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1

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

NASA Astrophysics Data System (ADS)

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

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

2009-09-01

2

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

3

Earthquakes, fluid pressures and rapid subduction zone metamorphism  

NASA Astrophysics Data System (ADS)

High-pressure/low-temperature (HP/LT) metamorphism is commonly incomplete, meaning that large tracts of rock can remain metastable at blueschist- and eclogite-facies conditions for timescales up to millions of years [1]. When HP/LT metamorphism does take place, it can occur over extremely short durations (<<1 Myr) [1-2]. HP/LT metamorphism must be associated with processes that allow large volumes of rock to remain unaffected over long periods of time, but then suddenly undergo localized metamorphism. Existing models for HP/LT metamorphism have focussed on the role of fluids in providing heat for metamorphism [2] or catalyzing metamorphic reactions [1]. Earthquakes in subduction zone settings can occur to depths of 100s of km. Metamorphic dehydration and the associated development of elevated pore pressures in HP/LT metamorphic rocks has been identified as a cause of earthquake activity at such great depths [3-4]. The process of fracturing/faulting significantly increases rock permeability, causing channelized fluid flow and dissipation of pore pressures [3-4]. Thus, deep subduction zone earthquakes are thought to reflect an evolution in fluid pressure, involving: (1) an initial increase in pore pressure by heating-related dehydration of subduction zone rocks, and (2) rapid relief of pore pressures by faulting and channelized flow. Models for earthquakes at depth in subduction zones have focussed on the in situ effects of dehydration and then sudden escape of fluids from the rock mass following fracturing [3-4]. On the other hand, existing models for rapid and incomplete metamorphism in subduction zones have focussed only on the effects of heating and/or hydration with the arrival of external fluids [1-2]. Significant changes in pressure over very short timescales should result in rapid mineral growth and/or disequilibrium texture development in response to overstepping of mineral reaction boundaries. The repeated process of dehydration-pore pressure development-earthquake-pore pressure relief could conceivably produce a record of episodic HP/LT metamorphism driven by rapid pressure pulses. A new hypothesis is presented for the origins of HP/LT metamorphism: that HP/LT metamorphism is driven by effective pressure pulses caused by localized, earthquake-related modifications to fluid pressures in the subducted slab. In other words, HP/LT metamorphism marks abrupt changes in stress state within the subducted slab, driven by earthquake rupture and fluid flow, and involving a rapid return toward lithostatic pressure from effective pressures well below lithostatic. References: 1. Bjørnerud, MG, Austrheim, H & Lund, MG, 2002. Processes leading to eclogitization (densification) of subducted and tectonically buried crust. Journal of Geophysical Research 107, 2252. 2. Camacho, A, Lee, JKW, Hensen, BJ & Braun, J, 2005. Short-lived orogenic cycles and the eclogitization of cold crust by spasmodic hot fluids. Nature 435, 1191-1196. 3. Green, HW & Houston, H, 1995. The mechanics of deep earthquakes. Annual Reviews of Earth and Planetary Sciences 23, 169-213. 4. Hacker, BR, Peacock, SM, Abers, GA & Holloway, SD, 2003. Subduction factory 2. Are intermediate-depth earthquakes in subducting slabs linked to metamorphic dehydration reactions?. Journal of Geophysical Research 108, 2030.

Viete, D. R.

2013-12-01

4

Does subduction zone magmatism produce average continental crust  

NASA Technical Reports Server (NTRS)

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

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

1988-01-01

5

Experimental modeling of subduction zone metamorphism and devolatilization  

NASA Astrophysics Data System (ADS)

Subduction zones are characterized by valuable thermal gradients owing to descend of cold oceanic plate into the hot mantle. We propose for the first time to apply high-gradient zones of the piston-cylinder apparatus to study the interaction between subducting plate and hanging wall mantle. In this contribution, we present peculiar features of the glaucophane schist-olivine interaction as analogs of crust and mantle, respectively. The glaucophane schist is from the Atbashi complex, Tien Shan, the olivine is collected from the quarry Ahaim, Norway. ?-? conditions of the runs correspond to so-called "warm" geotherm. We present results of the two runs carried out at pressure of 2.4 and 2.6 GPa and both at the temperature of 1050 grad ? at the upper range of the capsule. Temperature of ~700 grad C the lower edge of the 7 mm capsule was inferred from the numerical modeling based on the real sizes of materials after the runs. The experiments reveal that transformation of the glaucophane schist is strongly controlled by the pressure. The transformation was resulted in growth of barroisitic amphibole rims around the primary glaucophane at 2.4 GPa and patchy omphacite+quartz aggregate at 2.6 GPa. Thus we didn't observe the typical eclogite assemblage (omphacite+garnet) in the metabasic rock well within the eclogite facies P-T conditions. The devolatalization owing to the breakdown of hydrous mineral of the glaucophane schist lead to the formation of thin (30 micron) orthopyroxene layer at the contact of olive with glaucophane schist. In addition, first portions of melt (dacite composition) appear in between the orthopyroxene layer and the glaucophane schist. The melt (fluid) was migrated above the opx layer but owing to the low Si-content didn't react with the host olivine. The study demonstrated that high-gradient zones of the "piston-cylinder" apparatus are very efficient for study mantle-crust interaction in the subduction zones. Financial support by RFBR grants N 09-05-01217 and 09-05-00991.

Perchuk, Alexei L.; Korepanova, Olga S.

2010-05-01

6

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

NASA Astrophysics Data System (ADS)

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

Laurie, A.; Stevens, G.

2011-12-01

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

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.

2007-12-01

9

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

NASA Technical Reports Server (NTRS)

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

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

1989-01-01

10

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

NASA Astrophysics Data System (ADS)

We studied the Zhimafang ultrahigh-pressure metamorphic (UHP) peridotite from pre-pilot drill hole PP-1 of Chinese Continental Scientific Drilling project in the Sulu UHP terrane, eastern China. The peridotite occurs as lens within quartofeldspathic gneiss, and has an assemblage of Ol + Opx + Cpx + Phl + Ti-clinohumite (Ti-Chu) + Grt (or chromite) ± magnesite (Mgs). Zircons were separated from cores at depths of 152 m (C24, garnet lhezolite), 160 m (C27, strongly retrograded phlogopite-rich peridotite) and 225 m (C50, banded peridotite), and were dated by SHRIMP mass spectrometer. Isometric zircons without inherited cores contain inclusions of olivine (Fo 91-92), enstatite (En 91-92), Ti-clinohumite, diopside, phlogopite and apatite. The enstatite inclusions have low Al 2O 3 contents of only 0.04-0.13 wt.%, indicating a UHP metamorphic origin. The weighted mean 206Pb / 238U zircon age for garnet lherzolite (C24) is 221 ± 3 Ma, and a discordia lower intercept age for peridotite (C50) is 220 ± 2 Ma. These ages are within error and represent the time of subduction-zone UHP metamorphism. A younger lower intercept age of 212 ± 3 Ma for a foliated wehrlite (C27) was probably caused by Pb loss during retrograde metamorphism. The source of zirconium may be partially attributed to melt/fluid metasomatism within the mantle wedge. Geochronological and geochemical data confirm that the mantle-derived Zhimafang garnet peridotites (probably the most representative type of Sulu garnet peridotites) were tectonically inserted into a subducting crustal slab and subjected to in situ Triassic subduction-zone UHP metamorphism.

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

2005-09-01

11

High-pressure metamorphism in the Aegean, eastern Mediterranean: Underplating and exhumation from the Late Cretaceous until the Miocene to Recent above the retreating Hellenic subduction zone  

NASA Astrophysics Data System (ADS)

We report 40Ar/39Ar ages from various tectonic units in the Aegean and westernmost Turkey. On the basis of published geochronologic data and our 40Ar/39Ar ages we propose that the Aegean is made up of several high-pressure units, which were successively underplated from the Late Cretaceous until the Miocene. Ages for high-pressure metamorphism range from 80-83 Ma in parts of the Vardar-Izmir-Ankara suture zone in the north to 21-24 Ma for the Basal unit in the Cyclades and the external high-pressure belt on Crete in the south. Published seismic data suggest that high-pressure metamorphism is currently occurring underneath Crete. Younging of high-pressure metamorphism in a southerly direction mimics the southward retreat of the Hellenic subduction zone. We propose that distinct stages of high-pressure metamorphism were controlled by the underthrusting of fragments of mainly thinned continental crust and that these punctuated events were superposed on progressive slab retreat. By far most of the exhumation of the high-pressure units occurred early during the orogenic history in a forearc position.

Ring, Uwe; Layer, Paul W.

2003-06-01

12

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

NASA Astrophysics Data System (ADS)

The Chilean Andes extend north-south for about 3000 km over the subducting Nazca plate, and show evidence of local rheological controls on first-order tectonic features. Here, rheological parameters are tested with numerical models of a subduction driven by slab-pull and upper plate velocities, and which calculate the development of stress and strain over a typical period of 4 Myr. The models test the effects of subduction interface strength, arc and fore-arc crust rheology, and arc temperature, on the development of superficial near-surface faulting as well as viscous shear zones in the mantle. Deformation geometries are controlled by the intersection of the subduction interface with continental rheological heterogeneities. Upper plate shortening and trench advance are both correlated, and favored, to a first-order by upper plate weakness, and to a second-order by interface strength. In cases of a strong interface, a weak fore-arc crust is dragged downward by “tectonic erosion”, a scenario for which indications are found along the northern Chilean margin. In contrast for a resistant fore-arc, the slab-pull force transmits to the surface and produces topographic subsidence. This process may explain present-day subsidence of the Salar de Atacama basin and/or the persistence of a Central Depression. Specific conditions for northern Chile produce a shear zone that propagates from the subduction zone in the mantle, through the Altiplano lower crust into the Sub-Andean crust, as proposed by previous studies. Models with a weak interface in turn, allow buoyant subducted material to rise into the continental arc. In case of cessation of the slab-pull, this buoyant material may rise enough to change the stress state in the continental crust, and lead to back-arc opening. In a case of young and hydrated oceanic plate forced by the slab-pull to subduct under a resistant continent, this plate is deviated and indented by the continental mantle, and stretches horizontally at ˜100 km depth. This situation might explain the flat Wadati-Benioff zone of Central Chile.

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

2009-12-01

13

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

14

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

NASA Astrophysics Data System (ADS)

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

Meyer, Melanie; Klemd, Reiner; Konopelko, Dmitry

2013-09-01

15

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

NASA Astrophysics Data System (ADS)

Depending on the magnitude of the poorly constrained C flux in ultramafic rocks, on a global basis, sediments and altered oceanic crust (AOC) together deliver 70-95% of the C currently entering subduction zones. We are investigating extents of retention and metamorphic release of C in deeply subducted AOC and carbonate-rich sediment represented by HP/UHP meta-ophiolitic and metasedimentary rocks in the Italian Alps and in the Tianshan. Study of metapelite devolatilization in the same W. Alps suite (Bebout et al., 2013, Chem. Geol.) provides a geochemical framework for study of C behavior along prograde P-T paths similar to those experienced in forearcs of most modern subduction margins. Study of veins in the Tianshan affords examination of C mobility in UHP fluids, in later stages as metabasaltic rocks were fragmented in the subduction channel. Our results for sediments and AOC indicate impressive retention of oxidized C (carbonate) and reduced C (variably metamorphosed organic matter) to depths approaching those beneath arc volcanic fronts. In metasedimentary rocks, extensive isotopic exchange between the oxidized and reduced C resulted in shifts in both reservoirs toward upper mantle compositions. Much of the carbonate in metabasalts has C and O isotopic compositions overlapping with those for carbonate in AOC, with some HP/UHP metamorphic veins showing greater influence of organic C signatures from metasedimentary rocks. Calculations of prograde devolatilization histories using Perple-X demonstrate that, in most forearcs, very little decarbonation occurs in the more carbonate-rich rocks unless they are flushed by H2O-rich fluids from an external source, for example, from the hydrated ultramafic section of subducting slabs (cf. Gorman et al., 2006; G3) or from more nearby rocks experiencing dehydration (e.g., metapelites). A comparison of the most recently published thermal models for modern subduction zones (van Keken et al., 2011, JGR) with calculated and experimentally determined phase relations indicates that significant C loss during devolatilization (and partial melting) should occur as subducting sections traverse depths beneath arcs. The extent of C mobility due to carbonate dissolution remains uncertain. On a global basis, imbalance between subducted C input and C return flux by magmatism (excluding ultramafic inputs, ~40×20% of subducted C return via arcs and ~80×20% by all magmatism; Bebout, 2013, Treat. Geochem.) indicates net modern C return to the mantle, perhaps a reversal of Archean net outgassing (despite more rapid subduction). Global C cycle models predict that relatively small (and geologically plausible) change in the subduction/volcanic C flux could significantly affect atmospheric CO2 levels and thus global climate.

