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1

UHP impure marbles from the Dabie Mountains: Metamorphic evolution and carbon cycling in continental subduction zones  

NASA Astrophysics Data System (ADS)

Impure marbles from ultra-high pressure (UHP) metamorphic belts bear significant information on the metamorphic evolution and carbon cycling during continental subduction and exhumation. In this study, detailed petrological data are presented and a P-T-X(CO2) path is constructed for the impure marbles from the Dabie UHP terrane. Coesite relicts are discovered as inclusions within dolomite from the selected samples, which have a peak assemblage of dolomite, aragonite, garnet, omphacite, phengite, coesite, allanite and rutile. Estimated with the compositions of peak minerals, a P-T condition of 4.05-4.45 GPa at 740-820 °C is obtained by conventional geothermobarometry. The modeled fluid compositions have a low X(CO2) (0.01-0.02) at the peak conditions, while the X(CO2) firstly increased during isothermal exhumation and then decreased at later retrogression. The discovery of coesite within dolomite underscores the role of the 'pressure vessel' models and highlights the significance of fluid unavailability in preserving coesite in UHP rocks. Neither petrological evidence nor independent peak P-T estimations support the breakdown of dolomite in the studied marbles, which contests recent suggestions. Analysis on the phase relations in the CaO-MgO-SiO2-H2O-CO2 system shows that the bulk rock compositions have a large control on the stable UHP carbonate associations in carbonate-bearing rocks. The low X(CO2) in the peak fluids indicates a weak decarbonation of the impure marbles under sub-arcs. In the last, a large fraction of CO2 is shown to be sequestrated during regional retrogression of clinopyroxene marbles, which has a profound influence and must be considered for the global carbon cycling.

Liu, Penglei; Wu, Yao; Chen, Yi; Zhang, Junfeng; Jin, Zhenmin

2015-01-01

2

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

NASA Astrophysics Data System (ADS)

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

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

2006-12-01

3

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

NASA Astrophysics Data System (ADS)

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

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

2007-05-01

4

Metamorphic chemical geodynamics of subduction zones Gray E. Bebout  

E-print Network

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

Bebout, Gray E.

5

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

E-print Network

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

6

Dating Subduction Zone Metamorphism with Garnet and Lawsonite Geochronology  

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

7

Does subduction zone magmatism produce average continental crust  

NASA Technical Reports Server (NTRS)

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.

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

1988-01-01

8

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

E-print Network

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

Pearce, Frederick Douglas

9

Growth of early continental crust by water-present eclogite melting in subduction zones  

NASA Astrophysics Data System (ADS)

The geochemistry of well preserved Paleo- to Meso-Archaean Tonalite-Trondhjemite-Granodiorite (TTG) suite rocks, such as the ca 3.45 Ga trondhjemites from the Barberton greenstone belt in South Africa, provides insight into the origins of Earth's early felsic continental crust. This is particularly well demonstrated by the high-Al2O3 variety of these magmas, such as the Barberton rocks, where the geochemistry requires that they are formed by high pressure (HP) melting of a garnet-rich metamafic source. This has been interpreted as evidence for the formation of these magmas by anatexis of the upper portions of slabs within Archaean subduction zones. Most of the experimental data relevant to Archaean TTG genesis has been generated by studies of fluid-absent melting of metabasaltic sources. However, water drives arc magmatism within Phanerozoic subduction zones and thus, understanding the behaviour of water in Archaean subduction zones, may have considerable value for understanding the genesis of these TTG magmas. Consequently, this study investigates the role of HP water-present melting of an eclogite-facies starting material, in the production of high-Al2O3 type TTG melts. Water-saturated partial melting experiments were conducted between 1.9 and 3.0GPa; and, 870°C and 900°C. The melting reaction is characterized by the breakdown of sodic Cpx, together with Qtz and H2O, to form melt in conjunction with a less sodic Cpx: Qtz + Cpx1 + Grt1 + H2O = Melt + Cpx2 + Grt2. In many of the experimental run products, melt segregated efficiently from residual crystals, allowing for the measurement of a full range of trace elements via Laser Ablation Inductively Coupled Plasma Mass Spectroscopy. The experimental glasses produced by this study have the compositions of peraluminous trondhjemites; and they are light rare earth element, Zr and Sr enriched; and heavy rare earth element, Y and Nb depleted. The compositions of the experimental glasses are similar to high-Al2O3 type Archaean TTG rocks in general and similar to the Barberton trondhjemites in particular. Additionally, due to Cpx being a major reactant, Ni and Cr contents of the glasses are high and match those of high-Al2O3 type TTG compositions. This challenges the notion that this aspect of TTG geochemistry indicates interaction of the magma with the mantle wedge. Consequently, we propose that water-present melting of an eclogitic source is a viable mechanism for the genesis of Paleo- to Meso-Archaean felsic continental crust. Importantly, this mechanism of TTG formation involves the upper surface of the subducting slab acting as an anatectic capture site for metamorphic fluid which evolved from cooler domains slightly deeper within the hydrated upper portion of the slab. This explains both TTG genesis and the lack of characteristic products of mantle wedge metasomatism, such as andesites, concurrent with TTG magmatism of this type during the Paleo- to Meso-Archaean. Cooling of the upper mantle by only a small amount towards to end of the Archaean Eon acted to "turn off" water-present melting of the slab, allowing water to metasomatise the mantle wedge and induce calc-alkaline magmatism in association with volcanic arcs.

Laurie, A.; Stevens, G.

2011-12-01

10

Microstructural evidence for dissolution precipitation creep in high pressure metamorphic serpentinites from subduction zones  

NASA Astrophysics Data System (ADS)

Serpentinite is generally believed to constitute weak material in subduction zones and to play an essential role for the development of a subduction channel. Information on deformation mechanisms and appropriate rheological models to describe these large scale flow processes is obtained from natural serpentinites exhumed from ancient subduction zones. In this study, we investigate the microstructural record of serpentinites exposed in the Zermatt-Saas-Zone, Western Alps. The metamorphic mineral assemblage comprises antigorite, forsterite, diopside, tremolite, chlorite, oxide phases, and in places titanclinohumite. Original mantle minerals are only locally preserved as relics. The conditions of Alpine metamorphism can be estimated from associated eclogites; the pressures are mostly between 1.5 and 2.5 GPa at temperatures of c. 500-600°C. The serpentinites show a complex structure with several generations of folds and foliations. An early foliation is defined by the combined shape and crystallographic preferred orientation (SPO and CPO) of antigorite and metamorphic diopside. These patterns are undistinguishable from the SPO and CPO of the same minerals in strain shadows, where the CPO must be developed by oriented growth from an aqueous solution. Therefore we suspect that the SPO and CPO in the polyphase matrix layers also result from oriented growth. Also, there is no microstructural evidence for any crystal plastic deformation of diopside. In places, antigorite flakes with SPO and CPO are overgrown by single crystals or aggregates of metamorphic forsterite, locally titanoclinohumite, and tremolite. The aggregates of forsterite exhibit a foam structure without CPO. All microfabrics indicate that dissolution precipitation creep was predominant in the investigated serpentinites, and most finite strain was accumulated by this mechanism. We see no evidence for a significant contribution of dislocation creep, both based on microstructure and on the CPO patterns. This does not preclude dislocation creep and a power law rheology to hold for higher stress levels, as expected for short episodes of postseismic creep. For the long term flow of serpentinites in subduction zones, however, Newtonian behaviour and a low viscosity are indicated.

Wassmann, S.; Trepmann, C.; Krohe, A.; Stoeckhert, B.

2009-12-01

11

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

PubMed

It is thought that the first continental crust formed by melting of either eclogite or amphibolite, either at subduction zones or on the underside of thick oceanic crust. However, the observed compositions of early crustal rocks and experimental studies have been unable to distinguish between these possibilities. Here we show a clear contrast in trace-element ratios of melts derived from amphibolites and those from eclogites. Partial melting of low-magnesium amphibolite can explain the low niobium/tantalum and high zirconium/samarium ratios in melts, as required for the early continental crust, whereas the melting of eclogite cannot. This indicates that the earliest continental crust formed by melting of amphibolites in subduction-zone environments and not by the melting of eclogite or magnesium-rich amphibolites in the lower part of thick oceanic crust. Moreover, the low niobium/tantalum ratio seen in subduction-zone igneous rocks of all ages is evidence that the melting of rutile-eclogite has never been a volumetrically important process. PMID:12075348

Foley, Stephen; Tiepolo, Massimo; Vannucci, Riccardo

2002-06-20

12

Thermal and metamorphic environment of subduction zone episodic tremor and slip  

Microsoft Academic Search

Episodic tremor and slip (ETS) have been detected in the Cascadia and southwest Japan subduction zones, where the subducting crust is relatively warm because of the young incoming lithosphere (<20 Ma) and modest plate convergence rates (?40–60 mm\\/a). In the southwest Japan subduction zone, low-frequency earthquakes occur on the plate interface at depths of 30–35 km beneath Shikoku where finite

Simon M. Peacock

2009-01-01

13

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

USGS Publications Warehouse

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

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

2005-01-01

14

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

NASA Astrophysics Data System (ADS)

Temperature is the primary control parameter of several processes occurring at subduction zones, such as slab metamorphism and dehydration, arc volcanism and the rupture width of megathrust earthquakes. The thermal state depends on the temperature of the oceanic slab and the flow pattern of the overlying mantle wedge. In most previous studies, mantle flow was modeled as two-dimensional (2D) corner flow, driven by the subducting plate. However, recent studies have shown the limitations of the 2D corner flow scheme, as a three-dimensional (3D) oceanic plate structure can generate along-strike pressure gradients, producing a trench-parallel flow component. One region where 3D effects may be important is the Middle America Subduction Zone (MASZ). Here, the dip of the oceanic plate varies from 0 to 70 degrees along the margin, with abrupt changes in slab dip in Central Mexico and Costa Rica-Nicaragua. Seismic anisotropy and arc magma geochemistry variations suggest a significant along-margin component of flow in these areas. Further, offshore surface heat flow measurements show that there may be along-margin variations in the temperature of the subducting oceanic plate, due to variations in plate age and hydrothermal circulation. In this study, we quantify the changes in the thermal structure of a subduction zone that result from along-margin variations in oceanic plate structure. We use 3D numerical models that consist of kinematically-defined subducting and overriding plates, and a flowing mantle wedge driven by drag exerted by the subducting plate. The finite-element code PGCtherm-3D is used to solve the steady-state governing equations for mantle wedge flow and the 3D thermal structure of the subduction zone. The models employ an oceanic plate that smoothly dips into the mantle and has along-margin variations in the deep dip of 40 and 70 degrees over a distance of 50km to 300km, as observed in some regions of the MASZ. Using an isoviscous mantle wedge, our preliminary results show that the introduction of a 3D oceanic plate geometry causes along-margin variations in slab surface and inslab temperatures, with differences as much as 50C from 2D models. The differences increase as the along-margin transition in slab geometry becomes more abrupt. Future models will explore a non-Newtonian, temperature-dependent mantle wedge rheology and will include a three-dimensional oceanic geotherm that fits the offshore surface heat flow for the MASZ. The combined effect of a 3D oceanic geotherm and temperature-dependent flow in the wedge is expected to have a strong impact on metamorphic reactions within the oceanic plate, such as the transformation of oceanic basalt into eclogite, as well as the width of megathrust seismogenic zone.

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

2013-12-01

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)

The Western Gneiss Region (WGR) of Norway lies within the Caledonian collision zone, which was subjected to the eclogites-facies metamorphism during the Silurian-Devonian continental collision of Baltica and Laurentia. Exhumed from depths of 80-200 km, the peridotites enclosed in Proterozoic gneisses in the WGR can be divided into two groups: garnet-free Archean dunites with extremely depleted compositions, and garnet-bearing peridotites that resulted from refertilization of the dunites during the Proterozoic time (Beyer et al., 2006). Therefore peridotites from the WGR provide a unique window to investigate petrofabrics of the upper mantle with different compositions and deformation history. Using the electron backscatter diffraction (EBSD) technique, we measured lattice-preferred orientations of 10 peridotite samples from the WGR. The water contents of olivine were determined by Fourier transform infrared spectroscopy in Paterson’s calibration (1982). For dunites from Almklovdalen in the southern WGR, one sample with water content of 0.5 ppm H2O displays the [100](010) (the A-type) olivine fabric, while the other two samples with water contents of 5-7 ppm H2O developed [001](010) (the B-type) fabric. In contrast, garnet lherzolites and wehrlites with various water content of 0.2-7 ppm H2O from Almklovdalen and Gurskøy yield the B-type fabric. The [001](100) (the C-type) fabric of olivine was observed in garnet harzburgite with very low water content (1.4 ppm H2O) from Otrøy in the northern WGR, where coesite, majoritic garnet and microdiamond were found by previous studies. Compared with the fabric diagram of olivine proposed by Jung and Karato (2001), all 10 peridotite samples came from dry conditions, under which the A-type or the D-type fabric is expected at low or high stress, respectively. Clearly, our results indicate that water is not the key to producing the different fabrics in the WGR peridotites, and the Proterozoic refertilization did not significantly affect the fabric development of peridotites during the continental collision. Combined with field observations in the WGR and recent deformation experiments on olivine, we propose that the B- and C-type fabrics of olivine were formed during the subduction of the Baltic plate in fluid-limited conditions. The combination of UHP and low temperature plays a more important role than water to promote [001] slip in continental subduction zones. The spatial distribution of olivine fabrics in the WGR could be related with the increasing pressure from south to north, i.e., the HP to UHP metamorphism transition. It is probable that in continental subduction zones, the B- and C-type fabrics will predominate over the A-type fabric with increasing depths of the subducting lithospheric mantle, and the C-type fabric is more easily to activated at pressure higher than 4 GPa on low geothermal gradients. Therefore the olivine C-type fabric may be a marker of ultradeep subduction.

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

2010-12-01

16

Partial melting, fluid supercriticality and element mobility in ultrahigh-pressure metamorphic rocks during continental collision  

NASA Astrophysics Data System (ADS)

Partial melting at continental lithosphere depths plays an important role in generating geochemical variations in igneous rocks. In particular, dehydration melting of ultrahigh-pressure (UHP) metamorphic rocks during continental collision provides a petrological link to intracrustal differentiation with respect to the compositional evolution of continental crust. While island arc magmatism represents one end-member of fluid-induced large-scale melting in the mantle wedge during subduction of the oceanic crust, the partial melting of UHP rocks can be viewed as the other end-member of fluid-induced small-scale anatexis during exhumation of the deeply subducted continental crust. This latter type of melting is also triggered by metamorphic dehydration in response to P-T changes during the continental collision. It results in local occurrences of hydrous melts (even supercritical fluids) as felsic veinlets between boundaries of and multiphase solid inclusions in UHP metamorphic minerals as well as local accumulation of veinlet-like felsic leucosomes in foliated UHP metamorphic rocks and metamorphically grown zircons in orogenic peridotites. Thus, very low-degree melts of UHP rocks provide a window into magmatic processes that operated in continental subduction zones. This article presents a review on available results from experimental petrology concerning the possibility of partial melting under conditions of continental subduction-zone metamorphism, and petrological evidence for the occurrence of dehydration-driven in-situ partial melting in natural UHP rocks during the continental collision. Although the deeply subducted continental crust is characterized by a relative lack of aqueous fluids, the partial melting in UHP rocks commonly takes place during decompression exhumation to result in local in-situ occurrences of felsic melts at small scales. This is caused by the local accumulation of aqueous fluids due to the breakdown of hydrous minerals and the exsolution of structural hydroxyl and molecular water from nominally anhydrous minerals in UHP rocks during the exhumation. The dehydration melting of UHP rocks would not only have bearing on the formation of supercritical fluids during subduction-zone metamorphism, but also contribute to element mobility and ultrapotassic magmatism in continental collision orogens. Therefore, the study of dehydration melting and its effects on element transport in UHP slabs, rocks and minerals is a key to chemical geodynamics of continental subduction zones.

Zheng, Yong-Fei; Xia, Qiong-Xia; Chen, Ren-Xu; Gao, Xiao-Ying

2011-08-01

17

Fluid processes in subduction zones.  

PubMed

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

Peacock, S A

1990-04-20

18

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)

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

Baldwin, S.; Das, J. P.

2013-12-01

19

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

NASA Astrophysics Data System (ADS)

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

Craig, Timothy J.; Copley, Alex

2014-04-01

20

Evolution of a Subduction Zone  

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

21

Geodynamic models of terrane accretion: Testing the fate of island arcs, oceanic plateaus, and continental fragments in subduction zones  

NASA Astrophysics Data System (ADS)

Crustal growth at convergent margins can occur by the accretion of future allochthonous terranes (FATs), such as island arcs, oceanic plateaus, submarine ridges, and continental fragments. Using geodynamic numerical experiments, we demonstrate how crustal properties of FATs impact the amount of FAT crust that is accreted or subducted, the type of accretionary process, and the style of deformation on the overriding plate. Our results show that (1) accretion of crustal units occurs when there is a weak detachment layer within the FAT, (2) the depth of detachment controls the amount of crust accreted onto the overriding plate, and (3) lithospheric buoyancy does not prevent FAT subduction during constant convergence. Island arcs, oceanic plateaus, and continental fragments will completely subduct, despite having buoyant lithospheric densities, if they have rheologically strong crusts. Weak basal layers, representing pre-existing weaknesses or detachment layers, will either lead to underplating of faulted blocks of FAT crust to the overriding plate or collision and suturing of an unbroken FAT crust. Our experiments show that the weak, ultramafic layer found at the base of island arcs and oceanic plateaus plays a significant role in terrane accretion. The different types of accretionary processes also affect deformation and uplift patterns in the overriding plate, trench migration and jumping, and the dip of the plate interface. The resulting accreted terranes produced from our numerical experiments resemble observed accreted terranes, such as the Wrangellia Terrane and Klamath Mountain terranes in the North American Cordilleran Belt.

Tetreault, J. L.; Buiter, S. J. H.

2012-08-01

22

Reply to Comment on "Corundum-bearing garnet peridotites from northern Dominican Republic: A metamorphic product of an arc cumulate in the Caribbean subduction zone" by Richard N. Abbott and Grenville Draper  

NASA Astrophysics Data System (ADS)

In our Reply to the Comment by Abbott R.N., Jr., Draper, G., 2010. Comment on "Corundum-bearing garnet peridotite from northern Dominican Republic: A metamorphic product of an arc cumulate in the Caribbean subduction zone" by Hattori et al. Lithos 114 (2010) 437-450]. Lithos 117, 322-326 (this issue), we clarify several points concerning the origin of garnet (Grt)-bearing ultramafic rocks in Dominican Republic, and provide new trace element data from additional samples that contain varying amounts of loss on ignition (LOI). These new data verify that the trace element signature of these bulk rocks reflect those of their primary rocks, and are not significantly affected by low-temperature alteration. These new geochemical data reconfirm the interpretations in our paper (Hattori et al., 2010a) that Grt-bearing ultramafic rocks crystallized as cumulates of arc magmas at shallow levels in the mantle wedge, and were later metamorphosed in the subduction channel to form Grt.

Hattori, Kéiko H.; Guillot, Stéphane; Tubrett, Mike N.; Saumur, Benoit-Michel; Vidal, Olivier; Morfin, Samuel

2010-06-01

23

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

Microsoft Academic Search

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

Riccardo Tribuzio; Bruno Messiga; Riccardo Vannucci; Piero Bottazzi

1996-01-01

24

SUBDUCTION ZONES Robert J. Stern  

E-print Network

to andesites [Rieder et al., 1997] in the ancient southern highlands of Mars suggest that plate tectonics of convergent plate mar- gins and are the geodynamic system that builds island arcs. Excess density Zone, subduction, tectonics Citation: Stern, R. J., Subduction zones, Rev. Geophys., 40(4), 1012, doi

Stern, Robert J.

25

Deep crustal metamorphism during continental extension: modern and ancient examples  

Microsoft Academic Search

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

Michael Sandiford; Roger Powell

1986-01-01

26

Modeling the Migration of Fluids in Subduction Zones  

NASA Astrophysics Data System (ADS)

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 system of mantle and fluid flow equations, introducing complex feedbacks between the rheology, temperature, permeability, strain rate and porosity. Using idealized subduction zone geometries we investigate the effects of this non-linearity and explore the sensitivity of fluid flow paths for a range of fluid flow parameters with emphasis on variability of the location of the volcanic arc with respect to flow paths. We also estimate the expected degrees of hydrous melting using a variety of wet-melting parameterizations (e.g., Katz et al, 2003, Kelley et al, 2010). The current models only include dehydration reactions but work continues on the next generation of models which will include both dehydration and hydration reactions as well as parameterized flux melting in a consistent reactive-flow framework.

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

2011-12-01

27

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

NASA Astrophysics Data System (ADS)

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. This discontinuous exhumation is likely permitted by the specific thermal regime following the onset of a young, warm subduction (group 1), by continental subduction (group 2) or by a major, geodynamic modification of convergence across the subduction zone (group 3; change of kinematics, subduction of asperities, etc). Understanding what controls this short-lived exhumation and the detachment and migration of oceanic crustal slices along the subduction channel will provide useful insights into the interplate mechanical coupling in subduction zones.

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

2009-01-01

28

Accretionary processes at subduction zones in the eastern mediterranean  

NASA Astrophysics Data System (ADS)

The tectonic history of the eastern Mediterranean as well as the present tectonic style are affected by the collision and accretion of oceanic plateaus at subduction zones. Small but distinct oceanic plateaus exist today in the eastern Mediterranean away from the plate boundary. Many other oceanic plateaus crowd the plate boundary, particularly in the Hellenic arc area. Their presence results in the unusual complexity of this subduction zone, as is evidenced by the seismicity and bathymetry patterns. The present two arcs system in this region, the Hellenic and Cyprean arcs, is probably the consequence of the collision of a large continental plateau with an older arc. This plateau has been thrusted over the continental mass of Anatolia and pieces of it are to be found in the region between the two arcs. Active subduction in the Hellenic arc takes place in the outer trenches, particularly in the Ionian and Strabo trenches. Subduction of Mediterranean lithosphere in the Pliny and Ptolemy trenches is limited to their western parts. Inner segments of these trenches served as the main subduction trenches before the subduction zones migrated outward following the collision of oceanic plateaus.

Rotstein, Yair; Ben-Avraham, Zvi

1985-03-01

29

Arc magmas sourced from mélange diapirs in subduction zones  

NASA Astrophysics Data System (ADS)

At subduction zones, crustal material enters the mantle. Some of this material, however, is returned to the overriding plate through volcanic and plutonic activity. Magmas erupted above subduction zones show a characteristic range of compositions that reflect mixing in the magma source region between three components: hydrous fluids derived from the subducted oceanic crust, components of the thin veneer of subducted sediments and peridotite mantle rocks. The mechanism for mixing and transport of these components has been enigmatic. A combination of results from the fields of petrology, numerical modelling, geophysics and geochemistry suggests a two-step process. First, intensely mixed metamorphic rock formations--mélanges--form along the interface between the subducted slab and the mantle. As the mélange contains the characteristic three-component geochemical pattern of subduction-zone magmas, we suggest that mélange formation provides the physical mixing process. Then, blobs of low-density mélange material--diapirs--rise buoyantly from the surface of the subducting slab and transport the well-mixed mélange material into the mantle beneath the volcanoes.

Marschall, Horst R.; Schumacher, John C.

2012-12-01

30

Permeabilities of Subduction Zone Sediments  

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

31

A numerical approach to melting in warm subduction zones  

NASA Astrophysics Data System (ADS)

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

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

2015-02-01

32

Subduction of continental margins and the uplift of high-pressure metamorphic rocks  

NASA Astrophysics Data System (ADS)

The mechanism by which high-pressure metamorphosed continental material is emplaced at high structural levels is a major unsolved problem of collisional orogenesis. We suggest that the emplacement results from partial subduction of the continental margin which, because of its high flexural rigidity, produces a rapid change in the trajectory of the descending slab. We assume a two-fold increase in effective elastic thickness of the lithosphere as the continental margin approaches the subduction zone, and calculate the flexural profile of a thin plate for progressive downward migration of the zone of increased rigidity. We assess the effect of changes in the flexural profile on the overlying accretionary prism and mantle wedge as the continent approaches by estimating the extra stresses that are imposed on the wedge due to the bending moment exerted by the continental part of the plate. The wedges overlying the subduction zones, and the subducting slab itself, experience substantial extra compressional stress at depths of around 100 km, and extensional stress at shallower depths, as the continental margin passes through the zone of maximum curvature. The magnitudes of such extra stresses are probably adequate to effect significant deformation of the wedge and/or the descending plate, and are experienced in a time interval of less than 5 m.y. for typical subduction rates. The spatial variation of yield stresses in the region of the wedge and descending slab indicates that much of this deformation may be taken up in the crustal part of the descending slab, which is the weakest region in the deeper parts of the subduction zone. This may result in rapid upward migration of the crust of the partially subducted continental margin, against the flow of subduction. High-pressure metamorphosed terranes emplaced by the mechanism envisaged in this paper would be bounded by thrust faults below and normal faults above. Movement on the faults would have been coeval, and would have resulted in rapid unroofing of the high-pressure terranes, synchronous with arrival of the continental margin at the subduction zone and, therefore, relatively early in the history of a collisional orogen.

Hynes, Andrew; Arkani-Hamed, Jafar; Greiling, Reinhard

1996-05-01

33

Seismic coupling and uncoupling at subduction zones  

NASA Technical Reports Server (NTRS)

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.

Ruff, L.; Kanamori, H.

1983-01-01

34

Silent Earthquakes, Structure, and Seismotectonics of the Mexican Subduction Zone  

NASA Astrophysics Data System (ADS)

Based on tide gauge, levelling and GPS data, we find evidence for a sequence of silent earthquakes in 1972, 1979, 1996, 1998, 2002, and 2003 in the central part of the Mexican subduction zone (Guerrero and Oaxaca states). Characteristic duration of these events was 4-6 months and the maximum equivalent magnitude exceeded Mw7.5. In all cases, with the exception of the event of 1996, the slow aseismic slips initiated in the Guerrero seismic gap and propagated laterally along the strike of the subduction zone. However, the propagation velocity of ~2 km/day could be estimated reliably only for the most recent 2002 event. The observations indicate that the total area affected by the 1972 and 2002 slow events may have been greater than ~300x700 km2. The shallow, subhorizontal configuration of the plate interface in Guerrero and partly in Oaxaca appears to be a controlling factor for the physical conditions favorable for such extensive slow slip. The entire partially coupled interplate zone in Guerrero is of ~160 km width (starting ~55 km inland from the trench) while the seismogenic, shallowest part of it is only ~40 km wide. The elastic half space dislocation models (EHSDM) applied to invert the observed slow aseismic slip displacements (2002 event) can not distinguish between the two main scenarios: (a) slow slip of ~10 cm occurring on the entire coupled interface, and (b) slip of 15-20 cm taking place only on the transition part of the plate interface from ~90 to 180 km. In the first case the anticipated large thrust earthquake in the Guerrero seismic gap should be somewhat delayed, while on the second case the seismic rupture may be advanced. Thermo-mechanical modeling of the Mexican subduction zone shows that the coupling cutoff of ~450oC on the plate interface at ~180-205 km from the trench is achievable only for the subhorizontal configuration of the subduction zone. In this case the predominant metamorphic facies on the surface of subducted crust should be blueschists. There are, however, several observations which can not be explained in the frame of the EHSDM, e.g., a considerable tilt observed in the coastal area and a relatively large displacement on the Popocatepetl volcano (~400 km from the trench) during the 2002 silent earthquake.

Kostoglodov, V.; Larson, K. M.; Singh, S.; Lowry, A. R.; Santiago, J.; Franco, S.; Bilham, R.

2004-05-01

35

Heterogeneous coupling along Makran subduction zone  

Microsoft Academic Search

The Makran subduction zone, located in the southeast of Iran and southern Pakistan, extends for almost 900 km along the Eurasian-Arabian plate boundary. The seismic activities in the eastern and western Makran exhibit very different patterns. The eastern Makran characterized by infrequent large earthquakes and low level of seismicity. The only large instrumentally recorded earthquake in the eastern Makran, the

Z. Zarifi; M. Raeesi

2010-01-01

36

Nonvolcanic tremors in the Mexican subduction zone  

NASA Astrophysics Data System (ADS)

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.

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

2007-05-01

37

Seismicity, metamorphism and rheology of the lower continental crust  

NASA Astrophysics Data System (ADS)

Seismological data document that both normal earthquakes and tremors occur in the lower continental crust. Pseudotachylytes (frictional melts and ultracommunited rocks) have been described from several high grade metamorphic terrains and may be the geological manifestation of this seismicity. The Grenville (c. 930Ma) granulite facies complex (T: 800 °C; P: ?10kbar) of the Lindås Nappe in the Bergen Arcs, W-Norway underwent a fluid induced partial eclogite (T: 600-650 °C; P: 15-20 kbar) and amphibolite facies metamorphism during the Caledonian (c.400-430 Ma) continent collision. Pseudotachylyte fault and injection veins formed in the dry granulites at or close to the reaction fronts both in the eclogitized (western parts) and the amphibolitized (eastern parts) of the Nappe. They are locally recrystalized with the development of amphibolite and eclogite facies assemblages demonstrating that they formed pre or syn the Caledonian metamorphism. The pseudotachylytes transect lithologies ranging from peridotite to anorthosite and consequently the influence of the seismic energy release on a range of granulite facies minerals including garnet, pyroxenes, olivine, plagioclase, hornblende and scapolite can be observed. The seismic energy released promotes the Caledonian metamorphism and change the petrophysical properties of the lower crust in the following ways: The melting and the ultracommunition of the granulite facies minerals increased the reactive surface area and produce local pathways for fluid. S-rich scapolite, a common mineral in granulities play a key role in this process by releasing S and C to form sulfides and carbonates. Small sulfide grains impregnate the pseudotachylyte veins which may lead to an increased electrical conductivity of the deep crust. The pseudotachylyte veins impose inhomogeneities in the massive rocks through grain size reduction and lead to strain localization with development of amphibolite and eclogite facies shear zones. Formation of eclogite facies breccias where meter size blocks of rotated granulites are enclosed in eclogite may have initiated by the seismic events as indicated by fractures in the relict granulite facies garnet. The seismic events may have been important in large scale transport of fluid required to bring about the metamorphism of the dry granulite facies complex.

Austrheim, Håkon

2014-05-01

38

The redox budget of subduction zones  

NASA Astrophysics Data System (ADS)

Elements that can occur in more than one valence state, such as Fe, C and S, play an important role in Earth's systems at all levels, and can drive planetary evolution as they cycle through the various geochemical reservoirs. Subduction introduces oxidised Fe, C and S in sediments, altered ocean crust, and partially serpentinised lithospheric mantle to the relatively reduced mantle, with short- and long-term consequences for the redox state of the mantle. The distribution of redox-sensitive elements in the mantle controls the redox state of mantle-derived material added to the lithosphere and atmosphere, such as arc volcanic gases and the magmas that form arc-related ore deposits. The extent of mantle oxidation induced by subduction zone cycling can be assessed, albeit with large uncertainties, with redox budget calculations that quantify the inputs and outputs to subduction zones. Literature data are augmented by new measurements of the chemical composition of partially serpentinised lithospheric mantle from New Caledonia and ODP 209. Results indicate that there is a net addition of Fe (55 ± 13 × 1012 mol year- 1), C (4.6 ± 4.0 × 1012 mol year- 1), S (2.4 ± 0.9 × 1012 mol year- 1), and redox budget (5-89 × 1012 mol year- 1) at subduction zones. Monte Carlo calculations of redox budget fluxes indicate that fluxes are 46 ± 12 × 1012 mol year- 1 entering subduction zones, if input and output parameters are assumed to be normally distributed, and 46-58 × 1012 mol year- 1 if input and output parameters are assumed to be log-normally distributed. Thus, inputs into subduction zones for Fe, C, S and redox budget are in excess of subduction zone outputs. If MORB and plume-related fluxes are taken into account then Fe, C and S fluxes balance, within error. However, the redox budget does not balance, unless the very lowest estimates for the extent of slab oxidation are taken. Thus it is likely that subduction continuously increases the redox budget of the mantle, that is, there is addition of Fe, C and S that are oxidised relative to the Fe, C and S in the mantle. The fate of this redox budget can be constrained by consideration of element mobility under mantle conditions. If slab fluids are assumed to be dominantly aqueous and relatively low salinity then fluxes of Fe3 +, C4 +, and S6 + are limited to less than 109, 2.3 × 1012 mol year- 1 and 2 × 1012 mol year- 1 respectively by the low solubility of these elements in slab-derived fluids. Nevertheless, such fluxes can produce the increased fO2 inferred for sub-arc mantle from arc lavas after around 10 Ma subduction. The rest of the redox budget added by the subduction process is likely to be carried to the deep mantle by the slab, and mix slowly with the whole mantle reservoir, depending on the timescale of reincorporation of subducted lithosphere into the mantle. Simple mixing calculations indicate that these fluxes will only cause a measurable difference to mantle redox on a 1 Ga timescale, which is longer than the 550 Ma during which redox budget fluxes are likely to have been at present day levels. However, measurable effects, with potential consequences for the Earth's evolution may be expected in the future.

Evans, K. A.

2012-06-01

39

Comments on “Corundum-bearing garnet peridotite from northern Dominican Republic: A metamorphic product of an arc cumulate in the Caribbean subduction zone,” by Hattori et al. [Lithos 114 (2010) 437-450  

NASA Astrophysics Data System (ADS)

The Cuaba Gneiss in northern Dominican Republic hosts an unusual suite of Grt-ultramafites. Two hypotheses for their origin are distinguished by the depth of crystallization of an igneous protolith. In the first hypothesis the protolith crystallized under ultra high pressure (UHP) magmatic conditions (> 3.2 GPa) in the field of stability for Grt + Spl + Crn. The protolith was then modified by subsolidus processes. In a new hypothesis the protolith crystallized under low pressure (LP) magmatic conditions (< 1.1 GPa) in the field of stability for plagioclase. Grt + Crn was produced during prograde metamorphism. The LP hypothesis depends on a small Eu anomaly, limited fluid interaction, REE modeling, and a magmatic composition for clinopyroxene. Arguments against the LP hypothesis address, (1) the source of the Eu anomaly, (2) the use of clinopyroxene in assessing provenance and modeling REEs, (3) the proposed prograde mineral reactions, and (4) thermodynamic calculations.

Abbott, Richard N., Jr.; Draper, Grenville

2010-06-01

40

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

NASA Astrophysics Data System (ADS)

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.

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

2012-03-01

41

Opening and closing slab windows in congested subduction zones  

NASA Astrophysics Data System (ADS)

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

Moresi, Louis

2013-04-01

42

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

NASA Astrophysics Data System (ADS)

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 deformation front, and (3) on the relations of chemistry, tectonic events, and flow rates. The formation fluid Li concentrations and isotope data at the ODP reference Site 1253 support mixing between seawater and a deep-sourced fluid from the forearc of the Costa Rica subduction zone, implying that the uppermost permeable basement serves as pathway of fluid expulsion from the forearc. At the Nankai Trough décollement the Li concentrations are significantly higher and the ?7Li value is lower than at Costa Rica, reflecting the different sediment inputs and geothermal gradients at these two subduction zones.

Kastner, M.

2008-12-01

43

The fate of the downgoing oceanic plate: Insight from the Northern Cascadia subduction zone  

NASA Astrophysics Data System (ADS)

In this study, we use teleseismic receiver function analysis to image the seismic structure of the Juan de Fuca oceanic plate during its subduction beneath the North American plate. Seismic data have been recorded at 58 seismic stations deployed along the northern Cascadia subduction zone. Harmonic decomposition of the receiver function data-set along a trench-normal profile allows us to image both the isotropic and the anisotropic structure of the plate (slab). Our images highlight the presence of a highly anisotropic region at 40-70 km depths across the Cascadia subduction zone. The detected seismic anisotropy is interpreted to be related to both metamorphic facies (e.g. blueschists) and fluid released during the dehydration of the subducting mantle. The processes of dehydration and metamorphism produce the variations of the seismic properties within each lithologic unit that constitutes the subducted slab, i.e. basalts, gabbro layer and upper mantle, as the oceanic plate sinks in the upper mantle. Such variations make it almost impossible to recognize the “plate boundary” as a characteristic “velocity-jump” at depth (neither positive nor negative) along the Cascadia subduction zone. Based on the comparative interpretation of both the isotropic and the anisotropic structures retrieved, we propose a 4-stage model of the evolution of the Juan de Fuca oceanic plate during its subduction beneath the North American plate.

Piana Agostinetti, Nicola; Miller, Meghan S.

2014-12-01

44

Acceleration spectra for subduction zone earthquakes  

USGS Publications Warehouse

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

Boatwright, J.; Choy, G.L.

1989-01-01

45

Diapiric Flow at Subduction Zones: A Recipe for Rapid Transport  

Microsoft Academic Search

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 ~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.

Paul S. Hall; Chris Kincaid

2001-01-01

46

The dynamics of reactive fluid escape in subduction zones  

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

47

Tomography and Dynamics of Western-Pacific Subduction Zones  

NASA Astrophysics Data System (ADS)

We review the significant recent results of multiscale seismic tomography of the Western-Pacific subduction zones and discuss their implications for seismotectonics, magmatism, and subduction dynamics, with an emphasis on the Japan Islands. Many important new findings are obtained due to technical advances in tomography, such as the handling of complex-shaped velocity discontinuities, the use of various later phases, the joint inversion of local and teleseismic data, tomographic imaging outside a seismic network, and P-wave anisotropy tomography. Prominent low-velocity (low-V) and high-attenuation (low-Q) zones are revealed in the crust and uppermost mantle beneath active arc and back-arc volcanoes and they extend to the deeper portion of the mantle wedge, indicating that the low-V/low-Q zones form the sources of arc magmatism and volcanism, and the arc magmatic system is related to deep processes such as convective circulation in the mantle wedge and dehydration reactions in the subducting slab. Seismic anisotropy seems to exist in all portions of the Northeast Japan subduction zone, including the upper and lower crust, the mantle wedge and the subducting Pacific slab. Multilayer anisotropies with different orientations may have caused the apparently weak shear-wave splitting observed so far, whereas recent results show a greater effect of crustal anisotropy than previously thought. Deep subduction of the Philippine Sea slab and deep dehydration of the Pacific slab are revealed beneath Southwest Japan. Significant structural heterogeneities are imaged in the source areas of large earthquakes in the crust, subducting slab and interplate megathrust zone, which may reflect fluids and/or magma originating from slab dehydration that affected the rupture nucleation of large earthquakes. These results suggest that large earthquakes do not strike anywhere, but in only anomalous areas that may be detected with geophysical methods. The occurrence of deep earthquakes under the Japan Sea and the East Asia margin may be related to a metastable olivine wedge in the subducting Pacific slab. The Pacific slab becomes stagnant in the mantle transition zone under East Asia, and a big mantle wedge (BMW) has formed above the stagnant slab. Convective circulations and fluid and magmatic processes in the BMW may have caused intraplate volcanism (e.g., Changbai and Wudalianchi), reactivation of the North China craton, large earthquakes, and other active tectonics in East Asia. Deep subduction and dehydration of continental plates (such as the Eurasian plate, Indian plate and Burma microplate) are also found, which have caused intraplate magmatism (e.g., Tengchong) and geothermal anomalies above the subducted continental plates. Under Kamchatka, the subducting Pacific slab shortens toward the north and terminates near the Aleutian-Kamchatka junction. The slab loss was induced by friction with the surrounding asthenosphere, as the Pacific plate rotated clockwise 30 Ma ago, and then it was enlarged by the slab-edge pinch-off by the asthenospheric flow. The stagnant slab finally collapses down to the bottom of the mantle, which may trigger upwelling of hot mantle materials from the lower mantle to the shallow mantle. Suggestions are also made for future directions of the seismological research of subduction zones.

