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1

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

NASA Astrophysics Data System (ADS)

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

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

2009-05-01

2

Protolith control on fluid availability for zircon growth during continental subduction-zone metamorphism in the Dabie orogen  

NASA Astrophysics Data System (ADS)

Different episodes of zircon growth are recognized by a combined study of CL images, mineral inclusions, U–Pb ages, trace elements and Lu–Hf isotopes for ultrahigh-pressure (UHP) eclogite-facies metamorphic rocks in the Dabie orogen. The results provide insights into the effect of protolith property on fluid liberation during continental collision. Fluid availability from premetamorphic protoliths of different origins is recognized as a key to the zircon growth. Zircon U–Pb dating for UHP metabasalt and metasediment (eclogite and its host paragneiss) yields two groups of ages at 244 ± 3 and 225 ± 2 Ma, respectively. Mineral inclusion, trace element and Lu–Hf isotope analyses also suggest that these two groups of zircon grew from hydrous melt during the final subduction and supercritical fluid during the initial exhumation, respectively. In contrast, zircon U–Pb dating for UHP metaintrusive rocks (granitic orthogneiss and its hosted eclogite) gave only one group of age at 222 ± 2 Ma. Mineral inclusion, trace element and Lu–Hf isotope analyses suggest that the metamorphic zircon grew from aqueous fluid during the initial exhumation. The difference between the two groups of zircon U–Pb dates is attributed to the difference in their protolith origin. Volcanic and sedimentary rocks contain large amounts of water primarily in the form of molecular water (pore fluid), so that considerable amounts of aqueous fluid can be released from them during subduction. This episode of fluid action was recorded by the growth of anatectic zircon at ˜244 Ma. In contrast, intrusive rock only contains small amounts of water primarily in the form of structural hydroxyl in crystalline minerals, so that little fluid can be released from them during subduction. Nevertheless, large amounts of retrograde fluid were released from UHP metamorphic rocks regardless of their protolith origin during decompression exhumation. This episode of fluid action was recorded by the growth of metamorphic and anatectic zircons at 220–225 Ma. Therefore, the protolith property is a key to the liberation of aqueous fluid from metamorphosing rocks during subduction, which has great bearing on partial melting, element transport and mineral growth during continental subduction-zone metamorphism.

Xia, Qiong-Xia; Zheng, Yong-Fei; Chen, Yi-Xiang

2013-05-01

3

Metamorphic chemical geodynamics of subduction zones  

Microsoft Academic Search

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

Gray E. Bebout

2007-01-01

4

Thermodynamic Computation of Metamorphic Decarbonation for Marine Sediments Entering the Izu-Bonin-Mariana and Central America Subduction zones  

NASA Astrophysics Data System (ADS)

Release of metamorphic fluids from subducted slabs plays an important role in subduction zone earthquakes, arc magmas and volatile recycling. Recycling of CO2 in subduction zones is vital to the global carbon cycle and paleoclimate. Marked contrasts in the carbonate-bearing marine sediment lithologies entering the Izu-Bonin-Mariana (I-B-M) and Central America (C-A) subduction zones provide a dichotomy for evaluating the role of bulk-rock composition in subduction-zone metamorphic decarbonation. Sedimentary carbonate entering the I-B-M system is primarily contained in marls and volcanoclastics. In contrast, carbonate in the marine sediment of the C-A subduction zone is primarily within a nearly-pure carbonate ooze. We have utilized PerpleX (www.perplex.ethz.ch) to compute decarbonation for protoliths entering the I-B-M and C-A subduction zones. Because the bulk compositions of marls and volcanoclastics are conducive for metamorphic silicate-carbonate decarbonation reactions, decarbonation of these lithologies is significant in the I-B-M system. However, because the carbonate ooze entering the C-A subduction zone is nearly pure, decarbonation is predicted to be minimal. Barring the unlikely scenario of massive silica metasomatism in subducted limestone of the C-A system, we conclude that the marine sediments entering the C-A subduction zone are an infertile source for CO2. Consequently, CO2 emitted from C-A volcanoes and thermal springs is unlikely to be generated from metamorphism of subducted marine sediments. Subducted upper mantle ophicarbonates and/or continental crustal carbonate lithologies provide alternative CO2 sources.

Kerrick, D. M.; Connolly, J.

2006-12-01

5

Fractionation of trace elements by subduction-zone metamorphism — effect of convergent-margin thermal evolution  

Microsoft Academic Search

Differential chemical\\/isotopic alteration during forearc devolatilization can strongly influence the cycling of volatile components, including some trace elements, in subduction zones. The nature and magnitude of this devolatilization effect are likely to be strongly dependent on the thermal structure of individual convergent margins. A recent model for metamorphism of the Catalina Schist, involving progressive underplating (at ?45 km depths) of

Gray E. Bebout; Jeffrey G. Ryan; William P. Leeman; Ann E. Bebout

1999-01-01

6

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

7

Crustal Recylcing at Ocean Margin and Continental Subduction Zones and the Net Accumulation of Continental Crust  

NASA Astrophysics Data System (ADS)

CRUSTAL RECYCLING PROCESSES AND VOLUMES: At convergent ocean margins large volumes of rock and sediment are missing from the global length of submerged forearcs. Material is removed by the kindred tectonic process of sediment subduction and subduction erosion, both of which insert sediment and eroded crustal debris into the subduction channel separating the upper and lower plates. The channel transports entrained debris toward the mantle where it is ultimately recycled. Large tracks of exposed high P/T rocks are exposed remnants of subduction channels. Over the past 100-200 my, the average solid-rock volume of recycled crust is estimated to have averaged globally 2.5-3.0 km3/yr--or 2.5 to 3 Armstrong Units (AU). Exposed tracts of UHP rocks at collisional orogens document that crustal material is subducted deep into the mantle at continental subduction zones. Based on missing terranes of extended lower plate, a volume of recycled continental crust detached by slab failure can be estimated at ~5000 km3 for each km of the early Proterozoic Wopmay orogen of the NW Canadian Shield (Hildrenbrand and Bowring, 1999, Geology, v. 27, p.11-14). Averaged over an orogenic episode of ~40 my, the corresponding rate is ~125 km3/my/km of margin. Using the Wopmay- rate as a guide, and assuming that similar to the Cenozoic, collisional orogenic margins averaged 10-12,000 km in global length, then since the early Proterozoic crustal recycling at collisional subduction zones has averaged close to 1.5 AU (i.e., 1.5 km3/yr). Crustal losses from the upper plate have also been recognized for sectors of the Variscan orogen (Oncken, 1998, Geology, v. 26, p. 1975-1078). The missing crust is roughly 40 km3/my for each km of upper plate, thus globally tallying an additional ~0.5 AU. CRUSTAL GROWTH: New information implies that at intra-oceanic subduction zones the long-term (~50 my), global rate of arc magmatic productivity has averaged close to 5 AU, a much higher rate than formerly estimated (~1 AU). It is not clear that this rate, which is based on the growth of the Aleutian and Izu-Bonin-Mariana arc massifs corrected for subduction erosion losses, can be applied to continental or Andean arcs. But allowing that it can, then the combined global rate of additions of juvenile igneous rock to build continents ( 5 AU) is similar to that recycled at ocean margin (2-3 AU) and continental subduction zones (2 AU). Additional losses can arise from delamination of magmatically or tectonically thickened convergent-margin crust. The implication of these estimates and linked assumptions support the Armstrong posit that since the early Archean the yang of magmatic additions to the continents has been matched by the yin of recycling losses.

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

2004-12-01

8

Quantification of Subduction Zone Metamorphic Devolatilization From Computed High Pressure Phase Equilibria  

NASA Astrophysics Data System (ADS)

Devolatilization in subduction zones is essential to arc magmatism, seismicity and volatile recycling. A premise of our research is that realistic modeling of metamorphic devolatilization of subducted lithologies is only possible with phase equilibria for chemical systems that closely approximate actual bulk compositions. Volatile components are introduced into subduction zones by three contrasting lithologies: marine sediments, and hydrothermally altered mantle ultramafic rocks and oceanic metabasalts. Using free energy minimization (Perplex programs - web address: erdw.ethz.ch/ ~jamie/perplex), phase equilibria were computed to 6 GPa ( ~160 km) to quantify the evolution of CO2 and H20 by prograde metamorphism of these lithologies. Geotherms for NW and SE Japan [1] were adopted as thermal extremes along the tops of subducted slabs. The following summarizes our conclusions regarding the correlation between the depth of devolatilization, bulk rock composition and P-T locations geotherms. Serpentinites: for carbonate-bearing serpentinites (ophicarbonates) little CO2 is released; for high-temperature (high-T) geotherms complete dehydration occurs under forearcs, and for low-temperature (low-T) geotherms major dehydration occurs under subarcs. Siliceous limestones: little devolatilization for all geotherms. Marls: with high-T geotherms devolatilization is complete under subarcs; in contrast, little devolatilization occurs with low-T geotherms. Carbonate-free pelites and turbidites: with high-T geotherms most dehydration occurs under forearcs, whereas along low-T geotherms substantial H20 is released under subarcs. Carbonate-bearing oceanic metabasalts: decarbonation is negligible along low-T and intermediate-T geotherms and is limited along high-T geotherms; dehydration is complete under forearcs for high-T geotherms, significant under subarcs for intermediate-T geotherms, and very limited along low-T geotherms. Carbonate-free oceanic metabasalts: dehydration is complete under forearcs for high-T geotherms, and widespread under subarcs for intermediate-T and low-T geotherms. Metamorphic devolatilization of subducted metabasalts and metasediments is continuous; thus, we do not expect pulses of fluid release (corresponding to univariant devolatilization) for these lithologies. Our study emphasizes that because of differences in the bulk compositions of volatile-bearing lithologies and in the P-T location of geotherms, generalized modeling of subduction zone volatile recycling is questionable. References: [1] Peacock, S.M. and Wang, K. (1999) Science, 268, 937-939.

Kerrick, D. M.; Connolly, J. D.

2001-12-01

9

Polyphase Formation and Exhumation of HP-UHP Rocks in Continental Subduction Zone: Numerical Modeling  

Microsoft Academic Search

High- to ultrahigh-pressure (HP-UHP) metamorphic rocks commonly form and exhume during the early continental collision, with the protoliths mainly derived from subducted upper and middle continental crust. While the geodynamic significance of HP-UHP complexes is widely recognized and their appearance in the Neoproterozoic is considered as a \\

Z. Li; T. Gerya

2008-01-01

10

Vestiges of a 3.2 Ga Subduction Zone, as Evidenced by High-Pressure, Low-Temperature Metamorphism in the Barberton Greenstone Belt  

NASA Astrophysics Data System (ADS)

The lack of high-pressure, low-temperature metamorphism in the Archaean rock record, one of the most diagnostic fingerprints of subduction-driven tectonics, is commonly considered to indicate that plate tectonics was absent during the Archean, or at least very different in form and function from what it is now. Here we report evidence for high-pressure, low-temperature metamorphism in supracrustal amphibolites from the mid-Archaean Barberton granitoid-greenstone terrain in South Africa that challenges this paradigm. In the Inyoni shear zone, located between the 3.45 Ga Stolzburg homogeneous tronhjemite pluton and the ca. 3.2 Ga Badplaas granodioritic to trondhjemitic gneissic complex, garnet-bearing amphibolites are preserved within the 3.22 Ga greenschist to amphibolite facies high-strain structure. Peak temperature conditions for these amphibolites are recorded by epidote-amphibole-plagioclase (An30) ± clinopyroxene assemblages, which yield PT conditions via conventional thermobarometry of 750 to 850 °C and 0.7 to1.0 GPa. However, the cores of garnet crystals, that are of more calcic and less magnesian composition than the rims, contain inclusions of quartz, clinopyroxene, amphibole, epidote and plagioclase (An10), which also differ in composition from the matrix generations of the same minerals. This earlier assemblage constrains peak pressure conditions to between 1.2 and 1.5 GPa at temperatures below 650-700 °C. This points to very low apparent geothermal gradients of 12-15 °C. These values are similar to those found in recent subduction zone settings. These highest-pressure metamorphic rocks occur on the eastern margin of a coherent granitoid terrain, at least some 500 km2 in extent, which has experienced peak metamorphic pressures of 0.8 to 1.1 GPa. This clearly demonstrates that the ca 3.23 crust in this area was cold enough and stiff enough to allow for tectonic doubling of crustal thickness. The timing of high-pressure metamorphism coincides with the proposed main phase of terrane accretion in the structurally overlying Barberton greenstone belt. We suggest that these high-pressure, low-temperature conditions represent the first metamorphic evidence for subduction-driven burial of continental crust in the mid-Archaean and are the deep crustal manifestation of the terrane accretion proposed on the grounds of structural, geochronological and sedimentological evidence from the fold and thrust belt.

Stevens, G.; Moyen, J.; Kisters, A.

2005-12-01

11

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

12

Detrital fingerprints of fossil continental-subduction zones (Axial Belt Provenance, European Alps)  

Microsoft Academic Search

Alpine-type collision orogens are generated by attempted subduction of thinned continental margins. Because of complex tectonic structure, orogenic detritus is characterized by a range of detrital signatures, making its recognition an arduous task (Dickinson and Suczek, 1979). Among the various orogenic sub-provenances, Axial Belt Provenance, derived from the erosion of the neometamorphic axial pile, can be regarded as the most

Alberto Resentini; Eduardo Garzanti; Giovanni Vezzoli; Sergio Andò; Marco G. Malusà; Marta Padoan; Paolo Paparella

2010-01-01

13

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

Microsoft Academic Search

This thesis consists of four chapters that evaluate the location, orientation, and strength of seismic anisotropy in the lithosphere and mantle in several tectonic settings, including two western Pacific subduction zones, the tectonically stable region of eastern North America, and two regions of the core-mantle boundary beneath the Pacific Ocean. The analyses in these chapters not only utilize existing methods,

Matthew James Fouch

1999-01-01

14

Petrology of HP metamorphic veins in coesite-bearing eclogite from western Tianshan, China: Fluid processes and elemental mobility during exhumation in a cold subduction zone  

NASA Astrophysics Data System (ADS)

A petrological study was carried out for high pressure (HP) veins which cut through the host coesite-bearing eclogites at Habutengsu-Kebuerte in western Tianshan, China. The results place constraints on the origin and property of metamorphic fluids during subduction-zone metamorphism. Omphacite-, clinozoisite- and quartz-dominated veins occur on centimeter to meter scales within lens-shaped and layered eclogites, or cutting into the country rocks of garnet phengite schists. Coesite-bearing eclogites mainly consist of fibrous fine-grained omphacite and porphyroblastic garnet, with minor amounts of amphibole (mainly barroisite), clinozoisite, white mica (mainly paragonite) and rutile. The veins are pronouncedly coarse-grained compared to the host eclogites and commonly consist of quartz, clinozosite, rutile, white mica (phengite and paragonite) and garnet, with or without omphacite, titanite, apatite, carbonate (mainly dolomite) and glaucophane. Fluid inclusions are abundant in vein omphacite, titanite and apatite, but are rare in the equivalent minerals of host eclogites. Rounded vein garnets usually occur close to the sharp interface of vein and host eclogite. Omphacite in the veins is characterized by its euhedral form surrounded by quartz, or coarse bladed aggregates in contrast to the fibrous or patchy one, suggesting dynamic recrystallization in the host rocks. Omphacite in both veins and host eclogites has similar jadeite contents (Jd40-50), indicating formation at eclogite-facies metamorphic conditions. Vein phengite uniformly contains certain amounts of Ba with maximum BaO content of 3.16-4.25 wt.%, suggesting that Ba was mobilized during the exhumation of UHP rocks. Specific textures of vein minerals, such as the enclosure of magnesite (or calcite) in dolomite, rutile in titanite, and the occurrence of zoned Ba-rich phengite, indicate the chemical variability of channelized fluids over time. Based on Zr content in rutile and the presence of paragonite, omphacite, phengite, glaucophane as well as the little deformed textures of HP veins, it is estimated that the vein-forming fluids would flow at about 1.3-2.1 GPa and 540-580 °C, corresponding to the stage of retrograde eclogite-facies recrystallization during exhumation of the UHP eclogites that formed at peak P-T conditions of > 2.7 GPa and 460-520 °C. The HP veins occur as a consequence of a regional tectonothermal event, triggering breakdown of lawsonite within the UHP eclogites. Based on the petrology of vein minerals, it is inferred that the HP fluids were enriched in Si, Ca, Na, Al and Ba. This suggests that these elements could be mobilized during the retrograde metamorphism of UHP eclogites in a cold subduction zone. Coeval pervasive flow of HP metamorphic fluids through the UHP eclogites at this stage may be an important process to eliminate most mineralogical evidence of the UHP metamorphism.

Lü, Zeng; Zhang, Lifei; Du, Jinxue; Yang, Xin; Tian, Zuolin; Xia, Bin

2012-04-01

15

Mineralogical Evidence for the Bulk Transformation of Continental Crust to Ultrahigh-Pressure Conditions in Subduction Zones  

NASA Astrophysics Data System (ADS)

Evidence for (ultra)high-pressure --(U)HP-- metamorphism in modern orogenic belts and the preservation of exhumed (U)HP terranes around the world suggest that subduction and exhumation of continental crust plays an important role in Phanerozoic plate tectonics. The Western Gneiss region (WGR) of Norway, a major (U)HP province extending over 60,000 km2, provides an excellent opportunity to study how subduction to depths >100 km affects continental crust. By studying a ~60 km wide transect bounded to the north by Vartdalsfjorden and Rovdefjorden and the south by the Möre og Romsdal county boundary, we are able to examine mineralogical changes that occurred during subduction and exhumation within a rock composed predominantly of orthogneiss and variably transformed mafic bodies, which indicate the depths to which these rocks were subducted. Previous studies (e.g. Hacker et al., 2005) have suggested that Caledonian deformation in WGR host gneisses is primarily limited to brittle-ductile fabrics characterized by greenschist to lower-amphibolite facies metamorphism; the majority of the deformation in the rocks, including the pervasive foliation and foliation-parallel isoclinal folds, occurred between 1200 and 900 Ma. On the northern half of our study area, however, locally occurring neoblastic garnet crosscuts the foliation in the gneiss. The boundary of this garnet zone coincides with the local HP-UHP boundary, as determined by the presence of coesite in eclogite. Because garnet can retain information about changes in pressure and temperature, as well as the availability of water within the crust to catalyze chemical reactions, our findings suggest that 1) portions of the orthogneiss did transform at high pressures, 2) the presence of garnet within the orthogneiss may indicate conditions that approximate UHP and can therefore be useful in defining the boundaries between UHP and HP conditions, and 3) the growth of garnet during (U)HP metamorphism may be controlled by hydration of the crust, thus explaining the partial transformation to (U)HP mineral assemblages throughout the WGR.

Peterman, E. M.; Hacker, B. R.; Kylander-Clark, A. R.

2005-12-01

16

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

Microsoft Academic Search

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,

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

2005-01-01

17

Magnetotelluric image of the fluid cycle in the Costa Rican subduction zone  

Microsoft Academic Search

Fluids entering the subduction zone play a key role in the subduction process. They cause changes in the dynamics and thermal structure of the subduction zone, and trigger earthquakes when released from the subducting plate during metamorphism. Fluids are delivered to the subduction zone by the oceanic crust and also enter the oceanic plate as it bends downwards at the

Tamara Worzewski; Marion Jegen; Heidrun Kopp; Heinrich Brasse; Waldo Taylor Castillo

2011-01-01

18

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.

19

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

Microsoft Academic Search

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

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

2009-01-01

20

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

21

Polyphase formation and exhumation of high- to ultrahigh-pressure rocks in continental subduction zone: Numerical modeling and application to the Sulu ultrahigh-pressure terrane in eastern China  

Microsoft Academic Search

High- to ultrahigh-pressure (HP-UHP) metamorphic rocks commonly form and exhume during the early continental collision, and many questions related to their origin still remain unresolved. We focus our study on explaining the poly metamorphic origins of many HP-UHP terranes composed of tectonic units having strongly variable ages, peak metamorphic conditions, and P-T paths. These features are especially well characterized for

Zhonghai Li; Taras V. Gerya

2009-01-01

22

Subduction-zone magnetic anomalies and implications for hydrated forearc mantle  

Microsoft Academic Search

Continental mantle in subduction zones is hydrated by release of water from the underlying oceanic plate. Magnetite is a significant byproduct of mantle hydration, and forearc mantle, cooled by subduction, should contribute to long-wavelength magnetic anomalies above subduction zones. We test this hypothesis with a quantitative model of the Cascadia convergent margin, based on gravity and aeromagnetic anomalies and constrained

Richard J. Blakely; Thomas M. Brocher; Ray E. Wells

2005-01-01

23

Subduction-zone magnetic anomalies and implications for hydrated forearc mantle  

Microsoft Academic Search

Continental mantle in subduction zones is hydrated by release of water from the un- derlying oceanic plate. Magnetite is a significant byproduct of mantle hydration, and fore- arc mantle, cooled by subduction, should contribute to long-wavelength magnetic anom- alies above subduction zones. We test this hypothesis with a quantitative model of the Cascadia convergent margin, based on gravity and aeromagnetic

Richard J. Blakely; Thomas M. Brocher; Ray E. Wells

2005-01-01

24

Electromagnetic images of modern and ancient subduction zones  

Microsoft Academic Search

Jones, A.G., 1993. Electromagnetic images of modern and ancient subduction zones. In: A.G. Green, A. KrGner, H.-J. Gijtze and N. Pavlenkova (Editors), Plate Tectonic Signatures in the Continental Lithosphere. Tectonophysics, 219: 29-45. One question frequently posed since the advent of plate tectonic theory is \\

Alan G. Jones

1993-01-01

25

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

NASA Astrophysics Data System (ADS)

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

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

2011-12-01

26

Numerical simulations of subduction zones  

Microsoft Academic Search

In oceanic subduction zones, dehydration of slab’s minerals may favor asthenospheric flow in the mantle wedge by decreasing rocks strength. This should enhance the upper plate base reheating and markedly alter its thermal structure. To quantify this phenomenon, we model slab subduction within a viscous mantle, dehydration–hydration processes, and the rock strength dependence on water content. We use accurate phase

D. Arcay; E. Tric; M.-P. Doin

2005-01-01

27

Earthquake hazards on the cascadia subduction zone  

Microsoft Academic Search

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

T. H. Heaton; S. H. Hartzell

1987-01-01

28

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

29

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

NASA Astrophysics Data System (ADS)

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

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

2009-12-01

30

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

NASA Astrophysics Data System (ADS)

In order to better understand the role of fluids during subduction and subsequent exhumation, we have investigated whole-rock and mineral chemistry (major and trace elements) and Li, B as well as O, Sr, Nd, Pb isotopes on selected continuous drill-core profiles through contrasting lithological boundaries from the Chinese Continental Scientific Drilling Program (CCSD) in Sulu, China. Four carefully selected sample sets have been chosen to investigate geochemical changes as a result of fluid mobilization during dehydration, peak metamorphism, and exhumation of deeply subducted continental crust. Our data reveal that while O and Sr-Nd-Pb isotopic compositions remain more or less unchanged, significant Li and/or B isotope fractionations occur between different lithologies that are in close contact during various metamorphic stages. Samples that are supposed to represent prograde dehydration as indicated by veins formed at high pressures (HP) are characterized by element patterns of highly fluid-mobile elements in the veins that are complementary to those of the host eclogite. A second sample set represents a UHP metamorphic crustal eclogite that is separated from a garnet peridotite by a thin transitional interface. Garnet peridotite and eclogite are characterized by a >10% difference in MgO, which, together with the presence of abundant hydroxyl-bearing minerals and compositionally different clinopyroxene grains demonstrate that both rocks have been derived from different sources that have been tectonically juxtaposed during subduction, and that hydrous silicate-rich fluids have been added from the subducting slab to the mantle. Two additional sample sets, comprising retrograde amphibolite and relatively fresh eclogite, demonstrate that besides external fluids, internal fluids can be responsible for the formation of amphibolite. Li and B concentrations and isotopic compositions point to losses and isotopic fractionation during progressive dehydration. On the other hand, fluids with isotopically heavier Li and B are added during retrogression. On a small scale, mantle-derived rocks may be significantly metasomatized by fluids derived from the subducted slab. Our study indicates that during high-grade metamorphism, Li and B may show different patterns of enrichment and of isotopic fractionation.

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

2011-10-01

31

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

Microsoft Academic Search

Fluids are an integral part of metamorphic and igneous processes in the deep crust and within subduction zones. The presence of fluids in deep settings is no longer in dispute, but major research frontiers of global significance remain. One of these is the problem of tracing flow paths from deep metamorphic settings into the shallow crust. This is important to

J. J. Ague

2006-01-01

32

The Role of H2O in Subduction Zone Magmatism  

NASA Astrophysics Data System (ADS)

Water is a key ingredient in the generation of magmas in subduction zones. This review focuses on the role of water in the generation of magmas in the mantle wedge, the factors that allow melting to occur, and the plate tectonic variables controlling the location of arc volcanoes worldwide. Water also influences chemical differentiation that occurs when magmas cool and crystallize in Earth's continental crust. The source of H2O for arc magma generation is hydrous minerals that are carried into Earth by the subducting slab. These minerals dehydrate, releasing their bound H2O into overlying hotter, shallower mantle where melting begins and continues as buoyant hydrous magmas ascend and encounter increasingly hotter surroundings. This process is controlled by plate tectonic variables that ultimately influence the location of the active volcanic arc above subduction zones. Water also modifies the thermodynamic properties of melts, leading to the unique chemical composition of arc volcanic rocks and Earth's continental crust.

Grove, Timothy L.; Till, Christy B.; Krawczynski, Michael J.

2012-05-01

33

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

Microsoft Academic Search

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

Hemendra Acharya

1992-01-01

34

Scientific teams analyze earthquake hazards of the Cascadia Subduction Zone  

Microsoft Academic Search

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

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

1997-01-01

35

Multistage mixing in subduction zone: Application to Merapi volcano, Indonesia  

Microsoft Academic Search

Basalts sampling subduction zone volcanism (IAB) often show binary mixing relationship in classical Sr-Nd, Pb-Pb, Sr-Pb isotopic diagrams, generally interpreted as reflecting the involvement of two components in their source. However, several authors have highlighted the presence of minimum three components in such a geodynamical context: mantle wedge, subducted and altered oceanic crust and subducted sediments. The overlying continental crust

V. Debaille; R. Doucelance; D. Weis; P. Schiano

2003-01-01

36

Earthquake hazards on the Cascadia subduction zone  

Microsoft Academic Search

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

Thomas H. Heaton; Stephen H. Hartzell

1987-01-01

37

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

Microsoft Academic Search

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

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

2008-01-01

38

Fluid and deformation induced metamorphic processes around continental Moho  

NASA Astrophysics Data System (ADS)

Exposed High Pressure (HP) and Ultra High Pressure (UHP) metamorphic terrains have been studied in order to assess the metamorphic processes and their role in changing petrophysical properties near Moho depth in continental root zones. The investigation points to the critical role of fluid and deformation in metamorphic transformation in the deep crust and upper mantle. This applies to a) formation of granulite facies areas, b) transformation of granulites to eclogites, c) retrogression of eclogite facies rocks to amphibolite and greenschist facies rocks and d) the spinel lherzolite to garnet lherzolite transition. Dry rocks, both feldspar bearing and ultramafic, remain with their pre-HP and UHP structures and anhydrous mineralogy preserved while reactions occur where fluid has been introduced along deformation zones. Metamorphic rocks of different ages and stable at different P-T conditions are mixed on a metre to km scale and such mixtures will be present throughout the crust and upper mantle. This lack of equilibration may lead to error in geothermal gradients calculated on minerals from xenoliths; b) misinterpretation of rock composition inferred from geophysical signature and c) induce extra uncertainty in geodynamic models that assume continuous metamorphic equilibration. Pseudotachylytes (frictional melts or ultracomminuted material) are observed in both ultramafic and feldspar bearing lithologies spatially associated with HP and UHP rocks, suggesting that rock properties at Moho depth allow earthquakes. Seismicity enhances the metamorphic and metasomatic transitions through fragmentation and by opening the rock for fluid influx. Ductile eclogite facies shear zones nucleate along the brittle structures. These observations point to Moho as a rock processing zone with the following facets: 1. A metastable dry and strong lower crust and upper mantle 2. Earthquakes and tremors result in fluid flow and HP metamorphism. 3. A pronounced weakening of the hydrated and transformed rocks allows the development of new fabrics (crystallographic preferred orientation, CPO) in the transformed rocks. Deep tremors and earthquakes at Moho depth may record ongoing metamorphic transitions.

Austrheim, Håkon

2013-04-01

39

Double-Sided Wedge Model For Retreating Subduction Zones: Applications to the Apenninic and Hellenic Subduction Zones (Invited)  

Microsoft Academic Search

We propose a new model for the evolution of accreting wedges at retreating subduction zones. Advance and retreat refer to the polarity of the velocity of the overriding plate with respect to subduction zone. Advance indicates a velocity toward the subduction zone (e.g., Andes) and retreat, away from the subduction zone (e.g. Apennines, Crete). The tectonic mode of a subduction

M. T. Brandon; S. Willett; J. M. Rahl; D. S. Cowan

2009-01-01

40

Serpentine in active subduction zones  

NASA Astrophysics Data System (ADS)

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

Reynard, Bruno

2013-09-01

41

Observe an animation of volcanism at a subduction zone  

NSDL National Science Digital Library

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

Education, Terc. C.; Littell, Mcdougal

2003-01-01

42

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

NASA Astrophysics Data System (ADS)

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

Nelson, Bruce K.

1991-10-01

43

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

SciTech Connect

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

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

1991-10-01

44

Earthquake hazards on the cascadia subduction zone.  

PubMed

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

Heaton, T H; Hartzell, S H

1987-04-10

45

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

NASA Astrophysics Data System (ADS)

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

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

2010-12-01

46

Geochemistry of subduction zone serpentinites: A review  

NASA Astrophysics Data System (ADS)

Over the last decades, numerous studies have emphasized the role of serpentinites in the subduction zone geodynamics. Their presence and role in subduction environments are recognized through geophysical, geochemical and field observations of modern and ancient subduction zones and large amounts of geochemical database of serpentinites have been created. Here, we present a review of the geochemistry of serpentinites, based on the compilation of ~ 900 geochemical data of abyssal, mantle wedge and exhumed serpentinites after subduction. The aim was to better understand the geochemical evolution of these rocks during their subduction as well as their impact in the global geochemical cycle.

Deschamps, Fabien; Godard, Marguerite; Guillot, Stéphane; Hattori, Kéiko

2013-09-01

47

Temperature Models for the Mexican Subduction Zone  

NASA Astrophysics Data System (ADS)

It is well known that the temperature is one of the major factors which controls the seismogenic zone. The Mexican subduction zone is characterized by a very shallow flat subducting interplate in its central part (Acapulco, Oaxaca), and deeper subduction slabs northern (Jalisco) and southern (Chiapas). It has been proposed that the seismogenic zone is controlled, among other factors, by a temperature. Therefore, we have developed four two-dimensional steady state thermal models for Jalisco, Guerrero, Oaxaca and Chiapas. The updip limit of the seismogenic zone is taken between 100 §C and 150 §C, while the downdip limit is thought to be at 350 §C because of the transition from stick-slip to stable-sliding. The shape of the subducting plate is inferred from gravity and seismicity. The convergence velocity between oceanic and continental lithospheric plates is taken as the following: 5 cm/yr for Jalisco profile, 5.5 for Guerrero profile, 5.8 for Oaxaca profile, and 7.8 for Chiapas profile. The age of the subducting plates, since they are young, and provides the primary control on the forearc thermal structure, are as the following: 11 My for Jalisco profile, 14.5 My for Guerrero profile, 15 My for Oaxaca profile, and 28 My for Chiapas profile. We also introduced in the models a small quantity of frictional heating (pore pressure ratio 0.98). The value of 0.98 for pore pressure ratio was obtained for the Guerrero profile, in order to fit the intersection between the 350 §C isotherm and the subducting plate at 200 Km from trench. The value of 200 km coupling zone from trench is inferred from GPS data for the steady interseismic period and also for the last slow aseismic slip that occurred in Guerrero in 2002. We have used this value of pore pressure ratio (0.98) for all the other profiles. For the others three profiles we obtained the following coupling extents: Jalisco - 100 km, Oaxaca - 170 km and Chiapas - 125 km (from the trench). Independent constrains of the thermal models come from the surface thermal measurements (offshore - Prol-Ledesma et al (1989) and onshore - Ziagos et al. (1985)). Unfortunately these measurements are very sparse, and present an important dispersion and have large uncertainties. In our models, all profiles show a decrease in heat flow from the trench towards the continent, which is characteristic for subduction zones. We also have included a mantle wedge flow current in order to keep a constant depth to the lithosphere-asthenosphere boundary. This mantle wedge convection provides an increase in heat flow near the volcanic arc which is consistent with the surface observations. Our models indicate that the seismogenic zone in Mexico comprised between 100 §C and 350 §C is in good agreement with the coseismic rupture width inferred from the megathrust earthquake aftershocks and seismic models of rupture. These thermal models will be used to calculate the thermal stresses induced by the subducting plate.