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

2013-12-01

16

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

NASA Astrophysics Data System (ADS)

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 typifying signature of collision orogens (Garzanti et al., 2007). In the Austroalpine Cretaceous and Penninic Eocene axial belts of the Alps, we ideally distinguish three structural levels, each characterized by diagnostic detrital fingerprints. The shallow level chiefly consists of offscraped remnant-ocean turbidites and unmetamorphosed continental-margin sediments, and mostly produces lithic to lithoquartzose sedimentaclastic sands yielding very-poor heavy-mineral suites including ultrastable minerals. The intermediate level includes low-grade metasediments and polymetamorphic basements, and sheds lithoquartzose to quartzolithofeldspathic metamorphiclastic sands yielding moderately-rich epidote- amphibole suites with chloritoid or garnet. The deep level contains eclogitic remnants of continent- ocean transitions, and supplies quartzofeldspathic to quartzolithic high-rank metamorphiclastic to lithic ultramaficlastic sands yielding rich to extremely-rich suites dominated by garnet, hornblende, or epidote depending on protoliths (continental vs. oceanic) and pressure/temperature paths followed during exhumation. Although widely overprinted under greenschist-facies or amphibolite-facies conditions, occurrence of ultradense eclogite in source areas is readily revealed by the Heavy Mineral Concentration (HMC) index, which mirrors the average density of source rocks in the absence of hydraulic-sorting effects (Garzanti and Andò 2007). The Metamorphic Index (MI, Garzanti and Vezzoli, 2003) and Hornblende Colour Index (HCI) reflect peak temperatures reached at later stages, when subduction is throttled by arrival of thicker continental crust and geothermal gradients increase. Experience gained from modern sediments provides fundamental help to decrypt the innumerable pieces of information stored in the sedimentary record, and thus to identify and reconstruct subduction events of the past. Dickinson, W., R., Suczek, C.A., 1979.Plate tectonics and sandstone compositions. AAPG Bull. 63, 2164-2182. Garzanti, E. and Andò, S. 2007. Heavy-mineral concentration in modern sands: implications for provenance interpretation. In Mange, M., and Wright, D., eds. Heavy Minerals in Use. Developments in Sedimentology Series 58. Amsterdam, Elsevier, p. 517-545. Garzanti, E., and Vezzoli, G. 2003. A classification of metamorphic grains in sands based on their composition and grade. J. Sediment. Res. 73:830-837. Garzanti, E., Doglioni, C., Vezzoli, G., Andò, S., 2007. Orogenic belts and orogenic sediment provenante. J. Geology, 115:315-334.

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

2010-05-01

17

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

NASA Astrophysics Data System (ADS)

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

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

2011-08-01

18

Continental Arc subduction Zone Processes: Insights from Quartz-Bearing High Mg# Metasomatic Selvages in Mantle Xenoliths  

NASA Astrophysics Data System (ADS)

Subduction zones are fundamental in generating continental crust. An important step in the formation of an arc is the passage of slab derived fluids or melts through the mantle wedge. However, access to the mantle wedge or sub-arc lithosphere is generally rare in such environments. Here, we examine xenoliths from the Pleistocene Big Pine volcanic field in Owens Valley, California in effort to describe the metasomatic influences of the slab-derived fluids on the mantle underlying the Sierra Nevada batholith, the remnants of a Mesozoic continental arc. These mantle xenoliths are plagioclase-quartz-feldspar-clinopyroxene-phlogopite selvages and veins, which contain abundant accessory zircon and sphene. The plagioclases are calcic and the clinopyroxenes have high Mg#s greater than 85, which suggest equilibrium with very primitive magmas or at least magmas with relatively high Mg#. Parodoxically, however, the presence of quartz requires a more felsic, and hence, evolved magma. We thus interpret these lithologies to represent cumulates from a high Mg# felsic magma. Such magmas could have originated as partial melts of mafic lithologies, such as subducting oceanic crust, delaminating lower crust, or garnet pyroxenite veins. As these felsic melts pass through the mantle wedge or lithospheric mantle, they may have reacted with peridotite, allowing them to inherit high Mg#s. The phlogopites require a hydrous origin for these lithologies, and such evidence for water as well as subduction-like trace-element systematics suggest a subducting slab as the likely candidate for the origin of these melts. Quartz and sphene thermometry reveal that the crystallization temperatures were between 650° C and 750° C while the presence of plagioclase constrains the maximum pressure to 1 GPa, indicating these cumulates formed near or just below the Moho. Preliminary U- Pb dating of sphenes and zircons suggest these cumulates formed in the early Tertiary. Further geochronology of the zircons will be useful in better understanding the link between these xenoliths and the subducting slab.

Dyer, B.; Lee, C.; Leeman, W.

2007-12-01

19

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

NASA Astrophysics Data System (ADS)

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

Baldwin, S.; Das, J. P.

2013-12-01

20

Constraints on amount and Composition of Subduction Zone Fluids from Partially Overprinted High-Pressure Metamorphic Rocks and Minerals  

Microsoft Academic Search

The Sesia Lanzo Zone (SLZ, Western Alps, Italy) is a sliver of eclogite facies continental rocks that was exhumed in the hanging wall of a subducted oceanic slab. Fluids derived from the dehydrating oceanic crust affected the SLZ rocks to different extents at blueschist-facies conditions which enables tracing of the effects of infiltrating subduction- related fluids on major and trace

M. Konrad-Schmolke; T. Zack; P. J. O'Brien

2010-01-01

21

Pocho volcanic rocks and the melting of depleted continental lithosphere above a shallowly dipping subduction zone in the central Andes  

NASA Astrophysics Data System (ADS)

The Late Miocene (7.9 to 4.5 Ma) Pocho volcanic field in Argentina occurs 700 km east of the Chile trench over the modern shallowly dipping Andean Wadati-Benioff zone near 32° S latitude in Argentina. The field is located in the Sierra de Cordoba which is the easternmost Laramide-style, block-faulted range in the Sierras Pampeanas (Pampean ranges). The arrival of the shallowly dipping slab initiated both volcanism and the uplift of the Sierra de Cordoba. Pocho rocks (52% to 68% SiO2; FeO*/MgO>2.2) comprise an older (7.5±0.5 Ma) high-K and a younger (5.3±0.7 Ma) shoshonitic series. Mineralogic data and fractionation models show that crystallization occurred under hydrous, oxidizing conditions, which were most extreme in the high-K series. An unusual pattern of successively lower REE at higher SiO2 concentrations can be modeled by sphene, apatite and amphibole removal. An arc-like trace element signature attributed to an arc component is strongest in the younger shoshonitic series. An important depleted lower crustal/mantle lithospheric source component in both series is indicated by non-radiogenic Sr and Pb isotopic ratios at ?Nd= 0 to + 2, low Rb/Sr ratios, and low U and Th concentrations. This depleted signature contrasts with the enriched one in potassic back-arc Central Volcanic Zone (CVZ) lavas over the steeper subduction zone to the north and is attributed to several processes in the shallow subduction zone. First, deep crustal (MASH) processes in the nearly normal thickness crust beneath Pocho incorporated depleted Proterozoic basement components, and not complexly mixed structurally thickened crustal components as in the CVZ. Second, the association of Pocho volcanism with the arrival of the slab allowed little time for modification of the mantle by subduction components. Third, Miocene shallowing of the subduction zone beneath the “flat-slab” required thinning of both the astenosphere and the subcontinental lithosphere. Thus, an important subcrustal component could be from blocks removed from the base of the lithosphere to the west and recycled into the asthenosphere. Similar magmatic sources would have existed during Laramide shallow subduction in western North America.

Kay, Suzanne Mahlburg; Gordillo, Carlos E.

1994-06-01

22

Evolution of a Subduction Zone  

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

23

Cascadia Subduction Zone  

USGS Publications Warehouse

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

Frankel, Arthur D.; Petersen, Mark D.

2008-01-01

24

Ultrahigh-pressure metamorphism: tracing continental crust into the mantle  

NASA Astrophysics Data System (ADS)

More and more evidence is being discovered in Phanerozoic collision belts of the burial of crustal rocks to previously unsuspected (and ever increasing) depths, presently on the order of 150-200 km, and of exhumation from such depths. This extends by almost one order of magnitude the depth classically ascribed to the metamorphic cycling of continental crust, and demonstrates its possible subduction. The pieces of evidence for this new, ultrahigh-pressure (UHP) metamorphism exclusively occur in the form of relics of high-pressure minerals that escaped back-transformation during decompression. The main UHP mineral indicators are the high-pressure polymorphs of silica and carbon, coesite and microdiamond, respectively; the latter often demonstrably precipitated from a metamorphic fluid and is completely unrelated to kimberlitic diamond or any shock event. Recent discoveries of pyroxene exsolutions in garnet and of coesite exsolutions in titanite suggest a precursor garnet or titanite containing six-fold coordinated silicon, therefore still higher pressures than implied by diamond stability, on the order of 6 GPa. The UHP rocks raise a formidable geological problem: that of the mechanisms responsible for their burial and, more pressingly, for their exhumation from the relevant depths. The petrological record indicates that large tracts of UHP rocks were buried to conditions of low T/ P ratio, consistent with a subduction-zone context. Decompression occurred in most instances under continuous cooling, implying continuous heat loss to the footwall and hangingwall of the rising body. This rise along the subduction channel - an obvious mechanical discontinuity and weak zone - may be driven by buoyancy up to mid-crustal levels as a result of the lesser density of the acidic crustal rocks (even if completely re-equilibrated at depth) after delamination from the lower crust, in a convergent setting. Chronological studies suggest that the rates involved are typical plate velocities (1-2 cm/yr), especially during early stages of exhumation, and bear no relation to normal erosion rates. Important observations are that: (i) as a result of strain partitioning and fluid channelling, significant volumes of subducted crust may remain unreacted (i.e. metastable) even at conditions as high as 700°C and 3 GPa - with implications as to geophysical modeling; (ii) subducted continental crust shows no isotopic or geochemical evidence of interaction with mantle material. An unknown proportion of subducted continental crust must have escaped exhumation and effectively recycled into the mantle, with geochemical implications still to be explored, bearing in mind the above inefficiency of mixing. The repeated occurrence of UHP metamorphism, hence of continental subduction, through time and space since at least the late Proterozoic shows that it must be considered a common process, inherent to continental collision. Evidence of older, Precambrian UHP metamorphism is to be sought in high-pressure granulite-facies terranes.

Chopin, Christian

2003-07-01

25

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.

