The MARS (MApping the Rivera Subduction zone) project consists of the deployment of 50 broadband seismometers covering the Jalisco block from the coast to the Tepic-Chapala rift in the north and about 150 km to the west of the Colima rift. The instruments were deployed in January, 2006 and removed in June, 2007. The goal of the project is to seismically image the subducting Rivera and Cocos plates at depth as well as the mantle wedge above the plates. We present the results of a P-wave tomography inversion using teleseisms recorded by MARS. The inversion used 10,495 residuals and finite frequency theory to back project the kernels through the model. At shallow depths it is difficult to discern the subducting Cocos and Rivera plates but at depths deeper than about 80 km both plates are clearly imaged in the tomography model. Below a depth of 150 km, a clear gap between the Rivera and Cocos slabs is apparent that increases in size at further depths. The images indicate that the deeper

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

    PubMed

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

    2007-03-15

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

  13. Slab anisotropy from subduction zone guided waves in Taiwan

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

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

  14. 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

  15. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

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

  17. 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.

    2016-12-01

    Deformation in rocks and sediments is controlled by multiple mechanisms, each governed by its own pressure- (P), temperature- (T), and slip velocity- (v) dependent kinetics. Frictional behavior depends on which of these mechanisms are dominant, and, thus, varies with P, T, and v. Carbonates are a useful material with which to interrogate the PTv controls on friction due to the fact that a wide range of mechanisms can be easily accessed in the lab at geologically relevant conditions. In addition, carbonate-rich layers make up a significant component of subducting sediments around the world and may impact the frictional behavior of shallow subduction zones. In order to investigate the effect of carbonate subduction and the evolution of friction at subduction zone conditions, we conducted deformation experiments on 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 σeff = 1-100 MPa and T = 20-100 °C with sliding velocities of 1-300 μm/s and hold times of 1-1000 s. We test the changes in velocity dependence and healing over these PT conditions to elucidate the frictional behavior of carbonates in subduction zone settings. The mechanical results are complemented by microstructural analysis. In lower stress experiments, there is no obvious shear localization; however, by 25 MPa, pervasive boundary-parallel shears become dominant, particularly in the Peru samples. Optical observations of these shear zones under cross-polarized light show evidence of plastic deformation (CPO development) while SEM-EDS observations indicate phase segregation in the boundary shears. Degree of microstructural localization appears to correspond with the trends observed in velocity-dependence. Our

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

    NASA Technical Reports Server (NTRS)

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

    1985-01-01

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

  19. 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

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

    NASA Astrophysics Data System (ADS)

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

    2007-05-01

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

  1. 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.

  2. Subduction zone decoupling/retreat modeling explains south Tibet (Xigaze) and other supra-subduction zone ophiolites and their UHP mineral phases

    NASA Astrophysics Data System (ADS)

    Butler, Jared P.; Beaumont, Christopher

    2017-04-01

    The plate tectonic setting in which proto-ophiolite 'oceanic' lithosphere is created remains controversial with a number of environments suggested. Recent opinions tend to coalesce around supra-subduction zone (SSZ) forearc extension, with a popular conceptual model in which the proto-ophiolite forms during foundering of oceanic lithosphere at the time of spontaneous or induced onset of subduction. This mechanism is favored in intra-oceanic settings where the subducting lithosphere is old and the upper plate is young and thin. We investigate an alternative mechanism; namely, decoupling of the subducting oceanic lithosphere in the forearc of an active continental margin, followed by subduction zone (trench) retreat and creation of a forearc oceanic rift basin, containing proto-ophiolite lithosphere, between the continental margin and the retreating subduction zone. A template of 2D numerical model experiments examines the trade-off between strength of viscous coupling in the lithospheric subduction channel and net slab pull of the subducting lithosphere. Three tectonic styles are observed: 1) C, continuous subduction without forearc decoupling; 2) R, forearc decoupling followed by rapid subduction zone retreat; 3) B, breakoff of subducting lithosphere followed by re-initiation of subduction and in some cases, forearc decoupling (B-R). In one case (BA-B-R; where BA denotes backarc) subduction zone retreat follows backarc rifting. Subduction zone decoupling is analyzed using frictional-plastic yield theory and the Stefan solution for the separation of plates containing a viscous fluid. The numerical model results are used to explain the formation of Xigaze group ophiolites, southern Tibet, which formed in the Lhasa terrane forearc, likely following earlier subduction and not necessarily during subduction initiation. Either there was normal coupled subduction before subduction zone decoupling, or precursor slab breakoff, subduction re-initiation and then decoupling

  3. 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.

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

    NASA Astrophysics Data System (ADS)

    Contreras-Reyes, Eduardo; Carrizo, Daniel

    2011-05-01

    We discuss the earthquake rupture behavior along the Chile-Peru subduction zone in terms of the buoyancy of the subducting high oceanic features (HOF's), and the effect of the interplay between HOF and subduction channel thickness on the degree of interplate coupling. We show a strong relation between subduction of HOF's and earthquake rupture segments along the Chile-Peru margin, elucidating how these subducting features play a key role in seismic segmentation. Within this context, the extra increase of normal stress at the subduction interface is strongly controlled by the buoyancy of HOF's which is likely caused by crustal thickening and mantle serpentinization beneath hotspot ridges and fracture zones, respectively. Buoyancy of HOF's provide an increase in normal stress estimated to be as high as 10-50 MPa. This significant increase of normal stress will enhance seismic coupling across the subduction interface and hence will affect the seismicity. In particular, several large earthquakes (Mw ≥ 7.5) have occurred in regions characterized by subduction of HOF's including fracture zones (e.g., Nazca, Challenger and Mocha), hotspot ridges (e.g., Nazca, Iquique, and Juan Fernández) and the active Nazca-Antarctic spreading center. For instance, the giant 1960 earthquake (Mw = 9.5) is coincident with the linear projections of the Mocha Fracture Zone and the buoyant Chile Rise, while the active seismic gap of north Chile spatially correlates with the subduction of the Iquique Ridge. Further comparison of rupture characteristics of large underthrusting earthquakes and the locations of subducting features provide evidence that HOF's control earthquake rupture acting as both asperities and barriers. This dual behavior can be partially controlled by the subduction channel thickness. A thick subduction channel smooths the degree of coupling caused by the subducted HOF which allows lateral earthquake rupture propagation. This may explain why the 1960 rupture propagates

  5. 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

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

    NASA Astrophysics Data System (ADS)

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

    2015-10-01

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

  7. The Global Range of Subduction Zone Thermal Structures From Exhumed Blueschists and Eclogites: Rocks are Hotter than Models

    NASA Astrophysics Data System (ADS)

    Penniston-Dorland, S.; Kohn, M. J.; Manning, C. E.

    2015-12-01

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

  8. Volcanism and aseismic slip in subduction zones

    SciTech Connect

    Acharya, H.

    1981-01-10

    The spatial and temporal relationship of volcanism to the occurrence of large earthquakes and convergent plate motion is examined. The number of volcanic eruptions per year in a convergent zone is found to be linearly related to the aseismic slip component of plate motion. If the aseismic slip rate is low (coupling between converging plates is strong), then the primary manifestation of tectonic activity is the occurrence of large earthquakes with only infrequent volcanic activity. If, however, the aseismic slip rate is high (coupling is weak), then there are few large earthquakes, and volcanism is the principal manifestation of tectonic activity. This model is consistent with the spatial distribution of large earthquakes and active volcanoes in the circum-Pacific area. It is tested by examining the extent of volcanic activity in the rupture zones of the 1952--1973 sequence of earthquakes in the Japan--Kurile Islands area. The number of volcanic euptions along these zones during the interval between large earthquakes is used to compute the aseismic slip rates for these segments, based on the relationship developed in this study. The aseismic slip rates so computed agree with those determined from the earthquake history of the area and rates of plate motion. The agreement suggests that in the interval between large earthquakes, the aseismic plate motion is manifested in a specific number of volcanic eruptions. Therefore in areas with adequate historial data it should be possible to use the model developed in this study to monitor volcanic eruptions for long-term prediction of large earthquakes.

  9. A subduction zone reference frame based on slab geometry and subduction partitioning of plate motion and trench migration

    NASA Astrophysics Data System (ADS)

    Schellart, W. P.

    2011-08-01

    The geometry of subducted slabs that interact with the transition zone depends critically on the partitioning of the subduction velocity (vS$\\perp$) at the surface into its subducting plate motion component (vSP$\\perp$) and trench migration component (vT$\\perp$). Geodynamic models of progressive subduction demonstrate such dependence with five distinct slab geometries and corresponding partitioning ratios (vSP$\\perp$/vS$\\perp$): slab draping (vSP$\\perp$/vS$\\perp$ ≤ 0.5), slab draping with recumbent folds (0.5 < vSP$\\perp$/vS$\\perp$ < ˜0.8), slab piling (˜0.8 ≤ vSP$\\perp$/vS$\\perp$ ≤ ˜1.2), slab roll-over with recumbent folds (˜1.2 < vSP$\\perp$/vS$\\perp$ < ˜1.5) and slab roll-over (vSP$\\perp$/vS$\\perp$ ≥ ˜1.5). The model findings have been applied to subduction zones in nature with well-resolved slab geometries, for which subduction partitioning ratios have been calculated during the last 20 million years in two global reference frames: the Indo-Atlantic and Pacific hotspot reference frames. The model-nature comparison determines in which reference frame subduction partitioning ratios are most in agreement with observed slab geometries. In the Indo-Atlantic frame, five (out of five) selected subduction zone segments with well-resolved slab geometries, plate velocities and trench velocities (Japan, Izu-Bonin, Mariana, Tonga, Kermadec) agree with the geodynamic model predictions, as calculated subduction partitioning ratios match the observed slab geometries. In the Pacific frame the partitioning ratio of only one subduction zone segment (Izu-Bonin) matches observations. It is thus concluded that the Indo-Atlantic hotspot reference frame is preferred over the Pacific one as a subduction zone reference frame in which to describe plate motions, subduction kinematics and mantle flow.

  10. Zinc isotope systematics of subduction-zone magmas

    NASA Astrophysics Data System (ADS)

    Huang, J.; Zhang, X. C.; Huang, F.; Yu, H.

    2016-12-01

    Subduction-zone magmas are generated by partial melting of mantle wedge triggered by addition of fluids derived from subducted hydrothermally altered oceanic lithosphere. Source of the fluids may be sediment, altered oceanic crust and serpentinized peridotite/serpentinite. Knowledge of the exact fluid source can facilitate our better understanding of the mechanism of fluid flux, element cycling and crust-mantle interaction in subduction zones. Zinc isotopes have the potential to place a constraint on this issue, because (1) Zn has an intermediate mobility during fluid-rock interaction and is enriched in subduction-zone fluids (e.g., Li et al., 2013); (2) sediment, altered oceanic crust and serpentinite have distinct Zn isotopic compositions (Pons et al., 2011); and (3) the mantle has a homogeneous Zn isotope composition (δ66Zn = 0.28 ± 0.05‰, Chen et al., 2013). Thus, the Zn isotopic composition of subduction-zone magmas reflects the characteristics of slab-derived fluids of different sources. Here, high-precision Zn isotope analyses were conducted on igneous rocks from arcs of Central America, Kamchatka, South Lesser Antilles, and Aleutian. One rhyolite with 75.1 wt.% SiO2 and 0.2 wt.% FeOT displays the heaviest δ66Zn value of 0.394‰ (relative to JMC Lyon) that probably results from the crystallization of Fe-Ti oxides during the late-stage differentiation. The rest of rocks have Zn isotopic compositions (0.161 to 0.339‰) similar to or lighter than those of the mantle. In an individual arc, the δ66Zn values of rocks show broad negative correlations with Ba/Th and 87Sr/86Sr ratios, suggesting that the slab-derived fluids should have lighter δ66Zn as well as higher Ba/Th and 87Sr/86Sr ratios relative to the mantle. These features are in accordance with those of serpentinites. Thus, addition of serpentinite-derived 66Zn-depleted fluids into the mantle wedge can explain the declined δ66Zn of subduction-zone magmas. ReferenceChen et al. (2013) EPSL 369

  11. 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

  12. Repeating earthquakes analysis in the Taiwan-Ryukyu subduction zone

    NASA Astrophysics Data System (ADS)

    Chang, Y. E.; Chen, K. H.; Chen, S.

    2012-12-01

    The understanding of interplate slip has been limited in the Taiwan-Ryukyu subduction zone due to sparse sampling of seismic and geodetic stations. Repeating earthquakes study has been proposed to improve the understanding of deep fault behavior. However, in this particular regions where spatial coverage of seismic station is sparse and one sided with high noise level or where starting catalog origin times are inaccurate; establishing waveform cross-correlation method (wcc) for rapid detection of repeating earthquakes is problematic. When the wcc method is applied over a large area of 100 Km x 300 km for M>2 earthquakes at a depth range of 0-50 km (12932 events in total), a large number of false detections occurred due to low-amplitude signal templates. This leads additional time-consumingefforts on manual re-identification. In this study we propose an efficient repeating sequence identification scheme in the regions where the station coverage is sparse with low signal to noise ratio. Using this identification method, we found 9 (M2.5~4.5) interplate repeating sequences in the Taiwan-Ryukyu subduction zone at 35-45 km depth. The interplate repeating earthquakes tend to be short-lived in the form of doublet or multiplet, which the association between doublet/multiplet generation and subduction zone processes requires further study. Other crustal repeating earthquakes occurred at shallow depth above 20 km; these sequences have occurrence rate strongly associated with the nearby M6 earthquakes. This identification scheme will be applied to deeper events (> 50 km depth) for a more complete image of spatial and temporal distribution of interpolate aseismic slip in the Taiwan-Ryukyu subduction zone .

  13. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2014-07-01

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

  15. 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

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

    PubMed

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

    2014-07-14

    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.

  17. 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

  18. 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

  19. 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)

  20. 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

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

    PubMed

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

    2011-05-17

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

  2. Satellite Gravity Gradients Bound Rupture Domains on Subduction Zone Megathrusts

    NASA Astrophysics Data System (ADS)

    Wells, R. E.; Blakely, R. J.

    2016-12-01

    Sea surface altimetry from satellites provides a synoptic view of the bathymetry, gravity, and structure of the world's ocean basins and its subduction zones. Researchers over the past decade have documented apparent relations between forearc structures revealed by gravity and the distribution of slip in recent subduction zone earthquakes. In 2003, we hypothesized that asperities, or loci of greatest seismic moment release in great megathrust earthquakes preferentially occurred beneath the Deep Sea Terrace gravity Low (DSTL), which lies between two maximum gravity gradients; one along the lower slope break near the trench and another on the upper slope, landward of the offshore forearc basins. We further suggested the asperities tended to occur beneath the forearc basins. Thirteen subsequent great earthquakes, some with large slip beneath local gravity highs, have falsified the second hypothesis, most notably off Tohoku in 2011. Bassett and Watts in 2015 incorporated these new data in a global synthesis of the relation between megathrust seismic slip and forearc gravity. They documented a clear relation between the down dip limit of seismic slip and the axis of the Trench Parallel Forearc gravity high, or Ridge (TPFR), and they showed that stick-slip behavior was bracketed by the TPFR and the lower slope gradient. We note that these strong gravity gradients and the TPFR may outline the 4 depth-delimited domains of subduction zone rupture behavior described by Lay et al. (2012). For the 2011 Tohoku and 2010 Maule earthquakes: Domain A, the great tsunami launcher, lies between the trench and the lower slope gravity gradient. Domain B, the source of large slip earthquakes and modest short period energy lies between the lower and the upper slope gradients. Domain C, with its short period seismic sources, lies between the upper slope gradient and the axis of the TPFR. Domain D, landward of the axis of the TPFR, is the locus of seismic tremor and slow slip events in warm

  3. 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.

  4. Crust and subduction zone structure of Southwestern Mexico

    NASA Astrophysics Data System (ADS)

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

    2015-02-01

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

  5. Low velocity layer (LVL) in subduction zones: elasticity of lawsonite

    NASA Astrophysics Data System (ADS)

    Chantel, J.; Mookherjee, M.; Frost, D. J.

    2010-12-01

    As the oceanic plates subduct, they undergoes a series of phase transformations. The hydrated oceanic crust undergoes dehydrations and eventually transforms to eclogite. However, in cold subduction zones such transformations are kinetically hindered. Eclogite is dense, and its elastic properties are similar to the normal peridotitic mantle. On the other hand, the seismic wave speeds in basalts are 10-15% slower than harzburgite. In certain subduction zones, including southern Japan, a 5-10 km think low velocity layer (LVL) has been observed. The LVL is around 5-7% slower than the surrounding mantle and cannot be readily explained by the presence of meta-stable basalts. Instead, a metamorphic rock such as lawsonite-blueschist is a likely candidate for explaining the observed LVL. We have conducted high-pressure ultrasonic interferometric measurements to gain insight into the elastic properties of lawsonite [CaAl2(Si2O7)(OH)2.H2O]. In addition, we have also computed the full elastic constant tensor at elevated pressures, using electronic structure calculations. The bulk and shear modulus obtained from theory and experiments are in good agreement with an adiabatic bulk (K) and shear (G) moduli of 126.2 ± 0.3 GPa and 52.7 ± 0.2 GPa respectively. The pressure derivaitive of bulk modulus (K’) and shear modulus (G’) are 3.5 and 1.1 respectively. Indeed, lawsonite has unusually low shear modulus and might be a suitable candidate phase to explain the observed LVL in subduction zones.