Zhao, D.

2012-01-01

48

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

NASA Astrophysics Data System (ADS)

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

Plunder, Alexis; Agard, Philippe; Chopin, Christian

2014-05-01

49

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

NASA Astrophysics Data System (ADS)

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

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

1995-06-01

50

Subduction zone earthquake probably triggered submarine hydrocarbon seepage offshore Pakistan  

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

51

Seismotectonics of Central Mexico Subduction Zone From Crustal Deformation Studies  

NASA Astrophysics Data System (ADS)

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

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

2006-12-01

52

Controls on earthquake rupture and triggering mechanisms in subduction zones  

E-print Network

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

Llenos, Andrea Lesley

2010-01-01

53

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

NASA Astrophysics Data System (ADS)

The ratio of \\{long-term-average seismic moment production per unit length of plate boundary\\} to \\{relative plate velocity\\} is determined by the "coupled thickness" of seismogenic lithosphere, and also by elastic moduli and geometric factors that are fairly well known. It is generally assumed that coupled thickness is constant within a given class of plate boundary, such as Bird's [2003, G3]: CCB Continental Convergent Boundary, CRB Continental Rift Boundary, CTF Continental Transform Fault, OCB Oceanic Convergent Boundary, OSR Oceanic Spreading Ridge, OTF Oceanic Transform Fault, or SUB Subduction zone. However, Bird et al. [2002, Geodyn. Ser.] and Bird & Kagan [2004, BSSA] found two exceptions: OSR and OTF both have greater coupled thickness at low relative plate velocities. We test for variation of coupled thickness with relative plate velocity in each of the 7 classes of plate boundary. We use shallow (<70 km) earthquakes from the Harvard CMT catalog, 1982.01.01-2007.03.31, above magnitude MW threshold of 5.51 or 5.66. In order to reduce the influence of aftershock swarms, we estimate the probability of independence of each earthquake according to the likelihood stochastic declustering method of Kagan & Jackson [1991; GJI] and use this as a weight. We use the algorithm of Bird & Kagan [2004, BSSA] to assign 95% of shallow earthquakes to plate boundary steps and plate boundary classes, rejecting all earthquakes that fall into one of the 13 orogens of Bird [2003, G3]. We order the plate-boundary steps outside orogens in each class by relative plate velocity according to the PB2002 model of Bird [2003]. Then, we plot cumulative earthquake count as a function of cumulative model tectonic moment (assuming constant coupled thickness and other parameters within each plate boundary class). The null hypothesis is a linear relation; we use 2 measures (Kolmogorov-Smirnov, and Cramer-von Mises) to quantify departures from this line. We use 10,000 simulations of each class with random Poissonian seismicity in each plate boundary step (with expectations based on the tectonic model) to assess the significance of the measures obtained. Subduction zones have velocity-dependent coupled thickness: P < 0.001 for the null hypothesis. Subduction zones with relative plate velocity <67 mm/a (which would comprise 35% of the model tectonic moment rate, in the null hypothesis) actually produce only 20% of the global subduction zone earthquakes (outside orogens), and thus have a coupled thickness about half that of faster subduction zones (if corner magnitude and spectral slope are constant). This result contradicts the uniform coupling of subduction zones inferred by Kreemer et al. [2002, Geodyn. Ser.]; the difference may be due to their exclusion of several slow subduction zones including Aegean, Cascadia, New Zealand, Caribbean, and South Shetland. Continental CCBs show a similarly strong relation (P < 0.001), with an increase in coupled thickness when velocity exceeds 25 mm/a. OSRs show coupled thickness declining with velocity, as in previous studies. OTFs and OCBs give complex results with significant variations (P < 0.01; P < 0.05) that are not easy to interpret. For CRBs and CTFs we do not reject the null hypothesis of constant coupled thickness.

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

2007-12-01

54

Earthquake size distribution in subduction zones linked to slab buoyancy  

NASA Astrophysics Data System (ADS)

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

Nishikawa, Tomoaki; Ide, Satoshi

2014-12-01

55

The Sulfur Cycle at Subduction Zones  

NASA Astrophysics Data System (ADS)

We present sulfur (S) isotope data for magmatic gases emitted along the Central American (CA) Arc (oxidizing conditions ?QFM ~+ 1.5) and at the East African Rift (reduced conditions ?QFM ~0). The results are interpreted through mass balance calculations to characterize the S cycle through subduction zones with implications for the redox conditions of arc magmas. Voluminous gas emissions from Masaya, an open vent basaltic volcano in Nicaragua, represent >20% of the SO2 flux from the CA arc [1]. Samples from the Masaya plume have S isotope compositions of + 4.8 × 0.4 ‰ [2]. Degassing fractionation modeling and assessment of differentiation processes in this oxidized volcano suggest that this value is close to that of the source composition. High T gas samples from other CA volcanoes (Momotombo, Cerro Negro, Poas, Turrialba) range from + 3 ‰ (Cerro Negro) to + 7 ‰ (Poas; [3]). The high ?34S values are attributed to recycling of subducted oxidized sulfur (sulfate ~ + 20 ‰) through the CA arc. The ?34S values of the more reduced samples from East African Rift volcanoes, Erta Ale - 0.5 × 0.6 ‰ [3] and Oldoinyo Lengai -0.7 ‰ to + 1.2 ‰) are far lower and are probably sourced directly from ambient mantle. The subduction of oxidized material at arcs presents a likely explanation for the oxidized nature of arc magmas relative to magmas from spreading centers. We observe no distinguishable change in melt fO2 with S degassing and attribute these differences to tectonic setting. Monte Carlo modeling suggests that subducted crust (sediments, altered oceanic crust, and serpentinized lithospheric mantle) delivers ~7.7 × 2.2 x 1010 mols of S with ?34S of -1.5 × 2.3‰ per year into the subduction zone. The total S output from the arc is estimated to be 3.4 × 1.1 x 1010 mols/yr with a ?34S value similar to that of Masaya gas (+5 × 0.5 ‰). Considering ?34S values for ambient upper mantle (0 ‰ [4]) and slab-derived fluids (+14 ‰ [5]) allows calculation of the flux of S released from slab into the mantle wedge. Based on these constraints, we calculate that 1.2 × 0.4 x 1010 mols of S/yr is released from the slab. If slab-derived S is in the S6+ oxidation state, this flux is enough to oxidize the entire mantle wedge to the Fe3+/Fe2+ observed in typical arc rocks in ~ 20 million years. [1] Hilton et al. (2002) Noble Gases in Geochemistry and Cosmochemistry. pp. 319-370 [2] de Moor et al., (in review) G-cubed [3] Rowe (1994) Chem. Geol., 236:303-322 [4] Sakai et al. (1984) J. Petrol., 52: 1307-1331 [5] Alt et al. (2012) Earth Plan. Sci. Lett., 327: 50-60

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

2013-12-01

56

Seismicity of the eastern Hellenic Subduction Zone  

NASA Astrophysics Data System (ADS)

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

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

2012-04-01

57

Diapiric Flow at Subduction Zones: A Recipe for Rapid Transport  

NASA Astrophysics Data System (ADS)

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 ~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.

Hall, Paul S.; Kincaid, Chris

2001-06-01

58

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

NASA Astrophysics Data System (ADS)

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

Chen, Yi-Xiang; Zheng, Yong-Fei

2015-02-01

59

Comparative Thermal Structures of Circum-Pacific Subduction Zones  

NASA Astrophysics Data System (ADS)

Subduction zone thermal structure influences Benioff Zone (BZ) seismicity, slab dehydration/metamorphism, geochemical fluxes, and arc magma production. To evaluate effects of relative differences in slab-surface temperature (SST), we have simulated thermal structures of ten circum-Pacific cross-arc transects (Andean, Central American, Cascades, E Aleutian, Kurile, NE Japan, and Mariana arcs) showing large variations in slab age (9-134 Ma), convergence rate (2-10 cm/yr), duration (40-226 Ma), slab length (200-1200 km), and slab geometry. A finite-element method was used to simulate mantle convection and a finite-difference solver used to compute heat transfer. A staggered grid for discretization enhanced accurate modeling of mantle convection. All models assume realistic curved slab geometries as constrained by BZ earthquake hypocenters. Shear-heating is evaluated using rheology of wet quartzite to simulate the presence of a veneer of subducted sediment near upper slab surfaces. Error propagation analysis indicates that the major uncertainty in SST profile is associated with (1) uncertainty in thickness of the overriding lithospheric plate, which determines the size of the convecting wedge domain, and (2) shear-heating contributions. Assuming constant lithosphere thickness (100 km), we estimate a nearly two-fold range in SST (~300 to 600 oC) at BZ depths below the respective volcanic fronts. Moreover, for most transects similar SSTs (~800+/-50 oC) are predicted at depths of the deepest BZ earthquakes. Even where slow subduction of young slabs occur, Ts approaching the wet solidus of oceanic crust or sediments are predicted only if the lithospheric lid is relatively thin or there is very strong shear-heating. Thus, we consider direct melting of subducted materials unlikely in any of the model transects. Assuming that highly fluid-mobile B is derived largely from subducted slabs, B-enrichment data help constrain our thermal models. We show that B-enrichment (e.g., B/Zr ratios) in arc volcanic rocks is anti-correlated with SSTs at 100 km depth. This relation is consistent with progressive loss of fluids and fluid-mobile elements as descending slabs warm, and with variation in slab thermal structure from arc to arc.

Huang, S.; Leeman, W. P.; Sisson, V. B.

2001-12-01

60

GEOPHYSICAL RESEARCH LETTERS, VOL. 27, NO. 3, PAGES 433-436, FEBRUARY 1, 2000 Wave-guide effects in subduction zones: evidence from  

E-print Network

seismograms recorded by stations along the Pacific coast of Mexico for subduction earthquakes. Introduction of conver- gence between the oceanic and the continental plates some of the largest earthquakes in any countries. Seismic radiation from subduction zone earthquakes can be strongly affected by the heterogeneity

Shapiro, Nikolai

61

Great earthquakes of variable magnitude at the Cascadia subduction zone  

Microsoft Academic Search

Comparison of histories of great earthquakes and accompanying tsunamis at eight coastal sites suggests plate-boundary ruptures of varying length, implying great earthquakes of variable magnitude at the Cascadia subduction zone. Inference of rupture length relies on degree of overlap on radiocarbon age ranges for earthquakes and tsunamis, and relative amounts of coseismic subsidence and heights of tsunamis. Written records of

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

2006-01-01

62

Cyclic stressing and seismicity at strongly coupled subduction zones  

Microsoft Academic Search

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

Mark A. J. Taylor; Gutuan Zheng; James R. Rice; William D. Stuart; Renata Dmowska

1996-01-01

63

Earthquake swarms in circum-Pacific subduction zones  

Microsoft Academic Search

We systematically and manually search through clusters of earthquakes along circum-Pacific subduction zones to identify potential earthquake swarms. In total, we find 266 potential earthquake swarms: 180 we classify as megathrust and 68 we classify as volcanic due to their proximity to the megathrust or to volcanoes. We focus on the megathrust swarms and demonstrate that: (1) the number of

S. G. Holtkamp; M. R. Brudzinski

2011-01-01

64

Invited review paper: Seismicity along the South American subduction zone: Review of large earthquakes, tsunamis, and subduction zone complexity  

Microsoft Academic Search

Earthquakes along the shallow South American subduction zone exhibit heterogeneous rupture characteristics, going back several centuries of the earthquake record in this area. This heterogeneity is manifest in several ways, such as changes in rupture mode from magnitude >8 events during one century followed by smaller ones in other time periods, as well as unusual tsunami events. There is also

Susan L. Bilek

2010-01-01

65

Supra-subduction zone tectonic setting of the Muslim Bagh Ophiolite, northwestern Pakistan: Insights from geochemistry and petrology  

NASA Astrophysics Data System (ADS)

The geology of the Muslim Bagh area comprises the Indian passive continental margin and suture zone, which is overlain by the Muslim Bagh Ophiolite, Bagh Complex and a Flysch Zone of marine-fluvial successions. The Muslim Bagh Ophiolite has a nearly-complete ophiolite stratigraphy. The mantle sequence of foliated peridotite is mainly harzburgite with minor dunite and contains podiform chromite deposits that grade upwards into transition zone dunite. The mantle rocks (harzburgite/dunite) resulted from large degrees of partial melting of lherzolite and have also been affected by melt-peridotite reaction. The Muslim Bagh crustal section has a cyclic succession of ultramafic-mafic cumulate with dunite at the base, that grades into wehrlite/pyroxenite with gabbros (olivine gabbro, norite and hornblende gabbro) at the top. The sheeted dykes are immature in nature and are rooted in crustal gabbros. The dykes are mainly metamorphosed dolerites, with minor intrusions of plagiogranites. The configuration of the crustal section indicates that the crustal rocks were formed over variable time periods, in pulses, by a low magma supply rate. The whole rock geochemistry of the gabbros, sheeted dykes and the mafic dyke swarm suggests that they formed in a supra-subduction zone tectonic setting in Neo-Tethys during the Late Cretaceous. The dykes of the mafic swarm crosscut both the ophiolite and the metamorphic sole rocks and have a less-marked subduction signature than the other mafic rocks. These dykes were possibly emplaced off-axis and can be interpreted to have been generated in the spinel peridotite stability zone i.e., < 50-60 km, and to have risen through a slab window. The Bagh Complex is an assemblage of Triassic-Cretaceous igneous and sedimentary rocks, containing tholeiitic, N-MORB-like basalts and alkali basalts with OIB-type signatures. Nb-Ta depletion in both basalt types suggests possible contamination from continental fragments incorporated into the opening Tethyan oceanic basin during break-up of Gondwana. The lithologies and ages of the Bagh Complex imply that these rocks formed in an area extending from the continental margin over the Neo-Tethyan ocean floor. The Bagh Complex was then juxtaposed with the Muslim Bagh Ophiolite in the final stage of tectonic emplacement.

Kakar, Mohammad Ishaq; Kerr, Andrew C.; Mahmood, Khalid; Collins, Alan S.; Khan, Mehrab; McDonald, Iain

2014-08-01

66

Interplay of lower crustal metamorphism and continental lithosphere dynamics  

NASA Astrophysics Data System (ADS)

Crustal rocks densify if they are subjected to higher pressures, consistent with the common perception of compressibility of solid materials, direct observations and simple and more sophisticated calculations. The response of typical lower crustal rocks to heating, however, is more complicated. While the crust expands with increasing temperature at some pressure-temperature conditions - in line with conventional use of a positive thermal expansion coefficient - it contracts upon heating at certain P-T conditions if it contains some water. The loss of volume due to dehydration reactions can be larger than the tendency of the solid to swell upon heating, leading to an effective negative coefficient of thermal expansion for these rocks under those conditions. Systematic calculations of phase relations and densities using internally consistent thermodynamic data and Gibb's free energy minimization show that negative thermal expansion (densification upon heating) is most pronounced at pressures larger 1 GPa and temperatures between 500 and 800 °C. These conditions prevail in lower continental crust that is pushed into the mantle due to flexure and loading (e.g. in foreland basins) or due to thickening (e.g. in the crustal root of a mountain range), and in subducted oceanic crust. We propose that the density increase of the lower crust due to dehydration reactions may be the driving force for the subsidence in cratonic basins, and can explain the larger than predicted subsidence of foreland basins as well as the preservation of orogenic roots over long periods of time in eroded mountain chains. In addition, we predict a higher than usually assumed density increase in the initially hydrated oceanic crust during subduction. Simple isostatic modelling predicts that intra-cratonic basins will subside rapidly as a response to pressure increase, followed by slow subsidence on the time-scale of thermal equilibration. Differences in the late stage evolution of orogens may be due to variations in bulk lower crustal compositions. Mafic lower crust becomes very dense if buried to sub-Moho conditions and may delaminate on a timescale that is determined by the initial level of hydration (in addition to rheological paramters). Dry basaltic crust is densest at high-P, but low-T conditions while wet mafic rocks reach densities higher than those of the mantle at high-P and high-T conditions. Lower crustal rocks with more felsic compositions are slightly lighter than mantle under high-P, high-T conditions and may stabilize basins and orogenic roots. However, some water-bearing felsic rocks (meta-pelitic restites) display larger density variations than mafic rocks along a wide range of P-T-paths and consequently cause larger isostatic vertical motions.

Simon, Nina S. C.; Podladchikov, Yuri Y.

2010-05-01

67

Subduction-related metamorphism beneath ophiolites (Oman) and during early stages of continental collision (Himalaya)  

NASA Astrophysics Data System (ADS)

Subduction-related metamorphism occurs beneath ophiolites (Oman), beneath island arcs (Kohistan) and during the early stages of continental collision (Kaghan, Tso Morari; Himalaya). Ophiolite obduction necessarily involves subduction of first oceanic, then continental crust to mantle depths beneath the ophiolite. In Oman an inverted pressure and temperature profile is exposed beneath the Semail ophiolite from garnet+clinopyroxene-bearing granulite to hornblende+plagioclase amphibolite down through epidote amphibolite and a variety of greenschist facies meta-sediments, dominantly cherts, marbles and quartzites. Thermobarometry on Grt+Cpx-bearing amphibolites immediately beneath the contact with mantle sequence harzburgites shows that the upper sole rocks formed at PT conditions of 770-900°C and 11-13 kbar, equivalent to depths of 30-40 km in oceanic lithosphere. Heat for metamorphism can only have been derived from the overlying mantle peridotites. Pressures are higher than can be accounted for by the thickness of the preserved ophiolite (15-20 km). Timing of peak metamorphism was synchronous with formation of the ophiolite gabbroic - trondhjemite crustal sequence and eruption of the pillow lavas (Cenomanian; 96-95 Ma). During the later stages of obduction the continental margin was dragged down to depths of nearly 100 km and basaltic sills within calc-schists were converted to eclogites (20-25 kbar; 500-560oC; 79.1 Ma), then exhumed back up the same subduction channel. Apparent 'extensional' fabrics throughout the HP units are related to upward flow of deeply buried rocks in a wholly compressional environment. Eclogites in a similar structural position occur along the Himalaya in the northernmost exposures of Indian plate rocks. These eclogites formed either during the latest stage of ophiolite obduction or the earliest stage of continental collision.

Searle, Mike; Waters, David; Cowan, Robert; Cherry, Alan; Cooper, Charles

2014-05-01

68

Early Earth melt production in a subduction zone, a petrological model  

NASA Astrophysics Data System (ADS)

A large part of the Archean continental crust is made of a composite rock assemblage dominated by granitoids belonging to the TTG series (tonalite-trondhejmeite-granodiorite). The modus operandi of this sodic granitoids still disputed. If the modern processes leading to continental crust formation at convergent margins are well constrained, the extrapolation to early Earth conditions is hazardous, because the composition of Earth's early crust can be achieved through several processes. However, an 'arc' signature seems to be present in TTGs, suggesting a formation of continental crust in subduction zone settings. Moreover, they show strong similarities with modern adakites, which are thought to be formed by melting of the oceanic subducting crust. We present the results of a study where numerical models of subduction are integrated with a thermodynamic database. Our goal is to investigate under which conditions slab melting can be achieved if at all. We particularly focus our attention on the fate of water, since it is a component that is essential to the formation of TTG series, independently of the petrogenetical scenario preferred. The amount and composition of water bearing fluids in a subduction zone is controlled by slab devolatilization, and influence both the melting regime and the melt composition. Our reference model of an early Earth regime, with a high mantle potential temperature, show that the slab dehydrates early, ending up being composed of a dry eclogites. Importantly, our models show that dehydration melting is not achieved in the slab crust; yet, water-present melting of the 'dry' eclogites can be achieved if a dehydration reaction occurs in the deeper portion of the slab, fuelling the melting reaction with water. Moreover, the dehydration reactions that occurred within the slab are able to metasomatize the overlying mantle wedge, forming hydrated peridotites, that becomes a melt source when dragged down by corner-flow. Our results show the crucial role of dehydration and re-hydration reactions on slab and mantle wedge melting potential. We investigate the effect of different important parameters, such as, mantle potential temperature, subduction velocity and slab composition, on the dehydration and melting processes, to be able to specify the different types of magmas that can be generated in an early-Earth subduction zone.

Magni, V.; Bouilhol, P.; Van Hunen, J.; Moyen, J.

2013-12-01

69

The global range of subduction zone thermal models  

NASA Astrophysics Data System (ADS)

We model 56 segments of subduction zones using kinematically defined slabs based on updated geometries from Syracuse and Abers (2006) to obtain a comprehensive suite of thermal models for the global subduction system. These two-dimensional thermal models provide insight to the dehydration and melting processes that occur in subduction zones. Despite the wide range of slab geometries, ages, convergence velocities and upper plates the predicted thermal structures share many common features. All models feature partial coupling between the slab and the overriding plate directly downdip of the thrust zone, invoked to replicate the cold nose observed in measurements of heat flow and seismic attenuation. We test four separate assumptions about the causes of the partial coupling: (1) the downdip end of the partial coupling is at a constant depth, (2) it is at constant distance trenchward from the arc, (3) it is defined by a critical surface slab temperature, or (4) it is adjusted such that the hottest part of the mantle wedge beneath the arc is at a constant temperature for all subduction zones. In all of these models, slabs reach temperatures where the top of the oceanic crust and sediments dehydrate before they reach subarc depths, and the overlying mantle wedge is too hot for hydrous minerals to be stable at subarc depths. By contrast, the interior of the oceanic crust and underlying mantle within the downgoing plate remains cold enough for hydrous phases to be stable beyond the arc in all but the hottest subduction zones, allowing water to be carried beyond the arc in the slab.

Syracuse, Ellen M.; van Keken, Peter E.; Abers, Geoffrey A.; Suetsugu, Daisuke; Bina, Craig; Inoue, Toru; Wiens, Douglas; Jellinek, Mark

2010-11-01

70

Ups and downs in western Crete (Hellenic subduction zone)  

NASA Astrophysics Data System (ADS)

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.

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

2014-07-01

71

Ups and downs in western Crete (Hellenic subduction zone)  

PubMed Central

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

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

2014-01-01

72

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

NASA Astrophysics Data System (ADS)

For more than two decades, seismologists at the Pacific Geoscience Centre puzzled over the episodic appearance of simultaneous "noise" on seismographs from stations located over the Cascadia subduction zone in southwestern British Columbia. With the 1992 initiation of continuous GPS monitoring in Victoria, B.C., another puzzle presented itself in an inexplicable 5-mm westward displacement of the then solitary regional continuous GPS monument over a period of about a week in October 1994. These observations remained unexplained (and unrelated) until recently. Beginning in 1996, as a result of improvements in GPS orbits and regional densification of continuous GPS networks, transient aseismic crustal motions lasting from periods of a few days to over a year were starting to be recognized. For the Pacific Northwest, detailed analyses of continuous GPS data successfully resolved a spatially coherent, transient signal that occurred in August 1999. Unrelated to after-slip that can follow great thrust earthquakes or to shallow slow-slip "tsunami" earthquakes, this signal, detected at 7 contiguous GPS sites, was characterized by a change in site positions ranging from 2 to 5 mm over a period of 6 to 15 days in a direction opposite to long-term deformation motions. This brief reversal was modeled by ˜2 cm of slip on the plate interface, providing the first evidence for discrete "silent" slip events occurring on the deeper Cascadia subduction zone. In early 2002, researchers at Central Washington University established the surprising regularity of Cascadia slip events on the plate interface underlying southern Vancouver Is. and northwestern Washington State. Eight slip events were identified between 1992 and 2002, with a recurrence interval of 14.5 +/-2 months. Next, Japanese scientists discovered the episodic occurrence of unique, non-volcanic tremors at average depths of about 30 km along the Nankai Subduction Zone. The similarity of the average depth of slip and the migration velocity of slip for the GPS-determined Cascadia slip events, to the depth and migration velocity of the Japanese tremors triggered the search for seismic signatures for the Cascadia slip events. An examination of seismic records from 1996 to 2002 for sites on Vancouver Is. revealed that what had previously been deemed surface noise was signal from seismic tremors that accompanied slip events. The Cascadia tremors were found to be similar in character to the Japanese deep tremors. In addition, their source region was found to coincide with, or directly overlie, the region of the subducting slab interface where transient slip occurs. The close correlation of tremors with slip coined the naming of the phenomenon as Episodic Tremor and Slip (ETS). The physical processes which give rise to this dynamic behavior on the deeper plate interface are not yet well understood. To date, only the Nankai and Cascadia subduction zones have been observed to share aspects of this behavior, suggesting that this phenomenon may be restricted to young subduction zones. The release of fluids, contact with a hydrated mantle wedge, and episodic changes in shear strength or mechanical coupling may all play a part in governing this behavior. Possible connections of ETS with the development of "E-zone" reflector bands, basal erosion, and pulsating metamorphism await further research. In the context of seismic hazard, the ETS zone may mark the down-dip limit of coseismic rupture of the next megathrust earthquake. Also, since it is conceivable for a slip event to trigger a large subduction thrust earthquake, the onset of ETS activity could identify times of higher probability for the occurrence of megathrust earthquakes.

Dragert, H.

2003-12-01

73

The formation of deep basins in High Arctic from metamorphism in continental crust  

NASA Astrophysics Data System (ADS)

In the East Barents and North Chukchi basins, 16-20 km deep, the crystalline crust is attenuated to 12-18 km (reference profiles 2-AR, 4-AR and 5-AR). P-wave velocities and densities in this layer are characteristic of the oceanic crust. However, the subsidence history in the basins is quite different from that typical of the oceanic crust. In both basins the subsidence continued for several hundred million years and one half of the deposits or more was formed long after the start of the subsidence when cooling of the oceanic plate would be already over. Moreover, the basins are 4-5 km deeper than it could be expected according to the thickness of the crystalline crust above the Moho boundary. In the absence of large free-air gravity anomalies, joint analysis of the gravity and seismic data indicates the existence under the Moho of thick layers of high-density and high-velocity eclogites. As can be seen in high resolution seismic profiles, the intensity of crustal stretching did not exceed 10% in the basins, and their formation can be predominantly attributed to a high-grade metamorphism in the mafic lower part of continental crust. At some episodes, strong increase in the rate of subsidence occurred in the basins. This indicates acceleration of metamorphism catalyzed by infiltration of mantle fluids. A set of the above features, abnormally large depth, long subsidence history with its acceleration at the late stages, and episodes of pronounced acceleration of the subsidence represent characteristic features of some other large hydrocarbon basins, e.g., of the North and South Caspian basins. These features can be used for prospecting new prolific provinces on the Arctic shelf. The Lomonosov ridge, Mendeleev high and the Makarov basin pertain to the same structural type. In the Oligocene they underwent erosion near to sea level with the formation of pronounced unconformity. Then at the end of Oligocene deep-water basins were formed in these regions. Rapid crustal subsidence after a long period of relative stability is atypical of oceanic crust. It can be produced either by intense stretching of continental crust or by a density increase due to metamorphism in this layer. Recent seismic reflection profiles demonstrate only minor stretching of the crystalline basement in the regions. Then metamorphism should be the main cause of formation of deep basins in these regions. This can explain attenuation of crystalline crust and an increase in P-wave velocities in this layer that are typical for many deep basins formed due to intense metamorphism in continental crust.

Artyushkov, Eugene; Belyaev, Igor; Chekhovich, Peter; Petrov, Eugene; Poselov, Viktor

2014-05-01

74

The dynamics of intra-oceanic subduction zones: A direct comparison between fossil petrological evidence (Rio San Juan Complex, Dominican Republic) and numerical simulation  

NASA Astrophysics Data System (ADS)

Dispersed blocks of various types of metamorphic rocks in serpentinite mélanges of the northern Dominican Republic (Hispaniola) provide fossil evidence for the dynamics of the subduction zone channel in the intra-oceanic Caribbean subduction zone system between 120 and 55 Ma. Comprehensive petrological and geochronological data on three exemplary samples of eclogite and blueschist are presented that allow a series of different but interrelated pressure-temperature-time paths to be delineated. Eclogites indicate a low P/T gradient during subduction and record conditions in the nascent stages of the subduction zone. Lu-Hf data yield 103.6 ± 2.7 Ma for peak metamorphic conditions of 23 kbar/750 °C. An anticlockwise P-T path is defined. Other blocks record the continuous cooling of the evolving subduction zone and show typical clockwise P-T-paths. Omphacite blueschists reach maximum P-T-conditions of 17-18 kbar/520 °C at 80.3 ± 1.1 Ma (Rb-Sr age data). The mature subduction zone is typified by jadeite blueschists recording very high ("cold") P/T gradients. A Rb-Sr age of 62.1 ± 1.4 Ma dates peak metamorphic P-T conditions at 16-18 kbar/340-380 °C. The array of P-T-t data allows overall cooling rates of the subduction zone at depths of c. 60 km to be constrained at 9 °C/Ma. Cooling rates and exhumation rates (i.e., vertical component of retrograde trajectories) of the metamorphic blocks are 9-20 °C/Ma and 5-6 mm/a, respectively. The derived P-T-t array is compared with a 2-D numerical subduction-zone model published by Gerya et al. [Gerya, T.V., Stöckhert, B. and Perchuk, A.L., 2002. Exhumation of high-pressure metamorphic rocks in a subduction channel: a numerical simulation. Tectonics 142, 6-1-6-19.; 45° slab dip, 40 Ma lithosphere age, convergence rates of 10-40 mm/a], which incorporates weakening of lithospheric mantle of the hanging wall by fluids emanating from the downgoing slab, resulting in an increasingly more funnel-shaped subduction channel system with time. The numerically derived array of simulated P-T-t paths as well as the calculated rates of exhumation and cooling agree well with the P-T-t data derived from the metamorphic blocks of the Rio San Juan serpentinite mélanges when convergence rates of 15 to 25 mm/a are chosen. This value is also in accord with available paleogeographic reconstructions calling for a long-term average of 22 mm/a of orthogonal convergence. On the basis of the comparison, the onset of subduction in the Rio San Juan segment of the Caribbean Great Arc can be constrained to approximately 120 Ma. This segment was thus obviously active for more than 65 Ma. An orthogonal convergence rate of 15-25 mm/a requires that a minimum amount of 975-1625 km of oceanic crust must have been subducted. Both petrological/geochronological data and numerical simulation underscore the broad spectrum of different P-T-t paths and peak conditions recorded by material subducted at different periods of time as the subduction zone evolved and matured.

Krebs, M.; Maresch, W. V.; Schertl, H.-P.; Münker, C.; Baumann, A.; Draper, G.; Idleman, B.; Trapp, E.

2008-06-01

75

Tectonics of the northernmost Tonga subduction zone and a comparative study of the tectonics of subduction zone terminations  

NASA Astrophysics Data System (ADS)

It is a geometric necessity for a plate to tear where a subduction plate boundary terminates at a transform boundary whose trends are at high angles to each other. The manifestation of these tears is a family of near-vertical faults that trend nearly parallel to the direction of convergence, known as hinge faults, which are characterized by near-vertical dip-slip earthquake focal mechanisms. To better understand the tectonic processes involved in hinge-fault zones, I used earthquake distribution patterns and inversion of long-period body waveforms of 56 moderate-to-large (5.0 ? Ms ? 7.5) events to analyze earthquakes in the highly active northern termination of the Tonga subduction zone at the "Samoa Corner". This inversion generated precise estimates of the focal mechanism parameters, including fault plane strike, dip, and slip, source-time function and focal depth for shallow (h ? 100 km) events in the northern region of the Tonga subduction zone. I find that this region is associated with distinct groups of earthquakes whose mechanisms document tearing of the Pacific Plate lithosphere, back-arc extension in the northern Lau Basin, strike-slip faulting along the Fiji Fracture Zone, and bending and underthrusting of the Pacific Plate at the Tonga trench. In addition, these mechanisms are associate with distinct bathymetric features, such as the Tonga Trench and the Tonga arc. In the second part of this thesis I consider other subduction zones that terminate or sharply change orientation in order to determine the conditions necessary for the existence of hinge-fault-type mechanisms and to compare the different regimes where these mechanisms occur. Using earthquake distribution patterns and Harvard Centroid moment tensor solutions I find that hinge-fault-type mechanisms are clearly evident in the northern subduction termination in Tonga as well as in the northern Scotia, southern Mariana, southern New Hebrides subduction zones. There is more limited evidence for hinge-fault-type mechanisms in the southern Caribbean and western Aleutian subduction zone terminations. In contrast, there is no evidence of hinge-faulting earthquakes in the southern Scotia, northern Caribbean, northern Mariana and eastern Alaska subduction zones. I find that the zones with clear evidence of hinge-fault-type mechanisms are always associated with (1) rapid convergence between the overriding and underthrusting plates, (2) subduction of relatively old (?60 m.y.) oceanic lithosphere, (3) subduction zones with active back-arc spreading, and (4) in areas where orthogonal subduction rapidly changes to orthogonal transform motion. Furthermore, there appears to be a correlation between the length of time since initiation of back-arc spreading and the depth at which hinge-fault-type mechanisms occur. These observations suggest that under normal circumstances oceanic lithosphere will bend and contort aseismically during subduction and that only under the conditions of very high strain rates combined with unusually strong lithosphere, will the plate tearing be manifested by hinge faulting earthquakes.

Millen, David Walter

2000-10-01

76

Gravity anomalies, forearc morphology and seismicity in subduction zones  

NASA Astrophysics Data System (ADS)

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

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

2012-12-01

77

Evolution and diversity of subduction zones controlled by slab width.  

PubMed

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

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

2007-03-15

78

The Global Range of Subduction Zone Thermal Models  

NASA Astrophysics Data System (ADS)

Two-dimensional thermal models provide insight to the dehydration and melting processes that occur in subduction zones. The wide range of slab geometries, ages, convergence velocities and upper plates result in an array of thermal structures that share many common features. We model forty-six arc sections in two dimensions using kinematically-defined slabs based on updated geometries from Syracuse and Abers [2006] to obtain a comprehensive suite of thermal models for the global subduction system. All models feature partial coupling between the slab and the overriding plate directly downdip of the thrust zone, invoked to replicate the cold nose observed in heat flow measurements and seismic attenuation. Four separate cases are tested with four separate sets of assumptions about the causes of the partial coupling: the downdip end of the partial coupling is at a constant depth, it is at constant distance trenchward from the arc, is defined by a critical surface slab temperature, or is adjusted such that the hottest part of the mantle wedge beneath the arc is at a constant temperature for all subduction zones. In all of these models, slabs reach temperatures where the top of the crust and sediments dehydrate before they reach sub-arc depths, and the overlying mantle wedge is too hot for hydrous minerals to be stable at sub-arc depths. The exceptions are cases where subduction is fast and coupling is controlled by a critical slab surface temperature (550°C); these cases also do not produce wedges hot enough to generate primitive arc magmas so probably underestimate temperatures. By contrast, the mantle within the downgoing plate remains cold enough for serpentine to be stable beyond the arc in all but the hottest subduction zones, allowing water to be carried beyond the arc in the slab.

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

2008-12-01

79

Large earthquake processes in the northern Vanuatu subduction zone  

NASA Astrophysics Data System (ADS)

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.

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

2014-12-01

80

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

NASA Astrophysics Data System (ADS)

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

Suarez, G.; Mortera, C.

2013-05-01

81

Water and the Oxidation State of Subduction Zone Magmas  

SciTech Connect

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

Kelley, K.; Cottrell, E

2009-01-01

82

Earthquake mechanisms and active tectonics of the Hellenic subduction zone  

NASA Astrophysics Data System (ADS)

We use improved focal mechanisms and centroid depth estimates of earthquakes, combined with GPS velocities, to examine the tectonics of the Hellenic subduction zone, and in particular the processes occurring at both ends of the Hellenic Arc. Nubia-Aegean convergence is accommodated by shallowly dipping thrust-faulting along the subduction-zone interface, as well as by steeper splay faults in the overriding material. From a comparison of observed and expected seismic moment release over the last 100 yr, combined with existing knowledge of the longer-term documented historical record, we confirm earlier suggestions that most (80 per cent) of this convergence is accommodated aseismically, that is, that the subduction zone is uncoupled. This conclusion is robust, even allowing for rare very large earthquakes on splay faults, such as that of AD 365, and also allowing for the contribution of small earthquakes. The downgoing Nubian plate deforms by arc-parallel contraction at all depths, from 200 km seaward of Crete to at least 100 km within the subducting slab. Extensional (T) axes of earthquakes are aligned downdip within the descending slab suggesting that, even if the aseismic prolongation of the slab has reached the 670 km mantle discontinuity, it does not transmit stresses to shallower depths. Shallow thrust-faulting earthquakes on the subduction interface show a divergence of slip vectors round the arc, and GPS measurements show that this is accommodated mainly by E-W extension on normal faults in the overriding Aegean material. The eastern end of the subduction zone, south of Rhodes, displays distributed deformation in the overriding material, including a mixture of strike-slip and splay-thrust faulting, and probably involves rotations about a vertical axes. Here slip on the interface itself is by thrust faulting with slip vectors oblique to the arc but parallel to the overall Nubia-Aegean convergence: there is no evidence for slip-partitioning in the traditional sense. In the west, the subduction zone terminates in a distributed zone of parallel NE-SW strike-slip faults, of which the most prominent is the Kefalonia Transform Fault (KTF). A flexural gravity anomaly confirms that the deep bathymetric escarpment of the KTF is a lateral ramp, formed as the Ionian islands are emplaced SW onto the Apulian lithosphere, and enhanced by minor thrust faulting with slip vectors perpendicular to the scarp. Distributed parallel strike-slip faults both SW and NE of mainland central Greece terminate in E-W graben in central Greece, which accommodate the overall NE-SW shear by clockwise block rotation. Central Greece therefore acts as a relay zone between the strike-slip faulting of the NE Aegean and the Ionian Islands-western Peloponnese.