26

A 3D Velocity Model of the Central Cascadia Subduction Zone beneath the Oregon Continental Shelf and Coast Range  

NASA Astrophysics Data System (ADS)

Signals from the R/V Langseth's tuned airgun array were recorded on 6 ocean-bottom seismometers and 35 EarthScope FlexArray seismometers deployed in the Oregon Coast Range from 43.5° to 45°N as a piggyback project on the 2012 Ridge2Trench experiment to image the structure of the Juan de Fuca plate. This section of the Juan de Fuca/North America plate boundary slipped in 2 moderate low-angle thrust earthquakes in 2004, and continuing seismicity has clustered around these events since that time. It has also been associated with a transition in megathrust recurrence rate based on paleoseismic data and exhibits anomalous locking characteristics in geodetic data. Previously acquired bathymetric, potential field, and seismic data indicate the presence of subducted seamounts in this region and hint at large velocity variations of the overlying forearc crust immediately above the plate boundary [Tréhu et al., 2012]. The sources for this study are ~18,000 airgun shots along 4 lines on the continental shelf and upper slope and a line oblique to the coast that extends across the margin to the trench (and ultimately to the ridge). Data quality is excellent, with strong Pg and PmP arrivals observed at most stations for all shots. A preliminary look at the data supports the presence of large along-strike velocity variations in this region. To date, ~2/3 of the data have been picked. We will present velocity models that merge the new data with constraints from 2D and 3D onshore/offshore data acquired in 1989 and 1996 to improve the resolution and extend the velocity model to the south to include Heceta Bank, a region of uplifted late Miocene and early Pliocene sediments underlain by a previously poorly resolved high density anomaly. [Tréhu, A.M., Blakely, R.J., Williams, M.C., Subducted seamounts and recent earthquakes beneath the central Cascadia forearc, Geology, v. 40, p. 103-106, 2012

Kenyon, C.; Trehu, A. M.; Toomey, D. R.; Wilcock, W. S.; Carbotte, S. M.; Carton, H. D.; Canales, J.

2013-12-01

27

Mass Flux of Continental Material at Cenozoic Subduction Zones--New Global and Trench-sector Calculations Using New Geological and Geophysical Observations  

NASA Astrophysics Data System (ADS)

INTRODUCTION: A decade ago, then available geophysical and geological data implied that more than 65 percent of ocean floor sediment entering most subduction zones (SZ) accompanied the oceanic crust to the mantle (= sediment subduction or SS). The underthrusting slab also eroded the margin's crustal framework and conveyed this material to the mantle (= subduction erosion or SE). Globally, the mass of continental material recycled to the mantle was estimated at 1.3-1.8 km3 / yr (SS. = 0.7 km3 + SE = 0.6-1.1 km3). SEDIMENT SUBDUCTION: New and enhanced seismic reflection data, new drilling observations, and reevaluation of older information stress that the efficacy of SS is higher than earlier assessed. In detail, it appears that 100 percent SS occurs at non-accreting margins (19,000 km), at least 80 percent at accreting margins (16,000 km) where small to moderate size accretionary prisms (width=5-40 km) are forming, and 40-45 percent where larger prisms are accumulating (8,000 km). At Cenozoic SZs (~43,000 km), it is now estimated that the long-term (i.e., >10 Myr) rate of SS is at least 1.0 km3 / yr (solid volume). SUBDUCTION EROSION: New and reassessed seismic, drilling, submersible, coastal mapping and arc-retreat observations suggest a higher long-term rate of SE than formerly estimated at 30 km3 / Myr / km of trench. We now estimate that, except perhaps where large accretionary bodies are forming, the long-term rate of forearc erosion averages at least 40 km3 / Myr (range = 28-62), which corresponds to a global recycling rate of 1.4 km3 / yr. The matching average rate of landward truncation of the submerged forearc is 2.5 km / Myr (range = 1.8-4.2). SUMMARY: The late Cenozoic rate at which continental crust is recycled at SZs is currently estimated at 2.4 km3 / yr (ss=1+ se=1.4) +/- 25 percent, which is basically that now approximated for arc magmatic additions. It can thus be inferred that at Cenozoic SZs rates of crustal addition and recycling have been in general balance. This quasi-stasis may be applicable to the Phanerozoic.

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

2001-12-01

28

Intra-oceanic Subduction Zones  

Microsoft Academic Search

\\u000a Modern intra-oceanic subduction zones comprise around 17,000 km (~40%) of the convergent margins of the Earth and are subjects\\u000a of intense cross-disciplinary studies that are reviewed in this chapter. Most of these subduction zones exhibit trench retreat,\\u000a do not accrete sediments and are affected by back-arc extension processes. Initiation of intra-oceanic subduction zones is\\u000a partly enigmatic although two major types of

T. V. Gerya

29

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

SciTech Connect

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

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

1988-12-01

30

Very-High-Pressure Metamorphism in the Western Alps: Implications for Subduction of Continental Crust  

NASA Astrophysics Data System (ADS)

The widespread occurrence of coesite and pure pyrope, the presence of several new mineral assemblages and new rock-forming minerals in common lithologies, but also crystal--chemical features such as the existence of silicate--phosphate solid solutions or the repeated occurrence among the new minerals of a new dense structure with face-sharing octahedra, are evidence that unusual pressures have been attained in part of the Dora Maira massif, western Alps, during Alpine regional metamorphism. Mineral assemblages suggest minimum pressures well in excess of 25 kbar (1 bar = 105 Pa) which, according to preliminary experimental data, may have reached 35 kbar in this at least 5 × 10 km2 continental terrain clearly of supra-crustal origin. Obviously even continental material may be buried to depths of the order 100 km and then uplifted to the surface. Whereas the burial in a low temperature regime is readily explained by subduction of the continental lithosphere (of a peninsula?) 110 Ma ago or earlier, the uplift is much more problematic because it is petrologically constrained to proceed without significant temperature increase. The progressive migration 'in-plate' of intra-continental thrusts once subduction was blocked, accounts for the stepwise decrease in age and grade of high-pressure metamorphism toward the external part of the chain; by the repeated underthrusting of cold material it might also have prevented a temperature increase in the most internal, early subducted zones. The widespread occurrence of coesite and pure pyrope, the presence of several new mineral assemblages and rock-forming minerals in common lithologies, but also more subtle crystal- chemical details such as the existence of silicate-phosphate solid solutions or the repeated occurrence among the new minerals of a dense, otherwise unknown structure, constitute compelling evidence that the metamorphic terrain considered must have endured exceptionally high pressures for continental crust. The mineral assemblages encountered suggest minimum pressures that are in excess of 25 kbar and may reach 35 kbar, according to preliminary experimental work. We therefore cannot escape the issue that a continental terrain may be buried to depths of the order of 100 km and then uplifted to reach the surface. Since this is a new result and often considered to be at variance with concepts prevailing in geodynamics, an attempt must be made at reconstructing the successive stages of this evolution. In the following, before envisaging consequences, we will shortly consider the few constraints imposed by the metamorphic records and geochromological data on the conditions and timing of burial and uplift.

Chopin, C.

1987-01-01

31

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

32

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

33

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. When studying serpentinites, it is essential to determine their protoliths and their geological history before serpentinization. The geochemical data of serpentinites shows little mobility of compatible and rare earth elements (REE) at the scale of hand-specimen during their serpentinization. Thus, REE abundance can be used to identify the protolith for serpentinites, as well as magmatic processes such as melt/rock interactions before serpentinization. In the case of subducted serpentinites, the interpretation of trace element data is difficult due to the enrichments of light REE, independent of the nature of the protolith. We propose that enrichments are probably not related to serpentinization itself, but mostly due to (sedimentary-derived) fluid/rock interactions within the subduction channel after the serpentinization. It is also possible that the enrichment reflects the geochemical signature of the mantle protolith itself which could derive from the less refractory continental lithosphere exhumed at the ocean-continent transition. Additionally, during the last ten years, numerous analyses have been carried out, notably using in situ approaches, to better constrain the behavior of fluid-mobile elements (FME; e.g. B, Li, Cl, As, Sb, U, Th, Sr) incorporated in serpentine phases. The abundance of these elements provides information related to the fluid/rock interactions during serpentinization and the behavior of FME, from their incorporation to their gradual release during subduction. Serpentinites are considered as a reservoir of the FME in subduction zones and their role, notably on arc magma composition, is underestimated presently in the global geochemical cycle.

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

2013-09-01

34

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

NASA Astrophysics Data System (ADS)

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 plate boundary. However, the amount of fluids entering subduction zones is not matched by that leaving through volcanic emissions or transfer to the deep mantle, implying possible storage of fluids in the crust. Here we use magnetotelluric data to map the entire hydration and dehydration cycle of the Costa Rican subduction zone to 120km depth. Along the incoming plate bend, we detect a conductivity anomaly that we interpret as sea water penetrating down extensional faults and cracks into the upper mantle. Along the subducting plate interface we document the dehydration of sediments, the crust and mantle. We identify an accumulation of fluids at ~20-30km depth at a distance of 30km seaward from the volcanic arc. Comparison with other subduction zones indicates that such fluid accumulation is a global phenomenon. Although we are unable to test whether these fluid reservoirs grow with time, we suggest that they can account for some of the missing outflow of fluid at subduction zones.

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

2011-02-01

35

Seismic coupling and uncoupling at subduction zones  

Microsoft Academic Search

Some of the correlations concerning the properties of subduction zones are reviewed. A quantitative global comparison of many subduction zones reveals that the largest earthquakes occur in zones with young lithosphere and fast convergence rates. Maximum earthquake size is directly related to the asperity distribution on the fault plane. This observation can be translated into a simple model of seismic

Larry Ruff; Hiroo Kanamori

1983-01-01

36

Seismological Investigation of Deep Subduction Zones  

Microsoft Academic Search

Models of subduction zones have a potential of impacting thinking about many of the central problems in the structure, dynamics, chemistry, and history of the solid earth, cummulative effects of which would reach across the earth sciences. The long-term goal of this work is to develop adaptive seismic imaging concepts for modeling and understanding of the subduction zone processes, taking

I. Lucifredi; M. Ishii

2008-01-01

37

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

NASA Astrophysics Data System (ADS)

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

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

2009-01-01

38

Permeabilities of Subduction Zone Sediments  

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

39

Density model of the Cascadia subduction zone  

USGS Publications Warehouse

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

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

2001-01-01

40

Central Cascadia subduction zone creep  

NASA Astrophysics Data System (ADS)

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

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

2014-04-01

41

Earthquake hazards on the cascadia subduction zone  

SciTech Connect

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

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

1987-04-10

42

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

43

Seismic coupling and uncoupling at subduction zones  

NASA Technical Reports Server (NTRS)

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

Ruff, L.; Kanamori, H.

1983-01-01

44

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

SciTech Connect

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

Bebout, G.E. (Carnegie Institution of Washington, DC (United States))

1991-01-25

45

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

46

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

47

Ancient subduction zone in Sakhalin Island  

NASA Astrophysics Data System (ADS)

The northern part of Sakhalin Island is an area of recent intensive tectonic movements and hydrothermal processes, as well as a place of accumulation of useful minerals. The deep structure of the lithosphere beneath the region of the Neftegorsk earthquake of May 27, 1995 in North Sakhalin, which killed residents and caused significant destruction, is examined in this paper. Our geodynamic model shows that North Sakhalin consists of the North Sakhalin Basin, Deryugin Basin and an ophiolite complex located between them. The Deryugin Basin was formed in place of an ancient deep trench after subducting the Okhotsk Sea Plate under Sakhalin in the Late Cretaceous-Paleogene. The North Sakhalin Basin was formed on the side of the back-arc basin at that time. The ophiolite complex is fixed in the position of ancient subduction zone that was active in the Late Cretaceous-Paleogene. Approximately in the Miocene, the subduction of the Okhotsk lithosphere apparently ceased. The remains of the subduction zone in the form of an ophiolite complex have been identified from geological and geophysical data. On the surface, the subduction zone is manifested as deep faults stretched along Sakhalin. It is probable that the Neftegorsk earthquake was a result of activation of this ancient subduction zone.