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

2002-12-01

48

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

Microsoft Academic Search

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

Zhi Zhang; Thorne Lay

1993-01-01

49

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

50

Temperature Models for the Mexican Subduction Zone  

Microsoft Academic Search

It is well known that the temperature is one of the major factors which controls the seismogenic zone. The Mexican subduction zone is characterized by a very shallow flat subducting interplate in its central part (Acapulco, Oaxaca), and deeper subduction slabs northern (Jalisco) and southern (Chiapas). It has been proposed that the seismogenic zone is controlled, among other factors, by

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

2002-01-01

51

Nonvolcanic tremors in the Mexican subduction zone  

Microsoft Academic Search

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

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

2007-01-01

52

Oregon Subduction Zone: Venting, Fauna, and Carbonates  

Microsoft Academic Search

Transects of the submersible Alvin across rock outcrops in the Oregon subduction zone have furnished information on the structural and stratigraphic framework of this accretionary complex. Communities of clams and tube worms, and authigenic carbonate mineral precipitates, are associated with venting sites of cool fluids located on a fault-bend anticline at a water depth of 2036 meters. The distribution of

L. D. Kulm; E. Suess; J. C. Moore; B. Carson; B. T. Lewis; S. D. Ritger; D. C. Kadko; T. M. Thornburg; R. W. Embley; W. D. Rugh; G. J. Massoth; M. G. Langseth; G. R. Cochrane; R. L. Scamman

1986-01-01

53

Strain accumulation along the Cascadia subduction zone  

Microsoft Academic Search

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

Mark H. Murray; Michael Lisowski

2000-01-01

54

Serpentine and the subduction zone water cycle  

Microsoft Academic Search

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

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

2004-01-01

55

Serpentine and the subduction zone water cycle  

Microsoft Academic Search

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

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

2004-01-01

56

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

NASA Astrophysics Data System (ADS)

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

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

2013-08-01

57

Decoupling of Pacific subduction zone guided waves  

NASA Astrophysics Data System (ADS)

Subduction zone guided wave arrivals have been observed in many circum Pacific subduction zones and have been attributed to the presence of a low velocity layer (LVL) in the subducting slab. This LVL acts as a waveguide for the high frequency energy, while lower frequency energy is not retained and travels in the higher velocity surrounding mantle. This leads to the characteristic dispersion of seismic waves observed. The commonly accepted model for the LVL is the persistence of basaltic oceanic crust to a depth of greater than 150 km. This basaltic oceanic crust has not yet undergone phase transformation to eclogite due to kinetic hindering, and so still has a distinguishably lower velocity than the surrounding mantle. It has been shown that guided waves are only seen from events that occur in or near to the low velocity layer. Similarly it would be expected that guided waves are only seen when the receiver is on the wave guide. However in a subduction zone setting it has been shown that guided wave energy is decoupled from the waveguide, due to the bend of the slab (Martin et al., 2003). Therefore high frequency guided wave energy escapes the waveguide and so can be observed at receivers placed in specific positions on the overriding plate. This decoupling mechanism allows guided waves from intermediate and deep Wadati-Benioff zone earthquakes to be observed. We use a two dimensional finite difference model to investigate the decoupling of wave guide energy due to the geometry of various Pacific subduction zones in order to predict the occurrence of guided wave arrivals along up-dip and along-strike propagation paths. The slab geometry is inferred from the USGS slab contour model slab 1.0. An explosive source is used so that frequency effects of the source do not complicate the results. The thickness of the LVL is inferred from published observations of Pacific subduction zone guided waves. For the along-strike profile we concentrate on the observations of guided waves, particularly from the events reported on by Abers et al. (2005). In this work the distance that the high frequency guided wave travels in the slab is inferred by ray tracing, and the velocity of the LVL is calculated from the time lag. We investigate where the guided wave energy decouples from the slab in these arrivals. This could potentially affect the estimated time that the wave spent in the slab, and so affect the estimated velocity of the wave guide. For the up-dip profiles we investigate the effect of slab bending on the dispersed waveform and its influence on the stability of the characteristic wave dispersion.

Garth, T.; Rietbrock, A.

2010-12-01

58

Growth of sediment diapirs in subduction zones  

NASA Astrophysics Data System (ADS)

We calculate the instability times of diapirs forming in 2D numerical models of a buoyant sediment layer subducting beneath a dense mantle wedge. Subducted sediments are incorporated into arc magmas where they play a key role in influencing the composition of arc lavas. Our understanding of the processes by which these sediments are transported from the slab-top to the Earth’s surface remains uncertain. Recent geochemical data for high- to ultrahigh-pressure rocks that underwent subduction suggest that significant melting of a metasedimentary component in arc magmas does not occur until temperatures significantly exceed those predicted for the slab-surface at similar pressures in recent thermal models. These data suggest that subducting sediments detach from the slab and rise into the mantle wedge, where they melt during their ascent to the surface. Petrophysical calculations indicate that subducted metasediments are buoyant with respect to the overlying mantle wedge and thus may form solid-state diapirs. Here, we use a finite-element code to calculate the growth of instabilities forming in a buoyant sediment layer beneath a dense mantle half-space. To simulate the effects of the subduction process on diapir growth, we impose temperature and background strain-rate values from subduction-zone thermal models at each time-step, assuming different non-linear rheologies for each layer. In accordance with previous scaling relations, we find that instability time for subducting sediment diapirs is primarily sensitive to sediment layer thickness and the absolute layer viscosities. However, we also show that changes in the relative viscosity contrast between the sediment and mantle-wedge due to the evolving subduction-zone thermal and strain-rate fields are also important. Instability times from these numerical models suggest that, while sediment diapirs may form in hot and cold subduction-zones for a range of sediment layer thicknesses, instabilities only detach and rise into the mantle wedge in hot subduction systems with relatively thick sediment layers.

Miller, N. C.; Behn, M. D.

2010-12-01

59

Formation of metamorphic and metamorphosed garnets in the low-T/UHP metagranite during continental collision in the Dabie orogen  

NASA Astrophysics Data System (ADS)

A combined study of major and trace elements in garnet was carried out for low-T/UHP metagranite in the Dabie orogen. The results show different fashions of element zoning in the garnet, suggesting occurrence of both metamorphic and metamorphosed garnets. Three generations of garnet growth are distinguished on the basis of the assumption that Ca contents, Mn contents and Fe/Mg ratios of garnet in metagranites are a function of pressure and temperature. The first generation of garnet (Grt-I) occurs in the core of a skeletal garnet, showing homogeneously low XGrs values and Fe/Mg ratios but high XSps values. This implies that the core garnet grew at the highest temperature but the lowest pressure, representing a metamorphosed residue of magmatic garnet in protolith granite. The second generation of garnet (Grt-II) occurs in the mantles and cores of many garnet grains, exhibiting increased XGrs values but decreased Fe/Mg ratios. This is ascribed to a continuous increase in temperature and pressure till the peak pressure, corresponding to metamorphic growth (or overgrowth) during the prograde subduction. The third generation of garnet (Grt-III) occurs in the rims of all garnet grains, displaying decreased XGrs values and Fe/Mg ratios in response to a pressure decrease but a temperature increase till the peak temperature. These rims overgrew subsequent to the peak pressure with continuous heating during the initial exhumation. Grt-I shows steep MREE-HREE patterns and profoundly negative Eu anoalies, consistent with growth from granitic melt. This kind of metamorphosed garnet from protolith granite has still preserved very high contents of many trace elements (such as REE, Rb, Ba, Sr, Pb, Th, U, Nb and Ta) despite the low-T/UHP metamorphism. Grt-II and Grt-III in one sample exhibit steep MREE-HREE patterns, with a continuous decrease in REE contents. This suggests their growth from the almost same matrix of mineral assemblages (plagioclase + K-feldspar + muscovite) during metamorphism. However, Grt-II in the other sample displays flat to steep MREE-HREE patterns, with an increase of REE contents from core to mantle. This implies that the matrix of mineral assemblages for Grt-II changes from common rock-forming minerals (e.g., feldspar, muscovite and biotite) to REE-rich minerals (e.g., epidote, allanite, zircon, amphibole, apatite and titanite). Grt-III in the two samples all displays significantly lowered REE contents compared to those of Grt-I and Grt-II. This may be due to a decrease in pressure and an increase in temperature during the initial exhumation. Therefore, the two-stage growth of metamorphic garnet is evident in the low-T/UHP metagranite during the continental subduction-zone metamorphism. In addition, the highest pressure occurs in the cores or mantles whereas the highest temperature occurs in the rims. This suggests that the peak pressure (Pmax) did not occur contemporarily at the peak temperature (Tmax), corresponding to the "hot" exhumation.

Xia, Qiong-Xia; Zheng, Yong-Fei; Lu, Xiao-Nan; Hu, Zhaochu; Xu, Haijun

2012-04-01

60

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

61

Hydrologic control of forearc strength and seismicity in the Costa Rican subduction zone  

NASA Astrophysics Data System (ADS)

Subduction zones can exhibit variable seismic behaviour, ranging from great earthquakes to slow slip. This variability may be linked to fault frictional properties, and the rheology and structure of the upper plate. The subduction zone beneath the Nicoya Peninsula, Costa Rica, is characterized by strong variations in fault-slip behaviour and a lateral change in the origin of the subducting plate. In the northwest, the plate interface is locked, and experiences large, infrequent earthquakes, and the subducting plate is formed at the East Pacific Rise. In contrast, in the southeast, slow-slip events occur frequently and the subducting plate is formed at the Cocos-Nazca spreading centre. Here we use seismic receiver-function data to analyse the structure of the subduction zone beneath the Nicoya Peninsula. We find extremely high P-S seismic-velocity ratios within the entire subducting oceanic crust that we interpret as high pore-fluid pressure. Velocity ratios in the overriding continental crust, however, change from lower values in the northwest to higher ones in the southeast, indicating a disparity in fluid accumulation. We infer that this disparity is caused by a higher supply of fluid from the subducting slab in the southeast, owing to the permeability structure of oceanic crust formed at the Cocos-Nazca spreading centre. We suggest that the spatial gradient in fluid content influences upper-plate strength and controls the segmentation of seismogenic behaviour in this subduction zone.

Audet, Pascal; Schwartz, Susan Y.

2013-10-01

62

3-D Seismic Refraction Tomography in Sumatra Subduction Zone  

NASA Astrophysics Data System (ADS)

A seismic refraction survey was conducted as part of the major UK and international project to image the 3-D structures and the seismic velocity of the Sumatra subduction zone. The 3-D seismic refraction tomography mainly focusses on the two segment boundaries identified by the earthquake ruptures in 2004 and 2005. High quality seismic datasets (refraction, reflection, gravity and magnetics) were collected in the two survey areas on the vessel R/V Sonne in 2008. The northern area, around the island of Simeulue, is about 196 km long and 185 km wide. 50 Ocean Bottom Seismometers (OBS) were deployed in this area, and 10462 air-gun shots were fired along 1550 km of profiles. 47 OBSs were then installed near the island of Nias, in an area of 246 km long and 180 km wide, and 9134 shots were fired on 1408 km of profiles. During the OBS deployment, air-gun shooting, and OBS recovery, high resolution swathe bathymetry data were recorded, and XBT data were collected in each OBS deployment location. Gravity data were also recorded during the whole survey and magnetics data collected during the air-gun shooting. The 3-D refraction tomography successfully sampled the two survey areas. Refractions from the oceanic and continental crust are clear and easy to pick, and refractions from the mantle lithosphere are also visible at some locations at an offset up to 150 km, which enables us to image the deeper structures of the Sumatra subduction zone. A tomographic inversion program JIVE-3D (Hobro et al. 2003) will be applied to the refraction/reflection travel times to invert them into a minimum-structure velocity model. The high resolution bathymetry will be smoothed and put into the model as a known interface. The XBT data will be used to calibrate the acoustic velocity in the water. During the shooting period, several earthquakes of magnitude 5.0 and above occurred near the survey area, which also provide extra information for the inversion. The well resolved 3-D structure models obtained will give insight into the possible rupture barriers causing the observed segmentation of the subduction zone.

Tang, G.; Barton, P.; Dean, S.; Vermeesch, P.; Jusuf, M. D.; Henstock, T.; Djajadihardja, Y. S.; McNeill, L.; Permana, H.; Party, S. S.

2008-12-01

63

Geochemistry of serpentinites in subduction zones: A review  

NASA Astrophysics Data System (ADS)

Over the last decades, numerous studies have emphasized the role of serpentinites in the subduction zones geodynamics. Their presence and effective role in this environment is acknowledged notably by geophysical, geochemical and field observations of (paleo-) subduction zones. In this context, with the increasing amount of studies concerning serpentinites in subduction environments, a huge geochemical database was created. Here, we present a review of the geochemistry of serpentinites, based on the compilation of ~ 900 geochemical analyses of abyssal, mantle wedge and subducted serpentinites. The aim was to better understand the geochemical evolution of these rocks during their subduction history as well as their impact in the global geochemical cycle. When studying serpentinites, it is often a challenge to determine the nature of the protolith and their geological history before serpentinisation. The present-day (increasing) geochemical database for serpentinites indicates little to no mobility of incompatible elements at the scale of the hand-sample in most serpentinized peridotites. Thus, Rare Earth Elements (REE) distribution can be used to identify the initial protolith for abyssal and mantle wedge serpentinites, as well as magmatic processes such as melt/rock interactions taking place before serpentinisation. In the case of subducted serpentinites, the interpretation of trace element data is more difficult due to secondary enrichments independent of the nature of the protolith, notably in (L)REE. We propose that these enrichments reflect complex interactions probably not related to serpentinisation itself, but mostly to fluid/rock or sediment/rock interactions within the subduction channel, as well as intrinsic feature of the mantle protolith which could derive from the continental lithosphere exhumed at the ocean-continent transition. Additionally, during the last ten years, numerous studies have been carried out, notably using in situ approaches, to better constrain the geochemical budget of fluid-mobile elements (FME; e.g. B, Li, Cl, As, Sb, U, Th, Sr) stored in serpentinites and serpentine phases. These elements are good markers of the fluid/rock interactions taking place during serpentinisation. Today, the control of serpentinites on the behaviour of these elements, from their incorporation to their gradually release during subduction, is better understood. Serpentinites must be considered as a component of the FME budget in subduction zones and their role, notably on arc magmas composition, is undoubtedly underestimated presently in the global geochemical cycle.

Deschamps, Fabien; Godard, Marguerite; Guillot, Stéphane; Hattori, Kéiko

2013-04-01

64

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

65

Subduction Zone Waveguides in Tonga-Fiji  

NASA Astrophysics Data System (ADS)

Data collected on the islands of Tonga and Fiji from a single deep, 650 km, earthquake shows dramatically contrasting styles. Recordings from Fiji, mantle wedge, show relatively simple waveforms including P and S arrivals. In opposition, recordings from Tonga, slab updip, show a secondary coda arrival with a frequency around ~ 0.3 Hz. Representing the data with synthetics computed from a 3D velocity (Zhao et al., 1997) and attenuation (Roth et al. 1999, 2000) works well for the simple mantle wedge paths but not the complex updip paths. The addition of a low velocity zone (LVZ) atop the subducted plate produces a isolated secondary arrival dependent on the properties of the LVZ (Abers, 2005), but not the secondary coda arrival seen in the data. Creation of subduction zone coda arrivals was successfully accomplished from Japanese seismic data by Furumura and Kennett, (2005) using scatters within the subducting plate. The coda arrivals seen by Furumura and Kennett (2005) were high frequency, < 2 Hz, whereas the data from Tonga-Fiji shows a much lower frequency, ~ 0.3 Hz. After we show a scattering waveguide produces the secondary coda arrivals, we vary the scattering properties, further isolate the size, orientation, and properties of these scatters. Synthetics are computed using either in 2D using the finite difference method or in 3D with the Spectral Element Method. A selection of events within the Tonga Fiji subduction zone show similar features as the deep event and will be used to test the scattering waveguide model.

Savage, B.

2006-12-01

66

Development of Forearcs of Intraoceanic Subduction Zones  

NASA Astrophysics Data System (ADS)

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

Lundberg, Neil

1983-02-01

67

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

68

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

Microsoft Academic Search

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

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

2000-01-01

69

Fluid flow in ocean crust cools the Cascadia subduction zone  

Microsoft Academic Search

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

B. D. Cozzens; G. A. Spinelli

2010-01-01

70

Do microplates in subduction zones leave a geological record?  

Microsoft Academic Search

Active microplate boundaries in ocean-continent subduction zones may induce deformation of the overlying plate and spatial or geochemical variations in the volcanic arc. We discuss two modern cases. The first is the South Gorda-Juan de Fuca plate boundary in the Cascadia subduction zone, where there is little or no effect on the overriding plate and the oceanic plate takes up

Joann M. Stock; Jeffrey Lee

1994-01-01

71

Cyclic stressing and seismicity at strongly coupled subduction zones  

USGS Publications Warehouse

We use the finite element method to analyze stress variations in and near a strongly coupled subduction zone during an earthquake cycle. Deformation is assumed to be uniform along strike (plane strain on a cross section normal to the trench axis), and periodic earthquake slip is imposed consistent with the long-term rate of plate convergence and degree of coupling. Simulations of stress and displacement rate fields represent periodic fluctuations in time superimposed on an average field. The oceanic plate, descending slab, and continental lithosphere are assumed here to respond elastically to these fluctuations, and the remaining mantle under and between plates is assumed to respond as Maxwell viscoelastic. In the first part of the analysis we find that computed stress fluctuations in space and time are generally consistent with observed earthquake mechanism variations with time since a great thrust event. In particular, trench-normal extensional earthquakes tend to occur early in the earthquake cycle toward the outer rise but occur more abundantly late in the cycle in the subducting slab downdip of the main thrust zone. Compressional earthquakes, when they occur at all, have the opposite pattern. Our results suggest also that the actual timing of extensional outer rise events is controlled by the rheology of the shallow aseismic portion of the thrust interface. The second part of the analysis shows the effects of mantle relaxation on the rate of ground surface deformation during the earthquake cycle. Models without relaxation predict a strong overall compressional strain rate in the continental plate above the main thrust zone, with the strain rate constant between mainshocks. However with significant relaxation present, a localized region of unusually low compressional, or even slightly extensional, strain rate develops along the surface of the continental plate above and somewhat inland from the downdip edge of the locked main thrust zone. The low strain rate starts in the middle or late part of the cycle, depending on position. This result suggests that the negligible or small contraction measured on the Shumagin Islands, Alaska, during 1980 to 1991, may not invalidate an interpretation of that region as being a moderately coupled subduction zone. In contrast, mantle relaxation causes only modest temporal nonuniformity of uplift rates in the overriding plate and of extensional stress rates in the subducting plate, even when the Maxwell time is an order of magnitude less than the recurrence interval.

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

1996-01-01

72

Seismic structure across the active subduction zone of western Canada  

NASA Astrophysics Data System (ADS)

The Vancouver Island Seismic Project was conducted in 1980 to study the structure of the subducting oceanic Juan de Fuca plate and the overriding continental America plate. The principal seismic refraction line (line I) was a 350-km onshore-offshore profile perpendicular to the continental margin. An array of 32 receivers was located on the America plate on the mainland and across Vancouver Island and extended offshore with three ocean bottom seismometers (OBS's). Two shots were fired at the eastern end of the line, and 17 shots were located along the westernmost 100 km of the profile. Control for the interpretation of the onshore-offshore profile was provided by a reversed refraction profile along the length of Vancouver Island and by a marine refraction profile recorded on the OBS'S. Modeling of the seismic structure of this complex region utilized an iterative inversion method for travel times from explosions in which shots at several locations are recorded on the same set of receivers and utilized an algorithim based on asymptotic ray theory for the calculation of synthetic seismograms through two-dimensional media. The major features of the refraction structural model are that (1) the oceanic lithosphere dips at 3° or less beneath the continental slope, so the bend in the subducting slab occurs landward of the foot of the slope. (2) the oceanic lithosphere dips at 14°-16° beneath the continental shelf until it passes beneath the continental Moho at 37 km depth below western Vancouver Island, (3) an upper mantle reflector may correspond to the base of the subducting lithosphere, and (4) a segment of high-velocity material above the downgoing crust, with velocity 7.7 km/s and depth range 20-25 km, may represent a remnant of subducted lithosphere, perhaps detached when the subduction zone jumped westward to its present position.

Spence, G. D.; Clowes, R. M.; Ellis, R. M.

1985-07-01

73

Mechanism of décollement formation in subduction zones  

NASA Astrophysics Data System (ADS)

The mechanism of décollement formation was investigated through a particle-based simulation model assuming homogeneity (e.g. no weak layer or pore fluid). A décollement-like structure appeared as a spontaneously localized shear deformation near the bottom of the sediment when the thickness of the sediment was sufficient to balance the gravitational force and tectonic loading. In contrast, no such décollement-like structure was formed when the sediment was too thin; in this case, the entire prism was deformed because of plate motion. These results are consistent with various observations in real subduction zones. A precise analysis of the stress state evolution during accretion reveals that the formation of a décollement-like structure is controlled by the spatio-temporal distribution of isotropic compression states.

Hori, Takane; Sakaguchi, Hide

2011-12-01

74

Permeability anisotropy of serpentinite and fluid migration in subduction zones  

NASA Astrophysics Data System (ADS)

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

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

2010-12-01

75

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

76

Magnetic petrology of high Fe-Ti eclogites from the CCSD main hole: Implications for subduction-zone magnetism  

NASA Astrophysics Data System (ADS)

The high Fe-Ti eclogites with exsolved lamellar in 530-600 m depths from the Chinese Continental Scientific Drilling (CCSD) main hole in the Sulu Ultra-High Pressure (UHP) metamorphic belt, eastern China, record an anomalously high susceptibility (?), natural remanent magnetization (NRM) and Köenigsberger ratio Q (NRM/Ji, Ji is induced magnetization). This provides us with a good opportunity to study the effects of magnetic minerals and exsolution lamellae on the magnetic properties of deep rocks. In this paper, we have measured systematically magnetic properties and mineral assemblage and structure for three special samples (No. 83, No.86 and No.89). Results show that these Fe-Ti-rich eclogites are the result of the fine-grained (titano)-magnetite exsolution in pyroxene and lamellar structure consisting of finely interlayered ilmenite and hematite in exsolved hemo-ilmenite. We found that the dominant Fe-bearing oxide minerals in samples studied are ilmenite, hematite (Hem + Ilm up to 25%), little (titano)magnetite and pyrite. The ferromagnetic susceptibility is mainly controlled by fine grained (titano)magnetite and NRM is closely related to the exsolved lamellar structure. We hence propose that the anomalous magnetism of these eclogites observed in our samples is the result of exsolution from homogenous pyroxene and ilmeno-hematite during cooling and decompression processes. These Fe-Ti-rich eclogites might be one of the sources of high-magnetic anomalies observed in the Sulu subduction zone, eastern China.

Liu, Qingsheng; Frost, B. Ronald; Wang, Hongcai; Zheng, Jianping; Zeng, Qingli; Jin, Zhenmin

2012-07-01

77

Origin and serpentinization of ultramafic rocks of Manipur Ophiolite Complex in the Indo-Myanmar subduction zone, Northeast India  

NASA Astrophysics Data System (ADS)

The Manipur Ophiolite Complex (MOC) is part of the Manipur-Nagaland ophiolite belt (MNOB). The belt is exposed in the eastern margin of the Indo-Myanmar Ranges (IMRs), which formed by the collision between the India and Myanmar continental plates. Several contrasting views were put forward concerning the origin of the MNOB. The complex represents a dismembered ophiolite sequence with serpentinite as the largest litho-unit formed. Petrography and Raman spectroscopy of the serpentinite suggest that they are serpentinized ultramafic cumulate and peridotite. The serpentinization may have occurred at a condition of low pressure and low temperature metamorphism. Geochemical signatures of the rocks and spinel grains revealed that the protolith be an abyssal peridotite, derived from a less depleted fertile mantle melt at a MORB setting after low degree (10-15%) partial melting. The study concluded that the serpentinite may have been created at a slow-spreading ridge, rather than a supra-subduction-zone setting. These rocks were later obducted and incorporated into the IMR of Indo-Myanmar suture zone.

Ningthoujam, P. S.; Dubey, C. S.; Guillot, S.; Fagion, A.-S.; Shukla, D. P.

2012-05-01

78

Low Poisson Ratios in Subduction Zones  

NASA Astrophysics Data System (ADS)

The growing capability to measure seismic velocities in subduction zones has led to an unusual observation: VP/VS ratios as low as 1.65 with VS ~ 4.7 km/s. This is difficult to explain because most minerals have VP/VS ratios exceeding 1.75, and some of the likely alteration phases, like antigorite, clinohumite and chlorite have high VP/VS ratios. Here we explore how these unusually low VP/VS ratios might be produced, using three methods. 1. Velocity anisotropy was calculated for deformed peridotite, using the Christoffel equation and crystal preferred orientations (CPOs) reported in the literature. Peridotite with ‘Type-A’ olivine CPOs can reproduce the observed values for VP/VS1, but not VP/VS2, and only at temperatures <1000°C; other types of olivine CPOs yield VP/VS1 > 1.73. Anisotropy may be effective in situations where measurements of Vs are dominated by the fast, first-arriving signal, as may be the case in some local tomography studies. 2. Isotropic velocities were calculated for rocks other than peridotite. Although the presence of free quartz produces low VP/VS, it requires extensive SiO2 metasomatism, produces rocks too buoyant to remain in the wedge, is stable only over a narrow 1000-1342°C temperature interval at 3 GPa, and yields VS that are too slow. Brucite and talc are also too slow. Less SiO2 metasomatism produces orthopyroxene, and this yields peridotite with appropriate VP/VS ratios, but only at 600-900°C and only for pure enstatite, not the Mg# 93-95 expected. 3. Mineralogy was calculated as a function of P & T for a range of rock compositions (including metasomatized peridotite, igneous rocks, and sediment) using Perple_X, and velocity anisotropy calculations were made using the Christoffel equation and typical crystal preferred orientations for the constituent phases. Metasomatized mantle containing talc can produce the appropriate VP/VS ratios and VS at 600°C as can mantle with mica + amphibole + pyroxene at 800°C. At 1000°C, no calculated velocities were found that match those observed. Thus, the unusually low VP/VS ratios observed in some subduction zones can be explained as the result of metasomatized peridotite with “Type-A” olivine CPOs at temperatures <1000°C.

Hacker, B. R.; Abers, G. A.

2010-12-01

79

Petrological and experimental study of the effect of Fe on the stability of dolomite and magenesite in subduction zone  

NASA Astrophysics Data System (ADS)

Subdcution zone plays a very important role in deep carbon cycle. After the altered oceanic crusts were subjected to eclogite-phase metamorphism in the subduction zone, the carbonates were preserved and formed the carbonated eclogites. In the southwest Tianshan organic belt, the largest low-temperature and high-pressure oceanic metamorphic subduction zone, carbonates with various proportions (5%-30%) were found among the three types of eclogites. Solid-state reaction "dolomite (ankerite) = magnesite (siderite) + aragonite" was identified in the three carbonated eclogite samples from western Tianshan, China. In comparison with other carbonates in high-pressure metamorphic rocks, the carbonates from Southwest Tianshan have high Fe content. In order to understand the role of Fe in the carbonate reactions, we have performed experiments up to 8 GPa and in the temperature range of 900 - 1500 K to examine the effect of Fe on the stability of dolomite. We have also observed the disproportionation reaction of siderite to magnetite and graphite in the same carbonated eclogite specimen. Magnesite and siderite are the reaction products from the decomposition of dolomite and ankerite, respectively. The breakdown reactions of carbonates could be an important mechanism for the formation of graphite or diamond in the carbonated subduction zone.

Tao, R.; Fei, Y.; Zhang, L.

2011-12-01

80

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

Microsoft Academic Search

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

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

2000-01-01

81

Fluid flow in ocean crust cools the Cascadia subduction zone  

NASA Astrophysics Data System (ADS)

Temperatures along subduction zone plate boundary faults have been used to estimate the area and extent of the seismogenic zone. Recent studies of the well-constrained Nankai margin of Japan show that hydrothermal circulation in the subducting crust cools the subduction zone and widens the area of the plate boundary fault that is between the key temperatures of 150 and 350 °C. Here, we present new thermal models for the Cascadia subduction zone that include the effects of fluid flow in the subducting crust. This fluid circulation cools the subduction zone and widens the thermally-defined seismogenic zone by shifting the intersection of 350 °C with the plate boundary fault ˜35-50 km landward. Temperatures in the region of episodic tremor and slip are ~350-450 °C, ˜100 °C cooler than based on estimates that do not include fluid circulation. In contrast to the Nankai margin, the observed surface heat flux pattern for the thickly sedimented Cascadia margin provides only a weak constraint on subduction zone temperature. We use the tomographically-defined basalt-to-eclogite transition in the subducting slab as an additional constraint on the Cascadia subduction zone thermal models. The model most consistent with both the slab alteration observations and surface heat flux measurements includes moderate to vigorous fluid flow in an ocean crust aquifer with permeability ~10-10 to 10-9 m2, consistent with previous observations and inferences.

Cozzens, B. D.; Spinelli, G. A.

2010-12-01

82

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

NASA Astrophysics Data System (ADS)

The Trinity peridotite was emplaced over metabasalts and metasedimentary rocks of the central metamorphic belt along the Devonian Trinity thrust zone. Three metamorphic events can be recognized in the Trinity peridotite: (1) antigorite ( ?D= -63 to -65%.) formation related to regional underthrusting of the central metamorphic belt; (2) contact metamorphism associated with Mesozoic dioritic plutons; and (3) late-stage formation of lizardite ± brucite and chrysotile ( ?D= -127 to -175%.) due to infiltration of meteoric waters. Abundant relict phases indicate incomplete reactions and strongly suggest that the availability of H2O was a controlling factor during serpentinization. Antigorite (event 1) formed as a result of infiltration into the Trinity peridotite of mixed H2O-CO2 fluids derived from the underlying central metamorphic belt. Foliation defined by magnetite veins and shear zones within antigorite serpentinites are subparallel to the Trinity thrust. The assemblage Fo + Atg + Chl + Mag ± Tr ± Carb reflects partial hydration of peridotite at 425 570° C. Talc-rich serpentinite formed along the thrust as a result of the infiltration of silica-bearing fluids. Metasomatic mass-balance calculations based on silica solubilities and the extent of antigorite serpentinization suggest that 80 175 volumes of fluid have passed through a given volume of original peridotite at the Trinity thrust. The Trinity thrust probably represents a Devonian subduction zone. Thermodynamic calculations suggest that hydration reactions account for ˜30 35% of the total heat released by the cooling Trinity peridotite. By analogy, similar hydration reactions are to be expected in the overlying mantle wedge of a subduction zone which act to retard cooling of the hanging wall, just as dehydration reactions delay heating of the downgoing slab. Metasomatic zones formed in peridotite at the Trinity thrust may reflect similar metasomatic processes to those proposed to occur in the mantle wedge above a subducting slab.

Peacock, Simon M.

1987-03-01

83

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

NASA Astrophysics Data System (ADS)

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

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

2010-12-01

84

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

85

Autocorrelation analysis of ambient noise in northeastern Japan subduction zone  

NASA Astrophysics Data System (ADS)

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

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

2012-10-01

86

The Origin of Global Mountain Belts: Hot Subduction Zone Backarcs  

NASA Astrophysics Data System (ADS)

Our understanding of the origin of current mountain belts and ancient orogenic belts was revolutionized by the simple and elegant plate tectonic model of continental collision and crustal thickening. However, it is not widely appreciated that there are significant discrepancies between some of the predictions of the simple collision model and observations. Examples are: (1) many mountain belts show little shortening deformation of the exposed crust; they are eroded high plateaux, e.g., Tibet, (2) a number of major mountain belts have normal or thin crust, e.g., most of N. American Cordillera, (3) several major mountain belts occur where there is no current or recent continental or terrane collision, e.g., S. American Cordillera. We deal with the first two; the last may be related to shallow angle flat slab subduction. These discrepancies can be understood by recognizing that most major mountain belts are in subduction zone backarcs, and that most backarcs, not just extensional, are very hot and have uniformly thin weak lithospheres over considerable widths. Backarcs may be hot because shallow asthenosphere convection results from viscosity reduction by water rising from the underlying subducting plate. Thermal expansion due to the high lithosphere temperatures, in contrast to the cold stable cratons and platforms, accounts for about 2500 m of backarc mountain belt elevations with no crustal thickening. Crustal thickening does occur in some mountain belts with and without collision (e.g., Tibet and central S. America Cordillera), but the shortening and thickening appear to be primarily in the weak lower crust. The upper crust is uplifted as a plateau and overthrusts adjacent stable areas, but remains largely undeformed. Most mountain belts are also broad mobile belts with a long history of distributed deformation. They are mobile because they are sufficiently weak to be deformed by the forces developed at plate boundaries, and usually they are in backarcs. Moho temperatures are 800-900C and lithosphere thicknesses are 50-60 km, compared to 400-500C and 200-300 km for cratons. The temperature differences result in backarc lithospheres being more than a factor of 10 weaker than cratons. Consequences include the ongoing complex histories of deformation in response to changing plate boundary forces, and that hot weak former backarcs are the locus of most deformation during continent or terrane collision orogeny, i.e., the vice or inherited weakness model.

Hyndman, R. D.; Currie, C.; Mazzotti, S.

2005-12-01

87

Evolution of passive continental margins and initiation of subduction zones  

Microsoft Academic Search

The initiation of subduction is\\u000aa key element in plate tectonic schemes\\u000afor the evolution of the Earth's lithosphere.\\u000aNevertheless, up to present,\\u000athe underlying mechanism has not been\\u000avery well understood (e.g. Dickinson\\u000aand Seely, 1979; Hager, 1980; Kanamori,\\u000a1980). The insight into the initiation\\u000aof subduction process has lagged far\\u000abehind the progress made in the last\\u000afew

S. A. P. L. Cloetingh; M. J. R. Wortel; N. J. Vlaar

1982-01-01

88

Geodetic Constraints on Fault Coupling on the Cascadia Subduction Zone  

Microsoft Academic Search

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

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

2002-01-01

89

The Cascadia subduction zone: Two contrasting models of lithospheric structure  

Microsoft Academic Search

The Pacific margin of North America is one of the most complicated regions in the world in terms of its structure and present day geodynamic regime. The aim of this work is to develop a better understanding of lithospheric structure of the Pacific Northwest, in particular the Cascadia subduction zone of Southwest Canada and Northwest USA (Figure 1). The goal

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

1998-01-01

90

Evidence for large earthquakes at the Cascadia subduction zone  

Microsoft Academic Search

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

John J. Clague

1997-01-01

91

Radiocarbon test of earthquake magnitude at the Cascadia subduction zone  

Microsoft Academic Search

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

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

1991-01-01

92

Crustal Deformation in the Southcentral Alaska Subduction Zone  

Microsoft Academic Search

The study of crustal deformation in the subduction zone of the 1964 Prince William Sound, Alaska earthquake reveals a temporally and spatially complex pattern of surface motions and interplate coupling. This temporal–spatial pattern provides fundamental information on the elastic and inelastic processes associated with strain energy accumulation and release in the seismic cycle and development of geological structures in southcentral

Steven C. Cohen; Jeffrey T. Freymueller

2004-01-01

93

Slow slip events at the Alaska Subduction Zone  

Microsoft Academic Search

A large slow slip event (SSE) occurred at the Alaska subduction zone during 1998-2001 [Ohta et al., 2006, EPSL]. The SSE occurs downdip of the Prince William Sound asperity, on a section that accumulated some slip deficit before and after the SSE. During the SSE, >20,000 sq. km of the plate interface slipped >10 cm, for a cumulative moment magnitude

J. T. Freymueller; Y. Ohta

2007-01-01

94

Seismotectonics of Central Mexico Subduction Zone From Crustal Deformation Studies  

Microsoft Academic Search

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

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

2006-01-01

95

Plate detachment, asthenosphere upwelling, and topography across subduction zones  

Microsoft Academic Search

This study analyzes the topography across subduction zones, considering the separate contributions of the crust and the mantle lithosphere to the observed surface elevation. We have found a transition from a region where the overriding plate is coupled to the descending slab and pulled down along with it to a region where the overriding plate floats freely on the asthenosphere.