Shaw, Beth; Jackson, James

2010-05-01

83

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

USGS Publications Warehouse

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

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

2000-01-01

84

Imaging Hydration and Dehydration of the Costa Rican Subduction Zone based on an amphibious Magnetotelluric Profile  

Microsoft Academic Search

We present an image of the electrical structure of the Costa Rica subduction zone based on an amphibious magnetotelluric (MT) profile. For the first time a large scale amphibious MT data set was evaluated to provide a full electrical image of deep Costa Rican erosional subduction zone. The aim was to image hydration of and subsequent fluid release from the

T. W. Worzewski; M. D. Jegen; L. Muetschard; H. Brasse; H. Kopp; W. Taylor

2009-01-01

85

The role of hydrothermal fluids in the production of subduction zone magmas: Evidence from siderophile and chalcophile trace elements and boron  

NASA Astrophysics Data System (ADS)

In order to evaluate the processes responsible for the enrichments of certain siderophile/ chalcophile trace elements during the production of subduction-related magmas, representative lavas from seven subduction zones have been analyzed for Pb, As, Sb, Sn, W, Mo, Tl, Cu, and Zn by inductively coupled plasma-mass spectrometry (ICP-MS), radiochemical epithermal neutron activation analysis (RENA), and atomic absorption (AA). The siderophile/chalcophile elements are compared to the highly fluid-mobile element B, the light rare earth elements (LREEs), U, and Th in order to place constraints on their behavior in subduction zones. Boron, As, Sb, and Pb are all enriched in arc lavas and continental crustal rocks more so than expected assuming normal magmatic processes (melting and crystallization). Tin, W, and Mo show little evidence of enrichment. Correlations of Pb/Ce, As/Ce, and Sb/Ce with B/La are statistically significant and have high correlation coefficients (and, more importantly, slopes approaching one) suggesting that Pb, As, and Sb behave similarly to B (i.e., that they are fluid-mobile). In addition, across-arc traverses show that B/La, As/Ce, Pb/Ce, and Sb/Ce ratios decrease dramatically with distance towards the back-arc basin. W/Th, Tl/La, Sn/Sm, and Mo/Ce ratios and Cu and Zn concentrations have much less systematic across-arc variations and correlations with B/La are not as strong (and in some cases, not statistically significant) and the regression lines have much lower slopes. Mixing models between upper mantle, slab-derived fluid, and sediment are consistent with a fluid-derived component in the arcs displaying extra enrichments of B, Pb, As, and Sb. These observations imply efficient mobilization of B, Pb, As, Sb, and possibly Tl into arc magma source regions by hydrothermal fluids derived from metamorphic dehydration reactions within the slab. Tin, W, and Mo show little, if any, evidence of hydrothermal mobilization. Copper appears to be slightly enriched in arc lavas relative to mid-ocean ridge basalts (MORBs) whereas Zn contents of arc lavas, MORB, ocean island basalts (OIBs), and continental crustal samples are similar suggesting that the bulk partition coefficient for Zn is approximately equal to one. However, Zn contents of the upper mantle are lower than these reservoirs implying an enrichment of the source region in Zn prior to melting. These nonigneous enrichments have implications not only for arc magma genesis but also for continental crust formation and crust-mantle evolution. The mobility of Pb, As, Sb, and B in hot, reducing, acidic hydrothermal fluids may be greatly enhanced relative to the large-ion lithophile elements (LILEs; including U) as a result of HS -, H 2S, OH -, or other types of complexing. In the case of Pb, continued transport of Pb from subducted slabs into arc magma source regions throughout Earth history coupled with a U fluxing of the mantle a the end of the Archean may account for the depletion of Pb in the upper mantle, the low U/Pb of most arc volcanics and continental crustal rocks, and provide an explanation for the Pb- Paradox (Hofmann et al., 1986;McCulloch, 1993;Miller et al., 1994). Recycled slabs will then retain high U/Pb ratios upon entering the deep mantle and may eventually become incorporated into the source regions of many OIBs; some with HIMU (high 238U/ 204Pb) signatures.

Noll, P. D.; Newsom, H. E.; Leeman, W. P.; Ryan, J. G.

1996-02-01

86

Slab melting versus slab dehydration in subduction-zone magmatism  

PubMed Central

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

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

2011-01-01

87

Interplate coupling along segments of the Central America Subduction zone  

NASA Astrophysics Data System (ADS)

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.

Zarifi, Zoya; Raeesi, Mohammad; Atakan, Kuvvet

2013-04-01

88

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

PubMed

Findings of coesite and microdiamond in metamorphic rocks of supracrustal protolith led to the recognition of continental subduction to mantle depths. The crust-mantle interaction is expected to take place during subduction of the continental crust beneath the subcontinental lithospheric mantle wedge. This is recorded by postcollisional mafic igneous rocks in the Dabie-Sulu orogenic belt and its adjacent continental margin in the North China Block. These rocks exhibit the geochemical inheritance of whole-rock trace elements and Sr-Nd-Pb isotopes as well as zircon U-Pb ages and Hf-O isotopes from felsic melts derived from the subducted continental crust. Reaction of such melts with the overlying wedge peridotite would transfer the crustal signatures to the mantle sources for postcollisional mafic magmatism. Therefore, postcollisonal mafic igneous rocks above continental subduction zones are an analog to arc volcanics above oceanic subduction zones, providing an additional laboratory for the study of crust-mantle interaction at convergent plate margins. PMID:24301173

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

2013-01-01

89

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

PubMed Central

Findings of coesite and microdiamond in metamorphic rocks of supracrustal protolith led to the recognition of continental subduction to mantle depths. The crust-mantle interaction is expected to take place during subduction of the continental crust beneath the subcontinental lithospheric mantle wedge. This is recorded by postcollisional mafic igneous rocks in the Dabie-Sulu orogenic belt and its adjacent continental margin in the North China Block. These rocks exhibit the geochemical inheritance of whole-rock trace elements and Sr-Nd-Pb isotopes as well as zircon U-Pb ages and Hf-O isotopes from felsic melts derived from the subducted continental crust. Reaction of such melts with the overlying wedge peridotite would transfer the crustal signatures to the mantle sources for postcollisional mafic magmatism. Therefore, postcollisonal mafic igneous rocks above continental subduction zones are an analog to arc volcanics above oceanic subduction zones, providing an additional laboratory for the study of crust-mantle interaction at convergent plate margins. PMID:24301173

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

2013-01-01

90

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

USGS Publications Warehouse

Turbidite systems along the continental margin of Cascadia Basin from Vancouver Island, Canada, to Cape Mendocino, California, United States, have been investigated with swath bathymetry; newly collected and archive piston, gravity, kasten, and box cores; and accelerator mass spectrometry radiocarbon dates. The purpose of this study is to test the applicability of the Holocene turbidite record as a paleoseismic record for the Cascadia subduction zone. The Cascadia Basin is an ideal place to develop a turbidite paleoseismologic method and to record paleoearthquakes because (1) a single subduction-zone fault underlies the Cascadia submarine-canyon systems; (2) multiple tributary canyons and a variety of turbidite systems and sedimentary sources exist to use in tests of synchronous turbidite triggering; (3) the Cascadia trench is completely sediment filled, allowing channel systems to trend seaward across the abyssal plain, rather than merging in the trench; (4) the continental shelf is wide, favoring disconnection of Holocene river systems from their largely Pleistocene canyons; and (5) excellent stratigraphic datums, including the Mazama ash and distinguishable sedimentological and faunal changes near the Pleistocene-Holocene boundary, are present for correlating events and anchoring the temporal framework. Multiple tributaries to Cascadia Channel with 50- to 150-km spacing, and a wide variety of other turbidite systems with different sedimentary sources contain 13 post-Mazama-ash and 19 Holocene turbidites. Likely correlative sequences are found in Cascadia Channel, Juan de Fuca Channel off Washington, and Hydrate Ridge slope basin and Astoria Fan off northern and central Oregon. A probable correlative sequence of turbidites is also found in cores on Rogue Apron off southern Oregon. The Hydrate Ridge and Rogue Apron cores also include 12-22 interspersed thinner turbidite beds respectively. We use 14C dates, relative-dating tests at channel confluences, and stratigraphic correlation of turbidites to determine whether turbidites deposited in separate channel systems are correlative - triggered by a common event. In most cases, these tests can separate earthquake-triggered turbidity currents from other possible sources. The 10,000-year turbidite record along the Cascadia margin passes several tests for synchronous triggering and correlates well with the shorter onshore paleoseismic record. The synchroneity of a 10,000-year turbidite-event record for 500 km along the northern half of the Cascadia subduction zone is best explained by paleoseismic triggering by great earthquakes. Similarly, we find a likely synchronous record in southern Cascadia, including correlated additional events along the southern margin. We examine the applicability of other regional triggers, such as storm waves, storm surges, hyperpycnal flows, and teletsunami, specifically for the Cascadia margin. The average age of the oldest turbidite emplacement event in the 10-0-ka series is 9,800±~210 cal yr B.P. and the youngest is 270±~120 cal yr B.P., indistinguishable from the A.D. 1700 (250 cal yr B.P.) Cascadia earthquake. The northern events define a great earthquake recurrence of ~500-530 years. The recurrence times and averages are supported by the thickness of hemipelagic sediment deposited between turbidite beds. The southern Oregon and northern California margins represent at least three segments that include all of the northern ruptures, as well as ~22 thinner turbidites of restricted latitude range that are correlated between multiple sites. At least two northern California sites, Trinidad and Eel Canyon/pools, record additional turbidites, which may be a mix of earthquake and sedimentologically or storm-triggered events, particularly during the early Holocene when a close connection existed between these canyons and associated river systems. The combined stratigraphic correlations, hemipelagic analysis, and 14C framework suggest that the Cascadia margin has three rupture modes: (1) 19-20 full-length or nearly full length ruptures; (

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

2012-01-01

91

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

USGS Publications Warehouse

Ductilely deformed amphibolite facies tectonites comprise two adjacent terranes in east-central Alaska: the northern, structurally higher Taylor Mountain terrane and the southern, structurally lower Lake George subterrane of the Yukon-Tanana terrane. The pressure, temperature, kinematic and age data are interpreted to indicate that the metamorphism of the Taylor Mountain terrane and Lake George subterrane took place during different phases of a latest Palaeozoic through early Mesozoic shortening episode resulting from closure of an ocean basin now represented by klippen of the Seventymile-Slide Mountain terrane. High- to intermediate-pressure metamorphism of the Taylor Mountain terrane took place within a SW-dipping (present-day coordinates) subduction system. High- to intermediate-pressure metamorphism of the Lake George subterrane and the structural contact zone occurred during NW-directed overthrusting of the Taylor Mountain, Seventymile-Slide Mountain and Nisutlin terranes, and imbrication of the continental margin in Jurassic time. -from Authors

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

1995-01-01

92

Stability and dynamics of serpentinite layer in subduction zone  

NASA Astrophysics Data System (ADS)

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

Hilairet, Nadege; Reynard, Bruno

2009-02-01

93

Indications of Fluid Venting Along the Middle American Subduction Zone of Guatemala  

NASA Astrophysics Data System (ADS)

A 2003 multibeam bathymetric survey with R/V SONNE mapped the continental margins of Guatemala and El Salvador revealing a detailed image of the continental slope morphology and of the incoming ocean plate. The continental slope displays three morphotectonic units that roughly correspond to upper, middle and lower slope. The upper slope is characterized by numerous small canyons and gullies that begin at the eroded shelf edge high. The middle slope is characterized by a rough terrain of variable width and dip sculptured by pervasive normal faults and mass wasting. The lower slope is formed by en echelon terraces striking parallel to the rough terrain of the incoming plate that mimicks the half graben morphology of the underthrusting plate. The incoming plate morphology, inherited from seafloor spreading processes, is reactivated by bending of the oceanic lithosphere into the subduction zone. Widespread fluid venting has been reported along the adjacent Middle America Margin of Costa Rica and Nicaragua, and was expected off Guatemala and El Salvador. Typical manifestations of fluid venting, such as pockmarks, mud mounds, mud volcanoes or seeps, are difficult to document with swath mapping surveys if only the bathymetry is taken into account, as the size of these features are in the order of the footprint size of one acoustic beam. The backscatter signal, however, resolves different reflectivity of the seafloor and thus gives information on lithology. Fluid venting is often associated with the formation of carbonate structures characterized by high acoustic reflectivity. A detailed analysis of the backscatter data, compiled into a mosaic of the area, revealed several small topographic features (mounds) at mid slope depth characterized by high backscatter indicating carbonate precipitation and thus fluid venting. Several small pinnacles in the trench, showing a mound-like shape and very high backscatter might also indicate fluid venting structures associated with oceanic plate tectonism.

Weinrebe, W.; Ranero, C. R.; Grevemeyer, I.; Vannucchi, P.; von Huene, R.

2005-12-01

94

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

PubMed

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

Calvert, Andrew J

2004-03-11

95

Permeability-Porosity Relationships of Subduction Zone Sediments  

NASA Astrophysics Data System (ADS)

Permeability-porosity relationships for sediments from Northern Barbados, Costa Rica, Nankai, and Peru subduction zones were examined based on their sediment type and grain size distribution. Greater correlation was observed between permeability and porosity for siliciclastic sediments, diatom oozes, and nannofossil chalk than for nannofossil oozes. For siliciclastic sediments, grouping of sediments by clay content yields relationships that are generally consistent with results from other marine settings and suggest decreasing permeability for a given porosity as clay content increases. Correction of measured porosities for smectite content generally improves the quality of permeability-porosity relationships. The relationship between permeability and porosity for diatom oozes may be controlled by the amount of clay present in the ooze, causing diatom oozes to behave similarly to siliciclastic sediments. For a given porosity the nannofossil oozes have higher permeability values by 1.5 orders of magnitude than the siliciclastic sediments. However, the use of a permeability-porosity relation may not be appropriate for unconsolidated carbonates such as nannofossil oozes. This study provided insight to the effects of porosity correction for smectite, variations in lithology and grain size in permeability-porosity relationships. However, further progress in delineating controls on permeability will require more careful and better documented permeability tests on characterized samples.

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

2008-12-01

96

Downgoing plate controls on overriding plate deformation in subduction zones  

NASA Astrophysics Data System (ADS)

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.

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

2014-05-01

97

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

NASA Astrophysics Data System (ADS)

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

Métois, Marianne; Vigny, Christophe; Socquet, Anne

2014-05-01

98

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

USGS Publications Warehouse

In April and May 1996, a geophysical study of the Cascadia continental margin off Oregon and Washington was conducted aboard the German R/V Sonne. This cooperative experiment by GEOMAR and the USGS acquired wide-angle reflection and refraction seismic data, using ocean-bottom seismometers (OBS) and hydrophones (OBH), and multichannel seismic reflection (MCS) data. The main goal of this experiment was to investigate the internal structure and associated earthquake hazard of the Cascadia subduction zone and to image the downgoing plate. Coincident MCS and wide-angle profiles along two tracks are presented here. The plate boundary has been imaged precisely beneath the wide accretionary wedge close to shore at c13km depth. Thus, the downgoing plate dips more shallowly than previously assumed. The dip of the plate changes from 2?? to 4?? at the eastern boundary of the wedge on the northern profile, whereas approximately 3km of sediment is entering the subduction zone. On the southern profile, where the incoming sedimentary section is about 2.2km thick, the plate dips about 0.5?? to 1.5?? near the deformation front and increases to 3.5?? further landwards. On both profiles, the deformation of the accretionary wedge has produced six ridges on the seafloor, three of which represent active faulting, as indicated by growth folding. The ridges are bordered by landward verging faults which reach as deep as the top of the oceanic basement. Thus, the entire incoming sediment package is being accreted. At least two phases of accretion are evident, and the rocks of the older accretionary phase(s) forms the backstop for the younger phase, which started around 1.5 Ma ago. This documents that the 30 to 50km wide frontal part of the accretionary wedge, which is characterized by landward vergent thrusts, is a Pleistocene feature which was formed in response to the high input of sediment building the fans during glacial periods. Velocities increase quite rapidly within the wedge, both landward and downward. At the toe of the deformation front, velocities are higher than 4.0 km/s, indicating extensive dewatering of deep, oceanic sediment. Further landward, considerable velocity variation is found, which indicates major breaks throughout the accretionary history.

Flueh, E.R.; Fisher, M.A.; Bialas, J.; Childs, J. R.; Klaeschen, D.; Kukowski, N.; Parsons, T.; Scholl, D. W.; ten Brink, U.; Trehu, A.M.; Vidal, N.

1998-01-01

99

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

NASA Astrophysics Data System (ADS)

In active subduction zones, when the converging plates cannot slip freely past each other, "plate coupling" occurs. The moving subducting slab and therefore the coupling/decoupling relationship between plates control both short- and long-term deformation of the upper plate. Short-term deformation is dominantly elastic, occurs at human timescales and can be directly associated with earthquakes. Long-term deformation is cumulative, permanent and prevails at the geological timescale (Hoffman-Rothe et al., 2006, Springer Berlin Heidelberg). Here we used 3D numerical simulations to test oblique subduction zones and to investigate: 1) how long-term deformation and coupling relationship vary along the trench-axis; 2) how this relationship influences erosion and down-drag of upper plate material. Our models are based on thermo-mechanical equations solved with finite differences method and marker-in-cell techniques combined with a multigrid approach (Gerya, 2010, Cambridge Univ. Press). The reference model simulates an intraoceanic subduction close to the continental margin (Malatesta et al., 2013, Nature Communications, 4:2456 DOI:10.1038/ncomms3456). The oceanic crust is layered with a 5-km-thick layer of gabbro overlain by a 3-km-thick layer of basalt. The ocean floor is covered by 1-km-thick sediments. Plates move with a total velocity of 3.15 cm/yr; the oblique convergence is obtained using velocity vectors that form an angle of 45° with the initial starting point of subduction (weak zone in the lithosphere). After initiation of plate convergence, part of sediments on top of the incoming plate enters the subduction zone and is buried; another part is suddenly transferred along strike at shallow depths and along the subducting slab according to the direction of the along-trench velocity component of subduction. The lateral migration of sediment causes the evolution of the trench along its strike from sediment-poor to sediment-rich. As soon as subduction starts, where the sedimentary infill of the trench is almost nonexistent, short-term shallow coupling occurs and friction between the frontal sector of the overriding plate and the downgoing plate triggers upper-plate bending. In this sector, after the early short-term coupling, the overriding plate is hereafter decoupled from the subducting slab. Moving along trench-strike, where sediments amount increases, the upper plate couples with the subducting plate and is dragged coherently downwards. If a large amount of sediments is stored in the trench the overriding plate is scraped off and incorporated as fragments along the plate interface. Our results suggest that a) one main parameter controlling coupling at convergent plate margins is the occurrence and the amount of sediment at the trench; b) the upper plate margin is dragged to depth or destroyed only where sediments thickness at the trench is large enough to promote interplate coupling, suggesting that a variation of sediment amount along the trench-axis influences the amount and style of transport of upper-plate material in the mantle.

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

2014-05-01

100

Heat flow distribution and thermal structure of the Nankai subduction zone off the Kii Peninsula  

Microsoft Academic Search

Detailed heat flow surveys were carried out in the Nankai subduction zoneSubsurface thermal structure was calculated based on the obtained heat flow dataFrictional heating along the subduction plate interface is estimated to be small

Hideki Hamamoto; Makoto Yamano; Shusaku Goto; Masataka Kinoshita; Keiko Fujino; Kelin Wang

2011-01-01

101

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

Microsoft Academic Search

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

George L. Choy; Stephen H. Kirby

2004-01-01

102

CASA GPS Results: Lithospheric Deformation and the Earthquake Cycle in the Ecuador-Colombia Subduction Zone  

Microsoft Academic Search

The Ecuador-Colombia subduction zone is a potentially important laboratory for observing the effects of variations in lithospheric buoyancy on seismic coupling. Subduction of the aseismic Carnegie Ridge is associated with pronounced margin-parallel variations in crustal thickness and buoyancy of the downgoing lithosphere. The subduction zone has been ruptured repeatedly in 1906 (Mw = 8.8), 1942 (Mw = 7.9), 1958 (Mw

J. Kellogg

2003-01-01

103

A sulfur isotope perspective of fluid transport across subduction zones  

NASA Astrophysics Data System (ADS)

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

Shimizu, N.; Mandeville, C. W.

2011-12-01

104

Controls on the Migration of Fluids in Subduction Zones  

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

105

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

NASA Astrophysics Data System (ADS)

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

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

2012-04-01

106

Rheology of magnesite and implications for subduction zone dynamics  

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

107

Sources of Tsunami and Tsunamigenic Earthquakes in Subduction Zones  

NASA Astrophysics Data System (ADS)

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

Satake, K.; Tanioka, Y.

108

Multiscale seismic imaging of the Western-Pacific subduction zone  

NASA Astrophysics Data System (ADS)

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

Zhao, D.

2011-12-01

109

Mélange Formation, Mantle-Wedge Diapirs And Subduction Zone Magmatism  

NASA Astrophysics Data System (ADS)

Components derived from the subducting slab contribute to the source region of magmas produced at convergent plate margins. The characteristic range of compositions of these magmas is commonly attributed to three-component mixing in the source regions of these magmas: hydrous fluids derived from subducted altered oceanic crust and components derived from the thin sedimentary veneer are added to the depleted peridotite in the mantle wedge, which subsequently undergoes melting. However, the physical processes of transport and mixing of these components are largely enigmatic. In this presentation, we outline an integrated physico-chemical model of subduction zones: mélange formation at the slab-mantle interface is considered the physical mixing process that is responsible for the geochemical three-component pattern of arc magmas. Low-density mantle-wedge diapirs or plumes transport the well-mixed materials into the hot corner of the mantle wedge beneath arcs, where melt is produced by heating and decompression of the hydrous, low-density mélange plumes. Studies of exhumed subduction mélanges suggest that hybrid rocks with newly grown minerals concentrate, sequester and redistribute water and key trace elements. The strong petrologic and chemical contrast at the slab-mantle interface, produces these hybrid rock compositions by metasomatic reactions, diffusion and mechanical mixing: the Al-, Si- and alkali-rich slab that carries crustal isotopic signatures and trace-element abundances is juxtaposed with the Mg-rich ultramafic rocks of the harzburgitic mantle. Mechanical mixing of crustal and mantle rocks will propagate the formation of hybrid rocks, and fluxing by hydrous fluids derived from the dehydrating slab will enhance reactivity and lead to fluid saturation of the newly formed rocks. The rise of low-density plumes in the mantle wedge provides a mechanism to transport buoyant hybrid rocks from the slab-mantle interface toward the source region of arc magmas. Mélange rocks rising into the mantle wedge in 'wet' diapirs would be subjected to P-T conditions dramatically different from those at the slab surface. Partial melting of hybrid rocks may produce the large range of major and trace-element compositions found in modern island arc volcanic rocks.

Schumacher, J. C.; Marschall, H. R.

2012-12-01

110

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

USGS Publications Warehouse

The location and timing of metamorphic reactions in subducting lithosph??re are influenced by thermal effects of fluid circulation in the ocean crust aquifer. Fluid circulation in subducting crust extracts heat from the Nankai subduction zone, causing the crust to pass through cooler metamorphic faci??s than if no fluid circulation occurs. This fluid circulation shifts the basalt-to-eclogite transition and the associated slab dehydration 14 km deeper (35 km farther landward) than would be predicted with no fluid flow. For most subduction zones, hydrothermal cooling of the subducting slab will delay eclogitization relative to estimates made without considering fluid circulation. Copyright 2009 by the American Geophysical Union.

Spinelli, G.A.; Wang, K.

2009-01-01

111

Diagenesis of volcanic ashes in the Central American subduction zone: an experimental and field study approach  

NASA Astrophysics Data System (ADS)

There is a significant interest in the interaction between volcanic glass and seawater at the Nicaraguan and Costa Rican fore arc sediments, because of the extensive volcanism that takes places along the volcanic chain of Nicaragua and Costa Rica. Large effort has been focused on understanding dissolution rates and alteration mechanisms of basaltic glass [e.g. Berger et al., 1987, Staudigel &Hart, 1983]. In contrast, only few studies investigating alteration rates and mechanisms of intermediate or felsic ashes [e.g. de la Fuente et al. 2000], which play a major role in the Nicaraguan volcanism, were performed. Volcanic arc ashes from Nicaragua of intermediate and basaltic origin, have been reacted with seawater in a temperature (20-100°C) and pressure (1-200 bar) controlled reaction chamber. Element ratios of major elements (e.g. Ca, Mg, K, Ba) between fluid and solid phase as well as alkalinity and pH variations with time were determined. Experimental results were compared with ash layers and related pore water data from continental slope off Nicaragua, collected during research cruise with RV Meteor (M54/2). First results show clearly different dissolution behavior of ashes of various chemical composition and a significant increase of dissolution rates with increasing pressure and temperature. Future studies, also performed on volcanic ashes of Costa Rica, comparing field data and experimental data will improve our understanding of volcanic material input and output in the Central America subduction zone. References: Berger et al., 1987: Fundamental processes controlling the first stage of alteration of a basalt glass by seawater: an experimental study between 200° and 320°C, Earth and Planetary Science Letters 84, 431-455 de la Fuente et al., 2000: Electron microscopy of volcanic tuff alteration to illite-smectite under hydrothermal conditions, Clays and Clay Minerals 48/3, 339-350 Staudigel &Hart, 1983: Alteration of basaltic glass: Mechanisms and sigificance for oceanic crust-seawater budget Geochimica et Cosmochimica Acta 47, 337-350

Schacht, U.; Kutterolf, S.; Hensen, C.; Schmidt, M.; Wallmann, K.

2003-04-01

112

Multiple garnet growth in garnet-kyanite-staurolite gneiss, Pangong metamorphic complex, Ladakh Himalaya: New constraints on tectonic setting  

NASA Astrophysics Data System (ADS)

Garnet-kyanite-staurolite gneiss in the Pangong complex, Ladakh Himalaya, contains porphyroblastic euhedral garnets, blades of kyanite and resorbed staurolite surrounded by a fine-grained muscovite-biotite matrix associated with a leucogranite layer. Sillimanite is absent. The gneiss contains two generations of garnet in cores and rims that represent two stages of metamorphism. Garnet cores are extremely rich in Mn (XSps = 0.35-0.38) and poor in Fe (XAlm = 0.40-0.45), whereas rims are relatively Mn-poor (XSps = 0.07-0.08), and rich in Fe (XAlm = 0.75-0.77). We suggest that garnet cores formed during prograde metamorphism in a subduction zone followed by abrupt exhumation, during early collision of the Ladakh arc and Karakoram block. The subsequent India-Asia continental collision subducted the metamorphic rocks to a mid-crustal level, where the garnet rims overgrew the Mn-rich cores at ca. 680 °C and ca. 8.5 kbar. PT calculations were estimated from phase diagrams calculated using a calculated bulk chemical composition in the Mn-NCKFMASHT system for the garnet-kyanite-staurolite-bearing assemblage. Muscovites from the metamorphic rocks and associated leucogranites have consistent K-Ar ages (ca. 10 Ma), closely related to activation of the Karakoram fault in the Pangong metamorphic complex. These ages indicate the contemporaneity of the exhumation of the metamorphic rocks and the cooling of the leucogranites.

Thanh, N. X.; Sajeev, K.; Itaya, T.; Windley, B. F.

2011-12-01

113

Dehydrated fluid and seismic deformation in deep subduction zone  

NASA Astrophysics Data System (ADS)

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

Okamoto, K.

2013-12-01

114

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

USGS Publications Warehouse

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

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

1993-01-01

115

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

NASA Astrophysics Data System (ADS)

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

Liu, Y.; Rubin, A. M.

2009-12-01

116

Serpentine rheology and its significance on subduction zone processes  

NASA Astrophysics Data System (ADS)

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

Katayama, I.; Hirauchi, K.

2011-12-01

117

Metamorphism and melting of picritic crust in the early Earth  

NASA Astrophysics Data System (ADS)

Partial melting experiments with models of Archean oceanic crust (MAOC; with 11, 13 and 15 wt.% MgO) are used to investigate the role of metamorphism and melting of primary picritic compositions in the formation of TTG-like melts and continental crust on the early Earth. The approach investigates the possibility that the average early crust composition was comparatively MgO-rich and evolved to lower magnesium content during the secular cooling of the Earth. High-pressure partial melting experiments indicate a transition of melt compositions from aluminous basaltic melts in MAOC 15 to predominantly tonalitic melts in MAOC 11 and higher melting temperatures with increasing magnesium in the bulk composition. Tonalitic melts were generated in MAOC 11 and 13 at pressures ? 12.5 kbar along with the residual phases garnet + clinopyroxene + plagioclase ± quartz (± orthopyroxene in the presence of quartz and at lower pressures) in the absence of amphibole. Basaltic melts were generated at pressures ? 15 kbar predominantly in the presence of granulite facies residues such as amphibole ± garnet ± plagioclase + orthopyroxene that lack quartz in all MAOC compositions. The tonalitic melts generated in MAOC 11 and 13 indicate that thicker oceanic crust with more magnesium than that of a modern MORB is a viable source for the generation of early Archean high-Si, low-MgO melts, and therefore TTG-like continental crust in the Archean. The favoured settings for the generation of these melts at pressures up to 15 kbar are the base of oceanic crust much thicker than today or melting of slabs in shallow subduction zones, both without interaction of the melts with the mantle during passage to the surface. Tonalitic melts may have formed in deeper subduction zones at 20 kbar beyond plagioclase stability but it is unlikely that such melts could migrate to shallower levels without further mantle interaction. This process may have become more important during the progressive cooling of the Earth.

Ziaja, Karen; Foley, Stephen F.; White, Richard W.; Buhre, Stephan

2014-02-01

118

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

NASA Astrophysics Data System (ADS)

The interseismic deformation associated with plate coupling at a subduction zone is commonly simulated by the steady-slip model in which a reverse dip-slip is imposed on the down-dip extension of the locked plate interface, or by the backslip model in which a normal slip is imposed on the locked plate interface. It is found that these two models, although totally different in principle, produce similar patterns for the vertical deformation at a subduction zone. This suggests that it is almost impossible to distinguish between these two models by analysing only the interseismic vertical deformation observed at a subduction zone. The steady-slip model cannot correctly predict the horizontal deformation associated with plate coupling at a subduction zone, a fact that is proved by both the numerical modelling in this study and the GPS (Global Positioning System) observations near the Nankai trough, southwest Japan. It is therefore inadequate to simulate the effect of the plate coupling at a subduction zone by the steady-slip model. It is also revealed that the unphysical assumption inherent in the backslip model of imposing a normal slip on the locked plate interface makes it impossible to predict correctly the horizontal motion of the subducted plate and the stress change within the overthrust zone associated with the plate coupling during interseismic stages. If the analysis made in this work is proved to be correct, some of the previous studies on interpreting the interseismic deformation observed at several subduction zones based on these two models might need substantial revision. On the basis of the investigations on plate interaction at subduction zones made using the finite element method and the kinematic/mechanical conditions of the plate coupling implied by the present plate tectonics, a synthesized model is proposed to simulate the kinematic effect of the plate interaction during interseismic stages. A numerical analysis shows that the proposed model, designed to simulate the motion of a subducted slab, can correctly produce the deformation and the main pattern of stress concentration associated with plate coupling at a subduction zone. The validity of the synthesized model is examined and partially verified by analysing the horizontal deformation observed by GPS near the Nankai trough, southwest Japan.

Zhao, Shaorong; Takemoto, Shuzo

2000-08-01

119

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

Microsoft Academic Search

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

L. McNeill; T. Henstock; D. Tappin

2005-01-01

120

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

NASA Astrophysics Data System (ADS)

Recent knowledge of the great geometric and dynamic complexity insubduction zones, combined with new capacity for analogue mechanical and numerical modeling has sparked a number of studies on subduction processes. Not unexpectedly, such models reveal a complex relation between physical conditions during subduction initiation, strength profile of the subducting plate, the thermo-dynamic conditions and the subduction zones geometries. One rare geometrical complexity of subduction that remains particularly controversial, is the potential for polarity shift in subduction systems. The present experiments were therefore performed to explore the influence of the architecture, strength and strain velocity on complexities in subduction zones, focusing on along-strike variation of the collision zone. Of particular concern were the consequences for the geometry and kinematics of the transition zones between segments of contrasting subduction direction. Although the model design to some extent was inspired by the configuration along the Iberian - Eurasian suture zone, the results are also of significance for other orogens with complex along-strike geometries. The experiments were set up to explore the initial state of subduction only, and were accordingly terminated before slab subduction occurred. The model wasbuilt from layers of silicone putty and sand, tailored to simulate the assumed lithospheric geometries and strength-viscosity profiles along the plate boundary zone prior to contraction, and comprises two 'continental' plates separated by a thinner 'oceanic' plate that represents the narrow seaway. The experiment floats on a substrate of sodiumpolytungstate, representing mantle. 24 experimental runs were performed, varying the thickness (and thus strength) of the upper mantle lithosphere, as well as the strain rate. Keeping all other parameters identical for each experiment, the models were shortened by a computer-controlled jackscrew while time-lapse images were recorded. After completion, the models were saturated with water and frozen, allowing for sectioning and profile inspection. The experiments were invariably characterized by different along-strike patterns of deformation, so that three distinct structural domains could be distinguished in all cases. Model descriptions are subdivided accordingly, including domain CC, simulating a continent-continent collision, domain OC, characterized by continent-ocean-continent collision and domain T, representing the transition zone between domain CC and domain OC. The latter zone varied in width and complexity depending on the contrast in structural style developed in the two other domains; in cases where domain OC developed very differently from domain CC, the transition zone was generally wider and more complex. A typical experiment displayed the following features and strain history: In domain CC two principal thrust sheets are displayed, which obviously developed in an in-sequence foreland-directed fashion. The lowermost detachment nucleated at the base of the High Strength Lithospheric Mantle analogue, whereas the uppermost thrust was anchored within the "lower crust". The two thrusts operated in concert, the surface trace of the deepest dominating in the west, and the shallowest in the east. The kinematic development of domain CC could be subdivided into four stages, including initiation of a symmetrical anticline with a minute amplitude and situated directly above the velocity discontinuity defined by the plate contact (stage 1), contemporaneous development of the two thrusts (stage 2) and an associated asymmetrical anticline (stage 3) with a central collapse graben in the latest phase (stage 4). It is noted that the segment CC as seen in a clear majority of the experiments followed this pattern of development. In contrast, the configuration of domain OC displayed greater variation, and included north and south-directed subduction, folding, growth of pop-up-structures and triangle zones. In the "ocean crust" domain, stage 1 was characterized by the growt

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

2012-04-01

121

Interseismic Strain Along the Sumatra Subduction Zone: A Case for a Locked Fault Portion Extending Well Below the Forearc Moho  

Microsoft Academic Search

A current and most accepted view about the seismogenic zone along subduction zones is that the downdip extent of the locked fault portion would correspond either to the 350° C isotherm if this temperature is reached above the Moho, or to the intersection with the forearc Moho for colder subduction zones [Oleskevich et al., 1999]. This limit would reflect the

M. Simoes; J. Avouac; R. Cattin; P. Henry; D. H. Natawidjaja

2003-01-01

122

Inversions of Geodetic Data for Rotation Pole and Deformation Mechanism at Subduction Zones: Applications to Cascadia, Sumatra, and Alaska  

Microsoft Academic Search

We use horizontal and vertical geodetic data to perform linear inversions at the Cascadia subduction zone in Washington and Oregon, the Kenai Peninsula in Alaska, and the Sumatra subduction zone. The total velocity field is viewed as the superposition of plate convergence deformation and rigid block motion, and we use a linear inversion method that simultaneously solves for both of

C. A. Williams; R. McCaffrey

2001-01-01

123

SOURCE MODELING OF SUBDUCTION-ZONE EARTHQUAKES FOR LONG-PERIOD GROUND MOTION VALIDATION  

Microsoft Academic Search

The national seismic hazard map for Japan indicates 30-year probability in the Tokyo metropolitan area to be controlled by megathrust earthquakes along the Sagami and Nankai troughs of the Philippine Sea plate. This indicates that source modeling and realistic ground motion prediction for distant subduction-zone earthquakes are quite important. We have proposed two types of source modeling for four major

Hiroe Miyake; Kazuki Koketsu

124

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

NASA Astrophysics Data System (ADS)

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.

Ujiie, Kohtaro; Kimura, Gaku

2014-12-01

125

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

NASA Astrophysics Data System (ADS)

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.

Holm, N. G.

2010-04-01

126

Nonvolcanic tremor along the Oaxaca segment of the Middle America subduction zone  

Microsoft Academic Search

The Oaxaca subduction zone is an ideal area for detailed studies of plate boundary deformation as rapid convergent rates, shallow subduction, and short trench-to-coast distances bring the thermally defined seismogenic and transition zones of the plate interface over 100 km inland. Previous analysis of slow slip events in southern Mexico suggests that they may represent motion in the transition zone,

Michael R. Brudzinski; Héctor R. Hinojosa-Prieto; Kristen M. Schlanser; Enrique Cabral-Cano; Alejandra Arciniega-Ceballos; Oscar Diaz-Molina; Charles DeMets

2010-01-01

127

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

E-print Network

Stress drop as a criterion to differentiate subduction zones where Mw 9 earthquakes can occur segment in which at least one Mw 9 earthquake has occurred, and class 3: earthquakes in a sub- duction zone segment in which no Mw 9 earthquake has occurred. A total of 53 earthquakes are analyzed

Seno, Tetsuzo

128

Earth Planets Space, 62, 665673, 2010 Construction of semi-dynamic model of subduction zone  

E-print Network

Earth Planets Space, 62, 665­673, 2010 Construction of semi-dynamic model of subduction zone with given plate kinematics in 3D sphere M. Morishige1 , S. Honda1 , and P. J. Tackley2 1Earthquake Research; Kneller and van Keken, 2007, 2008). The other is to calcu- late slab movement more or less dynamically (e

Tackley, Paul J.