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

2013-07-01

48

Seismicity, metamorphism and rheology of the lower continental crust  

NASA Astrophysics Data System (ADS)

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

Austrheim, Håkon

2014-05-01

49

Systematic biases in subduction zone hypocenters  

Microsoft Academic Search

Earthquake hypocenters from global datasets commonly have regional biases in their locations due to the use of a one-dimensional velocity model in their location. To analyze this effect, we group intermediate-depth earthquakes into forty-four 500-km-long sections of subduction zone. We relocate earthquakes in each group relative to one another and in a three-dimensional global velocity model. Hypocenters shift up to

Ellen M. Syracuse; Geoffrey A. Abers

2009-01-01

50

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

51

Satellite magnetic anomalies over subduction zones - The Aleutian Arc anomaly  

NASA Technical Reports Server (NTRS)

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

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

1985-01-01

52

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

53

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

54

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

55

Serpentine and the subduction zone water cycle  

NASA Astrophysics Data System (ADS)

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 an efficient 'transport-lithology' to recycle chemically bound water into the deeper mantle. In fact, an old slab may remain sufficiently cold during subduction to retain up to 40% of its initial 'mantle' water at 8 GPa (˜240-km depth) after serpentine transforms to higher pressure hydrous phase A. Furthermore, deep water recycling at subduction zones is parameterized in terms of slab age and speed. Coupling this parameterization to a parameterized mantle convection evolution model allows us to calculate the mantle-surface geologic water cycle throughout the Earth's history. We find that the present-day Earth mantle may be highly outgassed containing only a small fraction of the Earth's water, which would mostly be recycled water from the exosphere.

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

2004-06-01

56

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

57

Décollement and its formation in subduction zones  

NASA Astrophysics Data System (ADS)

Accretionary prisms in subduction zones have important correlation with earthquake occurrences, and further, the shapes of overlying accretionary prisms are believed to be controlled by the friction of a décollement. In this study, décollement-like structure formation near the sediment bottom during accretion has been numerically simulated by a particle-based method, and its formation mechanism has been investigated. In our simulations, the décollement-like structure appears as spontaneously localized shear deformation near the sediment bottom when the sediment thickness is sufficient to balance gravitational force and tectonic loading. The mechanical behavior of the décollement-like structure is similar to that of densely packed granular material in direct shear tests—the shear resistance increases first and then decreases with dilation depending on the shear displacement. In contrast, no such décollement-like structure is formed when the sediment is so thin that tectonic loading dominates; no localization occurs and the entire prism is deformed in shear due to plate motion. In such differential stress fields, shear failure with dilation frequently occurs, thereby weakening the prism. Thus, the décollement-like structure formation is controlled by the stress state and the material strength attained during an accretionary prism formation process. Our simulation results for thick and thin sediments are consistent with various observations in real subduction zones. Hence, a décollement in nature can also be considered to be a shear zone that is spontaneously formed in sediment whose material strength dynamically changes due to consolidation and dilation during accretion.

Hori, T.; Sakaguchi, H.

2010-12-01

58

Mineralogical Models of Subduction Zone Tomography  

NASA Astrophysics Data System (ADS)

Subduction zones have long been recognized by their significant role in the Earth's dynamics and plate tectonics. Mantle seismicity and volcanoes highlight their importance. Seismic tomographic studies and thermal models have been built. Here we model the seismic velocity and density of subduction zones using different thermal profile models, mantle chemical composition models, laboratory phase diagrams, and elasticity data from previous studies. With these we calculate the velocity and density impact and the buoyancy in the subducting slab region. This allows us to understand the effect of different physical and chemical parameters on the distribution of mineral phases in this area. Two major discontinuities have been globally observed at average depths 410km and 660km respectively by seismic wave data study. The 410 discontinuity occurs at a shallower depth within the colder subducting slab compared to the warmer surrounding ambient mantle. This phenomenon agrees well with seismic models based on deep earthquake velocity observations (Krishna & Kaila, 1995). The generally known discontinuity at 660km is shifted deeper down to 700km in our model. Similar results were presented by previous seismology studies (Thirot et al., 1998). By considering different mantle chemical models, the P-wave velocity jump at 410km in the middle region of the subducting slab is considerably larger for a harzburgite composition than for a pyrolite one. We also find a significant signature of the ilmenite forming reaction within the slab.

Yu, T.; Weidner, D.

2003-12-01

59

Enrichment of trace elements in garnet amphibolites from a paleo-subduction zone: Catalina schist, southern California  

SciTech Connect

The abundance, P-T stability, solubility, and element-partitioning behaviour of minerals such as rutile, garnet, sphene, apatite, zircon, zoisite, and allanite are critical variables in models for mass transfer from the slab to the mantle wedge in deep regions of subduction zones. The influence of these minerals on the composition of subduction-related magmas has been inferred (and disputed) from inverse modelling of the geochemistry of island-arc basalt, or by experiment. Although direct samples of the dehydration + partial-melting region of a mature subduction zone have not been reported from subduction complexes, garnet amphibolites from melanges of circumpacific and Caribbean blueschist terranes reflect high T (>600{degree}C) conditions in shallower regions. Such rocks record geochemical processes that affected deep-seated, high-T portions of paleo-subduction zones. In the Catalina Schist, a subduction-zone metamorphic terrane of southern California, metasomatized and migmatitic garnet amphibolites occur as blocks in a matrix of meta-ultramafic rocks. This mafic and ultramafic complex may represent either slab-derived material accreted to the mantle wedge of a nascent subduction zone or a portion of a shear zone closely related to the slab-mantle wedge contact, or both. The trace-element geochemistry of the complex and the distribution of trace element among the minerals of garnet amphibolites were studied by INAA, XRF, electron microprobe, and SEM.

Sorensen, S.S. (Smithsonian Institution, Washington, DC (USA)); Grossman, J.N. (Geological Survey, Reston, VA (USA))

1989-12-01

60

Heat sources and melting in subduction zones  

NASA Astrophysics Data System (ADS)

In order to understand the mechanism of melt generation and the origin of high heat flow in subduction zones, a series of numerical models of the thermal and flow structures in the mantle have been tested in two-dimensional boxes with an inclined subducting slab of constant velocity. In contrast to previous models, (1) large convecting cells are used without imposing a high-temperature profile on the backarc boundary, which enables us to discuss the global heat balance and to seek for the heat sources responsible for the melting and high heat flow observed in subduction zones, and (2) various mechanical conditions (e.g., coupling between the slab and overlying mantle wedge, buoyancy associated with melting) and heat sources (e.g., heat flux from below, internal heat generation, viscous heating) are tested in varying proportions. In all the calculations, steady state or near-steady state with a small instability periodically occurring at the upper thermal boundary layer is achieved. Under these conditions, the global heat balance can be described by a simple boundary layer argument. The results show that, in order to attain a high enough temperature for melting and a high heat flux, a large amount of internal heating (i.e., more than 2.5×10-7 W/m3) is required if the convection is limited within the upper mantle. The high internal heating required may be explained if the radioactive nuclides in fluids expelled from the subducting slab are added to the wedge and circulate in the convection cell for a sufficient time. Another possible explanation for melting and the high heat flux is that the hot material is supplied from the lower mantle. Based on the thermal and flow structures obtained, melting regimes in subduction zones are discussed, in which the following key processes take place: (1) melting associated with pressure change of a rock packet with its movement, especially compression melting in the downward flow of the mantle wedge along the slab, and (2) melting due to compositional change of the rock packet associated with migration of H2O and melt. To solve these problems in detail, further studies on distribution and migration of the fluids will be required.

Iwamori, Hikaru

1997-07-01

61

Seismic Survey of the Locked and Unlocked Sumatra Subduction Zone  

Microsoft Academic Search

The Sumatra subduction zone is the most seismically active region on Earth. In the past 5 years, it has been the site of three great earthquakes, including the 26 December 2004 Sumatra-Andaman earthquake. That event produced a devastating tsunami around the Indian Ocean that claimed approximately 230,000 lives and caused terrible damage and destruction. Part of the subduction zone still

Satish C. Singh; Stephan Midenet; Yusuf Djajadihardja

2009-01-01

62

Transportation of H 2O and melting in subduction zones  

Microsoft Academic Search

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

Hikaru Iwamori

1998-01-01

63

Diapiric Flow at Subduction Zones: A Recipe for Rapid Transport  

Microsoft Academic Search

Recent geochemical studies of uranium-thorium series disequilibrium in rocks from subduction zones require magmas to be transported through the mantle from just above the subducting slab to the surface in as little as ~30,000 years. We present a series of laboratory experiments that investigate the characteristic time scales and flow patterns of the diapiric upwelling model of subduction zone magmatism.

Paul S. Hall; Chris Kincaid

2001-01-01

64

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

65

Seismological Investigation of Deep Subduction Zones  

NASA Astrophysics Data System (ADS)

Models of subduction zones have a potential of impacting thinking about many of the central problems in the structure, dynamics, chemistry, and history of the solid earth, cummulative effects of which would reach across the earth sciences. The long-term goal of this work is to develop adaptive seismic imaging concepts for modeling and understanding of the subduction zone processes, taking into account the modification of the propagating seismic field due to reflection, refraction, and scattering from complex geological structures such as slabs, low velocity layers , scatterers, and topography. Different from the relatively small-scale approaches employed so far in reflection seismology, our approach provides the resultant field from deep geological structures, and surface topography spanning hundreds of kilometers, incorporating the signatures of the large-scale features typical in subduction zones. A part of the objective consists of obtaining a fundamental understanding of the 3D structure such as the high velocity slab, through the development of efficient physically based propagation and scattering models. This includes features such as aspect-dependent scattering objects, geological structures, and topography. Moreover, the analysis addresses the resultant physical mechanisms such as waveguide multipath effects produced by wave-trapping slabs. Additionally, the effort aims to analyze the existing experimental data and develop novel signal processing techniques for spatial, temporal, and spectral identification of different classes of waves, including early slab arrivals. The initial phase consists of the development and testing of the modeling and simulation work to explore the underlying physical mechanisms of seismic propagation through slabs beneath the ocean floor, scattering from complex structure and closed boundary objects, and the prediction of the resultant field to the surface receivers. For the representation of range-dependent propagation, we employ a hybrid coupled wavenumber integration approach to range-dependent seismic modeling based on the OASES environmental modeling framework. The generic model is a suite of modules covering a variety of range-dependent waveguide, topography, and source/receiver representations. Furthermore, the representation and the analysis of scattering from arbitrarily shaped structure as well as from closed boundary scatterers is performed by developing a wave theory based computational model combining a virtual source approach to open and closed boundary scattering with an established range- dependent wavenumber integration model for propagation. The current subduction zone modeling has shown that for specific receivers, early arrivals are a manifestation of the slab-trapped traveling waves reaching the surface first, while the rest of the response follows significantly later - a concept which will be further explored in the actual data. The spaciotemporal and frequency characteristics of data recorded by the Japanese seismic network is analyzed and compared to the numerical model. Time-frequency, array processing, and event classification methods for extracting properties of slab structure and scatterer signatures are developed and implemented on the data as well as on the synthetics. The model and experimental data time-frequency analysis exhibits frequency dependence and intricate time of arrival features. The understanding of these, and other manifestations of the underlying physical processes, are further explored and analyzed using signal processing techniques.

Lucifredi, I.; Ishii, M.

2008-12-01

66

Strain accumulation along the Cascadia subduction zone  

USGS Publications Warehouse

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

Murray, M. H.; Lisowski, M.

2000-01-01

67

The earthquake cycle in subduction zones  

NASA Technical Reports Server (NTRS)

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

Melosh, H. J.; Fleitout, L.