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

    NASA Astrophysics Data System (ADS)

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

    2015-06-01

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

  7. Subduction Channel Thickening and Thinning: Implications for Interplate Seismicity

    NASA Astrophysics Data System (ADS)

    Cloos, Mark

    2013-04-01

    Reconciling the viscous behavior inferred along the plate interface zone in the subduction channel model with the global variations in subduction zone seismicity is a matter of geodynamic importance. Thermal modeling indicates that where subduction is slow (<2 cm/yr) or the incoming plate is very young (<5 Ma), 300°C temperatures are present at depths as shallow as 20 km. Consequently, intracrystalline creep dominates in the shear zone and earthquakes are limited to shallow depths. Where subduction is fast (> 4 cm/yr), the plate interface zone cools to great depth and interplate earthquakes occur to depths as great as 60 km. Thermal modeling and many petrological observations indicate temperature/depth trajectories near the plate interface can become as cold as 6°C/km. As first emphasized by Uyeda and Kanamori (1979), there is a wide range in the fraction of the plate convergence that manifests itself as thrust-type seismicity at rapidly convergent plate margins. They characterized the end-member behaviors as Mariana-type where only a small fraction of the plate convergence is evident from seismogenic movements and Chilean-type where a large fraction of the plate convergence is accomodated by slip during large earthquakes (M>7.5). Mariana-type margins are sites of subduction erosion because sediment supply is less than channel capacity, the shear zone is thin and shear stresses are high near the inlet. The long-term mechanical behavior of Chilean-type margins is accretionary because sediment supply is greater than channel capacity. Shear stresses are lower where the shear zone is thicker. The association of infrequent large earthquakes with thicker zones of subducting sediment is especially problematic if the build up of large elastic strains is attributed to friction along a planer interface (decollement) within compacting and metamorphosing sediments. The subduction channel concept postulates that the shear from convergence becomes distributed in the subducting

  8. Subducting an old subduction zone sideways provides insights into what controls plate coupling

    NASA Astrophysics Data System (ADS)

    Reyners, Martin; Eberhart-Phillips, Donna; Bannister, Stephen

    2017-05-01

    The Hikurangi Plateau has had two episodes of subduction beneath New Zealand - firstly at ca. 100 Ma during north-south convergence with Gondwana, and currently during east-west convergence between the Pacific and Australian plates. As a result of this ninety-degree change in convergence direction, an old subduction zone is now being subducted sideways, and the tectonic history of the subducted plate varies dramatically along the strike of the Hikurangi Margin. Here we identify the location of the underplated Hikurangi Plateau along the shallow part of the Hikurangi Margin, using results from both relocated seismicity and seismic tomography. Next we decipher the tectonic history of the plateau along strike, particularly in terms of the hydration state of the plateau, and the nature of any sedimentary rock units capping the plateau. We then use this information to understand plate coupling at two scales: on the large scale, the southward transition from typical subduction in the North Island to continental collision in the South Island; and at a smaller scale, the strong lateral change from a high deficit in slip rate at the plate interface in the southern North Island to a low deficit in slip rate in the northeastern North Island. We find that the southward transition from subduction to continental collision is controlled by the plateau being more dehydrated to the south, as a result of being more deeply subducted at the Gondwana margin. The southward transition from localized slip at the plate interface to distributed upper plate deformation with no active plate interface occurs in Cook Strait and is relatively sharp. The high deficit in slip rate at the plate interface in the southern North Island is likely due to a relatively smooth plate interface from sedimentary rocks capping the Hikurangi Plateau, an impermeable terrane in the overlying plate, and the hydrated plateau acting in concert to produce an interseismically sealed plate interface. Further northeast

  9. 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.

  10. Deep crustal fracture zones control fluid escape and the seismic cycle in the Cascadia subduction zone

    NASA Astrophysics Data System (ADS)

    Tauzin, Benoît; Reynard, Bruno; Perrillat, Jean-Philippe; Debayle, Eric; Bodin, Thomas

    2017-02-01

    Seismic activity and non-volcanic tremors are often associated with fluid circulation resulting from the dehydration of subducting plates. Tremors in the overriding continental crust of several subduction zones suggest fluid circulation at shallower depths, but potential fluid pathways are still poorly documented. Using receiver function analysis in the Cascadia subduction zone, we provide evidence for a seismic discontinuity near 15 km depth in the crust of the overriding North American plate. This interface is segmented, and its interruptions are spatially correlated with conductive regions of the forearc and shallow swarms of seismicity and non-volcanic tremors. These observations suggest that fluid circulation in the overriding plate is controlled by fault zones separating blocks of accreted terranes. These zones constitute fluid escape routes that may influence the seismic cycle by releasing fluid pressure from the megathrust.

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

    NASA Astrophysics Data System (ADS)

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

    2016-03-01

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

  12. 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.

  13. 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.

  14. Water and the oxidation state of subduction zone magmas.

    PubMed

    Kelley, Katherine A; Cottrell, Elizabeth

    2009-07-31

    Mantle oxygen fugacity exerts a primary control on mass exchange between Earth's surface and interior at subduction zones, but the major factors controlling mantle oxygen fugacity (such as volatiles and phase assemblages) and how tectonic cycles drive its secular evolution are still debated. We present integrated measurements of redox-sensitive ratios of oxidized iron to total iron (Fe3+/SigmaFe), determined with Fe K-edge micro-x-ray absorption near-edge structure spectroscopy, and pre-eruptive magmatic H2O contents of a global sampling of primitive undegassed basaltic glasses and melt inclusions covering a range of plate tectonic settings. Magmatic Fe3+/SigmaFe ratios increase toward subduction zones (at ridges, 0.13 to 0.17; at back arcs, 0.15 to 0.19; and at arcs, 0.18 to 0.32) and correlate linearly with H2O content and element tracers of slab-derived fluids. These observations indicate a direct link between mass transfer from the subducted plate and oxidation of the mantle wedge.

  15. Geoid anomalies in the vicinity of subduction zones

    NASA Technical Reports Server (NTRS)

    Mcadoo, D. C.

    1980-01-01

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

  16. The thermal effect of fluid circulation in the subducting crust on slab melting in the Chile subduction zone

    NASA Astrophysics Data System (ADS)

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

    2016-01-01

    Fluids released from subducting slabs affect geochemical recycling and melt generation in the mantle wedge. The distribution of slab dehydration and the potential for slab melting are controlled by the composition/hydration of the slab entering a subduction zone and the pressure-temperature path that the slab follows. We examine the potential for along-strike changes in temperatures, fluid release, and slab melting for the subduction zone beneath the southern portion of the Southern Volcanic Zone (SVZ) in south central Chile. Because the age of the Nazca Plate entering the subduction zone decreases from ∼14 Ma north of the Guafo Fracture Zone to ∼6 Ma to the south, a southward warming of the subduction zone has been hypothesized. However, both north and south of Guafo Fracture Zone the geochemical signatures of southern SVZ arc lavas are similar, indicating 3-5 wt.% sediment melt and little to no contribution from melt of subducted basalt or aqueous fluids from subducted crust. We model temperatures in the system, use results of the thermal models and the thermodynamic calculation code Perple_X to estimate the pattern of dehydration-derived fluid release, and examine the potential locations for the onset of melting of the subducting slab. Surface heat flux observations in the region are most consistent with fluid circulation in the high permeability upper oceanic crust redistributing heat. This hydrothermal circulation preferentially cools the hottest parts of the system (i.e. those with the youngest subducting lithosphere). Models including the thermal effects of fluid circulation in the oceanic crust predict melting of the subducting sediment but not the basalt, consistent with the geochemical observations. In contrast, models that do not account for fluid circulation predict melting of both subducting sediment and basalt below the volcanic arc south of Guafo Fracture Zone. In our simulations with the effects of fluid circulation, the onset of sediment

  17. Resolution experiments for NW Pacific subduction zone tomography

    NASA Technical Reports Server (NTRS)

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

    1989-01-01

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

  18. 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

  19. 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

  20. 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

  1. Earthquake supercycle in subduction zones controlled by the downdip width of the seismogenic zone

    NASA Astrophysics Data System (ADS)

    Herrendörfer, Robert; van Dinther, Ylona; Gerya, Taras; Dalguer, Luis Angel

    2015-04-01

    Supercycles describe a long-term cluster of megathrust earthquakes that consist of partial ruptures before a complete failure of a subduction zone segment (Sieh et al. 2008, Goldfinger et al. 2013). The controls on supercycles remain unclear, although structural or fault frictional heterogeneities were proposed (Sieh et al. 2008). We recognize that supercycles have been suggested in those subduction zones (Sieh et al. 2008, Goldfinger et al. 2013, Metois et al. 2014, Chlieh et al. 2014) for which the seismogenic zone downdip width is estimated (Heuret et al. 2011, Hayes et al. 2012, Hayes et al. 2013) to be larger than average. Here we assess this potential link between the seismogenic zone downdip width and supercycles. For this purpose we use the continuum-based seismo-mechanical model of megathrust earthquake cycles in subduction zones (Van Dinther et al. 2013), which was validated through a comparison against scaled analogue subduction experiments (Corbi et al. 2013). The two-dimensional numerical model setup consists of a visco-elastic wedge underthrusted by a rigid plate and a frictional boundary layer simulating the megathrust. In this boundary layer, we evaluate a non-associative Drucker-Prager plasticity with pressure dependent yield strength and a strongly rate-dependent friction formulation. The velocity-weakening seismogenic zone with a downdip width W is limited up-and downdip by velocity-strengthening regions. In our numerical models, an increasing seismogenic zone downdip width leads to a transition from ordinary cycles of similar sized complete ruptures to supercycles. For supercycles in wide seismogenic zones, we demonstrate how interseismic deformation accompanied by partial ruptures effectively increases the stress throughout the seismogenic zone until a crack-like superevent releases most of the accumulated stresses. Our findings suggest that supercycles are more likely to occur in subduction zones with a large seismogenic downdip width due to a

  2. 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

  3. 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.

  4. Geodynamic Modeling of the Subduction Zone around the Japanese Islands

    NASA Astrophysics Data System (ADS)

    Honda, S.

    2017-06-01

    In this review, which focuses on our research, we describe the development of the thermomechanical modeling of subduction zones, paying special attention to those around the Japanese Islands. Without a sufficient amount of data and observations, models tended to be conceptual and general. However, the increasing power of computational tools has resulted in simple analytical and numerical models becoming more realistic, by incorporating the mantle flow around the subducting slab. The accumulation of observations and data has made it possible to construct regional models to understand the detail of the subduction processes. Recent advancements in the study of the seismic tomography and geology around the Japanese Islands has enabled new aspects of modeling the mantle processes. A good correlation between the seismic velocity anomalies and the finger-like distribution of volcanoes in northeast Japan has been recognized and small-scale convection (SSC) in the mantle wedge has been proposed to explain such a feature. The spatial and temporal evolution of the distribution of past volcanoes may reflect the characteristics of the flow in the mantle wedge, and points to the possibility of the flip-flopping of the finger-like pattern of the volcano distribution and the migration of volcanic activity from the back-arc side to the trench side. These observations are found to be qualitatively consistent with the results of the SSC model. We have also investigated the expected seismic anisotropy in the presence of SSC. The fast direction of the P-wave anisotropy generally shows the trench-normal direction with a reduced magnitude compared to the case without SSC. An analysis of full 3D seismic anisotropy is necessary to confirm the existence and nature of SSC. The 3D mantle flow around the subduction zone of plate-size scale has been modeled. It was found that the trench-parallel flow in the sub-slab mantle around the northern edge of the Pacific plate at the junction between

  5. Modelling Subduction Zone Magmatism Due to Hydraulic Fracture

    NASA Astrophysics Data System (ADS)

    Lawton, R.; Davies, J. H.

    2014-12-01

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

  6. Kinematics and Dynamics of the Makran Subduction Zone

    NASA Astrophysics Data System (ADS)

    Penney, C.; Tavakoli, F.; Sobouti, F.; Copley, A.; Priestley, K. F.; Jackson, J. A.

    2016-12-01

    The Makran subduction zone, along the southern coasts of Iran and Pakistan, hosts the world's largest exposed accretionary prism. In contrast to the circum-Pacific subduction zones, the Makran has not been extensively studied, with seismic data collected in the offshore region presenting only a time-integrated picture of the deformation. We investigate spatio-temporal variations in the deformation of the accretionary prism and the insights these offer into subduction zone driving forces and megathrust rheology. We combine seismology, geodesy and field observations to study the 2013 Mw 6.1 Minab earthquake, which occurred at the western end of the accretionary prism. We find that the earthquake was a left-lateral rupture on an ENE-WSW plane, approximately perpendicular to the previously mapped faults in the region. The causative fault of the Minab earthquake is one of a series of left-lateral faults in the region which accommodate a velocity field equivalent to right-lateral shear on N-S planes by rotating clockwise about vertical axes. Another recent strike-slip event within the Makran accretionary wedge was the 2013 Mw 7.7 Balochistan earthquake, which occurred on a fault optimally oriented to accommodate the regional compression by thrusting. The dominance of strike-slip faulting within the onshore prism, on faults perpendicular to the regional compression, suggests that the prism may have reached the maximum elevation which the megathrust can support, with the compressional forces which dominated in the early stages of the collision now balanced by gravitational forces. This observation allows us to estimate the mean shear stress on the megathrust interface and its effective coefficient of friction.

  7. Some aspects of the tectonics of subduction zones

    NASA Astrophysics Data System (ADS)

    Aubouin, Jean

    1989-03-01

    The main structures of a subduction zone are as follows. (1) On the outer wall: faults, formed either by reactivation of the structural grain of the oceanic plate, when the latter is slightly oblique to the trench, or by a new fault network parallel to the trench, or both. The width of the faulted zone is about 50 miles. (2) On the inner wall: either an accretionary prism or an extensional fault network, or both; collapsed structures and slumps are often associated, sometimes creating confusion with the accretionary structures. (3) The overall structure of the trench itself is determined by the shape of the edge of the continental crust or of the island arc. Its detailed structure, however, is related to the oceanic plate, namely when the structural grain of the latter is slightly oblique to the trench, which then takes an "en echelon" form. Collapsed units can fill up the trench which is, in that case, restricted to an irregular narrow depression; the tectonic framework of the trench can be buried under a sedimentary blanket when the sedimentation rate is high and the trench bottom is a large, flat area. Two extreme types of active margins can be distinguished: convergent compressive margins, when the accretionary mechanism is strongly active; and convergent extensional margins where the accretionary mechanism is absent or only weakly active. The status of a given margin between these two extreme types is related to the convergence rate of the plates, the dip of the subduction zone, the sedimentation activity and the presence of a continental obstacle, because oceanic seamounts and aseismic ridges are easily subducted. Examples are taken from the Barbados, Middle America, Peru, Kuril, Japan, Nankai, Marianna, Manila, New Hebredes and Tonga trenches.

  8. Large earthquake processes in the northern Vanuatu subduction zone

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

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

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

    NASA Astrophysics Data System (ADS)

    Nishikawa, Tomoaki; Ide, Satoshi

    2015-09-01

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

  10. Postglacial rebound at the northern Cascadia subduction zone

    NASA Astrophysics Data System (ADS)

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

    2000-10-01

    Postglacial rebound is the response of the Earth to the decay of ice-sheets. A postglacial rebound model explains crustal tilting and rapid uplift at the northern Cascadia subduction zone that occurred during retreat of the Cordilleran ice-sheet. Observations explained by the model include the shoreline tilts of two proglacial lakes that formed at 13.5-14 ka ( 14C yr ago) and rapid sea level fall (land uplift) at 12-12.5 ka. Modelled mantle viscosity values range from 5×10 18 to 5×10 19 Pa s, and are consistent with previous viscosity inferences from observations of crustal deformation following subduction zone earthquakes (10 18-10 19 Pa s). No lower limit to subduction zone mantle viscosity is apparent from our model, but viscosity values equal to or larger than 10 20 Pa s are definitely ruled out. Our modelled subduction zone viscosity values are smaller than most upper-mantle viscosity estimates derived from postglacial rebound studies of tectonically less-active regions (10 20-10 21 Pa s). The rapid observed uplift at 12 ka requires, in addition to a low mantle viscosity, rapid unloading from a sudden collapse of remaining coastal portions of the southern Cordilleran ice-sheet. The sudden collapse provides 0.18 m of global eustatic sea level rise, approximately 0.7% of the sea level rise associated with melt-water pulse IA. Predictions of a global postglacial rebound model (ICE-3G) with a 10 21 Pa s upper-mantle viscosity were previously applied to geodetic data from this region to isolate signals associated with the earthquake cycle. Owing to the low-viscosity values, and resulting rapid recovery of glacial deformation, our model predicts present-day postglacial rebound uplift rates at least 10 times smaller than ICE-3G (less than about 0.1 mm/yr). As the ICE-3G adjustments were substantial, this indicates the need for re-evaluation of the geodetic data.

  11. Tectonics of the IndoBurma Oblique Subduction Zone

    NASA Astrophysics Data System (ADS)

    Steckler, M. S.; Seeber, L.; Akhter, S. H.; Betka, P. M.; Cai, Y.; Grall, C.; Mondal, D. R.; Gahalaut, V. K.; Gaherty, J. B.; Maung Maung, P.; Ni, J.; Persaud, P.; Sandvol, E. A.; Tun, S. T.

    2016-12-01

    The Ganges-Brahmaputra Delta (GBD) is obliquely colliding with the IndoBurma subduction zone. Most of the 42 mm/y of arc-parallel motion is absorbed in a set of dextral to dextral-convergent faults, the Sagaing, Kabaw and Churachandpur-Mao Faults. The 13-17 mm/y of convergence with the delta has built a 250-km wide active accretionary prism. The upper part of the 19-km sediment thickness consists of a shallowing-up stack of prograding strata that has shifted the shelf edge 3-400 km since the Himalayan orogeny at 50 Ma. The upper 3-5 km sandy shelf to fluvial strata are deformed into a broad fold and thrust belt above an overpressured décollement. It forms a flat shallow roof thrust in the frontal accretionary prism. The structure of the deeper part of the accretionary prism, which must transfer the incoming sediments to the upper plate, is unknown. GPS indicates the downdip end of the megathrust locked zone is 25 km at 92.5°E. The deformation front, marked by nascent detachment folds above the shallow décollement reaches the megacity of Dhaka in the middle of the GBD. The seismogenic potential of this portion of the prism is unknown. Arc volcanism in Myanmar, 500 km east of the deformation front, is sparse. Limited geochemical data on the arc volcanics are consistent with hot slab conditions. One possibility is that the deep GBD slab and basement are metamorphosed and dewatered early in the subduction process whereby most of the fluids are transferred to the growing prism by buoyancy driven migration or accretion of fluid-rich strata. Since it is entirely subaerial this little-studied region crossing Bangladesh, India and Myanmar provides an opportunity for a detailed multidisciplinary geophysical and geological investigation. It has the potential to highlight the role of fluids in subduction zones, the tectonics of extreme accretion and their seismic hazards, and the interplay between driving and resistance forces of a subduction zone during a soft collision.

  12. 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.

  13. 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

  14. 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

  15. Detection of earthquake swarms in subduction zones around Japan

    NASA Astrophysics Data System (ADS)

    Nishikawa, T.; Ide, S.

    2015-12-01

    Earthquake swarms in subduction zones are likely to be related with slow slip events (SSEs) and locking on the plate interface. In the Boso-Oki region in central Japan, swarms repeatedly occur accompanying SSEs (e.g, Hirose et al., 2012). It is pointed out that ruptures of great earthquakes tend to terminate in regions with recurring swarm activity because of reduced and heterogeneous locking there (Holtkamp and Brudzinsiki, 2014). Given these observations, we may be able to infer aseismic slips and spatial variations in locking on the plate interface by investigating swarm activity in subduction zones. It is known that swarms do not follow Omori's law and have much higher seismicity rates than predicted by the ETAS model (e.g., Llenos et al., 2009). Here, we devised a statistical method to detect unexpectedly frequent earthquakes using the space-time ETAS model (Zhuang et al., 2002). We applied this method to subduction zones around Japan (Tohoku, Ibaraki-Boso-oki, Hokkaido, Izu, Tonankai, Nankai, and Kyushu) and detected swarms in JMA catalog (M ≥ 3) from 2001 to 2010. We detected recurring swarm activities as expected in the Boso-Oki region and also in the Ibaraki-Oki region (see Figures), where intensive foreshock activity was found by Maeda and Hirose (2011). In Tohoku, regions with intensive foreshock activity also appear to roughly correspond to regions with recurring swarm activity. Given that both foreshocks and swarms are triggered by SSEs (e.g., Bouchon et al., 2013), these results suggest that the regions with foreshock activity and swarm activity such as the Ibaraki-Oki region are characterized by extensive occurrences of SSEs just like the Boso-Oki region. Besides Ibaraki-Oki and Boso-Oki, we detected many swarms in Tohoku, Hokkaido, Izu, and Kyushu. On the other hand, swarms are rare in the rupture areas of the 1944 Tonankai and 1946 Nankai earthquakes. These variations in swarm activity may reflect variations in SSE activity among subduction zones

  16. Strong motions in Alaska-type subduction zone environments

    SciTech Connect

    Jacob, K.H.; Mori, J.