129

Long-term seismogenic process for major earthquakes in subduction zones  

Microsoft Academic Search

A qualitative physical process for the long-term seismogenesis of major earthquakes in subduction zones is proposed on the basis of quantitative empirical evidence that swarms, mainshocks and aftershocks are closely related phenomena. The relations, which have been identified in the comprehensive, long-term catalogues of New Zealand and Japan, represent swarms as predictors of mainshocks with respect to location, time and

F. F Evison; D. A Rhoades

1998-01-01

130

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

E-print Network

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

Simons, Mark

131

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

E-print Network

Subduction zone coupling and tectonic block rotations in the North Island, New Zealand Laura M field in the North Island of New Zealand is dominated by the long- term tectonic rotation of the eastern and tectonic block rotations in the North Island, New Zealand, J. Geophys. Res., 109, B12406, doi:10

McCaffrey, Robert

132

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

E-print Network

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

Billen, Magali I.

133

Scaling Relationships for Slow Slip Events and Tremor in Subduction Zones  

Microsoft Academic Search

Global compilations of faulting parameters for slow slip phenomena in subduction zones reveal that event duration is proportional to seismic moment rather than its cube-root, as for earthquakes (Schwartz and Rokosky, 2007; Ide et al., 2007). Ide et al. (2007) proposed two different models consistent with this scale dependent behavior that either assume direct proportionality between fault slip (d) and

S. Y. Schwartz; J. M. Rokosky; K. Obara

2007-01-01

134

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

Microsoft Academic Search

Earthquake and tsunami hazard for northern California and southern Oregon is dominated by estimates of recurrence for earthquakes on the Cascadia subduction zone (CSZ) and upper plate thrust faults. Recurrence interval (RI)estimates derived from site based terrestrial data (270-1,370 years) are inconsistent with the regional marine record of great earthquakes (RI = ~240 years). Reconciling these differences reveals information regarding

T. H. Leroy; J. R. Patton

2010-01-01

135

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

NASA Astrophysics Data System (ADS)

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

Nishikawa, T.; Ide, S.

2013-12-01

136

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

USGS Publications Warehouse

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

Masterlark, T.

2003-01-01

137

Guatemala jadeitites and albitites were formed by deuterium-rich serpentinizing fluids deep within a subduction zone  

USGS Publications Warehouse

Jadeitites and albitites from the Motagua Valley, Guatemala, are high-pressure-low-temperature metasomatic rocks that occur as tectonic inclusions in serpentinite-matrix melange. Metasomatism was driven by a fluid with a ??18OH2O value of 6???, and a ??DH2O value that is high in comparison with metamorphic fluids at other high-pressure-low-temperature localities of similar grade. We infer that the fluid was originally seawater that was entrained during subduction either as mineral-bound H2O or as free pore waters. The fluid drove serpentinization reactions in ultramafic rocks, possibly leading to deuterium enrichment of H2O, prior to forming the Jadeitites and albitites at a depth of 29 ?? 11 km. There are isotopic and fluid-inclusion similarities to rodingites, which are Ca-rich metasomatites found at other serpentinite localities. Our results suggest that the serpentinization process, whether it occurs within subduction zones or on the flanks of oceanic spreading ridges, may produce residual fluids that are H2O rich, have 1-8 wt% equivalent NaCl, and have high, perhaps sea water-like, ??D values.

Johnson, C.A.; Harlow, G.E.

1999-01-01

138

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

NASA Astrophysics Data System (ADS)

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

Bottrill, A. D.; van Hunen, J.; Cuthbert, S. J.; Brueckner, H. K.; Allen, M. B.

2014-05-01

139

Metamorphic rocks of the Yap arc-trench system  

Microsoft Academic Search

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

J. Hawkins; R. Batiza

1977-01-01

140

Anatexis and metamorphism in tectonically thickened continental crust exemplified by the Sevier hinterland, western North America  

NASA Technical Reports Server (NTRS)

This paper presents a thermal and petrologic model of anatexis and metamorphism in regions of crustal thickening exemplified by the Sevier hinterland in western North America, and uses the model to examine the geological and physical processes leading to crustally derived magmatism. The results of numerical experiments show that anatexis was an inevitable end-product of Barrovian metamorphism in the thickened crust of the late Mesozoic Sevier orogenic belt and that the advection of heat across the lithosphere, in the form of mantle-derived mafic magmas, was not required for melting of metasedimentary rocks. It is suggested that, in the Sevier belt, as in other intracontinental orogenic belts, anatexis occurred in the midcrust and not at the base of the crust.

Patino Douce, Alberto E.; Humphreys, Eugene D.; Johnston, A. Dana

1990-01-01

141

Elevation of volcanoes and their edifice heights at subduction zones  

SciTech Connect

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

Ben-Avraham, Z.; Nur, A.

1980-08-10

142

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

NASA Astrophysics Data System (ADS)

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

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

2006-12-01

143

High-resolution seismic imaging of the western Hellenic subduction zone using teleseismic scattered waves  

NASA Astrophysics Data System (ADS)

The active Hellenic subduction system has long been considered an ideal setting for studying subduction dynamics because it is easily accessible and of limited spatial extent. It has been the focus of numerous seismological studies over the last few decades but, nonetheless, the detailed structure of both the slab and the surrounding mantle remain poorly constrained in an intermediate depth range from 30 to 150 km. The objective of this paper is to fill this gap. The intermediate depth regime is of particular interest because it is pivotal for improving our understanding of the dynamic interaction between subducting lithosphere and the surrounding mantle. An interdisciplinary effort aimed at addressing this challenge is currently undertaken by the `Multidisciplinary Experiments for Dynamic Understanding of Subduction under the Aegean Sea' (MEDUSA) project. As part of the MEDUSA initiative, a temporary array consisting of 40 densely spaced broad-band seismometers from the IRIS-PASSCAL pool has been deployed in southern Greece. We process the teleseismic data recorded by this array with a migration algorithm based on the generalized radon transform to obtain high-resolution images of the subduction zone in 2-D. The images reveal a sharp Mohorovi?i? discontinuity (Moho) at depths ranging from 30 km beneath the western margin of the Aegean Sea to 40 km beneath the central Peloponnesus, where it outlines the crustal root of the Hellenides. To the west of the Hellenides, the continental Moho is not identified, but we interpret a pronounced discontinuity imaged at ~20 km depth as the contact between low-velocity sediments and high-velocity crystalline basement. The images also show the subducted oceanic crust as a low-velocity layer that plunges at a constant angle of 21° from west to east. The oceanic crust exhibits low velocities to at least 90 km depth, indicating that the bulk of fluid transfer from the subducted slab into the mantle wedge occurs below this depth. A detailed comparison of images constructed for distinct backazimuthal illuminations reveals deviations in the geometry of the subducted slab. These deviations are attributed to structural and/or compositional changes taking place directly to the north of the MEDUSA array, and are consistent with the existence of a slab tear beneath the Central Hellenic Shear Zone.

Suckale, J.; Rondenay, S.; Sachpazi, M.; Charalampakis, M.; Hosa, A.; Royden, L. H.

2009-08-01

144

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

NASA Astrophysics Data System (ADS)

Knowledge of convergent margin systems evolved from hypothesis testing with marine geophysical technology that improved over decades. Wegener's drift hypothesis, Holmes mantle convection, and marine magnetic anomaly patterns were integrated into an ocean spreading concept that won wide acceptance after ocean drilling confirmed the crustal younging trend toward the Mid-Atlantic ridge. In contrast, the necessary disposal of oceanic and trench sediment at convergent margins remained largely hypothetical. Fresh interpretations of some coastal mountains as exposing ancient convergent margin rock assemblages and the seismologist's "Wadati-Benioff" zone were combined into a widely-accepted hypothesis. A convergent margin upper plate was pictured as an imbricate fan of ocean sediment thrust slices detached from the lower plate. During the 1980s ocean drilling to test the hypothesis revealed what then were counter-intuitive processes of sediment subduction and subduction erosion. Rather than the proposed seaward growth by accretion, many margins had lost material from erosion. In current concepts, individual margins are shaped by the net consequences of subduction accretion, sediment subduction, and subduction erosion. Similarly, recently acquired age data from ancient subduction complexes reveal periods dominated by accretion separated by periods dominated by tectonic erosion. Globally, the recycling of continental crustal material at subduction zones appears largely balanced by magmatic addition at volcanic arcs. The longevity of the original imbricate fan model in text books confirms its pictorial simplicity, because geophysical images and drill core evidence show that it commonly applies to only a relatively small frontal prism. A better understanding of convergent margin dynamics is of urgent societal importance as coastal populations increase rapidly and as recent disastrous earthquakes and tsunamis verify. The shift in convergent margin concepts has developed through 50 years of improved acquisition techniques, analysis capabilities, and imaging that greatly improves resolution in observational data. The first observatories in boreholes that place instruments closer to earthquake rupture zones have been deployed. Technological improvement should be strongly pursued to meet future challenges. Advanced seismic imaging and the new riser drilling vessel Chikyu are tools to significantly advance understanding of earthquake mechanics but availability is restricted by current global science budgets. The present scientific knowledge leaves great earthquakes and tsunamis an unpredictable "stealth" natural hazard of great proportions.

von Huene, Roland

2010-05-01

145

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

NASA Astrophysics Data System (ADS)

While the northern and central part of the South American Subduction zone has been intensively studied the southern part has attracted less attention, which might be caused by its difficult accessibility and lower seismic activity. However, the southern part exhibits strong seismic and tsunamogenic potential with the prominent example of the Mw=9.5~May~22, 1960 Valdivia earthquake. Here we present data from an amphibious seismic array (Project TIPTEQ) located between 41.5--43.5°S reaching from the trench to the active magmatic arc incorporating the Island of Chiloé and the magmatic arc with the N-S trending Liquiñe-Ofqui fault zone~(LOFZ). 364 local events were observed in a 11-month period from November 2004 until October 2005 with magnitudes between 0.5 to 5.1~Ml. The observed seismicity allows us to constrain the current state of stress of the subducting plate and magmatic arc as well as the local seismic velocity structure. The downgoing Wadati-Benioff zone is readily identifiable as an eastward dipping plane with an inclination of about 29-33°. Besides events in in the Benioff Zone, 75~shallow crustal events with depths shallower than 25~km were observed mainly occurring in different clusters along the magmatic arc. These crustal clusters of seismicity are related to the LOFZ, to the volcanoes Chaitén, Michinmahuida and Corcovado, and to active faulting on secondary faults. Further activity along the LOFZ is indicated by individual events located in direct vicinity of the LOFZ. Focal mechanisms were calculated using moment tensor inversion of amplitude spectra for body waves which mostly yield strike slip mechanisms with SE-NW oriented direction of~?1~for the LOFZ at this latitude. In contrast to the strike-slip mechanism of the events along the LOFZ, the focal mechanisms of the stronger events in the Benioff Zone yield mainly thrust mechanism down to 55~km depth. Focal Mechanism Stress Inversion (FMSI) was carried out for the crustal events and the events in the Benioff zone indicating a strike slip regime along the arc and thrust regime in the Benioff zone, respectively. We suggest that the observed deformation along the LOFZ combined with teleseismic observations is a confirmation for the proposed northward movement of the forearc sliver acting as a detached continental micro-plate as outlined by Forsythe~& Nelson (1985) and Beck et al.~(1993). A high quality subset of events was inverted for a 2-D velocity model using the SIMULPS inversion code. First results suggest a 10~km high mantle bulge (vp>7.8~km/s) below the longitudinal valley and low velocity structure below the coastal cordillera.

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

2007-12-01

146

Areas of slip of recent earthquakes in the Mexican subduction zone  

NASA Astrophysics Data System (ADS)

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

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

2012-12-01

147

Strength of megathrust faults and its control on subduction-zone seismotectonics  

NASA Astrophysics Data System (ADS)

Predicting where devastating subduction-zone earthquakes could occur requires identification of key fault parameters from geophysical observations. Global and regional-scale comparisons of subduction-zone seismicity against gravity and bathymetry anomalies show that a large percentage of the moment released by co-seismic slip during megathrust earthquakes concentrates over regions characterized by gravity and bathymetry lows. This has been interpreted after assuming that high interplate friction implies high mechanical coupling between the subducting slab and the overriding forearc and, therefore, a depressed forearc topography leading to low gravity anomalies. This interpretation supports the accepted paradigm as to that seismic asperities (areas of large co-seismic slip) are strong patches of the plate interface characterized by high friction. Attempting to test this idea from a physically compelling perspective, we apply a wavelet formulation of the classical spectral isostatic analysis to invert grids of gravity anomalies and bathymetry/topography into maps of flexural rigidity for several subduction zones worldwide. Flexural rigidity explicitly links those surface observables with the integrated mechanical strength of the lithosphere. For the special case of subduction zones where two tectonic plates are in contact along a low-angle fault, it can be argued that local- to regional-scale lateral variations of flexural rigidity are mostly due to spatial changes of the shear strength along the interplate fault caused by the physical conditions of the subduction channel, which supersede large-scale variations of thermo-mechanical properties of both converging plates. In our interpretation, high/low flexural rigidity along subduction zones means high/low shear stress supported by the megathrust fault and, therefore, high/low friction and/or low/high pore pressure along the subduction channel. We use the gravity model EIGEN-GL04C, which combine data from satellite missions with marine and land gravity measurements, to calculate flexural rigidity for most subduction zones worldwide. In addition, we compile a seismicity catalogue for moderate to giant earthquakes (5.5 < Mw < 9.6) occurred during the last hundred years along those subduction zones. Comparing flexural rigidity at the epicentral location of subduction earthquakes, with their moment magnitude, released seismic moment and the slip distribution of giant megathrust events, we confirm that subduction-zone earthquakes tend to nucleate at regions of the interplate fault characterized by high strength. However, a large portion of the total seismic moment accounted by subduction earthquakes during the last hundred years was released by rupturing weak segments of the plate interface characterized by low flexural rigidity, a tendency which is dominated by the growing pattern characterizing the three giant earthquakes (Mw>9) occurred during the twenty century (Kamchatka 1952, Chile 1960, Alaska 1964). The unexpected Mw9.3 Sumatra-Andaman 2004 earthquake is anomalous with respect to other giants because it propagated throughout several high-strength segment of the subduction zone. Paleoseismological studies in Chile and Sumatra suggest that giant earthquakes in the later region have a recurrence interval that is 2-3 times higher than the interval between giants in the former region. This intriguing behavior fits into a model suggested by our results, as to that the common way for an earthquake to grow toward giant magnitudes is for the rupture front entering a conditionally-stable, weak segment of the megathrust (the case of twenty century giants); a less common scenario (typified by the Sumatra-Andaman 2004 earthquake) supposes the synchronized rupture of several high strength patches of the interplate fault that are all loaded near a critical shear stress. Our findings partially contradicts previous interpretations based on the correlation of gravity and bathymetry anomalies with seismogenic behaviour and can potentially change paradigms forming the physical b

Tassara, A.; Hackney, R.

2009-04-01

148

Evaluating the Extent of C Cycling Through a Cold Subduction Zone: New Clues from Izu- Bonin Melt Inclusions  

NASA Astrophysics Data System (ADS)

Subduction zones provide our best window into C cycling processes between Earth's surface reservoirs and the mantle. The efficiency of this process can be constrained through volatile studies of melt inclusions, where measured pre-eruptive CO2 contents are combined with magma production rates to obtain an output CO2 flux. These outputs can then be compared to C inputs from the subducting slab (sedimentary, organic and altered oceanic crust) to evaluate budgets through a given arc system. Decarbonation of the various C components within a slab are strongly controlled by temperature, pressure and fluid availability. The Izu-Bonin subduction zone system is a cold subduction zone and modeled CO2 behaviour for low temperature geotherms suggest that little decarbonation would occur at subarc depths 1. However, fluids can effectively promote decarbonation. Trace element ratios of Izu arc rocks 2 predict that a significant amount of fluid is fluxed through the Izu-Bonin arc system. This study aims to evaluate the extent of C recycling through a cold, yet fluid-rich arc system. Here we report new CO2 melt inclusions abundance data from 4 volcanoes in the Izu-Bonin arc: Nijima, Oshima, Hachijojima and Aogashima. Concentrations of CO2, along with other volatiles (H2O, F, SO2, Cl), were determined using SIMS techniques at the Carnegie Institution of Washington. Various processes can modify intrinsic volatile contents such as degassing, fractional crystallization, crustal contamination and extent of melting, thereby masking true source values. CO2 contents of Izu-Bonin melt inclusions show positive trends with other volatiles (H2O and SO2) and with MgO contents (with the exception of Nijima). This indicates that differentiation and degassing have occurred simultaneously. In this case, we assume that the highest CO2 concentration samples (up to 1200 ppm CO2 from Nijima volcano) best represent pre-eruptive magma compositions. Comparing a total CO2 input of 10.35 Mmol/yr3 to our calculated output of 17.6 Mmol/yr (assuming a magma production rate of 60 km3/km/Myr1) we estimate that approximately 17% of C subducted at the trench is recycled at the arc front. This value is remarkably similar to the C recycling efficiency found at the Central American arc (14-18%)5, where thermal conditions are significantly warmer. 1Kerrick, D.M. and Connolly, J.A.D. Metamorphic devolatization of subducted marine sediments and the transport of volatiles into the Earth's mantle, Nature v. 411, 293-296 (2001). 2Stern, R. J., Fouch, M. J. & Klemperer, S. in Inside the Subduction Factory 175-222 (2003). 3Hilton, D. R., Fischer, T. P. & Marty, B. Rev. in Mineral v. 47 319-370. 4Dimalanta, C., Taira, A., Yumul, G. P., Jr., Tokuyama, H. & Mochizuki, K. EPSL, v. 202, 105-115 (2002). 5Shaw, A. M., Hilton, D. R., Fischer, T. P., Walker, J. A. & Alvarado, G. EPSL v. 214, 499-513 (2003).

Shaw, A. M.; Hauri, E. H.; Fischer, T. P.; Hilton, D. R.

2006-05-01

149

Marine Gravity Measurements at the Subduction Zone offshore Central Chile  

NASA Astrophysics Data System (ADS)

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

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

2003-12-01

150

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

NASA Astrophysics Data System (ADS)

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

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

2013-04-01

151

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

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

152

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

Microsoft Academic Search

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

Ian J. Parkinson; Richard J. Arculus

1999-01-01

153

Comparison of earthquake source parameters and interseismic plate coupling variations in global subduction zones (Invited)  

NASA Astrophysics Data System (ADS)

Geodetically determined interseismic coupling variations have been found in subduction zones worldwide. These coupling variations have been linked to heterogeneities in interplate fault frictional conditions. These connections to fault friction imply that observed coupling variations are also important in influencing details in earthquake rupture behavior. Because of the wealth of newly available geodetic models along many subduction zones, it is now possible to examine detailed variations in coupling and compare to seismicity characteristics. Here we use a large catalog of earthquake source time functions and slip models for moderate to large magnitude earthquakes to explore these connections, comparing earthquake source parameters with available models of geodetic coupling along segments of the Japan, Kurile, Kamchatka, Peru, Chile, and Alaska subduction zones. In addition, we use published geodetic results along the Costa Rica margin to compare with source parameters of small magnitude earthquakes recorded with an onshore-offshore network of seismometers. For the moderate to large magnitude earthquakes, preliminary results suggest a complex relationship between earthquake parameters and estimates of strongly and weakly coupled segments of the plate interface. For example, along the Kamchatka subduction zone, these earthquakes occur primarily along the transition between strong and weak coupling, with significant heterogeneity in the pattern of moment scaled duration with respect to the coupling estimates. The longest scaled duration event in this catalog occurred in a region of strong coupling. Earthquakes along the transition between strong and weakly coupled exhibited the most complexity in the source time functions. Use of small magnitude (0.5 < Ml < 4) regionally recorded earthquakes in Costa Rica is also fruitful for comparing with coupling estimates. We find systematic variations in earthquake spectra, with higher corner frequencies and higher mean apparent stress for earthquakes that occur in along the Osa Peninsula relative to the Nicoya Peninsula, mimicking the along-strike variations in calculated interplate coupling.

Bilek, S. L.; Moyer, P. A.; Stankova-Pursley, J.

2010-12-01

154

Paleoseismic Investigations of Subduction Zone Earthquakes on the Southeastern Coast of the Kenai Peninsula, Alaska  

NASA Astrophysics Data System (ADS)

Rupture extents for prehistoric subduction zone earthquakes on the Alaska margin are poorly documented. For example, the 1964 great Alaska earthquake ruptured the subduction zone from Prince William Sound westward to the southwest end of Kodiak Island, but an outstanding question is whether or not earlier subduction earthquakes ruptured a shorter segment in the Kodiak region with the eastern margin located in the Kenai Peninsula. To address the question, we investigated several embayments on the Pacific (southeastern) coast of the Kenai Peninsula to reconstruct the late Holocene paleoseismic history. Constraints to constructing a paleoseismic history on the outer Kenai coast include recent deglaciation of embayments that limits a stratigraphic record to approximately the last 1 to 3 thousand years and the high wave-energy environment that results in frequent closure of embayments by barrier bars. Recognizing these constraints, two approaches have proved productive in investigating paleoseismic history on the outer Kenai coast. One involves understanding how beach ridges can be formed and modified by coseismic subsidence and documenting timing and number of late Holocene prehistoric earthquakes based on beach ridge genesis and evolution. The other approach, which is the more conventional paleoseismic approach for subduction zones, is investigating stratigraphic evidence for coseismic subsidence in peaty and muddy sediment within embayments. We illustrate these two approaches using two coastal sites in Aialik Bay within Kenai Fjords National Park, a strandplain complex at Verdant Cove and a drowned embayment at Quicksand Cove, respectively. Both sites record two prehistoric subduction zone earthquakes prior to the historically known 1964 great Alaska earthquake.

Kelsey, H. M.; Witter, R. C.; Briggs, R. W.; Engelhart, S. E.; Nelson, A. R.; Haeussler, P. J.

2013-12-01

155

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

Microsoft Academic Search

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

Stefano Poli; Max W. Schmidt

1995-01-01

156

3D extension of remarkable characteristics at the front of the Colombia-Ecuador subduction zone  

Microsoft Academic Search

During the last century, four great earthquakes shook the Colombia-Ecuador subduction zone. The 500 km long rupture zone of the first of this seismic events, the 1906 earthquake (Mw= 8.8), was partially reactivated, from south to north, by a sequence of 3 thrust events in 1942 (Mw = 7.8), 1958 (Mw = 7.7) and 1979 (Mw = 8.2). The rupture

L. Garcia Cano; A. Galve; P. Charvis

2009-01-01

157

GPS Application : Theoretical Analysis of Coseismic Crustal Deformation of the Subduction Zone Colombia - Ecuador  

Microsoft Academic Search

The Colombia - Ecuador trench is located on the northwestern subduction zone along the Nazca and South America plate boundary. The zone is considered tectonically complex and it is continuously under large crustal deformation. The rapid convergence rate (58mm\\/a) in the zone caused a series of four large earthquakes (Mw>7.5) in the 20th century: 1906(Mw=8.8), 1942 (Mw=7.6), 1958 (Mw=7.7) and

A. L. Ramos Barreto; S. I. Franco; A. Iglesias

2010-01-01

158

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

NASA Astrophysics Data System (ADS)

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

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

2012-04-01

159

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

PubMed Central

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

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

2007-01-01

160

Mantle peridotites from continental rifts to ocean basins to subduction zones  

Microsoft Academic Search

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

Enrico Bonatti; Peter J. Michael

1989-01-01

161

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

NASA Astrophysics Data System (ADS)

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

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

2005-12-01

162

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

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

163

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

NASA Astrophysics Data System (ADS)

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

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

2014-12-01

164

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

NASA Astrophysics Data System (ADS)

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

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

2014-10-01

165

Seismic evidence for the nature of the Vrancea zone of the Eastern Carpathians: an oceanic subduction zone  

NASA Astrophysics Data System (ADS)

The Vrancea region of the southeastern Carpathians is one of the most active seismic zones in Europe and it is well-known for its strong intermediate depth earthquakes. Seismic tomography had revealed a high-velocity body beneath Vrancea and the Moesian platform that extends to a depth of at least 350 km and can be interpreted as descending lithosphere. The strong earthquakes occur within the northeastern part of this high-velocity body, in a very limited seismogenic volume at intermediate depth (70-180 km). Several geodynamic models have been proposed for this area. They can be split into two main categories, in terms of the nature of the high-velocity anomaly, which may (a) be associated with descending relic oceanic lithosphere beneath the bending zone of the SE-Carpathians, either attached or already detached from the continental crust; or (b) it may represent continental lithosphere that has been delaminated, after continental collision and orogenic thickening. Based on currently available information, it appears difficult to distinguish between these two types of models. In this paper we attempt to shed more light on the nature of the seismic anomaly, as well as that of the origin of the intermediate depth seismicity in the Vrancea zone, by investigating the waveform character of P-waves excited by local earthquakes beneath this area, and in particular the dependence of group arrival times on frequency. We present observations of such a dispersion from stations situated at the bending zone of the SE-Carpathians. On the other hand, signals from the same earthquakes, but observed at reference stations outside of the anomalous zone do not show that frequency dependence. A natural explanation for these observations is that they are caused by the presence of a low-velocity (oceanic crustal) channel at the top of the seismic anomaly, which is too thin to be resolved by classical seismic tomographic techniques. Similar observations of dispersed first-arriving P-waves have been made above subduction zones around the world, in which low-velocity layers with a thickness of several kilometers are known to exist. This suggests that a tabular slab of subducted oceanic crust is present within the seismic anomaly under the Vrancea region, and that the anomaly consists of subducted oceanic lithosphere rather than continental lithosphere, at least at depths shallower than the seismically active zone.

Bokelmann, Götz; Rodler, Fee-Alexandra

2014-05-01

166

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

NASA Astrophysics Data System (ADS)

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 excess 40Ar*. Three of six serpentinites analysed for helium also have measurable excess 4He contents that cannot be explained by in situ production. The data show that serpentinites trap noble gases and halogens that originate from seawater, organic matter and diverse crustal lithologies. Combined with previous analyses of metamorphosed serpentinites, the new data suggest that approximately 60-70% of the 36Ar entering subduction zones in serpentinites is lost from chrysotile and/or antigorite and could potentially escape through the forearc. An additional, ˜20-30% of the 36Ar entering subduction zones in serpentinites is lost during antigorite breakdown and may be cycled through the arc or back-arc, and ˜1-10% of the 36Ar entering subduction zones in serpentinites may be subducted into the deeper mantle. The data demonstrate decoupling of noble gases, halogens and water during subduction and suggest that subduction-zone fluid fluxes can produce especially high concentrations of noble gases and iodine in newly formed forearc serpentinites. The distinctive I/Cl enrichment of forearc serpentinites suggest that halogen abundance ratios provide a plausible means for inferring the geotectonic setting of serpentinisation in ophiolite samples. The exceptional Cl, Br, I and noble gas concentrations of serpentinites, the potential subduction of the forearc serpentinites and the stability of serpentine minerals to mantle depths of >200 km, imply that serpentinites could dominate the deep recycling budgets of both the heavy halogens and atmospheric noble gases.

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

2013-03-01

167

In-situ formation of Indian Mantle in global subduction zones  

NASA Astrophysics Data System (ADS)

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

Nebel, Oliver; Arculus, Richard; Davies, Rhodri

2014-05-01

168

Plate coupling along the Manila subduction zone between Taiwan and northern Luzon  

NASA Astrophysics Data System (ADS)

We use GPS data, trench parallel gravity anomaly (TPGA), and bathymetry to infer plate coupling patterns along the Manila subduction zone. Using a block model and a fault geometry constrained by seismicity, we simultaneously solve for the location of Euler pole and angular velocity between the Sunda and Luzon blocks as well as the slip-deficit rate on plate interface. Our estimates show that the Euler pole between the Sunda and Luzon blocks is situated at southern Palawan near 8.3°N and 119.4°E with the angular velocity of 4.6 Myr-1. The estimated convergence rate along the Manila Trench continuously decreases southward from 91 mm/yr at the northern tip of Luzon to 55 mm/yr north of Mindoro. The inversion of GPS data reveals partially locked fault patches extending from the West Luzon Trough to the east of Scarborough Seamount chain. The slip-deficit rate in this region is in the range of 20-30 mm/yr corresponding to a coupling ratio of 0.4. However, the fault slip behavior is not well resolved near the North Luzon Trough. Based on a good correlation between locations of large subduction zone earthquakes and areas possessing gravity low, we investigate a variety of TPGA-based plate coupling models assuming different scaling between TPGA values and plate coupling ratios. The TPGA-based plate coupling models offer plausible rupture scenarios which are not constrained by current GPS data. The partially locked fault zone near 15-16.5°N may be associated with the subducted Scarborough Seamount wherein oceanic floor is highly fractured. The great subduction zone earthquake propagates beneath the Scarborough Seamount seems to be unlikely. The densification of GPS network in central Luzon and seafloor geodetic observations close to trench axis are crucial to distinguish the detailed fault coupling patterns and the role of subducted seamounts.

Hsu, Ya-Ju; Yu, Shui-Beih; Song, Teh-Ru Alex; Bacolcol, Teresito

2012-06-01

169

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

USGS Publications Warehouse

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

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

1989-01-01

170

The global range of subduction zone thermal models: consequences for slab devolatilization  

NASA Astrophysics Data System (ADS)

Two- and three-dimensional thermal models provide insight into the dehydration and melting processes that occur in subduction zones. The wide range of slab geometries, ages, convergence velocities and upper plates result in an array of thermal structures that share many common features. We model forty-six arc sections in two dimensions using kinematically-defined slabs based on updated geometries from Syracuse and Abers [2006] to obtain a comprehensive suite of thermal models for the global subduction system. All models feature partial coupling between the slab and the overriding plate directly downdip of the thrust zone, invoked to replicate the cold nose observed in heat flow measurements and seismic attenuation. Four separate cases are tested with four separate sets of assumptions about the causes of the partial coupling: the downdip end of the partial coupling is at a constant depth, it is at constant distance trenchward from the arc, is defined by a critical surface slab temperature, or is adjusted such that the hottest part of the mantle wedge beneath the arc is at a constant temperature for all subduction zones. In all of these models, slabs reach temperatures where the top of the crust and sediments dehydrate before they reach sub-arc depths, and the overlying mantle wedge is too hot for hydrous minerals to be stable at sub-arc depths. The exceptions are cases where subduction is fast and coupling is controlled by a critical slab surface temperature (550°C); these cases also do not produce wedges hot enough to generate primitive arc magmas so probably underestimate temperatures. By contrast, the mantle within the downgoing plate remains cold enough for serpentine to be stable beyond the arc in all but the hottest subduction zones, allowing water to be carried beyond the arc in the slab.

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

2009-04-01

171

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

NASA Astrophysics Data System (ADS)

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

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

2011-12-01

172

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

NASA Astrophysics Data System (ADS)

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

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

2012-12-01

173

Understanding seismogenic zone transitions in New Zealand’s subduction zones (Invited)  

NASA Astrophysics Data System (ADS)

In both of New Zealand’s subduction zones, the seismogenic part of the plate interface underlies the land area or is just offshore. Thus while these seismogenic zones present a significant seismic hazard, they are also amenable to detailed study with land-based instruments. We have approached the study of these seismogenic zones from the viewpoint that the mechanical behaviour of the subduction thrust will depend not only on the relatively small-scale complexity within the thrust zone itself, but also on the large-scale three-dimensional distribution of rock types surrounding the thrust. So in recent years we have used dense seismograph deployments to derive detailed 3-D tomographic images of seismic velocities and attenuation surrounding the subduction thrusts. In the Hikurangi subduction zone in the North Island, we have no record of large subduction thrust earthquakes in historical times (last 175 years). Yet GPS data suggest that currently there is a major along-strike transition in interseismic geodetic coupling at the plate interface, from strongly coupled in the southern North Island to weakly coupled further north. By comparing this coupling distribution with seismicity data and structural data from tomography, we propose a model where plate coupling is controlled by the ability of fluid to cross the plate interface. When an impermeable terrane in the overlying plate prevents such fluid flow, plate coupling appears to be strong. The Fiordland subduction zone in the southwestern South Island is not as densely instrumented as the Hikurangi subduction zone (largely because few people live there). But it has the advantage of having experienced a recent sequence of subduction thrust events, culminating in the Mw 7.6 Dusky Sound earthquake of 15 July 2009. The northern limit of the rupture zone of this recent large event abuts the southern limit of the Mw 7.2 subduction thrust event of 21 August 2003. This rupture zone boundary appears to be structurally controlled. It marks a distinct transition in both the strike and dip of the seismogenic zone of the plate interface, the structure of the overlying plate crust, and the structure of the mantle wedge.

Reyners, M.; Eberhart-Phillips, D. M.

2009-12-01

174

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

NASA Astrophysics Data System (ADS)

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

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

2012-03-01

175

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

NASA Astrophysics Data System (ADS)

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

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

1996-05-01

176

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

NASA Astrophysics Data System (ADS)

Petrologic, structural and age investigations of the Lolotoi Complex of East Timor indicate that it is part of a group of thin metamorphic klippe found throughout the region that were detached from the Banda forearc and accreted to the NW Australian continental margin during Late Miocene to Present arc-continent collision. Metamorphic rock types are dominated by (in order of abundance), greenschist, graphitic phyllite quartz-mica schist, amphibolite and pelitic schist. Geochemical analyses show metamorphic protolith compositions similar to overlying unmetamoprhosed tholeiitic basalt and andesite with oceanic arc affinities, and turbidities conglomerates and limestone. Fragments of lherzolite and hazburgite are also found. The series of deformation as indicated by field and microstructural measurements is similar to those reported from other klippen throughout the Timor region. 1) S1 foliation formed parallel to original bedding. 2) S2 formed axial- planar to small scale isoclinal folds of S1 during the main phase of metamorphism. 3) Subsequent, mostly broad wavelength SE verging folds with axes oriented NE-SW deformed S2 into a steeper and more variably dipping orientation. 4.) Late brittle deformation in the form of normal and thrust/reverse faults cut across earlier structures. Thrust faults are mostly top to the NW, while normal faults have top to the SW, S and SE sense of shear. Contact relationships with adjacent units indicate that the metamorphic terrain is in thrust contact with underlying Gondwana sequence rocks. Mostly unmetamorphosed volcanic and sedimentary cover units are found locally in fault contact on the edges of the klippen. Geothermobarometric calculations show peak metamorphic temperatures in pelitic rocks range from 564°C to 596° C for garnet-biotite pairs and peak pressures of 4.3 to 5.8 kbar for garnet-aluminosilicate-quartz-plag assemblages. Analysis of amphibole in amphibolites yield temperatures of 570°C to 670°C and pressures of 4.3 to 6.8 kbar. Ar/Ar age analyses of amphibole from amphibolite in West Timor yield ages of 34-39 Ma, which are interpreted as the age of metamorphism. The Lolotoi Complex formed part of the eastern Great Indonesian Arc, which collapsed in the Eocene, was incorporated into the Banda Arc in the Miocene, and accreted to the Austrailian continental margin from Pliocene to Present.

Standley, C. R.

2006-12-01

177

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

NASA Astrophysics Data System (ADS)

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

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

2014-12-01

178

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

NASA Astrophysics Data System (ADS)

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

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

2005-12-01

179

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

NASA Astrophysics Data System (ADS)

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

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

2013-08-01

180

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

NASA Astrophysics Data System (ADS)

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

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

2014-06-01

181

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

PubMed

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

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

2015-01-01

182

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

USGS Publications Warehouse

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

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

2001-01-01

183

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

NASA Astrophysics Data System (ADS)

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

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

2012-04-01

184

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

NASA Astrophysics Data System (ADS)

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

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

2011-12-01

185

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

NASA Astrophysics Data System (ADS)

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

Spinelli, G. A.; Wada, I.

2012-12-01

186

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

NASA Astrophysics Data System (ADS)

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

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

2014-12-01

187

Ambient seafloor noise excited by earthquakes in the Nankai subduction zone  

PubMed Central

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

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

2015-01-01

188

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

NASA Astrophysics Data System (ADS)

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

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

2008-12-01

189

Effects of subducting buoyant oceanic ridges on subduction zones: Area of influence and rotational effects  

NASA Astrophysics Data System (ADS)

The subduction of buoyant oceanic ridges into subduction zones is typically manifested by doming of arc rocks, shallowing of the trench, and diffuse or shallowly-dipping Benioff zones. Two important questions include: 1) what distance inboard of the overriding plate are effects observed; and 2) what is the degree that colliding ridges can induce large-scale rotations of forearc terranes and consequent "back-arc opening" behind rotated forearc blocks. I describe regional effects from five relatively narrow ridges actively entering subduction zones: 1) Carnegie; 2) Cocos; 3) Emperor seamount chain; 4) Louisville, and 5) D'Entrecasteaux. GPS from all areas shows a characteristic outward flow pattern in map view indicative of the strong landward push on the ridge along radial thrust systems within the overriding plate. This area of influence can extend 100s of kms. The pattern of outward flow from GPS vectors is consistent with the of bathymetry, gravity and earthquakes show some of these ridges act as strong indentors that push into the arc along strike-slip systems at their edges. In other cases likely related to thinner crust, no strong disruption of the outer forearc high or forearc basin is observed and adjacent to subducting ridges. Rotating forearc blocks are most expressed by examples where the direction of subduction is highly oblique and the least rotational effects are expressed where the direction of subduction is orthogonal. Wider ridges also appear to have fewer rotational effects.

Mann, W. P.