Zohar Gvirtzman; Amos Nur

1999-01-01

96

Subduction zone evolution and low viscosity wedges and channels  

Microsoft Academic Search

Dehydration of subducting lithosphere likely transports fluid into the mantle wedge where the viscosity is decreased. Such a decrease in viscosity could form a low viscosity wedge (LVW) or a low viscosity channel (LVC) on top of the subducting slab. Using numerical models, we investigate the influence of low viscosity wedges and channels on subduction zone structure. Slab dip changes

Vlad Manea; Michael Gurnis

2007-01-01

97

3-D Seismic Refraction Tomography in Sumatra Subduction Zone  

Microsoft Academic Search

A seismic refraction survey was conducted as part of the major UK and international project to image the 3-D structures and the seismic velocity of the Sumatra subduction zone. The 3-D seismic refraction tomography mainly focusses on the two segment boundaries identified by the earthquake ruptures in 2004 and 2005. High quality seismic datasets (refraction, reflection, gravity and magnetics) were

G. Tang; P. Barton; S. Dean; P. Vermeesch; M. D. Jusuf; T. Henstock; Y. S. Djajadihardja; L. McNeill; H. Permana; S. S. Party

2008-01-01

98

Noncharacteristic behavior and complex recurrence of large subduction zone earthquakes  

Microsoft Academic Search

The last few years have been remarkable with respect to the number of large underthrusting earthquakes in subduction zones that reruptured plate boundary segments that failed in previous great events. Availability of modern seismic data for two consecutive large earthquakes rupturing the same portion of the plate interface provides the opportunity to compare the spatial distribution of moment release for

Susan Y. Schwartz

1999-01-01

99

Upper mantle discontinuities around circum-Pacific subduction zones  

NASA Astrophysics Data System (ADS)

Characterization of mantle discontinuities near subducted slabs is one approach to understanding thermal structure and mantle dynamics around subduction zones. The ubiquitous presence of the "410-km" and ``660-km" discontinuities has previously been established and effects of downwellings on these major transition zone discontinuities has been studied. Shallower upper mantle discontinuities near subduction zones have been reported in the literature but their nature is still elusive, so we focus on these structures. Precise determination of the depth and impedance contrasts of these near-subduction zone discontinuities is important to understanding their origins. We compiled a large three-component teleseismic body wave dataset for moderate size (mb~5.5-7.0) deep (>400km) earthquakes for circum-Pacific subduction zones. The data consist of digitized WWSSN long-period records (1960s-1980s) and IRIS FARM broadband data (1988-2005) and a long-period digital dataset (1976-2005). We seek and model underside reflections from upper mantle discontinuities at depth, x, (e.g., pxP, sxP and sxSH), applying linear stacking along predicted travel time curves relative to surface reflected phases (pP, sP, sS) calculated for a reference Earth velocity model to enhance any weak reflected signals. In order to assess spatial coherence of the upper mantle discontinuities, we developed a Kirchhoff migration algorithm to project the scattered energy back to its 'correct' position for an assumed migration structure. Application of these algorithms to data from South American deep events indicates that there is substantial energy reflected from a "300-km" discontinuity, as well as a "200-km" discontinuity for both P and SH -waves. Strong underside reflections from the Moho are also apparent as precursors to the surface reflections. Analyses for other subduction zones will be presented as well.

Zheng, Y.; Lay, T.; Flanagan, M.

2005-12-01

100

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

Microsoft Academic Search

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

Donald O. West; Dennis R. McCrumb

1988-01-01

101

Geochemical Tracing of Mantle Flow above Subduction Zones  

NASA Astrophysics Data System (ADS)

Geochemical tracing may be used to track mantle flow above and behind subduction zones and so provide an independent test of the applicability of seismic anisotropy measurements. The theory is that, if mantle flow is accompanied by decompression, then extraction of small degree melts from multi-component mantle leads to compositional gradients in the mantle in both isotope and trace element space. These gradients may be obtained by inverting geochemical data from the products of mantle melting. If mantle flow is accompanied by addition of a subduction fluid, then simultaneous melting and subduction component addition may also produce compositional gradients. Numerical experiments enable compositional gradients to be quantified in terms of the extent of melt extraction, mantle temperature and other variables. In addition, isotope and trace element systematics provide evidence for the provenance of the mantle entering the subduction system, with Hf isotopes and immobile trace elements providing a means of establishing provenance even from magmas generated directly above the dehydrating subducted plate. This work focuses on a series of geochemical maps which enable mantle flow to be traced for range of oceanic arc basin systems (Izu-Bonin-Mariana, Tonga-Vanuatu, Scotia, Manus) and, provisionally, some continental systems (Japan, Cascades). Using maps based on geochemical proxies for melt extraction (such as Ta/Yb), subduction-addition (such as Th/Ta) and mantle provenance (such as epsilon-Hf v epsilon Nd), it is possible to demonstrate the existence of a wide range of mantle flow regimes. Thus, the Izu and Japan systems appear to be characterised by simple trench-orthogonal flow, the Mariana system by dispersion away from several separate centers of mantle upwelling, the Tonga-Vanuatu system by unidirectional flow from beneath the Pacific plate in the north, and the Scotia system by bi-directional flow from both north and south. In a number of these cases, isotopic fingerprinting using immobile isotope ratios is critical for establishing the ultimate source of the mantle: for example, the mantle entering the Scotia system may be seen to originate from the Atlantic Bouvet domain rather than the Atlantic Tristan or the Pacific domains. These results are broadly in keeping with seismic anisotropy measurements to date while providing greater coverage but, of course, geochemical tracing is restricted to areas of magmatic activity. Integration of geophysical and geochemical methods may therefore be necessary to provide the maximum information on mantle flow.

Pearce, J. A.; Barry, T. L.; Millar, I. L.; Leat, P. T.; Stern, R. J.

2004-12-01

102

A continuum of stress, strength and slip in the Cascadia subduction zone  

NASA Astrophysics Data System (ADS)

As oceanic lithosphere subducts beneath continental lithosphere it experiences variable degrees of interaction with the overriding plate and movement is accommodated by a continuum of slip modes. At shallow depths, the plates are locked and movement occurs intermittently as earthquakes. By contrast, at large depths the down-going plate slips into the mantle continually. In the transition zone between locked and stable slip, plate movement is accommodated by slow slip, which generates tectonic tremor. Here we use tectonic tremor to infer the location and duration of slow slip in the Cascadia subduction zone from 2006 to 2011. We find that individual slow-slip events are initiated deep on the plate interface and migrate upwards. With decreasing depth, we observe a gradation from small, frequent slip, to large, infrequent slip. These observations fill in the transition zone with a continuum of slip size and periodicity, and indicate that the fault weakens with depth, which we attribute to lower friction. We suggest that stable sliding loads the fault at depth and transfers stress to the base of the transition zone, causing the initiation of slow slip. In a self-similar process, slow slip migrates upwards and ratchets stress up the fault, towards the shallower seismogenic zone. Our conceptual model provides an intuitive understanding of subduction zone dynamics.

Wech, Aaron G.; Creager, Kenneth C.

2011-09-01

103

Relating seismic observables to fluid migration in subduction zones  

NASA Astrophysics Data System (ADS)

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

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

2011-12-01

104

Electromagnetic Images of the South and Central American Subduction Zones  

Microsoft Academic Search

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

Heinrich Brasse

105

S wave velocity structure of the northern Cascadia subduction zone  

Microsoft Academic Search

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

John F. Cassidy; Robert M. Ellis

1993-01-01

106

Postglacial rebound at the northern Cascadia subduction zone  

Microsoft Academic Search

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

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

2000-01-01

107

Absolute Gravity Measurements in the Cascadia Subduction Zone  

Microsoft Academic Search

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

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

2001-01-01

108

S wave velocity structure of the Northern Cascadia subduction zone  

Microsoft Academic Search

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

John F. Cassidy; Robert M. Ellis

1993-01-01

109

Homogeneous vs heterogeneous subduction zone models: Coseismic and postseismic deformation  

Microsoft Academic Search

A finite-element model (FEM) incorporating geologic properties characteristic of a subduction zone is compared with FEMs approximating homogeneous elastic half-spaces (HEHS)s to investigate the effect of heterogeneity on coseismic and postseismic deformation predictions for the 1995 Colima-Jalisco Mw =8.0 earthquake. The FEMs are used to compute a coefficient matrix relating displacements at observation points due to unit dislocations of contact-node

T. Masterlark; C. DeMets; H. F. Wang; O. Sánchez; J. Stock

2001-01-01

110

Homogeneous vs heterogeneous subduction zone models: Coseismic and postseismic deformation  

Microsoft Academic Search

A finite-element model (FEM) incorporating geologic properties characteristic of a subduction zone is compared with FEMs approximating homogeneous elastic half-spaces (HEHS)s to investigate the effect of heterogeneity on coseismic and postseismic deformation predictions for the 1995 Colima-Jalisco Mw=8.0 earthquake. The FEMs are used to compute a coefficient matrix relating displacements at observation points due to unit dislocations of contact-node pairs

T. Masterlark; C. DeMets; H. F. Wang; O. Sánchez; J. Stock

2001-01-01

111

Imaging Interseismic Locking at the Nankai Subduction Zone, Southwest Japan  

Microsoft Academic Search

Recent geodetic and seismological observations have revealed that brittle-plastic transition zones at subduction zone interfaces\\u000a are loci of slow slip episodes and nonvolcanic harmonic tremors. It is therefore important to estimate the depth range of\\u000a brittle-plastic transition zones and how interplate locking changes with space and time within the brittle-plastic transition\\u000a zone, not only for understanding the interseismic stress accumulation

Yosuke Aoki; Christopher H. Scholz

112

Anchor-like force proposed for subduction zones  

NASA Astrophysics Data System (ADS)

Two plate tectonics researchers have turned to a simple maritime tool to explain a 25-year-old mystery. Christopher Scholz and Jaime Campos have proposed that the action of a sea anchor, which is used to steady ships in deep water, provides an apt model of the forces at work when the Earth's crustal plates collide and why those collisions provoke powerful earthquakes in some subduction zones and just minor temblors in others.

Carlowicz, Michael

113

Subduction zone seismic anisotropy from antigorite crystal preferred orientations (CPOs)  

NASA Astrophysics Data System (ADS)

Seismic anisotropy is often used to infer flow directions in the mantle, but anisotropy in the tip of a subduction zone mantle wedge might most plausibly be related to alteration of the mantle (e.g., hydration of olivine). Understanding the mechanisms responsible for anisotropy in subduction zones provides a link between seismic observations and the processes occurring in subduction zones. Kneller et al. [2008] suggested that a B-type olivine crystal preferred orientation (CPO) in the tip of a mantle wedge can explain some, but not all, of the observed trench-parallel shear-wave splitting observations, and suggested that the growth of antigorite might explain the remainder. We investigate the potential contribution of antigorite to subduction-zone anisotropy by measuring the CPO of antigorite in 7 serpentinites from central Guatemala using electron-backscatter diffraction (EBSD) and calculating the seismic properties using the single-crystal elastic constants from Bezacier et al. [2010]. Each sample was mapped twice, once with a very fine step size (1-2 ?m) to image the microstructure of a small region (1 mm^2), and once with a coarse step size (50-100 ?m) to obtain 1-point-per-grain measurements to characterize the antigorite CPO over most of each thin section (1-3 cm^2). Fine-scale band contrast images show a range of microstructures, from slightly elongated plates to submicron needles. Our results show two types of antigorite CPO - one related to deformation and one related to the CPO of the parent material - which result in calculated anisotropies of 6-28% in Vp and 5-33% in Vs. In one sample a dolomite-antigorite intergrowth may provide insight into anisotropy associated with carbonation reactions.

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

2011-12-01

114

Deformation processes in great subduction zone earthquake cycles  

NASA Astrophysics Data System (ADS)

Crustal deformation associated with great subduction zone earthquakes yields important information on mantle rheology and slip evolution of the megathrust. We have used three-dimensional viscoelastic finite element models to study the contemporary crustal deformation of three margins, Sumatra, Chile, and Cascadia, that are presently at different stages of their great earthquake cycles. At Sumatra where an Mw 9.2 earthquake occurred in 2004, all the GPS stations are moving seaward. At Chile where an Mw 9.5 earthquake occurred in 1960, coast GPS stations are moving landward, obviously due to the re-locking of the fault, while the inland stations are still moving seaward. At Cascadia where an Mw 9.0 earthquake occurred in 1700, all the GPS stations are moving landward. The earthquake cycle deformation at Alaska where an Mw 9.2 earthquake occurred in 1964 is similar to that of Chile, and the deformation at NE Japan where an Mw 9.0 earthquake occurred in 2011 is similar to that of Sumatra. Model results indicate that the earthquake cycle deformation of different margins is governed by a common physical process. A great earthquake causes the upper plate to move towards the trench and induces shear stresses in the upper mantle. After the earthquake, the fault is re-locked, causing the upper plate to move landward. However, portions of the fault undergo aseismic afterslip for a short duration, causing the overriding areas to move seaward. At the same time, the viscoelastic stress relaxation of the upper mantle causes prolonged seaward motion in inland areas including the forearc and the back arc. After a long time when the earthquake-induced stresses have mostly relaxed, the upper plate moves landward due to the re-locking of the fault. The model of the 2004 Sumatra earthquake indicates that the afterslip must be at work immediately after the earthquake, and the characteristic time of the afterslip is ~1 yr. With the incorporation of the transient (biviscous) rheology, the model well explains the near-field and far-field postseismic deformation within a few years after the 2004 Sumatra event. For all the margins modeled, the steady-state (Maxwell) viscosity of the continental upper mantle is determined to be ~1019 Pa s, two orders of magnitude lower than that of the global value obtained through global postglacial rebound analyses. Based on the model for the 2004 Sumatra earthquake, the transient (Kelvin) viscosity of the continental mantle is one to two orders of magnitude lower than that of the stead-state viscosity. Long-term postseismic deformation is controlled mainly by the steady-state viscosity of the mantle and is relatively better understood. For the short-term postseismic deformation, the interaction of the afterslip of the fault and the transient deformation of the mantle is still poorly understood. Geodetic monitoring following the 2010 Mw 8.8 Maule earthquake and 2011 Mw 9.0 Tohoku earthquakes is expected to improve greatly our understanding the short-term deformation over the next few years.

Hu, Y.; Wang, K.; He, J.

2011-12-01

115

Common Maximum Depth of Slab-Mantle Wedge Decoupling: Understanding Variations in Fluid Supply and Thermal-petrologic Processes Among Subduction Zones  

NASA Astrophysics Data System (ADS)

In subduction zones, aqueous fluid is critical to a diversity of processes such as intraslab earthquakes, mantle-wedge metamorphism, and arc volcanism. Fluid availability at depth depends on the thermal structure of the subducting slab. The age of the slab is the primary factor that controls its thermal structure, but the thermal effect of slab-driven mantle wedge flow is large enough to affect the depths of dehydration and even melting of the slab. Geophysical observations indicate that the shallow part of the forearc mantle wedge does not participate in this flow and thus is decoupled from the subducting slab. We develop 2-D steady-state thermal models for Cascadia, NE Japan, and Kamchatka, where there are sufficient heat flow data to constrain the maximum depth of decoupling. In the models, we use temperature- and stress- dependent mantle rheology, and the age-controlled thermal structure of the slab is advected into the subduction system. Cascadia has a very young (< 10 Ma) and warm slab whereas NE Japan and Kamchatka have a very old (> 100 Ma) and cold slab. These end members show dramatic contrast in their metamorphic, volcanic, and seismological characteristics, but the heat flow data require the subduction interface to be decoupled to a depth of 70-80 km for all the three subduction zones regardless of their slab age. We thus hypothesize that the common depth of decoupling of 70-80 km applies to most, if not all, subduction zones, and we develop models for fourteen other subduction zones to test this hypothesis against available observations. For subduction zones with a very warm slab, such as Cascadia and Nankai, the models predict that slab dehydration peaks in the forearc where the mantle wedge temperature is low enough for serpentinization but too low for melt production. For most other subduction zones, slab dehydration is predicted to peak in the depth range of 80-140 km beneath the high-temperature part of the mantle wedge, promoting arc volcanism. In all models, the mantle-wedge temperatures above where the slab is about 100-km deep reach the melting temperatures estimated by laboratory and geochemical studies. The common maximum depth of decoupling explains multidisciplinary observations, and our modeling results are consistent with conclusions drawn by various research groups through modeling individual subduction zones.

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

2008-12-01

116

Lithospheric Scale Deformation in Mega-thrust Subduction Zones  

NASA Astrophysics Data System (ADS)

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

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

2008-12-01

117

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

NASA Astrophysics Data System (ADS)

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

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

2011-12-01

118

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

NASA Astrophysics Data System (ADS)

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

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

2002-12-01

119

Rheological control in subduction zones: slab dynamics, fluid flow and seismic anisotropy  

NASA Astrophysics Data System (ADS)

The strong temperature, strain-rate and compositional gradients in subduction zones provide extreme challenges to the modeling of their dynamics. Major questions remain regarding the state of stress in the slab, the nature of the downdip transition of the seismogenic zone, the rheological control on the release of fluids and the role of fluids in magma generation, earthquake source processes and the formation of seismic anisotropy. We use high resolution finite element models of convergent margins to address some of these topics. First, we explore the role of fluids on wedge rheology and how the presence or absence of fluids changes the dynamics of the wedge. The presence of fluids in the cold fore-arc may be the cause of trench-parallel anisotropy due to B-type olivine fabric or due to shape preferred orientation by the alignment of serpentinite filled cracks. Second, The cold fore-arc generally extends to where the slab is at ~80 km depth. This depth has a strong sensitivity to the depth where the overriding wedge couples with the slab. Weak phases such as chlorite, serpentinite and talc may play a critical control on the region where the slab remains decoupled, but the strong non-linearities involved make it difficult to determine a single process for all subduction zones. Finally, the increasing metamorphic grade of rocks in the subducting slab cause progressive fluid production, which has been linked to intermediate depth seismicity. The fluids also are considered the primary cause for arc volcanism, but it is not yet clear how the fluids escape from the slab and travel to the zones of arc magmatism. We use a new set of coupled solid state and porous flow models to determine how fluids affect the shear and bulk viscosity of these rocks and how this in turn controls the flow of fluids from the slab.

Van Keken, P. E.; Spiegelman, M.; Wilson, C. R.

2011-12-01

120

A perspective view on ultrahigh-pressure metamorphism and continental collision in the Dabie-Sulu orogenic belt  

Microsoft Academic Search

The study of continental deep-subduction has been one of the forefront and core subjects to advance the plate tectonics theory\\u000a in the twenty-first century. The Dabie-Sulu orogenic belt in China crops out the largest lithotectonic unit containing ultrahigh-pressure\\u000a metamorphic rocks in the world. Much of our understanding of the world’s most enigmatic processes in continental deep-subduction\\u000a zones has been deduced

YongFei Zheng

2008-01-01

121

Interaction of metamorphism, fluids and deformation in exhuming subducted continental crust, field evidence and a model  

NASA Astrophysics Data System (ADS)

Exhuming pieces of continental crust from the depths of a subduction channel involves a competition between buoyancy and viscosity, so that lighter and less viscous units will exhume more efficiently. In terms of geological evolution, two conditions are required: (1) once they have been dragged down to large depth, tectonic units must be decoupled from the subducting lithosphere and, (2) they must be allowed to move upward within the subduction channel. Both processes involve considerable deformation along the units margins and this deformation is aided by fluids and metamorphism. It is a frequent observation that deformation is totally absent or very weak along the prograde path and this is not only because it has been erased by later events. The examples of the Norwegian Caledonides or the Aegean, or even the Alps, show that the first significant deformation event is recorded near the peak of pressure, thus at maximum depth. This is in line with the idea that the more resistant a rock unit is, the deeper is can be dragged along with the subducting plate, and UHP metamorphics are indeed basement rocks in most cases. The first deformation is sometimes partly brittle as suggested by the Bergen Arcs example. Fluids then invade fractures or shear zones and initiate metamorphic recrystallisation at peak pressure. This further induces a local density increase associated with a drop in strength and strain localisation ensues, that in turns promotes recristallisation. Large and weak shear zones progressively form and the tectonic unit is then decoupled from the subducting lithosphere. Afterward, the piece of continental crust makes its way upward in the subduction channel, limited by a thrust along its base and a normal "extensional" shear zone at its top. Then again, the interaction between fluid and deformation is important during retrogression into the blueschists (Aegean, Alps) or the amphibolite facies (Caledonides). Large scale ductile shear zones accommodate the exhumation during progressively less and less severe metamorphic conditions. In the vicinity of the brittle-ductile transition zone fluids from the surface invade the shear zones and further facilitate strain localisation. We discuss a simple kinematic model that integrates this sequence of events in various geodynamic contexts.

Jolivet, L.; Labrousse, L.; Hetényi, G.; Huet, B.; Raimbourg, H.; Cattin, R.

2008-12-01

122

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

123

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

NASA Astrophysics Data System (ADS)

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

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

2011-12-01

124

Variable Rupture Mode at Subduction Zones Around the Pacific  

NASA Astrophysics Data System (ADS)

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

Satake, K.

2005-12-01

125

Methane generation in subduction zones: A cause for fluid overpressures?  

NASA Astrophysics Data System (ADS)

The nature of the fluids involved in the deep plate interface in subduction zones is difficult to constrain, as it incorporates many potential sources (sea water trapped in pores, water from dehydration reactions, fluid from the depths of the subduction channel or from the slab). Using Raman analysis of fluid inclusions in quartz veins from the deep domains of the Shimanto paleo-accretionary complex, Japan, we first show that at temperatures of ~250°C, the fluid is a mixture of water and methane, in agreement with literature on similar terranes. In most of the studied area, we could observe only one, water-rich, kind of inclusion, while in a restricted region a second, methane-rich, kind of inclusion was also present, suggesting in the first case the circulation at depth of a single fluid and in the second case the coexistence of two fluid phases. We used then isochores of the methane-rich fluid inclusions to constrain the paleo- fluid pressure. In the present case, methane-rich inclusions are distributed as planes, i.e. along healed microcracks, hence they provide a record of the conditions that prevailed during a short period of time. Within a single plane of inclusions, homogeneization temperatures of the methane phase show large variations between inclusions, which we interpret as the record of large and rapid variations in fluid pressure. To account for this diversity in the fluid state (single- vs. two-phased) as well as for the rapid variations in pressure, we developed a model of methane generation by thermal cracking of organic matter during burial. In spite of the low average organic matter content of subducted sediments, the porosity, hence the water content of deep sediments is sufficiently low for the oversaturation of the water in methane, hence unmixing of a free, methane-rich phase, to be a realistic scenario. Predicted overpressures resulting from rapid unmixing of methane can be significant with respect to ambient fluid pressure and constitute therefore a potential source for rapid drop in effective pressure, possibly resulting in seismic events. Finally, the evaluation of the potential to generate methane at depth over worldwide subduction zones shows that the classical distinction between aseismic, Marianna-type subduction zones and seismic, Chilean-type subduction zones coincides with a division between methane-poor and methane-rich subducted sediments.

Raimbourg, Hugues; Disnar, Jean-Robert; Thiery, Regis; Ramboz, Claire; Yamaguchi, Asuka; Kimura, Gaku

2013-04-01

126

Flexural modelling of gravity anomalies seaward of Pacific subduction zones  

NASA Astrophysics Data System (ADS)

The strength of the lithosphere is determined by its flexural rigidity, which is commonly expressed through the effective elastic thickness, Te. In oceanic regions, it is widely accepted that Te increases as a function of age at the time of loading, due to cooling and thickening of the lithosphere. Evidence for this comes from studies of free-air gravity anomalies and bathymetry at seamounts, fracture zones and subduction zone outer-rises. 75% of Te estimates from seamounts lie between the 300 ° C and 600 ° C isotherms as predicted by a cooling plate model, whilst the majority of subduction zone and fracture zone estimates lie between the 500 ° C and 800 ° C isotherms. Recent outer-rise investigations, however, have questioned whether such a simple relationship exists and suggested that either the strength of the lithosphere is independent of plate age, or that Te cannot be measured with sufficient accuracy to reveal such a relationship. In order to reassess the relationship between lithospheric strength and age, we use trench-normal, ensemble-averaged profiles of satellite-derived free-air gravity anomalies to model the outer-rise of all the major Pacific subduction zones. Profiles are corrected for sediment loading, as well as for thermal cooling effects. A broken elastic plate model is used, with a finite difference solution that allows Teto vary as a function of distance from the trench. We use an inverse approach, iterating Te values and inverting for end-conditioning forces (a vertical shear force and a bending moment). Results show that oceanic lithosphere younger than 90 Ma clearly strengthens with age, with Te roughly following the 550 ° C isotherm. For example, the Middle America trench (4 - 25 Ma) has a mean Te of 14.1 ± 2.8 km, whilst the Alaska-Aleutian trench (43 - 60 Ma) has a mean Te of 34.3 ± 8.0 km. For older lithosphere, the pattern is not as clear. We suggest that this is due to thermal rejuvenation, which has two effects: it weakens the lithosphere, lowering Te estimates; the associated magmatism masks the flexural signal, producing scatter. For many of the subduction zones, gravity profiles require that the lithosphere has been weakened in the region of the seaward wall of the trench. We attribute this to inelastic yielding - a combination of brittle fracture of the upper lithosphere and ductile flow of the lower lithosphere - due to high bending moments. Evidence for this can be seen in swath bathymetry data, which reveals zones of pervasive extensional faulting. Hydrothermal alteration of the lithosphere might also contribute to weakening, if bend faulting allows hydration and serpentinization of the upper mantle.

Hunter, Johnny; Watts, Anthony; Bassett, Daniel

2013-04-01

127

Serpent: Magnetic signatures of serpentinized mantle and mesoscale oceanic variability along the Alaska/Aleutian subduction zone  

NASA Astrophysics Data System (ADS)

NASA recently solicited suborbital missions as a part of its new Earth Venture program element. These missions are designed as complete PI-led investigations to conduct innovative, integrated, hypothesis or scientific question driven approaches to pressing questions in Earth System science. The missions should require sustained observations (<5 years) and significant resources (<30 million USD). The submitted mission proposals have been under evaluation since last November, and NASA is scheduled to make a decision in April. We, a team led by Raytheon's Photon Research Associates, propose to carry out a suborbital magnetic survey of the Aleutian subduction zone using NASA's Global Hawk to test the magnetic serpentinite hypothesis. This hypothesis states that dewatering of the descending slab within subduction zones produces an observable static magnetic signature through the formation of serpentinite in the overriding mantle. This signature may serve as a predictor of the location of large megathrust earthquakes and their associated tsunamis. Magnetic field measurements from 20 km (sub-orbital) altitude are essential to the testing of this hypothesis; analysis shows orbital and/or near-surface measurements are not likely to provide sufficient sensitivity and uniform calibration to confirm or reject the hypothesis, nor to consistently map its presence around the world. Static and dynamic magnetic signatures from the motion of seawater in the earth's magnetic field have the potential to confound an evaluation of the magnetic serpentinite hypothesis. Through a combination of modeling and exact repeat surveys over the subduction zone, spaced weeks to as much as six months apart, we can study the magnetic signature of the motion that characterizes the mesoscale oceanic circulation in order to develop the best possible corrections for lithospheric imaging, and elucidating the intrinsic and unique oceanic information content in the magnetic fields for the first time ever. The role of water in subduction zones, and in the overlying ocean, can be traced by sustained suborbital observations of the magnetic field. At critical depths of 40 to 50 km, subducting ocean crust goes through important metamorphic changes that release large amounts of water into overriding mantle rocks. Introduction of water into the mantle produces serpentinite, a highly magnetic, low-density rock. Thermal models indicate that, in many of the world's subduction zones, this part of the mantle is cooler than the Curie temperature of magnetite, the most important magnetic mineral in serpentinite, and thus large volumes of mantle in subduction-margin settings should be magnetic. Indeed, analysis of magnetic data from some subduction zones indicates that magnetic mantle can be detected in long-wavelength magnetic anomalies. The presence of serpentinite in subduction margins has two important links to large within-slab and giant megathrust earthquakes, and associated tsunamis. First, release of water from the subducting slab is thought to embrittle the slab, thereby promoting within-slab earthquakes (M 7-8). Thus, we expect to see a spatial association between this type of earthquake and mantle magnetic anomalies. Second, in cool subduction margins, the down-dip limit of megathrust earthquakes (M 8.0-9.6) is controlled by the slab's first encounter with serpentinized mantle. Again, we expect to see a spatial association between these devastating earthquakes and magnetic anomalies. The magnetic serpentinite hypothesis can be tested by comparison to free-air gravity, geologic, topographic, and bathymetric data of comparable resolution. Significant static and dynamic magnetic fields also originate as a consequence of oceanic flow in electrically conducting ocean water above the subduction zone. Although these signals are of much lower amplitude than the magnetic field associated with serpentinite, they can have significant power at short spatial scales, and thus have the potential to confound estimated magnetic source depths that rely on inferences from the

Purucker, Michael; Serpent Team

2010-05-01

128

Fore- and Back-Arc Structures Along the Hikurangi-Kermadec Subduction Zone  

NASA Astrophysics Data System (ADS)

The Hikurangi-Kermadec subduction zone northeast of New Zealand represents an ideal target to study lateral variations of subduction zone processes. The incoming Pacific plate changes from being a large igneous province, called the Hikurangi Plateau, in the south to normal oceanic plate north of the Rapuhia Scarp. The overriding Australian plate is continental in the south, forming the North Island of New Zealand, and changes to an island arc in the north. Further lateral variability exists in changes in volcanic and hydro-thermal activity, transitions from accretion to subduction erosion, backarc spreading and rifting, and is accompanied by northward increasing seismicity. As part of the MANGO project (Marine Geoscientific Investigations on the Input and Output of the Kermadec Subduction Zone), four marine geophysical transects of largely seismic reflection and refraction data provide constraints on the upper lithospheric structures across the Hikurangi-Kermadec Trench between 29-38 degrees South. On MANGO profile 1 in the south, the initially shallow subduction of the incoming plateau coincides with crustal underplating beneath the East Cape ridge. To the west lies the 100 km wide and over 10 km deep Raukumara Basin. Seismic velocities of the upper arc mantle are around 8 km/s and are considered normal. In contrast, on MANGO profile 4, about 1000 km to the north around the volcanically active Raoul Island, the incoming oceanic crust appears to bend considerably steeper and thus causes a 50 km narrower forearc with a smaller forearc basin. Furthermore, the upper mantle velocities in both plates are relatively low (7.4-7.7 km/s), likely indicating strong bending related deformation of the incoming plate and thermal activity within the arc possibly due to spreading. Here, arc volcanism is relatively active, with many large volcanoes directly on the ridge. The central two transects MANGO 2 and 3, though without data coverage of the structure of the incoming plate, are more similar to MANGO 4. The arc regions appear to be strongly affected by the activity of the arc. The arc crust of the northern MANGO 3 becomes significantly thinner in the backarc region due to extension, and much reduced volcanism behind the ridge. The structures on MANGO 2, on the other hand, cover strong and densely spaced thermal activity from the adjacent arc volcanism, possibly linked to a recent, fluid-rich passage of the Hikurangi Plateau.

Scherwath, M.; Kopp, H.; Flueh, E. R.; Henrys, S. A.; Sutherland, R.