1982-01-01

68

The dynamics of reactive fluid escape in subduction zones  

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

69

Dehydration-driven topotaxy in subduction zones  

NASA Astrophysics Data System (ADS)

Mineral replacement reactions play a fundamental role in the chemistry and the strength of the lithosphere. When externally or internally derived fluids are present, interface-coupled dissolution-precipitation is the driving mechanism for such reactions [1]. One of the microstructural features of this process is a 3D arrangement of crystallographic axes across internal interfaces (topotaxy) between reactant and product phases. Dehydration reactions are a special case of mineral replacement reaction that generates a transient fluid-filled porosity. Among others, the dehydration serpentinite is of special relevance in subduction zones because of the amount of fluids involved (potentially up to 13 wt.%). Two topotatic relationships between olivine and antigorite (the serpentine mineral stable at high temperature and pressure) have been reported in partially hydrated mantle wedge xenoliths [2]. Therefore, if precursor antigorite serpentine has a strong crystallographic preferred orientation (CPO) its dehydration might result in prograde peridotite with a strong inherited CPO. However for predicting the importance of topotactic reactions for seismic anisotropy of subduction zones we also need to consider the crystallization orthopyroxene + chlorite in the prograde reaction and, more importantly, the fact that this dehydration reaction produces a transient porosity of ca. 20 % vol. that results in local fluctuations of strain during compaction and fluid migration. We address this issue by a microstructural comparison between the CPO developed in olivine, orthopyroxene and chlorite during high-pressure antigorite dehydration in piston cylinder experiments (at 750ºC and 20 kbar and 1000ºC and 30 kbar, 168 h) and that recorded in natural samples (Cerro del Almirez, Betic Cordillera, Spain). Experimentally developed CPOs are strong. Prograde minerals show a significant inheritance of the former antigorite foliation. Topotactic relations are dominated by (001)atg//(100)ol// (100)opx//(001)chl. The relation [010]atg// [001]ol //[001]opx can also be inferred but it is weaker. Similar topotactic relations are observed in the Cerro del Almirez samples, but the CPOs are weaker and more complex. The complexity arises from constant interfacial angles and systematic low-index interfacial contacts between orthopyroxene-olivine-chlorite (e.g. (001)chl // (100)opx). As a consequence the inheritance from the antigorite serpentinite is partially obliterated. Compaction-related microstructural features are also present including: (1) smooth bending of the former foliation and diffuse olivine veinlets perpendicular to it, (2) gradual crystallographic misorientation (up to 15º) of prismatic enstatite due to buckling, (3) localized orthoenstatite(Pbca)/low clinoenstatite (P21/c) inversion, and (4) brittle fracturing of prismatic enstatite wrapped by plastically deformed chlorite. These observations suggest that topotactic crystrallographic relations are dominant in undrained systems, but that the mechanisms allowing for compaction and fluid draining significantly affect the final texture in drained systems. Because the second case prevails in subduction zones, compaction mechanisms need to be better understood for modelling the development of CPOs after foliated protoliths in the slab and the mantle wedge. [1] Putnis, A., 2009. Reviews in Mineralogy and Geochemistry 70, 87-124. [2] Boudier, F., et al. 2010 J. Petrology 51, 495-512.

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

2014-05-01

70

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

NASA Astrophysics Data System (ADS)

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

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

1995-06-01

71

HP-UHP metamorphism as an indicator of slab dip variations in the Alpine arc  

NASA Astrophysics Data System (ADS)

HP/UHP and LT metamorphic units that commonly occur in the inner parts of mountain belts result from the subduction of continental and oceanic material, most often exhumed prior to continental collision. The prograde pressure-temperature history of HP-UHP rocks strongly depends on the convergence rate and on the subduction zone geometry. The maximum pressure recorded provides a proxy for the depth of shearing off and stacking of HP metamorphic nappes. A 2-D thermal model of continental subduction at lithospheric scale is used to compute the length and pressure peak of detached HP metamorphic units as a function of the slab dip angle and the convergence rate. Model results are applied to the metamorphic nappe pile of the inner Alps. A mean convergence rate of 1 cm/year during the subduction of the Briançonnais terrane is indicated by the paleogeographic reconstructions between 46 and 38 Ma. On this basis, the available petrological data and lengths of metamorphic units are used to compute the variations of the slab dip angle. The slab dip angle is shown to increase, from the northeast to the southwest, along the Alpine arc with estimated values of 20° for Suretta, 30-45° for Monte Rosa and Gran Paradiso, and 60° for Dora Maira. From Eocene to Oligocene times, the increase in slab dip angle is controlled by changes of buoyancy, due to the spatial configuration of the Valaisan trough and the incoming of crustal material within the subduction zone.

Carry, Nicolas; Gueydan, Fredéric; Marquer, Didier; Brun, Jean Pierre

2011-07-01

72

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

NASA Astrophysics Data System (ADS)

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

Plunder, Alexis; Agard, Philippe; Chopin, Christian

2014-05-01

73

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

NASA Astrophysics Data System (ADS)

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

Lamb, Simon

2006-07-01

74

Subduction zone earthquake probably triggered submarine hydrocarbon seepage offshore Pakistan  

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

75

Earthquake mechanisms and active tectonics of the Hellenic subduction zone  

Microsoft Academic Search

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.

Beth Shaw; James Jackson

2010-01-01

76

Splay Fault Branching Along the Nankai Subduction Zone  

Microsoft Academic Search

Seismic reflection profiles reveal steeply landward-dipping splay faults in the rupture area of the magnitude (M) 8.1 Tonankai earthquake in the Nankai subduction zone. These splay faults branch upward from the plate-boundary interface (that is, the subduction zone) at a depth of ~10 kilometers, ~50 to 55 kilometers landward of the trough axis, breaking through the upper crustal plate. Slip

Jin-Oh Park; Tetsuro Tsuru; Shuichi Kodaira; Phil R. Cummins; Yoshiyuki Kaneda

2002-01-01

77

Accretion of continental extensional zone and development of Rehamna metamorphic dome (Morocco)  

NASA Astrophysics Data System (ADS)

The Rehamna massif is a part of the Morocco Variscan belt where the metamorphic infrastructure has been exhumed in a continental accretionary wedge. The relationships between infra- and supra-structure tectonics originate by thickening of intra-continental Devonian and Carboniferous (Mississipian) basin. Two superposed deformations probably related to plate configuration changes have been identified. The first one corresponds to the development of the Barrovian metamorphism with a climax estimated at 0.5-0.7 Gpa, 500-550°C (litt.). The thickening of thermally preheated crust is related to a SW vergent nappe stacking in the orogenic infrastructure. This event terminates with large scale folding and extrusion of high grade rocks forming a large scale E-W trending metamorphic dome surrounded by un-metamorphosed Lower Palaeozoic rocks of orogenic suprastructre indicating complete decoupling between the two crustal layers. The second E-W shortening along Western Meseta Shear Zone event is responsible for the development of important deformation gradient orthogonal to the metamorphic dome axis. It is marked by an increase of the strain intensity towards the Cambrian rigid buttress located further west. This deformation gradient is marked by mechanical coupling and gentle folding of infra- and supra-structure in the east, development of slaty cleavage in the central part of the dome and intense deformation front close to the western buttress. Here the superposed fold pattern results in accentuation of asymmetrical NE-SW trending metamorphic dome close to the buttress and to further exhumation of deepest rocks associated with subsequent elevation and folding of metamorphic isograds. 40Ar/39Ar dating reveals that the cooling associated with the first thermal event occurred at around 310 Ma (biotite) while the second thermal event yields typical Alleghanian age around 280 Ma (muscovite). This last thermal event is contemporaneous with magmatic intrusions and continental basins opening in a context of Permo-Carboniferous dextral shearing.

Chopin, F.; Schulmann, K.; Corsini, M.; El Houicha, M.; Ghienne, J. F.; El Attari, M.

2009-04-01

78

Dynamic topography in subduction zones: insights from laboratory models  

NASA Astrophysics Data System (ADS)

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

Bajolet, Flora; Faccenna, Claudio; Funiciello, Francesca

2014-05-01

79

Thermobarometric and fluid expulsion history of subduction zones  

NASA Astrophysics Data System (ADS)

Phanerozoic, unmetamorphosed, weathered, and altered lithotectonic complexes subjected to subduction exhibit the prograde metamorphic facies sequence: zeolite ? prehnite-pumpellyite ? glaucophane schist ? eclogite. Parageneses reflect relatively high-P trajectories, accompanied by semicontinuous devolatilization. The thermal evolution of convergent plate junctions results in early production of high-rank blueschists, high-P amphibolites, and eclogues at depth within narrow subduction zones while the hanging wall lithosphere is still hot. Protracted underflow drains heat from the nonsubducted plate and, even at profound depths, generates very low-T/high-P parageneses. Inclusion studies suggest that two-phase immiscible volatiles (liquid H2O, and gaseous high-hydrocarbons, CH4 and CO2) are evolved in turn during progressive metamorphism of the subducted sections. Expulsion of pore fluids and transitions from weathered and altered supracrustal rocks to zeolite facies assemblages release far more fluid than the better understood higher-grade transformations. Many blueschist parageneses, such as those of the internal Western Alps, have been partially overprinted by later greenschist and/or epidote-amphibolite facies assemblages. Alpine-type postblueschist metamorphic paths involved fairly rapid, nearly adiabatic decompression; some terranes even underwent modest continued heating and fluid evolution during early stages of ascent. Uplift probably occurred as a consequence of the underthrusting of low-density island arc or microcontinental crust along the convergent plate junction, resulting in marked deceleration or cessation of lithospheric underflow, decoupling, and nearly isothermal rise of the recrystallized subduction complex. Other, less common blueschist terranes, such as the eastern Franciscan belt of western California, preserve metamorphic aragonite and other high-P minerals, and lack a low-pressure overprint; physical conditions during retrogression approximately retraced the prograde path or, for early formed high-grade blocks, reflect somewhat higher pressures and lower temperatures. Subducted sections constituting portions of the Franciscan-type of metamorphic belt evidently moved slowly back up the inclined lithospheric plate junction during continued convergence and sustained refrigeration. Upward motion due to isostatic forces was produced by tectonic imbrication of fault suces, laminar return flow in melange zones, and lateral extension of the underplated accretionary prism. The ease with which volatiles are expelled from a subduction complex and migrate upward along the plate junction zone is roughly proportional to the sandstone/shale ratio: low-permeability mudstones tend to maintain fluid values approaching lithostatic, lose strength, and deform chaotically (forming melange belts), whereas permeable sandstone-rich sections retain structural/stratigraphic coherence and fail brittlely (forming coherent terranes). Because of substantial updip expulsion of volatiles during prograde recrystallization, only small amounts of H2O and CO2 are available to support hydration and carbonation of the accretionary complex during its return toward the surface; thus limited back reaction takes place and occurs at low Pfluid/Plithostatic ratios, unless an abundance of volatiles is introduced during uplift.

Ernst, W. G.

1990-06-01

80

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

NASA Astrophysics Data System (ADS)

The maximum earthquake magnitude recorded for subduction zone plate boundaries varies considerably on Earth, with some subduction zone segments producing giant subduction zone thrust earthquakes (e.g. Chile, Alaska, Sumatra-Andaman, Japan) and others producing relatively small earthquakes (e.g. Mariana, Scotia). Here we show how such variability might depend on various subduction zone parameters. We present 24 physical parameters that characterize these subduction zones in terms of their geometry, kinematics, geology and dynamics. We have investigated correlations between these parameters and the maximum recorded moment magnitude (MW) for subduction zone segments in the period 1900-June 2012. The investigations were done for one dataset using a geological subduction zone segmentation (44 segments) and for two datasets (rupture zone dataset and epicenter dataset) using a 200 km segmentation (241 segments). All linear correlations for the rupture zone dataset and the epicenter dataset (|R| = 0.00-0.30) and for the geological dataset (|R| = 0.02-0.51) are negligible-low, indicating that even for the highest correlation the best-fit regression line can only explain 26% of the variance. A comparative investigation of the observed ranges of the physical parameters for subduction segments with MW > 8.5 and the observed ranges for all subduction segments gives more useful insight into the spatial distribution of giant subduction thrust earthquakes. For segments with MW > 8.5 distinct (narrow) ranges are observed for several parameters, most notably the trench-normal overriding plate deformation rate (vOPD?, i.e. the relative velocity between forearc and stable far-field backarc), trench-normal absolute trench rollback velocity (vT?), subduction partitioning ratio (vSP?/vS?, the fraction of the subduction velocity that is accommodated by subducting plate motion), subduction thrust dip angle (?ST), subduction thrust curvature (CST), and trench curvature angle (?T). The results indicate that MW > 8.5 subduction earthquakes occur for rapidly shortening to slowly extending overriding plates (-3.0 ? vOPD? ? 2.3 cm/yr), slow trench velocities (-2.9 ? vT? ? 2.8 cm/yr), moderate to high subduction partitioning ratios (vSP?/vS? ? 0.3-1.4), low subduction thrust dip angles (?ST ? 30°), low subduction thrust curvature (CST ? 2.0 × 10-13 m-2) and low trench curvature angles (-6.3° ? ?T ? 9.8°). Epicenters of giant earthquakes with MW > 8.5 only occur at trench segments bordering overriding plates that experience shortening or are neutral (vOPD? ? 0), suggesting that such earthquakes initiate at mechanically highly coupled segments of the subduction zone interface that have a relatively high normal stress (deviatoric compression) on the interface (i.e. a normal stress asperity). Notably, for the three largest recorded earthquakes (Chile 1960, Alaska 1964, Sumatra-Andaman 2004) the earthquake rupture propagated from a zone of compressive deviatoric normal stress on the subduction zone interface to a region of lower normal stress (neutral or deviatoric tension). Stress asperities should be seen separately from frictional asperities that result from a variation in friction coefficient along the subduction zone interface. We have developed a global map in which individual subduction zone segments have been ranked in terms of their predicted capability of generating a giant subduction zone earthquake (MW > 8.5) using the six most indicative subduction zone parameters (vOPD?, vT?, vSP?/vS?, ?ST, CST and ?T). We identify a number of subduction zones and segments that rank highly, which implies a capability to generate MW > 8.5 earthquakes. These include Sunda, North Sulawesi, Hikurangi, Nankai-northern Ryukyu, Kamchatka-Kuril-Japan, Aleutians-Alaska, Cascadia, Mexico-Central America, South America, Lesser Antilles, western Hellenic and Makran. Several subduction segments have a low score, most notably Scotia, New Hebrides and Mariana.