2012-12-01

190

Geophysical signature of hydration-dehydration processes in active subduction zones  

NASA Astrophysics Data System (ADS)

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

Reynard, Bruno

2013-04-01

191

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

192

Fabric kinematics of the ultrahigh-pressure metamorphic rocks from the main borehole of the Chinese Continental Scientific Drilling Project: Implications for continental subduction and exhumation  

NASA Astrophysics Data System (ADS)

The 5158-m-deep main borehole of the Chinese Continental Scientific Drilling Project (CCSD-MH) penetrated granitic gneisses, paragneisses, eclogites, retrograde eclogites, amphibolites and ultramafic rocks in the Sulu ultrahigh-pressure (UHP) metamorphic terrane, eastern China. The CCSD-MH consists of four petro-structural units separated by three SE-dipping ductile shear zones DFa (835-1280 m), DFb (2010-2280 m) and DFc (2920-3225 m), which are correspondent with the regional shear zones in the northern Sulu UHP supracrustal zone. Using the electron backscatter diffraction (EBSD) technique, we investigated the lattice-preferred orientations (LPOs) of omphacite, diopside and quartz in core samples from the CCSD-MH. Omphacite from eclogites and diopside from garnet pyroxenites display very strong LPOs, which are characterized by the maximum concentration of [001]-axes parallel to the lineation and (010)-poles normal to the foliation. Quartz in para- and granitic mylonites/gneisses from the shear zones DFa, DFb and DFc developed multiple slip systems. 40Ar/ 39Ar dating of biotite in para- and granitic gneisses from the CCSD-MH yields 223-202 Ma, which constrains the formation ages of the quartz high-temperature prism slip systems {m} and {m}[c]. The asymmetric LPOs of omphacite, diopside, olivine and quartz with respect to the structural frame reveal three deformation phases in the Sulu terrane. In the Middle Triassic, the northward subduction of the Yangtze plate to depths > 100 km produced a top-to-the-south shear sense in LPOs of omphacite, diopside and olivine, and a nearly N-S-striking foliation and a subhorizontal N-S-trending lineation in eclogites and ultramafic rocks. In the Late Triassic, the UHP rocks were exhumed to the lower crust and quartz developed high-temperature slip systems with a top-to-the-NW shear sense, which is consistent with the regional SE-dipping foliation and SE-plunging lineation in the ductile shear zones. In the Cretaceous the UHP rocks were exhumed to the middle crust when the migmatization and granitic intrusion formed a NE-striking antiform structure. As a result, the activation of quartz low-temperature basal slip (0001) is characterized by a top-to-the-SE shear sense in the south, but a top-to-the-NW shear sense in the north.

Xu, Zhiqin; Wang, Qin; Tang, Zhemin; Chen, Fangyuan

2009-09-01

193

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

NASA Astrophysics Data System (ADS)

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

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

2014-10-01

194

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

NASA Astrophysics Data System (ADS)

We invert for the distribution of locking along the Oregon portion of the Cascadia subduction zone using an updated dataset of the interseismic vertical displacement rates. Uplift rates are inferred from spirit leveling that is tied into an absolute vertical reference frame using tide gauge records. With absolute uncertainties less than 1 mm/yr, the data provide good constraints on the accumulation of interseismic strain along the Oregon coast. The slip rate deficit on the subduction interface is modeled using a backslip calculation in an elastic half-space. Static Green's functions are calculated using a triangular fault model that approximates the 3D geometry of the plate interface. We assume a slip rate deficit that is equivalent to the full convergence rate in an offshore locked zone and tapers to zero at depth across a transition zone. The convergence rate is calculated using the published Euler vector for motion of the Juan de Fuca oceanic plate relative the Oregon continental forearc. To minimize the number of free parameters in the inversion, the down-dip extent of locking is defined by the lower edge of the fully locked zone and the lower edge of the transition zone. These two free parameters are optimized at various latitudes by minimizing the misfit of the east-west leveling lines through a grid search of the parameter space. The north-south leveling route that runs along the coast is then used to further optimize the model by interpolating the slip deficit distribution along-strike. The inversion prefers models where the locked zone is forced up-dip (i.e. farther offshore) at the east-west profile near Newport on the central Oregon coast. This is in contrast to inversion results near Astoria where the locked and transition zones extend farther down-dip relative to a constant depth contour on the plate interface. Our optimal model of the slip-rate deficit distribution is compared to previously published models of strain accumulation constrained by only horizontal displacement rates. We also compare the distribution of locking to the extent of Siletzia in the continental forearc, gravity lows, and forearc basins to better understand the potential location of long-lived asperities on the subduction interface.

Schmidt, D.; Burgette, R.; Weldon, R.

2007-12-01

195

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

NASA Astrophysics Data System (ADS)

We invert for the distribution of locking along the Oregon portion of the Cascadia subduction zone using an updated dataset of the interseismic vertical displacement rates. Uplift rates are inferred from spirit leveling that is tied into an absolute vertical reference frame using tide gauge records. With absolute uncertainties less than 1 mm/yr, the data provide good constraints on the accumulation of interseismic strain along the Oregon coast. The slip rate deficit on the subduction interface is modeled using a backslip calculation in an elastic half-space. Static Green's functions are calculated using a triangular fault model that approximates the 3D geometry of the plate interface. We assume a slip rate deficit that is equivalent to the full convergence rate in an offshore locked zone and tapers to zero at depth across a transition zone. The convergence rate is calculated using the published Euler vector for motion of the Juan de Fuca oceanic plate relative the Oregon continental forearc. To minimize the number of free parameters in the inversion, the down-dip extent of locking is defined by the lower edge of the fully locked zone and the lower edge of the transition zone. These two free parameters are optimized at various latitudes by minimizing the misfit of the east-west leveling lines through a grid search of the parameter space. The north-south leveling route that runs along the coast is then used to further optimize the model by interpolating the slip deficit distribution along-strike. The inversion prefers models where the locked zone is forced up-dip (i.e. farther offshore) at the east-west profile near Newport on the central Oregon coast. This is in contrast to inversion results near Astoria where the locked and transition zones extend farther down-dip relative to a constant depth contour on the plate interface. Our optimal model of the slip-rate deficit distribution is compared to previously published models of strain accumulation constrained by only horizontal displacement rates. We also compare the distribution of locking to the extent of Siletzia in the continental forearc, gravity lows, and forearc basins to better understand the potential location of long-lived asperities on the subduction interface.

Schmidt, D.; Burgette, R.; Weldon, R.

2004-12-01

196

Boron isotopes of sediments and pore-fluids in the Costa Rica subduction zone and their implications for fluid-rock interaction and geochemical cycling  

Microsoft Academic Search

Only very little is known about boron isotopic compositions of continuous siliceous sedimentary successions in the forearcs of subduction zones. In order to evaluate the B geochemical cycle and fluid-rock interactions at convergent margins, the knowledge of the boron isotopic composition of the subducting sediment component is important. The Costa Rican subduction zone offshore the Nicoya Peninsula is non-accretionary and

A. Deyhle; A. Kopf; A. Eisenhauer; K. Wallmann

2003-01-01

197

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

E-print Network

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

Biggs, Juliet

198

Subduction zone Hf-anomalies: Mantle messenger, melting artefact or crustal process?  

NASA Astrophysics Data System (ADS)

The origin of Hf elemental depletions in subduction zone magmas is investigated using new major- and trace-element data for cumulate xenoliths from the Mariana arc, and deep sea sediments recovered by the DSDP and ODP drilling programmes. Results indicate that most of the rare earth element (REE) and Hf inventory in the xenoliths is contained within two minerals—clinopyroxene and titanomagnetite—and that removal of a typical gabbroic fractionating assemblage reduces the depletion in Hf relative to neighbouring REE on a mantle normalised trace element diagram (commonly denoted Hf/Hf*) in the evolving magmas. Confirmation of this observation is provided by a variety of literature data from different subduction zones in which bulk-rock samples also define a positive correlation between Hf/Hf* and the silica content of the magmas. In agreement with experimental studies on REE-HFSE partitioning, we observe that the ability of clinopyroxene to influence the Hf/Hf* of fractionating magmas is associated with its aluminium content. This decoupling of Hf from the REE in differentiating arc magmas suggests that bulk rock Hf/Hf* values, when used in isolation, are unlikely to provide a robust measure of source REE-Hf characteristics, even when suites are filtered to exclude all but the most mafic samples. It may be possible to normalise data to a constant degree of fractionation, and in this way distinguish subtle changes in source Hf/Hf* but most existing datasets are of neither the size nor quality to attempt such calculations. Modification of Hf/Hf* is also seen when modelling mantle melting processes and there are strong suggestions that source variations are influenced by not only subducted sediment, which exhibits a remarkably wide range in Hf/Hf*, but also subduction zone fluids. These observations remove some of the constraints imposed on recent models that attempt to reconcile Hf isotope data with Hf-REE abundance data in some arc suites. Although a case may be made for the involvement of residual, minor, phases in the downgoing slab, fluid and sediment addition, and the role of major phases during partial melting, in particular clinopyroxene, in the mantle wedge can also exert a strong influence on Hf-REE relationships.

Woodhead, Jon; Hergt, Janet; Greig, Alan; Edwards, Louise

2011-04-01

199

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

200

Metamorphism and Continental Collision  

NSDL National Science Digital Library

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

Howard, Kenneth

201

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

NASA Astrophysics Data System (ADS)

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.

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

2012-12-01

202

Seismicity and state of stress within the overriding plate of the Tonga-Kermadec subduction zone  

NASA Astrophysics Data System (ADS)

To reassess the main tectonic units and to quantify the slip partitioning within the overriding plate of the Tonga-Kermadec subduction zone, a seismotectonic study was performed using global seismicity and focal mechanisms catalogs. (1) New tectonic features were identified within the Lau Basin and the volcanic arc by remarkable shallow hypocenters alignments. (2) The Centroid Moment Tensor solutions catalog was processed in order to map the stress tensor variation in the upper plate. We found the tectonic features characterized by a diffuse seismicity are subjected to a composite stress regime and they are interpreted as diffuse immature plate boundaries controlled by the high thermal anomaly lying beneath the Lau Basin. (3) We quantified the margin-parallel rates of motion using the azimuth of the maximum compressive stress component computed within the interplate zone. The results highlight a major tectono-kinematic segmentation related to the subduction of the Louisville Seamount Chain.

Bonnardot, M.-A.; RéGnier, M.; Ruellan, E.; Christova, C.; Tric, E.

2007-10-01

203

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

USGS Publications Warehouse

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

Satake, K.; Atwater, B.F.

2007-01-01

204

Unusually large shear wave anisotropy for chlorite in subduction zone settings  

NASA Astrophysics Data System (ADS)

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

Mookherjee, Mainak; Mainprice, David

2014-03-01

205

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

USGS Publications Warehouse

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

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

2009-01-01

206

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

NASA Astrophysics Data System (ADS)

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

Satake, Kenji; Atwater, Brian F.

2007-05-01

207

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

USGS Publications Warehouse

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

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

2008-01-01

208

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

NASA Astrophysics Data System (ADS)

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-Miocene or Pliocene. We suggest that the first episode predates intracontinental subduction, when deformation could have been distributed throughout the region. Following the onset of subduction, perhaps linked to ca. 25 Mya breakoff of the Indian slab, most of the shortening between the NP and Tien Shan was accommodated in a narrow zone bounded by the MPT. As thicker, more rigid crust and / or a thicker sedimentary section entered the subduction zone, the degree of coupling between the two plates would have increased, causing the mid-Miocene - Pliocene deformation in the Alai Basin, westernmost Tarim, and Tien Shan. Progressively greater coupling can explain the difficulty in dating this collision. Intracontinental subduction is absent on the north flank of Tibet, likely because the Tarim block is too buoyant to be subducted. The consequences of the different boundary conditions may explain the different Neogene deformation styles of these regions.

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

2011-12-01

209

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

NASA Astrophysics Data System (ADS)

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

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

2012-12-01

210

The influence of slab-length variations on mantle circulations in the southern Chilean subduction zone  

NASA Astrophysics Data System (ADS)

The depth of the slab base may vary from less than 100 kilometers to several hundred kilometers along the strike of the southern Chilean subduction zone. The variation in slab length would play an important role on shaping the flow pattern. In this study dynamical subduction zone models were constructed to examine the combined effects of slab geometry, plate age, mantle rheology and heterogeneities of plate thickness on the regional mantle circulations. In the models various degrees of slab rollback occur along strike, significantly influenced by the slab length. The differential slab rollback leads to the along-arc pressure gradient causing trench-parallel flow components, while the lower portion of the deep segments acts as a slab edge in part to induce three-dimensional edge flow. These factors together give rise to complex flow patterns, and lead to a great extent of trench-parallel components in the mantle wedge and sub-slab mantle and significant upwelling in the back arc. The length scales of both the poloidal and toroidal components are variable and generally smaller, but the 3D edge flow affects a significantly wider region, compared with flow associated with a slab with a constant length. The model results may help to provide explanations for the complicated anisotropy patterns in the mantle wedge and the sub-slab mantle, the heat sources of the northern Patagonian basalts and the subduction-related components of the MORB from the Chile Ridge. The results may also help to constrain the detailed flow structure in the vicinity of the slab window and the Antarctic slab geometry.

Lin, Shu-Chuan

2014-05-01

211

Along-strike variability of rupture duration in subduction zone earthquakes  

NASA Astrophysics Data System (ADS)

Subduction zone earthquakes exhibit a wide spectrum of rupture times that reflect conditions on the megathrust fault. Tsunami earthquakes are examples of slower than expected ruptures that produce anomalously large tsunamis relative to the surface-wave magnitude. One model explaining tsunami earthquakes suggests slip within patches of low rigidity material at shallow depths. Heterogeneous fault conditions, such as having patches of low rigidity material surrounded by higher strength material, should produce heterogeneous earthquake rupture parameters. Here we investigate along-strike variation in rupture duration for 427 shallow thrust earthquakes (Mw = 5.0-7.0) in the Peru, Chile, Alaska, Tonga, Kuril, Izu, and Java-Sumatra subduction zones to explore how heterogeneous seismic and tectonic characteristics, such as differences in sediment type, thickness, and roughness of subducting bathymetry, affect earthquake properties. Earthquake source parameters, including rupture durations, are estimated using multi-station deconvolution of teleseismic P and SH waves to solve for earthquake source time functions, and all events are relocated using additional depth phase information. We classify events into shallow (?26 km) and deep (>26 km and ?61 km) groups based on the overall mean depth and focus on the longest duration events with moment normalized rupture durations of >1 standard deviation above the mean duration for each group. We find long-duration events at all depths within the study regions except Peru and Chile. We find no correlation with incoming sediment thickness or type, and limited spatial correlation with regions of past tsunami earthquakes, regions of observed afterslip, and subducting bathymetric features.

El Hariri, Maya; Bilek, Susan L.; Deshon, Heather R.; Engdahl, E. Robert; Bisrat, Shishay

2013-02-01

212

Interplate Earthquake Fault Slip During Periodic Earthquake Cycles in a Viscoelastic Medium at a Subduction Zone  

NASA Astrophysics Data System (ADS)

A 2-D finite-element-method (FEM) numerical experiment of earthquake cycles at a subduction zone is performed to investigate the effect of viscoelasticity of the earth on great interplate earthquake fault slip. We construct a 2-D viscoelastic FEM model of northeast Japan, which consists of an elastic upper crust and a viscoelastic mantle wedge under gravitation overlying the subducting elastic Pacific plate. Instead of the dislocation model prescribing an amount of slip on a plate interface, we define an earthquake cycle, in which the plate interface down to a depth is locked during an interseismic period and unlocked during coseismic and postseismic periods by changing the friction on the boundary with the master-slave method. This earthquake cycle with steady plate subduction is periodically repeated to calculate the resultant earthquake fault slip.As simulated in a previous study (Wang, 1995), the amount of fault slip at the first earthquake cycle is smaller than the total relative plate motion. This small amount of fault slip in the viscoelastic medium was considered to be one factor explaining the small seismic coupling observed at several subduction zones. Our simulation, however, shows that the fault slip grows with an increasing number of repeated earthquake cycles and reaches an amount comparable to the total relative plate motion after more than ten earthquake cycles. This new finding indicates that the viscoelasticity of the earth is not the main factor in explaining the observed small seismic coupling. In comparison with a simple one-degree-of-freedom experiment, we demonstrate that the increase of the fault slip occurs in the transient state from the relaxed initial state to the stressed equilibrium state due to the intermittent plate loading in a viscoelastic medium.

Hirahara, K.

213

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

NASA Astrophysics Data System (ADS)

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

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

2015-02-01

214

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

NASA Astrophysics Data System (ADS)

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

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

2012-12-01

215

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

NASA Astrophysics Data System (ADS)

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

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

2014-12-01

216

Interseismic deformation along the Philippine Fault system and Manila subduction zone  

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

217

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

Microsoft Academic Search

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

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

2009-01-01

218

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

Microsoft Academic Search

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

Gail M. Atkinson; David M. Boore

2008-01-01

219

GPS-derived coupling estimates for the Central America subduction zone and volcanic arc faults: El Salvador, Honduras and Nicaragua  

Microsoft Academic Search

We invert GPS velocities from 32 sites in El Salvador, Honduras and Nicaragua to estimate the rate of long-term forearc motion and distributions of interseismic coupling across the Middle America subduction zone offshore from these countries and faults in the Salvadoran and Nicaraguan volcanic arcs. A 3-D finite element model is used to approximate the geometries of the subduction interface

F. Correa-Mora; C. Demets; D. Alvarado; H. L. Turner; G. Mattioli; D. Hernandez; C. Pullinger; M. Rodriguez; C. Tenorio

2009-01-01

220

An investigation of deformation and fluid flow at subduction zones using newly developed instrumentation and finite element modeling  

Microsoft Academic Search

Detecting seafloor deformation events in the offshore convergent margin environment is of particular importance considering the significant seismic hazard at subduction zones. Efforts to gain insight into the earthquake cycle have been made at the Cascadia and Costa Rica subduction margins through recent expansions of onshore GPS and seismic networks. While these studies have given scientists the ability to quantify

Alison Louise Labonte

2007-01-01

221

Do intermediate- and deep-focus earthquakes occur on preexisting weak zones? An examination of the Tonga subduction zone  

Microsoft Academic Search

It has been proposed that intermediate-and deep-focus earthquakes occur on preexisting planes of weakness that were created at shallow depth. The purpose of this study is to test this hypothesis. We examined fault plane solutions for 360 events that occurred in the Tonga subduction zone from January 1976 to February 1999. For events in the slab, each fault plane solution

Wenjie Jiao; Paul G. Silver; Yingwei Fei; Charles T. Prewitt

2000-01-01

222

Second Critical Endpoint in Peridotite-H2O System and its Bearing on the Magmatism in Subduction Zones  

Microsoft Academic Search

Migration of fluid\\/melt through mantle wedge plays an important role on the magmatism in subduction zones. In general, both the solubility of water in silicate melts and the solubility of silicate materials in aqueous fluid increase with increasing pressure. This could suggest that, above a certain critical pressure and temperature (the second critical endpoint), silicate melts and aqueous fluid become

K. Mibe; M. Kanzaki; T. Kawamoto; K. N. Matsukage; Y. Fei; S. Ono

2004-01-01

223

Segmentation of the Nazca and South American plates along the Ecuador subduction zone from wide angle seismic profiles  

E-print Network

Segmentation of the Nazca and South American plates along the Ecuador subduction zone from wide describe the deep structure of the south Colombian ­ northern Ecuador convergent margin using travel time contrasting zones. In the North Zone, affected by four great subduction earthquakes during the 20th century

Paris-Sud XI, Université de

224

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

Microsoft Academic Search

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

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

2003-01-01

225

Metamorphic density controls on early-stage subduction dynamics  

NASA Astrophysics Data System (ADS)

Subduction is primarily driven by the densification of the downgoing oceanic slab, due to dynamic P-T-fields in subduction zones. It is crucial to unravel slab densification induced by metamorphic reactions to understand the influence on plate dynamics. By analyzing the density and metamorphic structure of subduction zones, we may gain knowledge about the driving, metamorphic processes in a subduction zone like the eclogitization (i.e., the transformation of a MORB to an eclogite), the breakdown of hydrous minerals and the release of fluid or the generation of partial melts. We have therefore developed a 2D subduction zone model down to 250 km that is based on thermodynamic equilibrium assemblage computations. Our model computes the "metamorphic density" of rocks as a function of pressure, temperature and chemical composition using the Theriak-Domino software package at different time stages. We have used this model to investigate how the hydration, dehydration, partial melting and fractionation processes of rocks all influence the metamorphic density and greatly depend on the temperature field within subduction systems. These processes are commonly neglected by other approaches (e.g., gravitational or thermomechanical in nature) reproducing the density distribution within this tectonic setting. The process of eclogitization is assumed as being important to subduction dynamics, based on the very high density (3.6 g/cm3) of eclogitic rocks. The eclogitization in a MORB-type crust is possible only if the rock reaches the garnet phase stability field. This process is primarily temperature driven. Our model demonstrates that the initiation of eclogitization of the slab is not the only significant process that makes the descending slab denser and is responsible for the slab pull force. Indeed, our results show that the densification of the downgoing lithospheric mantle (due to an increase of pressure) starts in the early subduction stage and makes a significant contribution to the slab pull, where eclogitization does not occur. Thus, the lithospheric mantle acts as additional ballast below the sinking slab shortly after the initiation of subduction. Our calculation shows that the dogma of eclogitized basaltic, oceanic crust as the driving force of slab pull is overestimated during the early stage of subduction. These results improve our understanding of the force budget for slab pull during the intial and early stage of subduction. Therefore, the complex metamorphic structure of a slab and mantle wedge has an important impact on the development and dynamics of subduction zones. Further Reading: Duesterhoeft, Oberhänsli & Bousquet (2013), submitted to Earth and Planetary Science Letters

Duesterhoeft, Erik; Oberhänsli, Roland; Bousquet, Romain

2013-04-01

226

Dehydration Behavior of Metapelites and Metabasites at Very low to low Grade Metamorphic Conditions  

NASA Astrophysics Data System (ADS)

Thermodynamic calculations have been undertaken in the system Na-Ca-K-Fe-Mg-Al-Si-Ti-H-O with the PERPLE_X software package (Connolly, 1990 and updates) for a better understanding of the dehydration behavior of metapelites and metabasites during prograde metamorphism. To obtain reasonable results for the temperature range 150-450° C at pressures up to 25 kbar, the subsequent solid solution models were introduced being compatible with the applied thermodynamic data set of Holland & Powell (1998 and updates): a three component model for Mg-Fe2+-Fe3+-pumpellyite, a two component model for Fe2+-Mg- stilpnomelane, a four component amphibole model (tremolite - Fe2+-tremolite - glaucophane - Mg- riebeckite), and a four component Na-pyroxene model (acmite - jadeite - diopside - hedenbergite). The water contents released by prograde metamorphism up to 450° C from MORB and psammopelitic compositions on top of oceanic crust, were obtained by calculating P-T pseudosections. Metabasite contains 6-7 wt% H2O bound to minerals at 150° C depending on the oxidation state. Along geotherms lower than 7° C/km typical for young subduction zones, no water is released up to 400° C. However, reduction of the rock causes release of small amounts of water. Metapsammopelitic rocks also store about 6 wt% H2O in minerals at 150° C. Considerable amounts of water are liberated by mineral reactions already in the temperature range 150-250° C also at the above mentioned low geotherms. This behavior determines the rheological characteristics of the upper oceanic crust during early subduction. If water is exclusively released in the sedimentary portion of the downgoing crust only this material gets weakened to be scraped off to form accretionary wedges. At geotherms of 15-20° C/km both lithologies show significant dehydration at very low metamorphic grade. For instance, in cold frontal paleoaccretionary prisms of the Chilean Coastal Cordillera metapelites by far dominate whereas in hotter basal accretionary prisms both low grade basic oceanic crust and continental sediments occur. We also hypothesize that accretionary wedge complexes with a clear dominance of sediments should not have formed in hot subduction zones typical for Precambrian Earth.

Massonne, H.; Willner, A. P.

2007-12-01

227

The effect of sediment recycling in subduction zones on the Hf isotope character of new arc crust, Banda arc, Indonesia  

NASA Astrophysics Data System (ADS)

A large portion of Earth's crust is formed at convergent plate boundaries that are accompanied by the subduction of sediments that can contain evolved crust-derived detritus. Partial melting of such sediments can strongly affect the trace element and isotope geochemistry of new arc rocks. Here, we present high-precision Lu-Hf-Zr concentration data and Hf isotope compositions for a series of volcanic rocks from the Banda arc, East Indonesia, to quantify the transfer of subducted Hf to the Banda arc crust and address the influence of recycled Hf in subduction zones on the Hf isotope systematics of arc rocks. Along-arc from NE to SW, the 176Hf/177Hf decreases from 0.28314 to 0.28268 ranging from predominantly mantle-like ratios towards more crustal signatures. Hf-Nd isotope co-variations require low Nd/Hf in the arc magma source, inconsistent with fluid addition to the arc melts, but in agreement with the involvement of partial sediment melts. The systematic decrease in Hf-Nd isotopes infers a NE-SW along-arc increase in the involvement of subducted continental material (SCM) in the arc magma source, consistent with ?18O and Nd-Sr-Pb isotope constraints. The along-arc decrease in Hf isotopes coupled with increasing Zr/Hf (from 31.9 to 36.1) and decreasing Lu/Hf suggests that the newly produced arc lavas contain crustal-derived Hf as a result of partial melting of SCM associated with the breakdown of zircon. Based on recent experimental estimates of temperatures required to achieve zircon breakdown, we infer that slab surface temperatures in the Banda arc region need to be as high as 925 °C. As a consequence of the inheritance of non-radiogenic Hf from SCM, the juvenile Banda arc crust exhibits Hf isotope model ages biased by hundreds of millions of years. We conclude that crust-formation ages derived from Hf isotope ratios of convergent margin rocks and their constituent minerals (such as zircon) can be geologically meaningless mixing ages, even when they readily preserve low ?18O values (i.e., < 6.5). These findings are discussed with respect to the inferred origin of Hadaean zircons at convergent plate boundaries, which appear consistent with an origin in a convergent margin setting.

Nebel, O.; Vroon, P. Z.; van Westrenen, W.; Iizuka, T.; Davies, G. R.

2011-03-01

228

Neogene Sediment Transport, Deposition, and Exhumation from the Southern Alaska Syntaxis to the Eastern Aleutian Subduction Zone  

NASA Astrophysics Data System (ADS)

Over 5 km of Neogene sedimentary strata are well exposed in the Chugach-St. Elias Ranges within the southern Alaska syntaxis. This syntaxis forms where the Pacific-North America plate boundary changes from the northwest-trending Queen Charlotte-Fairweather transform system to the southwest-trending Alaska-Aleutian subduction zone. Active collision and subduction of the buoyant Yakutat microplate in the syntaxis results in a wide collisional zone defined by active mountain belts, extensive glaciation, and thick packages of synorogenic strata. New stratigraphic and U-Th/He thermochronologic data from Neogene synorogenic strata, named the Yakataga and Redwood Formations, provide insights on collisional tectonics, glacial erosion, and sediment transport, deposition, burial, and exhumation from the onshore Chugach and St. Elias Ranges to the exposed accretionary prism of the Aleutian trench. Stratigraphic analyses show that along the southeastern part of the syntaxis, Neogene strata are characterized by deposition in braid delta, shallow marine, and glaciomarine slope apron depositional systems that resulted in construction of a broad continental shelf. In the central part of the syntaxis, marine shelf and upper slope environments deposited thick-bedded sandstone and mudstone in a thrust belt/foreland basin system. Along the southwestern part of the syntaxis, Neogene strata were deposited in a regional submarine fan system that filled the easternmost part of the Aleutian trench. Geologic mapping of the contact between the Yakataga Formation and underlying strata along the syntaxis document an angular unconformity with maximum stratigraphic separation (> 5 km) in the central part of the syntaxis. Along strike, this unconformity becomes conformable along both the southwestern and southeastern parts of the syntaxis. The regional angular unconformity and facies transitions both point to the importance of the central part of the syntaxis in the generation and distribution of synorogenic sediment. Apatite and zircon U-Th/He thermochronologic data from granitoid and gneissic clasts in conglomerate suggest that Neogene sediments were buried no deeper than ~2 km in the central and southeastern parts of the syntaxis, and that burial temperatures did not exceed ~40-45°C. In contrast, Neogene sediment deposited by submarine fans in the Aleutian trench along the southwestern part of the syntaxis were buried at depths of 5 to 7.5 km and reached temperatures between ~120-160°C. These strata were subsequently exhumed as the trench fill was incorporated into the growing accretionary prism. Collectively, our data show that the first-order sediment pathway along a glaciated syntaxis is dynamically linked to tectonic uplift, focused glacial erosion, deposition of thick packages of glacial marine sediment, and rapid exhumation along thrust belts and accretionary prisms.

Ridgway, K. D.; Witmer, J. W.; Enkelmann, E.; Plafker, G.; Brennan, P. R.

2011-12-01

229

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

230

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

231

Towards understanding carbon recycling at subduction zones - lessons from Central America  

NASA Astrophysics Data System (ADS)

Subduction zones provide the essential pathways for input of carbon from Earth’s external reservoirs (crust, sediments, oceans) to the mantle. However, carbon input to the deep interior is interrupted by outputs via the fore-arc, volcanic front, and back-arc regions. Coupled CO2 and He isotope data for geothermal fluids from throughout Central American (CA) are used to derive estimates of the output carbon flux for comparison with inputs estimated for the subducting Cocos Plate. The carbon flux carried by the incoming sediments is ~1.6 × 109 gCkm-1yr-1[1], as is the ratio of input carbon derived from pelagic limestone (L) and organic sediment (S), i.e., L/S ~10.7. Additionally, the upper 7 km of oceanic (crustal) basement supplies ~9.1 × 108 gCkm-1yr-1[2]: this flux is dominated by L-derived CO2. In terms of output, measured carbon concentrations coupled with flow rates for submarine cold seeps sites at the Costa Rica outer forearc yield CO2 and CH4 fluxes of ~ 6.1 × 103 and 8.0 × 105 (gCkm-1yr-1), respectively [3]. On the Nicoya Peninsula, the Costa Rica Pacific coastline (including the Oso Peninsula) and the Talamanca Mountain Range, coupled CO2-He studies allow recognition of a deep input (3He/4He up to 4RA) and resolution of CO2 into L- and S-components. There is an increase in the L/S ratio arc-ward with the lowest values lying close to diatomaceous ooze in the uppermost sequence of subducting sediment package. This observation is consistent with under-plating and removal of the uppermost organic-rich sediment from deeper subduction. As the input carbon fluxes of the individual sedimentary layers are well constrained [1], we can limit the potential steady-state flux of carbon loss at the subaerial fore-arc to ~ 6 × 107 gCkm-1yr-1, equivalent to ~88% of the input flux of the diatomaceous ooze, or < 4% of the total incoming sedimentary carbon. The greatest loss of slab-derived carbon occurs at the volcanic front. Estimates of the output CO2 flux along the CA front - 2-5 (× 108 gCkm-1yr-1) [4-5] together with identification of a slab origin (~90%) of the CO2, gives output estimates between 12% (Costa Rica) and 29% (El Salvador) of the sedimentary input [6]. The low L/S ratio found along the entire strike of the volcanic front precludes a significant C-contribution from oceanic basement of the subducting slab. Finally, arc-like L/S ratios behind the volcanic front in Honduras [6] indicates the back-arc inventory is composed of either entrained or ancient CO2 but not slab carbon released beyond the region of arc magma generation. Thus, at the CA subduction zone, significant carbon influx to the mantle can occur due to limited fore-arc and back-arc losses and modest C-outputs via the volcanic front. These observations are compared with other subduction zones where sediment lithologies, thermal conditions and water budgets differ, to address the question of understanding intrinsic and extrinsic controls on the mass balance of the mantle carbon reservoir. [1] Li and Bebout, JGR, 2005; [2] Hilton et al., Rev. Min. Geochem., 2002; [3] Furi et al., G-cubed, 2010; [4] Rodriguez et al., JVGR, 2004; [5] Zimmer et al., G-cubed, 2004; [6] De Leeuw et al., EPSL, 2007.

Hilton, D. R.; Barry, P. H.; Fischer, T. P.

2010-12-01

232

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

NASA Astrophysics Data System (ADS)

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

Papadakis, Giorgos; Vallianatos, Filippos; Sammonds, Peter

2014-05-01

233

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

NASA Astrophysics Data System (ADS)

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], but much lower than the estimated ˜{6} km/day of the 1999 Cascadia subduction zone event [Dragert et al., 2001]. Some just rupture limited areas below the gap regions. They tend to weaken the locking intensity (i.e., increase the slip rates) around the gap regions, which may become potential nucleation sites for future large events. Furthermore, nucleation, rupture propagation and re-locking behaviors, much like during a large earthquake, are also observed in some of these aseismic events, although they take place only near or below the downdip end of seismogenic zone. The transitional behavior of the friction law around the downdip end of the seismogenic zone may be a cause of the observed aseismic slip events. We are investigating the influence of constitutive parameters a and a - b (magnitude and distribution) on the aseismic slip patterns, and also of the length scale Dc which must be chosen larger than realistic to make simulations possible.

Liu, Y.; Rice, J. R.

2004-05-01

234

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

E-print Network

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

Paris-Sud XI, Université de

235

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

NASA Astrophysics Data System (ADS)

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

Yabe, Suguru; Ide, Satoshi; Yoshioka, Shoichi

2014-12-01

236

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

NASA Astrophysics Data System (ADS)

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

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

2009-12-01

237

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

NASA Astrophysics Data System (ADS)

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

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

2008-05-01

238

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

USGS Publications Warehouse

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

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

2004-01-01

239

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

240

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

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.

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

2015-01-01

241

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

NASA Astrophysics Data System (ADS)

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

Swapna, M.; Srivastava, Kirti

2014-12-01

242

Detailed Image of the Subducting Plate and Upper mantle Seismic Discontinuities in the Mariana Subduction Zone  

NASA Astrophysics Data System (ADS)

We use P-to-S converted teleseismic phases recorded at island and ocean bottom stations in Mariana to image the subducting plate and the upper mantle seismic discontinuities in the Mariana subduction zone. The land and seafloor stations which operated from June 2003 to May 2004, were deployed within the framework of the MARGINS Subduction Factory experiment of the Mariana system. The crust in the sudducting plate is observed at about 80--90 km depth beneath the islands of Saipan, Tinian and Rota. For most of the island stations, a low velocity layer is imaged in the forearc at depth between about 20 and 60 km, with decreasing depths toward the arc. The nature of this feature is not yet clear. We found evidence for double seismic discontinuities at the base of the transition zone near the Mariana slab. A shallower discontinuity is imaged at depths of ~650--715 km, and a deeper interface lies at ~740-- 770 km depth. The amplitudes of the seismic signals suggest that the shear velocity contrasts across the two features are comparable. These characteristics support the interpretation that the discontinuities are the results of the phase transformations in olivine (ringwoodite to post-spinel) and garnet (ilminite to perovskite), respectively, for the pyrolite model of mantle composition.

Tibi, R.; Wiens, D. A.; Shiobara, H.; Sugioka, H.; Yuan, X.

2006-12-01

243

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

NASA Astrophysics Data System (ADS)

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

Liu, Xin; Zhao, Dapeng

2014-12-01

244

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

NASA Astrophysics Data System (ADS)

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

Yabe, Suguru; Ide, Satoshi

2014-11-01

245

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

PubMed

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

Kushiro, Ikuo

2007-02-01

246

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

PubMed Central

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

Kushiro, Ikuo

2007-01-01

247

Forecast model for great earthquakes at the Nankai Trough subduction zone  

USGS Publications Warehouse

An earthquake instability model is formulated for recurring great earthquakes at the Nankai Trough subduction zone in southwest Japan. The model is quasistatic, two-dimensional, and has a displacement and velocity dependent constitutive law applied at the fault plane. A constant rate of fault slip at depth represents forcing due to relative motion of the Philippine Sea and Eurasian plates. The model simulates fault slip and stress for all parts of repeated earthquake cycles, including post-, inter-, pre- and coseismic stages. Calculated ground uplift is in agreement with most of the main features of elevation changes observed before and after the M=8.1 1946 Nankaido earthquake. In model simulations, accelerating fault slip has two time-scales. The first time-scale is several years long and is interpreted as an intermediate-term precursor. The second time-scale is a few days long and is interpreted as a short-term precursor. Accelerating fault slip on both time-scales causes anomalous elevation changes of the ground surface over the fault plane of 100 mm or less within 50 km of the fault trace. ?? 1988 Birkha??user Verlag.

Stuart, W.D.

1988-01-01

248

Resonant slow fault slip in subduction zones forced by climatic load stress  

NASA Astrophysics Data System (ADS)

Global Positioning System (GPS) measurements at subduction plate boundaries often record fault movements similar to earthquakes but much slower, occurring over timescales of ~1week to ~1year. These `slow slip events' have been observed in Japan, Cascadia, Mexico, Alaska and New Zealand. The phenomenon is poorly understood, but several observations hint at the processes underlying slow slip. Although slip itself is silent, seismic instruments often record coincident low-amplitude tremor in a narrow (1-5cycles per second) frequency range. Also, modelling of GPS data and estimates of tremor location indicate that slip focuses near the transition from unstable (`stick-slip') to stable friction at the deep limit of the earthquake-producing seismogenic zone. Perhaps most intriguingly, slow slip is periodic at several locations, with recurrence varying from 6 to 18months depending on which subduction zone (or even segment) is examined. Here I show that such periodic slow fault slip may be a resonant response to climate-driven stress perturbations. Fault slip resonance helps to explain why slip events are periodic, why periods differ from place to place, and why slip focuses near the base of the seismogenic zone. Resonant slip should initiate within the rupture zone of future great earthquakes, suggesting that slow slip may illuminate fault properties that control earthquake slip.

Lowry, Anthony R.

2006-08-01

249

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

NASA Astrophysics Data System (ADS)

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

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

2014-12-01

250

Stable Cl isotopes in subduction-zone pore waters: Implications for fluid-rock reactions and the cycling of chlorine  

Microsoft Academic Search

Stable Cl isotope ratios, measured in marine pore waters associated with the Barbados and Nankai subduction zones, extend significantly (to ˜-80\\/00) the range of delta37Cl values reported for natural waters. These relatively large negative values, together with geologic and chemical evidence from Barbados and Nankai and recent laboratory data showing that hydrous silicate minerals (i.e., those with structural OH sites)

Barbara Ransom; Arthur J. Spivack; Miriam Kastner

1995-01-01

251

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

Microsoft Academic Search

Multi-component sediment recovered from numerous drilling legs off SW Japan allow to characterize the input into the Nankai subduction zone. The succession approach- ing the large accretionary wedge consists of fine-grained turbidites interspersed with tephra, the first of which are dominated by smectite- (S) or illite-rich (I) clayey in- tervals within silts and fine sands. Combined ring shear and direct

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

2002-01-01

252

Character of slip and stress due to interaction between fault segments along the dip direction of a subduction zone  

Microsoft Academic Search

We have performed a two-dimensional numerical simulation to elucidate the physical processes governing earthquake behavior when significant stress perturbations are produced by interaction between fault segments. Our model involves two seismogenic segments separated down-dip on a subduction zone plate boundary and incorporates a rate- and state-dependent friction law. Based on repeated simulations under different scenarios, we find that the rate-

Keisuke Ariyoshi; Toru Matsuzawa; Yasuo Yabe; Naoyuki Kato; Ryota Hino; Akira Hasegawa; Yoshiyuki Kaneda

2009-01-01

253

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

SciTech Connect

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)

Hilde, T.W.C.