2009-04-01

129

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

NASA Astrophysics Data System (ADS)

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

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

2010-12-01

130

Serpentinites and Boron Isotope Evidence for Shallow Fluid Transfer Across Subduction Zones  

NASA Astrophysics Data System (ADS)

In subduction zones, fluid-mediated chemical exchanges between subducting plates and overlying mantle dictate volatile and incompatible element cycles in earth and influence arc magmatism. One of the outstanding issues is concerned with the sources of water for arc magmas and mechanisms for its slab-to-mantle wedge transport. Does it occur by slab dehydration at depths directly beneath arc front, or by hydration of fore-arc mantle and subsequent subduction of the hydrated mantle? Historically, the deep slab dehydration hypothesis had strong support, but it appears that the hydrated mantle wedge hypothesis is gaining ground. At the center of this hypothesis are studies of fluid-mobile element tracers in volatile-rich mantle wedge peridotites (serpentinites) and their subducted high-pressure equivalents. Serpentinites are key players in volatile and fluid-mobile element cycles in subduction zones. Their dehydration represents the main event for fluid and element flux from slabs to mantle, though direct evidence for this process and identification of dehydration environments have been elusive. Boron isotopes are known markers of fluid-assisted element transfer during subduction and can be the tracers of these processes. Until recently, the altered oceanic crust has been considered the main 11B reservoir for arc magmas, which largely display positive delta11B. However, slab dehydration below fore-arcs transfers 11B to the overlying hydrated mantle and leaves the residual mafic crust very depleted in 11B below sub-arcs. The 11B-rich composition of serpentinites candidate them as the heavy B carriers for subduction. Here we present high positive delta11B of Alpine high-pressure (HP) serpentinites recording subduction metamorphism from hydration at low gades to eclogite-facies dehydration: we show a connection among serpentinite dehydration, release of 11B-rich fluids and arc magmatism. In general, the delta11B of these rocks is heavy (16‰ to + 24‰ delta11B). No B loss and no 11B fractionation occurs in these rocks with progressive burial: their high B and 11B compositions demonstrate that initially high budgets acquired during shallow hydration are transferred and released to fluids at arc magma depths, providing the high-boron component requested for arcs. Interaction of depleted mantle-wedge with de-serpentinization fluids and/or serpentinite diapirs uprising from the slab-mantle interface thus provide an efficient self-consistent mechanism for water and B transfer to many arcs. The boron compositions documented here for Erro-Tobbio serpentinites are unexpected for slabs, deputed to loose much B and 11B during subduction dehydration. Their isotopic compositions can be achieved diluting through the mantle the subduction-fluids released during shallow dehydration (30 km) of a model slab. Moreover their delta11B is close to values measured in Syros eclogite blocks, hosted in mélanges atop of the slab and metasomatized by uprising subduction-fluids. The nature of serpentinizing fluids and the fluid-transfer mechanism in Erro-Tobbio is further clarified integrating B isotopes with O-H and Sr isotopic systems. Low deltaD (-102‰), high delta18O (8‰) of early serpentinites suggest low-temperature hydration by metamorphic fluids. 87Sr/86Sr ranges from 0.7044 to 0.7065 and is lower than oceanic serpentinites formed from seawater. Our data indicate that alteration occurred distant from mid-ocean ridges: we propose metamorphic environments like the slab-mantle interface or the fore-arc mantle fed by B- and 11B-rich slab fluids. We therefore provide field-based evidence for delivery of water and 11B at sub-arcs by serpentinites formed by subduction-fluid infiltration in mantle rocks atop of the slab since the early stages of burial, witnessing shallow fluid transfer across the subduction zone.

Scambelluri, M.; Tonarini, S.

2012-04-01

131

Examining Models of Melt Transport at Subduction Zones  

NASA Astrophysics Data System (ADS)

Several models have been proposed to describe melt transport at subduction zones, including diapiric upwellings (e.g., Marsh, 1979) and networked flow (Hall and Kincaid, 2001), flow via fracture propagation (e.g., Furukawa, 1993), and distributed porous flow (e.g., Spiegelman and McKenzie, 1987). The flow morphologies associated with each of these models have characteristic length and time scales which bear on the thermal and chemical evolution of the melt during its ascent, in particular controlling the degree to which the melt will equilibrate, both thermally and chemically, with the surrounding mantle. We use a combination of laboratory tank experiments and numerical geodynamic models of subduction to explore the thermochemical evolution of melts at subduction zones for the various melt transport models. Predicted melt characteristics are compared to geophysical and geochemical observations of arc volcanism, including the frequency and volume of eruptions and the trace element chemistry of arc rocks, to constrain the viability of the melt transport models. Of particular interest, the coexistence of large stratovolcanoes and small monogenetic volcanoes in close geographic proximity within the Central Mexican Volcanic Belt (CMVB) provides strong constraints on melt transport mechanisms.

Hall, P. S.; Kincaid, C.; Langmuir, C. H.

2005-12-01

132

Modeling of Mantle Convection in 3D Subduction Zones  

NASA Astrophysics Data System (ADS)

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

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

2010-12-01

133

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

134

Polyphase growth of accessory minerals during continental collision: Geochemical evidence from ultrahigh-pressure metamorphic gneisses in the Sulu orogen  

NASA Astrophysics Data System (ADS)

An integrated study of petrology, geochronology and geochemistry was performed for ultrahigh-pressure metamorphic gneisses in the Sulu orogen. The results are used to elucidate the polyphase growth of such metamorphic minerals as zircon, titanite and garnet in response to pressure-temperature changes and fluid/melt action during continental collision. This provides insights into the property of metamorphic fluid/melt and their effects on trace element mobility. A combined result from REE patterns, mineral inclusions and Ti-in-zircon temperatures indicates three stages of zircon growth. Prograde growth occurred at ~ 237 Ma primarily at eclogite-facies, retrograde growth at ~ 222 Ma mostly at eclogite-facies, and the last growth at ~ 205 Ma at granulite-facies. The three stages of zircon growth are deciphered by distinct REE patterns and trace element compositions, reflecting the differences in the property of metamorphic fluid/melt. The episodic growth of metamorphic zircon is primarily dictated by the episodic concentration of Zr and Si in metamorphic fluid/melt. Relict domains of magmatic titanite are distinguished from metamorphosed and metamorphic domains by their distinctive REE patterns and trace element compositions. The metamorphic titanite exhibit variably elevated Nb contents and Nb/Ta ratios, suggesting Nb/Ta fractionation due to breakdown of amphibole and/or biotite during metamorphism. Polyphase growth of garnet is suggested by an integrated analysis of mineral inclusions, and major and trace elements in large garnet grains. Trace element abundances vary in different zones of garnet, which is ascribed to changes in the paragenesis and composition of matrix minerals involved in garnet-forming reactions at different P-T conditions. Therefore, the metamorphic growth of zircon, titanite and garnet would have occurred not only during prograde subduction but also during retrograde exhumation in the continental collision zones. Both metamorphic dehydration and partial melting would have taken place episodically during the collisional orogeny. The breakdown of hydrous minerals at high- to ultrahigh-pressure conditions is a key to fluid liberation and element supply for the growth of these accessory minerals.

Chen, Yi-Xiang; Zheng, Yong-Fei; Hu, Zhaochu

2013-09-01

135

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-, H2S, 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

136

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

Microsoft Academic Search

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

Briant T. R. Lewis; Guy C. Cochrane

1990-01-01

137

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

Microsoft Academic Search

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

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

2009-01-01

138

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

Microsoft Academic Search

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

Brian T. R. Lewis; Guy C. Cochrane

1990-01-01

139

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

Microsoft Academic Search

Shear stresses tau on a subduction megathrust play an important role in determining the forces available for mountain building adjacent to a subduction zone. In this study, the temperatures and shear stresses on megathrusts in 11 subduction zones around the Pacific rim (Hikurangi, Tonga, Izu-Ogasawara, western Nankai, northeastern Japan, Aleutians, western Alaska, Cascadia, northern Chile, southern Chile) and SE Asia

Simon Lamb

2006-01-01

140

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

Microsoft Academic Search

Shear stresses ? on a subduction megathrust play an important role in determining the forces available for mountain building adjacent to a subduction zone. In this study, the temperatures and shear stresses on megathrusts in 11 subduction zones around the Pacific rim (Hikurangi, Tonga, Izu-Ogasawara, western Nankai, northeastern Japan, Aleutians, western Alaska, Cascadia, northern Chile, southern Chile) and SE Asia

Simon Lamb

2006-01-01

141

The seismic structure of the Rivera subduction zone  

NASA Astrophysics Data System (ADS)

The subduction zone of western Mexico is a unique region on Earth where microplate capture and overriding plate disruption are occurring today. The small Rivera plate is subducting beneath western most Mexico primarily beneath Jalisco state while to the east it is the Cocos plate that is subducting. Above the Rivera plate the Jalisco block of Mexico is bounded by the north trending Colima Rift and the northwest trending Tepic-Chapala Rift and may form a microplate in its own right. Magmatism is present throughout the region and is unusual for a subduction zone in that geochemical analyses indicate an ocean island basalt component to some of the lavas. Also, Colima volcano is offset trenchward from other volcanoes in the Mexican Volcanic Belt. Little is known of the subducting Rivera plate geometry due to the paucity of seismicity within the plate yet the geometry of the Rivera and Cocos plates at depth are likely critical for understanding the tectonic evolution of western Mexico. The MARS (MApping the Rivera Subduction zone) project consists of the deployment of 50 broadband seismometers covering the Jalisco block from the coast to the Tepic-Chapala rift in the north and about 150 km to the west of the Colima rift. The instruments were deployed in January, 2006 and removed in June, 2007. The goal of the project is to seismically image the subducting Rivera and Cocos plates at depth as well as the mantle wedge above the plates. We present the results of a P-wave tomography inversion using teleseisms recorded by MARS. The inversion used 10,495 residuals and finite frequency theory to back project the kernels through the model. At shallow depths it is difficult to discern the subducting Cocos and Rivera plates but at depths deeper than about 80 km both plates are clearly imaged in the tomography model. Below a depth of 150 km, a clear gap between the Rivera and Cocos slabs is apparent that increases in size at further depths. The images indicate that the deeper Rivera plate is subducting more steeply than the adjacent Cocos plate and also has a more northerly trajection. The break between the two plates is just to the west of Colima graben. The Rivera and western most Cocos plates reach depths exceeding 200 km to the south of the main Mexican Volcanic Belt. The Rivera plate is at roughly 140 km depth beneath the young (<3Ma) central Jalisco Volcanic lineament indicating the Rivera plate has likely recently rolled back and steepened.

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

2007-12-01

142

Late holocene tectonics and paleoseismicity, southern cascadia subduction zone.  

PubMed

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

Clarke, S H; Carver, G A

1992-01-10

143

Plate detachment, asthenosphere upwelling, and topography across subduction zones  

NASA Astrophysics Data System (ADS)

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

Gvirtzman, Zohar; Nur, Amos

1999-06-01

144

The earliest mantle fabrics formed during subduction zone infancy  

NASA Astrophysics Data System (ADS)

Harzburgites obtained from the oldest crust–mantle section in the Philippine Sea plate (˜52 Ma) along the landward slope of the southern Izu–Ogasawara Trench, preserve mantle fabrics formed during the infancy of the subduction zone; that is during the initial stages of Pacific plate subduction beneath the Philippine Sea plate. The harzburgites have relatively fresh primary minerals despite of their heavy serpentinizations, and show inequigranular interlobate textures, and crystal preferred orientation patterns in olivine (001)[100] and Opx (100)[001]. The harzburgites have the characteristics of residual peridotites, whereas the dunites, obtained from the same location as the harzburgites, provide evidence for the earliest stages of arc volcanism during the inception of subduction. We propose that the (001)[100] olivine patterns began forming in immature fore-arc mantle with an increase in slab-derived hydrous fluids during the initial stages of subduction in in situ oceanic island arc.

Harigane, Yumiko; Michibayashi, Katsuyoshi; Morishita, Tomoaki; Tani, Kenichiro; Dick, Henry J. B.; Ishizuka, Osamu

2013-09-01

145

Slab melting versus slab dehydration in subduction-zone magmatism.  

PubMed

The second critical endpoint in the basalt-H(2)O system was directly determined by a high-pressure and high-temperature X-ray radiography technique. We found that the second critical endpoint occurs at around 3.4 GPa and 770?°C (corresponding to a depth of approximately 100 km in a subducting slab), which is much shallower than the previously estimated conditions. Our results indicate that the melting temperature of the subducting oceanic crust can no longer be defined beyond this critical condition and that the fluid released from subducting oceanic crust at depths greater than 100 km under volcanic arcs is supercritical fluid rather than aqueous fluid and/or hydrous melts. The position of the second critical endpoint explains why there is a limitation to the slab depth at which adakitic magmas are produced, as well as the origin of across-arc geochemical variations of trace elements in volcanic rocks in subduction zones. PMID:21536910

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

2011-05-02

146

Late Holocene Tectonics and Paleoseismicity, Southern Cascadia Subduction Zone  

NASA Astrophysics Data System (ADS)

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

Clarke, Samuel H., Jr.; Carver, Gary A.

1992-01-01

147

Serpentine preferred orientation and variation in subduction zone anisotropy  

NASA Astrophysics Data System (ADS)

Serpentine has a strong crystallographic anisotropy, which is several times greater than olivine, and the occurrence of serpentine is expected where water infiltrates into the mantle. Accordingly, one possible explanation of the spatial variation of seismic anisotropy in subduction zones is the crystal-preferred orientation of serpentine. In the Japan subduction systems, delay time of shear wave splitting is ~1-2 sec beneath the Ryukyu arc, whereas northeast Japan shows much shorter delay time ~0.1-0.2 sec. Although seismic anisotropy in the upper mantle is generally attributed to the crystal-preferred orientation of olivine, the strong anisotropy (dt ~1-2 sec) observed in the Ryukyu arc cannot be explained in terms of olivine anisotropy, even if the entire mantle wedge were to act as an anisotropic source. Here we report that the crystal-preferred orientation of serpentine can produce the strong trench-parallel anisotropy and results variations of seismic anisotropy observed in the subduction systems. High-pressure deformation experiments reveal that the serpentine c-axis tends to rotate to an orientation normal to the shear plane; consequently, seismic velocity propagating normal to the shear plane (plate interface) is much slower than that in other directions. The seismic anisotropy estimated for deformed serpentine aggregates (AVs of ~32%) is much greater than that for olivine; consequently, the alignment of serpentine in the hydrated mantle wedge results in a strong trench-parallel seismic anisotropy in the case of a steeply subducting slab. This hypothesis is consistent with the presence of a hydrous phase in the mantle wedge beneath the Ryukyu arc, as inferred from anomalously low seismic velocities. In contrast, if the subduction angle is a relatively flat, the serpentine slowest c-axis is oriented normal to the subducting plate, resulting a much smaller detectable seismic anisotropy. This may explain the weak seismic anisotropy in the Cascadia subduction zone where the presence of serpentine is also inferred.

Katayama, I.; Hirauchi, K.; Michibayashi, K.; Ando, J.

2010-12-01

148

Depth-varying rupture properties of subduction zone megathrust faults  

NASA Astrophysics Data System (ADS)

Subduction zone plate boundary megathrust faults accommodate relative plate motions with spatially varying sliding behavior. The 2004 Sumatra-Andaman (Mw 9.2), 2010 Chile (Mw 8.8), and 2011 Tohoku (Mw9.0) great earthquakes had similar depth variations in seismic wave radiation across their wide rupture zones - coherent teleseismic short-period radiation preferentially emanated from the deeper portion of the megathrusts whereas the largest fault displacements occurred at shallower depths but produced relatively little coherent short-period radiation. We represent these and other depth-varying seismic characteristics with four distinct failure domains extending along the megathrust from the trench to the downdip edge of the seismogenic zone. We designate the portion of the megathrust less than 15 km below the ocean surface as domain A, the region of tsunami earthquakes. From 15 to ˜35 km deep, large earthquake displacements occur over large-scale regions with only modest coherent short-period radiation, in what we designate as domain B. Rupture of smaller isolated megathrust patches dominate in domain C, which extends from ˜35 to 55 km deep. These isolated patches produce bursts of coherent short-period energy both in great ruptures and in smaller, sometimes repeating, moderate-size events. For the 2011 Tohoku earthquake, the sites of coherent teleseismic short-period radiation are close to areas where local strong ground motions originated. Domain D, found at depths of 30-45 km in subduction zones where relatively young oceanic lithosphere is being underthrust with shallow plate dip, is represented by the occurrence of low-frequency earthquakes, seismic tremor, and slow slip events in a transition zone to stable sliding or ductile flow below the seismogenic zone.

Lay, Thorne; Kanamori, Hiroo; Ammon, Charles J.; Koper, Keith D.; Hutko, Alexander R.; Ye, Lingling; Yue, Han; Rushing, Teresa M.

2012-04-01

149

Interacting Earthquakes Along the Northern Vanuatu Subduction Zone  

NASA Astrophysics Data System (ADS)

Located in the southwest Pacific, the Vanuatu subduction zone displays diverse seismic behavior, characterized by frequent moderately-sized earthquakes (Mw 5-7), but much fewer events larger than Mw 7.0. The seismic character also varies regionally along the length of the trench. On 07 October 2009 the northern segment of the Vanuatu subduction zone (11° to 14°S) ruptured in three large, shallow, thrust earthquakes. All three (Mw 7.7, 7.8, and 7.4) occurred in close proximity and over the duration of about one hour. Historical seismicity shows this region of the trench has experienced multiple shallow, large (Mw >7.0), thrust events during the past century, including large shallow doublets in 1966 and 1980. The July 1980 earthquake doublet (Mw 7.5, 7.7) ruptured the plate boundary in approximately the same location as the 2009 sequence. The focal mechanisms of these earlier events are similar to one another and to the mechanisms of the 2009 events. The close spatial proximity and similarity in focal mechanisms of these earthquake sequences provides an opportunity to explore the possibility of characteristic multiplet earthquakes. Large, shallow, thrust earthquakes have been observed to occur in pairs and multiples in various tectonic regimes, but a unifying tectonic characteristic producing these sequences is poorly understood. We present rupture models for the larger events in the region (for which teleseismic body waves can be isolated), and explore the relative locations of the 2009 events and earlier events using regional and teleseismic surface waves. Our goal is to map the region of the plate boundaries failing in the large multiplets of the region in hopes of understanding the spatial and temporal relationships between these potentially interacting earthquakes.

Cleveland, M.; Ammon, C. J.; Lay, T.

2010-12-01

150

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

151

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

USGS Publications Warehouse

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

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

1996-01-01

152

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

NASA Astrophysics Data System (ADS)

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

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

2010-12-01

153

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

USGS Publications Warehouse

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

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

2012-01-01

154

Mineral Growth Controlled By Aperture Of Fluid-filled Cracks In Subduction Zones: An Example From The Sanbagawa Belt, Japan  

NASA Astrophysics Data System (ADS)

Sealed cracks in high-pressure metamorphic rocks have been regarded as a direct evidence of the fracture- controlled fluid flow in subduction zones. Although various growth microstructures of vein minerals have been reported, the relationship between microstructures and fluid flow remains unclear. The pelitic schists from Nagatoro area of the Sanbagawa metamorphic belt, Japan contain two types of veins composed of quartz + albite + K-feldspar + chlorite (type I) and quartz + albite + calcite (type II). Both veins cut the foliation and the stretching lineation of the host rocks, indicating the formation at the exhumation stage (about 300 degC). Within type I veins, elongate quartz and albite grains grew from the fractured quartz and albite grains of the vein wall, respectively, and K-feldspar and chlorite form corresponding to muscovite + chlorite-rich layers of host rocks. In contrast, type II veins have euhedral quartz grains with concentric zoning, and the mineral distribution is independent of those of host rocks. The veins systematically change from type I to type II with increasing vein width, and the critical width is about 1 mm. Both veins show the evidences of multiple crack- seal events, but the aperture width of each crack are 0.01 - 0.05 mm for type I, whereas 0.5 - 3.0 mm for type II. Considering the cubic dependence of the permeability on crack aperture, the permeability of type II veins was 103 - 106 times larger than type I veins. The materials for forming vein minerals come from deeper parts of the subduction zones, or from the surrounding host rocks. The mineral distribution of type I veins suggests that material diffused into fluid-filled cracks from the host rocks, and that the effect of fluid advection was very small. In contrast, for type II veins with wide aperture, the upward fluid flow would have brought the high concentration of Si into the crack, that leaded to the homogeneous nucleation of quartz. Although the frequency of type II veins is about 10 % of total veins in the Nagatoro area, this type veins would have played an important roll on fluid and material transport within the subduction zone.

Okamoto, A.; Kikuchi, T.; Tsuchiya, N.

2006-12-01

155

Automated Tremor Analysis From the Cascadia Subduction Zone  

NASA Astrophysics Data System (ADS)

Reanalysis of geodetic GPS time series from the Cascadia subduction zone have revealed at least 30 resolvable slow slip events along the megathrust since 1997, ranging from northern California to southern British Columbia. Many of the smaller and more recent events are barely detectable with GPS, but stand out clearly as tremor sequences on band-pass filtered record sections of surface seismic data. With the intent of assessing whether a fixed frequency-magnitude relationship applies to subduction zone slow slip events, and assuming that tremor and GPS transients are different manifestations of the same faulting process at depth, we have implemented automated processes to identify the frequency-magnitude relationship of slow slip events that are not resolvable with GPS. Our methodology is as follows. We first consolidate daily seismic files from the northern Puget Basin of Washington State and SW British Columbia, where GPS density is highest. Seismic traces are included from the Pacific Northwest Seismic Network, the Plate Boundary Observatory borehole seismic network, the Earthscope-funded CAFÉ experiment, and locally deployed seismometers around long-baseline tiltmeters, all running at 100 sps. We then band-pass the data between 1-6 Hz and run it through several tremor-detection algorithms. The envelopes of rectified traces are then regionally averaged to provide a single metric indicative of tremor activity. We also compute spectrograms of daily files and use power in the 1-6 Hz range to assess tremor onset and duration. To validate that this automated approach works as expected, we first manually inspected time periods of slow slip events, particularly the Sept 6-30 of 2005, when GPS transients are known. Secondly, hours per week of tremor identified on at least three stations separated by 25 Km were summed manually and show clear spikes during the GPS-inferred timing, indicating that envelope averaging does accurately identify the largest known events. We are currently applying this process to all of 2005 and 2006 data sets, the results of which will be presented and discussed in the background context of known measurements from the GPS, borehole strain, and long-baseline tiltmeter measurements.

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

2006-12-01

156

Earthquake swarms in circum-Pacific subduction zones  

NASA Astrophysics Data System (ADS)

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

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

2011-05-01

157

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

158

Continental-scale rheological heterogeneities and complex intraplate tectono-metamorphic patterns: insights from a case-study and numerical models  

Microsoft Academic Search

Continental plates are built over long periods of time through successive extensional and compressional cycles. They are therefore rheologically heterogeneous. This heterogeneity should significantly influence the mechanical response of the continental lithosphere during collision processes. The study of the Neoproterozoic Borborema shear zone system of northeast Brazil highlights a systematic link between marked changes in its tectono-metamorphic pattern and the

Andréa Tommasi; Alain Vauchez

1997-01-01

159

Developing a comprehensive seismic velocity model of the Cascadia subduction zone  

NASA Astrophysics Data System (ADS)

A comprehensive and high-resolution velocity model is essential to understand the subduction dynamics and continental accretion in the Cascadia subduction zone. In this study, we image the seismic structure of the crust and upper mantle with the application of a full-wave tomographic method. The tomographic result provides important insights on addressing such scientific questions as the along-strike segmentation of Cascadia and the role of (de-)hydration of the slab/crust, which is thought to be critical for the occurrence of the episodic tremor and slip. We use continuous seismic data recorded between 1995 and 2011 by about 600 stations in an area covering from northernmost California to northern Vancouver Island, Canada. The empirical Green's functions are recovered from inter-station cross correlation at periods of 7-200 seconds. At the periods of our interest, the empirical Green's functions from cross correlation of vertical-vertical channels are primarily Rayleigh waves. We simulate full-wave propagation within a 3D reference velocity model. The travel time anomalies are measured from the observed and synthetic Green's functions at stations. The shear and compressional velocities are inverted jointly as Rayleigh waves are sensitive to both Vp and Vs. The solution from inversion is used to iteratively update the 3D reference model. The integration of various waves in a physically consistent way refines absolute P and S velocities and Vp/Vs ratio, which allows for more robust and reliable geodynamic interpretations.

Gao, H.; Shen, Y.

2011-12-01

160

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

PubMed

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

Calvert, Andrew J

2004-03-11

161

Numerical modelling of lithosphere asthenosphere interaction in a subduction zone  

NASA Astrophysics Data System (ADS)

We developed a new 2-D numerical approach to study solid fluid coupling applied to subduction zones. The lithosphere is characterised by an elastic or elastoplastic behaviour and the asthenosphere by a homogeneous isoviscous fluid. The temperature effects are ignored and viscosity and density are constant in time. The solid and the fluid problem are discretised by the finite elements method (FEM). The same solid code used in Hassani et al. [Hassani, R., Jongmans, D., Chery, J., Study of plate deformation and stress in subduction processes using two-dimensional numerical models, J. Geophys. Res. (1997) 102 17951 17965.] has been used to compute the solution of the solid problem. The Stokes problem is solved by a direct solver with a stabilisation procedure. We used a very simple staggered coupling method where the fluid domain is regularly re-meshing. We observed numerical instabilities when the time step is not sufficiently small, especially when strong coupling between the solid and the fluid occurs. We have tested different configurations where the lithosphere is elastic or elastoplastic and show how the slab geometry, the topography and the stress regime in the plates are affected by the viscous resistance of the mantle. We observed that the asthenosphere viscosity is a fundamental parameter in the subduction process. For subduction with an extensional regime in the upper plate, we observe a linear decrease of the extensional stress as a function of the asthenospheric viscosity.

Bonnardot, M.-A.; Hassani, R.; Tric, E.

2008-08-01

162

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

163

Depth-varying Rupture Properties of Subduction Zone Megathrust Faults  

NASA Astrophysics Data System (ADS)

Earth's largest earthquakes rupture megathrust faults at subduction zone plate boundaries. The 2004 Sumatra (Mw 9.2), 2010 Chile (Mw 8.8) and 2011 Tohoku (Mw 9.0) great earthquakes exhibit similarities in spatial variations of seismic wave radiation from their rupture zones, with coherent short-period radiation preferentially emanating from the deeper portion of the megathrust while the largest fault displacements occur at shallower megathrust depths with relatively less short-period radiation. In numerous regions, large tsunami earthquakes rupture the shallowest region of the plate boundary with large displacements and very low short-period radiation, while low-frequency earthquakes, seismic tremor and slow slip events occur down-dip of the primary seismogenic portion of some megathrusts. These observations indicate that interplate thrust faults rupture with systematic depth-variations characterized by four distinct domains from the trench to the down-dip edge of the seismogenic zone. Attributes of large ruptures such as seismic moment scaled radiated energy vary with the domain or domains ruptured. Fundamental differences in rupture behavior exist between these domains, with variations in frictional properties, pore fluids, sediments, mineral phase, rock-type contrast, and temperature likely contributing to depth-dependent rupture behavior.

Lay, T.; Kanamori, H.; Ammon, C. J.; Yue, H.; Koper, K. D.; Hutko, A. R.; Brodsky, E. E.

2011-12-01

164

S wave velocity structure of the northern Cascadia subduction zone  

NASA Astrophysics Data System (ADS)

The shear velocity structure across the northern Cascadia subduction zone is examined using three-component broadband digital seismographs. Locally generated P-to-S conversions are analyzed to estimate the S velocity structure to upper mantle depths and to constrain the subduction geometry of the Juan de Fuca plate. The oceanic crust is at 47-53 km beneath central Vancouver Island, 60-65 km beneath Georgia Strait, and 70-75 km beneath EGM station on the Sechelt Peninsula. The dip direction of the plate at each site provides new evidence that the Juan de Fuca plate is arched upward in the Puget Sound region. The dip angle increases from 15 deg +/- 5 deg to 22 deg +/- 5 deg near LAS station, where a concentration of seismicity occurs near 60 km depth. This study provides the first definitive evidence that the reflective zones imaged beneath Vancouver Island are regions of low S velocity and that they extend into the lower crust and upper mantle beneath the British Columbia mainland.

Cassidy, John F.; Ellis, Robert M.

1993-03-01

165

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

NASA Astrophysics Data System (ADS)

High precision U Pb geochronology of rutile from quartz carbonate white mica rutile veins that are hosted within eclogite and schist of the Monte Rosa nappe, western Alps, Italy, indicate that the Monte Rosa nappe was at eclogite-facies metamorphic conditions at 42.6 ± 0.6 Ma. The sample area [Indren glacier, Furgg zone; Dal Piaz (2001) Geology of the Monte Rosa massif: historical review and personal comments. SMPM] consists of eclogite boudins that are exposed inside a south-plunging overturned synform within micaceous schist. Associated with the eclogite and schist are quartz carbonate white mica rutile veins that formed in tension cracks in the eclogite and along the contact between eclogite and surrounding schist. Intrusion of the veins at about 42.6 Ma occurred at eclogite-facies metamorphic conditions (480 570°C, >1.3 1.4 GPa) based on textural relations, oxygen isotope thermometry, and geothermobarometry. The timing of eclogite-facies metamorphism in the Monte Rosa nappe determined in this study is identical to that of the Gran Paradiso nappe [Meffan-Main et al. (2004) J Metamorphic Geol 22:261 281], confirming that these two units have shared the same Alpine metamorphic history. Furthermore, the Gran Paradiso and Monte Rosa nappes underwent eclogite-facies metamorphism within the same time interval as the structurally overlying Zermatt-Saas ophiolite [˜50 40 Ma; e.g., Amato et al. (1999) Earth Planet Sci Lett 171:425 438; Mayer et al. (1999) Eur Union Geosci 10:809 (abstract); Lapen et al. (2003) Earth Planet Sci Lett 215:57 72]. The nearly identical P T t histories of the Gran Paradiso, Monte Rosa, and Zermatt-Saas units suggest that these units shared a common Alpine tectonic and metamorphic history. The close spatial and temporal associations between high pressure (HP) ophiolite and continental crust during Alpine orogeny indicates that the HP internal basement nappes in the western Alps may have played a key role in exhumation and preservation of the ophiolitic rocks through buoyancy-driven uplift. Coupling of oceanic and continental crust may therefore be critical in preventing permanent loss of oceanic crust to the mantle.

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

2007-02-01

166

Supra-subduction zone (SSZ) ophiolites: the Fore-arc connection (Invited)  

NASA Astrophysics Data System (ADS)

Ophiolites are distinct assemblages of submarine volcanic rocks and plutonic rocks that include cumulate dunite, wehrlite, and gabbro, as well as isotropic gabbro and diorite, and peridotite tectonite, representing the underlying refractory mantle. They were originally thought to represent oceanic crust formed at mid-oceanic spreading centers, but their connection with island arcs has become increasingly apparent ever since it was proposed by Miyashiro (1973). Recognition that ophiolites are not normal arc assemblages, but form during unique, transient episodes of arc formation, has led to the concept of supra-subduction zone ophiolites (Pearce, 1984). SSZ ophiolites display a consistent development history from birth through death that implies a common origin and evolution in response to systematic, non-random processes (Shervais 2001). A review of modern volcanic rocks formed at mid-ocean ridges and back-arc basins shows that they have a limited range in major element compositions, and trace element systematics that range from depleted (“normal”) to enriched MORB, in which ratios of fluid-mobile LFS elements to fluid-immobile HFS elements are relatively constant. In contrast, volcanic rocks formed within regionally-extended fore-arcs (which may also form the basement of later arc complexes) have wider range in major element compositions and trace element systematics that are depleted in the HFS elements and enriched in fluid-mobile LFS elements (Metcalf and Shervais 2008). Most ophiolite volcanic suites are dominated by major and trace element systematics that are identical to those displayed by fore-arc volcanic suites, including the occurrence of boninites, which are only found within forearc settings. These systematics are consistent with fluid-enrichment of the mantle source region that had seen a prior extreme melt depletion event. Some ophiolites display more complex relations, with both SSZ and MORB or BAB-like compositions, but the SSZ components are generally dominant. These observations of volcanic rock chemistry are reinforced by recent studies of the mantle peridotite tectonites that underlie ophiolites. These studies show that ophiolite tectonites are strongly depleted in HFS and rare earth elements, requiring extensive melt extraction, and enriched in fluid-mobile elements, requiring a significant fluid flux that can only be sustained in a supra-subduction zone setting. This conclusion is enforced by recent isotopic studies, which document subduction-enriched isotopic compositions of Sr and Pb in SSZ ophiolites, and by ICP-MS studies of fluid mobile elements in relict pyroxene, which document enrichment in all fluid mobile elements. We conclude that ophiolites provide an analogue to modern fore-arc settings, and that their position in the upper plate of a subduction-zone leads to their preferential emplacement by obduction onto passive continental margins, or by accretionary uplift along continually active margins.

Shervais, J. W.; Metcalf, R. V.

2009-12-01

167

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

SciTech Connect

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

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

1995-11-10

168

Simulation of Tsunamis from Great Earthquakes on the Cascadia Subduction Zone  

Microsoft Academic Search

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

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

1990-01-01

169

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

Microsoft Academic Search

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

Paul M. Whitmore

1993-01-01

170

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

Microsoft Academic Search

Subduction zone magmatism plays an important role in material recycling of the earth's interior, and it is imperative to understand melt generation mechanisms in the wedge mantle. The subduction zone is more complicated and diverse as a magma generation site than that of mid-ocean ridge or hot spot mostly because of the more significant and variable contribution of H2O-rich fluid

I. Ogitsu; K. Ozawa

2009-01-01

171

The proportionality between relative plate velocity and seismicity in subduction zones  

NASA Astrophysics Data System (ADS)

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

Ide, Satoshi

2013-09-01

172

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

173

Earth's free oscillations recorded by free-fall OBS ocean-bottom seismometers at the Lesser Antilles subduction zone  

NASA Astrophysics Data System (ADS)

Three unburied ocean bottom seismometers (OBS) equipped with Trillium 240 s broad-band seismometers recorded spheroidal free oscillations of the Earth out to periods over 1000 s period, for the M = 8.1, April 1, 2007 Solomon Islands earthquake. In contrast to broadband observatories of the global network that operate in quiet continental locations, these instruments were dropped on the several-km thick layer of sediments of the forearc and accretionary wedge of the Lesser Antilles subduction zone. Furthermore, a high ambient noise level due to the ocean surface infragravity waves is expected to cover the frequency band of Earth's normal modes band when recorded at these sites. In spite of these hostile environmental conditions, the frequency of clearly defined peaks of the Earth's normal modes were measured after the earthquake. This suggests that the recording of normal modes and long period waves can be extended to parts of the hitherto inaccessible ocean with currently available OBS technology.

Bécel, A.; Laigle, M.; Diaz, J.; Montagner, J.-P.; Hirn, A.