    1984-01-01

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

  17. 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.

  18. Solubility and Speciation of CO2 in Natural Rhyolitic Melts at 1.5-3.0 GPa - Implications for Carbon Flux in Subduction Zones via Sediment Partial Melts

    NASA Astrophysics Data System (ADS)

    Duncan, M. S.; Dasgupta, R.

    2012-12-01

    Sediment partial melt derived from the subducting plate is thought to be a critical agent in transport of trace elements and water to arc basalt source regions. Sediment melts may also act as significant carrier of CO2 released from subducting carbonates. However, the CO2 carrying capacity of rhyolitic melts, similar to those derived from partial melting of subducting pelitic sediments, at sub-arc depths remains unconstrained. Here we measured the solubility of CO2 in a model sediment partial melt; experiments were conducted at 1.5-3.0 GPa and 1300°C with variable water contents. The rhyolitic melt compositions were constructed with reagent grade oxides, carbonates, and hydroxides, with carbonates as the source of excess CO2 and Al(OH)3 as the source of variable water content, and were contained in AuPd capsules. All experiments produced glasses with bubbles, the latter being taken as the evidence of equilibrium vapor saturation at experimental P-T conditions. Micro-Fourier Transform Infrared spectroscopy was employed to measure the concentrations of CO2 and H2O in doubly polished, bubble-free sections of the glasses. The total CO2 solubility, CO2tot. (= CO2mol. + CO32-), of experimental melts increases with pressure and water content. For melts with H2O of ~0.5 wt.%, CO2tot. values increase and CO2mol./CO2tot. values decrease with increasing pressure from 0.68 to 1.2 wt.% and 0.95 to 0.60 from 1.5 to 3.0 GPa, respectively. In contrast to the water-poor melts, the hydrous melts with ~2 and 3.5 wt.% H2O showed the opposite CO2mol./CO2tot.-pressure trend (0.18 to 0.55 for 2 wt.% H2O and 0.04 to 0.18 for 3.5 wt.% H2O from 1.5 to 3.0 GPa). Total CO2 contents of hydrous melts were also higher at a given pressure and ranged from 0.8 to 1.5 wt.% from 1.5 to 3.0 GPa for experiments with 2 wt.% H2O and 0.9 to >1.6 wt.% from 1.5 to 3.0 GPa for experiments with 3.5 wt.% H2O. Measurements of melt inclusions and gases determined that primary arc magmas contain ≥0.3 wt.% CO2

  19. The earliest mantle fabrics formed during subduction zone infancy

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

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

  20. Thermal modeling of the seismogenic zone: a new view of end-member subduction zones (flat slabs, ultra-slow subduction) in P-T-t space

    NASA Astrophysics Data System (ADS)

    Gutscher, Marc-Andre

    2017-04-01

    Assessing the pressure and temperature conditions along megathrust plate boundaries is crucial to understanding the rheology and the mineralogical transformations that occur here and can be expected to have a major impact on seismogenic behavior. Numerical modeling of forearc thermal structure has been applied to most subduction zones around the world (Syracuse et al., 2010, PEPI). Though their work examined a vast range of subducting plate ages and subduction velocities, most P-T paths were in fact shown to rather similar, with strong increases in temperature occuring in the 80-100 km depth range, as the convecting corner of asthenosphere is reached. Two groups of end-member subduction zones (undersampled in existing work), however, show consistently different P-T paths: flat-slabs and ultra-slow subduction zones. Their P-T paths are flatter, with temperatures increasing at shallower depths. Even more interesting is to examine the P-T conditions in P-T-t space along a time axis. For a flat-slab, pressure conditions remain constant over a much longer period, and depending on the exact depth this can have a strong impact on the seismogenic zone. Ultra-slow subduction zones are unique in the first place due to the very slow transit time of sediments along the plate interface. Secondly, because most ultra-slow subduction zones are marked by very thick, broad accretionary wedges, this also strongly impacts the thermal structure along the upper portion of the seismogenic zone.

  1. 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.

  2. Three-dimensional imaging of impact of a large igneous province with a subduction zone

    NASA Astrophysics Data System (ADS)

    Reyners, Martin; Eberhart-Phillips, Donna; Upton, Phaedra; Gubbins, David

    2017-02-01

    How the thickened crust of a large igneous province on an incoming oceanic plate is accommodated at a subduction zone remains an open question. New Zealand is one of the few places to study this, as at ca. 105 Ma the ca. 35 km-thick Hikurangi Plateau impacted the Gondwana subduction zone in what is now the South Island. Here we report on results from a forty-station portable seismograph array in the southern South Island, designed to delineate the leading edge of the subducted plateau. Three-dimensional images of Vp and Vp/Vs reveal the southwestern part of the plateau was a relatively narrow salient, and the first part to be subducted. The plateau then rotated clockwise about this salient until the southern edge of the plateau was parallel to subduction strike and subduction ceased at ca. 100 Ma. Our results suggest that the global-scale plate reorganization event at 105-100 Ma was due to a cessation of subduction caused by the Hikurangi Plateau choking the Gondwana subduction zone, rather than the subduction of mid ocean ridges as previously proposed. The choking of Gondwana subduction by the plateau also led to a concentration of slab pull in the adjacent subducted oceanic crust, explaining the episode of basin opening and intraplate magmatism there that occurred at the same time. Our study underlines the havoc caused by impact of a large igneous province with a subduction zone.

  3. Interseismic deformation and moment deficit along the Manila subduction zone and the Philippine Fault system

    NASA Astrophysics Data System (ADS)

    Hsu, Ya-Ju; Yu, Shui-Beih; Loveless, John P.; Bacolcol, Teresito; Solidum, Renato; Luis, Artemio; Pelicano, Alfie; Woessner, Jochen

    2016-10-01

    We examine interseismic coupling of the Manila subduction zone and fault activity in the Luzon area using a block model constrained by GPS data collected from 1998 to 2015. Estimated long-term slip rates along the Manila subduction zone show a gradual southward decrease from 90-100 mm/yr at the northwest tip of Luzon to 65-80 mm/yr at the southern portion of the Manila Trench. We provide two block models (models A and B) to illustrate possible realizations of coupling along the Manila Trench, which may be used to infer future earthquake rupture scenarios. Model A shows a low coupling ratio of 0.34 offshore western Luzon and continuous creeping on the plate interface at latitudes 18-19°N. Model B includes the North Luzon Trough Fault and shows prevalent coupling on the plate interface with a coupling ratio of 0.48. Both models fit GPS velocities well, although they have significantly different tectonic implications. The accumulated strain along the Manila subduction zone at latitudes 15-19°N could be balanced by earthquakes with composite magnitudes of Mw 8.8-9.2, assuming recurrence intervals of 500-1000 years. GPS observations are consistent with full locking of the majority of active faults in Luzon to a depth of 20 km. Inferred moments of large inland earthquakes in Luzon fall in the range of Mw 6.9-7.6 assuming a recurrence interval of 100 years.

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

    PubMed Central

    Chaves, Esteban J.; Schwartz, Susan Y.

    2016-01-01

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

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

    PubMed

    Chaves, Esteban J; Schwartz, Susan Y

    2016-01-01

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

  6. Slab2 - Updated subduction zone geometries and modeling tools

    NASA Astrophysics Data System (ADS)

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

    2016-12-01

    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 for others to reproduce the models it describes. Now near completion, Slab2 will update and replace Slab1.0 by: (1) extending modeled slab geometries to the full extent of all known global subduction zones; (2) incorporating regional data sets (e.g., tomography models) that may describe slab geometry more comprehensively 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) adding further layers to the base geometry dataset, such as historic moment release, earthquake tectonic providence, and interface coupling; (6) migrating the slab modeling code base to a more universally distributable language, Python; and (7) 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 will describe our progress made in creating Slab2, and provide information on

  7. Variability in Coastal Neotectonics Along the Kamchatka Subduction Zone

    NASA Astrophysics Data System (ADS)

    Bourgeois, J.; Pinegina, T.; Ponomareva, V.

    2004-12-01

    The eastern coast of Kamchatka can be divided into several morphotectonic zones, which appear to correspond primarily to variations in the subducting crust, rather than to characteristics of the subduction zone such as rate of subduction or subduction angle. Pleistocene marine terraces and Holocene coastal stratigraphy along Kamchatka show variability in uplift (and subsidence) rate both at the degree-latitude scale, and at a scale of kilometers to 10s of km. The southern coast (51-53oN, to Petropavlovsk) is primarily rocky headlands with narrow embayments, some filled with volcaniclastics. North of Petropavlovsk, the coast is subdivided into a series of broad embayments separated by four peninsulas--Zhupanovskiy, Kronotskiy, Kamchatskiy, Ozernoi, from south to north. These peninsulas are landward, respectively, of the Kruzenstern fracture zone, the Meiji seamounts, the Aleutian-Komandorskiy island chain, and the Beta Rise. Our field studies of Quaternary coastal evolution and seismotectonic regime over the last decade have included: 1) tephra chronology for dating and correlation 2) study of paleotsunami deposits to estimate recurrence rates of tsunamigenic earthquakes; and 3) analysis of the modern and paleo-topography of marine terraces and beach ridges in order to determine the direction and intensity of tectonic deformation over different spatial and temporal scales. The shorter the time interval we are able to specify (decades to hundreds of years), the higher the rate of vertical movements we tend to obtain. Estimates of net deformation for longer periods (10s to 100s of thousands of years) are commonly an order of magnitude slower, because seismic cycles are averaged out. The net Quaternary deformation of eastern Kamchatka is uplift, with peninsulas exhibiting the highest rates, up to 2 mm/yr for the last 0.5 my on Kamchatskiy Peninsula. The large embayments between peninsulas typically exhibit evidence for subsidence or stasis on a Holocene time scale. On

  8. Subduction zone forearc serpentinites as incubators for deep microbial life

    PubMed Central

    Plümper, Oliver; Geisler, Thorsten; Liu, Yang; Pabst, Sonja; Savov, Ivan P.; Rost, Detlef; Zack, Thomas

    2017-01-01

    Serpentinization-fueled systems in the cool, hydrated forearc mantle of subduction zones may provide an environment that supports deep chemolithoautotrophic life. Here, we examine serpentinite clasts expelled from mud volcanoes above the Izu–Bonin–Mariana subduction zone forearc (Pacific Ocean) that contain complex organic matter and nanosized Ni–Fe alloys. Using time-of-flight secondary ion mass spectrometry and Raman spectroscopy, we determined that the organic matter consists of a mixture of aliphatic and aromatic compounds and functional groups such as amides. Although an abiotic or subduction slab-derived fluid origin cannot be excluded, the similarities between the molecular signatures identified in the clasts and those of bacteria-derived biopolymers from other serpentinizing systems hint at the possibility of deep microbial life within the forearc. To test this hypothesis, we coupled the currently known temperature limit for life, 122 °C, with a heat conduction model that predicts a potential depth limit for life within the forearc at ∼10,000 m below the seafloor. This is deeper than the 122 °C isotherm in known oceanic serpentinizing regions and an order of magnitude deeper than the downhole temperature at the serpentinized Atlantis Massif oceanic core complex, Mid-Atlantic Ridge. We suggest that the organic-rich serpentinites may be indicators for microbial life deep within or below the mud volcano. Thus, the hydrated forearc mantle may represent one of Earth’s largest hidden microbial ecosystems. These types of protected ecosystems may have allowed the deep biosphere to thrive, despite violent phases during Earth’s history such as the late heavy bombardment and global mass extinctions. PMID:28396389

  9. Subduction zone forearc serpentinites as incubators for deep microbial life

    NASA Astrophysics Data System (ADS)

    Plümper, Oliver; King, Helen E.; Geisler, Thorsten; Liu, Yang; Pabst, Sonja; Savov, Ivan P.; Rost, Detlef; Zack, Thomas

    2017-04-01

    Serpentinization-fueled systems in the cool, hydrated forearc mantle of subduction zones may provide an environment that supports deep chemolithoautotrophic life. Here, we examine serpentinite clasts expelled from mud volcanoes above the Izu-Bonin-Mariana subduction zone forearc (Pacific Ocean) that contain complex organic matter and nanosized Ni-Fe alloys. Using time-of-flight secondary ion mass spectrometry and Raman spectroscopy, we determined that the organic matter consists of a mixture of aliphatic and aromatic compounds and functional groups such as amides. Although an abiotic or subduction slab-derived fluid origin cannot be excluded, the similarities between the molecular signatures identified in the clasts and those of bacteria-derived biopolymers from other serpentinizing systems hint at the possibility of deep microbial life within the forearc. To test this hypothesis, we coupled the currently known temperature limit for life, 122 °C, with a heat conduction model that predicts a potential depth limit for life within the forearc at ˜10,000 m below the seafloor. This is deeper than the 122 °C isotherm in known oceanic serpentinizing regions and an order of magnitude deeper than the downhole temperature at the serpentinized Atlantis Massif oceanic core complex, Mid-Atlantic Ridge. We suggest that the organic-rich serpentinites may be indicators for microbial life deep within or below the mud volcano. Thus, the hydrated forearc mantle may represent one of Earth’s largest hidden microbial ecosystems. These types of protected ecosystems may have allowed the deep biosphere to thrive, despite violent phases during Earth’s history such as the late heavy bombardment and global mass extinctions.

  10. Subduction zone forearc serpentinites as incubators for deep microbial life.

    PubMed

    Plümper, Oliver; King, Helen E; Geisler, Thorsten; Liu, Yang; Pabst, Sonja; Savov, Ivan P; Rost, Detlef; Zack, Thomas

    2017-04-25

    Serpentinization-fueled systems in the cool, hydrated forearc mantle of subduction zones may provide an environment that supports deep chemolithoautotrophic life. Here, we examine serpentinite clasts expelled from mud volcanoes above the Izu-Bonin-Mariana subduction zone forearc (Pacific Ocean) that contain complex organic matter and nanosized Ni-Fe alloys. Using time-of-flight secondary ion mass spectrometry and Raman spectroscopy, we determined that the organic matter consists of a mixture of aliphatic and aromatic compounds and functional groups such as amides. Although an abiotic or subduction slab-derived fluid origin cannot be excluded, the similarities between the molecular signatures identified in the clasts and those of bacteria-derived biopolymers from other serpentinizing systems hint at the possibility of deep microbial life within the forearc. To test this hypothesis, we coupled the currently known temperature limit for life, 122 °C, with a heat conduction model that predicts a potential depth limit for life within the forearc at ∼10,000 m below the seafloor. This is deeper than the 122 °C isotherm in known oceanic serpentinizing regions and an order of magnitude deeper than the downhole temperature at the serpentinized Atlantis Massif oceanic core complex, Mid-Atlantic Ridge. We suggest that the organic-rich serpentinites may be indicators for microbial life deep within or below the mud volcano. Thus, the hydrated forearc mantle may represent one of Earth's largest hidden microbial ecosystems. These types of protected ecosystems may have allowed the deep biosphere to thrive, despite violent phases during Earth's history such as the late heavy bombardment and global mass extinctions.

  11. 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

  12. Seismogenic zone structure along the Middle America subduction zone, Costa Rica

    NASA Astrophysics Data System (ADS)

    Deshon, Heather Rene

    Most large (MW > 7.0) underthrusting earthquakes nucleate along a shallow region of unstable frictional stability on or near the subducting plate interface termed the seismogenic zone. The studies presented here investigate along-strike spatial and temporal variability in microseismicity and seismic velocity and provide spatial constraints on the updip and downdip limits of microseismicity within the Middle America subduction offshore western Costa Rica. All chapters utilize data recorded by the Costa Rica Seismogenic Zone Experiment (CRSEIZE), a collaborative seismic and geodetic study undertaken from September 1999--June 2001 to better understand subduction zone behavior near the Osa and Nicoya Peninsulas, Costa Rica. Chapter 1 serves as a broad introduction to the thesis while Chapter 2 provides an overview of Costa Rica seismicity, the CRSEIZE experiment setup, data processing, and data quality. Chapter 3 discusses simultaneous inversion for 1D P- and S-wave velocity models, station corrections, and hypocenter parameters for both the Nicoya and Osa experiments and presents a refined location for the continental Moho in northern Costa Rica. Chapter 4 presents absolute and relative relocations of ˜300 aftershocks of the 1999 Quepos, Costa Rica, underthrusting earthquake and analyzes seismogenic zone structure offshore central Costa Rica during a period of increased seismicity rate. Subduction of highly disrupted seafloor north of the Osa Peninsula has established a set of conditions that presently limit the seismogenic zone to be between 10--35 km below sea level, 30--95 km from the trench axis. Chapter 5 presents high resolution earthquake locations and P-wave and P-wave/S-wave 3D velocity models for the locked Nicoya Peninsula segment of the Middle America subduction zone calculated using an iterative, damped least squares local tomography method. In the southern Nicoya Peninsula, microseismicity along the plate interface extends from 12--26 km depth, 73

  13. 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.

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

    PubMed

    Clarke, S H; Carver, G A

    1992-01-10

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

  15. 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.

  16. Subslab seismic anisotropy and mantle flow in the western Pacific subduction zones

    NASA Astrophysics Data System (ADS)

    Peng, C. C.; Kuo, B. Y.; Chen, C. W.

    2014-12-01

    We present source-side anisotropy for a few subduction zones in an attempt to map the mantle flow beneath the slab. Shear-wave splitting parameters of S were measured at stations towards the back of the subduction with the receiver-side anisotropy removed. We examined the observed fast directions against tilting/rotation of olivine fabric relative to the geometry of the subduction. We found that at the SW edge of the Ryukyu subduction zone the olivine fabric in the subslab mantle must rotate clockwise by 25 degrees from the slab subduction trajectory to explain the observed pattern of shear-wave splitting. This rotation echoes the deformation model of the slab when it is impinging against the Eurasian lithosphere. In the Vanuatu (New Hebrides) subduction zone, the olivine fabric may rotate dramatically to accommodate the rapid retreat of the trench and flipping of subduction polarity in the past a few Mys.

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

    NASA Astrophysics Data System (ADS)

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

    2010-12-01

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

  18. 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

  19. 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

  20. Dynamic Stress Drop of Recent Earthquakes: Variations within Subduction Zones

    NASA Astrophysics Data System (ADS)

    Ruff, L. J.