Schellart, W. P.; Rawlinson, N.

2013-12-01

81

On subduction zone earthquakes and the Pacific Northwest seismicity  

SciTech Connect

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

Chung, Dae H.

1991-12-01

82

Exploring Interactions Between Subduction Zone Earthquakes and Volcanic Activity in the South Central Alaskan Subduction Zone  

NASA Astrophysics Data System (ADS)

Although great earthquakes such as the recent moment-magnitude (M) 9 Tohoku-Oki earthquake have been shown to trigger remote seismicity in volcanoes, the extent to which subduction zone earthquakes can trigger shallow seismic swarms at volcanoes is largely unexplored. Unknowns in this relationship include the upper limit of distance, the lower limit of magnitude, the upper time limit between events, and the effects of rupture directivity. We searched the Advanced National Seismic System earthquake catalog from 1989 - 2011 for correlations in space and time between M > 5.0 earthquakes in the south central Alaskan subduction zone (between 58.5°N and 62.5°N, and 150.7°W and 154.7°W) and volcanic activity at Mt. Redoubt, Mt. Iliamna, and Mt. Spurr volcanoes. There are 48 earthquakes M > 5 in this catalog; five of these are M > 6. The depths of the 48 M>5 events range from 49km to 220km, and they are all between 100km and 350km of the three volcanoes. Preliminary analysis of our catalog shows that four of the five M > 6 earthquakes are followed by a volcanic earthquake swarm at either Redoubt or Spurr within 100 days, and three of them are followed by a volcanic earthquake swarm within a month. None of these events correlated in space and time with swarms at Mt. Iliamna. We are also searching for swarms and moderate earthquakes occurring in time windows far removed from each other. The likeliest case of remotely triggered seismicity in our search area to date occurred on January 24 2009, when a magnitude 5.8 earthquake beneath the Kenai Peninsula at 59.4°N, 152.8°W, and 95km depth was immediately followed by an increase of volcanic activity at Mt. Redoubt approximately 153km away. The first swarm began on Jan 25 2009. On Jan 30 2009, volcanologists at the Alaskan Volcano observatory determined the increased volcanic seismicity was indicative of an impending eruption. Mt. Redoubt erupted on March 15 2009. Proposed mechanisms for triggering of volcanoes by earthquakes include dynamic and static stress changes in the magmatic system, which could affect pressure in the magma chamber and overpressure, or affect phenocryst settling and bubble growth inside the chamber. However, these models have generally not been connected to specific events; expanding our catalog will help to refine these models to describe the mechanics of this relationship.

Lanagan, K. M.; Richardson, E.

2012-12-01

83

Seismic Imaging of the Middle America Subduction Zone Beneath Mexico  

NASA Astrophysics Data System (ADS)

P-wave coda from teleseismic events were used to compute receiver functions followed by formal inversions for discontinuous variations in elastic properties beneath a dense seismic array that crosses Mexico from Acapulco on the Pacific coast, through Mexico City, almost to Tempico on the Gulf of Mexico. Broadband data from the Meso-America Subduction Experiment (MASE) line were used to image the subducted Cocos plate and the overriding continental lithosphere beneath central Mexico using a generalized radon transform based migration. Our images provide insight into the process of subducting relatively young oceanic lithosphere. We observe nearly horizontal tectonic underplating of the Cocos oceanic lithosphere beneath the North American continent for a distance of approximately 300 km from the Middle America Trench, with a clear image of a very thin low-velocity oceanic crust (7-8 km) which dips at 15-20 degrees then flattens and slightly thickens (~10 km). At approximately 250 km inland the inferred subducting crust undergoes a change in seismic character, specifically a disruption in the crustal velocity signature, which may reflect the initiation of partial eclogitization of the subducting crust or release of fluids via dehydration that would result in a reduced velocity contrast at the Moho. Farther inland the slab then appears to abruptly change from nearly horizontal to a steeply dipping geometry of approximately 75 degrees underneath the Trans-Mexican Volcanic Belt (TMVB). The image of the steeply subducted Cocos slab underneath the TMVB is enhanced by using the P-to-S converted phases, following the method used in southern Central America to image a steeply dipping subducted slab (> 60 degrees) for the TUCAN experiment (MacKenzie et al, 2010), however is complicated by the wide active volcanic arc and deep sedimentary basins in the middle of the array. The continental Moho is clearly imaged at ~40 km deep beneath the TMVB and shallows (~25 km) towards the Gulf of Mexico. The deeper seismic structure underneath the TMVB shows a prominent negative discontinuity (fast-to-slow) at ~70-80 km within the upper mantle. This feature, which spans horizontally beneath the entire arc (~150 km), may delineate the top of a layer of ponded partial melt, which is consistent with previous geodynamic modeling of melt migration, and also evidenced from P-wave velocity tomographic images of this and other subduction zones although images of this layer was not sharply resolved.

Miller, M. S.; Kim, Y.; Pearce, F. D.; Clayton, R. W.

2011-12-01

84

Continental margin deformation along the Andean subduction zone: Thermomechanical models  

Microsoft Academic Search

The Chilean Andes extend north–south for about 3000km over the subducting Nazca plate, and show evidence of local rheological controls on first-order tectonic features. Here, rheological parameters are tested with numerical models of a subduction driven by slab-pull and upper plate velocities, and which calculate the development of stress and strain over a typical period of 4Myr. The models test

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

2009-01-01

85

What controls the seismogenic plate interface in subduction zones?  

NASA Astrophysics Data System (ADS)

The greatest earthquakes occur on the seismogenic plate interface of subduction zones. We need to understand what controls the updip and downdip edges of this seismogenic zone. For the circum-Pacific subduction zones that generate great earthquakes, the downdip edge is at a depth of about 40 km, but with significant variations. Several mechanisms might control this transition from seismogenic to aseismic slip. Tichelaar and Ruff [1993] argue that one or two critical temperatures can explain the global observations. The model with two critical temperatures invokes two different upper plate rock types, crust and mantle rocks. In this paper, we investigate the correlation between the location of the downdip edge with the location of the coastline. We find a statistically significant correlation between these two variables for the major circum-Pacific subduction zones. Is this correlation a coincidence, or is it indicative of deeper fundamental processes? We offer a simple unifying explanation: the intersection of the overlying plate's Moho with the top of the subducting slab determines both the downdip edge of the seismogenic zone, and the coastline above. This explanation implies that rocks in the subduction zone mantle wedge are aseismic.

Ruff, Larry J.; Tichelaar, Bart W.

86

Subduction zone earthquakes and the Pacific Northwest seismicity.  

National Technical Information Service (NTIS)

A short review of subduction zone earthquakes and the seismicity of the Pacific Northwest region of the United States is provided for the purpose of a basis for assessing issues related to earthquake hazard evaluations for the region. This review of seism...

D. H. Chung

1991-01-01

87

Great earthquakes of variable magnitude at the Cascadia subduction zone  

Microsoft Academic Search

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

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

2006-01-01

88

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

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

89

Assessing the Seismic Potential Hazard of the Makran Subduction Zone  

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

90

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

91

Structure of the Northern Cascadia Subduction Zone  

NASA Astrophysics Data System (ADS)

As part of the POLARIS (Portable Observatories for Lithospheric Analysis and Research Investigating Seismicity - www.polarisnet.ca) project, we have initiated a broadband seismic experiment across southwestern British Columbia and northwestern Washington. The objective of this work is to better understand the structure of the subducting plate and mantle wedge in northern Cascadia, and its relation to intra-slab (Wadati-Benioff) seismicity. There are currently 30 broadband stations extending across southern Vancouver Island, the Gulf and San Juan Islands, Watcom county and the British Columbia lower mainland in an approximately linear array. We have employed the P-wave coda from 28 teleseismic events recorded on this array in a formal inversion to image fine-scale shear-velocity structure. Preliminary results indicate a structure very similar to that identified across a comparable profile in Oregon. The continental Moho is evident at the eastern end of the profile near 30 km depth but disappears towards the Georgia Strait/Puget sound. Farther west, a horizontal boundary at a similar depth with an "inverted" shear-velocity contrast appears and persists to its projected intersection with the subducting plate. The oceanic crust is clearly evident below southern Vancouver Island as a dipping low-velocity zone but its signature diminishes at depths below 45 km. As for Oregon, we interpret these observations to signal the presence of a serpentinized forearc mantle hydrated, in part, through eclogitization of oceanic crust.

Nicholson, T. A.; Bostock, M. G.; Cassidy, J. F.

2003-12-01

92

Upper plate deformation and seismic barrier in front of Nazca subduction zone: The Chololo Fault System and active tectonics along the Coastal Cordillera, southern Peru  

NASA Astrophysics Data System (ADS)

The South America plate boundary is one of the most active subduction zone. The recent Mw = 8.4 Arequipa 2001 earthquake ruptured the subduction plane toward the south over 400 km and stopped abruptly on the Ilo Peninsula. In this exact region, the subduction seismic crisis induced the reactivation of continental fault systems in the coastal area. We studied the main reactivated fault system that trends perpendicular to the trench by detailed mapping of fault related-geomorphic features. Also, at a longer time scale, a recurrent Quaternary transtensive tectonic activity of the CFS is expressed by offset river gullies and alluvial fans. The presence of such extensional fault systems trending orthogonal to the trench along the Coastal Cordillera in southern Peru is interpreted to reflect a strong coupling between the two plates. In this particular case, stress transfer to the upper plate, at least along the coastal fringe, appears to have induced crustal seismic events that were initiated mainly during and after the 2001 earthquake. The seafloor roughness of the subducting plate is usually thought to be a cause of segmentation along subduction zones. However, after comparing and discussing the role of inherited structures within the upper plate to the subduction zone segmentation in southern Peru, we suggest that the continental structure itself may exert some feedback control on the segmentation of the subduction zone and thus participate to define the rupture pattern of major subduction earthquakes along the southern Peru continental margin.

Audin, Laurence; Lacan, Pierre; Tavera, Hernando; Bondoux, Francis

2008-11-01

93

Structure of the Northern Cascadia Subduction Zone.  