2011-12-01

174

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

175

Slow slip events at the Alaska Subduction Zone  

NASA Astrophysics Data System (ADS)

A large slow slip event (SSE) occurred at the Alaska subduction zone during 1998-2001 [Ohta et al., 2006, EPSL]. The SSE occurs downdip of the Prince William Sound asperity, on a section that accumulated some slip deficit before and after the SSE. During the SSE, >20,000 sq. km of the plate interface slipped >10 cm, for a cumulative moment magnitude of Mw=7.2. Slip during this event was accompanied by significant non-volcanic tremor, which located in the same area as the slip. The area of the SSE did not have significant slip in the 1964 earthquake, compared to the slip of the main asperity. Our previous work did not attempt to determine the time history of slip in the SSE, because of a lack of continuous GPS data, but a reanalysis of the data that includes a few new sites allows us to estimate the approximate time history. Based on a preliminary solution, it appears that the initiation time probably varied over the area of the SSE, implying some spatial propagation, but our ability to resolve this is very limited. The continuous GPS record suggests that that may have been at least one and possibly several smaller SSEs in the same area. These smaller events appear to last for several weeks, but because most appear at a similar time of year, it is possible that they represent an unmodeled seasonal signal instead. We will compare the displacement pattern and time history of the large SSE to the displacements to evaluate whether these smaller events actually represent SSEs on the same part of the plate interface.

Freymueller, J. T.; Ohta, Y.

2007-05-01

176

A possible source of water in seismogenic subduction zones  

NASA Astrophysics Data System (ADS)

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

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

2010-12-01

177

Seismic anisotropy and heterogeneity in the Alaska subduction zone  

NASA Astrophysics Data System (ADS)

We determined P- and S-wave tomography and P-wave anisotropic structure of the Alaska subduction zone using 259 283 P- and 73 817 S-wave arrival times from 7268 local shallow and intermediate-depth earthquakes recorded by more than 400 seismic stations. The results show strong velocity heterogeneities in the crust and upper mantle. Low-velocity anomalies are revealed in the mantle wedge with significant along-arc variations under the active volcanoes. In the mantle wedge, the low-velocity zone extends down to 100-150 km depth under the backarc. The results indicate that H2O and fluids brought downwards by the subducting Pacific slab are released to the mantle wedge by dehydration and they are subsequently transported to the surface by the upwelling flow in the mantle wedge. Significant P-wave anisotropic anomalies are revealed under Alaska. The predominant fast velocity direction (FVD) is trench-parallel in the shallow part of the mantle wedge (<90 km depth) and in the subslab mantle, whereas the FVD is trench-normal within the subducting Pacific slab. The trench-parallel FVDs in the mantle wedge and subslab mantle may be caused by 3-D mantle flow that is induced by the complex geometry and strong curvature of the Pacific slab under Alaska. The flat and oblique subduction of the Pacific slab may play a key role in forming the trench-parallel FVD under the slab. The trench-normal FVD in the subducting Pacific slab may reflect the original fossil anisotropy when the Pacific Plate was produced at the mid-ocean ridge.

Tian, You; Zhao, Dapeng

2012-07-01

178

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

179

Fluid migration in continental subduction: The Northern Apennines case study  

NASA Astrophysics Data System (ADS)

Subduction zones are the place in the world where fluids are transported from the foredeep to the mantle and back-to-the-surface in the back-arc. The subduction of an oceanic plate implies the transportation of the oceanic crust to depth and its methamorphization. Oceanic sediments release water in the (relatively) shallower part of the subduction zone, while dehydration of the subducted basaltic crust allows fluid circulation at larger depths. While the water budget in oceanic subduction has been deeply investigated, less attention has been given to the fluids implied in the subduction of a continental margin (i.e. in continental subduction). In this study, we use teleseismic receiver function (RF) analysis to image the process of water migration at depth, from the subducting plate to the mantle wedge, under the Northern Apennines (NAP, Italy). Harmonic decomposition of the RF data-set is used to constrain both isotropic and anisotropic structures. Isotropic structures highlight the subduction of the Adriatic lower crust under the NAP orogens, from 35-40 km to 65 km depth, as a dipping low S-velocity layer. Anisotropic structures indicate the presence of a broad anisotropic zone (anisotropy as high as 7%). This zone develops in the subducted Adriatic lower crust and mantle wedge, between 45 and 65 km depth, directly beneath the orogens and the more recent back-arc extensional basin. The anisotropy is related to the metamorphism of the Adriatic lower crust (gabbro to blueschists) and its consequent eclogitization (blueschists to eclogite). The second metamorphic phase releases water directly in the mantle wedge, hydrating the back-arc upper mantle. The fluid migration process imaged in this study below the northern Apennines could be a proxy for understanding other regions of ongoing continental subduction.

Agostinetti, Nicola Piana; Bianchi, Irene; Amato, Alessandro; Chiarabba, Claudio

2011-02-01

180

Mantle melting and crustal recycling in subduction zones  

NASA Astrophysics Data System (ADS)

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

Plank, Terry Ann

181

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

182

Using garnet peridotites as tools to reconstruct paleo-geodynamic settings of fossil continental collision zones  

NASA Astrophysics Data System (ADS)

Orogenic garnet peridotites (metamorphic rocks containing the characteristic HP garnet-olivine mineral assemblage) form volumetrically minor, but important components of (ultra)high pressure (UHP) metamorphic terranes. Such terranes form along convergent plate margins where two adjacent plates collide and one of the plates is subducted below the other. After eduction back to the surface such fossil continental subduction/collision zones form the basic components of exposed (U)HP metamorphic terranes. In the absence of significant amounts of tectonic overpressure (Vrijmoed et al., 2010), the discovery of diamond and majoritic garnet in (U)HP metamorphic terranes provide evidence that subduction of continental crust into the mantle was deep enough to reach the garnet stability field in the overlying mantle wedge above the subduction zone. Brueckner (1998) was the first author who noticed that garnet peridotite bodies, present in such mantle wedges, could be transferred during collision from the mantle wedge into the subducted continental crust. Subsequent buoyancy, most likely generated by slab break off of previously subducted oceanic crust, is the most likely candidate to enable the subducted continental crust and its garnet peridotite "cargo" to return back to (sub)crustal levels. During the latter process mantle wedge garnet peridotite may recrystallize (partly or completely) into what will be called here subduction zone garnet peridotite. Alternatively subduction zone garnet peridotite may be formed by prograde subduction of ultramafic protoliths (serpentinites, Fe-Ti peridotite) that may be present in subducting continental crust prior to subduction. Subdivision between these two basic types of orogenic garnet peridotites (mantle wedge - versus subduction zone peridotite) allows however that in mantle wedge garnet peridotite the subcontinental lithospheric mantle (SCLM) - versus crustal-incorporation processes can be identified which a.o. has lead to the recent recognition of a complete new, deep-seated, subcratonic, lithospheric mantle setting. In addition better characterization of SCLM processes in mantle wedge garnet peridotite will also allow for further subdivision of SCLM wedges into different subtypes that all may be present during collision in the hanging wall of a fossil collision/subduction system. In the following we will present the basic outlines of such a mantle wedge classification system. A simple "conceptual" model will be presented that will allow orogenic mantle wedge garnet peridotite to be used as a tool to reconstruct the former paleo-geodynamic setting of the collision/subduction system. Using field, petrological, geochemical, geochronological and geothermobarometric criteria, all of which can be analysed directly in the mantle wedge garnet peridotite body itself, the model allows for discrimination between four different end-member types within the SCLM (equivalent to young/hot/dynamic- versus cold/old/static mantle in thick or thin garnet-olivine bearing mantle wedges). In addition our conceptual model is based on the fundamental assumption that all SCLM was once formed by rising, accretion and cooling of hot asthenospheric mantle. Note also that all mantle wedge end member types may become overprinted by the subduction zone type. The latter, when complete, may evidently erase all former mantle wedge evidences. To test the applicability of our model we have applied the proposed mantle wedge classification system to well studied orogenic garnet peridotites of the Caledonian Orogeny in Scandinavia and the Triassic Sulu-Dabie Orogeny in China. Results will be presented. References: Brueckner (1998). Geology 26, 631-634; Vrijmoed et al. (2010). Eur. J. Mineralogy.

Zhang, Cong; van Roermund, Herman; Zhang, Lifei

2010-05-01

183

Structure and Deformation of the Hikurangi-Kermadec Subduction Zone - Transitions Revealed by Seismic Wide-angle Data  

NASA Astrophysics Data System (ADS)

The Hikurangi-Kermadec subduction zone northeast of New Zealand represents an ideal target to study lateral variations of subduction zone processes. The incoming Pacific plate changes from being a large igneous province, called the Hikurangi Plateau, in the south to normal oceanic plate north of the Rapuhia Scarp. The overriding Australian plate of continental character in the south, forming the North Island of New Zealand, and changes to an island arc in the north. Further lateral variability exists in changes in volcanic and hydro-thermal activity, transitions from accretion to subduction erosion, backarc spreading and rifting, and is accompanied by northward increasing seismicity. As part of the MANGO project (Marine Geoscientific Investigations on the Input and Output of the Kermadec Subduction Zone), four marine geophysical transects of largely seismic reflection and refraction data provide constraints on the upper lithospheric structures across the Hikurangi-Kermadec Trench between 29-38 deg S. On MANGO profile 1 in the south, the initially shallow subduction of the incoming plateau coincides with crustal underplating beneath the East Cape ridge. To the west lies the 100 km wide and over 10 km deep Raukumara Basin. Seismic velocities of the upper mantle of both plates are around 8 km/s and are considered normal. In contrast, on MANGO profile 4, about 1000 km to the north around the volcanically active Raoul Island, the incoming oceanic crust appears to bend considerably steeper and thus causes a 50 km narrower forearc with a smaller forearc basin. Furthermore, the upper mantle velocities in both plates are relatively low (7.4-7.7 km/s), likely indicating strong bending related deformation of the incoming plate and thermal activity within the arc possibly due to spreading. The central two transects MANGO 2 and 3, though without data coverage of the structure of the incoming plate, are more similar to MANGO 4. The arc regions appear to be strongly affected by the activity of the arc. The arc crust of the northern MANGO 3 becomes significantly thinner in the backarc region due to extension, whereas the data from MANGO 2 likely show thermal activity from the adjacent arc volcanism.

Scherwath, M.; Kopp, H.; Flueh, E. R.; Henrys, S. A.; Sutherland, R.

2008-12-01

184

Potential Significant Tsunami Hazard in the Puysegur Subduction Zone, South of New Zealand  

NASA Astrophysics Data System (ADS)

Subduction zone seismogenesis and related tsunami potential have recently become a significant focus; yet none of the recent global studies have considered the Puysegur subduction zone, south of New Zealand, and its hazards. While several local studies have identified the southern and southwestern (Fiordland) margins as potential tsunami hazards, those models fail to take into account the oblique nature of subduction and the impact of that obliquity on earthquake slip and tsunami wave generation. We have undertaken a comprehensive study of the Puysegur subduction zone and its earthquake and tsunami hazards by analyzing the historical seismicity over the entire plate boundary region south of New Zealand and using that data to constrain the earthquake potential for the Puysegur trench. We have identified both seismicity clearly associated with the interplate megathrust, and using these events, determined the seismic moment deficit of the subduction plate boundary over the past ~100 years. These calculations imply unreleased moment equivalent to a magnitude Mw 8.4 earthquake, and thus suggest that this subduction zone has the potential to break in a great, tsunamigenic event. We model the tsunami hazard using this moment deficit and the location of the 1979 plate interface event, and find that a tsunami caused by a great earthquake on the Puysegur subduction zone poses a significant threat to the southern and western coasts of the South Island of New Zealand, the coasts of Tasmania, and also to the southeastern coast of Australia, nearly 2000 km distant.

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

2009-04-01

185

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

186

Initial water budget: the key to detaching large volumes of eclogitized oceanic crust in subduction zones?  

NASA Astrophysics Data System (ADS)

The Mesozoic, Neotethyan ophiolites and eclogites from the Zermatt-Saas area (ZS, Western Alps) represent a complete sequence of subducted lithosphere and the largest and deepest known piece of exhumed oceanic lithosphere so far (Bucher et al., 2005; Angiboust et al., 2009). Pervasive hydrothermal processes and seafloor alteration, which led to the incorporation of large amounts of fluid bound in the hydrated, upper layers of the oceanic crust, enabled the development of moderately (lawsonite eclogites) or strongly hydrated parageneses (glaucophanites, chloritoschists). Although their exhumation may have been facilitated by the highly buoyant continental units underlying the ZS ophiolite (e.g., Monte Rosa) and/or the mechanically weak (and light) surrounding serpentinites, none of the other major ophiolite bodies from the same subduction zone (eg, Monviso, Voltri) show the same characteristics (ie, continuous mafic slices and abundant lawsonite). We therefore investigated the extent to which the hydration of the oceanic lithosphere may be a major parameter controlling exhumation, a process largely overlooked up to now. Internally, the ZS ophiolite is made up of a series of tectonics slices of oceanic crust (150-300m thick) which are systematically separated by a 5 to 100-m thick serpentinite slivers. This stack of slices is separated from the underlying eclogitized continental crust (e.g., Monte Rosa) by a thick (~500m) serpentinite sole. Field observations, textural relationships and pseudosection modelling reveal that lawsonite (now pseudomorphed by clinozoisite) was abundant and widespread in mafic eclogites when the ophiolite detached from the slab at c. 550°C and 24 kbar. Comparison between fresh eclogitic samples and thermodynamic modelling suggests that (i) water remained in excess from burial to eclogitic peak conditions, (ii) the lightest eclogitized metabasalts correspond to the portions of oceanic crust where metasomatism was most intense, (iii) crystallization of widespread hydrated parageneses (such as lawsonite, glaucophane and phengite), instead of garnet and omphacite, decreased rock density by 5 to 10 % and subsequently enhanced its flottability. We propose that this density decrease acted as an efficient force to prevent these slices from irreversibly sinking into the mantle. Penetrative serpentinization of the slab mantle harzburgites during ridge-processes (Li et al., 2004) likely facilitated the detachment of theses slices from the downgoing slab and their stacking in the serpentinized subduction channel at pressures between 15-20 kbar. Exhumation of the underlying positively buoyant continental crust later dragged this "frozen" nappe-stack towards the surface.

Angiboust, Samuel; Agard, Philippe

2010-05-01

187

An Integrated Geophysical View of the Cascadia Subduction Zone  

NASA Astrophysics Data System (ADS)

The goal of this study is to evaluate correlations between new models of conductivity and seismic structure of the Cascadia subduction zone in the northwestern United States. These models were obtained using data from EarthScope's USArray long period magnetotelluric (MT) and broadband seismic Transportable Array (TA) systems. Initial MT deployments in 2006 and 2007 acquired long period (10-10000s) data at 110 sites covering the US Pacific Northwest with the same nominal spacing as the USArray seismic transportable array (~70 km). Three-dimensional inversion of this dataset reveals extensive areas of high conductivity in the lower crust beneath the Northwest Basin and Range, and beneath the Cascade Mountains, contrasting with very resistive crust in Siletzia, the accreted thick ocean crust which forms the basement rocks in the Cascadia forearc and the Columbia Embayment). The conductive lower crust beneath the southeastern part of the array is inferred to result from fluids (including possibly partial melt at depth) associated with magmatic underplating. Beneath the Cascades high conductivities probably result from fluids released by the subducting Juan de Fuca slab. Resistive Siletzia represents a stronger crustal block, accommodating deformation in the surrounding crust by rigid rotation. Significant variations in upper mantle conductivity are also revealed by the inversions, with the most conductive mantle beneath the Washington backarc in the northeastern part of the array, and the most resistive corresponding to subducting oceanic lithosphere. The Juan de Fuca slab is well imaged in body wave seismic tomography models, extending to depths of ~500 km and perhaps deeper [Roth et al., 2008]. Significant zones of focused reduced seismic velocities are evident beneath both the Newberry region and the surface expression of the Columbia River basalts. We find no evidence for a zone of low velocities beneath the Juan de Fuca slab. Further, we demonstrate that the absence of a slab signature beneath central Oregon, interpreted by some groups as a "hole" in the slab, is an inversion artifact due to imperfect ray coverage and the presence of the reduced velocity zone coincident with Newberry. While the MT analyses have so far focused on the lower crustal structure which this data is most sensitive to, and the seismic results to date have emphasized upper mantle structure, notable relationships between the models are already evident. For instance, the Juan de Fuca slab signature is clear in both models, and in both cases exhibits lateral continuity. In addition, the low velocity feature beneath Newberry coincides with a a circular zone of enhanced conductivity in the upper mantle. Interestingly, some features in the MT image, such as the high conductivity zone at shallow depths in SE Oregon, correlate well with structure of similar geometry in the seismic images at mantle depths

Egbert, G. D.; Fouch, M. J.; Schultz, A.

2009-04-01

188

Structure and seismic activity of the Lesser Antilles subduction zone  

NASA Astrophysics Data System (ADS)

Several active and passive seismic experiments conducted in 2007 in the framework of the European program "Thales Was Right" and of the French ANR program "Subsismanti" provided a unique set of geophysical data highlighting the deep structure of the central part of the Lesser Antilles subduction zone, offshore Dominica and Martinique, and its seismic activity during a period of 8 months. The region is characterized by a relatively low rate of seismicity that is often attributed to the slow (2 cm/yr) subduction of the old, 90 My, Atlantic lithosphere beneath the Caribbean Plate. Based on tomographic inversion of wide-angle seismic data, the forearc can clearly be divided into an inner forearc, characterised by a high vertical velocity gradient in the igneous crust, and an outer forearc with lower crustal velocity gradient. The thick, high velocity, inner forearc is possibly the extension at depth of the Mesozoic Caribbean crust outcropping in La Désirade Island. The outer forearc, up to 70 km wide in the northern part of the study area, is getting narrower to the south and disappears offshore Martinique. Based on its seismic velocity structure with velocities higher than 6 km/s the backstop consists, at least partly, of magmatic rocks. The outer forearc is also highly deformed and faulted within the subducting trend of the Tiburon Ridge. With respect to the inner forearc velocity structure the outer forearc basement could either correspond to an accreted oceanic terrane or made of highly fractured rocks. The inner forearc is a dense, poorly deformable crustal block, tilted southward as a whole. It acts as a rigid buttress increasing the strain within both the overriding and subducting plates. This appears clearly in the current local seismicity affecting the subducting and the overriding plates that is located beneath the inner forearc. We detected earthquakes beneath the Caribbean forearc and in the Atlantic oceanic plate as well. The main seismic activity is concentrated in the lower crust and in the mantle wedge, close to the island arc beneath the inner forearc domain. In comparison, little seismicity is observed beneath the outer forearc domain. We propose that the difference of the seismicity beneath the inner and the outer forearc is related to a difference of crustal structure between the inner forearc interpreted as a dense, thick and rigid crustal block and the lighter and more flexible outer forearc. Seismicity is enhanced beneath the inner forearc because it likely increases the vertical stress applied to the subducting plate. At depth, interplate earthquakes observed between 35 and 45 km depth, deeper than the Moho of the forearc (~30 km), possibly reveal the downdip limit of the seismogenic zone. The Thales Scientific Party is composed of: Bayrakci, G., Bécel, A., Charvis, P., Diaz, J., Evain, M., Flueh, E., Gallart, J., Gailler, A., Galve, A., Hello, Y., Hirn, A., Kopp, H., Krabbenhoeft, A., Laigle, M., Lebrun, J. F., Monfret, T., Papenberg, C., Planert, L., Ruiz, M., Sapin, M., Weinzierl, W.

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

2011-12-01

189

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

190

Complex interactions of slow earthquakes in Nankai subduction zone  

NASA Astrophysics Data System (ADS)

In southwest Japan, there exist some types of slow earthquakes along the plate interface of the subducting Philippine Sea plate. ETS (episodic tremor and slip) in the Nankai subduction zone is composed of tremor, deep very-low-frequency (VLF) earthquake, and short-term slow slip event (SSE). ETS is considered as stick-slip and seismic events at small patches triggered by slipping. ETS recurs at interval of a few to several months with duration of several days at each segment. The width of ETS in dip direction ranges from 20 to 50 km. Even in the narrow width, the activity has clear depth-dependent behavior. Minor tremor episodes frequently occur at the deeper part without detection of SSE because of its small size. Some of them migrate updip and grow up to major episode with detectable SSE. Therefore, the recurrence interval of tremor episode becomes longer from downdip to updip. The stress transfer from downdip to updip by frequent occurrence of downdip minor episodes may cause updip major episode. This is considered as interaction within ETS source area. The interaction between tremor and updip long-term SSE is very clear. In Bungo channel, the long-term SSE with duration of 0.5 year occurs every 6 years. The GPS displacement by the long-term SSE of 2003 and 2010 is well correlated to the cumulative number of tremor in the shallower side of the tremor zone. On the other hand, the tremor activity in the deeper part is very stable and seems not to be affected by the occurrence of the long-term SSE. This indicates that the transient slip of long-term SSE causes the stress concentration at the edge of SSE and triggers tremor at adjacent area to the SSE source. Similar tremor triggering was observed in Tokai during acceleration stage of long-term SSE in 2003 and 2004. The long-term SSE triggers not only deep tremor but also shallow VLF earthquake, which has no higher frequency energy and is detected along the Nankai trough. At the beginning of the acceleration of long-term SSE, the shallow VLF earthquakes 100 km away from the estimated fault plane of long-term SSE were activated in 2003 and 2010. We interpret that the shallow VLF earthquake was triggered by the undetectable extended SSE approaching to the shallow VLF source area. We have another example of relationship between slip event and shallow VLF earthquake. In Tokachi-oki area, northern part of Japan, the shallow VLF earthquakes were activated just after the occurrence of 2003 M8-class megathrust earthquake. The shallow VLF earthquakes were located at the peak distribution of afterslip. The time sequence of cumulative number of the shallow VLF earthquakes is very similar to the GPS displacement caused by the afterslip. These observational facts suggest that the shallow VLF earthquake is commonly triggered by transient slip, like as deep tremor. The interactions between different slow earthquakes are important to clarify the frictional property on the plate interface. It might be also important to understand the interaction between slow earthquakes and megathrust earthquake.

Obara, K.

2011-12-01

191

Deformation Patterns and Subduction Behavior of Continental Lithosphere Entering a Trench  

Microsoft Academic Search

We perform 2-D numerical simulations of continental lithosphere entering a subduction zone, to better understand deformation patterns resulting from subduction of a continental margin. The model consists of a subduction zone in which an attached slab drives subduction of a passive continental margin beneath an oceanic plate. A particle-based 2-D visco-elasto-plastic thermo-mechanical finite element code is employed to study the

C. E. Steedman; B. J. Kaus; T. W. Becker; D. Okaya

2007-01-01

192

Tectonometamorphic Evolution of Continental Crust During Ultrahigh-Pressure Metamorphism and Exhumation: Implications For Flow In Mature Collisional Orogens  

NASA Astrophysics Data System (ADS)

Numerical models that account for the exhumation of high-pressure rocks in collisional orogens describe expulsion of buoyant (U)HP slices along the subduction interface; this process may also be linked to channel flow within higher levels of the orogenic wedge as the exhuming bolus enters the mid to lower crust. Both of these scenarios rely on a viscosity and/or negative density gradient between rocks in the channel and surrounding material to drive upward movement; such gradients are thought to result from syn-tectonic partial melting and metamorphic transformation. Strain and thermal weakening is also required to initiate detachment of the (U)HP crust from its downgoing substrate; upward flow is then typically accommodated between a coeval basal thrust and overlying normal-sense shear zone. Geologically, these processes should result in mixing and inversion of structural and metamorphic features, pervasive strain throughout exhumed (U)HP terranes, and perhaps extensive transformation to high-pressure assemblages. The metamorphic and deformation history of mid- to lower crustal sections that contain exhumed (U)HP rocks can yield constraints on the various defining characteristics of channel flow mechanisms. The Scandinavian Caledonides are an example of a continental collision where, in a deeply subducted part of western Norway known as the Western Gneiss Region (WGR), large (>30,000 sq. km), imbricate sheets containing scattered inclusions of eclogite-facies rocks have been exhumed between lower-pressure units. In the Nordfjord area of the southern WGR, early thrusting established a tectonostratigraphy that was subducted in entirety to UHP depths. Limited strain, or transformation to eclogite-facies assemblages, occurred during metamorphism at peak conditions. Exhumation was at least partially achieved by an overlying extensional detachment(s), but no evidence exists of a coeval basal thrust unless one lies buried beneath the current erosion surface. A coherent metamorphic transition from high-P amphibolite facies to coesite-eclogite facies that crosses tectonostratigraphy is retained, and bulk deformation during exhumation is consistent with extensional kinematics only. The overall extent of partial melting is also low. Apparently then, large tracts of ultrahigh-pressure crust can remain relatively untransformed and coherent during exhumation.

Young, D. J.; Kylander-Clark, A. R.; Hacker, B. R.

2011-12-01

193

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

194

Triassic high-pressure metamorphism in the Huwan shear zone: Tracking the initial subduction of continental crust in the whole Dabie orogen  

NASA Astrophysics Data System (ADS)

LA-(MC)-ICPMS trace element, U-Pb and Hf isotope analyses were performed on zircons from continental-derived retrograde high-pressure metamorphic rocks, which were attested by the occurrence of HP mineral inclusions in metamorphic zircons, from the Huwan shear zone in the western Dabie orogen. Zircons in a schist sample are mostly detrital and gave U-Pb ages from 1212 to 2312 Ma, suggesting that the detritus were derived from multiple sources or suffered partial Pb-loss during subsequent metamorphism. Magmatic zircons in an amphibolite sample yielded an upper intercept U-Pb age of 1806 ± 63 Ma, which was interpreted as its protolith formation age. The zircons in the amphibolite show a weighted ?Hf (t) value of - 16.3, and a weighted Hf model age of 3455 Ma, suggesting that the protolith was derived from reworking of ancient continental materials. Metamorphic zircons in a quartz-rich leucosome sample and the amphibolite sample gave weighted mean 206Pb/238U ages of 241 ± 1 and 243 ± 4 Ma, respectively. These metamorphic zircons contain mineral inclusions of garnet and omphacite and are characterized by relatively flat HREE patterns with slight negative Eu anomalies. This indicates that they formed in the presence of garnet but the absence of plagioclase, and thus under eclogite facies conditions. The U-Pb age of ca. 243 Ma is consistent with eclogite-facies metamorphic ages of the juvenile oceanic crust in the western Dabie orogen, suggesting that the initial continental subduction for high-pressure metamorphism of the whole Dabie orogen can be traced back to ca. 243 Ma. In addition, the western Dabie HP-UHP orogenic belt might have suffered multi-slice subduction and exhumation during the Triassic.

Liu, Xiaochi; Wu, Yuanbao; Gao, Shan; Peng, Min; Wang, Jing; Wang, Hao; Gong, Hujun; Yuan, Honglin

2012-04-01

195

Shallow depth of seismogenic coupling in southern Mexico: implications for the maximum size of earthquakes in the subduction zone  

Microsoft Academic Search

Studies of locally recorded microearthquakes and the centroidal depths of the largest earthquakes analyzed using teleseismic data show that the maximum depth of thrust faulting along the Mexican subduction zone is anomalously shallow. This observed maximum depth of about 25 ± 5 km is about half of that observed in most subduction zones of the world. A leveling line that

Gerardo Suárez; Osvaldo Sánchez

1996-01-01

196

Kinematic Coupling Along the Mexican Subduction Zone From GPS Data  

Microsoft Academic Search

Kinematic coupling (alpha) between the subducting Cocos and Rivera oceanic plates, and the continental North America plate is estimated as a ratio between the back slip on the fault plane and the convergence rate (NUVEL1A model). To obtain a distribution of the back slip (\\

S. Franco; V. Kostoglodov; A. Iglesias; S. Singh

2007-01-01

197

Imaging of Upwelling Fluids and Partial Melt in the Subduction Zone with 3D Regional Electrical Resistivity Structure by the Network-MT Data  

NASA Astrophysics Data System (ADS)

Subduction zones are where oceanic plate and seawater (aqueous fluids) return to Earth's mantle and the fluids released into the mantle from the downgoing slab as a consequence of metamorphic reactions [e.g., Tatsumi, 1989]. Such the fluids trigger partial melt of upper mantle and crust. As a result, igneous activity forms volcanoes as typical surface expressions in subduction zones because the partial melt rises owing to its lower density. The island of Kyushu in the Southwestern Japan is a typical high angle subduction zone, at which the hot Shikoku basin (15-27 Ma) and the cold Philippine Sea plate (45-55 Ma) subduct beneath the Eurasian plate. Kyushu can be separated into three parts; northern Kyushu, central Kyushu and southern Kyushu, and there exist many quaternary active volcanoes, as the Aso and Kuju volcanoes in the northern part and Kirishima and Sakurajima volcanoes in the southern part, with defining a volcanic front. Moreover, the central Kyushu is devoid of active volcanoes, where it is considered that the buoyant Kyushu-Palau Ridge subducts. It is important to investigate structure beneath Kyushu for understanding the volcanic formation. In the Kyushu district, the Network-Magnetotelluric (MT) observations were carried out from 1993 to 1998 to cover the whole island of Kyushu. The Network-MT method employs metallic wires in a commercial telephone network to measure the electric potential difference with a dipole length of ten to several tens of kilometers. The Network-MT data provide valuable information on fluid and partial melt (magma) generation because the electromagnetic soundings are highly sensitive to the presence of a few percent of interconnected fluids (aqueous and/or melt). We analyzed the Network-MT data sets, which have geoelectromagnetic information from the crust to upper mantle, in order to determine regional scale electrical resistivity structure. We applied three-dimensional (3D) inversion analyses using the WSINV3DMT inversion code of the version for the Network-MT impedance responses [Uyeshima et al., 2008]. Two remarkable features are found that a conductive block exists beneath the volcano of which the bottom extends to the backarc side and the forearc side including the Philippine Sea plate is resistive. Moreover, the resistive region distributes along the hinge line of the subducting plate as imaged by seismicity. The former conductor is thought to represent fluids released from the slab or partial melt related to the fluid, constituting a magma source for subduction zone volcanoes. In this presentation, we would like to explain details on the 3D resistivity structure related to the subducting Philippine Sea plate and the active volcanoes.

Hata, M.; Oshiman, N.; Yoshimura, R.; Tanaka, Y.; Uyeshima, M.

2012-04-01

198

Modeling Open System Metamorphic Devolatilization of Subducted Metabasalts  

Microsoft Academic Search

Fluids derived from the devolatilization of a subducting slab play a critical role in the melting of the mantle wedge and consequent arc volcanism. In this study we present results from thermodynamic modeling of subduction zone metamorphic devolatilization of metabasalts for pressures up to 6 GPa using an approach which considers fluid fractionation from source lithologies and infiltration from subjacent

P. Gorman; D. M. Kerrick; J. A. Connolly

2004-01-01

199

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

200

Effect of a retreating subduction zone on deformation in simple regions of plate convergence  

Microsoft Academic Search

Plane strain, two-dimensional, finite element models are used to calculate deformation patterns within model mountain belt systems where subduction zone retreat (the process by which space is created between the subducting (``pro'') and overriding (``retro'') lithospheres either by motion of one or both) is active. The model uses Coulomb plastic and thermally activated power law viscous rheologies. Overall deformation is

Paula Waschbusch; Christopher Beaumont

1996-01-01

201

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

Microsoft Academic Search

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

Anthony Lambert; Honn Kao; Garry Rogers; Nicholas Courtier

2009-01-01

202

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

Microsoft Academic Search

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

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

2004-01-01

203

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

Microsoft Academic Search

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

C. Jurney

2002-01-01

204

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

Microsoft Academic Search

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

Stephane Mazzotti; John Adams

2004-01-01

205

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

Microsoft Academic Search

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

Anthony Lambert; Honn Kao; Garry Rogers; Nicholas Courtier

2009-01-01

206

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

Microsoft Academic Search

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

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

2001-01-01

207

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

Microsoft Academic Search

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

Ian Hutchinson; Alan D. McMillan

1997-01-01

208

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

Microsoft Academic Search

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

Chris Goldfinger; Lisa C. McNeill; Cheryl Hummon

1997-01-01

209

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

Microsoft Academic Search

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

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

2008-01-01

210

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

Microsoft Academic Search

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

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

2007-01-01

211

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

Microsoft Academic Search

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

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

2008-01-01

212

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

Microsoft Academic Search

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

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

2004-01-01

213

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

Microsoft Academic Search

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

H. Dragert; K. Wang; G. Rogers

2004-01-01

214

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

Microsoft Academic Search

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

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

2008-01-01

215

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

Microsoft Academic Search

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

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

2007-01-01

216

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

Microsoft Academic Search

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

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

2004-01-01

217

Structure of the Hellenic Subduction Zone from Gravity Gradient Functions and Seismology  

NASA Astrophysics Data System (ADS)

The Normalized Full Gradient (NFG) method has widespread applications in the analysis of potential fields, especially the gravity and magnetic fields. This method is used to identify the lateral and horizontal density variations in the crust and lithosphere. In this study, the NFG method was applied to the gravity data of the Cretan Arc and its surroundings. Because of the tectonic features of the eastern Mediterranean, the Cretan Arc and the neighboring areas are seismically very active. Especially the subduction zone and the complicated crustal features have been defined applying many different geophysical methods. In this study, first the NFG method is tested with synthetic prisms (two cubes). After that, the NFG method was applied to the Bouguer gravity data of the Cretan Arc and its subduction zone (Hellenic subduction zone) and Hellenic subduction zone was defined with the foci depth data (USGS) along the south-north direction. Thus, geometry of the focal depth distribution has been created to determine probable media depths and their localizations. According to the NFG results, vertical structural transitions were observed at a depth ranging between 10 and 180 km. Also, these results were compared with the foci depth model and the other results of the related publications. Finally, some considerations in vertical solution with the NFG method have been presented and locations of the different structures at horizontally have been defined with application of the NFG method.