    Stress drop is a fundamental parameter of earthquakes, but it is difficult to obtain reliable stress drop estimates for most earthquakes. Static stress drop estimates require knowledge of the seismic moment and fault area. Dynamic stress drop estimates are based entirely upon the observed source time functions. Based on analytical formulas that I derive for the crack and slip-pulse rupture models, the amplitude and time of the initial peak in source time functions can be inverted for dynamic stress drop. For multiple event earthquakes, this method only gives the dynamic stress drop of the first event. The Michigan STF catalog provides a uniform data base for all large earthquakes that have occurred in the past four years. Dynamic stress drops are calculated for the nearly 200 events in this catalog, and the resultant estimates scatter between 0.1 and 100 MPa. There is some coherent tectonic signal within this scatter. In the Sanriku (Japan) and Mexico subduction zones, underthrusting earthquakes that occur at the up-dip and down-dip edges of the seismogenic zone have correspondingly low and high values of stress drop. A speculative picture of the stress state of subduction zones emerges from these results. A previous study found that the absolute value of shear stress linearly increases down the seismogenic interface to a value of about 50 MPa at the down-dip edge. In this study, the dynamic stress drop of earthquakes at the up-dip edge is about 0.2 MPa, while large earthquakes at the down-dip edge of the seismogenic plate interface have dynamic stress drops of up to 5 MPa. These results imply that (1) large earthquakes only reduce the shear stress on the plate interface by a small fraction of the absolute level; and thus (2) most of the earthquake energy is partitioned into friction at the plate interface.

  1. Variation of interplate fault zone properties with depth in the japan subduction zone

    PubMed

    Bilek; Lay

    1998-08-21

    The depth dependence of physical properties along the Japan subduction zone interface was explored using teleseismic recordings of earthquake signals. Broadband body waves were inverted to determine the duration of rupture and source depth for 40 interplate thrust earthquakes located offshore of Honshu between 1989 and 1995. After scaling for differences in seismic moment, there is a systematic decrease in rupture duration with increasing depth along the subducting plate interface. This indicates increases in rupture velocity or stress drop with depth, likely related to variation in rigidity of sediments on the megathrust.

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

    NASA Astrophysics Data System (ADS)

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

    2015-05-01

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

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

    USGS Publications Warehouse

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

    2010-01-01

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

  4. The cold and relatively dry nature of mantle forearcs in subduction zones

    NASA Astrophysics Data System (ADS)

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

    2017-04-01

    Some of Earth's coldest mantle is found in subduction zones at the tip of the mantle wedge that lies between the subducting and overriding plates. This forearc mantle is isolated from the flow of hot material beneath the volcanic arc, and so is inferred to reach temperatures no more than 600 to 800 °C -- conditions at which hydrous mantle minerals should be stable. The forearc mantle could therefore constitute a significant reservoir for water if sufficient water is released from the subducting slab into the mantle wedge. Such a reservoir could hydrate the plate interface and has been invoked to aid the genesis of megathrust earthquakes and slow slip events. Our synthesis of results from thermal models that simulate the conditions for subduction zones globally, however, indicates that dehydration of subducting plates is too slow over the life span of a typical subduction zone to hydrate the forearc mantle. Hot subduction zones, where slabs dehydrate rapidly, are an exception. The hottest, most buoyant forearcs are most likely to survive plate collisions and be exhumed to the surface, so probably dominate the metamorphic rock record. Analysis of global seismic data confirms the generally dry nature of mantle forearcs. We conclude that many subduction zones probably liberate insufficient water to hydrate the shallower plate boundary where great earthquakes and slow slip events nucleate. Thus, we suggest that it is solid-state processes and not hydration that leads to weakening of the plate interface in cold subduction zones.

  5. 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

  6. Does subduction zone magmatism produce average continental crust

    NASA Technical Reports Server (NTRS)

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

    1988-01-01

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

  7. Monochromatic body waves excited by great subduction zone earthquakes

    NASA Astrophysics Data System (ADS)

    Ihmlé, Pierre F.; Madariaga, Raúl

    Large quasi-monochromatic body waves were excited by the 1995 Chile Mw=8.1 and by the 1994 Kurile Mw=8.3 events. They are observed on vertical/radial component seismograms following the direct P and Pdiff arrivals, at all azimuths. We devise a slant stack algorithm to characterize the source of the oscillations. This technique aims at locating near-source isotropic scatterers using broadband data from global networks. For both events, we find that the oscillations emanate from the trench. We show that these monochromatic waves are due to localized oscillations of the water column. Their period corresponds to the gravest ID mode of a water layer for vertically traveling compressional waves. We suggest that these monochromatic body waves may yield additional constraints on the source process of great subduction zone earthquakes.

  8. Interplate coupling along segments of the Central America Subduction zone

    NASA Astrophysics Data System (ADS)

    Zarifi, Zoya; Raeesi, Mohammad; Atakan, Kuvvet

    2013-04-01

    We analyzed 5 major earthquakes that occurred during 1992 to 2012 in a segment of the Central America subduction zone along the coasts of Guatemala and El Salvador. These events include 1992/09/02 (Mw 7.7), 1993/09/10 (Mw 7.2), 2001/01/13 (Mw 7.7), 2012/08/27 (Mw 7.3) and 2012/11/07 (Mw 7.3). We derived the asperities of these earthquakes using two completely independent methods of body-waveform inversion and a gravity-derived measure, Trench Parallel Bouguer Anomaly (TPBA). Using TPBA we discuss the status of interplate coupling along the segment and interpret each of the major earthquakes as a piece of the governing rupture process. We delineate the critical unbroken asperities along the segment that will likely generate great earthquake(s) in the future.

  9. The production of Barberton komatiites in an Archean Subduction Zone

    NASA Astrophysics Data System (ADS)

    Parman, S. W.; Grove, T. L.; Dann, J. C.

    Based upon their geochemical similarity, we propose that the 3.5 Ga Barberton basaltic komatiites (BK) are the Archean equivalents of modern boninites, and were produced by the same melting processes (i.e. hydrous melting in a subduction zone). The Barberton komatiites also share some geochemical characteristics with boninites, including petrologic evidence for high magmatic H2O contents. Experimental data indicates that the Archean sub-arc mantle need only be 1500-1600°C to produce hydrous komatiitic melts. This is considerably cooler than estimates of mantle temperatures assuming an anhydrous, plume origin for komatiites (up to 1900°C). The depleted mantle residue that generates the Barberton komatiites and BK will be cooled and metasomatised as it resides beneath the fore-arc, and may represent part of the material that formed the Kaapvaal cratonic keel.

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

    SciTech Connect

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

    1988-02-01

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

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

    USGS Publications Warehouse

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

    2006-01-01

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

  12. Autocorrelation analysis of ambient noise in northeastern Japan subduction zone

    NASA Astrophysics Data System (ADS)

    Ito, Yoshihiro; Shiomi, Katsuhiko; Nakajima, Junichi; Hino, Ryota

    2012-10-01

    We obtained ambient seismic noise interferograms as seismic reflection images using autocorrelation functions (ACFs) in the northeastern Japan subduction zone. The ACFs with a time window length of 120 s were calculated from the continuous seismic records obtained at each seismic station over an analysis period of 300 days. These ACFs show some distinct signals with relatively large amplitude without any significant temporal variations during the analysis period. The signals, which are stable, appear at both small lag times of less than 10 s and large lag times of 20-50 s during the analysis period. The lag time of 10 s corresponds to the travel time of the PP reflection arrival from the continental Mohorovičić discontinuity. The signals with the large lag times between 30 and 50 s correspond to the back-scattered signals from the mantle wedge or the plate boundary; these signals are identified clearly at the stations located on the back-arc side. In the ACFs calculated from the records obtained from the fore-arc side stations, weak signals (interpreted as the reflection from the plate boundary) with a lag time range of 20 to 30 s are observed. We constructed depth-migrated images using the ACFs to obtain the reflectivity profile by assuming that the ACFs represent Green's functions composed of a random wavefield excited by stochastic sources or scatterers distributed in the vertical or near-vertical direction from the stations. Further, we assumed that the ACFs can be treated as zero-offset seismic traces recorded at each of the stations. The depth-migrated images show a relatively seismically transparent structure within the subducting Pacific slab and a reflective structure within the mantle wedge; this reflective structure is characterized by low-velocity zones corresponding to the wedge flow imaged by 3-D seismic velocity tomography.

  13. 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

  14. Interrogating Seismic Anisotropy in Subduction Zones and Continental Interiors

    NASA Astrophysics Data System (ADS)

    Moriarty, Erin Wirth

    Knowledge regarding the dynamics of Earth's interior is critical to our understanding of past and present tectonic processes. In subduction zones, the geometry of the mantle flow field and the fine-scale structure of the mantle wedge remain poorly understood. This is a major deficit in our understanding, as the mantle wedge plays an important role in many of the processes associated with subduction, including the transport of water into Earth's deep interior and the generation of arc volcanism. Similarly, in continental interiors, an improved understanding of past tectonic processes is necessary to attain a better understanding of the formation and evolution of continental lithosphere, also poorly understood phenomena. In this work, I interrogate the character of seismic anisotropy in both the subduction zone mantle wedge and continental interiors to learn about dynamic processes both past and present. In Chapter 1, I measure shear wave splitting of local S-waves beneath Japan. I find that shear wave splitting due to anisotropy in the mantle wedge exhibits strong lateral variations in character, as well as a strong dependence on frequency. This ultimately points to heterogeneous anisotropic structure and a complex mantle flow field. A consequence of this work is the detection of anomalously small shear wave splitting delay times beneath northeast Japan, which implies extremely weak or heterogeneous mantle wedge anisotropy. In Chapter 2, I use receiver functions as a tool for interrogating mantle wedge anisotropy, specifically beneath the anomalous region of northeast Japan. Results from this work clearly reveal a layer directly above the subducting slab with an anisotropic symmetry axis that is oriented parallel to the convergence direction. There is also evidence for a region of relatively low seismic velocities in the central portion of the mantle wedge and an anisotropic layer beneath the crust of the overriding plate, all of which exhibit a relatively modest

  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. 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

  17. 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.

  18. Marine Gravity Measurements at the Subduction Zone offshore Central Chile

    NASA Astrophysics Data System (ADS)

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

    2003-12-01

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

  19. 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

  20. 3D Numerical modelling of topography development associated with curved subduction zones

    NASA Astrophysics Data System (ADS)

    Munch, Jessica; Ueda, Kosuke; Burg, Jean-Pierre; May, Dave; Gerya, Taras

    2017-04-01

    Curved subduction zones, also called oroclines, are geological features found in various places on Earth. They occur in diverse geodynamic settings: 1) single slab subduction in oceanic domain (e.g. Sandwich trench in the Southern Atlantic); 2) single slab subduction in continental domain, (e.g. Gibraltar-Alboran orocline in the Western Mediterranean) 3); multi-slab subduction (e.g. Caribbean orocline in the South-East of the Gulf of Mexico). These systems present various curvatures, lengths (few hundreds to thousands of km) and ages (less than 35 Ma for Gibraltar Alboran orocline, up to 100 Ma for the Caribbean). Recent studies suggested that the formation of curved subduction systems depends on slab properties (age, length, etc) and may be linked with processes such as retreating subduction and delamination. Plume induced subduction initiation has been proposed for the Caribbean. All of these processes involve deep mechanisms such as mantle and slab dynamics. However, subduction zones always generate topography (trenches, uplifts, etc), which is likely to be influenced by surface processes. Hence, surface processes may also influence the evolution of subduction zones. We focus on different kinds of subduction systems initiated by plume-lithosphere interactions (single slab subduction/multi-slab subduction) and scrutinize their surface expression. We use numerical modeling to examine large-scale subduction initiation and three-dimensional slab retreat. We perform two kinds of simulations: 1) large scale subduction initiation with the 3D-thermomechanical code I3ELVIS (Gerya and Yuen, 2007) in an oceanic domain and 2) large scale subduction initiation in oceanic domain using I3ELVIS coupled with a robust new surface processes model (SPM). One to several retreating slabs form in the absence of surface processes, when the conditions for subduction initiation are reached (c.f. Gerya et al., 2015), and ridges occur in the middle of the extensional domain opened by slab

  1. The upper-mantle transition zone beneath the Chile-Argentina flat subduction zone

    NASA Astrophysics Data System (ADS)

    Bagdo, Paula; Bonatto, Luciana; Badi, Gabriela; Piromallo, Claudia

    2016-04-01

    The main objective of the present work is the study of the upper mantle structure of the western margin of South America (between 26°S and 36°S) within an area known as the Chile-Argentina flat subduction zone. For this purpose, we use teleseismic records from temporary broad band seismic stations that resulted from different seismic experiments carried out in South America. This area is characterized by on-going orogenic processes and complex subduction history that have profoundly affected the underlying mantle structure. The detection and characterization of the upper mantle seismic discontinuities are useful to understand subduction processes and the dynamics of mantle convection; this is due to the fact that they mark changes in mantle composition or phase changes in mantle minerals that respond differently to the disturbances caused by mantle convection. The discontinuities at a depth of 410 km and 660 km, generally associated to phase changes in olivine, vary in width and depth as a result of compositional and temperature anomalies. As a consequence, these discontinuities are an essential tool to study the thermal and compositional structure of the mantle. Here, we analyze the upper-mantle transition zone discontinuities at a depth of 410 km and 660 km as seen from Pds seismic phases beneath the Argentina-Chile flat subduction.

  2. Episodic tremor and slip in Northern Sumatra subduction zone

    NASA Astrophysics Data System (ADS)

    Sianipar, Dimas; Subakti, Hendri

    2017-07-01

    The first reported observation of non-volcanic tremor in Sunda Arc in Sumbawa, Indonesia open a possibility of discovery of episodic tremor and slip (ETS) from out of Pacific Rim. Non-volcanic tremor gives some important information about dynamic of plate boundaries. The characteristics of these tremors are visually as non-impulsive, high frequency, long-duration and low-amplitude signals. Tectonic tremor occurred in a transition part of brittle-ductile of a fault and frequently associated with the shearing mechanism of slow slip. Tectonic tremor is a seismic case that also very interested, because it shows strong sensitivity to stress changes. Deep non-volcanic tremor is usually associated with episodic slow-slip events. Tectonic tremor is found in close association with geodetically observed slow-slip events (SSE) in subduction zones. One research found that there is possibility of SSE occurrence on Banyak Islands, North Sumatra revealed from coral observation. The SSE occurred on the Banyak Islands portion of the megathrust at 30-55 km depth, within the downdip transition zone. We do a systematic search of episodic tremor and its possible relationship with slow-slip phenomena in Northern Sumatra subduction zone. The spectrogram analysis is done to analyze the potential tremor signals. We use three component broadband seismic stations with 20, 25, and 50 sampling per second (BH* and SH* channels). We apply a butterworth 5 Hz highpass filter to separate the signal as local tremor and teleseismic/regional earthquakes. Before computing spectrogram to avoid high-frequency artifacts to remote triggering, we apply a 0.5 Hz filter. We also convert the binary seismic data into sound waves to make sure that these events meet the tectonic tremor criterion. We successfully examine 3 seismic stations with good recording i.e. GSI, SNSI and KCSI. We find there are many evidences of high frequency episodic tremor like signals. This include an analysis of potential triggered

  3. Controls on the Migration of Fluids in Subduction Zones

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

    Arc volcanism associated with subduction is generally considered to be caused by the transport in the slab of hydrated minerals to sub-arc depths. In a qualitative sense it appears clear that progressive dehydration reactions in the down-going slab release fluids to the hot overlying mantle wedge, causing flux melting and the migration of melts to the volcanic front. However, the quantitative details of fluid release, migration, melt generation and transport in the wedge remain poorly understood. In particular, there are two fundamental observations that defy quantitative modeling. The first is the location of the volcanic front with respect to intermediate depth earthquakes (e.g. 100+/-40 km; England et al., 2004, Syracuse and Abers, 2006) which is remarkably robust yet insensitive to subduction parameters. This is particularly surprising given new estimates on the variability of fluid release in global subduction zones (e.g. van Keken et al. 2011) which show great sensitivity of fluid release to slab thermal conditions. Reconciling these results implies some robust mechanism for focusing fluids and/or melts toward the wedge corner. The second observation is the global existence of thermally hot erupted basalts and andesites that, if derived from flux melting of the mantle requires sub-arc mantle temperatures of 1300 degrees C over shallow pressures of 1-2 GPa which are not that different from mid-ocean ridge conditions. These observations impose significant challenges for geodynamic models of subduction zones, and in particular for those that do not include the explicit transport of fluids and melts. We present a range of high-resolution models that include a more complete description of coupled fluid and solid mechanics (allowing the fluid to interact with solid rheological variations) together with rheologically consistent solution for temperature and solid flow. Focusing on end-members of a global suite of arc geometries and thermal histories we discuss how

  4. 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.

  5. 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.

  6. Zircon U-Pb geochronology and petrogenesis of metabasites from the western Beihuaiyang zone in the Hong'an orogen, central China: Implications for detachment within subducting continental crust at shallow depths

    NASA Astrophysics Data System (ADS)

    Liu, Yi-Can; Liu, Li-Xiang; Li, Yuan; Gu, Xiao-Feng; Song, Biao

    2017-09-01

    Deformed low-grade metabasites from the western segment of the Beihuaiyang zone in the Hong'an orogen, central China can be divided into two types, i.e. meta-gabbro and meta-basalt. These lithologies have been studied by using whole-rock elemental and Sr-Nd-Pb isotopic analyses, and zircon SHRIMP U-Pb dating. Concordant zircon U-Pb ages of 631 ± 5 Ma and 623 ± 14 Ma are obtained for the meta-gabbros, consistent with a previously reported U-Pb age of 635 ± 5 Ma. The meta-basalt was dated to have a protolith age of middle Neoproterozoic (∼750 Ma) and a metamorphic age at ca. 240 Ma. The all studied metabasites occur as block or slice within a metamorphosed Ordovician volcanic zone (originally named as the Dingyuan Formation) and are in tectonic contact to each other. The gabbro and basalt emplaced at ∼630 Ma and ∼750 Ma, respectively in a continental rifting setting, whereas their present country rocks were erupted at ∼465 Ma in an arc setting. The Pb-isotope compositions of the low-grade meta-gabbros and meta-basalts are similar to those from the Dabie ultrahigh-pressure (UHP) meta-igneous rocks with an upper continental crust affinity. The protolith ages of the studied relatively low-grade meta-basic rocks are in good agreement not only with ages for two episodes of middle and late Neoproterozoic mafic and felsic magmatism in the Suizhou to Zaoyang areas at the northern margin of the South China Block, but are also in agreement with the protolith ages of UHP meta-igneous rocks in the Dabie-Sulu orogenic belt. Therefore, these Neoproterozoic low-grade metabasites are considered to be exotic and they may have been detached and offscraped from the subducting upper crust of the South China Block at shallow depths during continental collision in the Triassic. They were subsequently exhumed in the initial stage of continental subduction, and thrusted over the Paleozoic metamorphosed rocks in the southern margin of the North China Block or as foreign slices

  7. 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.

  8. 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.

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

    NASA Astrophysics Data System (ADS)

    Arredondo, K.; Billen, M. I.