NASA Astrophysics Data System (ADS)

As part of the POLARIS (Portable Observatories for Lithospheric Analysis and Research Investigating Seismicity - www.polarisnet.ca) project, we have undertaken a broadband seismic experiment across southwestern British Columbia and northwestern Washington. The objective of this work is to better understand the structure of the subducting plate and mantle wedge in northern Cascadia, and its relation to intra-slab (Wadati-Benioff) seismicity. There are currently 30 broadband stations extending across southern Vancouver Island, the Gulf and San Juan Islands, Watcom county and the British Columbia lower mainland in an approximately linear array. We have employed the P-wave coda from 41 teleseismic events recorded on this array in a formal inversion to image fine-scale shear-velocity structure. Results indicate a structure very similar to that identified across a comparable profile in Oregon. The continental Moho is evident at the eastern end of the profile near 30 km depth but disappears towards the Georgia Strait/Puget Sound. A prominent, dipping, low S-velocity zone is clearly evident below southern Vancouver Island coincident with the E-reflection zone originally identified in earlier Lithoprobe studies. Structure below the E-layer is of significantly lower magnitude and is only intermittently visible along the array. Based on the observations and interpretations of similar structures beneath Oregon, Alaska and South America, we suggest that the E reflection zone represents the subducted oceanic crust. This view implies that the oceanic crust is 6-8 km shallower beneath Vancouver Island than previously thought, with implications for geodynamic models of the region. As in Oregon, we interpret the diminishing low velocity signature below 45 km to signal the presence of a serpentinized forearc mantle hydrated, in part, through eclogitization of oceanic crust.

Nicholson, T. A.; Bostock, M. G.; Cassidy, J. F.

2004-05-01

94

Uranium and thorium solubilities in subduction zone fluids  

Microsoft Academic Search

Uranium is enriched in depleted island arc magmas more than would be anticipated, due to the overall enrichment in large ion lithophile elements. To attempt to understand this enrichment calculations were performed to establish concentrations of uranium and thorium in fluids under the pressure and temperatures experienced in a subduction zone. The solubility and speciation of uranium(VI), uranium(IV) and thorium(IV)

Elizabeth H. Bailey; K. Vala Ragnarsdottir

1994-01-01

95

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

96

Ups and downs in western Crete (Hellenic subduction zone)  

PubMed Central

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

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

2014-01-01

97

Subduction zone guided waves: 3D modelling and attenuation effects  

NASA Astrophysics Data System (ADS)

Waveform modelling is an important tool for understanding complex seismic structures such as subduction zone waveguides. These structures are often simplified to 2D structures for modelling purposes to reduce computational costs. In the case of subduction zone waveguide affects, 2D models have shown that dispersed arrivals are caused by a low velocity waveguide, inferred to be subducted oceanic crust and/or hydrated outer rise normal faults. However, due to the 2D modelling limitations the inferred seismic properties such as velocity contrast and waveguide thickness are still debated. Here we test these limitations with full 3D waveform modelling. For waveguide effects to be observable the waveform must be accurately modelled to relatively high frequencies (> 2 Hz). This requires a small grid spacing due to the high seismic velocities present in subduction zones. A large area must be modelled as well due to the long propagation distances (400 - 600 km) of waves interacting with subduction zone waveguides. The combination of the large model area and small grid spacing required means that these simulations require a large amount of computational resources, only available at high performance computational centres like the UK National super computer HECTOR (used in this study). To minimize the cost of modelling for such a large area, the width of the model area perpendicular to the subduction trench (the y-direction) is made as small as possible. This reduces the overall volume of the 3D model domain. Therefore the wave field is simulated in a model ';corridor' of the subduction zone velocity structure. This introduces new potential sources of error particularly from grazing wave side reflections in the y-direction. Various dampening methods are explored to reduce these grazing side reflections, including perfectly matched layers (PML) and more traditional exponential dampening layers. Defining a corridor model allows waveguide affects to be modelled up to at least 2 Hz (needed for dispersion analysis) for the large model area that is considered. Simulations with a variety of quality factors (Q) at different parts of the subduction zone have been run to investigate how seismic attenuation affects the observed dispersed waveforms. We show that the low Q in the mantle wedge can improve the fit of the dispersed waveforms. A low Q in the low velocity waveguide structure however means that the delayed high frequency energy has very low amplitude, and so is not seen clearly at the surface. The Q of the low velocity crustal waveguide must therefore be greater than 250, suggesting that melting does not occur in the subducted oceanic crust at depths of 220 km or less. The velocity contrast seen at these depths must therefore be due to compositional variations. Benchmarking 2D elastic models with the 3D case shows that 2D models give a good approximation of 3D subduction zone waveguide structure. Visco-elastic simulations show that attenuation in the mantle wedge affects the observed dispersion, but the low velocity waveguide itself does not have significantly reduced Q. This work is an example of how the increasing computing power coupled with well-defined model boundaries can allow high resolution 3D modelling to be applied to specific structures of interest.

Garth, T.; Rietbrock, A.

2013-12-01

98

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

NASA Astrophysics Data System (ADS)

With ca. forty thousand kilometers of subduction zones and convergence rates from 30 km Ma-1 to 180 km Ma-1, subduction carries massive amounts of material into seafloor trenches, and beyond. Most of the subducting plate is made of mantle material returning to the depths from which it originated. The hydrated and altered upper oceanic section and the overlying sediments, however, carry a record of low-temperature interaction with the ocean, atmosphere, and continents. Subduction and recycling of these components into the mantle has the potential to change mantle composition in terms of volatile contents, heat-producing elements, radiogenic isotope systematics, and trace element abundances. Enrichments in volatile and potassium, uranium, and thorium contents could change the rheological, thermal, and geodynamical behavior of portions of the mantle. Changing isotope and trace-element systematics provide a means for tracking mantle mixing and the possible subduction modification of the deep mantle. A large number of studies point to possible contributions of subducted sediments and altered oceanic crust (AOC) to the mantle-source region for enriched mantle II (EMII) and high mu (HiMU) enriched oceanic island basalts. Transit through the subduction zone, however, changes the composition of the subducting sediment and AOC from that measured outboard of trenches.This chapter focuses on subduction zone processes and their implications for mantle composition. It examines subduction contributions to the shallow mantle that may be left behind in the wedge following arc magma genesis, as well as the changing composition of the slab as it is processed beneath the fore-arc, volcanic front and rear arc on its way to the deep mantle. Much of this chapter uses boron and the beryllium isotopes as index tracers: boron, because it appears to be completely recycled in volcanic arcs with little to none subducted into the deep mantle, and cosmogenic 10Be, with a 1.5 Ma half-life, because it uniquely tracks the contribution from the subducted sediments.The focus here is on subduction processes from trench to rear arc. This chapter starts with a brief discussion of recent thermal models for the downgoing plate and the prograde metamorphic mineralogy of the oceanic crust and sedimentary veneer; the reader is referred to Schmidt and Poli (Chapter 3.17), for an extensive discussion. In the next step it uses 10Be to estimate the absolute mass of sediments subducted to the volcanic arc, in comparison to that supplied to the subduction trenches. Flux balances for 10Be subducted in the sediments versus that erupted in the volcanic arc provide estimates of the fraction of 10Be extracted from the downgoing plate, which can be extrapolated to other elements (cf. Plank and Langmuir, 1993). It subsequently looks at chemical changes for selected elements across the subduction zone, using data from fore-arc serpentinite mud volcanoes, subduction-assemblage metamorphic rocks, high-pressure eclogites, and volcanic lavas from Kurile cross-arc transects, and examines boron-isotope systematics across the convergent margin. Lithium-isotope systematics and comparison of 10Be with uranium-series systematics sometimes delineate multiple stages of subduction modification of the mantle and pinpoint the compositional effects of prior subduction modification on the upper mantle. This contribution ends with estimates of the efficiency of arsenic, antimony, potassium, caesium, rubidium, barium, strontium, uranium, thorium, lead, cerium, samarium, neodymium, lutetium, and hafnium recycling from trench to rear arc, relative to that of boron and beryllium.2.11.2. Thermal Structure and Mineralogy of The Subducting PlateCentral to understanding the recycling of subducted elements in the arc or their subduction to the deep mantle is the temperature variation in the subducting slab, and the prograde mineral assemblages in the sediment, oceanic crust, and lithospheric mantle. Together, they determine where dehydration of the sediments, crust, and deeper subducting mantle occurs, a

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

2003-12-01

99

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

100

Late Quaternary deformation of marine terraces on the Cascadia Subduction Zone near Cape Blanco, Oregon  

NASA Astrophysics Data System (ADS)

The Cape Blanco region of south coastal Oregon sits on the upper plate of the Cascadia subduction zone about 60-70 km east of the base of the continental slope. Though the region has no historic coseismic deformation, late Quaternary deposits and landforms show abundant evidence of uplift, folding, and faulting. A set of five late Quaternary marine terraces and one uplifted Holocene beach berm are preserved in the Cape Blanco region. Stratigraphic and altitudinal surveys of these physiographic features, combined with several numerical and correlation age determinations, permit a reconstruction of tectonic deformation near Cape Blanco in approximately the last 200,000 years. The most cogent aspects of the neotectonics in this part of the Cascadia subduction zone are that an east-west trending anticline deforms the lower three terraces (80-125 ka) as well as accounting for uplift of the Holocene storm berm. Latest movement on the anticline was no more than 2,000 years ago, but the anticline also deforms sediments as old as Middle Miocene. The anticline is consistent with a north-south principal contraction axis at Cape Blanco in about the last 100,000 years. An older terrace at Cape Blanco (approximately 200 ka) is offset by two reactivated faults whose movements prior to the Eocene served to suture several terranes to the North American continent. Tectonic tilting in the older terrace is not consistent with a north-south principle axis of contraction, but the axis of tilt is parallel to north-northwest striking structures developed in the Quaternary fold belt offshore on the continental shelf. The observed deformation indicates that regional stress in the vicinity of Cape Blanco in the late Quaternary has been accommodated by a variable pattern of strain.

Kelsey, Harvey M.

1990-10-01

101

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

NASA Astrophysics Data System (ADS)

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

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

2008-06-01

102

Great thrust earthquakes and aseismic slip along the plate boundary of the Makran Subduction Zone  

NASA Astrophysics Data System (ADS)

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 aseismic slip along the plate boundary, and we assess the effects of the large forearc and accretionary wedge on the style of plate boundary slip. We apply body waveform inversions and, for small-magnitude events, use first motions of P waves to estimate earthquake source parameters. For the 1945 event we also employ dislocation modeling of uplift data. We find that the earthquake of 1945 in eastern Makran is an interplate thrust event that ruptured approximately one-fifth the length of the subduction zone. Nine smaller events in eastern Makran that are also located at or close to the plate interface have thrust mechanisms similar to that of the 1945 shock. Seaward of these thrust earthquakes lies the shallowest 70-80 km of the plate boundary; we find that this segment and the overlying accretionary wedge remain aseismic both during and between great earthquakes. This aseismic zone, as in other subduction zones, lies within that part of the accretionary wedge that consists of largely uconsolidated sediments (seismic velocities less than 4.0 km/s). The existence of thrust earthquakes indicates that either the sediments along the plate boundary in eastern Makran become sufficiently well consolidated and de watered about 70 km from the deformation front or older, lithified rocks are present within the forearc so that stick-slip sliding behavior becomes possible. This study shows that a large quantity of unconsolidated sediment does not necessarily indicate a low potential for great thrust earthquakes. In contrast to the east, the plate boundary in western Makran has no clear record of historic great events, nor has modem instrumentation detected any shallow thrust events for at least the past 25 years. Most earthquakes in western Makran occur within the downgoing plate at intermediate depths. The large change in seismicity between eastern and western Makran along with two shallow events that exhibit right-lateral strike-slip motion in central Makran suggest segmentation of the subduction zone. Two Paleozoic continental blocks dominate the overriding plate. The boundary between them is approximately coincident with the transition in seismicity. Although relative motion between these blocks may account for some of the differing seismic behavior, the continuity of the deformation front and of other tectonic features along the subduction zone suggests that the rate of subduction does not change appreciably from east to west. The absence of plate boundary events in western Makran indicates either that entirely aseismic subduction occurs or that the plate boundary is currently locked and experiences great earthquakes with long repeat times. Evidence is presently inconclusive concerning which of these two hypotheses is most correct. The presence of well-defined late Holocene marine terraces along portions of the coasts of eastern and western Makran could be interpreted as evidence that both sections of the arc are capable of generating large plate boundary earthquakes. If that hypothesis is correct, then western Makran could produce a great earthquake or it could rupture as a number of segments in somewhat smaller-magnitude events. Alternatively, it is possible that western Makran is significantly different from eastern Makran and experiences largely aseismic slip at all times. A knowledge of the velocity structure and nature of the state of consolidation or lithification of rocks at depth in the interior portion of the forearc of western Makran should help to ascertain whether that portion of the plate boundary moves aseismically or ruptures in large to great earthquakes. A resolution of this question has important implications for seismic hazard not