Gönenç, Tolga; Akgün, Mustafa

2012-07-01

218

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

NASA Astrophysics Data System (ADS)

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

Noguchi, Shinako; Maeda, Takuto; Furumura, Takashi

2013-01-01

219

Seismic attenuation and mantle wedge temperatures in the Alaska subduction zone  

Microsoft Academic Search

Anelastic loss of seismic wave energy, or seismic attenuation (1\\/Q), provides a proxy for temperature under certain conditions. The Q structure of the upper mantle beneath central Alaska is imaged here at high resolution, an active subduction zone where arc volcanism is absent, to investigate mantle thermal structure. The recent BEAAR experiment provides the first dense broadband seismic coverage of

Joshua C. Stachnik; Geoffrey A. Abers; Douglas H. Christensen

2004-01-01

220

Seismic attenuation and mantle wedge temperatures in the Alaska subduction zone  

Microsoft Academic Search

Anelastic loss of seismic wave energy, or seismic attenuation (1\\/Q), provides a proxy for temperature under certain conditions. The Q structure of the upper mantle beneath central Alaska is imaged here at high resolution, an active subduction zone where arc volcanism is absent, to investigate mantle thermal structure. The recent Broadband Experiment Across the Alaska Range (BEAAR) provides the first

Joshua C. Stachnik; Geoffrey A. Abers; Douglas H. Christensen

2004-01-01

221

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

Microsoft Academic Search

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

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

2006-01-01

222

Thermal and Hydraulic Controls on Serpentinization at the Outer Rise of Subduction Zones  

Microsoft Academic Search

Normal faulting that takes place during plate bending at the outer rise of subduction zones may provide pathways for fluids to pass through the crust, allowing serpentinization of the surrounding rocks. High resolution sonar images of the ocean floor, seismic reflection profiles, and normal faulting earthquakes imply that these faults may penetrate into the lithosphere at depths up to tens

G. S. Atalan; E. Screaton

2010-01-01

223

The Subduction Zone Flow Field from Seismic Anisotropy: A Global View  

Microsoft Academic Search

Although the morphologies of subducting slabs have been relatively well characterized, the character of the mantle flow field that accompanies subduction remains poorly understood. To analyze this pattern of flow, we compiled observations of seismic anisotropy, as manifested by shear wave splitting. Data from 13 subduction zones reveal systematic variations in both mantle-wedge and subslab anisotropy with the magnitude of

Maureen D. Long; Paul G. Silver

2008-01-01

224

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

NSDL National Science Digital Library

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

225

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

226

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

Microsoft Academic Search

We found that repeated slow slip events observed on the deeper interface of the northern Cascadia subduction zone, which were at first thought to be silent, have unique nonearthquake seismic signatures. Tremorlike seismic signals were found to correlate temporally and spatially with slip events identified from crustal motion data spanning the past 6 years. During the period between slips, tremor

Garry Rogers; Herb Dragert

2003-01-01

227

The time constant variations of slow slip events in the south Alaska subduction zone  

Microsoft Academic Search

Slow Slip events (SSEs), episodic periods of slip on the plate interface that are very slow in comparison to earthquakes, but much faster steady plate motions, have been identified at several subduction zones. One characteristic shared by all of these events is that they occur near or downdip of the base of the seismogenic zone as defined by the slip

Y. Ohta; J. T. Freymueller; S. Miura

2007-01-01

228

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

229

Comparison of slip distribution of large slow slip events in Guerrero subduction zone  

Microsoft Academic Search

Aseismic slow slip events (SSEs) have been reported in most of the well geodetically instrumented subduction zones worldwide (Japan, Cascadia, Mexico, New Zealand, Costa Rica, Alaska). For most of the observed SSEs, the slip distribution on the subduction interface was inferred from the surface GPS displacements to be located at the downdip extension of the seismogenic zone, in the conditionally

F. Cotton; M. Vergnolle; O. Thollon; M. Campillo; I. Manighetti; N. Cotte; A. Walpersdorf; V. Kostoglodov

2008-01-01

230

Regional Wave Propagation from Mexican Subduction Zone Earthquakes: The Attenuation Functions for Interplate and Inslab Events  

Microsoft Academic Search

The seismic waves from subduction zone earthquakes are significantly affected by the presence of 3D variation in crust and upper-mantle structure around the source area. These heterogeneous structures also profoundly modify the character of seismic waves as they propagate from the source area to regional distances. This is illustrated by studying shallow, interplate earthquakes along the Mexican subduc- tion zone,

T. Furumura; S. K. Singh

2002-01-01

231

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

Microsoft Academic Search

Patterns of faulting in the northern Chilean forearc are consistent with modeled stress fields resulting from the subduction zone earthquake cycle. We define positive Coulomb stress change as encouraging normal faulting motion on steeply-dipping planes striking approximately parallel to the plate boundary, as shown by fault kinematic data collected in the field. Simulations show that coastal regions experience positive Coulomb

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

2006-01-01

232

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

Microsoft Academic Search

To address the problem of the great variability of the mechanical state of subduction zones, we investigate the mechanics of back arc spreading and seismic decoupling. Back arc spreading is assumed to be due to rifting of the upper plate and hence occurs when trench-normal tension reaches a critical value. Seismic decoupling is assumed to occur when the normal stress

C. H. Scholz; J. Campos

1995-01-01

233

Overriding plate thinning in subduction zones: Localized convection induced by slab dehydration  

Microsoft Academic Search

In subduction zones, many observations indicate that the backarc thermal state is particularly hot and that the upper lithosphere is thin, even if no recent extension episode has occurred. This might result from free thermal convection favored by low viscosities in the hydrated mantle wedge. We perform 2-D numerical experiments of the convective mantle wedge interaction with both the downgoing

D. Arcay; M.-P. Doin; E. Tric; R. Bousquet; C. de Capitani

2006-01-01

234

Geodetic, teleseismic, and strong motion constraints on slip from recent southern Peru subduction zone earthquakes  

Microsoft Academic Search

We use seismic and geodetic data both jointly and separately to constrain coseismic slip from the 12 November 1996 Mw 7.7 and 23 June 2001 Mw 8.5 southern Peru subduction zone earthquakes, as well as two large aftershocks following the 2001 earthquake on 26 June and 7 July 2001. We use all available data in our inversions: GPS, interferometric synthetic

M. E. Pritchard; E. O. Norabuena; C. Ji; R. Boroschek; D. Comte; M. Simons; T. H. Dixon; P. A. Rosen

2007-01-01

235

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

Microsoft Academic Search

Subduction zones are one of the most important components of the Earth's plate tectonic system. Knowing the detailed seismic velocity structure within and around subducting slabs is vital to understand the constitution of the slab, the cause of intermediate depth earthquakes inside the slab, the fluid distribution and recycling, and tremor occurrence [Hacker et al., 2001; Obara, 2002].Thanks to the

H. Zhang; C. H. Thurber

2005-01-01

236

Geologic Evidence of Earthquakes and Tsunamis in the Mexican Subduction zone - Guerrero  

Microsoft Academic Search

A study of large historic and prehistoric earthquakes and their tsunamis using a multiproxy approach (geomorphic features, sediment deposits, microfossils, sediment geochemistry and more recently the use of magnetic properties) has provided valuable information in the assessment of earthquake and tsunami record. The Pacific coast of Mexico is located over the active subduction zone (~1000 km) that has experienced numerous

M. Ramirez-Herrera; M. Lagos; I. Hutchinson; A. Ruiz-Fernández; M. Machain; M. Caballero; V. Rangel; H. Nava; N. Corona; F. Bautista; V. Kostoglodov; A. Goguitchaichrili; J. Morales; P. Quintana

2010-01-01

237

Great thrust earthquakes and aseismic slip along the plate boundary of the Makran subduction zone  

Microsoft Academic Search

The Makran subduction zone of Iran and Pakistan exhibits strong variation in seismicity between its eastern and western segments and has one of the world's largest forearcs. We determine the source parameters for 14 earthquakes at Makran including the great (Mw 8.1) earthquake of 1945 (the only instrumentally recorded great earthquake at Makran); we determine the loci of seismic and

Daniel E. Byrne; Lynn R. Sykes; Dan M. Davis

1992-01-01

238

Plate bending at subduction zones: Consequences for the direction of plate motions  

Microsoft Academic Search

Bending of lithospheric plates at subduction zones is thought to be an important source of dissipation for convection in the Earth's mantle. However, the influence of bending on plate motion is uncertain. Here we use a variational description of mantle convection to show that bending strongly affects the direction of plate motion. Subduction of slabs and subsidence of oceanic lithosphere

Bruce A. Buffett; David B. Rowley

2006-01-01

239

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

Microsoft Academic Search

With ca. forty thousand kilometers of subduction zones and convergence rates from 30 km Ma-1 to 180 km Ma-1, subduction carries massive amounts of material into seafloor trenches, and beyond. Most of the subducting plate is made of mantle material returning to the depths from which it originated. The hydrated and altered upper oceanic section and the overlying sediments, however,

J. D. Morris; J. G. Ryan

2003-01-01

240

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

NASA Astrophysics Data System (ADS)

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

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

2010-12-01

241

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

NASA Astrophysics Data System (ADS)

Traditionally studies of plate bending in subduction zones have utilized elastic, viscous or elastic-plastic rheologies to model the deformation of subducting plates, yet they are based on averaged plate properties and do not take into account variations in plate strength. Direct measurements of plate strength at subduction zones could permit more detailed models of how plates deform during subduction and may allow differentiation between the elastic and viscous or plastic rheologies. Additionally, weakening of the subducting plate is important for understanding the degree of coupling of the surface plate to the negative buoyancy of descending slabs. To obtain quantitative measurements of changes in plate strength along profiles parallel to the trench, we use analysis of the gravity-topography admittance in three subduction zones: Tonga, Costa Rica and Japan. We show that the plate flexural rigidity decreases near and inside the trench of the Tonga and Japan subduction zones, in agreement with previous results for the Kermadec subduction zone (1). Near the trench the flexural rigidity values are consistently smaller than those predicted by an elastic rheology and the plate age (2). This degree of weakening, by up to 3 orders magnitude, suggests that the plate does not act elastically as it is subducted, possibly due to lithospheric-scale weakening by extensional faulting and plastic yielding at depth. In contrast lithospheric-scale weakening in the Costa Rica subduction zone is less clear. This may be due to the younger age of the subducting plate and the small age difference between the seamounts and surrounding plate, which limits the sensitivity of the gravity field to changes in the non-isostatic support of topographic feature. These results suggest that this technique is only applicable to older plates with large seamounts that are appreciably younger than the subducting plate. Comparison of the flexural rigidity results to the tectonic characteristics of all three subduction zones suggest that plate age (thickness) has a strong affect on the length scale and degree of weakening of the subducting plate. References: 1. Billen, M. I., Gurnis, M. 2005. Constraints on subducting plate strength within the Kermadec Trench. Journal of Geophysical Research, 110(B05407). 2. Caldwell, J. G., Turcotte, D. L. 1979. Dependence of the thickness of the elastic oceanic lithosphere on age. J. of Geophys. Res., 84(B13):7572-7576, December 1979.

Arredondo, K.; Billen, M. I.

2010-12-01

242

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

243

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

244

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

NASA Astrophysics Data System (ADS)

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 incoming oceanic Cocos and the overriding Caribbean plate. The study was carried out in the framework of large scale collaborative multidisciplinary research program (SFB574 - ‘Volatiles and Fluids in Subduction Zones’). We will present the marine data and its processing, show final modeling results and their interpretation. The MT data set revealed a very low electrically conducting oceanic lithosphere entering the subduction zone. Upon entering the subduction, near the trench, the oceanic lithosphere undergoes a moderate enhancement in conductivity down to deep crustal and upper mantle regions, which may be attributed to hydration caused by water penetrating along bending related faults and cracks. A first accumulation of free water caused by subsequent dewatering reactions is observed in the forearc sediments above the décollement updip of the seismogenic zone. Further inland, a well conducting zone in the overriding plate, at about 20-30 km depth, may be interpreted as a fluid trap, while a conductive zone underneath, at about 120 km depth, might hint at dehydration of crust and deserpentinization of mantle. The feature in 20-30 km depth is observed in land MT measurements on subduction zones all over the world in comparable distances to the volcanic arc, and thus seems to be a robust feature, but was yet not recognized as such. Such fluid accumulation might explain the general observed deficit between water input and output in a subduction cycle.

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

2009-12-01

245

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

NASA Astrophysics Data System (ADS)

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

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

2010-12-01

246

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

NASA Astrophysics Data System (ADS)

In general, the vast majority of global seismicity occurs within the upper 60 km of subduction zones. However, focused examination of earthquake locations within any given subduction zone shows a complex distribution, with high concentrations of seismicity adjacent to relatively quiet zones. In addition, there is a wide spectrum of earthquake rupture characteristics found in subduction zones, including complex slip patterns featuring multiple asperities and slip over a range of time periods. Features such as subducting bathymetry, characteristics of the incoming plate, and forearc structure can all impact locations and rupture characteristics of the shallow earthquakes. Here we review observations of along-strike variations of seismicity distributions and rupture characteristics in several subduction zones around the Pacific and discuss possible factors in causing these lateral transitions. For instance, in regions such as Central and South America and the western Pacific margins of Japan and Kuriles, along-strike changes in the subducting plate, such as temperature, plate origins, presence of ridges or fracture zones, produce along strike variations in the location and slip distribution of earthquakes. Individual earthquakes in Costa Rica, Peru, and Chile were likely influenced by the past subduction of seamounts, ridges, and fracture zones, although rupture histories determined for these events suggest these subducted features act as either rupture asperities or barriers. Small-scale variations are also observed, such as factor of 2 variations in earthquake apparent stress within a roughly 100-150 km segment along the Nicoya Peninsula in Costa Rica that correspond to variations in temperature, geodetically determined plate coupling, and past earthquake history. Along the Alaska-Aleutian subduction zone, lateral variation within forearc blocks appears to impart along-strike variations in the seismicity.

Bilek, S. L.; Stankova-Pursley, J.

2009-12-01

247

Measuring surface deformation in subduction zones with InSAR: Examples from South America and Cascadia  

NASA Astrophysics Data System (ADS)

Ground displacements in subduction zones provide several important constraints: the location and magnitude of fault slip on the megathrust during large earthquakes and slow slip events, as well as the nature and extent of inter-seismic, post-seismic, and even pre-seismic surface deformation. The development of satellite Interferometric Synthetic Aperture Radar (InSAR) has allowed such ground displacements to be measured in many areas without dense arrays of continuously recording instruments on the ground, such as South America and Indonesia. Even where such arrays exist (like in Cascadia), InSAR observations can complement ground observations with increased spatial coverage or by helping to reconstruct the full three-dimensional deformation field. We will demonstrate the capabilities of InSAR in subduction zones with examples of co- seismic, post-seismic, and inter-seismic ground displacement in the Peru-Chile subduction zone including nine subduction zone earthquakes (6.7 < Mw < 8.5). Because of the arid climate in southern Peru and northern Chile, conventional C-band (5.6 cm radar wavelength) InSAR is successful, although like all geodetic methods, InSAR can be effected by changes in the refractive properties of the troposphere and ionosphere. We demonstrate that these atmospheric effects and orbital errors must be removed or accounted for when measuring small amplitude deformation over large spatial scales. In other subduction zones with more vegetation ground cover, L-band (23 cm wavelength) InSAR is successful, but observations are infrequent and the available data starts in 2006. To develop a longer time series (starting in 1992), we will demonstrate the potential of persistent scatterer C-band InSAR to reveal surface deformation in vegetated areas like Cascadia.

Pritchard, M. E.; Lohman, R. B.; Fournier, T. J.; Holtkamp, S. G.

2009-05-01

248

The Cycle of Hydration and Fluid Release in the Costa Rican Subduction Zone imaged through electromagnetic soundings: Where has all the water gone? (Invited)  

Microsoft Academic Search

Fluids entering the subduction zone play an important role. They determine the onset of melting, weakening and changes in the dynamics and thermal structure of subduction zones and trigger earthquakes when being released from the subducting plate. However, the amount of water carried into the subduction zone and its distribution are not well constrained by existing data and are subject

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

2010-01-01

249

Across Arc Variation in Basaltic fO2: Influence of a Subduction Component in the Cascadia Subduction Zone  

NASA Astrophysics Data System (ADS)

Oxidation of the subarc mantle in subduction zones can greatly affect mineral phase equilibria, the speciation of volatiles, and the transfer of multivalent elements in basaltic magmas. While peridotite xenoliths provide the most direct approach to measuring mantle oxidation states, such xenoliths in continental arcs are rare. In this investigation, we applied an alternative method, the determination of sulfur speciation in olivine-hosted melt inclusions and chromite-olivine oxygen barometry. We present a first attempt to spatially correlate oxygen fugacity relative to the subduction zone in a continental arc. The overall range in oxygen fugacity, based on sulfur speciation measurements, is from <-0.25 log units to +1.9 log units (?FMQ). Sulfur oxidation and the concentration of fluid-mobile trace elements both generally increase from backarc to forearc. This correlation is interpreted to reflect a progressively greater proportion of fluid-rich, oxidized subduction component closer to the trench. Estimates of the amount of subduction component (up to ~6 wt%) required to generate the geochemical diversity based on flux melt modeling correlate with oxygen fugacity, with the exception of calc-alkaline basalts, with high oxygen fugacity and greater proportion subduction component closer to the trench. Two other important observations to come from the flux melt modeling are 1) as basalt oxygen fugacity increases, calculated mantle temperature decreases, and 2) shoshonitic basalts require a depleted mantle source, distinct from the more enriched mantle source of the low-K tholeiite, calc-alkaline and ocean island-like magmas. The potential mantle source for shoshonitic basalts has a predicted oxygen fugacity from +0.3 to +2.4 log units (?FMQ) while the mantle source for low-K tholeiite, calc-alkaline and ocean island-like basalts may range from -1.1 to +0.7 log units (?FMQ), consistent with estimates of oxidation state for oceanic lithosphere. Therefore, despite the volatile and fluid-mobile trace element enrichment, the subarc mantle need not be significantly oxidized relative to unmodified oceanic lithosphere to generate the diversity in Cascade arc basaltic magmas.

Rowe, M. C.; Kent, A. J.; Nielsen, R. L.

2007-12-01

250

Water release and rock volume change associated with smectite dehydration in the < 30 km depth seismcity of subduction zones  

NASA Astrophysics Data System (ADS)

Volatiles play important roles in the dynamics of subduction systems from the shallowest levels of the plate interface to the greatest depths of arc melting source region. Near the trench, the release of water during the breakdown of smectite into illite is often proposed to be responsible for the elevated pore pressure associated with low frequency earthquakes. The thermal breakdown of smectite in subduction zones is generally assumed to occur at 100-150°C, as generally observed in sedimentary basins. However, the upper thermal stability of smectite depends on many factors such as the availability and activity of pore water, which might be higher in the context of subduction than in burial diagenesis. Smectite is also observed to crystallize up to 300°C in geothermal environment. Various experiments have shown that the dehydration of smectite is a stepwise process that occurs at T up to 550°C at P > 3 kbar. At each dehydration step, changes of volume can be as high as 30% and a release of free water of about 150 kgH2O/m3 smectite are observed. The consequences of the smectite dehydration might be thus extremely important at shallow to moderate depth subduction zones, if this mineral remains present at T > 100°C. We have constrained the first macroscopic thermodynamic model that reproduces i) the experimentally observed 3 -> 2 -> 1 -> 0 water-layer transitions during dehydration and the associated volume changes as a function of the nature of interlayer cation, water activity, pressure and temperature, and ii) the stability and compatibility relations of smectite with other minerals at high temperature and pressure condition. This model is used to predict the variations of the solid volume of the subducting sediments and the amount of water expelled from the conditions of early diagenesis to metamorphism. Large, fast and abrupt changes are predicted to occur up to 250°C. This result suggests a possible role of smectite dehydration in the production of high and local fluid pressure that might trigger the < 30 km seismicity, before its breakdown into mica and chlorite.

Vidal, O.; Dubacq, B.

2010-12-01

251

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

252

Cenozoic volcanic rocks of North Kamchatka: In search of subduction zones  

NASA Astrophysics Data System (ADS)

Two belts of subaerial volcanic rocks—the Eocene Kinkil belt and the Neogene belt of the Sredinny Range—extend along the Kamchatka Isthmus. It is suggested that their formation is related to subduction of the oceanic lithosphere beneath the continental margin of North Kamchatka. The oceanic lithosphere consumed in the subduction zones could have been formed as a result of active spreading in the Komandorsky Basin. In the simplest case, both spreading and subduction reflect the northwestward motion of the lithosphere of the Komandorsky Plate relative to Kamchatka, the Shirshov Ridge, and the Aleutian Basin combined into one relatively immobile plate conventionally called the North American Plate. The authors perform a simulation of conjugate spreading and subduction. The most important parameter determining the regional geodynamics—the velocity of the Komandorsky Plate moving relative to the North American Plate—is taken as 2.5, 5.0, and 7.5 cm/yr. The calculated ages of the onset and end of volcanic activity in the aforementioned belts are compared with the dates obtained with the isotopic and paleontological methods. For the Eocene Kinkil belt, where volcanism started 44 Ma ago, the model age of the onset of subduction depends on the accepted velocity of the motion of the Komandorsky Plate and varies from 54 Ma at the velocity of 2.5 cm/yr to 47.5 Ma at the velocity of 7.5 cm/yr. It can be assumed that the model of fast subduction in this age interval is most consistent with the geological data. For the Miocene-Pliocene belt of the Sredinny Range, assuming the velocity of the motion of the Komandorsky Plate at 5.0 and 7.5 cm/yr, multiple rifting at the boundary with the Shirshov Ridge should be assumed. Therefore, for the end of the Neogene, a model with low velocity (2.5-5.0 cm/yr, i.e., about 4.0 cm/yr) is preferable.

Shapiro, M. N.; Solov'ev, A. V.

2011-05-01

253

Hf-Nd Isotope Geochemistry of Contrasting Global Sedimentary Columns in Subduction Zones  

NASA Astrophysics Data System (ADS)

Most terrestrial Hf-Nd isotopic compositions lie along a positively correlated Hf-Nd array [1]. However, some abyssal sediments (e.g., ferromanganese nodules, crusts, and metalliferous clays; [1, 2]) have strongly decoupled Hf-Nd isotopic compositions. In order to evaluate the range of Hf-Nd isotopic compositions in subducting sediments on a global scale, we have selected contrasting drill sites outboard of active subduction systems. The selected sections differ in characteristics such as age, thickness and type of sediment, convergence rate and accretionary prism development. An example of one endmember is the metalliferous clay-rich Tonga section (DSDP sites 595 and 596). These compositions show large deviations from the Hf-Nd array by having a higher ? Hf relative to ? Nd, and have highly elevated REE concentrations and Lu/Hf ratios compared to normal terrigenous samples. These systems have moderately negative Hf and Nd isotopic compositions that are (at least in the case of Nd) similar to seawater. This type of signature may be seen in other slowly accumulating systems dominated by metalliferous sediments, such as the Philippine (DSDP site 291) and Ryukyu (DSDP site 294/295) sections. The Central America (ODP site 844 and DSDP site 495) subduction system has a bimodal sediment input, consisting of carbonates overlain by hemipelagic diatomaceous muds. The muds are dominated by volcanic arc input and show little variation on the Hf-Nd array (? Hf +10 to +11 and ? Nd +2 to +6). Volumetrically the largest flux of sediments into subduction zones worldwide, however, are terrigenous sediments derived from active continental margins. An example of this is the Cascadia system (DSDP site 174). The Cascadia sediments all plot on the crustal portion of the Hf-Nd array, but show large variation (? Hf -6.5 to -20 and ? Nd -5.5 to -14) reflecting changing sediment sources. Nd and Hf model ages for these sediments range from 0.4 Ga to 1.3 Ga. These negative epsilon values and dominantly Precambrian model ages conflict with exposed crust in the Pacific NW proximal to site 174. Instead, these sediments may be the result of Pleistocene glaciation with pulses of catastrophic flooding, which brought older cratonic material from distal sources to the site. This illustrates that Hf-Nd isotopic composition is useful not only for applications such as mantle composition and arc petrogenesis, but also for sediment provenance. [1] Vervoort et al., 1999, EPSL, Vol 168: 79-99. [2] Albarède et al., 1998, GRL, Vol 25: 3895-3898. [3] Vervoort and Plank, 2002, EOS, Vol 83.

Prytulak, J.; Vervoort, J.; Plank, T.

2003-12-01

254

Subduction zone geometry and stress transfer during megathrust earthquakes, upper Cook Inlet basin, Alaska  

NASA Astrophysics Data System (ADS)

Field observations and elastic and viscoelastic dislocation models of the 1964 great Alaska earthquake, Mw 9.2, demonstrate how complex subduction zone geometry affects static and time-dependent stress transfer from megathrust ruptures to upper plate faults. A three-dimensional elastic dislocation model of the 1964 earthquake uses triangular subfaults to capture sharp changes in trend and spacing of Benioff contours that define the top of the subducting slab as it transitions from steep subduction of the Pacific Plate (PP) to shallow subduction of the Yakutat terrane (YT), an allochthonous continental fragment. The model demonstrates that slab geometry strongly influences coseismic stress transfer into the upper plate, ascertaining that the anomalous upper Cook Inlet stress field (maximum horizontal stress oriented ˜45° counterclockwise from subducting plate motion), is partially due to slip on the complex subduction interface. The model further ascertains that slip on the complex interface causes marked variations in coseismic stresses transferred to the Castle Mountain fault (CMF) which shows marked along-strike variability in seismogenic behavior and straddles the underlying edge of the YT. The western CMF is the only upper plate fault with unequivocal Holocene surface rupture in the greater Anchorage area. Herein, a previously unrecognized offset margin of a postglacial outwash channel is identified and used to establish a right lateral slip rate of 3.0+/-0.6 mm yr-1 and a potential rupture magnitude of 6.9-7.3 for that fault segment. A time-dependent viscoelastic Maxwell rheological model of the 1964 earthquake suggests a time-delayed rotation of principal stresses from a reverse to a strike-slip faulting regime along the CMF. The delay may indicate a temporal link between megathrust earthquakes and CMF rupture. The time-dependent model further suggest that megathrust-induced changes in shear and normal stresses, and thus in rupture potential, on the Castle Mountain fault and on transpressional faults of upper Cook Inlet basin will peak ˜60 years post-1964. The transpressional faults core hydrocarbon-producing anticlines of the Cook Inlet petroleum province. These faults and the western CMF pose a significant seismic hazard to the greater Anchorage area and to petroleum infrastructure of Cook Inlet basin.

Willis, Julie Barrott

255

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

256

Two-dimensional numerical modeling of tectonic and metamorphic histories at active continental margins  

Microsoft Academic Search

The evolution of an active continental margin is simulated in two dimensions, using a finite difference thermomechanical code with half-staggered grid and marker-in-cell technique. The effect of mechanical properties, changing as a function of P and T, assigned to different crustal layers and mantle materials in the simple starting structure is discussed for a set of numerical models. For each

Taras Gerya; Bernhard Stöckhert

2006-01-01

257

Metamorphic growth and recrystallization of zircons in extremely 18O-depleted rocks during eclogite-facies metamorphism: Evidence from U-Pb ages, trace elements, and O-Hf isotopes  

NASA Astrophysics Data System (ADS)

A combined in situ SIMS and LA-(MC)-ICPMS study of U-Pb ages, trace elements, O and Lu-Hf isotopes was conducted for zircon from eclogite-facies metamorphic rocks in the Sulu orogen. The two microbeam techniques sampled various depths of zircon domains, revealing different element and isotope relationships between residual magmatic cores and new metamorphic rims and thus the geochemical architecture of metamorphic zircons which otherwise cannot be recognized by the single microbeam technique. This enables discrimination of metamorphic growth from different subtypes of metamorphic recrystallization. Magmatic cores with U-Pb ages of 769 ± 9 Ma have positive ? 18O values of 0.1-10.1‰, high Th/U and 176Lu/ 177Hf ratios, high REE contents, and steep MREE-HREE patterns with negative Eu anomalies. They are interpreted as crystallizing from positive ? 18O magmas during protolith emplacement. In contrast, newly grown domains have concordant U-Pb ages of 204 ± 4 to 252 ± 7 Ma and show negative ? 18O values of -10.0‰ to -2.2‰, low Th/U and 176Lu/ 177Hf ratios, low REE contents, and flat HREE patterns with weak to no Eu anomalies. They are interpreted as growing from negative ? 18O fluids that were produced by metamorphic dehydration of high-T glacial-hydrothermally altered rocks during continental subduction-zone metamorphism. Differences in ? 18O between different domains within single grains vary from 0.8‰ to 12.5‰, suggesting different degrees of O isotope exchange between the positive ? 18O magmatic core and the negative ? 18O metamorphic fluid during the metamorphism. The magmatic zircons underwent three subtypes of metamorphic recrystallization, depending on their accessibility to negative ? 18O fluids. The zircons recrystallized in solid-state maintained positive ? 18O values, and REE and Lu-Hf isotopes of protolith zircon, but their U-Pb ages are lowered. The zircons recrystallized through dissolution exhibit negative ? 18O values similar to the metamorphic growths, almost completely reset U-Pb ages, and partially reset REE systems. The zircons recrystallized through replacement show variably negative ? 18O values, and partially reset REE, and U-Pb and Lu-Hf isotopic systems. Therefore, this study places robust constraints on the origin of metamorphic zircons in eclogite-facies rocks and provides a methodological framework for linking the different types of metamorphic zircons to petrological processes during continental collision.

Chen, Yi-Xiang; Zheng, Yong-Fei; Chen, Ren-Xu; Zhang, Shao-Bing; Li, Qiuli; Dai, Mengning; Chen, Lu

2011-09-01

258

Mesoproterozoic High-Pressure Metamorphism in the Llano Uplift, Central Texas, USA  

NASA Astrophysics Data System (ADS)

High-pressure (HP) terranes of Precambrian age are rare, and their rarity may reflect secular changes in the nature of subduction-zone processes. The Llano Uplift of central Texas contains evidence of HP metamorphism that dates back to the late Mesoproterozoic, documenting subduction and exhumation of the southern margin of the Laurentian continent. Collisional orogenesis in the Llano Uplift has been dated at ca. 1147-1116 Ma by U-Pb (zrc, ttn, mnz, rut) and Lu-Hf (grt-rut) geochronometry. This orogenic event drove metamorphism comprising both an initial high- pressure (HP) phase (610-775 C at 1.4-2.4 GPa) and a subsequent moderate-pressure (MP) phase (ca. 700 C at ~0.7 GPa). A low-pressure overprint (525-625 C at 0.3 GPa) took place under largely static conditions at ca. 1093-1070 Ma. Evidence for HP metamorphism is geographically widespread, but confined principally to boudins of mafic eclogite encased in felsic gneisses. The geographic distribution of P-T conditions for HP metamorphism inferred from the eclogites, combined with evidence from the relative degrees of homogenization of growth zoning in garnet, suggests a general increase from northeast to southwest in depths of burial during HP metamorphism. Exhumation of HP rocks to shallower depths prior to MP metamorphism appears to have been rapid: ages for the two phases of metamorphism overlap, on the basis of the very limited data now available. The region's early tectonometamorphic history shares numerous characteristics with well-documented examples of Phanerozoic continental-margin subduction, including: the presence of disrupted ophiolites; a paucity of coeval island-arc volcanics and plutonics; the involvement of precursors representing continental basement and supracrustal rocks; rapid exhumation to lower crustal levels with significant Barrovian-style overprinting; the preservation of the highest pressure record only in eclogitic pods and lenses within quartzofeldspathic gneisses; and the presence of post-collisional and late-stage anorogenic granitic plutons. HP metamorphism in the Llano Uplift is best explained by southwestward subduction of the Laurentian continental margin during collision with a still-unidentified continental mass, followed by buoyancy-driven uplift to lower crustal levels while collisional contraction continued.

Carlson, W. D.

2009-05-01

259

Tectonic sediment thickening, fluid expulsion, and the thermal regime of subduction zone accretionary prisms: The Cascadia margin off Vancouver island  

NASA Astrophysics Data System (ADS)

Tectonic sediment thickening, fluid expulsion, and the resulting thermal regime of the northern Cascadia subduction zone accretionary prism have been described by numerical models. Both the sediment thickening and the fluid expulsion are found to have important thermal effects. Constraints on the models are provided by (1) detailed heat flow profiles across the continental slope accretion zone from variations in the thermally controlled depth to a gas hydrate bottom-simulating reflector (BSR), and (2) porosity changes across the accretion zone inferred from the landward variation in the P wave velocity-depth profiles. The heat flow in the region of rapid prism thickening centered 15 km landward of the deformation front is 20% below that predicted by a larger scale thermal model that ignores the thermal effects of the sediment accretion processes. The porosity also is substantially higher in this region compared to at the same depths in the adjacent deep sea Cascadia basin. The model results are in agreement with both the thermal and velocity data if the incoming sediment section is initially simply tectonically shortened horizontally and thickened vertically, generating a high porosity underconsolidated section. The prism sediments are inferred to be substantially underconsolidated and pore pressures to be high for at least the seaward 30 km of the accretionary prism.

Hyndman, R. D.; Wang, K.; Yuan, T.; Spence, G. D.

1993-12-01

260

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

NASA Astrophysics Data System (ADS)

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

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

2013-03-01

261

Anatomy of the Andean subduction zone: three-dimensional density model upgraded and compared against global-scale models  

NASA Astrophysics Data System (ADS)

We present an upgraded version of a previously published 3-D density model of the Andean subduction zone between 18°S and 45°S. This model consists of 3-D bodies of constant density, which geometry is constrained by independent seismic data and is triangulated from vertical cross-sections. These bodies define the first-order morphology and internal structure of the subducted Nazca slab and South American Plate. The new version of the density model results after forward modelling the Bouguer anomaly as computed from the most recent version of the Earth Gravitational Model (EGM2008). The 3-D density model incorporates new seismic information to better constrain the geometry of the subducted slab and continental Moho (CMH) and has a trench-parallel resolution doubling the resolution of the previous model. As an example of the potential utility of our model, we compare the geometry of the subducted slab and CMH against the corresponding global models Slab1.0 and Crust2.0, respectively. This exercise demonstrates that, although global models provide a good first-order representation of the slab and upper-plate crustal geometries, they show large discrepancies (up to ±40 km) with our upgraded model for some well-constrained areas. The geometries of the slab, lithosphere-asthenosphere boundary below the continent, CMH and intracrustal density discontinuity that we present here as Supporting Information can be used to study Andean geodynamic processes from a wide range of quantitative approaches.