1984-08-01

254

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

255

A mega-trench survey of huge tsunami traces in Nemuro lowland along the Kuril subduction zone  

Microsoft Academic Search

A mega-trench survey (100m length, 20m width and 5m depth) for huge tsunami traces were conducted using by civil engineering techniques in Nemuro lowland near Lake Nanbutou along the Kuril subduction zone, eastern Hokkaido, northern Japan. According to our stratigraphic methods such as tephrochronology and AMS14C dating, we identified 15 tsunami sands in marsh deposits since 5500 yrBP and their

F. Nanayama; K. Shigeno; M. Ishii; Y. Shitaoka; R. Furukawa; K. Nishio; K. Takano; S. Inokuma

2008-01-01

256

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

Microsoft Academic Search

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

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

2007-01-01

257

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

NASA Astrophysics Data System (ADS)

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

Font, Yvonne; Segovia, Monica; Theunissen, Thomas

2010-05-01

258

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

NASA Astrophysics Data System (ADS)

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

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

2012-08-01

259

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

NASA Astrophysics Data System (ADS)

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

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

2008-12-01

260

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

Microsoft Academic Search

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

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

1996-01-01

261

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

NASA Astrophysics Data System (ADS)

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

Sverjensky, Dimitri; Manning, Craig

2014-05-01

262

Advanced computation for modeling fluid-solid dynamics in subduction zones  

NASA Astrophysics Data System (ADS)

Arc volcanism associated with subduction is generally considered to occur by a process where hydrous fluids are released from the slab, interact with the overlying mantle wedge to produce silicate rich magmas which are then transported to the arc. 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). This observation is remarkably robust yet insensitive to subduction parameters. This contrasts with new estimates on the variability of fluid release in global subduction zones which suggest a significant sensitivity of fluid release to slab thermal conditions. Reconciling these results implies some 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 comparable to P-T estimates for the dry solidus beneath mid-ocean ridges. 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. We discuss how successful these interactions are at focusing both fluids and hot solids to sub-arc regions worldwide. We also evaluate the efficacy of current wet melting parameterizations in these models. When driven by buoyancy alone, fluid migrates through the mantle wedge along nearly vertical trajectories. Only interactions with the solid flow at very low values of permeability or high values of fluid viscosity can cause deviations from this path. However, in a viscous, permeable medium, additional pressure gradients are generated by volumetric deformation due to variations in fluid flux. These pressure gradients can significantly modify the fluid flow paths. At shallow depths, compaction channels form along the rheological contrast with the overriding plate while in the mantle wedge itself porosity waves concentrate the fluid. When considering multiple, distributed sources of fluid, as predicted by thermodynamic models, interaction between layers in the slab itself can also cause significant focusing. As well as permeability, rheological controls and numerical regularizations place upper and lower bounds on the length-scales over which such interactions occur further modifying the degree of focusing seen. The wide range of behaviors described here is modeled using TerraFERMA (the Transparent Finite Element Rapid Model Assembler), which harnesses the advanced computational libraries FEniCS, PETSc and SPuD to provide the a flexible computational framework for exploring coupled multi-physics problems.

Spiegelman, Marc; Wilson, Cian; van Keken, Peter; Kelemen, Peter; Hacker, Bradley

2014-05-01

263

Interseismic coupling, segmentation and mechanical behavior of the Central Chile subduction zone.  

NASA Astrophysics Data System (ADS)

Global Positioning System (GPS) measurements carried out in Chile over the last two decades showed that an entire portion of the Nazca-South America subduction zone (38°S-24°S) was locked over this period of time. The induced accumulation of elastic deformation in the upper-plate was not released until the recent Maule earthquake of 27 February 2010 (Mw 8.8) that ruptured the southern part of this section. Locking or coupling between the two plates varies both with depth and along strike. Here we use our own GPS data (an updated solution of our extended network in central Chile), combined with other published data sets, to quantify the spatial variations of the coupling that prevailed before the Maule earthquake. Using a simple elastic model based on the back-slip assumption, we show that coupling variations on the subduction plane are sufficient to explain the observed surface deformation, with no need of a sliver in central Chile. We identify four segments characterized by higher coupling and separated by narrow areas of lower coupling. This segmentation is in good agreement with historical and recent seismicity in Chile. In particular, the La Serena Bay (30°S-28°S) where the locked zone vanishes is a stable boundary where historical events stopped. The Maule region that ruptured in feb. 27th of 2010 (Mw 8.8) from 38 S to 34 S, was characterized by a well developped fully locked zone that extents far indepth and narrows where the earthquake stopped propagating (San Antonio at 34°S and south of Arauco peninsula at 38°S). These narrow zones of lower coupling are often associated with irregular bathymetric or coastal features (fracture zones or peninsulas). Finally, coseismic and early post-seismic slip distribution of the Maule earthquake, occurring either in previously highly or weakly coupled zones, map a complex distribution of velocity-weakening and velocity-strengthening patches on the subduction interface.

Métois, M.; Socquet, A.; Vigny, C.

2012-04-01

264

Automatic picking and earthquake relocation for the Antilles subduction zone (1972-2013)  

NASA Astrophysics Data System (ADS)

Locations for earthquake recorded in the Antilles subduction zone are processed separately by regional observatories and ISC. There is no earthquake location catalog available compiling all available first arrival data. We aim to produce a best complete earthquake catalog by merging all available first arrival data for better constrains on earthquake locations. ISC provides the first arrival data of 29243 earthquakes (magnitude range from 1.4 to 6.4) recorded by PRSN (Porto Rico), SRC (British West Indies), and form FUNVISIS (Venezuela). IPGP provided the first arrival data of 68718 earthquakes (magnitude from 0.1 to 7.5) recorded by OVSG (Guadeloupe, 53226 earthquakes since 1981) and by OVSM (Martinique, 29931 earthquakes since 1972). IPGP also provides the accelerometer waveform data of the GIS-RAP network in the Antilles. The final catalog contains 84979 earthquakes between 1972 and 2013, 24528 of which we compiled additional data. We achieved automatic picking using the Component Energy Correlation Method. The CECM provide high precision phase detection, a realistic estimation of picking error and realistic weights that can be used with manual pick weights. The CECM add an average of 3 P-waves and 2 S-waves arrivals to 3846 earthquakes recoded by the GIS-RAP network since 2002. Cluster analysis, earthquake local tomography and relative locations are to be applied in order to image active faulting and migration of seismicity. This will help to understand seismic coupling in the seismogenic zone as well as triggering mechanisms of intermediate depth seismicity like fluid migration beneath the volcanic arc.

Massin, F.; Amorèse, D.; Bengoubou-Valerius, M.; Bernard, M.

2013-12-01

265

Interseismic coupling, segmentation and mechanical behavior of the central Chile subduction zone  

NASA Astrophysics Data System (ADS)

Global Positioning System (GPS) measurements carried out in Chile over the last two decades showed that an entire portion of the Nazca-South America subduction zone (38°S - 24°S) was locked over this period of time. The induced accumulation of elastic deformation in the upper-plate was not released until the recent Maule earthquake of 27 February 2010 (Mw 8.8) that ruptured the southern part of this section. Locking or coupling between the two plates varies both with depth and along strike. Here we use our own GPS data (an updated solution of our extended network in central Chile), combined with other published data sets, to quantify the spatial variations of the coupling that prevailed before the Maule earthquake. Using a simple elastic model based on the back-slip assumption, we show that coupling variations on the subduction plane are sufficient to explain the observed surface deformation, with no need of a sliver in central Chile. We identify four segments characterized by higher coupling and separated by narrow areas of lower coupling. This segmentation is in good agreement with historical and recent seismicity in Chile. In particular the narrow zones of lower coupling seem to have stopped most large seismic ruptures, including Maule's. These zones are often associated with irregular bathymetric or coastal features (fracture zones or peninsulas). Finally, coseismic and early post-seismic slip distribution of the Maule earthquake, occurring either in previously highly or weakly coupled zones, map a complex distribution of velocity-weakening and velocity-strengthening patches on the subduction interface.

MéTois, M.; Socquet, A.; Vigny, C.

2012-03-01

266

The South Sandwich "Forgotten" Subduction Zone and Tsunami Hazard in the South Atlantic  

NASA Astrophysics Data System (ADS)

While no large interplate thrust earthquakes are know at the "forgotten" South Sandwich subduction zone, historical catalogues include a number of events with reported magnitudes 7 or more. A detailed seismological study of the largest event (27 June 1929; M (G&R) = 8.3) is presented. The earthquake relocates 80 km North of the Northwestern corner of the arc and its mechanism, inverted using the PDFM method, features normal faulting on a steeply dipping fault plane (phi, delta, lambda = 71, 70, 272 deg. respectively). The seismic moment of 1.7*10**28 dyn*cm supports Gutenberg and Richter's estimate, and is 28 times the largest shallow CMT in the region. This event is interpreted as representing a lateral tear in the South Atlantic plate, comparable to similar earthquakes in Samoa and Loyalty, deemed "STEP faults" by Gover and Wortel [2005]. Hydrodynamic simulations were performed using the MOST method [Titov and Synolakis, 1997]. Computed deep-water tsunami amplitudes of 30cm and 20cm were found off the coast of Brazil and along the Gulf of Guinea (Ivory Coast, Ghana) respectively. The 1929 moment was assigned to the geometries of other know earthquakes in the region, namely outer-rise normal faulting events at the center of the arc and its southern extremity, and an interplate thrust fault at the Southern corner, where the youngest lithosphere is subducted. Tsunami hydrodynamic simulation of these scenarios revealed strong focusing of tsunami wave energy by the SAR, the SWIOR and the Agulhas Rise, in Ghana, Southern Mozambique and certain parts of the coast of South Africa. This study documents the potential tsunami hazard to South Atlantic shorelines from earthquakes in this region, principally normal faulting events.

Okal, E. A.; Hartnady, C. J. H.; Synolakis, C. E.

2009-04-01

267

South Sandwich: The Forgotten Subduction Zone and Tsunami Hazard in the South Atlantic  

NASA Astrophysics Data System (ADS)

While no large interplate thrust earthquakes are known at the South Sandwich subduction zone, historical catalogues include a number of earthquakes with reported magnitudes of 7 or more. We present a detailed seismological study of the largest one (27 June 1929; M (G&R) = 8.3). The earthquake relocates 80 km North of the Northwestern corner of the arc. Its mechanism, inverted using the PDFM method, features normal faulting on a steeply dipping fault plane (phi, delta, lambda = 71, 70, 272 deg.). The seismic moment, 1.7 10**28 dyn*cm, supports Gutenberg and Richter's estimate, and is 28 times the largest shallow CMT in the region. The 1929 event is interpreted as representing a lateral tear in the South Atlantic plate, comparable to similar earthquakes in Samoa and Loyalty, deemed "STEP faults" by Gover and Wortel [2005]. Hydrodynamic simulations using the MOST method [Titov and Synolakis, 1997] suggest deep-water tsunami amplitudes reaching 30 cm off the coast of Brazil, where it should have had observable run-up, and 20 cm along the Gulf of Guinea (Ivory Coast, Ghana). We also simulate a number of potential sources obtained by assigning the 1929 moment to the geometries of other known earthquakes in the region, namely outer-rise normal faulting events at the center of the arc and its southern extremity, and an interplate thrust fault at the Southern corner, where the youngest lithosphere is subducted. A common feature of these models is the strong focusing of tsunami waves by the SAR, the SWIOR, and the Agulhas Rise, resulting in amplitudes always enhanced in Ghana, Southern Mozambique and certain parts of the coast of South Africa. This study documents the potential tsunami hazard to South Atlantic shorelines from earthquakes in this region, principally normal faulting events.

Okal, E. A.; Hartnady, C. J.

2008-12-01

268

Depth variation of upper mantle seismic discontinuities in the region of the Tonga subduction zone  

NASA Astrophysics Data System (ADS)

In order to study the mantle transition zone structure near the Tonga subduction zone in the southwestern Pacific, we analyzed receiver functions from teleseismic P waves recorded by both island broadband seismic stations and ocean bottom seismographs deployed as part of the RIDGE2000 Lau Basin Imaging Project. First, we used an iterative deconvolution in the time domain to obtain the receiver functions [Ammon, 1999]. The orientations of the OBS's were derived from a combination of the results of ambient noise correlation and Rayleigh-Wave polarization methods. Then a 3-D stacking approach [T. J. Owens, 2000] is adopted to stack those receiver functions for all station-event pairs. We binned the study area and stacked the traces within a certain radius from each bin at depths with a 10-km increment. The Tonga slab subducts with the fastest known convergence velocity, and the tectonics of this region are very complex. A flat slab with scattered seismicity is found to the west of the main Tonga slab in tomographic models. An elevation of the 410-km and a depression of the 660-km discontinuities are expected at and near the subducting slab since the olivine transitions are perturbed by the cold slab at these depths. From the on-going receiver function study, we are expecting to present detailed structure of the mantle transition zone at this complex plate boundary. Thomas J. Owens, Andrew A. Nyblade, Harold Gurrola, Charles A. Langston. Mantle transition zone structure beneath Tanzania, East Africa. Geophysical research letters, Vol. 27, No.6, Pages 827-830, 2000.

Chen, J.; Wiens, D. A.; Emry, E.; Wei, S. S.; Cai, C.; Webb, S. C.; Menke, W. H.; Zha, Y.; Chen, Y. J.

2013-12-01

269

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

NASA Astrophysics Data System (ADS)

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

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

2014-01-01

270

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

USGS Publications Warehouse

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

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

2013-01-01

271

Nitrogen sources and recycling at subduction zones: Insights from the Izu-Bonin-Mariana arc  

NASA Astrophysics Data System (ADS)

We report new chemical and nitrogen isotopic data from 29 volcanic and hydrothermal gas samples covering eight centers in the Izu-Bonin-Mariana (IBM) arc to investigate the sources, flux, and mass balance of nitrogen at a "cool" convergent margin. The majority of samples have high N2/He (1217-17,300) and low CO2/N2,exc. (78-937), implying addition of nitrogen from the subducting slab. This inference is supported by positive (i.e., sediment-like) ?15N values (up to 5.5‰) in most samples. The exception to these trends is Agrigan in the Mariana arc, with low N2/He (˜200), high CO2/N2,exc. (˜1500), and negative ?15N. Mixing calculations suggest an average of 34% of the nitrogen in our samples is derived from subducted sediment, or 75% after correction for atmospheric contamination. Sediment-derived N2 fluxes estimated by three different methods range from 0.25 × 108 to 1.11 × 108 mol yr-1 N2, representing 4%-17% of the total nitrogen input flux or 11%-51% of the sedimentary nitrogen input flux. The altered oceanic crust is identified as an important contributor to the arc nitrogen budget, and the ?15N of the residual nitrogen subducted into the mantle is estimated at approximately -1.9‰. Despite similarities in gas chemistry and ?15N values, our conclusions regarding nitrogen recycling for IBM are markedly different than those for the Nicaraguan segment of the Central American arc, and we suggest that thermal regime is the major control on nitrogen recycling within subduction zones. The global nitrogen cycle is estimated to be in steady state, suggesting either that subducted sediments are an unlikely source for heavy nitrogen in plume-related rocks or secular variation in the isotopic composition of subducted sediments. Better constraints on nitrogen recycling at other arcs are required to test these conclusions.

Mitchell, Euan C.; Fischer, Tobias P.; Hilton, David R.; Hauri, Erik H.; Shaw, Alison M.; de Moor, J. Maarten; Sharp, Zachary D.; Kazahaya, Kohei

2010-02-01

272

The geochemical behaviour of W in subduction zones: constraints from high precision isotope dilution measurements  

NASA Astrophysics Data System (ADS)

Assessing the behaviour of W during silicate Earth's differentiation is hampered by low abundances of W in terrestrial reservoirs, making sufficiently precise and accurate measurements difficult. Previous results (e.g., Newsom et al. 1996) indicate a lower W/Th of the mantle (ca. 0.19) compared to the Earth's crust, (ca. 0.26), suggesting that W appears to be more incompatible than Th. New data for MORB (Munker et al. 2007), however, demonstrate that W/Th is not significantly fractionated during dry peridotite melting, tentatively suggesting a fractionation of the two elements during crust formation by subduction related processes. We present high precision W and Nb-Ta, Zr-Hf data obtained by isotope dilution, using a mixed 183W-180Ta- 94Zr-180Hf-176Lu tracer and multiple collector inductively coupled plasma mass spectrometry (MC-ICPMS). This enables the direct determination of W and HFSE from one sample digestion. For some samples, a "nugget effect" as previously reported for PGE was observed, reflecting sample heterogeneity. Measured Ta and W abundances determined in basaltic glasses and whole rock powders from various island arc settings yield Ta/W ratios of 0.6 to 1.7, significantly lower than the values reported for MORB (4-6). In contrast, Nb/Ta for the samples overlap with MORB values, suggesting that Nb and Ta were not mobile in the magma sources. These systematic differences indicate that W does not behave as other HFSE (Nb-Ta, Zr-Hf) in subduction zones but rather displays a higher mobility in slab components. Intra-oceanic arc suites involving subducted pelagic sediment in their sources generally display higher W/Th compared to magmas without sediment-derived components in their sources, reflecting the higher initial W abundances in subducted pelagic sediments. A fractionation of W/Th during crust formation could consequently be explained by a selective W enrichment relative to Th during subduction processes.

Konig, S.; Munker, C.; Schuth, S.

2007-12-01

273

3-D Imaging of the Northern Hikurangi Subduction Zone, New Zealand: Subducted Sediment Near Shallow SSEs  

NASA Astrophysics Data System (ADS)

We examine 3-D Vp and Vp/Vs along the northern and central Hikurangi subduction zone, New Zealand, where the subducting Pacific plate offshore is heterogeneous with many seamounts and the overlying Australian plate has rotation and extension. The downdip limit of interseismic coupling is shallower than 15-km, indicating that much of the plate interface (PI) is slipping aseismically, and large shallow slow-slip events (SSEs) and small deep SSEs have been observed. We selected 2600 spatially distributed earthquakes, incorporating upgraded permanent network, and temporary networks from 1993-94, 2001, and 2011-12. Onshore-offshore marine-seismic data were also included, helping to constrain the shallow velocities and the PI gradient, where sampled. Our method used earthquake differential times and receiver differential times with gradational inversions. The results show major variations along strike and regions of subducted sediment. Above the shallow plate interface (less than 20-km depth) and north of Gisborne there is a 70-km long zone of high Vp/Vs and low Vp, which is interpreted as subducted sediment with high fluid-pressure. This section also showed subducted sediment in shallower depth offshore seismic reflection data, and corresponds to an area of shallow SSE's. It is bounded to the north by a high Qp, higher Vp, moderate Vp/Vs feature which may be a seamount, and to the south by high Qp, high Vp features that may correspond to Cretaceaous volcanics or a subducted seamont. At the northeastern tip of the North Island, low Vp above the PI is also consistent with thick sediment. In deeper zones, where the PI is 25-45 km depth, there are northern and central zones of thick low Vp, low Qp material related to underplated sediments, which are uplifting the Raukumara and Kaimanawa Ranges. Small deep (25-45 km) SSEs are related to the central deep underplated sediment zone, but no SSEs have been observed in the northern underplated zone.

Eberhart-Phillips, D. M.; Bannister, S. C.

2013-12-01

274

Models of postseismic deformation after megaearthquakes: the role of various rheological and geometrical parameters of the subduction zone  

NASA Astrophysics Data System (ADS)

The postseismic deformations following subduction megaearthquakes are characterized by a horizontal velocity which, once non-dimensionalized by the coseismic displacement, increases with distance to the trench then presents an almost constant value for distances between 500 and 1500 km. The vertical velocity features a strong narrow peak on the trenchward side of the volcanic arc. Subsidence is observed in the far-field. In order to understand better the implications of these observations, the influence of the geometry of low viscosity regions in subduction zones on the postseismic deformations is analyzed using a 2-D finite element model with viscoelastic rheologies. The slab dip in the top 80 km Θtop, and deeper Θbottom and the locking depth all have a limited impact on the ratio of horizontal postseismic velocity over coseismic displacement. The smaller Θbottom, the smaller the amplitude of the predicted vertical velocity in the middle-field (200-500 km from the trench). The presence of the slab at asthenospheric depths affects very significantly both the horizontal and vertical velocities. Models with an 80 km thick lithosphere, where the relaxation occurs only in the asthenosphere, are characterized by a trenchward horizontal velocity decreasing very moderately in the middle-field and an uplift maximum on the continental side of the volcanic arc, at odds with the observations. A low viscosity channel (LVCh) over the deep parts of the subduction interface or a low viscosity wedge (LVW) have a considerable impact on the middle-field horizontal and vertical velocities: the trenchward horizontal velocities are very significantly increased while the vertical velocities are characterized by strong uplift over the deep parts of the subduction interface. In the case of a low viscosity wedge, a marked subsidence further away from the trench, on the continent side of the volcanic arc is predicted. While the low-viscosity wedge affects little the far-field horizontal velocities, the LVCh increases them significantly. The thicknesses of the lithosphere and the asthenosphere also have a strong impact on both the middle-field and the far-field velocities. The larger they are, the further from the trench are the maxima of the ratio of the postseismic over coseismic horizontal displacement and of the far-field subsidence. 3-D modeling with a geometry as precise as possible of the various zones with postseismic creep associated with each megaearthquake is necessary to derive more precise conclusions. However, the 2-D modeling results obtained here, compared with postseismic data, point towards lithospheres and asthenospheres surprisingly similar in various areas of the world, with thicknesses around 70 and 200 km respectively and towards the presence of a LVW and/or a LVCh. The systematic description of the role of each parameter presented here will facilitate the choice of the parameters to vary in 3-D models.

Trubienko, O.; Garaud, J.-D.; Fleitout, L.

2014-01-01

275

Structural and metamorphic evolution of the Orocopia Schist and related rocks, southern California: Evidence for late movement on the Orocopia fault  

NASA Astrophysics Data System (ADS)

The Pelona, Orocopia, and Rand Schists (POR schists) of southern California and southwesternmost Arizona are late Mesozoic or early Tertiary subduction complexes that underlie Precambrian to Mesozoic continental basement along the low-angle Vincent-Chocolate Mountains (VCM) fault system. The VCM faults are often considered to be remnants of the original subduction zone, but recent work indicates that many have undergone substantial postsubduction reactivation. In the Orocopia Mountains, for example, the Orocopia Schist exhibits an exceptionally complex structural and metamorphic history due to multiple periods of movement along the Orocopia fault. Structures in the schist include isoclinal folds with axial-planar schistosity, open-to-tight folds that fold schistosity, penetrative stretching lineations, and crenulation lineations, all of which show a nearly 360° range in trend. Folds and lineations that trend approximately NE-SW occur throughout the schist and are thought to be part of an early phase of deformation related to subduction. Folds of this orientation show no consistent vergence. Folds and lineations that trend approximately NW-SE are concentrated near the Orocopia fault and are interpreted to have formed during exhumation of the schist. The NW-SE trending folds, and shear indicators in late-stage mylonite at the top of the schist, consistently verge NE. The exhumation event culminated in emplacement of the schist against brittlely deformed upper plate. Exhumation of the Orocopia Schist was accompanied by retrograde replacement of garnet, biotite, epidote, and calcic amphibole by chlorite, calcite, and sericite. Matrix amphibole has a lower Na/Al ratio than amphibole inclusions in albite, consistent with a late-stage decrease in pressure. As NE vergence in the Orocopia Mountains is associated with exhumation of the schist, the NE movement along other segments of the VCM fault may also be late and therefore have no bearing on the facing direction of the VCM subduction zone, contrary to past interpretations.

Jacobson, Carl E.; Dawson, M. Robert

1995-08-01

276

Geodynamics of collision and collapse at the AfricaArabiaEurasia subduction zone an introduction  

E-print Network

; (2) continental drift, sea-floor spreading and formation of ocean basins; (3) subduction initiation and drifting in the Western Mediterranean (Dercourt et al. 1986), and with initiation of the Tyrrhenian oceanic subduction stage closes the oceanic basin, even- tually resulting in the arrival of a continental

Utrecht, Universiteit

277

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

USGS Publications Warehouse

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

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

1971-01-01

278

Separation of supercritical slab-fluids to form aqueous fluid and melt components in subduction zone magmatism.  

PubMed

Subduction-zone magmatism is triggered by the addition of H(2)O-rich slab-derived components: aqueous fluid, hydrous partial melts, or supercritical fluids from the subducting slab. Geochemical analyses of island arc basalts suggest two slab-derived signatures of a melt and a fluid. These two liquids unite to a supercritical fluid under pressure and temperature conditions beyond a critical endpoint. We ascertain critical endpoints between aqueous fluids and sediment or high-Mg andesite (HMA) melts located, respectively, at 83-km and 92-km depths by using an in situ observation technique. These depths are within the mantle wedge underlying volcanic fronts, which are formed 90 to 200 km above subducting slabs. These data suggest that sediment-derived supercritical fluids, which are fed to the mantle wedge from the subducting slab, react with mantle peridotite to form HMA supercritical fluids. Such HMA supercritical fluids separate into aqueous fluids and HMA melts at 92 km depth during ascent. The aqueous fluids are fluxed into the asthenospheric mantle to form arc basalts, which are locally associated with HMAs in hot subduction zones. The separated HMA melts retain their composition in limited equilibrium with the surrounding mantle. Alternatively, they equilibrate with the surrounding mantle and change the major element chemistry to basaltic composition. However, trace element signatures of sediment-derived supercritical fluids remain more in the melt-derived magma than in the fluid-induced magma, which inherits only fluid-mobile elements from the sediment-derived supercritical fluids. Separation of slab-derived supercritical fluids into melts and aqueous fluids can elucidate the two slab-derived components observed in subduction zone magma chemistry. PMID:23112158

Kawamoto, Tatsuhiko; Kanzaki, Masami; Mibe, Kenji; Matsukage, Kyoko N; Ono, Shigeaki

2012-11-13

279

Estimated damage from the Cascadia Subduction Zone tsunami: A model comparisons using fragility curves  

NASA Astrophysics Data System (ADS)

Building damage from a hypothetical Cascadia Subduction Zone tsunami was estimated using two methods and applied at the community scale. The first method applies proposed guidelines for a new ASCE 7 standard to calculate the flow depth, flow velocity, and momentum flux from a known runup limit and estimate of the total tsunami energy at the shoreline. This procedure is based on a potential energy budget, uses the energy grade line, and accounts for frictional losses. The second method utilized numerical model results from previous studies to determine maximum flow depth, velocity, and momentum flux throughout the inundation zone. The towns of Seaside and Canon Beach, Oregon, were selected for analysis due to the availability of existing data from previously published works. Fragility curves, based on the hydrodynamic features of the tsunami flow (inundation depth, flow velocity, and momentum flux) and proposed design standards from ASCE 7 were used to estimate the probability of damage to structures located within the inundations zone. The analysis proceeded at the parcel level, using tax-lot data to identify construction type (wood, steel, and reinforced-concrete) and age, which was used as a performance measure when applying the fragility curves and design standards. The overall probability of damage to civil buildings was integrated for comparison between the two methods, and also analyzed spatially for damage patterns, which could be controlled by local bathymetric features. The two methods were compared to assess the sensitivity of the results to the uncertainty in the input hydrodynamic conditions and fragility curves, and the potential advantages of each method discussed. On-going work includes coupling the results of building damage and vulnerability to an economic input output model. This model assesses trade between business sectors located inside and outside the induction zone, and is used to measure the impact to the regional economy. Results highlight critical businesses sectors and infrastructure critical to the economic recovery effort, which could be retrofitted or relocated to survive the event. The results of this study improve community understanding of the tsunami hazard for civil buildings.

Wiebe, D. M.; Cox, D. T.; Chen, Y.; Weber, B. A.; Chen, Y.

2012-12-01

280

Earthquake precise locations catalog for the Lesser Antilles subduction zone (1972-2013)  

NASA Astrophysics Data System (ADS)

Locations for earthquake recorded in the Lesser Antilles subduction zone are processed separately by regional observatories, NEIC and ISC. There is no earthquake location catalog available compiling all available phase arrival data. We propose a new best complete earthquake catalog by merging all available phase arrival data for better constrains on earthquake locations. ISC provides the phase arrival data of 29243 earthquakes (magnitude range from 1.4 to 6.4) recorded by PRSN (Porto Rico), SRC (British West Indies), and from FUNVISIS (Venezuela). We add phases data from IPGP observatories for 68718 earthquakes from magnitudes 0.1 to 7.5 (OVSG, Guadeloupe, recorded 53226 earthquakes since 1981, and OVSM, Martinique, recorded 29931 earthquakes since 1972). IPGP also provides the accelerometer waveform data of the GIS-RAP network. We achieved automatic picking on the GIS-RAP data using the Component Energy Correlation Method. The CECM provides high precision phase detection, a realistic estimation of picking error and realistic weights that can be used with manual pick weights. The CECM add an average of 3 P-waves and 2 S-waves arrivals to 3846 earthquakes recorded by the GIS-RAP network since 2002. The final catalog contains 84979 earthquakes between 1972 and 2013, 24528 of which we compiled additional data. We achieve earthquake location using NonLinLoc, regional P and S waves data and a set of one dimensional velocity models. We produce improved locations for 22974 earthquakes (better residuals, on equal or larger arrival dataset) and improved duration magnitudes for 6258 earthquakes (using duration data and improved locations). A subset of best constrained 15626 hypocenters (with more than 8 phases and an average RMS of 0.48±0.77s) could be used for structural analysis and earthquake local tomography. Relative locations are to be applied in order to image active faulting. We aim to understand coupling in the seismogenic zone as well as triggering mechanisms of intermediate depth seismicity like fluid migration beneath the volcanic arc.

Massin, Frederick; Amorese, Daniel; Beauducel, Francois; Bengoubou-Valérius, Mendy; Bernard, Marie-Lise; Bertil, Didier

2014-05-01

281

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

NASA Astrophysics Data System (ADS)

Long period magnetotelluric (MT) measurements were made in southwestern British Columbia in 2003 to investigate the electrical structure of the Cascadia subduction zone as part of the POLARIS project. Data were recorded in the period range 1 to 25000 seconds at 36 field sites using long-period MT systems with fluxgate magnetometers. The instruments used were the Geological Survey of Canada LIMS systems and University of Alberta NIMS systems. MT data were collected at many locations in the Canadian Cordillera during the Lithoprobe project, but the new MT data are the first to give the long period data needed for imaging deep structure. The stations were concentrated on a profile that extended from Port Renfrew on Vancouver Island to Shuswap Lake. Some MT stations were located in Washington State to investigate the 3-D effects of the low resistivity salt water in the Strait of Georgia and Puget Sound. The MT impedance data have been analyzed with 2-D inversions and the models reveal the following features: (1) The data on Vancouver Island detect low resistivities above the subducting Juan de Fuca plate, similar to previous results on a profile to the northwest. This anomaly corresponds to an area of high seismic reflectivity ('E-reflector') and low seismicity, and may be related to fluids originating from the oceanic plate. (2) A zone of low resistivity is present beneath the volcanic arc at a depth of 15-20 km below the surface. (3) The Intermontane and Omineca Belts are characterized by high resistivities in the upper crust and low resistivity in the lower crust, similar to models of previous studies such as Lithoprobe. The lower crustal resistivity decreases to the east, as also indicated by vertical magnetic transfer functions. Further analysis of these data are in progress to determine the resistivity of the underlying upper mantle and to evaluate the depth of the astenosphere. A second deployment of the NIMS instruments in summer 2004 will extend the line across the Rocky Mountains into the Foothills of Alberta, and will produce a continuous long-period MT profile across the entire Canadian Cordillera.

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

2004-05-01

282

Along-arc Variations in Volatile Cycling in the Nicaragua-Costa Rica Subduction Zone  

NASA Astrophysics Data System (ADS)

Recent seismic imaging in the Nicaragua-Costa Rica subduction zone, in combination with geochemical data, has revealed systematic along-arc variations in volatile cycling from the subducting slab, through the mantle wedge, to the melts generated at arc volcanoes. Velocity and attenuation tomography based on P and S phases from local events recorded by the 2004-2006 TUCAN Broadband Seismic Experiment resolve a high velocity, low attenuation subducting slab, a shallow wedge corner with intermediate attenuation, and a slower, more highly attenuating mantle wedge beneath the arc and back-arc. However, velocity and attenuation structures also manifest strong along-arc variations. The subducting slab beneath Nicaragua contains a shallow layer of slow velocities and is more attenuating than the slab beneath Costa Rica, consistent with greater slab hydration (for example, 10-20% mantle serpentinization). Continuing the along-arc trend, attenuation in the Nicaraguan mantle wedge at depths of 60-100 km is significantly higher than in Costa Rica, suggesting that enhanced slab hydration beneath Nicaragua results in a more hydrated wedge. Moving to the arc, olivine melt inclusions indicate that Nicaraguan magmas are more water-rich on average than those erupted in the Costa Rican arc within the region best-imaged by the TUCAN array. Assuming that these wet melts equilibrated with olivine in the mantle wedge, then the inferred decrease in wedge hydration from Nicaragua to Costa Rica can explain the along-arc variation in wedge attenuation. In addition, although bulk magma compositions in the vicinity of the Nicaragua and Costa Rica attenuation profiles are significantly different, they imply a similar temperature of 1265 ± 25°C at a depth comparable to the middle of the wedge (75 km) when primary melt water content is taken into account in calculating olivine-liquid temperatures. A roughly vertical column of high Vp/Vs extends from the slab surface to the arc in Nicaragua, but this feature dies away along the arc to Costa Rica. This anomaly could reflect larger melt fractions in the Nicaraguan wedge, possibly produced by the apparently greater wedge hydration.

Fischer, K. M.; Rychert, C. A.; Syracuse, E. M.; Abers, G. A.; Plank, T.

2008-12-01

283

Rare earth elements as indicators of hydrothermal processes within the East Scotia subduction zone system  

NASA Astrophysics Data System (ADS)

The East Scotia subduction zone, located in the Atlantic sector of the Southern Ocean, hosts a number of hydrothermal sites in both back-arc and island-arc settings. High temperature (>348 °C) ‘black smoker’ vents have been sampled at three locations along segments E2 and E9 of the East Scotia back-arc spreading ridge, as well as ‘white smoker’ (<212 °C) and diffuse (<28 °C) hydrothermal fluids from within the caldera of the Kemp submarine volcano. The composition of the endmember fluids (Mg = 0 mmol/kg) is markedly different, with pH ranging from <1 to 3.4, [Cl-] from ?90 to 536 mM, [H2S] from 6.7 to ?200 mM and [F-] from 35 to ?1000 ?M. All of the vent sites are basalt- to basaltic andesite-hosted, providing an ideal opportunity for investigating the geochemical controls on rare earth element (REE) behaviour. Endmember hydrothermal fluids from E2 and E9 have total REE concentrations ranging from 7.3 to 123 nmol/kg, and chondrite-normalised distribution patterns are either light REE-enriched (LaCN/YbCN = 12.8-30.0) with a positive europium anomaly (EuCN/Eu?CN = 3.45-59.5), or mid REE-enriched (LaCN/NdCN = 0.61) with a negative Eu anomaly (EuCN/Eu?CN = 0.59). By contrast, fluids from the Kemp Caldera have almost flat REE patterns (LaCN/YbCN = 2.1-2.2; EuCN/Eu?CN = 1.2-2.2). We demonstrate that the REE geochemistry of fluids from the East Scotia back-arc spreading ridge is variably influenced by ion exchange with host minerals, phase separation, competitive complexation with ligands, and anhydrite deposition, whereas fluids from the Kemp submarine volcano are also affected by the injection of magmatic volatiles which enhances the solubility of all the REEs. We also show that the REE patterns of anhydrite deposits from Kemp differ from those of the present-day fluids, potentially providing critical information about the nature of hydrothermal activity in the past, where access to hydrothermal fluids is precluded.

Cole, Catherine S.; James, Rachael H.; Connelly, Douglas P.; Hathorne, Ed C.

2014-09-01

284

GPS measurements and finite element modeling of the earthquake cycle along the Middle America subduction zone  

NASA Astrophysics Data System (ADS)

We model surface deformation recorded by GPS stations along the Pacific coasts of Mexico and Central America to estimate the magnitude of and variations in frictional locking (coupling) along the subduction interface, toward a better understanding of seismic hazard in these earthquake-prone regions. The first chapter describes my primary analysis technique, namely 3-dimensional finite element modeling to simulate subduction and bounded-variable inversions that optimize the fit to the GPS velocity field. This chapter focuses on and describes interseismic coupling of the Oaxaca segment of the Mexican subduction zone and introduces an analysis of transient slip events that occur in this region. Our results indicate that coupling is strong within the rupture zone of the 1978 Ms=7.8 Oaxaca earthquake, making this region a potential source of a future large earthquake. However, we also find evidence for significant variations in coupling on the subduction interface over distances of only tens of kilometers, decreasing toward the outer edges of the 1978 rupture zone. In the second chapter, we study in more detail some of the slow slip events that have been recorded over a broad area of southern Mexico, with emphasis on their space-time behavior. Our modeling indicates that transient deformation beneath southern Mexico is focused in two distinct slip patches mostly located downdip from seismogenic areas beneath Guerrero and Oaxaca. Contrary to conclusions reached in one previous study, we find no evidence for a spatial or temporal correlation between transient slip that occurs in these two widely separated source regions. Finally, chapter three extends the modeling techniques to new GPS data in Central America, where subduction coupling is weak or zero and the upper plate deformation is much more complex than in Mexico. Cocos-Caribbean plate convergence beneath El Salvador and Nicaragua is accompanied by subduction and trench-parallel motion of the forearc. Our GPS velocity field is best fit by a model with strongly locked faults in the volcanic arc and a weakly coupled subduction interface. In this region, seismic hazards associated with subduction are therefore low, but are high for crustal faults, in agreement with records of historic seismicity.