    2010-12-01

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

  10. 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

  11. Trench dynamics: Effects of dynamically migrating trench on subducting slab morphology and characteristics of subduction zones systems

    NASA Astrophysics Data System (ADS)

    Yoshida, Masaki

    2017-07-01

    Understanding the mechanisms of trench migration (retreat or advance) is crucial to characterizing the driving forces of Earth's tectonics plates, the origins of subducting slab morphologies in the deep mantle, and identifying the characteristics of subduction zones systems, which are among the fundamental issues of solid Earth science. A series of numerical simulations of mantle convection, focusing on plate subduction in a three-dimensional (3-D) regional spherical shell coordinate system, was performed to examine subduction zone characteristics, including geodynamic relationships among trench migration, back-arc stress, and slab morphology. The results show that a subducting slab tends to deflect around the base of the mantle transition zone and form a sub-horizontal slab because its front edge (its 'toe') is subject to resistance from the highly viscous lower mantle. As the sub-horizontal slab starts to penetrate into the lower mantle from its 'heel,' the toe of the slab is drawn into the lower mantle. The results for models with dynamically migrating trenches suggest that trench retreat is the dynamically self-consistent phenomenon in trench migration. The reason for this is that the strong lateral mantle flow that is generated as a sequence of events leading from corner flow at the subduction initiation to return flow of the formation of a sub-horizontal slab in the shallower part of mantle wedge produces the retreat of the subducting slab. In fact, a 'mantle suction force,' which is generated in the mantle wedge to fill space left by the retreating subducting plate, is enhanced by the subsequent trench retreat. Even when upwelling flow with significant positive buoyancy originates just above a mantle phase boundary at a depth of 410 km (as inferred from independent seismic tomographic, geodynamic, geochemical, and mineral physics), reaches the base of the overriding plate, and the overriding plate is slightly thinned, lithospheric stress tends to be

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

    NASA Astrophysics Data System (ADS)

    Ide, Satoshi

    2013-09-01

    Maximum earthquake magnitude and the rate of seismic activity apparently differ among subduction zones. This variation is attributed to factors such as subduction zone temperature and stress, and the type of material being subducted. The relative velocity between the downgoing and overriding plates controls their tectonic deformation. It is also thought to correlate with seismicity. Here I use the epidemic type aftershock sequence model to calculate the background seismicity rate--the frequency of seismic events above magnitude 4.5--for 117 sections of subduction zones worldwide, during the past century. I demonstrate a proportionality relationship whereby relative plate velocity correlates positively with seismicity rate. This relationship is prominent in the southwestern Pacific Ocean. However, although seismically active, this region has not experienced a magnitude 9 earthquake since 1900. In contrast, the Cascadia, Nankai, southern Chilean and Alaskan subduction zones exhibit low background seismicity rates, yet have experienced magnitude 9 earthquakes in the past century. Slow slip occurs in many of these regions, implying that slow deformation may aid nucleation of very large earthquakes. The proportionality relationship could be used to assess the seismic risk between two endmembers: active subduction zones that generate moderate earthquakes and quiet subduction zones that generate extremely large earthquakes.

  13. Subduction of fracture zones controls mantle melting and geochemical signature above slabs.

    PubMed

    Manea, Vlad C; Leeman, William P; Gerya, Taras; Manea, Marina; Zhu, Guizhi

    2014-10-24

    For some volcanic arcs, the geochemistry of volcanic rocks erupting above subducted oceanic fracture zones is consistent with higher than normal fluid inputs to arc magma sources. Here we use enrichment of boron (B/Zr) in volcanic arc lavas as a proxy 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 fracture zones in the relatively cool Aleutian and Andean subduction zones where fracture zone subduction locally enhances fluid introduction beneath volcanic arcs. Geodynamic models of subduction have not previously considered how fracture zones may influence the melt and fluid distribution above slabs. Using high-resolution three-dimensional coupled petrological-thermomechanical numerical simulations of subduction, we show that enhanced production of slab-derived fluids and mantle wedge melts concentrate in areas where fracture zones are subducted, resulting in significant along-arc variability in magma source compositions and processes.

  14. Rare earth element budgets in subduction-zone fluids

    NASA Astrophysics Data System (ADS)

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

    2012-12-01

    Subduction zone fluids play a fundamental role in the geochemical cycle of the Earth. The nature and composition of these fluids are determined by complex processes and still poorly understood. As a result of a variety of metasomatic and partial melting events, arc-related magmas display a typical trace element abundance spectrum, in which the rare earth elements' (REE) signature is an important record of petrogenetic processes. Therefore, investigating the behavior of REE in fluids at high pressure (P) and temperature (T) conditions is crucial for constraining fluid composition, as well as understanding subduction-zone processes in general. However, up to date, the experimental studies on REE solubility and speciation are limited to quite low P-T conditions (300 °C, saturated water vapor pressure) [1]. The theoretical predictions of the stability of REE complexes have been performed up to 350 °C [2] and 1000 °C, 0.5 GPa [3] by the extrapolation of thermodynamic data obtained at ambient conditions. In this study we present new experimental data on REE silicate (REE2Si2O7) solubility in aqueous quartz saturated fluids, containing various ligands, at conditions relevant for subducting slabs (600, 700, 800 °C, 2.6 GPa). The aim of the experiments was to investigate the relative effect of temperature and ligands on the solubility of REE. The experiments were conducted in an end-loaded piston-cylinder apparatus and the fluids were in situ sampled at P-T in the form of primary fluid inclusions in quartz [4]. The gold capsule was typically loaded with a chip of synthetic REE silicate (La,Nd,Gd,Dy,Er,Yb)2Si2O7, an aqueous fluid (~20 wt.%) and a piece of natural quartz. During the experiment (24-48 h) a thermal gradient along the capsule promoted intensive dissolution of quartz at the hottest part and precipitation of new quartz at the cooler part of the capsule, allowing the primary fluid inclusions to be trapped (~30-50 μm). Rubidium and cesium were added to the

  15. Tracing fluid transfer across subduction zones using iron and zinc stable isotopes

    NASA Astrophysics Data System (ADS)

    Williams, H. M.; Debret, B.; Pons, M. L.; Bouilhol, P.

    2016-12-01

    In subduction zones, serpentinite devolatilization within the downgoing slab and the fluids released play a fundamental role in volatile transfer as well as the redox evolution of the sub-arc mantle. Constraining subduction-related serpentinite devolatilisation is essential in order to better understand of the nature and composition of slab-derived fluids and fluid/rock interactions. Fe and Zn stable isotopes can trace fluid composition and speciation as isotope partitioning is driven by changes in oxidation state, coordination, and bonding environment. In the case of serpentinite devolatilisation, Fe isotope fractionation should reflect changes in Fe redox state and the formation of Fe-Cl- and SO42- complexes (Hill et al., GCA 2010); Zn isotope fractionation should be sensitive to complexation with CO32-, HS- and SO42- anions (Fujii et al., GCA 2011). We targeted samples from Western Alps ophiolite complexes, interpreted as remnants of serpentinized oceanic lithosphere metamorphosed and devolatilized during subduction (Hattori and Guillot, G3 2007; Debret et al., Chem. Geol. 2013). A striking negative correlation is present between bulk serpentinite Fe isotope composition and Fe3+/Fetot, with the highest grade samples displaying the heaviest Fe isotope compositions and lowest Fe3+/Fetot (Debret et al., Geology, 2016). The same samples also display a corresponding variation in Zn isotopes, with the highest grade samples displaying isotopically light compositions (Pons et al., in revision). The negative correlation between Fe and Zn isotopes and decrease in Fe3+/Fetot can explained by serpentinite sulfide breakdown and the release of fluids enriched in isotopically light Fe and heavy Zn sulphate complexes. The migration of these SOX-bearing fluids from the slab to the slab-mantle interface or mantle wedge has important implications for the redox evolution of the sub-arc mantle and the transport of metals from the subducting slab.

  16. 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.

  17. Trading Time with Space - Development of subduction zone parameter database for a maximum magnitude correlation assessment

    NASA Astrophysics Data System (ADS)

    Schaefer, Andreas; Wenzel, Friedemann

    2017-04-01

    Subduction zones are generally the sources of the earthquakes with the highest magnitudes. Not only in Japan or Chile, but also in Pakistan, the Solomon Islands or for the Lesser Antilles, subduction zones pose a significant hazard for the people. To understand the behavior of subduction zones, especially to identify their capabilities to produce maximum magnitude earthquakes, various physical models have been developed leading to a large number of various datasets, e.g. from geodesy, geomagnetics, structural geology, etc. There have been various studies to utilize this data for the compilation of a subduction zone parameters database, but mostly concentrating on only the major zones. Here, we compile the largest dataset of subduction zone parameters both in parameter diversity but also in the number of considered subduction zones. In total, more than 70 individual sources have been assessed and the aforementioned parametric data have been combined with seismological data and many more sources have been compiled leading to more than 60 individual parameters. Not all parameters have been resolved for each zone, since the data completeness depends on the data availability and quality for each source. In addition, the 3D down-dip geometry of a majority of the subduction zones has been resolved using historical earthquake hypocenter data and centroid moment tensors where available and additionally compared and verified with results from previous studies. With such a database, a statistical study has been undertaken to identify not only correlations between those parameters to estimate a parametric driven way to identify potentials for maximum possible magnitudes, but also to identify similarities between the sources themselves. This identification of similarities leads to a classification system for subduction zones. Here, it could be expected if two sources share enough common characteristics, other characteristics of interest may be similar as well. This concept

  18. Rethinking turbidite paleoseismology along the Cascadia subduction zone

    USGS Publications Warehouse

    Atwater, Brian F.; Carson, Bobb; Griggs, Gary B.; Johnson, H. Paul; Salmi, Marie

    2014-01-01

    A stratigraphic synthesis of dozens of deep-sea cores, most of them overlooked in recent decades, provides new insights into deep-sea turbidites as guides to earthquake and tsunami hazards along the Cascadia subduction zone, which extends 1100 km along the Pacific coast of North America. The synthesis shows greater variability in Holocene stratigraphy and facies off the Washington coast than was recognized a quarter century ago in a confluence test for seismic triggering of sediment gravity flows. That test compared counts of Holocene turbidites upstream and downstream of a deep-sea channel junction. Similarity in the turbidite counts among seven core sites provided evidence that turbidity currents from different submarine canyons usually reached the junction around the same time, as expected of widespread seismic triggering. The fuller synthesis, however, shows distinct differences between tributaries, and these differences suggest sediment routing for which the confluence test was not designed. The synthesis also bears on recent estimates of Cascadia earthquake magnitudes and recurrence intervals. The magnitude estimates hinge on stratigraphic correlations that discount variability in turbidite facies. The recurrence estimates require turbidites to represent megathrust earthquakes more dependably than they do along a flow path where turbidite frequency appears limited less by seismic shaking than by sediment supply. These concerns underscore the complexity of extracting earthquake history from deep-sea turbidites at Cascadia.

  19. Electrical conductivity of hydrous andesitic melts pertinent to subduction zones

    NASA Astrophysics Data System (ADS)

    Guo, Xuan; Li, Bin; Ni, Huaiwei; Mao, Zhu

    2017-03-01

    Andesitic magmatism and rocks are widespread at convergent plate boundaries. Electrically conductive bodies beneath subduction zone arc volcanoes, such as the Uturuncu Volcano, Bolivia, may correspond to active reservoirs of H2O-bearing andesitic magma. Laboratory measurements of electrical conductivity of hydrous andesitic melts are required to constrain the physicochemical conditions of these magma reservoirs in combination with magnetotelluric data. This experimental study investigates electrical conductivity of andesitic melts with 0.01-5.9 wt % of H2O at 1164-1573 K and 0.5-1.0 GPa in a piston cylinder apparatus using sweeping-frequency impedance spectroscopy. Electrical conductivity of andesitic melt increases with increasing temperature and H2O concentration but decreases with pressure. Across the investigated range of H2O concentration, electrical conductivity varies by 1.2-2.4 log units, indicating stronger influence of H2O for andesitic melt than for rhyolitic and dacitic melts. Using the Nernst-Einstein equation, the principal charge carrier is inferred to be Na in anhydrous melt but divalent cations in hydrous andesitic melts. The experimental data are regressed into a general electrical conductivity model for andesitic melt accounting for the pressure-temperature-H2O dependences altogether. Modeling results show that the conductive layer at >20 km depths beneath the surface of the Uturuncu Volcano could be interpreted by the presence of less than 20 vol % of H2O-rich andesitic melt (with 6-9 wt % H2O).

  20. Lithium Isotopic Fractionation in Subduction Zones: Clues From Clays

    NASA Astrophysics Data System (ADS)

    Williams, L. B.; Hervig, R. L.

    2003-12-01

    Lithium isotope ratios show such large variations in nature (>30 per mil), that many areas of geosciences are exploring the usefulness of this system in explaining the evolution of particular rocks. Here we show how the lithium isotope ratios change during the transformation of smectite clay minerals to illite during burial metamorphism. Such a transition may be a common feature in the shallow regions of subduction zones and may ultimately affect the Li isotope compositions of fluids contributing to arc magmatism. Lithium is a ubiquitous trace element in natural formation waters that, like B, shows large isotopic fractionation especially during interactions with clay minerals. Lithium is adsorbed in the interlayer region of expandable clay minerals but is easily exchanged. Lithium is also incorporated into the octahedral sites. The substitutions of Li in two crystallographic sites of clay minerals may complicate interpretations of bulk Li-isotope ratios. We suggest that the magnitude of the isotopic fractionation of Li between fluid and clay is different in the interlayer sites of clay minerals than in the octahedral sites of clay minerals. Examination of Li contents and isotope variations in experimental reactions of smectite to illite (300C, 100MPa) shows changes with structural re-arrangement of the clay layers. The Li-isotope trend declines (from ~+6 to -13 per mil, expressed as ratios of 7/6) throughout R1-ordering of the mixed-layered illite smectite (I/S). However, the equilibrium end products of the reaction have R3-ordering and show a heavier isotope ratio (~0 per mil). This observation is very similar to the trends we observed for B-isotopes, where the interlayer B initially overprinted the tetrahedral-layer B isotope composition, but as the interlayer sites were collapsed during illitization, the equilibrium isotope composition was approached. The significant Li and B isotopic changes that occur during ordering of I/S coincides with the temperatures

  1. Early abyssal- and late SSZ-type vestiges of the Rheic oceanic mantle in the Variscan basement of the Sakarya Zone, NE Turkey: Implications for the sense of subduction and opening of the Paleotethys

    NASA Astrophysics Data System (ADS)

    Dokuz, Abdurrahman; Uysal, İbrahim; Kaliwoda, Melanie; Karsli, Orhan; Ottley, Chris J.; Kandemir, Raif

    2011-11-01

    The boundary between the Gondwana and Laurussia, or a terrane separated from it, is traced by a suture recording the closure of the Rheic ocean in the Pulur, Beyçam and Kurtoğlu areas of the eastern Sakarya Zone of Turkey. Pulur lherzolite has a lower Cr# [100Cr/(Cr + Al) = 5-37] of the spinel and slightly enriched flat medium-heavy rare earth element (M-HREE) whole-rock pattern typical for those of the fertile abyssal-type lherzolite. By contrast, the Beyçam harzburgite displays the mineralogical and geochemical features of a moderately depleted residue at a supra subduction zone (SSZ). Some of spinels from the Beyçam peridotites show TiO2 contents (0.15-0.48 wt.%) identical to a later interaction with a melt of island-arc tholeiite (IAT) composition. Meta-basalt from the Beyçam area is the representative of such a melt and displays the whole-rock M-HREE pattern of a complementary product of the Beyçam harzburgite. Data presented in this study support the primarily northward directed closure of the Rheic ocean, but suggest that the Variscan basement in the eastern part of the Sakarya Zone consists of mantle peridotites formed in a south-vergent SSZ. This dual polarity of subduction in the Rheic ocean, at least in the second half of its closure, is also favored by the geological structure and lithological diversity of the Variscan rocks. These are: (1) spatial distribution of the peridotites, e.g., SSZ-type harzburgites to the north and abyssal-type lherzolites to the south, (2) lack of high-pressure metamorphic rocks, (3) decrease in the degree of Variscan metamorphism toward the north, and (4) emplacement of Variscan syn-collisional magmatism into the low grade metamorphic rocks to the north. Paleotethys was the other ocean of the region created in the Late Paleozoic. Northward subduction of the Rheic ocean from Early Devonian to Early Carboniferous appears to be the most plausible mechanism responsible for the opening of Paleotethys. In this scenario the

  2. Numerical modeling of fracture zone subduction and related volcanism in Southern Mexico

    NASA Astrophysics Data System (ADS)

    Constantin Manea, Vlad; Gerya, Taras; Manea, Marina

    2010-05-01

    Oceanic fracture zones are recognized as areas where parts of the oceanic lithosphere can be partially serpentinized. Therefore, when subducting, these fracture zones have the potential to carry significant amounts of fluids which are released at certain depths, depending on the slab dynamics. In the case of Southern Mexico, the Cocos plate hosts a large oceanic fracture zone named Tehuantepec FZ, currently subducting. Onshore a large stratovolcano, called El Chichon, intersects the prolongation of Tehuantepec FZ where the slab depth beneath is more than 200 km, an unusual depth for a subduction related volcanic arc. In this study we investigate numerically the influence of a serpentinized fracture zone rheology on the depth where hydrous instabilities (cold-plumes) are formed. Our preliminary results show that the subduction of serpentinized oceanic lithosphere plays an important depth control for the hydrous cold-plume formation, which is probable responsible for the unusual volcanism location in Southern Mexico.

  3. 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)

  4. 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.

  5. 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)

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

    NASA Astrophysics Data System (ADS)

    Honda, S.

    2014-12-01

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

  7. Fluids in the Deep Crust and in Subduction Zones: Frontiers for Research

    NASA Astrophysics Data System (ADS)

    Ague, J. J.

    2006-05-01

    Fluids are an integral part of metamorphic and igneous processes in the deep crust and within subduction zones. The presence of fluids in deep settings is no longer in dispute, but major research frontiers of global significance remain. One of these is the problem of tracing flow paths from deep metamorphic settings into the shallow crust. This is important to better understand crustal-scale transfer of ore-forming metals and the location and formation of valuable ore deposits. Furthermore, the long-term flux of greenhouse gases such as CO2 out of metamorphic systems remains as one of the most poorly constrained parts of the global C cycle. Relatively rapid metamorphic reaction and release of CO2 may be an agent for global warming, whereas deep burial without reaction may sequester CO2 from the atmosphere and lead to cooling. Another unresolved problem concerns how much heat is transfered by deep fluid flow. Large fluxes are required to transport heat by fluid advection, and it remains controversial whether or not such fluxes are acheived deep within mountain belts. Several examples of heating by regional metamorphic fluids have been proposed in recent years; can fluids cool a terrane as well? Progress on heat transfer will require a better understanding of rates of metamorphic heating/cooling, fluid fluxes, and timescales of fluid flow. It has long been known that elevated pore fluid pressures can weaken rocks and lead to seismogenic failure, but many questions remain regarding the role of metamorphism. The volume change (solid+fluid) for most crustal dehydration reactions is positive, such that metamorphism can play an active role in generating elevated pore fluid pressures if rates of reaction are kinetically rapid and rock permeability is relatively low. In mantle settings such as subducted slabs, total volume changes attending dehydration can be negative, causing collapse of pore space; the rheological implications of this mode of devolatilization deserve much

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

    NASA Astrophysics Data System (ADS)

    Ujiie, Kohtaro; Kimura, Gaku

    2014-12-01

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

  9. A detailed map of the 660-kilometer discontinuity beneath the izu-bonin subduction zone.

    PubMed

    Wicks, C W; Richards, M A

    1993-09-10

    Dynamical processes in the Earth's mantle, such as cold downwelling at subduction zones, cause deformations of the solid-state phase change that produces a seismic discontinuity near a depth of 660 kilometers. Observations of short-period, shear-to-compressional wave conversions produced at the discontinuity yield a detailed map of deformation beneath the Izu-Bonin subduction zone. The discontinuity is depressed by about 60 kilometers beneath the coldest part of the subducted slab, with a deformation profile consistent with the expected thermal signature of the slab, the experimentally determined Clapeyron slope of the phase transition, and the regional tectonic history.