Tassara, Andrés.; Echaurren, Andrés.

2012-04-01

262

Spontaneous Nucleation of Subduction Zones in the Western Pacific During Middle Eocene Time: Evidence From the IBM Forearc Ophiolite  

NASA Astrophysics Data System (ADS)

Subduction zones nucleate in two fundamentally different ways. Induced nucleation is a response to continuing plate convergence following a collision event and requires the lithosphere to fail under compression; no change in plate motion is expected. Spontaneous nucleation of a subduction zone (SNSZ) manifests failure of old lithosphere due to gravitation instability. SNSZ doesn't require plate convergence to occur but major changes in plate motion are expected. SNSZ is possible where old oceanic lithosphere is unusually dense (old continental margins) or weak (along fracture zones). The western edge of the Pacific plate spontaneously reorganized as a convergent margin during Middle Eocene time ( ~50-42 Ma) and is the best known example of SNSZ. The unusual nature of this episode is preserved in the Izu-Bonin-Mariana (IBM) forearc, where pillow basalts, dyke complexes, gabbro, and harzburgitic mantle define an in situ ophiolite. The IBM forearc ophiolite requires that SNSZ was accompanied by a strongly magmatic episode of seafloor spreading. Spreading so close to the present trench requires asthenospheric upwelling where strong mantle downwelling now occurs. Abundant boninite, formed by melting harzburgite, in IBM forearc sections further demonstrates the unique nature of the IBM subduction initiation event. IBM SNSZ spans the period from beginning of magmatic construction of the IBM `forearc ophiolite' about 50 Ma to the change in Pacific Plate motion at 43 Ma marking the start of true subduction. Events during this stage are very poorly understood but can only be explained by subsidence of part of the lithosphere to a depth such that asthenosphere flowed over it. Stern and Bloomer (1992 BGSA 104, 1621-1636) argue that this occurred along a zone of weakness associated with a N-S fracture zone but this has been criticized on the basis of paleomagnetic models requiring ~90 o CW rotation of the Philippine Sea Plate (PSP) since 43Ma. The youngest parts of the paleomagnetic reconstructions fail because geologic evidence refutes arguments for significant rotations since ~15 Ma. Regardless of paleogeographic controvesies, the essential geologic evidence for IBM SNSZ are robust. Furthermore, the prediction that SNSZ will simultaneously develop along the entire margin of the affected plate is satisfied. Development of physical models are now required in order to make further advances in understanding SNSZ.

Stern, R. J.

2001-12-01

263

Kinematic Coupling Along the Mexican Subduction Zone From GPS Data  

NASA Astrophysics Data System (ADS)

Kinematic coupling (?) between the subducting Cocos and Rivera oceanic plates, and the continental North America plate is estimated as a ratio between the back slip on the fault plane and the convergence rate (NUVEL1A model). To obtain a distribution of the back slip ("negative" displacement at the plate interface) along the coast of Jalisco, Guerrero, Oaxaca and Chiapas we performed an inversion of the surface crustal deformation data from the permanent GPS network "SSN-Sismologia-UNAM" and campaign GPS measurements. The displacements for each cell of the plate interface model were calculated using closed form expressions developed by Okada (1992). The inversion based on a simulated annealing algorithm reveals a high coupling ratio, ? ~ 1, at the seismogenic zone of 40-50 km width in Guerrero and Oaxaca, just beneath the coast line. Downdip from this area, a transition or partially coupled zone, ? ~ 0.28, extends for a distance of about 200 km. For the Isthmus of Tehuantepec zone, at the southeastern Oaxaca, the inversion model shows a very low coupling ratio. In Chiapas, at the southern Mexico Pacific coast, the sparse distribution of GPS stations does not allow us to constrain a satisfactory model. Nevertheless, some preliminary results show a coupled area close to the trench. At the northwest, along the coast of Jalisco, the observed surface crustal deformation is mostly a result of subduction of the Rivera Plate beneath the North America Plate. For this area, the campaign GPS data were obtained from the UNAVCO database. Preliminary results show a highly coupled seismogenic zone. Strongly coupled areas that were determined from the inversion models are in good agreement with the seismogenic zones defined with the rupture areas of large Mexican subduction thrust earthquakes (M > 6.5) occurred last century.

Franco, S.; Kostoglodov, V.; Iglesias, A.; Singh, S.

2007-05-01

264

Tectonic development of forearc basins along the Western Sunda\\/Andaman Subduction Zone  

Microsoft Academic Search

A prominent feature of the western Sunda\\/Andaman subduction zone is a series of forearc basins located immediately to the east (arcward) of the accretionary prism and outerarc ridge. Multichannel seismic (MCS) reflection lines and other geophysical data allow us to examine the structure of the forearc basin located from about 8°30'N to 11°40'N along the western boundary of the Andaman

J. R. Cochran; K. R. Kattoju

2010-01-01

265

Array measurements of deep tremor signals in the Cascadia subduction zone  

Microsoft Academic Search

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

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

2005-01-01

266

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

Microsoft Academic Search

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

Thomas H. Heaton; Stephen H. Hartzell

1989-01-01

267

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

Microsoft Academic Search

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

Thomas H. Heaton; Stephen H. Hartzell

1989-01-01

268

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

Microsoft Academic Search

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

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

2003-01-01

269

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

Microsoft Academic Search

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

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

2011-01-01

270

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

Microsoft Academic Search

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

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

2004-01-01

271

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

Microsoft Academic Search

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

Nathan Wood

2009-01-01

272

Electrical conductivity of the serpentinised mantle and fluid flow in subduction zones  

Microsoft Academic Search

In the mantle wedge of subduction zones, electromagnetic profiles reveal high electrical-conductivity bodies. In hot areas (>700°C), water released by dehydration of the slab induces melting of the mantle under volcanic arcs that can explain the observed high conductivities. In the cold (<700°C) melt-free fore-arc mantle wedge, fluid water migrates and causes serpentinisation detected as low seismic wave velocities in

Bruno Reynard; Kenji Mibe; Bertrand Van de Moortèle

2011-01-01

273

Seismic Structure of the Middle Japan Trench Subduction Zone by Airgun-OBS Experiment  

Microsoft Academic Search

In the middle part of the Japan trench subduction zone, the off-Miyagi area, large interplate earthquakes with M>7 is known to regularly recur with about 40 years interval. The most recent event occurred 25 years ago (the 1978 Miyagi-Oki earthquake M7.4) and the Japanese government evaluated that the next large earthquake may occur within 20 years from now with over

R. Hino; M. Nishino; K. Mochizuki; K. Uehira; T. Sato; M. Nakamura; S. Nakata; M. Shinohara; J. Kasahara

2003-01-01

274

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

275

Erosional Subduction Zone in the Northern Japan Trench: Review of Submersible Dive Reports  

Microsoft Academic Search

\\u000a Submersible studies of the oceanward and landward slopes of the northern Japan trench are reviewed, and the typical erosional\\u000a features of this subduction zone are discussed. On the oceanward slope, normal faults with tension cracks have created horst\\u000a and graben structures, forming a series of steps. On the landward slope, brecciated Miocene sedimentary rocks with calcite\\u000a veins and cement are

Yujiro Ogawa

276

Deep structure of Japan subduction zone as derived from local, regional, and teleseismic events  

Microsoft Academic Search

We have determined a detailed three-dimensional P wave velocity structure of the Japan subduction zone to 500-km depth by inverting local, regional, and teleseismic data simultaneously. We used 45,318 P wave arrivals from 1241 shallow and deep earthquakes which occurred in and around the Japan Islands. The arrival times are recorded by the Japan University Seismic Network which covers the

Dapeng Zhao; Akira Hasegawa; Hiroo Kanamori

1994-01-01

277

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

Microsoft Academic Search

Subduction of ocean lithosphere drives plate tectonics, large-scale mantle circulation and thermal-chemical recycling processes through arcs. Seismologists have made important advances in our ability to map circulation patterns in subduction zones though anisotropy data\\/methods and in providing detailed images of mantle density fields. Increasingly, seismic and geodynamic disciplines are combining to extend our understanding of time varying subduction processes and

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

2009-01-01

278

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

Microsoft Academic Search

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

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

2004-01-01

279

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

NASA Astrophysics Data System (ADS)

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

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

2012-06-01

280

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

NASA Astrophysics Data System (ADS)

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

Tilling, Robert I.

281

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

NASA Astrophysics Data System (ADS)

The fast flow of glaciers and rupture of earthquakes on subduction zones both occur at interfaces notoriously difficult to observe. When slip events occur on such interfaces, elastic energy is radiated in the form of seismic waves. Here, we analyze these and other signals, such as continuous GPS, for two glacier systems and a subduction zone. We find that the slip events at an Antarctic ice stream, a Greenland outlet glacier, and a subduction zone in Costa Rica are all modulated by the ocean tide, however, the manifestations of the modulations vary considerably. In Antarctica, we directly measure the bidaily rupture process of the Whillans Ice Plain using broadband seismometers. The average rupture speed of these events varies by a factor of 2, which is dependent upon the recurrence interval. Based on variations of rupture near the source region, we observe that rupture speed depends on loading conditions. In Greenland, the breakup of the sea ice and iceberg mixture that forms during the winter causes changes in the flow velocity of Store Glacier, West Greenland. We measure velocity using continuous GPS 16 km away from the terminus and time-lapse photography near the terminus. We observe a step-change in velocity near the terminus in response to the ice melange breakup, corresponding to a 30-60 kPa loss of buttressing stress. Further, we observe semi-diurnal periodicity in GPS speed perturbations 16 km from the terminus, likely due to ocean tides. At the subduction zone, we identify and locate tremor events, distinct from earthquakes in their low amplitude, which occur as shear failure on the plate interface. We locate the events on portions of the plate believed to be undergoing stable sliding, adjacent to locked portions of the plate. Furthermore, we provide multiple forms of geodetic and pressure evidence of an offshore event that occurred in 2008. Observations of various behaviors that include relatively small stresses may provide insight into the unique dynamics of glaciers and subduction zones. Small stresses that drive failure suggests either a weak basal interface or a system near its critical stress state.

Walter, Jacob I.

282

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

NASA Astrophysics Data System (ADS)

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

Cooper, Lauren Beth

283

The deep roots of volcanoes: Models of magma dynamics with applications to subduction zones  

NASA Astrophysics Data System (ADS)

In this thesis I present research on the dynamics of the partially molten mantle with applications to magma genesis and transport in subduction zones. Subduction zones are the most complex tectonic setting for magma genesis on Earth. To model and understand them requires a study of the fundamental physics and chemistry of the mantle beneath their associated volcanoes where melting occurs and magma migrates through the pores of the rock toward the surface. These partially molten regions of the mantle control the overall chemical evolution of the Earth as well as the characteristics of the volcanic systems that they feed. Subduction zones represent the most complex instance of partial melting in a tectonic/volcanic system. Observations of arc volcanoes raise a suite of questions about the dynamics active at depth below them. To address these questions requires comprehensive, self-consistent models of magma genesis and transport in subduction zones. Even a basic model that incorporates fluid and solid flow, reaction and temperature evolution will be complex, challenging to construct, and require advanced computational resources. In Chapter 2 I describe a computational approach for fluid flow in a deforming mantle matrix; in Chapter 3 for modeling mantle flow with highly variable non-Newtonian viscosity; in Chapter 4 for melting systematics of the hydrous magma/mantle system; and in Chapter 5 for temperature-dependent reactive flow of hydrous fluids/magma. Each of these contributes to the development of a comprehensive model of subduction by addressing computational challenges and exposing complex behavior such as sharp viscosity gradients and strong localization phenomena. The central themes of this thesis are localization phenomena and the effects of complex rheology on the magma-mantle system. Calculations presented here demonstrate that both of these are likely to play an important role in shaping the processes of magma genesis and transport in partially molten regions of the mantle and, particularly, in subduction zones. Many questions are raised by this work, especially regarding the observable signature of predicted localizations. This work represents fundamental progress on understanding the generation and organization of magma in the mantle. (Abstract shortened by UMI.)

Katz, Richard Foa

284

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

SciTech Connect

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

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

2007-01-01

285

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.

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

2007-01-01

286

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

NASA Astrophysics Data System (ADS)

We model seismic wave propagation from intermediate depth earthquakes in a subduction zone using a 2-D Chebyshev pseudospectral method. Particular attention is directed to the influence of a deep, low-viscosity subduction channel on top of the plate contact where metamorphic rocks may be exhumed by forced return flow. The study is motivated by observations of complicated dispersive and high-amplitude P- and S-wave trains in the forearc of the Hellenic Subduction Zone. The basic model is a subducted slab with a thin oceanic crust forming a low-velocity layer. Our model setup closely follows recent results on the structure of the Hellenic Subduction Zone obtained from receiver functions and surface wave studies. They exhibit an abrupt change of the dip of the downgoing slab at about 70 km depth. The subduction channel is modelled as a thin, wedge-shaped layer of intermediate seismic velocity above a slower oceanic crust and below a faster overlying mantle wedge. We also look into the effects of a continuous phase transition from basalt to eclogite in the subducted oceanic crust and near-surface crustal structures. In all models, wave propagation is characterized by dispersive guided channel waves trapped in the low-velocity subducted crust. They produce high-amplitude arrivals in the forearc. A fast guided wave train (gP) originates from the direct P wave and a slower one (gS) from the direct S wave. Guided waves are radiated into the overlying mantle where the dip of the slab is abruptly changing. Seismogram sections for models without a subduction channel typically show two spatially separated guided wave trains, one following the oceanic crust and one travelling more steeply towards the forearc high. A subduction channel above the plate contact enhances the radiation effect of gP waves at the slab bend due to the weaker velocity contrast and inhibits the separation of the wave trains. In models with additional near-surface crustal structures the wave field is dominated by reverberations. However, guided waves remain discernible in seismogram sections because of their high amplitudes. When a basalt to eclogite phase transition is considered, guided waves are preferentially generated in the presence of a subduction channel. A pronounced gP-wave train develops particularly for sources inside this channel. On the contrary, guided waves are hardly distinct for sources below the subducted crust.

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

2009-12-01

287

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

NASA Astrophysics Data System (ADS)

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

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

2006-12-01

288

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

289

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

290

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

291

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

PubMed

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

Rogers, Garry; Dragert, Herb

2003-05-08

292

Molecular modelling of rare earth element complexation in subduction zone fluids  

NASA Astrophysics Data System (ADS)

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

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

2009-07-01

293

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

294

Variations Lithospheric Scale Strain Accumulation in Mega-thrust Subduction Zones: Implications for Earthquake rupture  

NASA Astrophysics Data System (ADS)

Although the general plate tectonic model of subduction zone deformation and its relationship to the earthquake cycle for mega-thrust earthquakes is well known, there are in fact significant differences in terms of how inter-earthquake strain accumulation is considered by different communities. Most seismologic studies of mega-thrust earthquakes assume that the co-seismic slip is essentially symmetric across the fault surface - that is both the upper and lower plates moved equal amounts (but in opposite directions) during the rupture - implying s similar symmetric pattern of strain accumulation. Implicit in many geologic studies along convergent margins is the assumption that most permanent deformation is within the upper plate and the subducting slab basically transits the seismogenic zone with little permanent deformation. Based on the analyises of two recent great earthquakes, we conclude that the pattern of strain accumulation during the inter-earthquake period ranges from situations (akin to the ‘geologic' model) where most strain accumulates in the upper plate to situations where the subducting plate undergoes most of the recoverable elastic strain during the period between great earthquakes. Two subduction zone locales, the Kuriles and Solomon Islands, that have hosted recent Mw 8+ earthquakes demonstrate these two end-member styles of subduction zone processes. The November 2006 (thrust) and January 2007 (normal) earthquake pair in the Kuriles provide an opportunity to quantify the deformation within the subducting Pacific slab during the interseismic period. Based on the correspondence in slip during these events, we are able to both estimate the deformation (dominantly in the subducting slab and not in the overriding plate) and place a constraint on the static frictional strength of the megathrust interface of approximately 2-5 MPa. The 2007 Solomon Island Mw 8+ earthquake shows a distinctly different pattern of interseismic deformation. During this event, the propagating rupture traversed an active transform plate boundary between the separately subducting Australia and Solomon Sea plates. We interpret this to represent a situation in which interseismic deformation is primarily in the upper (Pacific) plate allowing the rupture to jump the fundamental barrier of a plate boundary. These two subduction regimes indicate that there is likely a full continuum in how deformation is accommodated during subduction, and implies that attempts to determine the megathrust (and associated tsunami) potential of subduction zones using solely observations of upper-plate deformation is problematic.

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

2009-04-01

295

Sensitivity of the short-to-intermediate wavelength geoid to rheologic structure in subduction zones  

NASA Astrophysics Data System (ADS)

It is well established that the long wavelength geoid and dynamic topography are responsive to the radial viscosity structure of the mantle, but recent studies indicate that lateral viscosity variations affect the geoid at shorter wavelengths. These studies, however, only consider Newtonian viscosity structures, although experimental deformation studies of mantle minerals and seismic observations of lattice preferred orientation in the upper mantle provide evidence that dislocation creep is an active deformation mechanism at mantle conditions. In addition, the lithosphere is expected to yield plastically at high stresses based on laboratory measurements of yield strength. To quantify the effects of lateral viscosity variations and realistic flow laws on the short-to-intermediate wavelength dynamic topography and geoid near subduction zones, we consider a composite viscosity that accounts both for Newtonian and stress-dependent deformation mechanisms, including plastic yielding. Regional models of instantaneous stokes flow models are computed on a variable resolution mesh using CitcomS, where the resolution ranges from 25 km away from the subduction zone to 5 km in the vicinity of the subducting slab. The slab is defined as an 80 million year old lithosphere temperature anomaly smoothed above and below by half-space cooling models, and extends 100 km into the lower mantle. The buoyancy and stress fields are expanded to spherical harmonic degree 360, corresponding to a spatial resolution of about 110 km. These fields include the effects of self-gravitation and are used to solve for the surface geoid, as well as for dynamic topography at the surface and core-mantle boundary. Results of preliminary, layered mantle viscosity models are consistent with previous geoid studies, the main conclusion being that a more positive geoid at subduction zones is the product of relative viscosity increases with depth. In layered models, increased viscous support of the down-going slab with depth decreases its velocity, resulting in lower pressure gradients above the slab, decreased dynamic topography at the surface and a more positive geoid relative to the case of a uniform viscosity mantle. In contrast, relatively stronger layers above the slab increase coupling to the surface, thereby increasing dynamic topography and leading to a more negative geoid. Future models with temperature-dependent viscosities will include lateral variations in viscosity due to the temperature anomaly of the slab. These model results will be compared with those from models that include both linear and non-linear viscosities to determine the sensitivity of the short-to-intermediate wavelength geoid to increasingly realistic subduction zone rheologies.

Hines, J. M.; Billen, M. I.

2010-12-01

296

Metamorphic history and geodynamic significance of the Early Cretaceous Sabzevar granulites (Sabzevar structural zone, NE Iran)  

Microsoft Academic Search

The Iranian ophiolites are part of the vast orogenic suture zones that mark the Alpine-Himalayan convergence zone. Few petrological and geochronological data are available from these ophiolitic domains, hampering a full assessment of the timing and regimes of subduction zone metamorphism and orogenic construction in the region. This paper describes texture, geochemistry and the pressure-temperature path of the Early Cretaceous

M. Nasrabady; F. Rossetti; T. Theye; G. Vignaroli

2011-01-01

297

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

USGS Publications Warehouse

Arc magmatism at subduction zones (SZs) most voluminously supplies juvenile igneous material to build rafts of continental and intra-oceanic or island arc (CIA) crust. Return or recycling of accumulated CIA material to the mantle is also most vigorous at SZs. Recycling is effected by the processes of sediment subduction, subduction erosion, and detachment and sinking of deeply underthrust sectors of CIA crust. Long-term (>10-20 Ma) rates of additions and losses can be estimated from observational data gathered where oceanic crust underruns modern, long-running (Cenozoic to mid-Mesozoic) ocean-margin subduction zones (OMSZs, e.g. Aleutian and South America SZs). Long-term rates can also be observationally assessed at Mesozoic and older crust-suturing subduction zone (CSSZs) where thick bodies of CIA crust collided in tectonic contact (e.g. Wopmay and Appalachian orogens, India and SE Asia). At modern OMSZs arc magmatic additions at intra-oceanic arcs and at continental margins are globally estimated at c. 1.5 AU and c. 1.0 AU, respectively (1 AU, or Armstrong Unit,= 1 km3 a-1 of solid material). During collisional suturing at fossil CSSZs, global arc magmatic addition is estimated at 0.2 AU. This assessment presumes that in the past the global length of crustal collision zones averaged c. 6000 km, which is one-half that under way since the early Tertiary. The average long-term rate of arc magmatic additions extracted from modern OMSZs and older CSSZs is thus evaluated at 2.7 AU. Crustal recycling at Mesozoic and younger OMSZs is assessed at c. 60 km3 Ma-1 km-1 (c. 60% by subduction erosion). The corresponding global recycling rate is c. 2.5 AU. At CSSZs of Mesozoic, Palaeozoic and Proterozoic age, the combined upper and lower plate losses of CIA crust via subduction erosion, sediment subduction, and lower plate crustal detachment and sinking are assessed far less securely at c. 115 km3 Ma-1 km-1. At a global length of 6000 km, recycling at CSSZs is accordingly c. 0.7 AU. The collective loss of CIA crust estimated for modern OMSZs and for older CSSZs is thus estimated at c. 3.2 AU. SZ additions (2.7 AU) and subtractions (23.2 AU) are similar. Because many uncertainties and assumptions are involved in assessing and applying them to the deep past, the net growth of CIA crust during at least Phanerozoic time is viewed as effectively nil. With increasing uncertainty, the long-term balance can be applied to the Proterozoic, but not before the initiation of the present style of subduction at c. 3 Ga. Allowing that since this time a rounded-down rate of recycling of 3 AU is applicable, a startlingly high volume of CIA crust equal to that existing now has been recycled to the mantle. Although the recycled volume (c. 9 ?? 109 km3) is small (c. 1%) compared with that of the mantle, it is large enough to impart to the mantle the signature of recycled CIA crust. Because subduction zones are not spatially fixed, and their average global lengths have episodically been less or greater than at present, recycling must have contributed significantly to creating recognized heterogeneities in mantle geochemistry. ?? The Geological Society of London 2009.

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

2009-01-01

298

Submarine slope failures along the convergent continental margin of the Middle America Trench  

Microsoft Academic Search

We present the first comprehensive study of mass wasting processes in the continental slope of a convergent margin of a subduction zone where tectonic processes are dominated by subduction erosion. We have used multibeam bathymetry along ˜1300 km of the Middle America Trench of the Central America Subduction Zone and deep-towed side-scan sonar data. We found abundant evidence of large-scale

Rieka Harders; César R. Ranero; Wilhelm Weinrebe; Jan H. Behrmann

2011-01-01

299

Subduction-zone earthquake complexity related to frictional anisotropy in antigorite  

NASA Astrophysics Data System (ADS)

Earthquakes generated in subduction zones are caused by unstable movements along faults. This fault-slip instability is determined by frictional forces that depend on the temperature, pressure, morphology and deformation state of the fault rocks. Fault friction may also be influenced by preferred mineral orientations. Over-thrusting of rocks at the interface between a subducting slab and the overlying mantle wedge generates shear deformation that causes minerals to align, and this preferred mineral orientation affects the propagation of shear seismic waves. Here we use laboratory experiments to simulate fault slip in antigorite, the most abundant hydrous mineral phase within Earth's upper mantle. Using atomic force microscopy, we show that antigorite single crystals possess strong frictional anisotropy on their basal slip surface and that preferred mineral alignment extends this property to a regional scale. Depending on the alignment, fault movements can occur along a high-friction direction, creating stick-slip behaviour that generates earthquakes. In contrast, if movements occur along a low-friction direction, the mantle wedge will deform aseismically. Our results imply that mantle rocks in subduction-zone thrust faults can exhibit two opposite frictional behaviours, seismic and aseismic.

Campione, Marcello; Capitani, Gian Carlo

2013-10-01

300

Geodetic, teleseismic, and strong motion constraints on slip from recent southern Peru subduction zone earthquakes  

NASA Astrophysics Data System (ADS)

We use seismic and geodetic data both jointly and separately to constrain coseismic slip from the 12 November 1996 Mw 7.7 and 23 June 2001 Mw 8.5 southern Peru subduction zone earthquakes, as well as two large aftershocks following the 2001 earthquake on 26 June and 7 July 2001. We use all available data in our inversions: GPS, interferometric synthetic aperture radar (InSAR) from the ERS-1, ERS-2, JERS, and RADARSAT-1 satellites, and seismic data from teleseismic and strong motion stations. Our two-dimensional slip models derived from only teleseismic body waves from South American subduction zone earthquakes with Mw > 7.5 do not reliably predict available geodetic data. In particular, we find significant differences in the distribution of slip for the 2001 earthquake from models that use only seismic (teleseismic and two strong motion stations) or geodetic (InSAR and GPS) data. The differences might be related to postseismic deformation or, more likely, the different sensitivities of the teleseismic and geodetic data to coseismic rupture properties. The earthquakes studied here follow the pattern of earthquake directivity along the coast of western South America, north of 5°S, earthquakes rupture to the north; south of about 12°S, directivity is southerly; and in between, earthquakes are bilateral. The predicted deformation at the Arequipa GPS station from the seismic-only slip model for the 7 July 2001 aftershock is not consistent with significant preseismic motion.

Pritchard, M. E.; Norabuena, E. O.; Ji, C.; Boroschek, R.; Comte, D.; Simons, M.; Dixon, T. H.; Rosen, P. A.

2007-03-01

301

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

302

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

SciTech Connect

Chemosynthetic benthic communities, which live symbiotically with microbes capable of metabolizing nutrients dissolved in water seeping out of the seafloor, are widespread along the Cascadia subduction zone. These seeps and vents are therefore indicative of one mode of fluid migration out of the subduction zone sediments. The authors have used deep-towed seismic methods, including hydrophones mounted on Alvin, to examine the detailed geologic structure under two of these vent sites. At one of the sites, located on a seaward dipping thrust zone, the benthic communities are associated with a disruption of the subsurface acoustic layering in the thrust zone. It appears that at this site, dewatering is occurring along fractures in the disrupted sediments which connect to permeable layers in the undeformed sediments and not along the main thrust fault. The other site is located near the top of a ridge which has been uplifted by thrusting along a landward dipping thrust. Most of the benthic communities are found to exist at the outcrop of a steeply dipping unconformity between recent slope-basin sediments and the older uplifted sediments, with the unconformity serving as the fluid pathway. Underlying this unconformity at the most active dewatering location is a complexly deformed structure which appears to enhance the flow of fluid to the unconformity. The source of the fluids could be the older uplifted sediments or the recent slope-basin turbidites. In either case the source is shallow, less than about 1 km.

Lewis, B.T.R. (Univ. of Washington, Seattle (United States)); Cochrane, G.C. (Geological Survey, Menlo Park, CA (United States))

1990-06-10

303

Deep non-volcanic tremor of circum-Pacific subduction zones  

NASA Astrophysics Data System (ADS)

Non-volcanic tremor was first discovered in SW Japan. The preponderance of evidence indicates that tremor consists of swarms of deep low frequency earthquakes (LFEs) that occur as shear slip on the deep extension of the plate interface. Since being discovered in Japan, non-volcanic tremor has been detected in many other circum-Pacific subduction zones. Studies to date indicate that the results from Japan will generalize to other locales. In this study we present an update of LFE activity during non-volcanic tremor in four subduction zones: April 2006 southwest Japan, July 2004 northern Cascadia, July 2000 south-central Alaska, and May 2007 Costa Rica. We detect LFEs within tremor using the network autocorrelation of tremor waveforms in vicinity of the tremor source, and once the LFEs are detected, we measure phase arrival times and locate them using methods that are used to locate ordinary earthquakes. Our preliminary results suggest non-volcanic tremor is primarily composed of LFEs. A more complete understanding of where tremor occurs, and just as importantly where it doesn’t, is key to developing a comprehensive understanding of tremor and its relationship to regular earthquakes.

Brown, J. R.; Beroza, G. C.

2009-12-01

304

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

NASA Astrophysics Data System (ADS)

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

Morozov, I. B.; Zheng, H.

2005-12-01

305

New constraints on megathrust slip stability under subduction zone P-T conditions  

NASA Astrophysics Data System (ADS)

To understand and model subduction zone seismogenesis, data are needed on the frictional properties of (meta)pelitic subduction zone materials under in situ megathrust conditions. Here, we report the results of rotary shear friction experiments on simulated illite-quartz fault gouge at an effective normal stress of 170 MPa, a pore fluid pressure of 100 MPa, at 150-500 °C and sliding velocities of 1-100 ?m/s. The results show three temperature-dependent regimes, characterized by velocity-strengthening at 150-250 °C, velocity-weakening at 250-400 °C and velocity-strengthening at 400-500 °C. The regimes are defined by a decrease in the rate and state friction (RSF) parameter (a-b) at 150-300 °C followed by an increase at 300-500 °C. These trends correlate with systematic changes in other RSF parameters, as well as an increase in friction coefficient above 300 °C. We explain the effects of increasing temperature in terms of a transition from frictional granular flow (150-250 °C), through granular flow whereby dilatation is balanced by thermally activated compaction involving stress corrosion cracking (SCC) of quartz clasts (250-400 °C), to non-dilatant slip on the phyllosilicates with accommodation at quartz clasts by SCC (400-500 °C). Taking into account the effects of sliding velocity, the observed velocity-weakening regime broadly explains the extent of the seismogenic zone within subduction megathrusts.

den Hartog, S. A. M.; Niemeijer, A. R.; Spiers, C. J.

2012-11-01

306

The potential for a great earthquake along the southernmost Ryukyu subduction zone  

NASA Astrophysics Data System (ADS)

Interseismic GPS data along the Hualien-Suao coast (NE Taiwan) shows a pattern of strain accumulation that is consistent with a potential future large shallow earthquake along the southernmost Ryukyu subduction zone. The measured shortening rate parallel to the Ryukyu Trench is 80 mm/yr, about twice of the shortening rate perpendicular to the Ryukyu Trench. We invert for slip-deficit rates and the geometric configuration of the plate interface. Our preferred fault model dips 10° northward and extends about 70 km from the Ryukyu Trench to a depth of 13 km. The slip-deficit rate exhibits a left-lateral motion of 78 mm/yr and a normal motion of 36 mm/yr on a 290°-trending fault. The slip rate budget of the southernmost Ryukyu subduction zone is close to the plate convergence rate, suggesting the plate interface is fully locked. Assessments of seismic hazard in this region need to consider the potential threat from Mw 7.5˜8.7 tsunami earthquakes generated by shallow ruptures.

Hsu, Ya-Ju; Ando, Masataka; Yu, Shui-Beih; Simons, Mark

2012-07-01

307

Seismic anisotropy of the Central Andean subduction zone derived from shear-wave splitting  

NASA Astrophysics Data System (ADS)

The occurrence of seismic anisotropy is usually explained by the preferred alignment of anisotropic crystals, e.g. olivine, due to asthenospheric mantle flow or frozen-in lithospheric anisotropy in relation to previous tectonic events. In subduction zones anisotropy is often found to be dominated by fast-polarisation axes oriented sub-parallel to the trench. This has led to the hypothesis of trench-parallel mantle flow due to pressure gradients induced by slab geometry and trench migration. However, the character of the mantle-flow field in subduction zones remains poorly understood. We investigate shear-wave splitting along two profiles in the Central Andes at 21° S and 25.5° S in the downdip direction of the subducting Nazca plate in order to clarify variations of the fast splitting directions and the delay times from the Pacific coast to the West. Using both, teleseismic SKS and local S phases, we aim to discriminate between effects of the crust/mantle wedge above and asthenospheric flow beneath the slab. First results of fast polarisations from SKS phases show a significant variability over relatively short distances along the northern profile and delay times ranging from 0.5 to 1.5 sec. We discuss our results in view of the recently derived dependence of olivine-crystal alignment on pressure and water content.

Wölbern, I.; Rümpker, G.

2012-04-01

308

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

NASA Astrophysics Data System (ADS)

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

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

2011-12-01

309

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

310

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

311

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

312

The potential influence of subduction zone polarity on overriding plate deformation, trench migration and slab dip angle  

NASA Astrophysics Data System (ADS)

A geodynamic model exists, the westward lithospheric drift model, in which the variety of overriding plate deformation, trench migration and slab dip angles is explained by the polarity of subduction zones. The model predicts overriding plate extension, a fixed trench and a steep slab dip for westward-dipping subduction zones (e.g. Mariana) and predicts overriding plate shortening, oceanward trench retreat and a gentle slab dip for east to northeastward-dipping subduction zones (e.g. Chile). This paper investigates these predictions quantitatively with a global subduction zone analysis. The results show overriding plate extension for all dip directions (azimuth ? = - 180° to 180°) and overriding plate shortening for dip directions with ? = - 90° to 110°. The wide scatter in data negate any obvious trend and only local mean values in overriding plate deformation rate indicate that overriding plate extension is somewhat more prevalent for west-dipping slabs. West-dipping subduction zones are never fixed, irrespective of the choice of reference frame, while east to northeast-dipping subduction zones are both retreating and advancing in five out of seven global reference frames. In addition, westward-dipping subduction zones have a range in trench-migration velocities that is twice the magnitude of that for east to northeastward-dipping slabs. Finally, there is no recognizable correlation between slab dip direction and slab dip angle. East to northeast-dipping slabs (? = 30° to 120°) have shallow (0 125 km) slab dip angles in the range 10 60° and deep (125 670 km) slab dip angles in the range 40 82°, while west-dipping slabs (? = - 60° to - 120°) have shallow slab dip angles in the range 19 50° and deep slab dip angles in the range 25 86°. Local mean deep slab dip angles are nearly identical for east and west-dipping slabs, while local mean shallow slab dip angles are lower by only 4.7 8.1° for east to northeast-dipping slabs. It is thus concluded that overall, there is no observational basis to support the three predictions made by the westward drift model, and for some sub-predictions the observational basis is very weak at most. Alternative models, which incorporate and underline the importance of slab buoyancy-driven trench migration, slab width and overriding plate motion, are better candidates to explain the complexity of subduction zones, including the variety in trench-migration velocities, overriding plate deformation and slab dip angles.