Correa Mora, Francisco

285

Imaging megathrust zone and Yakutat/Pacific plate interface in Alaska subduction zone  

NASA Astrophysics Data System (ADS)

We image the subducted slab underneath a 450 km long transect of the Alaska subduction zone. Dense stations in southern Alaska are set up to investigate (1) the geometry and velocity structure of the downgoing plate and their relation to slab seismicity, and (2) the interplate coupled zone where the great 1964 (magnitude 9.3) had greatest rupture. The joint teleseismic migration of two array datasets (MOOS, Multidisciplinary Observations of Onshore Subduction, and BEAAR, Broadband Experiment Across the Alaska Range) based on teleseismic receiver functions (RFs) using the MOOS data reveal a shallow-dipping prominent low-velocity layer at ~25-30 km depth in southern Alaska. Modeling of these RF amplitudes shows a thin (<6.5 km) low-velocity layer (shear wave velocity of ~3 km/s), which is ~20-30% slower than normal oceanic crustal velocities, between the subducted slab and the overriding North American plate. The observed low-velocity megathrust layer (with P-to-S velocity ratio (Vp/Vs) exceeding 2.0) may be due to a thick sediment input from the trench in combination of elevated pore fluid pressure in the channel. The subducted crust below the low-velocity channel has gabbroic velocities with a thickness of 11-12 km. Both velocities and thickness of the low-velocity channel abruptly increase as the slab bends in central Alaska, which agrees with previously published RF results. Our image also includes an unusually thick low-velocity crust subducting with a ~20 degree dip down to 130 km depth at approximately 200 km inland beneath central Alaska. The unusual nature of this subducted segment has been suggested to be due to the subduction of the Yakutat terrane. We also show a clear image of the Yakutat and Pacific plate subduction beneath the Kenai Peninsula, and the along-strike boundary between them at megathrust depths. Our imaged western edge of the Yakutat terrane, at 25-30 km depth in the central Kenai along the megathrust, aligns with the western end of the geodetically locked patch with high slip deficit, and coincides with the boundary of aftershock events from the 1964 earthquake. It seems plausible that this sharp change in the nature of the downgoing plate controls the slip distribution of great earthquakes on this plate interface.

Kim, Y.; Abers, G. A.; Li, J.; Christensen, D. H.; Calkins, J. A.

2013-05-01

286

Imaging the megathrust zone and Yakutat/Pacific plate interface in the Alaska subduction zone  

NASA Astrophysics Data System (ADS)

We image the subducted slab underneath a 450 km long transect of the Alaska subduction zone. Dense stations in southern Alaska are set up to investigate (1) the geometry and velocity structure of the downgoing plate and their relationship to slab seismicity, and (2) the interplate coupled zone where the great 1964 earthquake (magnitude 9.3) exhibited the largest amount of rupture. The joint teleseismic migration of two array datasets based on teleseismic receiver functions (RFs) reveals a prominent, shallow-dipping low-velocity layer at ~25-30 km depth in southern Alaska. Modeling of RF amplitudes suggests the existence of a thin (3-5 km) low-velocity layer (shear wave velocity of ~2.0-2.5 km/s) that is ~20-40% slower than underlying oceanic crustal velocities, and is sandwiched between the subducted slab and the overriding North America plate. The observed low-velocity megathrust layer (with P-to-S velocity ratio of 1.9-2.3) may be due to a thick sediment input from the trench in combination with elevated pore fluid pressure in the channel. The subducted crust below the low-velocity channel has gabbroic velocities with a thickness of ~15 km. Both velocities and thickness of the low-velocity channel abruptly increase downdip in central Alaska, which agrees with previously published results. Our image also includes an unusually thick low-velocity crust subducting with a ~20 degree dip down to 130 km depth at approximately 200 km inland beneath central Alaska. The unusual nature of this subducted segment has been suggested to be due to the subduction of the Yakutat terrane. We also show a clear image of the Yakutat and Pacific plate subduction beneath the Kenai Peninsula, and the along-strike boundary between them at megathrust depths. Our imaged western edge of the Yakutat terrane, at 25-30 km depth in the central Kenai along the megathrust, aligns with the western end of a geodetically locked patch with high slip deficit, and coincides with the boundary of aftershock events from the 1964 earthquake. It appears that this sharp change in the nature of the downgoing plate could control the slip distribution of great earthquakes on this plate interface.

Kim, Y.; Abers, G. A.; Li, J.; Christensen, D. H.; Rondenay, S.

2013-12-01

287

Convergent margin structure and tectonics of the Java subduction zone (105°E-122°E)  

NASA Astrophysics Data System (ADS)

The Java margin is the site of oceanic subduction of the Indo-Australian plate underneath the Indonesian archipelago. Data from a suite of geophysical experiments conducted between 1997-2006 using RV SONNE as platform include seismic and seismological studies, potential field measurements and high-resolution seafloor bathymetry mapping. Tomographic inversions provide an image of the ongoing deformation of the forearc and the deep subsurface. We investigate the role of various key mechanisms that shape the first-order features characterizing the present margin architecture. Our results show a high variability in subduction zone processes along the Java margin, ranging from accretionary subduction to erosive processes to zero-budget mass transfer. These variations are closely linked to changes in character of the incoming plate. Off Western Java (105°E -109°E), near-full accretion of the trench sediment fill is associated with a well-developed accretionary prism fronting a 4 km deep forearc basin. The Central Java segment (109°E -115°E) experiences the collision of an oceanic plateau dotted with numerous seamounts, causing large-scale uplift of the forearc, coupled with erosion of the frontal prism and correlated mass wasting processes. Intense deformation of the forearc basin results from thrusting and compressional forces. In the neighbouring segment farther to the east (115°E-119°E), the lack of significant sediment input to the trench supports the notion that recycling of upper plate material in the forearc sustains the massive outer high observed here adjacent to a mature forearc basin. The incoming oceanic plate of the Argo Abyssal plain is devoid of a sediment drape and the original spreading fabric overprinted by bending-related faulting near the trench shape its morphology. The transition zone from the Java margin to the Banda Arc (119°E-122°E) experiences the early stages of continent-island arc collision associated with the convergence of the Australian Scott Plateau and the Sumba Block. Our contribution evaluates the differences in architecture and evolution along the Java forearc from a marine perspective to better understand the variation in tectonic styles and segmentation of the convergent margin, including its onshore components.

Kopp, H.; Barckhausen, U.; Djajadihardja, Y.; Engels, M.; Flueh, E. R.; Hindle, D. A.; Lueschen, E.; Mueller, C.; Planert, L.; Reichert, C. J.; Shulgin, A. A.; Wittwer, A.

2009-12-01

288

Intense interface seismicity triggered by a shallow slow slip event in the Central Ecuador subduction zone  

NASA Astrophysics Data System (ADS)

document a 1 week long slow slip event (SSE) with an equivalent moment magnitude of 6.0-6.3 which occurred in August 2010 below La Plata Island (Ecuador), south of the rupture area of the 1906 Mw = 8.8 megathrust earthquake. GPS data reveal that the SSE occurred at a depth of about 10 km, within the downdip part of a shallow (<15 km), isolated, locked patch along the subduction interface. The availability of both broadband seismometer and continuous geodetic station located at the La Plata Island, 10 km above the SSE, enables a careful analysis of the relationships between slow and rapid processes of stress release along the subduction interface. During the slow slip sequence, the seismic data show a sharp increase of the local seismicity, with more than 650 earthquakes detected, among which 50 have a moment magnitude between 1.8 and 4.1. However, the cumulative moment released through earthquakes accounts, at most, for 0.2% of the total moment release estimated from GPS displacements. Most of the largest earthquakes are located along or very close to the subduction interface with focal mechanism consistent with the relative plate motion. While the earthquake sizes show a classical distribution (Gutenberg-Richter law with a b-value close to 1), the space-time occurrence presents a specific pattern. First, the largest earthquakes appear to occur randomly during the slow slip sequence, which further evidence that the seismicity is driven by the stress fluctuations related to aseismic slip. Moreover, the seismicity observed during the SSE consists in individual events and families of repeating earthquakes. These observations indicate that the stress increment induced by the episodic aseismic slip may lead both to sudden seismic moment release and to progressive rupture within small locked patches. This study offers an a posteriori interpretation of the seismogenesis in the Central Ecuador subduction zone, where intense seismic swarms have been regularly observed (1977, 1998, 2002, and 2005). These swarms have likely been triggered by large-magnitude slow slip events.

ValléE, Martin; Nocquet, Jean-Mathieu; Battaglia, Jean; Font, Yvonne; Segovia, Monica; RéGnier, Marc; Mothes, Patricia; Jarrin, Paul; Cisneros, David; Vaca, Sandro; Yepes, Hugo; Martin, Xavier; BéThoux, Nicole; Chlieh, Mohamed

2013-06-01

289

Consideration of optimum site distribution for detecting seafloor crustal deformation at the Nankai subduction zone  

NASA Astrophysics Data System (ADS)

The Philippine Sea plate subducts beneath the southwest Japan from the Nankai Trough with a rate of about 4-6 cm/yr, where great interplate earthquakes have repeatedly occurred every 100-150 years. To clarify the mechanism of earthquake occurrence at such subduction zones, we require the geodetic data obtained from not only onshore area but also offshore area. However it is difficult to estimate the strength of interplate coupling in offshore areas, due to the poverty of those data. For this issue, we have conducted seafloor geodetic observation using GPS/Acoustic techniques around the Nankai Trough since 2004. In this system, we estimate the position of a surveying vessel by Kinematic GPS analysis and measure the distance between the vessel and the benchmark on the seafloor by Acoustic measurements. Next, we determine the location of the benchmark and detected crustal movement on the seafloor. In the Kumano Basin, we have two seafloor benchmarks, which are located about 60 and 80 km away from the deformation front of the Nankai Trough. The observations from 2005 to 2008 have illustrated that those benchmarks are moving at rates of about 5-6 cm/yr toward west-northwest with velocity uncertainties of about 2 cm/yr relative to the Amurian plate. In this study, in order to estimate infer coupling at the Nankai Trough, we calculated surface deformations accompanied with subduction of the Philippine Sea plate in an elastic half-space and compared them with on- and offshore GPS velocities. Then, we checked the effect of seafloor geodetic observation on slip resolution on the plate interface. Moreover, we investigated optimum seafloor site distribution at the Nankai Trough using numerical simulation, because we require more seafloor sites to understand spatial variation of the slip and strain accumulation on the plate interface. We conclude that seafloor geodetic observation data provide good constraints for the estimation of slips at the shallower part of the plate interface, especially at the depths of 10-20 km, where slip resolution is low without using only offshore geodetic data.

Watanabe, T.; Tadokoro, K.; Sugimoto, S.; Okuda, T.; Miyata, K.; Kuno, M.

2009-12-01

290

An integrated geodynamic model of the Nankai subduction zone and neighboring regions from geophysical inversion and modeling  

NASA Astrophysics Data System (ADS)

I investigate large-scale deep crustal structures of the Nankai subduction zone and neighboring region using regional magnetic and gravity anomalies, heat flow measurements, and earthquake hypocenters. It is found that ages, dip angles, and geothermal states of the subducting slab have direct influences on mantle wedge serpentinization. The weakest serpentinization observed in the Nankai forearc region is associated with the youngest downgoing plate of the Shikoku Basin. Conspicuous gravity anomalies identified in the forearc region are coincidental spatially with magnetic anomalies after the reduction to the pole, a mathematical procedure that helps relocate magnetic sources and boundaries, and allows us to more easily interpret magnetic data. It is argued that these patches of magnetic and gravity anomalies are caused by the same sources of anomalous density and magnetization, and are linked directly to preexisting structures such as magnetic anomalies and their boundaries in the subducting oceanic crust. Since the gravity and magnetic anomaly patches are found to be closely related to interplate seismogenic behaviors in the Nankai subduction zone, I suggest that major magnetic boundaries in the Shikoku Basin are likely weak places for slab tears that trigger seismic segmentations along the subduction zone. Application of the Parker-Oldenburg algorithm to Bouguer gravity anomalies yields a 3D Moho topography. Curie-point depths are also estimated from the magnetic anomalies with reduction to the pole using a windowed wavenumber-domain algorithm. Window sizes are found to have little effects on the average Curie-point depths other than lowering lateral resolutions. A wide zone of deep Curie depths is identified in southwest Japan, relating to strong influence from the subduction of the relatively young and warm Shikoku Basin crust of the Philippine Sea plate. Curie depths so obtained can be correlated well with heat flow measurements, which cluster around a theoretical curve when the average thermal conductivity is about 3.0 W/(m °C). Using constraints from both Curie depths and heat flow, I also model the shallow geothermal field of the subduction zone. Earthquake hypocenters plotted against Moho and Curie depths and geothermal fields on three transects confirm early studies that downdip limits of seismogenic zones along the Nankai plate boundary do not extend down to the island arc Moho and their temperatures are more or less close to 350 °C. Geothermal field has direct influences on earthquake distributions in the overriding island arc and accretionary prism, within the subducting oceanic lithosphere, and along the interplate boundary.

Li, Chun-Feng

2011-01-01

291

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

E-print Network

peridotites from Pohorje (Eastern Alps, Slovenia) Jan C.M. De Hoog 1,* Marian Janák 2 Mirijam Vrabec 3 Keiko H Garnet peridotites from the Slovenska Bistrica Ultramafic Complex (SBUC) in the Pohorje Mts., Eastern depleted mantle of oceanic affinity. The metamorphic assemblage of the garnet peridotites consists

292

Neodymium and Hafnium Isotope Systematics of ~2.7 Ga Adakites and Magnesian Andesites, Superior Province, Canada: Implications for Archean Subduction Zone Petrogenetic Processes  

NASA Astrophysics Data System (ADS)

An association of adakites and magnesian andesites (MA) that erupted within typical intra-oceanic arc tholeiitic to calc-alkaline basalts has recently been documented in ~2.7 Ga Wawa greenstone belts. The adakites and MA yield 2670\\pm66 Ma (2? ) and 2623\\pm160 Ma (2? ) Sm-Nd and Lu-Hf isochron ages, respectively. Within error these ages agree with U-Pb zircon ages (2750-2695 Ma), suggesting that the Sm-Nd and Lu-Hf systems were not significantly disturbed by post-emplacement alteration processes. At 2.7 Ga the adakites have larger positive initial ?Nd values (+2.2 to +2.8) than the MA (+0.4 to +2.0). Calculated initial ?Hf values,using the 176Lu decay constant of Scherer et al.[1] of the MA (+2.6 to +4.9) overlap with those of the adakites (+3.5 to +4.9) but extend to slightly lower values. Clearly positive initial ?Nd and ?Hf values of the adakites show that their basaltic precursors, subducted late Archean oceanic crust with a short crustal residence, were derived from a long-term depleted mantle source. The lower initial ?Nd values of the MA with respect to that of the associated adakites can be explained by melting of a sub-arc mantle that had been variably enriched by recycling of continental material into the sub-arc mantle. Collectively, the Nd and Hf isotopic characteristics of the MA can be attributed to the decoupling of these elements during recycling of continental material into the sub-arc mantle prior to 2.7 Ga arc. During this recycling process, Hf was more conservative than Nd, suggesting sub-arc mantle enrichment by fluids [2-3]. Since the late Archean Wawa adakites and magnesian andesites were generated by subduction zone processes similar to those operating in Cenozoic arcs, it is likely that late Archean oceanic crust, and island arc crust, was also created and destroyed by geodynamic processes similar to modern plate tectonics. In the Late Archean, crustal recycling and slab melting therefore played an important role for the generation of heterogeneity in the Archean upper mantle. References [1]Scherer, E., Munker, C., Mezger, K., 2001. Calibration of the Lutetium-Hafnium clock. Science, 293: 683-687. [2]Pearce, J.A., Kempton, P.D., Nowell, G.M., and Noble, S.R., 1999. Hf-Nd element and isotope perspective on the nature and provenance of mantle and subduction components in western Pacific arc-basin systems. J. Petrol., 40: 1579-1611. [3]Woodhead, J.D., Hergt, J.M., Davidson, J.P., and Eggins, S.M., 2001. Hafnium isotope evidence for 'conservative' element mobility during subduction zone processes. Earth Planet. Sci. Lett., 192: 331-346.

Polat, A.; Munker, C.

2002-12-01

293

Relative impact of mantle densification and eclogitization of slabs on subduction dynamics: A numerical thermodynamic/thermokinematic investigation of metamorphic density evolution  

NASA Astrophysics Data System (ADS)

Understanding the relationships between density and spatio-thermal variations at convergent plate boundaries is important for deciphering the present-day dynamics and evolution of subduction zones. In particular, the interaction between densification due to mineralogical phase transitions and slab pull forces is subject to ongoing investigations. We have developed a two-dimensional subduction zone model that is based on thermodynamic equilibrium assemblage calculations and includes the effects of melting processes on the density distribution in the lithosphere. Our model calculates the 'metamorphic density' of rocks as a function of pressure, temperature and chemical composition in a subduction zone down to 250 km. We have used this model to show how the hydration, dehydration, partial melting and fractionation processes of rocks all influence the metamorphic density and greatly depend on the temperature field within the subduction system. These processes are largely neglected by other approaches that reproduce the density distribution within this complex tectonic setting. Our model demonstrates that the initiation of eclogitization (i.e., when crustal rocks reach higher densities than the ambient mantle) of the slab is not the only significant process that makes the descending slab denser and generates the slab pull force. Instead, the densification of the lithospheric mantle of the sinking slab starts earlier than eclogitization and contributes significantly to slab pull in the early stages of subduction. Accordingly, the complex metamorphic structure of the slab and the mantle wedge has an important impact on the development of subduction zones.

Duesterhoeft, Erik; Quinteros, Javier; Oberhänsli, Roland; Bousquet, Romain; de Capitani, Christian

2014-12-01

294

Nitrogen isotopes in ophiolitic metagabbros: A re-evaluation of modern nitrogen fluxes in subduction zones and implication for the early Earth atmosphere  

NASA Astrophysics Data System (ADS)

Nitrogen contents and isotope compositions together with major and trace element concentrations were determined in a sequence of metagabbros from the western Alps (Europe) in order to constrain the evolution and behavior of N during hydrothermal alteration on the seafloor and progressive dehydration during subduction in a cold slab environment (8 °C/km). The rocks investigated include: (i) low-strain metagabbros that equilibrated under greenschist to amphibolite facies (Chenaillet Massif), blueschist facies (Queyras region) and eclogite facies (Monviso massif) conditions and (ii) highly-strained mylonites and associated eclogitic veins from the Monviso Massif. In all samples, nitrogen (2.6-55 ppm) occurs as bound ammonium ( NH4+) substituting for K or Na-Ca in minerals. Cu concentrations show a large variation, from 73.2 to 6.4 ppm, and are used as an index of hydrothermal alteration on the seafloor because of Cu fluid-mobility at relatively high temperature (>300 °C). In low-strain metagabbros, ?15N values of +0.8‰ to +8.1‰ are negatively correlated with Cu concentrations. Eclogitic mylonites and veins display Cu concentrations lower than 11 ppm and show a ?15N-Cu relationship that does not match the ?15N-Cu correlation found in low-strain rocks. This ?15N-Cu correlation preserved in low-strain rocks is best interpreted by leaching of Cu-N compounds, possibly of the form Cu(NH 3) 22+, during hydrothermal alteration. Recognition that the different types of low-strain metagabbros show the same ?15N-Cu correlation indicates that fluid release during subduction zone metamorphism did not modify the original N and Cu contents of the parent hydrothermally-altered metagabbros. In contrast, the low Cu content present in eclogitic veins and mylonites implies that ductile deformation and veining were accompanied either by a loss of copper or that externally-derived nitrogen was added to the system. We estimate the global annual flux of N subducted by metagabbros as 4.2 (±2.0) × 10 11 g/yr. This value is about half that of sedimentary rocks, which suggests that gabbros carry a significant portion of the subducted nitrogen. The net budget between subducted N and that outgassed at volcanic arcs indicates that ˜80% of the subducted N is not recycled to the surface. On a global scale, the total amount of N buried to the mantle via subduction zones is estimated to be three times higher than that released from the mantle via mid-ocean ridges, arc and intraplate volcanoes and back-arc basins. This implies that N contained in Earth surface reservoirs, mainly in the atmosphere, is progressively transferred and sequestered into the mantle, with a net flux of ˜9.6 × 10 11 g/yr. Assuming a constant flux of subducted N over the Earth's history indicates that an amount equivalent to the present atmospheric N may have been sequestered into the silicate Earth over a period of 4 billion years.

Busigny, Vincent; Cartigny, Pierre; Philippot, Pascal

2011-12-01

295

Metamorphic Rocks  

NSDL National Science Digital Library

Here is an in-depth description of metamorphic rocks, from their classification to formation and identification. It covers types of metamorphism (including Barrovian, or regional rock changes), classification by foliation, and metamorphic processes (facies and zones). An alphabetical list of rocks with picture, composition, description, tectonic association, and type of metamorphism is given. Common metamorphic minerals are covered as well.

2007-12-12

296

Pattern and kinematic polarity of late Mesozoic extension in continental NE Asia: Perspectives from metamorphic core complexes  

NASA Astrophysics Data System (ADS)

Late Mesozoic extension in NE Asia resulted in the development of a large extensional province. Metamorphic core complexes (MCCs) are the major features in this province and have 40Ar/39Ar ages of 130-110 Ma for the mylonites and U-Pb zircon ages of 150-110 Ma for the integral granitic intrusions. Based on this and previous studies, this paper summarizes major characteristics of these MCCs and recognizes a regional kinematic shear sense. Most MCCs in the Transbaikalia region, Sino-Mongolia border tract, and the northwest-central portion of the North China craton (NCC) show a top-to-the-southeast (SE) shear, whereas those in the eastern and southern NCC locally underwent top-to-the-northwest (NW) shear. The three largest basins (Songliao, Huabei and Ordos) in North China are located in the transitional zone between domains of opposing shear sense. We interpret the extension in the Transbaikalia, Sino-Mongolia tract and northwestern part of the NCC to reflect late-orogenic collapse of thickened crust following Middle-Late Jurassic collision along the Okhotsk suture. The southeastward extension is probably controlled by crustal-scale top-to-the-SE tangential shear. The transition from contraction to extension is marked by detachment faults that nucleated as extensional crenulation cleavage (ecc, i.e., C') in sub-horizontal ductile shear zones late in orogenic crustal thickening. The combined effect of gravitational loading and thermal-uplifting is considered to be the origin of the late-or post-orogenic collapse. The top-to-the-NW extension in the NE of the NCC might reflect antithetic sub-extensional zones or Mesozoic back-arc extension as a far-field effect of Cretaceous Pacific plate subduction.

Wang, Tao; Zheng, Yadong; Zhang, Jinjiang; Zeng, Lingsen; Donskaya, Tatiana; Guo, Lei; Li, Jianbo

2011-12-01

297

In situ Raman study and thermodynamic model of aqueous carbonate speciation in equilibrium with aragonite under subduction zone conditions  

NASA Astrophysics Data System (ADS)

Carbonate minerals may be recycled into the mantle at subduction zones. However, the evolution of carbonate minerals in equilibrium with aqueous fluids as well as the nature of the chemical species of dissolved carbon in the deep crust and mantle at high PT conditions are still unknown. In this study, we report an integrated experimental and theoretical study of the equilibration of CaCO3 minerals with pure water at subduction zone conditions over the pressure and temperature ranges 5-80 kbar and 300-400 °C. The fluid speciation was studied using in situ Raman spectroscopy. The relative amounts of dissolved carbonate and bicarbonate were estimated from the corrected areas of the Raman bands of the carbonate and bicarbonate ions and used to constrain a theoretical thermodynamic model of the fluid speciation and solubility of aragonite. At 300-400 °C, our results indicate that the proportion of dissolved C present as CO2 strongly decreases in fluids in equilibrium with aragonite at P > 10 kbar. CO2 is replaced by HCO3- and CaHCO3+ which predominate until P > 40 kbar, where CO32- and CaCO30 become the dominant C-species. At higher temperatures, the theoretical model indicates that CO2 again becomes a major species in fluids in equilibrium with aragonite depending on the pressure.

Facq, Sébastien; Daniel, Isabelle; Montagnac, Gilles; Cardon, Hervé; Sverjensky, Dimitri A.

2014-05-01

298

Comparison of crustal and upper mantle heterogeneity in different time periods: Indonesian subduction zone to northern Australia  

NASA Astrophysics Data System (ADS)

Earthquake events from the Indonesian subduction zone recorded in northern Australia show a long and high-frequency coda associated with both P and S waves. Regional events recorded by Warramunga array in northern Australia can separate out wave propagation through the mantle by focusing on the coherent signal across the medium-aperture array. Most of the incoherent wave components result from structures in the vicinity of the array with small-scale lengths of 1-2 km or smaller. The coherent phases with relatively rapid changes in waveforms are associated with the scattering of seismic waves by crustal and mantle heterogeneity, but in some case can be related to structural effects near the source. As the depth of the source increases, the coherent portion of the seismic wavefield tends to become much simpler, which suggests that the heterogeneity tends to weaken at depth with larger-scale length. We compare the coherent signal features of earthquakes from the Indonesian subduction zone that have occurred in recent years with those in the early 1980s, first studied by Kennett (Phys Earth Planet Inter 47: 319-332, 1987). The general characteristics of the coherent signal variation with depth in recent years are the same as those observed in 1980s, but the variations are larger. This change suggests a stringer variation in heterogeneity with depth than before, which may bear important information about the dynamic processes and evolution of the crust and upper mantle.

Sun, Weijia; Fu, Li-Yun; Kennett, B. L. N.

2014-02-01

299

A new method to investigate the dynamic properties of hydrous minerals and melts pertaining to subduction zones (Invited)  

NASA Astrophysics Data System (ADS)

Hydrous phases and melts play an important role in the geochemical and geodynamical evolution of the Earth's interior, and in particular subduction zones. Melts form an important basis for understanding the current state of our planet, as they are responsible for forming the crust of our planet and may likely play a role in understanding low-velocity regions deep within our planet. In an effort to obtain a more detailed understanding of minerals under high-pressure temperature conditions, we have developed a novel metric for detecting the solid-liquid phase boundary of iron-bearing materials at high-pressures using synchrotron Mössbauer spectroscopy (SMS), also known as nuclear forward scattering. Focused synchrotron radiation with 1 meV bandwidth passes through a laser-heated Fe-bearing sample inside a diamond anvil cell. The characteristic SMS time signature vanishes when melting occurs. This process is described by the Lamb-Mössbauer factor, a quantity that is directly related to the mean-square displacement of the iron atoms. Therefore, we measure the dynamics of the atoms in the material, in contrast to a static diffraction measurement. As this method monitors the dynamics of the atoms, the SMS technique provides a new and independent means of melting point determination for materials under high-pressure, as well as access to vibrational properties of the solid near its melting point (Jackson et al. EPSL 2013). In this presentation, we will discuss the applications to phases relevant to subduction zones.

Jackson, J. M.

2013-12-01

300

Common Pb isotope mapping of UHP metamorphic zones in Dabie orogen, Central China: Implication for Pb isotopic structure of subducted continental crust  

NASA Astrophysics Data System (ADS)

We report Pb isotopic compositions for feldspars separated from 57 orthogneisses and 2 paragneisses from three exhumed UHPM slices representing the North Dabie zone, the Central Dabie zone and the South Dabie zone of the Dabie orogen, central-east China. The feldspars from the gneisses were recrystallized during Triassic continental subduction and UHP metamorphism. Precursors of the orthogneisses are products of Neoproterozoic bimodal magmatic events, those in north Dabie zone emplaced into the lower crust and those in central and south Dabie zones into middle or upper crust, respectively. On a 207Pb/204Pb vs. 206Pb/204Pb diagram, almost all orthogneisses data lie to the left of the 0.23 Ga paleogeochron and plot along the model mantle evolution curve with the major portion of the data plotting below it. On a 208Pb/204Pb vs. 206Pb/204Pb diagram the most of data of north Dabie zone extend in elongate arrays along the lower crustal curve and others extend between the lower crustal curve to near the mantle evolution curve for the plumbotectonics model. This pattern demonstrates that the Pb isotopic evolution of the feldspars essentially ended at 0.23 Ga and the orthogneiss protoliths were principally dominated by reworking of ancient lower crust with some addition of juvenile mantle in the Neoproterozoic rifting tectonic zone. According to geological evolution history of the locally Dabie orogen, a four-stage Pb isotope evolution model including a long time evolution between 2.0 and 0.8 Ga with a lower crust type U/Pb ratio (? = 5-6) suggests that magmatic emplacement levels of the protoliths of the orthogneisses in the Dabie orogen at 0.8 Ga also play an important role in the Pb evolution of the exhumed UHPM slices, corresponding to their respective Pb characters at ca. 0.8-0.23 Ga. For example, north Dabie zone requires low ? values (3.4-9.6), while central and south Dabie zones require high ? values (10.9-17.2). On the other hand, Pb isotopic mixing between north and central or south Dabie zones during retro-grade metamorphism enhanced by the extensive magmatism in the Cretaceous has also been observed in the 207Pb/204Pb vs. 206Pb/204Pb and 208Pb/204Pb vs. 206Pb/204Pb diagrams. A combined study of common Pb isotopic compositions of Dabie orthogneisses and Sulu UHPM rocks from the Chinese Continental Scientific Drilling project demonstrates that a slab marked by extremely unradiogenic Pb observed in the main hole was absent in the Dabie orogen. However, occurrence of some Mesozoic granitoids with such unradiogenic character in the Dabie orogen suggests that their source may be a buried unradiogenic unit underlying below north Dabie zone. This case study clearly shows that whether the position of the Dabie data relative to the orogen curve of the plumbotectonic model is helpful in understanding the Pb isotopic structure and evolution of subducted continental crust.

Shen, Ji; Wang, Ying; Li, Shu-Guang

2014-10-01

301

Tectonic history of subduction zones inferred from retrograde blueschist PT paths  

Microsoft Academic Search

Many Phanerozoic convergent plate junctions are marked by discontinuous blueschist belts, reflecting relatively high-pressure (P) prograde trajectories. Common blueschist paragneisses, such as those of the western Alps, exhibit widespread overprinting by greenschist and\\/or epidote-amphibolite facies assemblages. For this type of high-P belt, retrograde metamorphism involved fairly rapid, nearly isothermal decompression; some terranes underwent continued heating during early stages of pressure

W. G. Ernst

1988-01-01

302

Contrasting Protoliths of Cretaceous Metamorphic Rocks from the Luk Ulo Accretionary Wedge Complex of Central Java, Indonesia  

NASA Astrophysics Data System (ADS)

Rocks of the Lok Ulo Accretionary Complex crop out over a small (<100 km2) area in the Karangsambung residency of Central Java. They are part of Cretaceous accretionary wedge complexes in Central Indonesia, which are distributed sporadically in an arc extending from southwest and central Java to southeast Kalimantan and southern Sulawesi. The Lok Ulo complex consists of various types of rocks occurring as tectonic slabs in a black-shale matrix tectonic melange. The slabs are composed of a dismembered ophiolite, sedimentary rocks, and crystalline schists and gneisses. Detailed work on all the various metamorphic rock types in the Lok Ulo complex will be the focus of the study. Our investigations already show that the metamorphic rocks have two different kinds of protoliths and differ in P-T evolution as well. The first group (called `oceanic plate protolith') consists of fine-grained metabasites with metapelitic intercalations ranging from greenschist to amphibolite facies. High-pressure rocks such as eclogite, partially containing lawsonite, jadeite-glaucophane schist and blueschist crop out in a thin zone between the low-grade schists and a serpentinite zone along Kali Muncar. They are associated with a succession of metabasalt, serpentinite, chert and red limestone as common constituents of an ophiolite. The second group (called `continental crustal protolith') consists of low to high grade medium pressure metapelites, calc-silicate rocks, and metagranites (gneisses, quartzites, marbles, felsic granulites), and minor bimodal low grade metavolcanic. These rocks are presumably associated with a monotonous sequence of metapelites from the chlorite zone up to the garnet zone exposed in the northern and eastern part of the Karangsambung area (e.g. Kali Loning). Our findings suggest that the metamorphic rocks from the Lok Ulo complex are not the simple result of subduction metamorphism along the Indo-Australian oceanic plate (margin of the Sundaland craton) in the early Cretaceous as previously thought. The observed presence of low to high grade schists and gneisses of continental supracrustal parentage point to an early involvement of continental crust during the collisional event at least in the Karangsambung area (eastern part of the subduction zone).

Kadarusman, A.; Massone, H.; Permana, H.; Munasri, A.

2005-12-01

303

In situ Raman study of dissolved CaCO3 minerals under subduction zone conditions  

NASA Astrophysics Data System (ADS)

The fate and the characteristics of the Earth's deep carbon reservoirs are still not well understood [1]. The connection between the surficial and the deep-Earth carbon cycles occurs at subduction zones where carbon is transported into the mantle and where hydrous silicates and carbonate minerals break down releasing H2O and C-species in fluids associated with mantle metasomatism and the generation of arc volcanism [2]. In order to obtain mass balance between recycling and burial in the deep mantle, the study of the dissolution of CaCO3 minerals in equilibrium with aqueous fluids under mantle conditions is crucial. We report a novel integrated experimental and theoretical study of the equilibration of CaCO3 minerals with aqueous solutions (pure water or NaCl solutions) at high pressures and temperatures (0.5 to 8 GPa and 250 to 500 °C). The fluid speciation was studied using in situ Raman spectroscopy coupled to an externally heated membrane type DAC equipped with 500 ?m pure synthetic diamond anvils. In a typical experiment, the aqueous fluid and a calcite crystal [3] were loaded in a rhenium gasket. The pressure was determined from the calibrated shift of the carbonate ?1 symmetric stretching mode of aragonite and the temperature measured with a K-type thermocouple. Raman spectra were recorded using a Labram HR800 Raman spectrometer (Horiba Jobin-Yvon) coupled to a Spectra Physics Ar+ laser. At equilibrium with an aragonite crystal, the Raman data show that bicarbonate is the most abundant species in low-pressure fluids (below 4 GPa) whereas carbonate becomes progressively dominant at higher pressure. After correction from their Raman cross-sections [4], the relative amounts of dissolved carbonate and bicarbonate were estimated from the areas of the Raman bands of the carbonate and bicarbonate ions (?1 and ?5 symmetric stretching modes, respectively). The presence of sodium chloride influences the speciation by extending the pressure field where the bicarbonate species is dominant in the fluid. The Raman data were also used to constrain a theoretical thermodynamic model of the fluid speciation in equilibrium with CaCO3. Building on published calcite solubility data from 400 - 700 °C and 2 - 16 kb [3, 5], revised thermodynamic properties of aqueous CO2 and HCO3-, data for the aqueous CaHCO3+ complex from 4 to 90 °C [6], and estimated dielectric constants of water, enabled an equation of state characterization of the standard Gibbs free energy of CaHCO3+ at the conditions of the Raman study. At 300 - 400 °C, the Raman speciation results were used to constrain equilibrium constants involving the carbonate ion. The results indicate that CO2 is a minor species in fluids in equilibrium with aragonite at 300 - 500 °C and P > 1.0 GPa. Instead, the CaHCO3+ species becomes important at high pressures until carbonate ion becomes the dominant C-species. [1] Marty, B. and Tolstikhin, I.N., Chemical Geology 145, 233 (1998) [2] Schmidt, M.W. and Poli, S., Earth and Planetary Science Letters 163, 163 (1998) [3] Caciagli, N.C. and Manning, C.E., Contributions to Mineralogy and Petrology 146, 275 (2003) [4] Frantz, J., Chemical Geology 152, 211 (1998) [5] Fein, J.B. and Walther, J.V., Contributions to Mineralogy and Petrology 103, 317 (1989) [6] Plummer, L.N. and Busenberg, E., Geochimica et Cosmochimica Acta 46, 1011 (1982)

Facq, S.; Daniel, I.; Sverjensky, D. A.

2012-12-01

304

Shear-wave splitting and mantle anisotropy in the southern South American subduction zone  

NASA Astrophysics Data System (ADS)

The goal of this study is to constrain mantle flow above and below the subducting Nazca plate at latitudes of 30°-41° S. In this segment of the South American subduction zone, slab dip varies dramatically, including a region of flat slab subduction in the north and greater dip angles (~30°) in the south, where the segment ends at a slab gap associated with Chile Ridge. We measured shear-wave splitting in over 200 S arrivals from local earthquakes at permanent stations PLCA (USGS/GTSN) and PEL (Geoscope) and 14 stations of the 2000-2002 CHARGE (Chile Argentina Geophysical Experiment) PASSCAL array. We also made splitting measurements in 17 SKS and SKKS phases recorded by PLCA and permanent station TRQA (IRIS/GSN). Splitting parameters for a sub-set of local S, SKS and SKKS phases were determined using a range of filters from 0.05-0.2 to 0.05-2, and were generally stable as a function of frequency; frequency-dependence was observed in a small number of cases, and will be investigated further. The results reported below correspond to a 0.05-2 Hz bandpass filter. Local S splitting times range from 0.1-0.9 seconds, and for back-arc stations, splitting times correlate with path length in the mantle wedge. These results indicate that wedge anisotropy is a dominant factor in the observed splitting, although shallower anisotropy also appears to be present. Splitting fast polarizations at back-arc stations show a coherent variation with latitude. Fast polarizations vary from NE at 40°-41°S, to N (roughly slab-strike parallel) at 35°-36°S, to NE-ESE at 30°-33°S, curving as the slab flattens where the Juan Fernandez Ridge is subducting beneath the South American lithosphere. For SKS and SKKS phases at PLCA (in the western back-arc at 41°S), fast directions are predominantly ENE-ESE and splitting times range from 1.0-2.3 s. At TRQA (much farther to the east and at 38°S), teleseismic fast polarizations are E-SE and splitting times vary from 0.8-2.4 s. At PLCA, because SKS and SKKS splitting times are significantly larger than the local S splitting times, significant anisotropy must exist below and potentially within the Nazca slab. We plan to constrain the depth variation of anisotropy by modeling anisotropy above the slab with local S splitting, correcting SKS/SKKS phases from permanent stations and the CHARGE array for the effects of this shallow structure, and using the residual teleseismic splitting to model slab and sub-slab anisotropy. Below the slab and in back-arc regions where temperatures are likely too high, and stresses too low, to permit the formation of olivine B-type fabric, fast direction observations may be used as proxies for mantle flow. Under this assumption, the local S and SKS/SKKS splitting measurements indicate that flow both above and below the slab is neither purely parallel to slab-strike nor to the direction of subducting plate motion. The apparent three-dimensional flow pattern may be influenced by slab rollback, along-strike variations in slab dip, and/or influx of material into the wedge from the slab gap to the south of our study region.

MacDougall, J. G.; Fischer, K. M.; Anderson, M. L.