  10. Global analysis of the effect of fluid flow on subduction zone temperatures

    NASA Astrophysics Data System (ADS)

    Rotman, Holly M. M.; Spinelli, Glenn A.

    2013-08-01

    Knowledge of the controls on temperature distributions at subduction zones is critical for understanding a wide range of seismic, metamorphic, and magmatic processes. Here, we present the results of ˜220 thermal model simulations covering the majority of known subduction zone convergence rates, incoming plate ages, and slab dips. We quantify the thermal effects of fluid circulation in the subducting crust by comparing results with and without advective heat transfer in the oceanic crustal aquifer. We find that hydrothermal cooling of a subduction zone is maximized when the subducting slab is young, slowly converging, steeply dipping, and the crustal aquifer is ventilated near the trench. Incoming plate age is one of the primary controls on the effectiveness of advective heat transfer in the aquifer, and the greatest temperature effects occur with an incoming plate <50 Ma. The thermal effects of fluid circulation decrease dramatically with increasing age of the incoming plate. Temperatures in the Cascadia, Nankai, southern Chile, Colombia/Ecuador, Mexico, and Solomon Islands subduction zones are likely strongly affected by fluid circulation; for these systems, only thermal models of Cascadia and Nankai have included fluid flow in subducting crust.

  11. 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.

  12. Crustal basement controls granitoid magmatism, and implications for generation of continental crust in subduction zones: A Sr-Nd-Hf-O isotopic study from the Paleozoic Tongbai orogen, central China

    NASA Astrophysics Data System (ADS)

    Wang, Hao; Wu, Yuan-Bao; Yang, Jin-Hui; Qin, Zheng-Wei; Duan, Rui-Chun; Zhou, Lian; Yang, Sai-Hong

    2017-06-01

    Ascertaining the petrogenesis of granitoid rocks in subduction zones holds the key for understanding the processes of how continental crust is produced. The synchronous Taoyuan and Huanggang plutons occur in two different geological units of the Paleozoic Tongbai orogen of central China. They provide an optimal opportunity for a study to address the role of the crustal basement in generating voluminous granitoid magmatism in subduction zones. The Taoyuan and Huanggang plutons have identical U-Pb zircon crystallization ages of 440-444 Ma, which are temporally related to northward subduction of the Paleotethyan Ocean. The Taoyuan samples show high SiO2 (73.36-79.16%) and low Al2O3 (12.00-13.45%) contents, Mg numbers (20.6-38.2), and Sr/Y (2.04-10.1) and (La/Yb)N (2.34-7.32) ratios with negative Eu anomalies (Eu/Eu* = 0.33-0.93). They yielded positive εNd(t) (+ 3.0 to + 6.7) and εHf(t) (+ 11.8 to + 13.2) values, elevated initial Sr isotopic ratios (0.7040-0.7057) and relatively low zircon δ18O values of 4.62-5.39‰. These suggest that they were produced through partial melting of hydrothermally altered lower crust of the accreted Erlangping oceanic arc. In contrast, the Huanggang samples exhibit variable whole-rock geochemical and isotopic compositions with SiO2 contents of 57.01-64.42 wt.%, initial Sr isotopic ratios of 0.7065-0.7078, and εNd(t) values of - 5.7 to - 9.4. Additionally, they have high zircon δ18O values of 7.57-8.45‰ and strongly negative zircon εHf(t) values of - 14.4 to - 10.5. They were suggested to have been mainly derived from ancient continental crust of the Kuanping crustal unit with the addition of 20-40% juvenile, mantle-derived material. Accordingly, the granitoids in both oceanic and continental arcs are likely to be mainly derived from intracrustal melting of their crustal basement. It is revealed by the Huanggang pluton that little net continental crust growth occurs in continental arcs, and addition of new volume of continental

  13. Permeability-porosity relationships of subduction zone sediments

    USGS Publications Warehouse

    Gamage, Kusali; Screaton, Elizabeth; 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.

  14. 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.

  15. 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

  16. A fluid inclusion study of blueschist-facies lithologies from the Indus suture zone, Ladakh (India): Implications for the exhumation of the subduction related Sapi-Shergol ophiolitic mélange

    NASA Astrophysics Data System (ADS)

    Sachan, Himanshu Kumar; Kharya, Aditya; Singh, P. Chandra; Rolfo, Franco; Groppo, Chiara; Tiwari, Sameer K.

    2017-09-01

    The best occurrence of blueschist-facies lithologies in Himalaya is that of the Shergol Ophiolitic Mélange along the Indus suture zone in Ladakh region of north-western India. These lithologies are characterized by well preserved lawsonite-glaucophane-garnet-quartz assemblages. This paper presents for the first time the results of a detailed fluid inclusion study on these lithologies, in order to understand the fluid P-T evolution and its tectonic implications. The blueschist rocks from Shergol Ophiolitic Mélange record metamorphic peak conditions at ∼19 kbar, 470 °C. Several types of fluid inclusions are trapped in quartz and garnet, most of them being two-phase at room temperature. Three types of fluid inclusions have been recognised, basing on microtextures and fluid composition: Type-I are primary two-phase carbonic-aqueous fluid inclusions (VCO2 - LH2O); Type-II are two-phase (LH2O - VH2O) aqueous fluid inclusions, either primary (Type-IIa) or secondary (Type-IIb); Type-III are re-equilibrated fluid inclusions. In the Type-I primary carbonic-aqueous inclusions, H2O is strongly predominant with respect to CO2; the homogenization temperature of CO2 range from -7 to -2 °C. The clathrate melting temperature in such inclusions varies in between +7.1 and +8.6 °C. Type-II two-phase aqueous fluid inclusions show a wide range of salinity, from 7.8-14 wt.% NaCleq (Type-IIa) to 1.65-6.37 wt.% NaCleq (Type-IIb) with accuracy ±0.4 wt.% NaCleq. Type-I and Type-IIa primary fluid inclusions are hosted in peak minerals (garnet and quartz included in garnet), therefore they were likely entrapped at, or near to, peak P-T conditions. The dominantly aqueous fluid of both Type-I and Type-IIa inclusions was most likely produced through metamorphic devolatilization reactions occurring in the subducting slab. Despite their primary nature, the isochores of Type-I and Type-IIa inclusions do not intersect the peak metamorphic conditions of the blueschist mineral assemblage

  17. 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.

  18. 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

  19. Geochemistry of ocean floor serpentinites world-wide: constraints on the ultramafic input to subduction zones

    NASA Astrophysics Data System (ADS)

    Kodolányi, J.; Pettke, T.; Spandler, C.; Kamber, B.; Gméling, K.

    2009-04-01

    independent of geodynamic setting. Supra-subduction zone serpentinites reveal additional enrichments in Cs, Rb, ±Sr, identifying an alteration fluid source that is not pure seawater. In conclusion, precursor mineralogy and magmatic history together with hydration temperature govern the trace element budget of ocean floor serpentinites, which, apart from supplying H2O to the subduction zone, may also be a significant source of B and Cl to the arc magma source and, depending on geodynamic setting, may even influence the element budget for Cs, Rb, Pb, U and .Sr. References: Martin B, Fyfe WS (1970) Some experimental and theoretical observations on the kinetics of hydration reactions with particular reference to serpentinization. Chem Geol 6: 185-202 Pettke T, Spandler C, Kodolányi J, Scambelluri M (2009) The chemical signatures of progressive dehydration stages in subducted serpentinites (this volume) Salters VJM, Stracke A (2004) Composition of the depleted mantle. Geochem Geophys Geosyst 5 Doi: 10.1029/2003GC000597 Scambelluri M, Fiebig J, Malaspina N, Müntener O, Pettke T (2004) Serpentinite Subduction: Implications for Fluid Processes and Trace-Element Recycling. Int Geol Rev 46: 595-613 Ulmer P, Trommsdorff V (1995) Serpentine stability to mantle depths and subduction-related magmatism. Science 268: 858-861

  20. Modeling of Subduction Zone Slow/Slient Slip Events in Deeper Parts of the Seismogenic Zone

    NASA Astrophysics Data System (ADS)

    Liu, Y.; Rice, J. R.

    2004-05-01

    Recent high resolution GPS measurements have detected slow and silent (or aseismic) slip events near the downdip end of the seismogenic zone at Japan, Cascadia and Mexico subduction zones [Hirose et al., 1999; Ozawa et al., 2001; Dragert et al., 2001; Lowry et al., 2001; Ozawa et al., 2002]. To investigate possible physical mechanisms, we apply a Dieterich-Ruina rate and state friction law to a three dimensional shallow subduction fault, which is loaded by imposed slip at rate Vpl ( ˜{10-9} m/s) far downdip along the thrust interface. Friction properties are temperature, and hence depth, dependent, so that sliding is stable ( a - b > 0) at depths below about 30 km. The system is perturbed into a nonuniform slip mode by introducing small (0 to 5%) along-strike variations in the constitutive parameters a and (a-b). Simulation results show large events with multiple magnitudes at various along-strike locations on the fault, with different recurrence intervals, like natural interplate earthquakes. More interesting, we observe that the large heterogeneous slip at seismogenic depths (i.e., where a - b < 0) is sometimes accompanied by events that have clearly aseismic slip rates (10 to 102 Vpl), which are comparable to the 10-9 to 10-8 m/s slip rates inferred in Japan and Cascadia Subduction zones [Hirose et al., 1999; Ozawa et al., 2001; Dragert et al., 2001]. These aseismic slip events usually nucleate below the less well locked ``gap'' regions (slipping at order of 0.1 to 1 times plate convergence rate Vpl) between more firmly locked regions (slipping at 10-4 to 10-2 Vpl). Some have aseismic slip rate fronts that migrate more than 100 km in the strike direction with a maximum speed ˜{20} km/year, at depths near or below the downdip end of the seismogenic zone. This migration speed is of the same order as the along-strike slip propagation in 1997 Bungo Channel event, southwestern Japan [Ozawa et al., 2001] and 2001 Tokai region event, central Japan [Ozawa et al., 2002

  1. Connecting the Surface and the Deep: Evolving Role of Subduction Zone Fluids Through Time

    NASA Astrophysics Data System (ADS)

    Galvez, Matthieu Emmanuel

    2017-04-01

    The speciation of aqueous fluids controls the transport and exchange of metals and volatile elements on Earth. Subduction zones are the most important geodynamic setting for this fluid-mediated chemical exchange. Characterizing the ionic speciation and pH of fluids equilibrated with rocks at subduction zone conditions has been a major challenge in Earth science. I will first present thermodynamic predictions of fluid-rock equilibria that tie together models of mineralogy and fluid speciation along a range of model P-T paths. The pH of fluids in subducted crustal lithologies is uniform and confined to a mildly alkaline range, controlled by rock volatile and chlorine contents. In contrast, the pH of mantle wedge fluids exhibits marked sensitivity to minor variations in rock chemistry. These variations may be caused by intramantle differentiation, or by infiltration of fluids enriched in alkali components extracted from the subducted crust. The sensitivity of pH to carbon, alkali and halogens illustrates a top-down control of Earth's atmosphere - ocean chemistry on the speciation of subduction zone fluids via the hydrothermally altered oceanic lithosphere. These results provide a perspective on the physicochemical mechanisms that have coupled metal and volatile cycles in subduction zones for over 2.5 billion years.

  2. 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.

  3. 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

  4. 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

  5. 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

  6. 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

  7. 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

  8. 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.

  9. 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

  10. 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

  11. The Structural Evolution of the Gorda "Plate": Destruction of a Plate Fragment Prior to Subduction in the Cascadia Subduction Zone

    NASA Astrophysics Data System (ADS)

    Chaytor, J. D.; Goldfinger, C.; Dziak, R. P.

    2002-12-01

    The Gorda "plate" the southern-most fragment of the Juan de Fuca plate subducting beneath North America within the Cascadia Subduction Zone, represents an excellent example of the destruction of an oceanic lithospheric plate. Continuing structural mapping of the entire plate based on new and previously acquired multibeam bathymetric, backscatter, and seismic reflection data reveals a complex interplay of multiple generations, styles, and orientations of faulting. Primary spreading-center created faults dominate the plate, many of which are found to have undergone reactivation either as normal faults or as dip-slip faults that contain a significant component of strike-slip motion. Although fewer in number, the faults of most significance are northeast-southwest trending left-lateral strike-slip faults cutting through the central and northern parts of the plate, offsetting basement ridges, knolls and at least one inter-ridge channel. The most prominent of these is an approximately 100 km long fault originating at the spreading ridge, smoothly curving through the center of the plate laterally offsetting several large basement ridges, one knoll, and an inter-ridge channel, and joining a normal-reactivated basement ridge fault in the northern part of the plate. These second-generation strike-slip faults appear to be a major controlling factor in the absorption of deformation currently being imposed on the plate by continued subduction in the Cascadia Subduction Zone. In addition, recently acquired 3.5 KHz reflection data over several faults in the southern and central parts of the plate, including structures previously identified as inactive pseudofaults, shows clear evidence of recent activity. Relocated earthquake epicenters from several large Gorda intraplate events, achieved using the North Pacific SOSUS network, show excellent correlation with both the left-lateral strike slip and other mapped faults within the plate, attesting to the continued internal deformation

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

    NASA Astrophysics Data System (ADS)

    Ide, S.

    2013-12-01

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

  13. Abnormal high seismic moment release rate along the Ecuador-Colombia subduction zone

    NASA Astrophysics Data System (ADS)

    Nocquet, J. M.; Vallee, M.; Yepes, H. A.; Sladen, A.; Jarrin, P.; Rolandone, F.

    2016-12-01

    The Ecuador-Colombia subduction zone hosted a large seismic sequence that started with the great 1906 Mw8.8 earthquake, whose rupture area was then reactivated by large earthquakes with magnitude Mw 7.7-8.2. Unlike most subduction zones where the seismic budgets relies on historical records of past ruptures, suffering from large uncertainties, every event of the Ecuador-Colombia sequence since 1906 benefits from intensity, tsunami studies and global seismological records that enabled an epicenter location, fault parameters, aftershocks location and a moment magnitude to be derived. The 2016 earthquake further offers an opportunity to quantify the budgets of seismic moment versus plate motion and strain accumulation rates reliably estimated from GPS. The 2016 earthquake occurred in the same area as the 1942 event that had a similar size. In order to evaluate the level of overlap between the 1942 and 2016 ruptures, we carefully review every available observation. Despite the uncertainties inherent to the scarcity of records in 1942, we conclude to a significant overlap between the 2016 and 1942 earthquake slip distributions, suggesting the successive rupture of the same asperity. With co-seismic slip exceeding 6 m during the 2016 rupture, we find that the maximum slip largely overshoots the slip deficit accumulated since 1942 that is at most 3.2 m. Furthermore, regardless whether they broke the same asperity, both the 1942 and 2016 earthquakes had a moment exceeding significantly the moment accumulated since the previous earthquake. This characteristic is also shared by the Mw 7.7 1958 and Mw 8.2 1979 earthquakes, which had a seismic moment much larger than expected from the moment deficit accumulated from plate motion since 1906.These results have implications on the general description of the earthquake cycle but also call for a reappraisal seismic hazard assessment in the region on the short-term.

  14. Thermal regime along the Antilles subduction zone: Influence of the oceanic lithosphere materials subducted in the oceanic crust

    NASA Astrophysics Data System (ADS)

    Biari, Youssef; Marcaillou, Boris; Klingelhoefer, Frauke; Francis, Lucazeau; Fréderique, Rolandone; Arnauld, Heuret; Thibaud, Pichot; Hélène, Bouquerel

    2017-04-01

    Heat-flow measurements acquired during the Antithesis Cruise in the Northern Lesser Antilles reveal an atypical heat-flow trend, from the trench to the margin forearc, where the subducting crust consists of exhumed and serpentinized mantle rocks (see Marcaillou et al. same session). We investigate the thermal structure of the Lesser Antilles subduction zone along two transects perpendicular to the margin located off Antigua and Martinique Islands. We perform 2-D steady-state finite elements thermal modelling constrained by newly-recorded and existing data: heat flow measurements, deep multichannel reflection and wide angle seismic data as well as earthquake hypocenters location at depth. Along the Martinique profile, the heat-flow decreases from the trench (45 mW.m-2) to minimum in the outer fore-arc (30 mW.m-2) and increases to a plateau (50 mW.m-2) toward the back-arc area. These trend and values are typical for the subduction of a steep 80-MYr old oceanic plate beneath an oceanic margin. As a result, the 150°-350°C temperature range along the interplate contact, commonly associated to the thermally-defined seismogenic zone, is estimated to be located between 200 - 350km from the trench. In contrast, along the Antigua profile, the heat-flow shows an atypical "flat" trend at 40 ± 15 mW.m-2 from the trench to the inner forearc. Purely conductive thermal models fail at fitting both the measured values and the flat trend. We propose that the subducting crust made of serpentinized exhumed mantle rock strongly affecting the heat-flow at the surface and the margin thermal structure. The geothermal gradient in the 5-km-thick serpentinized layer is expected to be low compared to "normal" oceanic crust because of cold water percolation and peridotite alteration. Moreover, from 50km depth, serpentine dehydration reactions provide significant amounts of hot water expelled toward the upper plate, generated heat beneath the forearc. As a result, in our preferred model: 1/ A

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

    NASA Astrophysics Data System (ADS)

    Zhang, H.; Thurber, C. H.

    2005-12-01

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

  16. 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

  17. Thermodynamic Constraints on Carbonate Stability and Volatile Carbon Production in Subduction Zone Environments

    NASA Astrophysics Data System (ADS)

    Gorce, J. S.; Caddick, M. J.