Schellart, W. P.

2007-12-01

313

Thermochronologic evidence for the exhumational history of the Alpi Apuane metamorphic core complex, northern Apennines, Italy  

Microsoft Academic Search

The Apennine Range is a young convergent orogen that formed over a retreating subduction zone. The Alpi Apuane massif in the northern Apennines exposes synorogenic metamorphic rocks, and provides information about exhumation processes associated with accretion and retreat. (U-Th)\\/He and fission-track ages on zircon and apatite are used to resolve exhumational histories for the Apuane metamorphic rocks and the structurally

Maria Giuditta Fellin; Peter W. Reiners; Mark T. Brandon; Eliane Wüthrich; Maria Laura Balestrieri; Giancarlo Molli

2007-01-01

314

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

Microsoft Academic Search

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

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

2004-01-01

315

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

316

Seismic Velocity and Porosity Distribution within Underthrust Sediments at the Toe of the Nankai Subduction Zone: Inferences from PreStack Depth Migration Analysis  

Microsoft Academic Search

At the Nankai subduction zone offshore SW Japan, it has been inferred that rapid burial combined with the low permeability of underthrust sediments result in excess pore pressure, controlling the decollement strength. 2D PreStack Depth Migration (PSDM) was performed on seismic data collected in the seaward portion of the offshore Cape Muroto survey area within the Nankai subduction zone to

G. M. Kramer; P. C. Pisani; H. J. Tobin; G. F. Moore; D. Saffer

2005-01-01

317

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

NASA Astrophysics Data System (ADS)

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

Reynard, B.

2011-12-01

318

Elasticity of antigorite, seismic detection of serpentinites, and anisotropy in subduction zones  

NASA Astrophysics Data System (ADS)

Serpentinization of the mantle wedge is an important process that influences the seismic and mechanical properties in subduction zones. Seismic detection of serpentines relies on the knowledge of elastic properties of serpentinites, which thus far has not been possible in the absence of single-crystal elastic properties of antigorite. The elastic constants of antigorite, the dominant serpentine at high-pressure in subduction zones, were measured using Brillouin spectroscopy under ambient conditions. In addition, antigorite lattice preferred orientations (LPO) were determined using an electron back-scattering diffraction (EBSD) technique. Isotropic aggregate velocities are significantly lower than those of peridotites to allow seismic detection of serpentinites from tomography. The isotropic VP/VS ratio is 1.76 in the Voigt-Reuss-Hill average, not very different from that of 1.73 in peridotite, but may vary between 1.70 and 1.86 between the Voigt and Reuss bonds. Antigorite and deformed serpentinites have a very high seismic anisotropy and remarkably low velocities along particular directions. VP varies between 8.9 km s- 1 and 5.6 km s- 1 (46% anisotropy), and 8.3 km s- 1 and 5.8 km s- 1 (37%), and VS between 5.1 km s- 1 and 2.5 km s- 1 (66%), and 4.7 km s- 1 and 2.9 km s- 1 (50%) for the single-crystal and aggregate, respectively. The VP/VS ratio and shear wave splitting also vary with orientation between 1.2 and 3.4, and 1.3 and 2.8 for the single-crystal and aggregate, respectively. Thus deformed serpentinites can present seismic velocities similar to peridotites for wave propagation parallel to the foliation or lower than crustal rocks for wave propagation perpendicular to the foliation. These properties can be used to detect serpentinite, quantify the amount of serpentinization, and to discuss relationships between seismic anisotropy and deformation in the mantle wedge. Regions of high VP/VS ratios and extremely low velocities in the mantle wedge of subduction zones (down to about 6 and 3 km.s-1 for VP and VS, respectively) are difficult to explain without strong preferred orientation of serpentine. Local variations of anisotropy may result from kilometer-scale folding of serpentinites. Shear wave splittings up to 1-1.5 s can be explained with moderately thick (10-20 km) serpentinite bodies.

Bezacier, Lucile; Reynard, Bruno; Bass, Jay D.; Sanchez-Valle, Carmen; van de Moortèle, Bertrand

2010-01-01

319

The time constant variations of slow slip events in the south Alaska subduction zone  

NASA Astrophysics Data System (ADS)

Slow Slip events (SSEs), episodic periods of slip on the plate interface that are very slow in comparison to earthquakes, but much faster steady plate motions, have been identified at several subduction zones. One characteristic shared by all of these events is that they occur near or downdip of the base of the seismogenic zone as defined by the slip pattern of great earthquakes. A large slow slip event occurred at the Alaska subduction zone during 1998-2001 [Ohta et al., 2006, EPSL]. The SSE occurs downdip of the Prince William Sound asperity, on a section that accumulated some slip deficit before and after the SSE. During the SSE, >20,000 sq. km of the plate interfae slipped > 10cm, for a cumulative moment magnitude of Mw=7.2. Slip during this event was accompanied by significant non-volcanic tremor, which located in the same area as the slip. The area of the SSE did not have significant slip in the 1964 earthquake, compared to the slip of the main asperity. Our previous work did not attempt to determine the time history of slip in the SSE, because of a lack of continuous GPS data, but a reanalysis of the data that includes a few new sites allows us to estimate the approximate time history. Freymueller and Ohta (2007) found the short-term abnormal displacement in the south Alaska region during 2002-2006. The end of 2005 event may have the biggest event in the short-term abnormal displacements. We assumed the abnormal deformation as short-term SSE at the plate interface. The slip of short-term event reaches a maximum around 12-14 mm at plate boundary depths of 25-45 km, which corresponded to the transition zone of the plate interface. The short-term event fault located the neighboring of the long-term one during 1998-2001 investigated by Ohta et al., (2006.EPSL). The SSEs may play key role of strain accumulation in subduction zone. We will more precisely compare the displacement pattern and time history of the large SSE to the displacements to evaluate these smaller events actually represent SSEs on the same part of the plate interface.

Ohta, Y.; Freymueller, J. T.; Miura, S.

2007-12-01

320

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

NASA Astrophysics Data System (ADS)

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

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

2008-12-01

321

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

USGS Publications Warehouse

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

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

1997-01-01

322

Along-Strike Variations of Source Parameters for Subduction-Zone Earthquakes  

NASA Astrophysics Data System (ADS)

There is a continuum of rupture timescales for subduction zone earthquakes, from geodetically observed slow slip, tsunami earthquakes, and earthquakes of typical rupture velocities. Tsunami earthquakes, those interplate events that produce an anomalously large tsunami relative to the surface wave magnitude and are deficient in high frequency radiation, are of particular interest due to the associated hazards. One model to explain these events include large shallow seismic slip within low rigidity material, suggesting that patches of the fault which produce these large magnitude tsunami events could also produce related, long duration smaller events. Thus we investigate possible along-strike variation in rupture duration in the Chile, Alaska, Tonga, Izu and Java-Sumatra subduction zones, as these regions cover a subset of the subduction zones that have both produced tsunami earthquakes and regions that have not. These regions also have different tectonic conditions, such as differences in erosive or accretionary behavior, amounts of subducting bathymetry, sediment type and thickness, so we can explore how these tectonic differences may affect the along-strike earthquake source patterns. We use 289 shallow thrust earthquakes (Mw=5.0-7.0) relocated with the EHB method and an automated frequency-dependent phase detector to increase the number of depth phases used in the location. Source time functions, rupture durations and depths are determined using multi-station deconvolution of broadband teleseismic P- and SH-waves, and durations are normalized by moment for inter-event and along-strike comparisons. We group the events into shallow (? 25 km) and deep (26-57 km) long based on a t-test using the mean depth for the entire catalog. Here we focus on events with long normalized rupture duration (1 standard deviation above the mean) for comparison with tectonic features, with long events having durations of > 7.77 s (shallow) and >5.75 s (deep). Our results show that accretionary margins such Sumatra and Alaska have shallow long events in areas where terrigenous sediments are > 500 m, with the excess sediment acting to reduce earthquake rupture velocities. In erosive margins such as Java, Izu and Tonga, shallow long duration events correlate with subducting pelagic/hemipelagic (fluid-rich) sediments where fluids may also act to reduce rupture velocities. No long duration events in either depth category are observed in Chile, possibly due to little sedimentary cover in the event region and/or higher temperature of the young incoming slab. In both accretionary and erosional margins we observe deep long duration earthquakes occur in areas of subducting bathymetric features where fluids also play a role in increasing rupture duration. We also find that long duration earthquakes occur both in the shallow and deep sections of the seismogenic zone and subduction zones where tsunami earthquakes have occurred, shallow slow earthquakes also occur and are in the same region as tsunami earthquakes.

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

2011-12-01

323

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

324

Complex characteristics of slow slip events in subduction zones reproduced in multi-cycle simulations  

NASA Astrophysics Data System (ADS)

Since the discovery of slow slip events along subduction zone interfaces worldwide, dense geodetic and seismic networks have illuminated detailed characteristics of these events and associated tremor. High-resolution observations of tremor, where the spatial-temporal evolution is presumed to reflect that of the underlying slow slip events, show highly complex patterns in which the origins remain poorly understood. We present a new, computationally efficient modeling technique that reproduces many features of observed slow slip events, including slow initiation, coalescence of separate events, and rapid back-propagation of renewed slip over previously slipped regions. Rapid back propagation speeds are explained as a consequence of rate- and state-dependent frictional healing, consistent with analytical solutions developed in support of the simulations.

Colella, Harmony V.; Dieterich, James H.; Richards-Dinger, Keith; Rubin, Allan M.

2012-10-01

325

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

326

A slow slip event in the south central Alaska Subduction Zone and related seismicity anomaly  

NASA Astrophysics Data System (ADS)

We detected a slow slip event in the south central Alaska Subduction Zone by analyzing continuous GPS data from the Plate Boundary Observatory (PBO) network. The slow slip event started in early 2010 at a depth of 35 km beneath the Cook Inlet, near the down-dip end of the locked zone, and is ongoing as of November 2011 with an accumulated magnitude of Mw 6.9. Analysis of the earthquake catalog in the same area using the stochastic Epidemic Type Aftershock Sequence model (ETAS) shows a small but detectable seismicity increase during the slow slip event. We also find a change in seismicity rate around 1998, that may suggest an earlier slow slip event in the same region. Slow slip events in Alaska appear more widespread than previously thought but have remained undetected due to their long durations, the time intervals between them, and the limited time records from the continuous GPS.

Wei, Meng; McGuire, Jeffrey J.; Richardson, Eliza

2012-08-01

327

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

NSDL National Science Digital Library

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

328

Rutile/melt partition coefficients for trace elements and an assessment of the influence of rutile on the trace element characteristics of subduction zone magmas  

NASA Astrophysics Data System (ADS)

Fractionation of some or all of the high field strength elements (HFSE) Nb, Ta, Zr, Hf, and Ti relative to other trace elements occurs in igneous rocks from convergent margins and in the average continental crust, and is generally attributed to a process occurring during subduction. The experimental partitioning of an extensive array of trace elements between rutile/melt pairs is presented which enables the effect of rutile during melting in subduction zones to be directly assessed. D Nb and D Ta are in the range 100-500, D Zr and D Hf are about 5, whereas all other trace elements analyzed have D rutile/melt less than 0.1. Published D patterns for Nb and Ta between rutile and water-rich fluids are similar to those for melt, whereas the values for Zr and Hf are significantly higher. D Nb and D Ta values for clinopyroxene and garnet are much lower than for rutile, and cannot cause the fractionation of HFSE from other elements seen in island arcs. The presence of rutile in the subducted slab residue during dehydration may be essential in the production of the geochemical signatures of arc magmas, whereas that of the continental crust, including higher Zr/Sm, may be produced by melting of eclogite.

Foley, Stephen F.; Barth, Matthias G.; Jenner, George A.

2000-03-01

329

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

PubMed Central

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

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

1997-01-01

330

Velocity and Density Models Incorporating the Cascadia Subduction Zone for 3D Earthquake Ground Motion Simulations. Version 1.3.  

National Technical Information Service (NTIS)

In support of earthquake hazards and ground motion studies in the Pacific Northwest, three-dimensional P- and S-wave velocity (3D Vp and Vs) and density (3D rho) models incorporating the Cascadia subduction zone have been developed for the region encompas...

W. J. Stephenson

2007-01-01

331

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

Microsoft Academic Search

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

Shoichi Yoshioka; Kelin Wang; Stephane Mazzotti

2005-01-01

332

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

Microsoft Academic Search

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

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

2002-01-01

333

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

Microsoft Academic Search

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

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

2007-01-01

334

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

Microsoft Academic Search

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

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

2002-01-01

335

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

336

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

Microsoft Academic Search

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

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

1996-01-01

337

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

Microsoft Academic Search

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

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

1996-01-01

338

Times of increased probability of large earthquakes ( M s ? 7.5) along the Mexican subduction zone  

Microsoft Academic Search

Using the method of pattern recognition of infrequent events, we explore the possibility of identifying the times of increased probability (TIP) of occurrence of large earthquakes (Ms ? 7.5) along the Mexican subduction zone. A TIP refers to a 5 year period within which a strong earthquake has a high probability of occurrence. We analyzed the seismicity for the regions

David A. Novelo-Casanova; Jose Alvarez-Moctezuma

1995-01-01

339

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

Microsoft Academic Search

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

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

2009-01-01

340

Systematically Analyzing Relationships Between Three-Dimensional Subduction Zone Geometry and Slip During Great Earthquakes of the 21st Century  

NASA Astrophysics Data System (ADS)

Bathymetric and subduction interface structure have frequently been proposed to control the limits of rupture during large subduction zone earthquakes. However, in many instances the relationships between 3D subduction zone geometry and slip have not been robustly defined. Here we utilize Slab1.0, a new USGS compilation of the three-dimensional geometries of global subduction zones, in conjunction with USGS models of great (M > 8) earthquake ruptures extending back to the year 2000, to explore whether the limits of ruptures coincide with structural barriers. We correlate the spatial distribution of slip with the three-dimensional structure of the subducting slab and show that often, the ends of ruptures coincide with regional or local highs of the descending plate, which are in turn associated with bathymetric highs outboard of the subduction zone. We also investigate the effects of uncertainties in modeled slip distribution on these correlations. We use the March 11, 2011 Mw 9.0 Tohoku and the February 27, 2010 Mw 8.8 Maule earthquakes as examples (among others), and discuss some notable exceptions, such as the April 1, 2007 Mw 8.1 Solomon Islands and the September 25, 2003 Mw 8.3 Hokkaido events. We discuss the differences between these sets of events, and attempt to identify features elsewhere in these slab models that may have the potential to act as similar barriers to the rupture propagation of future earthquakes.

Hayes, G. P.; Wald, D. J.; Briggs, R. W.

2011-12-01

341

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

SciTech Connect

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

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

1995-11-10

342

Chlorine cycling in subduction zones: Insights from submarine glasses and melt inclusions from arc and back-arc basalts  

Microsoft Academic Search

Chlorine is an excellent tracer of contributions of slab-derived fluids to mantle-derived melts produced during subduction zone magmatism, as saline fluids released during slab dehydration are enriched in chlorine by several orders of magnitude over mantle peridotite. Chlorine and other halogens also strongly influence the solubility of many cations in high temperature solutions and thus play an important role in

A. J. Kent; M. C. Rowe

2004-01-01

343

The Role of Bending Resistance at Subduction Zones on the Force Balance of Plate Tectonics through the Cenozoic  

Microsoft Academic Search

The temperature-dependent rheology of mantle rock is such that cold lithosphere should have an effective viscosity several orders of magnitude greater than that of the underlying mantle. As a result, the deformation required to bend plates at subduction zones may have a slowing effect on plate motions comparable to the slowing associated with the deformation of the shearing mantle interior.

C. P. Conrad; C. Lithgow-Bertelloni

2001-01-01

344

On the relationships between slab dip, back-arc stress, upper plate absolute motion, and crustal nature in subduction zones  

Microsoft Academic Search

Statistical analysis of modern oceanic subduction zone parameters, such as the age of a downgoing plate or the absolute plate motions, is performed in order to investigate which parameter controls the dip of a slab and, conversely, what the influence of slab geometry is on upper plate behavior. For that purpose, parameters have been determined from global databases along 159

Serge Lallemand; Arnauld Heuret; David Boutelier

2005-01-01

345

Chapter 2.11: Subduction zone processes and implications for changing composition of the upper and lower mantle  

Microsoft Academic Search

With ca. forty thousand kilometers of subduction zones and convergence rates from 30 km Ma-1 to 180 km Ma-1, subduction carries massive amounts of material into seafloor trenches, and beyond. Most of the subducting plate is made of mantle material returning to the depths from which it originated. The hydrated and altered upper oceanic section and the overlying sediments, however,

J D Morris; Jeffrey G Ryan

2003-01-01

346

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

NASA Astrophysics Data System (ADS)

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

Myhill, R.; Warren, L. M.

2011-12-01

347

Bromine cycle in subduction zones through in situ Br monitoring in diamond anvil cells  

NASA Astrophysics Data System (ADS)

The geochemical partitioning of bromine between hydrous haplogranitic melts, initially enriched with respect to Br and aqueous fluids, has been continuously monitored in situ during decompression. Experiments were carried out in diamond anvil cells from 890 °C to room temperature and from 1.7 GPa to room pressure, typically from high P, T conditions corresponding to total miscibility (presence of a supercritical fluid). Br contents were measured in aqueous fluids, hydrous melts and supercritical fluids. Partition coefficients of bromine were characterized at pressure and temperature between fluids, hydrous melts and/or glasses, as appropriate: DBrfluid/melt = (Br) fluid/(Br) melt, ranges from 2.18 to 9.2 ± 0.5 for conditions within the ranges 0.66-1.7 GPa, 590-890 °C; and DBrfluid/glass = (Br) fluid/(Br) glass ranges from 60 to 375 at room conditions. The results suggest that because high pressure melts and fluids are capable of accepting high concentrations of bromine, this element may be efficiently removed from the slab to the mantle source of arc magmas. We show that Br may be highly concentrated in subduction zone magmas and strongly enriched in subduction-related volcanic gases, because its mobility is strongly correlated with that of water during magma degassing. Furthermore, our experimental results suggest that a non negligible part of Br present in the subducted slab may remain in the down-going slab, being transported toward the transition zone. This indicates that the Br cycle in subduction zones is in fact divided in two related but independent parts: (1) a shallower one where recycled Br may leave the slab with a water and silica-bearing "fluid" leading to enriched arc magmas that return Br to the atmosphere. (2) A deeper cycle where Br may be recycled back to the mantle maybe to the transition zone, where it may be present in high pressure water-rich metasomatic fluids.

Bureau, Hélène; Foy, Eddy; Raepsaet, Caroline; Somogyi, Andrea; Munsch, Pascal; Simon, Guilhem; Kubsky, Stefan

2010-07-01

348

Ascent paths of fluids and partial melts at the Sunda Arc compared with other subduction zones by geophysical parameters  

NASA Astrophysics Data System (ADS)

During the last decades many investigations were carried out at active continental margins to understand the link between the subduction of the fluid saturated oceanic plate and the process of ascent of fluids and partial melts forming a magmatic system that leads to volcanism at the earth surface. With seismic methods the physical structure of the down going slab and the part above it can be resolved. In particular the record of the natural seismicity and its distribution allows the three-dimensional imaging of the entire crust and lithosphere structure above the Wadati Benioff zone with the help of tomographic procedures, and therewith the entire ascent path region and reservoirs of fluids and melts, which are responsible for volcanism. If we consider statistically the distance between the volcanic chain at the earth surface down to the subducted plate and the Wadati Benioff zone then the mean value of the depth distance results of approximately 110 kilometers. Surprisingly, this depth range shows pronounced seismicity at most of all subduction zones. Additionally, mineralogical investigations in the lab have shown that the diving plate is maximal dehydrated around the 100 km depth because of temperature and pressure conditions at this depth range. However, assuming a vertical fluid ascent there are exceptions. For instance at the Sunda Arc beneath Central Java the vertical distance results in approximately 150 km, but, seismic investigations have shown that here the fluids do not ascend vertically, but inclined even from a source area at around the 100 km depth. With the seismic tomography areas where fluids ascent and the appearance of partial melts as well as the distribution of these materials in the crust can be detected and imaged by their lowered seismic velocities, high Vp/Vs ratios, as well as increased attenuation of seismic shear waves. The seismic velocity anomalies detected so far are within a range of a few per cent to more than 30% reduction. Discussed will be results with findings from structural investigations at Lake Toba, Sumatra, Kamchatka and parts of the Andes.

Luehr, B.-G.; Koulakov, I.; Kopp, H.; Rabbel, W.; Zschau, J.

2012-04-01

349

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

350

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

351

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

Microsoft Academic Search

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

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

2007-01-01

352

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

Microsoft Academic Search

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

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

2007-01-01

353

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

354

Redistribution of trace elements during prograde metamorphism from lawsonite blueschist to eclogite facies; implications for deep subduction-zone processes  

Microsoft Academic Search

The transfer of fluid and elements from subducting crust to the overlying mantle wedge is a fundamental process affecting arc magmatism and the chemical differentiation of the Earth. While the production of fluid by breakdown of hydrous minerals is well understood, the liberation of trace elements remains generally unconstrained. In this paper, we evaluate the behaviour of trace elements during

Carl Spandler; Jörg Hermann; Richard Arculus; John Mavrogenes

2003-01-01

355

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

NASA Astrophysics Data System (ADS)

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

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

2011-12-01

356

Coseismic Slip Distributions of Great or Large Earthquakes in the Northern Japan to Kurile Subduction Zone  

NASA Astrophysics Data System (ADS)

Slip distributions of great and large earthquakes since 1963 along the northern Japan and Kuril trenches are examined to study the recurrence of interplate, intraslab and outer-rise earthquakes. The main findings are that the large earthquakes in 1991 and 1995 reruptured the 1963 great Urup earthquake source, and the 2006, 2007 and 2009 Simshir earthquakes were all different types. We also identify three seismic gaps. The northern Japan to southern Kurile trenches have been regarded as a typical subduction zone with spatially and temporally regular recurrence of great (M>8) interplate earthquakes. The source regions were grouped into six segments by Utsu (1972; 1984). The Headquarters for Earthquake Research Promotion of the Japanese government (2004) divided the southern Kurile subduction zone into four regions and evaluated future probabilities of great interplate earthquakes. Besides great interplate events, however, many large (M>7) interplate, intraslab, outer-rise and tsunami earthquakes have also occurred in this region. Harada, Ishibashi, and Satake (2010, 2011) depicted the space-time pattern of M>7 earthquakes along the northern Japan to Kuril trench, based on the relocated mainshock-aftershock distributions of all types of earthquakes occurred since 1913. The space-time pattern is more complex than that had been considered conventionally. Each region has been ruptured by a M8-class interplate earthquake or by multiple M7-class events. In this study, in order to examine more detail space pattern, or rupture areas, of M>7 earthquakes since 1963 (WWSSN waveform data have been available since this year), we estimated cosiesmic slip distributions by the Kikuchi and Kanamori's (2003) teleseismic body wave inversion method. The WWSSN waveform data were used for earthquakes before 1990, and digital teleseismic waveform data compiled by the IRIS were used for events after 1990. Main-shock hypocenters that had been relocated by our previous study were used as initial rupture points. Some preliminary results are as follows. Offshore Urup Is. is source region of the 1963 Urup earthquake (M 8.5). Large interplate earthquakes occurred in the eastern and western part of the 1963 source region in 1991 (M 7.6) and 1995 (M 7.9), respectively. Their aftershock areas almost re-occupied the 1963 aftershock area. The 1963, 1991, and 1995 coseismic slip distributions show that the southwestern asperity of the 1963 event seems to be re-ruptured by the 1995 earthquake. The 2009 Simushir earthquake (M 7.4) with reverse faulting occurred within the aftershock area of the 2007 great outer-rise event (M 8.1). The 2007 and 2009 coseismic slip distributions show that the 2007 normal faulting occurred in the shallower part of the Pacific plate and the 2009 reverse intraplate faulting occurred in the deeper part. Giant (the 2011 Tohoku earthquake of M 9.0), great and large interplate earthquakes occurred in the Kurile to Japan subduction zone after 1990s successively. The aftershock areas and coseismic slip distributions clearly show that only three seismic gaps (offshore Aomori pref., offshore eastern Hokkaido to Etorofu Is., and offshore between Urup and Simushir Is.) have remained in this region.

Harada, T.; Satake, K.; Ishibashi, K.

2011-12-01

357

CO2 analysis in high-pressure melts and implications for carbon recycling in subduction zones  

NASA Astrophysics Data System (ADS)

Carbon recycling in subduction zones plays an important role in the evolution of climate through geological time and in the formation of diamonds in the deep mantle. Carbonates formed during seafloor alteration of the oceanic crust are buried into the deep Earth along subduction zones. We conducted experiments with a starting material made of a synthetic Potassium, CO2 and H2O-bearing basaltic composition at 3.0 and 3.5 GPa, 700 - 800°C and at 4.5 GPa, 800-900°C in a piston-cylinder apparatus, corresponding to typical conditions of the subducted slab at subarc conditions. The experimental results indicate that the solidus occurs between 700 and 750°C at both 3.0 and 3.5 GPa. In order to understand the CO2 transfer in subduction zones it is thus important to quantify the molar X(CO2)=CO2/(CO2+H2O) of such aqueous fluid or hydrous melt. However, so far this has not been possible because CO2 exsolves from the fluid/melt in the experiments during quenching. We have developed a new method where the experimental capsules are pierced under vacuum and the gas present after the experiment is mixed with He and directly analysed with a gas chromatograph equipped with a thermal conductivity detector (GC-TCD). Peak areas read in the chromatograms are proportional to the partial pressures of CO2 and H2O. Small quantities of these gasses can be determined with accuracy (±5%, on average) after the calibration of the GC-TCD using pure gas standards. For subsolidus runs at 700°C, GC-TCD analyses of the aqueous fluid show a decrease of the X(CO2) from 5.9±0.3% to 3.8±0.3% with increasing pressure from 3.0 to 3.5 GPa, respectively. It is more difficult to assess the CO2-H2O compositions of the hydrous melts. GC-TCD analysis of runs at 775°C show that approximately 75% (X(CO2)=3.6±0.1%) and 50% (X(CO2) =5.7±0.3%) of the volatiles dissolved in the melt are exsolved during the quench in the 3.5 and 3.0 GPa experiments, respectively. Additionally, the glass pools have been analysed by FTIR spectroscopy using an ATR objective, which provides an excellent means of comparison between relative amounts of dissolved volatile species. The main carbon species in the melt pool is the carbonate ion. The relative heights of the total water and carbonate peaks indicate that the glass at 3.5 GPa has about four times higher X(CO2) than the one at 3.0 GPa and X(CO2) values are 2-3% and 8-12% at 3 and 3.5 GPa, respectively. The resulting X(CO2) of the melt that was present at experimental conditions is thus ~3% at 3.0 GPa and ~7% at 3.5 GPa, 775°C (Fig. 3), providing evidence for a significant enhancement of carbonate solubility with increasing pressure in hydrous melts. Our results suggest that fluid fluxed partial melting of altered basalts provides an efficient mechanism by which significant amounts of subducted carbon can be brought back to the atmosphere via arc magmatism on relatively short time scales of less than 10 Ma.

Hermann, J.; Martin, L.

2012-04-01

358

Metamorphic devolatilization of subducted marine sediments and the transport of volatiles into the Earth's mantle.  

PubMed

Volatiles, most notably CO2, are recycled back into the Earth's interior at subduction zones. The amount of CO2 emitted from arc volcanism appears to be less than that subducted, which implies that a significant amount of CO2 either is released before reaching the depth at which arc magmas are generated or is subducted to deeper depths. Few high-pressure experimental studies have addressed this problem and therefore metamorphic decarbonation in subduction zones remains largely unquantified, despite its importance to arc magmatism, palaeoatmospheric CO2 concentrations and the global carbon cycle. Here we present computed phase equilibria to quantify the evolution of CO2 and H2O through the subduction-zone metamorphism of carbonate-bearing marine sediments (which are considered to be a major source for CO2 released by arc volcanoes). Our analysis indicates that siliceous limestones undergo negligible devolatilization under subduction-zone conditions. Along high-temperature geotherms clay-rich marls completely devolatilize before reaching the depths at which arc magmatism is generated, but along low-temperature geotherms, they undergo virtually no devolatilization. And from 80 to 180 km depth, little devolatilization occurs for all carbonate-bearing marine sediments. Infiltration of H2O-rich fluids therefore seems essential to promote subarc decarbonation of most marine sediments. In the absence of such infiltration, volatiles retained within marine sediments may explain the apparent discrepancy between subducted and volcanic volatile fluxes and represent a mechanism for return of carbon to the Earth's mantle. PMID:11357128

Kerrick, D M; Connolly, J A

2001-05-17

359

Earthquakes in Japan: A vital role of geofluids in earthquake generation in subduction zones  

NASA Astrophysics Data System (ADS)

Recent seismic observations based on a dense nationwide seismic network in Japan have shown that generations of all the three main types of earthquakes in this subduction zone (except for deep earthquakes) are closely related with geofluids. Studies on spatial distribution of earthquakes and seismic velocity structure within the subducted slab provide evidence which strongly supports the dehydration embrittlement hypothesis for generation of intermediate-depth intraslab earthquakes. Detailed imagings of seismic velocity structure in and around the plate boundary zones suggest that interplate coupling is mainly controlled by fluid overpressure there. Seismic tomography studies have shown the existence of inclined sheet-like seismic low-velocity zones in the mantle wedge not only in Tohoku but also in other areas in Japan, which perhaps correspond to the upwelling flow of the subduction-induced convection system. These upwelling flows reach the Moho right beneath the volcanic areas, suggesting that those volcanic areas are formed by the upwelling flows. Aqueous fluids derived from the slab are probably transported upward through the upwelling flows to reach the arc crust, where they might work to weaken the surrounding crustal rocks and finally cause shallow inland earthquakes. Generations of the 2011 M9.0 great Tohoku earthquake and earthquake activities induced by it also seem to be closely related with geofluids. Observed temporal change in stress field near the source area after the earthquake shows nearly complete stress drop by the earthquake, which suggests the plate interface is weak. Temporal change in stress field after the Tohoku earthquake is also observed for inland areas far from the source, suggesting that faults for shallow inland earthquakes are weak as well. These weak faults are inferred to be caused by overpressured fluids. Detailed imagings in the source area of a large (M7.1) intermediate-depth intraslab earthquake that occurred about one month after the Tohoku earthquake provide evidence for reactivation of a buried hydrated fault ,caused by the Tohoku earthquake, in the subducted Pacific slab. All these observations suggest that geofluids expelled from the subducting slab play an important role in the generation of earthquakes in subduction zones.

Hasegawa, A.; Nakajima, J.

2011-12-01

360

Forearcs and Supra-Subduction Zone Ophiolites: Some Similarities and Differences (Invited)  

NASA Astrophysics Data System (ADS)

Forearcs have long been recognised as important potential protoliths for ophiolites, given that they are the first lithologies to be obducted when a continent ‘collides’ with a trench in an intraoceanic setting. It is therefore perhaps surprising that oceanic forearcs and supra-subduction zone (SSZ) ophiolites are not more similar. One possible reason is that the best-understood present-day forearcs had time to evolve following the initiation of subduction of a large ocean (the Pacific) whereas most ophiolites represent extremely short-lived subduction events associated with the closure of small ocean basins. Thus only the first few million years of forearc evolution is critical to ophiolite formation and that is the least well understood part of forearc development in Western Pacific analogues. If there is anything systematic about supra-subduction zone ophiolites, it is that there is a general trend of increased depletion with time: this may be represented by a lava stratigraphy of true MORB, overlain by island arc tholeiites and then boninites (e.g. Pindos), or slightly anomalous MORB overlain by island arc tholeiities, then boninites (e.g. Oman) or island arc tholeiites overlain by boninites (Cyprus) or just boninites (e.g. Vourinos). The same progression can be seen in dykes, cumulates and (through spinel geochemistry) mantle tectonites. This may be explained by differences in subduction initation variables, best unravelled by detailed interpretation of each ophiolite in a forearc, subduction initiation context. We take as an example one ophiolite, the Troodos Massif, where we can use the large database of volcanic glass major element analyses to evaluate its setting in detail. We find that the lower (axis) lavas are high-Si8, medium-Fe tholeiities and that they are overlain by high-Ca boninites. This combination is characteristic of subduction initiation terranes and slab edges rather than ‘normal’ arc-basin systems. Low K8/H8 ratios support a near-trench setting where the mantle wedge can capture the water-rich, K-poor fluids normally lost through cold forearcs. Na8-Fe8 systematics demonstrate that the lower (axis) lavas of the ophiolite formed from mantle with a high potential temperature of about 1400 degrees-C, consistent with the rapid influx of hot mantle at a slab edge or during slab rollback after subduction initiation. Geological reconstructions indicate that slab tear (at a STEP) fault was likely needed to allow the influx of mantle into the wedge and hence slab roll-back. This was necessary in a small ocean bounded by continents where mantle flow was restricted, but may have been less important in the open ocean of the Western Pacific.

Pearce, J. A.; Robinson, P. T.

2009-12-01

361

Frictional properties of megathrust fault gouges under true in-situ subduction zone conditions  

NASA Astrophysics Data System (ADS)

The seismogenic zone of most subduction megathrusts spans a temperature range of ~150-350°C, while Slow Slip Events (SSEs) are reported to occur at temperatures up to ~500°C. Fault rocks at the corresponding depths are expected to consist of mixtures of phyllosilicates with quartz. At temperatures of ~100-200°C, the dominant phyllosilicate phase is illite, which is replaced at ~200-300°C by increasingly well-crystallized muscovite. However, relatively few data are available of the fric