2010-12-01

305

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

NASA Astrophysics Data System (ADS)

Thales LAST stands for Lesser Antilles Subduction zone Team which gathers the scientific teams of a cluster of surveys and cruises that have been carried out in 2007 and coordinated under the European Union THALES WAS RIGHT project (Coord. A. Hirn). This cluster is composed by the German cruise TRAIL with the vessel F/S Merian (PI E. Flueh and H. Kopp, IFM-GEOMAR), the French cruise SISMANTILLES 2 with the IFREMER vessel N/O Atalante (PI M. Laigle, IPG Paris and JF. Lebrun, Univ. Antilles Guyane), and French cruise OBSANTILLES with the IRD vessel N/O Antea (PI P. Charvis, Geoazur, Nice, France). During these cruises and surveys, 84 Ocean Bottom 3-components Seismometers (OBS) and 20 Hydrophones (OBHs) have been brought together from several pools (Geoazur, INSU, IPGP, IFM-GEOMAR, AWI,), with up to 30 land stations (CSIC Barcelone, IPG Paris, INSU-RLBM and -LITHOSCOPE) in addition to the permanent onshore arrays of IPGP and SRU. The deployment of all these instruments has been supported principally by ANR Catastrophes Telluriques et Tsunamis (SUBSISMANTI), by the EU SALVADOR Programme of IFM-GEOMAR, as well as by the EU project THALES WAS RIGHT on the Antilles and Hellenic active subductions to which contribute IPGP, Geoazur, IFM-GEOMAR (Germany), ETH Zurich (Switzerland), CSIC Barcelona (Spain), Univ. Trieste (Italy) and NOA Athens (Greece). The main goal of this large seismic investigation effort is the understanding of the behaviour of the seismogenic zone and location of potential source regions of mega-thrust earthquakes. Specific goals are the mapping of the subduction interplate in the range where it may be seismogenic along the Lesser Antilles Arc from Antigua to southern Martinique Islands, as a contribution to identification and localisation in advance of main rupture zones of possible future major earthquakes, and to the search for transient signals of the activity. The forearc region, commonly considered as a proxy to the seismogenic portion of the subduction mega-thrust fault plane, and which is here the main target has been localized along 3 transects to the Arc thanks to a preliminary survey in 2001, the French SISMANTILLES cuise. We will present the first results obtained during these experiments dedicated specifically to image at depth the seismic structure and activity of this region. To image faults at depth and the detailed upper-crustal structure, 3700 km of multi-beam bathymetry and multi-channel reflection seismic profiles have been collected along a grid comprising 7 strike-lines of up to 300 km long and spaced by 15 km and 12 transects of up to 150 km long and spaced by 25 km (SISMANTILLES 2). All these airgun shots dedicated to deep penetration have been recorded by the 84 OBSs and 20 OBHs deployed by the F/S Merian and N/O Atalante on the nodes of this grid of profiles. It will permit to get Vp constraints on the deep forearc region and mantle wedge by wide-angle refraction studies, as well as constraints on the updip and downdip limits of the seismogenic part of the mega-thrust fault plane. Two of these transects have been extended across the whole arc during the TRAIL survey, with up to 50 OBSs deployed along both 240 km long profiles. All these OBSs remained several months beyond the shot experiments for local earthquakes Vp and Vp/Vs tomography. They have been recovered and partly redeployed by N/O Antea during the OBSANTILLES survey. A significant number of those instruments had broadband seismometers, a notable originality in the case of the OBSs to detect low-frequency transient signals.

Last, T.

2007-12-01

306

A Model of Continental Growth and Mantle Degassing Comparing Biotic and Abiotic Worlds  

NASA Astrophysics Data System (ADS)

While examples for interaction of the biosphere with the atmosphere can be easily cited (e.g., production and consumption of O2), interaction between the biosphere and the solid planet and its interior is much less established. It has been argued (e.g., Rosing et al. 2006; Sleep et al, 2012) that the formation of continents could be a consequence of bioactivity harvesting solar energy through photosynthesis to help build the continents and that the mantle should carry a chemical biosignature. We present an interaction model that includes mantle convection, mantle water vapor degassing at mid-oceanic ridges and regassing through subduction zones, continental crust formation and erosion and water storage and transport in a porous oceanic crust that includes hydrous mineral phases. The mantle viscosity in this model depends on the water concentration in the mantle. We use boundary layer theory of mantle convection to parameterize the mantle convection flow rate and assume that the plate speed equals the mantle flow rate. The biosphere enters the calculation through the assumption that the continental erosion rate is enhanced by a factor of several through bioactivity and through an assumed reduction of the kinetic barrier to diagenetic and metamorphic reactions (e.g., Kim et al. 2004) in the sedimentary basins in subduction zones that would lead to increased water storage capacities. We further include a stochastic model of continent-to-continent interactions that limits the effective total length of subduction zones. We use present day parameters of the Earth and explore a phase plane spanned by the percentage of surface coverage of the Earth by continents and the total water content of the mantle. We vary the ratio of the erosion rate in a postulated abiotic Earth to the present Earth, as well as the activation barrier to diagenetic and metamorphic reactions that affect the water storage capacity of the subducting crust. We find stable and unstable fixed points in the phase area where the net degassing and continental growth rates are zero. Many of the parameter combinations result in one stable fixed point with a completely dry mantle that lacks continents altogether and a second stable fixed point with a continent coverage and mantle water concentration close to that of the present Earth. In addition, there is an unstable fixed point situated between the two. In general, the abiotic world has a larger zone of attraction for the fixed point with a dry mantle and no continents than the biotic world. Thus a biotic world is found to be more likely to develop continents and a have wet mantle. Furthermore, the biotic model is generally found to have a wetter mantle than an abiotic model with the same continent coverage. Through the effect of water on the mantle rheology, the biotic world would thus tend to be tectonically more active and have a more rapid long-term carbon silicate cycle. References: J. Kim, H. Dong, J. Seabaugh, S. W. Newell, D. D. Eberl, Science 303, 830-832, 2004 N. H. Sleep, D. K. Bird, E. Pope, Annu. Rev. Earth Planet. Sci. 40, 277-300, 2012 M. T. Rosing, D. K. Bird, N. H. Sleep, W. Glassley, F. Albarede, Paleo3 232, 90-113, 2006

Höning, D.; Hansen-Goos, H.; Spohn, T.

2012-12-01

307

Petrology of Ultra-high Pressure Metamorphosed Marble from the Dabie Mountains: Implications for Carbon Cycle during Continental Subduction  

NASA Astrophysics Data System (ADS)

In order to understand the carbon cycle in continental subduction zones, impure marble from Ganjialing area in the Dabie Mountains were selected for detail petrogenesis study. Impure marble contains an ultra-high pressure assemblage of dolomite + aragonite + garnet + omphacite + phengite + zoisite + coesite, whose peak P-T conditions estimated using garnet-omphacite-phengite pairs are comparable to those of enclosed eclogite puddings and also consistent with the occurrence of coesite (>2.7 GPa and >640 oC). A possible exhumation P-T-XCO2 path followed by impure marble was obtained. The XCO2 of fluid at UHP conditions (<0.03) gradually increased to >0.1 during retrogression (ca.1GPa and 600 oC), then reduced again to <0.01 with significant temperature decreasing. Due to retrogression reactions involving carbonates, the peak carbonate assemblage of dolomite + aragonite tends to be replaced by dolomite + calcite. Dolomite exsolution in calcite was discovered in some totally retrogressed tremolite marble. These results indicate that most carbon would be preserved in carbonates and hardly incorporated into the coexisting fluid during metamorphism. Thus, most sedimentary carbonates could be transported into the upper mantle through continental subduction.

Liu, P.; Wu, Y.; Jin, Z.; Wang, Y.

2011-12-01

308

The Slip History and Source Statistics of Major Slow Slip Events along the Cascadia Subduction Zone from 1998 to 2008  

NASA Astrophysics Data System (ADS)

We estimate the time dependent slip distribution of 16 prominent slow slip events along the northern half of the Cascadia subduction zone from 1998 to 2008. We process continuous GPS data from the PBO, PANGA and WCDA networks from the past decade using GAMIT/GLOBK processing package. Transient surface displacements are interpreted as slip on the plate interface using the Extended Network Inversion Filter. Of these 16 events, 10 events are centered north of Puget Sound, 4 events are resolved around the Columbia River and 1 event is located near Cape Blanco. The February 2003 event is complex, extending from Portland to southern Vancouver Island. Other smaller events beneath Northern Vancouver Island, Oregon and Northern California are not well resolved because of the limited station coverage. We identify two characteristic segments based on the along-strike extent of individual transient slip events in northern Washington. One segment is centered around Port Angeles. Another segment is between the Columbia River and the southern end of Puget Sound. The propagation direction of slow slip events is variable from one event to the next. The maximum cumulative slip for these 16 events is ~ 27 cm, which is centered beneath Port Angeles. This indicates that the strain release by transient slip is not uniform along-strike. In northwestern Washington where cumulative slip is a maximum, the subduction zone bends along-strike and dip of the plate is lower compared to the north and south. We hypothesize that the geometry of the slab plays an important role for focusing transient strain release at this location along the subduction zone. We explore the relationship of source parameters of slow slip using our catalogue of 16 events. The estimated moment magnitude ranges between 6.1 and 6.7. The average stress drop of 0.06-0.1 MPa is nearly two orders of magnitude smaller than that found for normal earthquakes (1-10 MPa). Standard earthquakes follow a scaling relationship where rupture length is proportional to slip amplitude resulting in a nearly constant stress drop. Slow slip events display the similar scaling law up to a possible limit in slip amplitude. We also explore the relationship of event duration to other source parameters.

Gao, H.; Schmidt, D. A.

2008-12-01

309

Mineralogy and fluid content of sediments entering the Costa Rica subduction zone - Results from Site U1414, IODP Expedition 344  

NASA Astrophysics Data System (ADS)

Subduction zones are characterized by the largest thrust earthquakes, as quantified by both rupture area and seismic moment release. Offshore Costa Rica, the oceanic Cocos Plate subducts under the Caribbean plate forming the southern end of the Middle America trench. A high convergence rate and almost complete subduction of incoming sediments make the Costa Rica convergent margin an extremely dynamic environment. The Costa Rica Seismogenesis Project (CRISP) is designed to understand the processes that control nucleation and seismic rupture of large earthquakes at erosional subduction zones. Site U1414 of IODP Exp.344 was drilled to investigate the material from the incoming Cocos Plate. A key parameter of incoming plate is fluid content and release because it impacts deformation within the subduction complex. The deposition, compaction and diagenesis of sedimentary rocks control the distribution of fluids, fluid pressures and fluid flow patterns within subduction zones. We therefore decided to characterize sediment composition and quantify the different types of water at Site U1414. Mineralogical investigations were performed using optical and electronic microscope observations, X Ray Diffraction (on bulk and clay fractions), Cation Exchange Capacity measurements, carbon analyses (to determine carbonate contents), and sequenced extractions in NaOH (to quantify the biogenic opal content). Fluid characteristics were approached by thermal gravimetric analyses. The entire sedimentary sequence was recovered at Site U1414 and can be divided into three major sedimentary units. The first one is a hemipelagic silty clay to clay with a gradual increase of calcareous nannofossils. The dominant mineral is smectite associated in the clay fractions with kaolinite and zeolites. Small amounts of biogenic opal have been analyzed. Other minerals like quartz, feldspar and calcite are also present. The second unit is composed of nannofossil-rich calcareous ooze. The proportion of biosilica is variable and can attain 15 wt.%. Smectite and zeolites are present in smaller amount. The third unit is a lithified sandstone. Biosilica and smectite are absent, but zeolites are still present in this unit. Fluid content that can be released varies from about 15 wt.% to 40 wt.%. In shallow levels a significant proportion is pore water fluid, whereas in deeper levels water stored within minerals comprises a greater proportion of the total fluid budget. The presence of smectite yields to fluid release by dehydration and dehydroxylation at temperatures less than approximately 100°C and 500°C respectively. Transformation of biogenic opal to diagenetic silice goes to completion at temperatures of 50-100°C. It seems to be an importance source of fluid in the second unit, whereas in unit three it is zeolite water.

Charpentier, D.; Buatier, M.; Kutterolf, S.; Straub, S. M.; Nascimento, D.; Millan, C.

2013-12-01

310

Tsunami history of an Oregon coastal lake reveals a 4600 yr record of great earthquakes on the Cascadia subduction zone  

USGS Publications Warehouse

Bradley Lake, on the southern Oregon coastal plain, records local tsunamis and seismic shaking on the Cascadia subduction zone over the last 7000 yr. Thirteen marine incursions delivered landward-thinning sheets of sand to the lake from nearshore, beach, and dune environments to the west. Following each incursion, a slug of marine water near the bottom of the freshwater lake instigated a few-year-to-several-decade period of a brackish (??? 4??? salinity) lake. Four additional disturbances without marine incursions destabilized sideslopes and bottom sediment, producing a suspension deposit that blanketed the lake bottom. Considering the magnitude and duration of the disturbances necessary to produce Bradley Lake's marine incursions, a local tsunami generated by a great earthquake on the Cascadia subduction zone is the only accountable mechanism. Extreme ocean levels must have been at least 5-8 m above sea level, and the cumulative duration of each marine incursion must have been at least 10 min. Disturbances without marine incursions require seismic shaking as well. Over the 4600 yr period when Bradley Lake was an optimum tsunami recorder, tsunamis from Cascadia plate-boundary earthquakes came in clusters. Between 4600 and 2800 cal yr B.P., tsunamis occurred at the average frequency of ??? 3-4 every 1000 yr. Then, starting ???2800 cal yr B.P., there was a 930-1260 yr interval with no tsunamis. That gap was followed by a ???1000 yr period with 4 tsunamis. In the last millennium, a 670-750 yr gap preceded the A.D. 1700 earthquake and tsunami. The A.D. 1700 earthquake may be the first of a new cluster of plate-boundary earthquakes and accompanying tsunamis. Local tsunamis entered Bradley Lake an average of every 390 yr, whereas the portion of the Cascadia plate boundary that underlies Bradley Lake ruptured in a great earthquake less frequently, about once every 500 yr. Therefore, the entire length of the subduction zone does not rupture in every earthquake, and Bradley Lake has recorded earthquakes caused by rupture along the entire length of the Cascadia plate boundary as well as earthquakes caused by rupture of shorter segments of the boundary. The tsunami record from Bradley Lake indicates that at times, most recently ???1700 yr B.P., overlapping or adjoining segments of the Cascadia plate boundary ruptured within decades of each other. ?? 2005 Geological Society of America.

Kelsey, H.M.; Nelson, A.R.; Hemphill-Haley, E.; Witter, R.C.

2005-01-01

311

Propagation of the 2001-2002 silent earthquake and interplate coupling in the Oaxaca subduction zone, Mexico  

NASA Astrophysics Data System (ADS)

The aseismic slow slip event of 2001-2002 in Guerrero, Mexico, with an equivalent magnitude MW ~ 7.5, is the largest silent earthquake (SQ) among many recently recorded by GPS in different subduction zones (i.e. Japan, Alaska, Cascadia, New Zealand). The sub-horizontal and shallow plate interface in Central Mexico is responsible for specific conditions for the ~100 km long extended transient zone where the SQs develop from ~80 to ~190 km inland from the trench. This wide transient zone and relatively large slow slips of 10 to 20 cm displacements on the subduction fault result in noticeable surface displacements of 5-6 cm during the SQs. Continuous GPS stations allow one to trace the propagation of SQs, and to estimate their arrival time, duration and geometric attenuation. These propagation parameters must be accounted in order to locate source of slow slips events and to understand the triggering effect that they have on large subduction earthquakes. We use long-baseline tiltmeter data to define new time limits (onset and duration) for the SQs and continuous records from 8 GPS stations to determine the propagation of the 2001-2002 SQ in Central Mexico. Data from the CAYA and IGUA GPS stations, separated by ~170 km and located along the profile perpendicular to the trench, are used to determine that the surface deformation from the 2001-2002 SQ started almost instantaneously. It propagated parallel to the coast at ~2 km/day with an exponential attenuation of the horizontal surface displacement and a linear decrease of its duration with distance. Campaign data obtained yearly from 2001 to 2005 at the Oaxaca GPS network have been modeled according to a propagation of the 2001-2002 SQ step-like displacement anomaly. This modeling shows that the SQ ceased gradually in the central part of the Oaxaca segment of the subduction zone (west of Puerto Angel, PUAN) and then it apparently triggered another SQ in SE Oaxaca (between PUAN and Salina Cruz, SACR). The estimated horizontal velocities for inter-event epochs at each GPS site are used to assess an average interplate coupling in the Central Oaxaca subduction zone.

Franco, S. I.; Kostoglodov, V.; Larson, K. M.; Manea, V. C.; Manea, M.; Santiago, J. A.

2005-10-01

312

Petrographical records of multiple fluid-infiltration during HP-LT metamorphism (Invited)  

NASA Astrophysics Data System (ADS)

In the subduction zone, flow of slab-derived fluids enriched in Si, Al, Na, and Ca enhances metamorphic + metasomatic reactions and precipitation of HP-LT minerals, and in some cases, the intraslab flow causes extensive elemental leaching (cf., Spandler & Pirard 2013). How does HP-LT rocks and minerals record subduction-zone fluid flow? Syn-metamorphic vein networks consisting of HP-LT minerals have been known in various blueschist- and low-temperature eclogites. The monomineralic and bimineralic veins includes a wide variety of HP-LT minerals: metamorphic pyroxenes, phengite, hydrous Ca-Al-silicate, and even rare garnet and rutile; oscillatory and sector zonings with resorption are common, suggesting precipitation from aqueous fluids. Natural observations imply that brittle fracturing and fluid infiltration during blueschist-to-eclogite-facies metamorphism are common phenomenon in the subducting oceanic crust attending progressive eclogitization. Another peculiar example of HP-LT rock preserving evidence of multiple fluid-infiltration is "lawsonitite", which is a metasomatic rock consisting of ~70-90 vol% lawsonite. Lawsonitite from the South Motagua Mélange consists principally of fluid precipitates and represents the extreme of lithological variation of "P-type" jadeitite (Tsujimori & Harlow 2012; Flores et al. 2013). Lawsonites show sector- and oscillatory-growth zoning on a millimeter to submicrometer scale. The rock shows millimeter-scale flow paths of Cr-rich fluids cross-cutting lawsonite crystals; the texture indicates that the fluid infiltration occurred after lawsonite crystallization/precipitation. The Cr-rich fluid flow-paths follow the track of Cr-bearing pinkish locally metasomatized lawsonite with inclusions of Cr-rich (kosmochloric) jadeite and Cr-rich phengite. These microtextures and mineral compositions indicate a chronological sequence of fluid infiltration events: lawsonite grains precipitated from subduction-zone aqueous fluids within lawsonitite-bearing thermal regime, and subsequently a Cr-rich fluid infiltrated from ultramafic protoliths into the lawsonitite, probably along microfractures, and crystallized Cr-rich jadeite. Except for fluid-inclusion studies, direct tracking of subduction-zone aqueous fluids is not simple. However, syn-metamorphic crack-seal HP-LT mineral veins and peculiar metasomatic rocks such as lawsonitite have a great potential for decoding the origin and history of subduction-zone fluids. These petrographical records should certainly narrow down the critical targets of in-situ geochemistry in the exploration of fluid in fossil slabs.

Tsujimori, T.

2013-12-01

313

Calculating in situ density and heat capacity of rocks with GMIN: new type of input data for thermomechanical modeling of subduction zones  

NASA Astrophysics Data System (ADS)

The technique of minimizing the Gibbs free energy of specified rock compositions by varying the amounts and compositions of coexisting phases has provided new insights and possibilities for petrological investigations. For thermomechanical modeling of subduction zone processes density, isobaric heat capacity, thermal expansion and compressibility can then be extracted as a function of the amounts and compositions of coexisting phases [1]. At present two programs offer the possibility of calculating phase diagrams with this technique, Holland and Powell's Thermocalc [2] and De Capitani's Theriac/Domino [3] package. The first one is a command line program producing a text output which can be converted to graphics with a user-written MathematicaTM program. It uses the Holland and Powell data base [2]. De Capitani's program is distributed as a Fortran 77 source code and uses the Berman database. Neither of these programs offers interfaces for use with geodynamic modeling programs. We have therefore enhanced our new Gibbs minimization program GMin [4] with an interface able to transfer density, entahalpy and volume values into databases or other programs. The minimization algorithm of our program is based on the de Capitani method [3], modified and adapted for the Holland and Powell database [2]. To achieve rapid convergence and stability of minimization we have optimized the computing strategy for models of solid solutions used in [2]. The standard program is divided in two parts, a front end and a calculation program controlled by the front end program. This construction opens the possibility of starting up several calculations in parallel on different computers and different operating systems. In this way large numbers of calculations can be performed on normal PC-networks (servers and clients) controlled by only one front end program. The front end is able to create pressure and temperature ordered databases containing density, enthalpy, volume and composition which can then be accessed by other programs. As a third component we have developed a dynamic link library which can be directly called on by any program; the library returns P,T- and X-dependent density, volume and enthalpy values to the main program. [1] Gerya et al. (2002) EJM, 14, 687-699. [2] Holland, T.J.B., Powell, R. (1998) J. Metamorph. Geol., 16, 309-344. [3] de Capitani, C., Brown, T.H. (1987) Geochim. Cosmochim. Acta, 51, 2639-2652. [4] Gerya, T.V., Burchard M. (2002) DMG 2002 Abstracts, Ber. DMG Beih. z. EJM, 14, 51.

Burchard, M.; Gerya, T.

2003-04-01

314

Niobium-enriched basalts from the Wabigoon subprovince, Canada: evidence for adakitic metasomatism above an Archean subduction zone  

NASA Astrophysics Data System (ADS)

Late Archean niobium-enriched basalts from the Central Sturgeon Lake assemblage and Neepawa group of the western Wabigoon subprovince have mantle-normalized Nb/La between 0.8 and 1.3 and Zr/Y between 4 and 7. They are compositionally similar to basalts attributed to adakite metasomatism of mantle wedge regions in Cenozoic subduction zones [Sajona et al., J. Petrol. 37 (1996) 693-726]. In detail, their Sc-REE systematics suggest the Archean basalts were generated above the garnet stability field. An association with adakite-like volcanic rocks, an absence of komatiites and the arc-like attributes of their host sequences suggest a subduction-related origin for the basalts. If current models of adakite and Niobium-enriched basalt genesis are valid, then additional examples of these rocks should be relatively common in other Archean greenstone belts.

Wyman, D. A.; Ayer, J. A.; Devaney, J. R.

2000-06-01

315

Empirical ground-motion relations for subduction-zone earthquakes and their application to Cascadia and other regions  

USGS Publications Warehouse

Ground-motion relations for earthquakes that occur in subduction zones are an important input to seismic-hazard analyses in many parts of the world. In the Cascadia region (Washington, Oregon, northern California, and British Columbia), for example, there is a significant hazard from megathrust earthquakes along the subduction interface and from large events within the subducting slab. These hazards are in addition to the hazard from shallow earthquakes in the overlying crust. We have compiled a response spectra database from thousands of strong-motion recordings from events of moment magnitude (M) 5-8.3 occurring in subduction zones around the world, including both interface and in-slab events. The 2001 M 6.8 Nisqually and 1999 M 5.9 Satsop earthquakes are included in the database, as are many records from subduction zones in Japan (Kyoshin-Net data), Mexico (Guerrero data), and Central America. The size of the database is four times larger than that available for previous empirical regressions to determine ground-motion relations for subduction-zone earthquakes. The large dataset enables improved determination of attenuation parameters and magnitude scaling, for both interface and in-slab events. Soil response parameters are also better determined by the data. We use the database to develop global ground-motion relations for interface and in-slab earthquakes, using a maximum likelihood regression method. We analyze regional variability of ground-motion amplitudes across the global database and find that there are significant regional differences. In particular, amplitudes in Cascadia differ by more than a factor of 2 from those in Japan for the same magnitude, distance, event type, and National Earthquake Hazards Reduction Program (NEHRP) soil class. This is believed to be due to regional differences in the depth of the soil profile, which are not captured by the NEHRP site classification scheme. Regional correction factors to account for these differences are proposed for Cascadia and Japan. The results of this study differ significantly from previous analyses based on more limited data and have important implications for seismic-hazard analysis. The ground-motion relations predict that a great megathrust earthquake (M ???8) at a fault distance of about 100 km would produce pseudoacceleration (PSA), 5% damped, horizontal component on soil sites of about 110 cm/sec2 at 0.5 Hz, 660 cm/sec2 at 2.5 Hz, and 410 cm/sec2 at 5 Hz, with a peak ground acceleration of about 180 cm/ sec2 . These damaging levels of motion would be experienced over a very large area, corresponding to a rectangular area about 300 km wide by 500 km long. Large in-slab events (M 7.5) would produce even higher PSA values within 100 km of the fault, but the in-slab motions attenuate much more rapidly with distance. Thus the hazard posed by moderate to large in-slab events such as the 2001 Nisqually earthquake is modest compared to that of a Cascadia megathrust earthquake of M ???8, in terms of the area that would experience damaging levels of ground motion.

Atkinson, G.M.; Boore, D.M.

2003-01-01

316

Neoproterozoic arc-back-arc system in the Central Eastern Desert of Egypt: Evidence from supra-subduction zone ophiolites  

NASA Astrophysics Data System (ADS)

Ophiolites are widely distributed in the Central Eastern Desert (CED) of Egypt, occurring as clusters in the northern (NCEDO) and southern (SCEDO) segments. Mineralogical and geochemical data on the volcanic sections of Wizer (WZO) and Abu Meriewa (AMO) ophiolites as representatives of the NCEDO and SCEDO, respectively, are presented. The WZO volcanic sequence comprises massive metavolcanics of MORB-like compositions intruded by minor boninitic dykes and thrust over island-arc metavolcanic blocks in the mélange matrix. Such transitional MORB-IAT-boninitic magmatic affinities for the WZO metavolcanics suggest that they most likely formed in a protoarc-forearc setting. Chemical compositions of primary clinopyroxene and Cr-spinel relicts from the WZO volcanic section further confirm this interpretation. The compositional variability in the WZO volcanic sequence is comparable with the associated mantle rocks that vary from slightly depleted harzburgites to highly depleted harzburgites containing small dunite bodies, which are residues after MORB, IAT and boninite melt formation, respectively. Source characteristics of the different lava groups from the WZO indicate generation via partial melting of a MORB source which was progressively depleted by melt extraction and variably enriched by subduction zone fluids. MORB-like magma may have been derived from ~ 20% partial melting of an undepleted lherzolite source, leaving slightly depleted harzburgite as a residuum. The generation of island-arc magma can be accounted for by partial melting (~ 15%) of the latter harzburgitic mantle source, whereas boninites may have been derived from partial melting (~ 20%) of a more refractory mantle source previously depleted by melt extraction of MORB and IAT melts, leaving ultra-refractory dunite bodies as residuum. The AMO volcanic unit occurs as highly deformed pillowed metavolcanic rocks in a mélange matrix. They can be categorized geochemically into LREE-depleted (La/Yb CN = 0.41-0.50) and LREE-enriched (La/Yb CN = 4.7-4.9) lava types that show an island arc to MORB geochemical signature, respectively, signifying a back-arc basin setting. This is consistent, as well, with their mantle section. Source characteristics indicate depleted to slightly enriched mantle sources with overall slight subduction zone geochemical affinities as compared to the WZO. Generally, CED ophiolites show supra-subduction zone geochemical signature with prevalent island arc tholeiitic and minor boninitic affinities in the NCEDO and MORB/island-arc association in the SCEDO. Such differences in geochemical characteristics of the NCEDO and SCEDO, along with the abundance of mature island arc metavolcanics which are close in age (~ 750 Ma) to the ophiolitic rocks, general enrichment in HFSE of ophiolites from north to south, and lack of a crustal break and major shear zones, is best explained by a geotectonic model whereby the CED represents an arc-back-arc system above a southeast-dipping subduction zone.

Farahat, E. S.

2010-12-01

317

Simulations of seismic hazard for the Pacific Northwest of the United States from earthquakes associated with the Cascadia subduction zone  

USGS Publications Warehouse

We investigate the impact of different rupture and attenuation models for the Cascadia subduction zone by simulating seismic hazard models for the Pacific Northwest of the U.S. at 2% probability of exceedance in 50 years. We calculate the sensitivity of hazard (probabilistic ground motions) to the source parameters and the attenuation relations for both intraslab and interface earthquakes and present these in the framework of the standard USGS hazard model that includes crustal earthquakes. Our results indicate that allowing the deep intraslab earthquakes to occur anywhere along the subduction zone increases the peak ground acceleration hazard near Portland, Oregon by about 20%. Alternative attenuation relations for deep earthquakes can result in ground motions that differ by a factor of two. The hazard uncertainty for the plate interface and intraslab earthquakes is analyzed through a Monte-Carlo logic tree approach and indicates a seismic hazard exceeding 1 g (0.2 s spectral acceleration) consistent with the U.S. National Seismic Hazard Maps in western Washington, Oregon, and California and an overall coefficient of variation that ranges from 0.1 to 0.4. Sensitivity studies indicate that the paleoseismic chronology and the magnitude of great plate interface earthquakes contribute significantly to the hazard uncertainty estimates for this region. Paleoseismic data indicate that the mean earthquake recurrence interval for great earthquakes is about 500 years and that it has been 300 years since the last great earthquake. We calculate the probability of such a great earthquake along the Cascadia plate interface to be about 14% when considering a time-dependent model and about 10% when considering a time-independent Poisson model during the next 50-year interval.

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

2002-01-01

318

GEOPHYSICAL RESEARCH LETTERS, VOL. 28, NO. 10, PAGES 2125-2128, MAY 1, 2001 Stress rates in the central Cascadia subduction zone  

E-print Network

- tions we perform a linear least-squares inversion of surface geodetic data to solve for the distribution for collecting geodetic data during interseismic periods at subduction zones is the determina- tion] have been successful in matching geodetic observations, from which the distribution of locking

McCaffrey, Robert

319

Cycling of B, Li, and LILE (K, Cs, Rb, Ba, Sr) into subduction zones: SIMS evidence from micas in high-P/T metasedimentary rocks  

E-print Network

Cycling of B, Li, and LILE (K, Cs, Rb, Ba, Sr) into subduction zones: SIMS evidence from micas of subducting sediments. Analysis of a suite of trace elements (Li, B, Rb, Sr, Cs, Ba) in micas obtained prograde P­T paths. SIMS mineral analyses confirm the inference from the whole-rock data that white mica

Bebout, Gray E.

320

Thermo-rheological, shear heating model for leucogranite generation, metamorphism, and deformation during  

E-print Network

Thermo-rheological, shear heating model for leucogranite generation, metamorphism, and deformation Accepted 20 June 2001 Abstract This paper evaluates thermotectonic models for metamorphism and leucogranite of regional deformation and staurolite-grade regional metamorphism. Published Consortium for Continental

Liu, Mian

321

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

USGS Publications Warehouse

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.

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

2003-01-01

322

Ultrahigh-pressure minerals and metamorphic terranes - The view from China  

NASA Astrophysics Data System (ADS)

Ultrahigh-pressure (UHP) metamorphism refers to mineralogical modifications of continental and oceanic crustal protoliths ± associated mafic-ultramafic rocks initially formed or emplaced in shallow levels of the lithosphere, but which subsequently have experienced P- T conditions within or above the coesite stability field (>˜2.7 GPa, ˜700 °C). Typical products include eclogite, garnet peridotite, and UHP varieties of metapelite, quartzite, marble, paragneiss, and orthogneiss. UHP metamorphic assemblages require relatively cold lithospheric subduction to mantle depths; some recrystallization even occurs under "forbidden" P- T conditions, characterized by a geotherm of <5 °C/km. In appropriate bulk compositions, UHP metamorphism produces coesite, microdiamond and other indicator phases such as majoritic garnet, TiO 2 with ?-PbO 2 structure, supersilicic clinopyroxene, high- P clinoenstatite, K-cymrite and stishovite. Globally, at least 20 coesite-bearing eclogitic, eight diamond-bearing, and five majoritic garnet-bearing UHP regions have been documented thus far; they are mostly of Phanerozoic ages. The presence of majoritic garnet, and even apparent stishovite pseudomorph in supracrustal rocks suggests continental subduction to mantle depths exceeding 300 km; such UHP metamorphic terranes should be distinguished from deep-seated mantle xenoliths that contain UHP minerals. Cold subduction zones may be sites of major recycling of H 2O back into the mantle; high- P experiments on mafic-ultramafic bulk compositions reveal that many important hydrous and formally anhydrous phases are stable under such UHP conditions. The current explosion of research on continental UHP terranes reflects their significance for mantle dynamics and the tectonics of continental subduction, collision, exhumation, mantle-slab interactions, and geochemical recycling. A further characterization of UHP phases and positive identification of UHP minerals requires new experimental studies coupled with state-of-the-art analyses. For example, the very rare occurrence of microdiamond inclusions in zircons from Dabie-Sulu UHP rocks may reflect high f attending recrystallization inasmuch as epidote is rather common. Rutile needles within garnets from Sulu UHP eclogitic rocks may not be the result of exsolution, so in such cases the apparent UHP pressure may have been over estimated. Hadean igneous microdiamond inclusions in Jack Hills detrital zircons could have originated from mantle xenoliths whereas abundant detrital Phanerozoic diamonds containing inclusions of coesite and other eclogitic minerals from New South Wales might have been derived from unexposed UHP metamorphic terranes. Micro-mineral intergrowth and nano-size minerals may hold important key to deciphering the actual P- T paths of subduction and mantle return flow. Although most exhumed terranes have returned surfaceward relatively rapidly after short time of UHP condition, the long duration of storage at great depth and slow exhumation for the largest UHP terranes remain as major problems.

Liou, J. G.; Ernst, W. G.; Zhang, R. Y.; Tsujimori, T.; Jahn, B. M.

2009-07-01

323

The Zermatt-Saas Ophiolite: a Continuous Slice of Oceanic Lithosphere Detached at 80 km Depth in the Subduction Zone  

NASA Astrophysics Data System (ADS)

The western Alps is a classic subduction-related collisional orogen, where large fragments of low-density continental crust (e.g. Dora Maira, Grand Paradis) were deeply subducted and then exhumed together with ophiolitic remnants of the Mesozoic Tethyan oceanic lithosphere (e.g. Monviso, Zermatt-Saas). Whereas the Monviso ophiolitic complex has been recognized as a paleao-subduction channel with tectonic blocks showing a wide range of pressure-temperature conditions, no comprehensive study has yet attempted to evaluate the metamorphic homogeneity of the extensive Zermatt-Saas ophiolite. Zermatt-Saas eclogitic assemblages are represented by omphacite-garnet-epidote-rutile +-lawsonite pseudomorphs +- glaucophane in "classical" MORB-derived metabasalts. Sea-floor hydrothermalized metabasalts are characterized by an unusual peak paragenesis characterized by garnet-chloritoid-talc +- lawsonite pseudomorphs +- glaucophane. Thermobarometric estimates with THERMOCALC and Raman Spectroscopy of carbonaceous material reveal homogeneous peak burial conditions at around 540 +- 20 °C and 23 +- 1kbar. These estimates are slightly lower (c.a. 50 °C - 4 kbar) than those from the rare, adjacent coesite-bearing metasediments, suggesting that most of the ophiolite detached from the slab at depths around 80 km. Our data indicate that the whole of the ophiolite, at least 50 km across, strikingly underwent similar tectonic patterns from burial to early exhumation. During exhumation, pervasive glaucophane recrystallization and later greenschist facies assemblages replaced the earlier eclogitic paragenesis. Early exhumation paths are homogeneous and characterized by nearly isothermal decompression between c.a. 23 and 10 kbar. The Zermatt-Saas ophiolite thus appears to be one of the world's largest oceanic lithosphere fragment exhumed from such depths. These results provide critical constraints on the migration of oceanic crustal slices along the subduction channel and on interplate coupling mechanisms.

Angiboust, S.; Agard, P.; Jolivet, L.; Burov, E.

2008-12-01

324

Geologic processes of accretion in the Cascadia subduction zone west of Washington State  

USGS Publications Warehouse

The continental margin west of Oregon and Washington undergoes a northward transition in morphology, from a relatively narrow, steep slope west of Oregon to a broad, midslope terrace off Washington. Multichannel seismic (MCS) reflection data collected over the accretionary complex show that the morphologic transition is accompanied by significant change in accretionary style: West of Oregon the direction of thrust vergence in the wedge toe flip-flops between landward and seaward, whereas off Washington, thrust faults in the toe verge consistently landward, except near the mouth of the Columbia River where detachment folding of accreted sediment is evident. Furthermore, rocks under the broad midslope terrace west of Washington appear to be intruded by diapirs. The combination of detachment folding, diapirs, and landward-vergent thrust faults all suggest that nearly as far landward as the shelf break, coupling along the interplate decollement is, or has been, low, as suggested by other lines of evidence.

Fisher, M.A.; Flueh, E.R.; Scholl, D. W.; Parsons, T.; Wells, R.E.; Trehu, A.; ten Brink, U.; Weaver, C.S.

1999-01-01

325

A dearth of intermediate melts at subduction zone volcanoes and the petrogenesis of arc andesites.  

PubMed

Andesites represent a large proportion of the magmas erupted at continental arc volcanoes and are regarded as a major component in the formation of continental crust. Andesite petrogenesis is therefore fundamental in terms of both volcanic hazard and differentiation of the Earth. Andesites typically contain a significant proportion of crystals showing disequilibrium petrographic characteristics indicative of mixing or mingling between silicic and mafic magmas, which fuels a long-standing debate regarding the significance of these processes in andesite petrogenesis and ultimately questions the abundance of true liquids with andesitic composition. Central to this debate is the distinction between liquids (or melts) and magmas, mixtures of liquids with crystals, which may or may not be co-genetic. With this distinction comes the realization that bulk-rock chemical analyses of petrologically complex andesites can lead to a blurred picture of the fundamental processes behind arc magmatism. Here we present an alternative view of andesite petrogenesis, based on a review of quenched glassy melt inclusions trapped in phenocrysts, whole-rock chemistry, and high-pressure and high-temperature experiments. We argue that true liquids of intermediate composition (59 to 66 wt% SiO(2)) are far less common in the sub-volcanic reservoirs of arc volcanoes than is suggested by the abundance of erupted magma within this compositional range. Effective mingling within upper crustal magmatic reservoirs obscures a compositional bimodality of melts ascending from the lower crust, and masks the fundamental role of silicic melts (>/=66 wt% SiO(2)) beneath intermediate arc volcanoes. This alternative view resolves several puzzling aspects of arc volcanism and provides important clues to the integration of plutonic and volcanic records. PMID:19865169

Reubi, Olivier; Blundy, Jon

2009-10-29

326

The Central Chilean Margin: Lower Plate Structure and Subduction Zone Geometry  

NASA Astrophysics Data System (ADS)

The central Chilean margin was the target of a combined on-/offshore seismic experiment using RV SONNE as platform for the marine data acquisition during cruise SO161. The along-strike segmentation of the margin results in areas of reduced slab dip ('Flat slab' segments). Segment b