    2016-12-01

    High pressure breakdown of carbonate minerals is frequently cited as an important mechanism for carbon release from subducted rocks. Previous estimates of carbon input into the atmosphere via subduction related volcanism, calculated through the use of helium and carbon isotopes, have suggested that up to 85% of the 0.3-5.6x1012 moles of carbon released per year at volcanic arcs originates from the subducting slab. Additionally, the presence of CO2-H2O-NaCl bearing fluid inclusions found in exhumed rocks from subduction zones provides us with direct evidence for complex fluid compositions in the subducting system. However, experimental data and thermodynamic models generally predict that the stability of carbonate minerals in the subducting slab extends deep into the mantle, implying that they may not be able to contribute sufficient C to match expectations. To address this, previous studies have suggested that the generation of volatile C from subducting slabs results from the addition of H2O-rich fluids that drive dissolution of carbonate rich lithologies. This study seeks to understand the potential for carbonate dissolution in subduction zones through the use of simple thermodynamic modeling. Because externally derived fluids may not be in equilibrium with the rock at a given P-T condition, they can facilitate water/rock interactions that ultimately break down carbonate minerals and liberate C. We thus calculated equilibrium mineral assemblages and fluid compositions (assuming a simple H2O-CO2 binary) for typical lithologies experiencing a range of subduction zone geotherms, exploring the addition of fluids at various stages. Results suggest that the addition of <100 liters of a hydrous fluid can fully decarbonate 1m3 of hydrated MORB during subduction and even a fluid with a composition of XH2O < 0.7 has the ability to break down a substantial amount of carbonate minerals. This interaction varies in efficacy with depth and temperature in the subduction system

  18. 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.

  19. 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

  20. 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.

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

    SciTech Connect

    Mccaffrey, R.

    1994-10-01

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

  2. A viscoplastic shear-zone model for episodic slow slip events in oceanic subduction zones

    NASA Astrophysics Data System (ADS)

    Yin, A.; Meng, L.

    2016-12-01

    Episodic slow slip events occur widely along oceanic subduction zones at the brittle-ductile transition depths ( 20-50 km). Although efforts have been devoted to unravel their mechanical origins, it remains unclear about the physical controls on the wide range of their recurrence intervals and slip durations. In this study we present a simple mechanical model that attempts to account for the observed temporal evolution of slow slip events. In our model we assume that slow slip events occur in a viscoplastic shear zone (i.e., Bingham material), which has an upper static and a lower dynamic plastic yield strength. We further assume that the hanging wall deformation is approximated as an elastic spring. We envision the shear zone to be initially locked during forward/landward motion but is subsequently unlocked when the elastic and gravity-induced stress exceeds the static yield strength of the shear zone. This leads to backward/trenchward motion damped by viscous shear-zone deformation. As the elastic spring progressively loosens, the hanging wall velocity evolves with time and the viscous shear stress eventually reaches the dynamic yield strength. This is followed by the termination of the trenchward motion when the elastic stress is balanced by the dynamic yield strength of the shear zone and the gravity. In order to account for the zig-saw slip-history pattern of typical repeated slow slip events, we assume that the shear zone progressively strengthens after each slow slip cycle, possibly caused by dilatancy as commonly assumed or by progressive fault healing through solution-transport mechanisms. We quantify our conceptual model by obtaining simple analytical solutions. Our model results suggest that the duration of the landward motion increases with the down-dip length and the static yield strength of the shear zone, but decreases with the ambient loading velocity and the elastic modulus of the hanging wall. The duration of the backward/trenchward motion depends

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

    SciTech Connect

    Barton, M.D.; Bebout, G.E. )

    1989-11-01

    On Santa Catalina Island, southern California, bluechist to amphibolite facies metasedimentary, metamafic, and meta-ultramafic rocks show veining and alteration that reflect fluid flow and mass transfer at 25-45 km depths in an Early Cretaceous subduction zone. Synkinematic and postkinematic veins record fluid transport and metasomatism during prograde metamorphism and uplift. Vein and host-rock mineralogy and whole-rock compositions demonstrate large-scale chemical redistribution, especially of Si and alkali elements. Veins and host rocks trend toward isotopic equilibration with aqueous fluids with {delta}{sup 18}O{sub SMOW}=+13{per thousand} {plus minus} 1{per thousand}. The likely source for these fluids is in lower temperature, sediment-rich parts of the subduction zone. Carbon isotope systematics support this conclusion and indicate the influence of an organic C source. Quartz solubility relations indicate the importance of fluid-flow paths in chemical redistribution during subduction. These results document large-scale fluid flow and the complexity of possible metasomatic and mechanical mixing processes at intermediate levels of subduction zones. The record of subduction-zone mass transfer in the Catalina Schist is compatible with the record inferred for greater depths from geochemical and petrologic studies of arc magmatism.

  4. 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.

  5. Advancing Subduction Zone Science After a Big Quake

    NASA Astrophysics Data System (ADS)

    Beck, Susan; Rietbrock, Andreas; Tilmann, Frederik; Barrientos, Sergio; Meltzer, Anne; Oncken, Onno; Bataille, Klause; Roecker, Steven; Vilotte, Jean-Pierre; Russo, Raymond M.

    2014-06-01

    After a long quiet period for earthquake activity with magnitude greater than 8.5, several great subduction megathrust earthquakes occurred during the past decade: Sumatra in 2004 and 2005, Chile in 2010, and Japan in 2011. Each of these events caused loss of life and damage to critical infrastructure on an enormous scale. And, in April, a Mw 8.2 earthquake occurred off the Chilean coast.

  6. 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.

  7. Gravity inversion of the Kermadec-Tonga subduction zone by GOCE data and seismic information

    NASA Astrophysics Data System (ADS)

    Rossi, Lorenzo; Hamdi Mansi, Ahmed; Reguzzoni, Mirko; Sampietro, Daniele

    2017-04-01

    The modelling of the main subducting plates is usually performed by combining different data sources, like active-source seismic and deep seismicity. Moreover, high-resolution bathymetry models and sediment thickness maps can contribute to define the geometry of the subducting slab at the trenches. Since in correspondence of the subducting plate there is generally a large mass density discontinuity, due to the contrast between the light crust and the heavy mantle, gravity data can be used to study the geometry and some physical properties of subduction zones. In particular, the recent availability of direct satellite observations, like the gravity gradients acquired by the ESA-GOCE mission, opens to the possibility of improving seismic models by imposing a consistency with the observed gravity field. In this work the Kermadec-Tonga subduction zone is studied, starting from the CRUST1.0 and the SLAB1.0 seismic models, and inverting the GOCE gravity gradients to improve our knowledge of the whole area. More specifically, the CRUST1.0 Moho discontinuity is refined, and the subducting plate is modelled for thickness and density contrast, since SLAB1.0 provides only the top surface. The inversion method is basically an iterative regularized least-squares adjustment and the main property of the obtained result is that it is at the same time close to the starting seismic models and consistent with the satellite gravity observations.

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

    PubMed

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

    1990-11-30

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

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

    NASA Astrophysics Data System (ADS)

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

    2012-12-01

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

  10. 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

  11. Splay Faults as Primary Targets for Drilling Within the Nankai Subduction Zone

    NASA Astrophysics Data System (ADS)

    LE PICHON, X.; LE PICHON, X.; Henry, P.; Lallemant, S.

    2001-12-01

    The Nankai subduction may be considered as an ideal case of fully and homogeneously locked subduction. Large earthquakes occur there on a quasi-periodic basis (100-150 yr.) and account for most of the subduction motion. It has been considered for many years as a prime target for investigations by drilling of the seismogenic zone. However, the important role played by subducting basement topography in the structuring of the Nankai wedge and subduction zone has only recently been realized. We propose that activation of splay thrusts is a mechanism by which subducting basement topography influences seismic activity in a decisive fashion and consider that these splay faults become primary drilling targets relatively easy to reach. As the geometrical asperity is impinging the accretionary wedge, it leaves a scar behind it in the form of an embayment. Decollement propagation restarts seaward of it at an accelerated rate. As the asperity is subducted below the backstop, one (or several) splay fault is formed and may propagate laterally beyond the geometrical asperity. From then on, a significant part of the subduction motion is diverted along the splay fault that is probably the true prolongation of the seismogenic zone to the seafloor. In this model, most of the motion is released along the splay fault during the earthquake and during the interseismic period, a significant part of it is slowly transferred to the seaward ductile decollement. Thus splay faults become very important targets for drilling as they can be reached with limited core length and as they have a decisive influence on the rupture distribution during large earthquakes and on the generation of tsunamis. In eastern Nankai, the impingement every two millions years or so of the wedge backstop by regularly spaced volcanic ridges associated with the Izu-Bonin arc activates splay faults with associated fluid seepage sites (Tokai Thrust and possibly Kodaiba Fault). Tokai thrust continues westward within the

  12. Frictional anisotropy of antigorite and its impact on thrust-fault mechanics at subduction zones

    NASA Astrophysics Data System (ADS)

    Campione, M.; Malaspina, N.; Borghesi, A.; Capitani, G. C.

    2012-04-01

    There is general consensus that the forearc mantle wedge at subduction environments is hydrated by aqueous fluids released during subduction by dehydration reactions occurring in the slab lithologies, at increasing pressure and temperature. At these conditions, the most abundant mineral phase characterising hydrated harzburgites is serpentine, stable at temperatures of 650-700 °C, at pressures corresponding to 30-60 km depths, whose occurrence may have important implications in the rheology of the forearc mantle wedge. In the last decades, efforts have been employed to unravel the seismic shear-wave anisotropy measured in many subduction systems, traditionally attributed to the crystal preferred orientation of olivine and/or serpentine in the mantle wedge. However, little is known about the implications of such preferred orientation, which likely has its origin at the slab-mantle interface, on the frictional behaviour of a large-scale thrust fault with cumulative slip. In this framework, we focussed on the mechanism of frictional anisotropy, which manifests through a dependence of the magnitude of friction force on slip direction and through the presence of friction force components transverse to the slip direction. This phenomenon is encountered for sliding surfaces having a structured, anisotropic morphology or nanoscopic corrugations related to the surface crystal structure. Among minerals with a prominent role in defining the mechanical behaviour of faulted regions, antigorite, the high-temperature, high pressure polymorph of serpentine, exhibits inherently nanostructured basal surfaces with an orthotropic symmetry. A new approach for the study of the crystal structure-frictional behaviour relationship is the nanotribological characterization performed with a scanning force microscope, now applied for the first time to natural antigorites. In this setup, a micrometric tip mounted on an elastic cantilever is scanned by piezoelectric actuation along all

  13. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2010-12-01

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

  15. A geophysical potential field study to image the Makran subduction zone in SE of Iran

    NASA Astrophysics Data System (ADS)

    Abedi, Maysam; Bahroudi, Abbas

    2016-10-01

    The Makran subduction wedge as one of the largest subduction complexes has been forming due to the Arabian oceanic lithosphere subducting beneath the Lut and the Afghan rigid block microplates. To better visualize the subducting oceanic crust in this region, a geophysical model of magnetic susceptibility from an airborne magnetic survey (line spacing about 7.5 km) over the Makran zone located at southeast of Iran is created to image various structural units in Iran plate. The constructed geophysical model from the 3D inverse modeling of the airborne magnetic data indicates a thin subducting slab to the north of the Makran structural zone. It is demonstrated that the thickness of sedimentary units varies approximately at an interval of 7.5-11 km from north to south of this zone in the Iranian plate, meanwhile the curie depth is also estimated approximately < 26 km. It is also shown the Jazmurian depression zone adjacent to the north of the Makran indicates high intensity magnetic anomalies due to being underlain by an ophiolite oceanic basement, while such intensity reduces over the Makran. The directional derivatives of the magnetic field data have subtle changes in the Makran, but strongly increase in the Jazmurian by enhancing and separating different structural boundaries in this region. In addition, the density variations of the subsurface geological layers were determined by 3D inversion of the ground-based gravity data over the whole study area, where the constructed density model was in good agreement with the magnetic one. According to the outputs of the magnetic susceptibility and the density contrast, the Arabian plate subducts to the north under the Eurasia with a very low dip angle in the Makran structural zone.

  16. 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

  17. 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.

  18. 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.

  19. 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.

  20. Depth-dependent rupture mode along the Ecuador-Colombia subduction zone

    NASA Astrophysics Data System (ADS)

    Yoshimoto, Masahiro; Kumagai, Hiroyuki; Acero, Wilson; Ponce, Gabriela; Vásconez, Freddy; Arrais, Santiago; Ruiz, Mario; Alvarado, Alexandra; Pedraza García, Patricia; Dionicio, Viviana; Chamorro, Orlando; Maeda, Yuta; Nakano, Masaru

    2017-03-01

    A large earthquake (Mw 7.7) occurred on 16 April 2016 within the source region of the 1906 earthquake in the Ecuador-Colombia subduction zone. The 1906 event has been interpreted as a megathrust earthquake (Mw 8.8) that ruptured the source regions of smaller earthquakes in 1942, 1958, and 1979 in this subduction. Our seismic analysis indicated that the spatial distribution of the 2016 earthquake and its aftershocks correlated with patches of high interplate coupling strength and was similar to those of the 1942 earthquake and its aftershocks, suggesting that the 2016 and 1942 earthquakes ruptured the same asperity. Our analysis of tsunami waveforms of the 1906 event indicated Mw around 8.4 and showed that large slip occurred near the trench off the source regions of the above three historical and the 2016 earthquakes, suggesting that a depth-dependent complex rupture mode exists along this subduction zone.

  1. Effects of decarbonation on elemental behaviors during subduction-zone metamorphism: Evidence from a titanite-rich contact between eclogite-facies marble and omphacitite

    NASA Astrophysics Data System (ADS)

    Xiao, Yuanyuan; Niu, Yaoling; Zhang, Hong-Fu; Wang, Kuo-Lung; Iizuka, Yoshiyuki; Lin, Jinyan; Tan, Yulong; Xu, Yongjiang

    2017-03-01

    In this paper, we show the effects of subducted carbonates on geochemical processes during subduction-zone metamorphism (SZM) through the study of an eclogite-facies marble coexisting with metabasite from the ultrahigh pressure metamorphic belt of the Chinese Western Tianshan orogen. Between the marble and metabasite is a titanite-rich contact resulting from fluid-facilitated metamorphic reactions between the two lithologies, and recording elemental changes of geodynamic significance. Because this titanite-rich contact is dominated by titanite (an important host for high field strength elements, HFSEs) without white micas (an important host for large ion lithophile elements, LILEs), HFSEs are largely conserved in titanite whereas LILEs are moved away. This observation emphasizes the potential significance of subducting carbonate in retaining HFSEs in the slab through the formation and stabilization of titanite, contributing to the characteristic "arc signature" unique to subduction-zone magmatism (i.e., high LILEs, low HFSEs). The implicit assumption in this interpretation is that the observed lithological assemblage represents residues of subducting oceanic crust that has undergone major episodes of dehydration. Subducted carbonates also have significant implications for the origin of mantle isotopic heterogeneity as revealed from oceanic basalts.

  2. 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).

  3. Structural and hydrologic controls of subduction zone seismogenic behavior along the Nicoya Peninsula, Costa Rica

    NASA Astrophysics Data System (ADS)

    Audet, P.; Schwartz, S. Y.

    2012-12-01

    Subduction zone thrust faults exhibit variations in rupture behavior that include potentially great (M>8) earthquakes and slow propagating (M~7) slip. The factors controlling transitions in frictional properties are loosely constrained and include variations in temperature, lithologies and pore-fluid pressures. Along the Nicoya Peninsula, Costa Rica, the seismogenic zone is characterized by strong heterogeneity in mechanical properties and a lateral change in the origin of the subducting plate. We use observations of scattered teleseismic waves to examine structural properties (compressional to shear velocity ratio, or Vp/Vs) of the subduction zone elements beneath the Nicoya Peninsula and report two findings: 1) evidence for inferred high pore fluid pressures within the subducting oceanic crust, in agreement with results globally and 2) contrasts from lower to higher forearc and oceanic Vp/Vs ratios from northwest to southeast that correlate with changes in interseismic locking, seismogenic behavior and the origin of the subducting plate. We interpret these results as representing differences in permeability and thus fluid overpressures in the oceanic crust. We suggest that enhanced permeability of the East Pacific Rise (EPR) crust being subducted beneath the northwest portion of the Nicoya Peninsula results in lower pore-fluid pressure, higher effective stress and strength compared with the Cocos Nazca Spreading Center (CNS) crust, subducted beneath the southeastern Nicoya Peninsula. The higher pore-fluid pressure within the CNS crust is consistent with the lower coupling and large slow slip events observed in this region. We posit that the elevated fluid pressures here are periodically released allowing fluids to migrate into the upper plate reducing its velocities. Changes in hydrologic properties resulting from differences in the structural integrity of the subducting oceanic crust appear to control the seismogenic segmentation along the Nicoya Peninsula. (a

  4. 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

  5. The role of subduction zone coupling and slip distribution on tsunamigenesis in Cascadia

    NASA Astrophysics Data System (ADS)

    Klein, E. C.; Barberopoulou, A.; Mahdyiar, M.; Shen-Tu, B.

    2016-12-01

    The slip distribution of recent great earthquakes like Sumatra, Tohoku, and Chili Maule earthquakes clearly demonstrated the very large spatial variation in the states of asperities and slip distribution for great earthquakes. Such variations have impact on the vertical displacement at the source and potentially for the tsunami footprint at various coastal areas. One of the very well-studied and not-yet ruptured subduction zones is the Cascadia Subduction Zone (CSZ). A number of published models using GPS data have provided information on the state of coupling for the CSZ, however, there are uncertainties in such results. We compare a suite of spatial slip distributions for the CSZ on the development and impact of tsunami on coastal areas of Washington, Oregon and California. More specifically, we investigate the role of size and slip of asperities in initial conditions used in the modeling of tsunamis. Our method utilizes modeled patterns of subduction zone coupling as a proxy for spatial variation in slip displacement. In addition we account for the uncertainties associated with inferred coupling field to explore a range of plausible slip distributions implied by the physical modeling of the subduction zone. We characterize the success of our tsunami source models based on current knowledge and similar studies for Cascadia.

  6. Megathrust and accretionary wedge properties and behaviour in the Makran subduction zone

    NASA Astrophysics Data System (ADS)

    Penney, Camilla; Tavakoli, Farokh; Saadat, Abdolreza; Nankali, Hamid Reza; Sedighi, Morteza; Khorrami, Fateme; Sobouti, Farhad; Rafi, Zahid; Copley, Alex; Jackson, James; Priestley, Keith

    2017-06-01

    We study the Makran subduction zone, along the southern coasts of Iran and Pakistan, to gain insights into the kinematics and dynamics of accretionary prism deformation. By combining techniques from seismology, geodesy and geomorphology, we are able to put constraints on the shape of the subduction interface and the style of strain across the prism. We also address the long-standing tectonic problem of how the right-lateral shear taken up by strike-slip faulting in the Sistan Suture Zone in eastern Iran is accommodated at the zone's southern end. We find that the subduction interface in the western Makran may be locked, accumulating elastic strain, and move in megathrust earthquakes. Such earthquakes, and associated tsunamis, present a significant hazard to populations around the Arabian Sea. The time-dependent strain within the accretionary prism, resulting from the megathrust earthquake cycle, may play an important role in the deformation of the Makran region. By considering the kinematics of the 2013 Balochistan and Minab earthquakes, we infer that the local gravitational and far-field compressive forces in the Makran accretionary prism are in balance. This force balance allows us to calculate the mean shear stress and effective coefficient of friction on the Makran megathrust, which we find to be 5-35 MPa and 0.01-0.03, respectively. These values are similar to those found in other subduction zones, showing that the abnormally high sediment thickness in the offshore Makran does not significantly reduce the shear stress on the megathrust.

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

    NASA Astrophysics Data System (ADS)

    Noguchi, Shinako; Maeda, Takuto; Furumura, Takashi

    2013-01-01

    We investigated the development of a distinct later phase observed at stations near the Japan Trench associated with shallow, outer-rise earthquakes off the coast of Sanriku, northern Japan based on the analysis of three-component broadband seismograms and FDM simulations of seismic wave propagation using a heterogeneous structural model of the Japan Trench subduction zone. Snapshots of seismic wave propagation obtained through these simulations clearly demonstrate the complicated seismic wavefield constructed by a coupling of the ocean acoustic waves and the Rayleigh waves propagating within seawater and below the sea bottom by multiple reflections associated with shallow subduction zone earthquakes. We demonstrated that the conversion to the Rayleigh wave from the coupled ocean acoustic waves and the Rayleigh wave as they propagate upward along the slope of seafloor near the coast is the primary cause of the arrival of the distinct later phase at the station near the coast. Through a sequence of simulations using different structural models of the Japan Trench subduction zone, we determined that the thick layer of seawater along the trench and the suddenly rising sea bottom onshore of the Japanese island are the major causes of the distinct later phase. The results of the present study indicate that for realistic modeling of seismic wave propagation from the subduction zone earthquakes, a high-resolution bathymetry model is very crucial, although most current simulations do not include a water column in their simulation models.

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

    NASA Technical Reports Server (NTRS)

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

    1989-01-01

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

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

    NASA Astrophysics Data System (ADS)

    Holm, N. G.

    2010-04-01

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

  10. 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

  11. 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.

  12. Geoid anomalies in the vicinity of subduction zones

    NASA Technical Reports Server (NTRS)

    Mcadoo, D. C.

    1981-01-01

    In the considered investigation, attention is given to the line source model, a surface source model, an application of the model, and a model of the thermal lithosphere associated with marginal basins. It is found that undulations in the altimetrically observed geoid of the southwest Pacific are strongly controlled by positive density anomalies in the subducting slabs of the region and the effects of elevation of the geotherm in behind arc lithosphere (corresponding to young marginal basins). Finer details of slab geometry do not obviously manifest themselves in the observed geoid. Such gravitational effects are quite attenuated at sea level and are apparently mixed with crustal effects, oceanographic noise, etc. It appears that slabs in global composite may contribute substantially to intermediate and long wavelength portions (down to spherical harmonic degree 3 or 4) of the earth's gravity field.

  13. Seismicity, shear failure and modes of deformation in deep subduction zones

    NASA Technical Reports Server (NTRS)

    Lundgren, Paul R.; Giardini, Domenico

    1992-01-01

    The joint hypocentral determination method is used to relocate deep seismicity reported in the International Seismological Center catalog for earthquakes deeper than 400 km in the Honshu, Bonin, Mariannas, Java, Banda, and South America subduction zones. Each deep seismic zone is found to display planar features of seismicity parallel to the Harvard centroid-moment tensor nodal planes, which are identified as planes of shear failure. The sense of displacement on these planes is one of resistance to deeper penetration.

  14. Seismicity, shear failure and modes of deformation in deep subduction zones

    NASA Technical Reports Server (NTRS)

    Lundgren, Paul R.; Giardini, Domenico

    1992-01-01

    The joint hypocentral determination method is used to relocate deep seismicity reported in the International Seismological Center catalog for earthquakes deeper than 400 km in the Honshu, Bonin, Mariannas, Java, Banda, and South America subduction zones. Each deep seismic zone is found to display planar features of seismicity parallel to the Harvard centroid-moment tensor nodal planes, which are identified as planes of shear failure. The sense of displacement on these planes is one of resistance to deeper penetration.

  15. Fluid escape from subduction zones controlled by channel-forming reactive porosity

    NASA Astrophysics Data System (ADS)

    Plümper, Oliver; John, Timm; Podladchikov, Yuri Y.; Vrijmoed, Johannes C.; Scambelluri, Marco

    2016-12-01

    Water within the oceanic lithosphere is returned to Earth's surface at subduction zones. Observations of metamorphosed veins preserved in exhumed slabs suggest that fluid can escape via channel networks. Yet, it is unclear how such channels form that allow chemically bound water to escape the subducting slab as the high pressures during subduction reduce the porosity of rocks to nearly zero. Here we use multiscale rock analysis combined with thermodynamic modelling to show that fluid flow initiation in dehydrating serpentinites is controlled by intrinsic chemical heterogeneities, localizing dehydration reactions at specific microsites. Porosity generation is directly linked to the dehydration reactions and resultant fluid pressure variations force the reactive fluid release to organize into vein networks across a wide range of spatial scales (μm to m). This fluid channelization results in large-scale fluid escape with sufficient fluxes to drain subducting plates. Moreover, our findings suggest that antigorite dehydration reactions do not cause instantaneous rock embrittlement, often presumed as the trigger of intermediate-depth subduction zone seismicity.

  16. Chlorine Stable Isotopes in Three Subduction Zones, With Inferences For Serpentinization and Fluid Flow

    NASA Astrophysics Data System (ADS)

    Wei, W.; Kastner, M.; Spivack, A. J.

    2005-12-01

    Pore fluids from Mariana, Nankai Trough, and Costa Rica subduction zones were analyzed for chlorine stable isotope ratios (37Cl/35Cl). At the Mariana subduction zone (ODP Site 1200), δ37Cl values are positive and range from 0 to 1.8‰. In contrast, at Costa Rica (ODP Sites 1039/1253, 1043/1255, 1040/1254) and Nankai (ODP Sites 808, 1174, and 1173) subduction zones, δ37Cl values are negative and exhibit the largest observed range in δ37Cl. At the Costa Rica subduction zone, δ37Cl values range from seawater value to -5.5‰ (Site 1040/1254) along the décollement and a fracture zone in the prism. At the Nankai subduction zone, δ37Cl ranges from seawater value of 0‰ to -7.8‰, the latter is the most negative value of all the ODP pore fluids analyzed so far. Chlorine isotopes fractionate when they are incorporated into diagenetic or metamorphic hydrous minerals where Cl substitutes for OH groups. Due to vibrational energy differences, the formation of high temperature (>250°C) hydrous minerals preferentially consumes 37Cl (Schauble et al., 2003), thus enriching the residual fresher fluid in 35Cl. Accordingly, when the minerals dehydrate, Cl, enriched in 37Cl, is released into the fluid. This behavior is well manifested by the Cl isotope data of the pore fluids and solids, mostly serpentines, at the Mariana subduction zone (ODP Site 1200); both are enriched in 37Cl. The pore fluid δ37Cl increases from seawater value at the seafloor to ~+1.8 % at 71 meter below sea floor, the deepest sample obtained. The pore fluids at this site originate at ~27 km, where dehydration of the subducting slab occurs (Mottl et al., 2003) and the Cl concentration in the upwelling fluid is diluted relative to bottom seawater by ~8-9%. The Mariana serpentines contain hundreds of ppm Cl, having δ37Cl values that range from 1 to 4‰. When they dehydrate, Cl with enriched 37Cl, as well as H2O, are released into the pore fluid. Therefore, the upwelling fluid exhibits the positive δ37

  17. Subduction zone fluxes of halogens and noble gases in seafloor and forearc serpentinites

    NASA Astrophysics Data System (ADS)

    Kendrick, Mark A.; Honda, Masahiko; Pettke, Thomas; Scambelluri, Marco; Phillips, David; Giuliani, Andrea

    2013-03-01

    Serpentinites form by hydration of ultramafic lithologies in a range of seafloor and shallow subduction zone settings. Serpentinites are recognised as major reservoirs of fluid mobile elements and H2O in subducting oceanic lithosphere, and together with forearc serpentinites formed in the mantle wedge, provide critical information about shallow-level volatile fluxes during subduction. The current study provides new Cl, as well as the first comprehensive Br, I and noble gas analyses reported for seafloor and forearc chrysotile-lizardite serpentinites. The samples were recovered from IODP drilling campaigns of mid-ocean ridge, passive margin and forearc settings (n=17), and ophiolites in the Italian Alps and Apennines (n=10). The aims of this study were to determine the compositional variability of noble gases and halogens in serpentinites entering subduction zones and evaluate the efficiency of gas loss during the early stages of serpentinite subduction. The chrysotile-lizardite serpentinites and serpentised peridotites contain 43-2300 ppm Cl and 3×10-13-2×10-11 mol g-136Ar, with the concentrations of these elements broadly related to the estimated degree of serpentinisation. The serpentinites have extremely variable Br/Cl and I/Cl ratios with many samples preserving compositions similar to organic-rich sedimentary marine pore fluids. Serpentinites from the Marianas Forearc have very high I concentrations of up to 45 ppm I and I/Cl ratios of ˜14,000 times the seawater value that is even higher than the maximum I/Cl enrichment observed in sedimentary marine pore fluids. The serpentinites have 130Xe/36Ar and 84Kr/36Ar ratios that are mostly close to or above seawater values, and 20Ne/36Ar ratios that range from seawater to lower values. The serpentinites contain <10-270 ppm K and, irrespective of age (0 Ma to ˜160 Ma), are characterised by 40Ar/36Ar ratios of 300-340 that are slightly higher than the seawater value of 296, thus indicating the presence of minor

  18. Streak Tectonics associated with the Irregular Slab Topography at Subduction Zones

    NASA Astrophysics Data System (ADS)

    Eguchi, T.

    2015-12-01

    We demonstrate the physical features of streak tectonics (or abrasion tectonics) associated with the irregular surface topography, such as local convex rise or seamount(s), on the downgoing slab at subduction zones. Marine surveys such as sophisticated multichannel seismic experiments have revealed the detailed vertical structure of the overriding lithosphere as well as the upper-most part of downgoing slab at the fore-arc zone from the trench axis through the inclined plate interface zone at a depth of 10 - 15km. As previously, some researchers (e.g., Eguchi, 1979, 1996; Hilde, 1983; Suzan, 2010) demonstrated the influence of the surface irregular topography of the slab on the occurrence regime of greater interplate seismic events with the low-angle underthrusting slip. However, the earlier studies didn't incorporate any effects due to the spherical buckling of oceanic lithosphere with the age-dependent elastic thickness at subduction zones. In the case of a subduction zone where the slab age has gradually been decreasing or increasing, the spherical buckling of elastic shell (e.g., Eguchi, 2012) suggests that the interplate mechanical coupling strength varies with time and space. Next, we argue some tectonic features of strain-rate dependent deformation at areas surrounding an isolated-seamount on the downgoing slab, such as the quasi-static fluid lubrication, boundary lubrication or plastic deformation. We then discuss how to represent mathematically the streak process during a larger interplate seismic event at the non-uniform plate interface zone.

  19. 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.

  20. 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.

  1. 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.

  2. 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

  3. 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

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

    NASA Astrophysics Data System (ADS)

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

    2012-12-01

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

  5. 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

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

    NASA Astrophysics Data System (ADS)

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

    2012-12-01

    The Mexican subduction zone is unusual: the width of the seismogenic zone is relatively narrow and a large portion of the co-seismic slip generally occurs below the coast, ~ 45 to 80 km from the trench. The earthquake recurrence interval is relatively short and almost the entire length of the zone has experienced a large (Mw≥7.4) earthquake in the last 100 years (Singh et al., 1981). In this study we present detailed analysis of the areas of significant slip during several recent (last 20 years) large earthquakes in the Mexican subduction zone. The most recent earthquake of 20 March 2012 (Mw7.4) occurred near the Guerrero/Oaxaca border. The slip was concentrated on the plate interface below land and the epicentral PGAs ranged between 0.2 and 0.7g. The updip portion of the plate interface had previously broken during the 25 Feb 1996 earthquake (Mw7.1), which was a slow earthquake and produced anomalously low PGAs (Iglesias et al., 2003). This indicates that in this region the area close to the trench is at least partially locked, with some earthquakes breaking the down-dip portion of the interface and others rupturing the up-dip portion. The Jalisco/Colima segment of the subduction zone seems to behave in a similar fashion. The 9 October 1995 (Mw 8.0) earthquake generated small accelerations relative to its size. The energy to moment ratio, E0/M0, is 4.2e-6 (Pérez-Campos, Singh and Beroza, 2003), a value similar to the Feb, 1996 earthquake. This value is low compared to other thrust events in the region. The earthquake also had the largest (Ms-Mw) disparity along the Mexican subduction zone, 7.4 vs 8.0. The event produced relatively large tsunami. On the contrary, the 3 June 1932 earthquake (Ms8.2, Mw8.0), that is believed to have broken the same segment of the subduction zone, appears to be "normal." Based on the available evidence, it may be concluded that the 1932 event broke a deeper patch of the plate interface relative to the 1995 event. The mode of rupture

  7. Subduction processes off chile (SPOC) - results from The amphibious wide-angle seismic experiment across The chilean subduction zone

    NASA Astrophysics Data System (ADS)

    Lueth, S.; Spoc Resaerch Group

    2003-04-01

    One component of the onshore-offshore, active-passive seismic experiment SPOC (Krawczyk et al., Stiller et al., this vol.) was a 2-D wide-angle seismic experiment covering the Chilean subduction zone from the Nazca Plate to the Magmatic Arc in the main cordillera. Three W-E-profiles of 52 stations each and up to 240 km long were deployed between 36° and 39° S. These profiles recorded chemical shots at their ends and, in order to extend the onshore profiles, the airgun pulses from RV SONNE cruising simultaneously on offshore profiles. On the southernmost of the three profiles OBHs/OBSs were deployed offshore, thus providing continuous wide-angle seismic data from the Nazca Plate to the South-American continent. Data examples, correlations, and velocity models along the three transects will be presented. The Moho of the subducted oceanic crust can be constrained by PmP-reflections down to 45 km depth under the coastal cordillera. The P-wave velocity field of the crust of the upper plate is characterized by gradually increasing P-wave velocities from East to West. Low seismic velocities (Vp < ~5 km/s) indicate the location of a young accretionary complex at the western tip of the continent. The highest seismic velocities (Vp > ~6.5 km/s below 10 km depth) are observed at the eastern margin of the investigated area.

  8. 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

  9. 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.

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

    USGS Publications Warehouse

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

    2004-01-01

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

  11. Exhumation of oceanic blueschists and eclogites in subduction zones: Timing and mechanisms

    NASA Astrophysics Data System (ADS)

    Agard, P.; Yamato, P.; Jolivet, L.; Burov, E.

    2009-01-01

    High-pressure low-temperature (HP-LT) metamorphic rocks provide invaluable constraints on the evolution of convergent zones. Based on a worldwide compilation of key information pertaining to fossil subduction zones (shape of exhumation P- T- t paths, exhumation velocities, timing of exhumation with respect to the convergence process, convergence velocities, volume of exhumed rocks,…), this contribution reappraises the burial and exhumation of oceanic blueschists and eclogites, which have received much less attention than continental ones during the last two decades. Whereas the buoyancy-driven exhumation of continental rocks proceeds at relatively fast rates at mantle depths (≥ cm/yr), oceanic exhumation velocities for HP-LT oceanic rocks, whether sedimentary or crustal, are usually on the order of the mm/yr. For the sediments, characterized by the continuity of the P- T conditions and the importance of accretionary processes, the driving exhumation mechanisms are underthrusting, detachment faulting and erosion. In contrast, blueschist and eclogite mafic bodies are systematically associated with serpentinites and/or a mechanically weak matrix and crop out in an internal position in the orogen. Oceanic crust rarely records P conditions > 2.0-2.3 GPa, which suggests the existence of maximum depths for the sampling of slab-derived oceanic crust. On the basis of natural observations and calculations of the net buoyancy of the oceanic crust, we conclude that beyond depths around 70 km there are either not enough serpentinites and/or they are not light enough to compensate the negative buoyancy of the crust. Most importantly, this survey demonstrates that short-lived (< ˜ 15 My), discontinuous exhumation is the rule for the oceanic crust and associated mantle rocks: exhumation takes place either early (group 1: Franciscan, Chile), late (group 2: New Caledonia, W. Alps) or incidentally (group 3: SE Zagros, Himalayas, Andes, N. Cuba) during the subduction history

  12. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2013-04-01

    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 206Pb/204Pb, 208Pb/204Pb, 86Sr/87Sr 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.

  14. Subducting-slab transition-zone interaction: Stagnation, penetration and mode switches

    NASA Astrophysics Data System (ADS)

    Agrusta, Roberto; Goes, Saskia; van Hunen, Jeroen

    2017-04-01

    Seismic tomography shows that subducting slabs can either sink straight into the lower mantle, or lie down in the mantle transition zone. Moreover, some slabs seem to have changed mode from stagnation to penetration or vice versa. We investigate the dynamic controls on these modes and particularly the transition between them using 2D self-consistent thermo-mechanical subduction models. Our models confirm that the ability of the trench to move is key for slab flattening in the transition zone. Over a wide range of plausible Clapeyron slopes and viscosity jumps at the base of the transition zone, hot young slabs (25 Myr in our models) are most likely to penetrate, while cold old slabs (150 Myr) drive more trench motion and tend to stagnate. Several mechanisms are able to induce penetrating slabs to stagnate: ageing of the subducting plate, decreasing upper plate forcing, and increasing Clapeyron slope (e.g. due to the arrival of a more hydrated slab). Getting stagnating slabs to penetrate is more difficult. It can be accomplished by an instantaneous change in the forcing of the upper plate from free to motionless, or a sudden decrease in the Clapeyron slope. A rapid change in plate age at the trench from old to young cannot easily induce penetration. On Earth, ageing of the subducting plate (with accompanying upper plate rifting) may be the most common mechanism for causing slab stagnation, while strong changes in upper plate forcing appear required for triggering slab penetration.

  15. 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

  16. 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.

  17. 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.

  18. 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.

  19. 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.

  20. 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

  1. RETRACTED: Signatures of downgoing plate-buoyancy driven subduction in motions and seismic coupling at major subduction zones

    NASA Astrophysics Data System (ADS)

    Capitanio, F. A.; Goes, S.; Morra, G.; Giardini, D.

    2007-10-01

    This article has been retracted at the request of the Editor-in-Chief and Authors. Please see Elsevier Policy on Article Withdrawal ( http://www.elsevier.com/locate/withdrawalpolicy). Reason: after publication, errors were discovered in the plate-motion database that it was based on. This dataset was an updated version of the dataset presented in Sdrolias and Muller (2006), provided to us by the first author. The errors in this version were in the away-from-trench/towards-trench assignment for subduction zones with back-arcs and also due to the fact that the next generation plate model had only partially been completed. These errors affect the conclusions about seismic coupling. They also change some of the points in most of the other plots, and although this does not invalidate the other conclusions, the discussion to reach them would be altered.

  2. Seismic tremor under the subduction zones: the rock-physics interpretation.

    NASA Astrophysics Data System (ADS)

    Burlini, L.; di Toro, G.; Meredith, P.; Mainprice, D.; Burg, J.

    2006-12-01

    Elevated pore fluid pressures in rocks leads to weakening and embrittlement through the principle of effective stress. Any material can fail by hydraulic fracture if pore pressure exceeds the confining pressure. We have measured the output of seismicity, as acoustic emission (AE