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

1992-01-01

103

Gravity anomalies, forearc morphology and seismicity in subduction zones  

NASA Astrophysics Data System (ADS)

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

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

2012-12-01

104

Unraveling topography around subduction zones from laboratory models  

NASA Astrophysics Data System (ADS)

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

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

2012-03-01

105

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

106

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

USGS Publications Warehouse

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

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

2000-01-01

107

Improved Teleseismic Locations of Shallow Subduction Zone Earthquakes  

NASA Astrophysics Data System (ADS)

Improved precision teleseismic earthquake locations in subduction zones are being used to better understand shallow megathrust frictional conditions and determine the global distribution of tsunami earthquakes. Most global teleseismic catalogs fail to accurately locate shallow subduction zone earthquakes, especially mid-magnitude events, leading to increased error in determining source time functions useful for identifying tsunami earthquakes. The Engdahl, van der Hilst and Buland (EHB) method had addressed this problem in part by including the teleseismic depth phases pP, pwP and sP in the relocation algorithm. The EHB catalog relies on phase times reported to the ISC and NEIC, but additional high quality depth phase onsets can be incorporated in the relocation procedure to enhance the robustness of individual locations. We present improvements to an automated frequency-based picker that identifies depth phases not reported in the standard catalogs. The revised autopicker uses abrupt amplitude changes of the power spectral density (PSD) function calculated at optimized frequencies for each waveform. It is being used to pick onsets for P and depth phases pP, pwP or sP for inclusion in the EHB phase catalog. In the case of events with an emergent P-wave onset or with a complex waveform consisting of sub-events, the autopicker may either overlook a relatively small change in frequency of the first arrival or misidentify the onset arrival time of associated later arrivals, leading to erroneous results. We track those waveforms by comparing the difference of the P-wave arrival time from ISC/NEIC and the autopicker. The phase arrivals can then be adjusted manually as they usually make up a few percent of the whole data. Epicentral changes following relocation using additional depth phases are generally small (<5 km). Changes in depth may be on the order of 10s of km for some events, though the standard deviation of depth changes within each subduction zone is ~5 km. We will present relocation results for Mb>5.7 events in a number of subduction zones, including Sumatra/Java, Alaska, Kuriles, Peru, and northern Japan.

Bisrat, S. T.; Deshon, H. R.; Engdahl, E. R.; Bilek, S. L.

2009-12-01

108

Rheology of serpentines and mass transfer in subduction zones  

NASA Astrophysics Data System (ADS)

Serpentinites have a lower density and lower viscosity than "dry" ultramafic rocks and it was proposed, based on numerical simulations, that they play a major role in mantle-slab decoupling, and in downward (sink) or upward (exhumation) motion of eclogites and ultra-high pressure (UHP) rocks in subduction zones (Schwartz et al., 2001). Numerical models either assume arbitrary viscosity or use rheological laws derived from low-pressure data (Rayleigh and Paterson, 1965). Extrapolation of less than 0.5 GPa data to subduction zone pressures may be the source of significant error in the physical models. We present here new rheological data on antigorite, the stable variety of serpentine in subduction zones, obtained over a P-T range of 1-4 GPa and 200-500 /deg C that cover most of its stability field. The experiments were carried out in a D-DIA apparatus installed at GSECARS on the 13-BM-D line of APS. Strain rates and stresses were obtained respectively from in-situ monitoring the sample length with X-ray radiographs, and azimuthal dependence of d-spacings on diffraction patterns. The deformed samples show textures resembling those of natural samples, suggesting similar deformation mechanisms. The determined stress-strain curves were fitted to a power-law equation including both temperature and pressure dependence. When compared with extrapolation of power-laws derived from Raleigh and Paterson (1965) data, the stress dependency and pre-exponential factor are identical within mutual uncertainties. However, our apparent activation energy at a given pressure remains less than half of those obtained in previous estimations. Thus the temperature dependence is very low, indicating that antigorite and antigorite-dominated rocks have low viscosities down to the lowest temperatures encountered in subducted slabs. The pressure-induced change in viscosity is significant but the viscosity increase between 1 and 5 GPa remains below 2 orders of magnitude. The present results confirm that serpentinites will indeed act as a weak layer that allows significant mass transfer along the "serpentinized channel" and dynamic processes such as mantle slab decoupling, and mantle wedge convection. They can be applied with little P-T extrapolation to the modeling of subduction zone processes. Raleigh CB, Paterson MS (1965) Experimental Deformation of Serpentinite and its Tectonic Implications J Geophys Res 70: 3965-3985 Schwartz S, Allemand P, Guillot S (2001) Numerical model of the effect of serpentinites on the exhumation of eclogitic rocks: insights from the Monviso ophiolitic massif (Western Alps) Tectonophysics 342: 193-206

Hilairet, N.; Reynard, B.; Daniel, I.; Wang, Y.; Nishiyama, N.; Petitgirard, S.; Merkel, S.

2006-12-01

109

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

110

Strong motions in Alaska-type subduction zone environments  

SciTech Connect

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

Jacob, K.H.; Mori, J.

1984-01-01

111

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

112

Enrichment of trace elements in garnet amphibolites from a paleo-subduction zone: Catalina Schist, southern California  

NASA Astrophysics Data System (ADS)

The abundance, P- T stability, solubility, and element-partitioning behavior of minerals such as rutile, garnet, sphene, apatite, zircon, zoisite, and allanite are critical variables in models for mass transfer from the slab to the mantle wedge in deep regions of subduction zones. The influence of these minerals on the composition of subduction-related magmas has been inferred (and disputed) from inverse modelling of the geochemistry of island-arc basalt, or by experiment. Although direct samples of the dehydration + partial-melting region of a mature subduction zone have not been reported from subduction complexes, garnet amphibolites from melanges of circumpacific and Caribbean blueschist terranes reflect high T (>600°C) conditions in shallower regions. Such rocks record geochemical processes that affected deep-seated, high- T portions of paleo-subduction zones. In the Catalina Schist, a subduction-zone metamorphic terrane of southern California, metasomatized and migmatitic garnet amphibolites occur as blocks in a matrix of meta-ultramafic rocks. This mafic and ultramafic complex may represent either slab-derived material accreted to the mantle wedge of a nascent subduction zone or a portion of a shear zone closely related to the slab-mantle wedge contact, or both. The trace-element geochemistry of the complex and the distribution of trace elements among the minerals of garnet amphibolites were studied by INAA, XRF, electron microprobe, and SEM. In order of increasing alteration from a probable metabasalt protolith, three common types of garnet amphibolite blocks in the Catalina Schist are: (1) non-migmatitic, clinopyroxene-bearing blocks, which are compositionally similar to MORB that has lost an albite component; (2) garnet-amphibolite blocks, which have rinds that reflect local interaction between metabasite, metaperidotite, and fluid; and (3) migmatites that are extremely enriched in Th, HFSE, LREE, and other trace elements. These trace-element enrichments are mineralogically controlled by rutile, garnet, sphene, apatite, zircon, zoisite, and allanite. Alkali and alkaline earth elements are much less enriched in the solid assemblage, and thus appear to be decoupled from the other elements in the inferred metasomatic process(es). The compositions of migmatitic garnet amphibolite blocks seem to complement that of "average" island-arc tholeiite. Trace-element metasomatism reflects fluid-solid, rather than melt-solid, interaction. The metasomatic effects indicate that H 2O-rich fluid, perhaps with a significant component of Na-Al silicate and alkalis, carried Th, U, Sr, REE, and HFSE. Fractionations of LREE in migmatites resemble those of migmatitic metasedimentary rocks underlying the mafic and ultramafic complex. "Exotic" LREE deposited in allanite in migmatites could have been derived from fluids in equilibrium with subducted sediment. If the paleo-subduction zone represented by the mafic and ultramafic complex of the Catalina Schist had continued its thermal and fluid evolution, a selvage of similarly enriched rocks might have been generated along the slab-mantle wedge contact between ~30 and 85 km depth. Rocks affected by "subduction-zone metasomatism," although rarely recognized at the surface, could be volumetrically significant products of the initiation of subduction and may prove to be geochemical probes of convergent margins that approach the significance of xenoliths in the study of other magmatic environments.

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

1989-12-01

113

Enrichment of trace elements in garnet amphibolites from a paleo-subduction zone: Catalina Schist, southern California  

USGS Publications Warehouse

The abundance, P-T stability, solubility, and element-partitioning behavior of minerals such as rutile, garnet, sphene, apatite, zircon, zoisite, and allanite are critical variables in models for mass transfer from the slab to the mantle wedge in deep regions of subduction zones. The influence of these minerals on the composition of subduction-related magmas has been inferred (and disputed) from inverse modelling of the geochemistry of island-arc basalt, or by experiment. Although direct samples of the dehydration + partial-melting region of a mature subduction zone have not been reported from subduction complexes, garnet amphibolites from melanges of circumpacific and Caribbean blueschist terranes reflect high T (>600??C) conditions in shallower regions. Such rocks record geochemical processes that affected deep-seated, high-T portions of paleo-subduction zones. In the Catalina Schist, a subduction-zone metamorphic terrane of southern California, metasomatized and migmatitic garnet amphibolites occur as blocks in a matrix of meta-ultramafic rocks. This mafic and ultramafic complex may represent either slab-derived material accreted to the mantle wedge of a nascent subduction zone or a portion of a shear zone closely related to the slab-mantle wedge contact, or both. The trace-element geochemistry of the complex and the distribution of trace elements among the minerals of garnet amphibolites were studied by INAA, XRF, electron microprobe, and SEM. In order of increasing alteration from a probable metabasalt protolith, three common types of garnet amphibolite blocks in the Catalina Schist are: (1) non-migmatitic, clinopyroxene-bearing blocks, which are compositionally similar to MORB that has lost an albite component; (2) garnet-amphibolite blocks, which have rinds that reflect local interaction between metabasite, metaperidotite, and fluid; and (3) migmatites that are extremely enriched in Th, HFSE, LREE, and other trace elements. These trace-element enrichments are mineralogically controlled by rutile, garnet, sphene, apatite, zircon, zoisite, and allanite. Alkali and alkaline earth elements are much less enriched in the solid assemblage, and thus appear to be decoupled from the other elements in the inferred metasomatic process(es). The compositions of migmatitic garnet amphibolite blocks seem to complement that of "average" island-arc tholeiite. Trace-element metasomatism reflects fluid-solid, rather than melt-solid, interaction. The metasomatic effects indicate that H2O-rich fluid, perhaps with a significant component of Na-Al silicate and alkalis, carried Th, U, Sr, REE, and HFSE. Fractionations of LREE in migmatites resemble those of migmatitic metasedimentary rocks underlying the mafic and ultramafic complex. "Exotic" LREE deposited in allanite in migmatites could have been derived from fluids in equilibrium with subducted sediment. If the paleo-subduction zone represented by the mafic and ultramafic complex of the Catalina Schist had continued its thermal and fluid evolution, a selvage of similarly enriched rocks might have been generated along the slab-mantle wedge contact between ~30 and 85 km depth. Rocks affected by "subduction-zone metasomatism," although rarely recognized at the surface, could be volumetrically significant products of the initiation of subduction and may prove to be geochemical probes of convergent margins that approach the significance of xenoliths in the study of other magmatic environments. ?? 1989.

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

1989-01-01

114

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

115

Metamorphism in Plate Boundary Zones  

NASA Astrophysics Data System (ADS)

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

Brown, M.

2005-12-01

116

Seismic evidence for massive silica addition in some subduction zones  

NASA Astrophysics Data System (ADS)

Subduction zones alter the shallow mantle composition in a variety of ways, including melt removal, addition of metasomatic fluids, and tectonic mixing. These processes should produce distinctive signatures in seismic structure. The mantle wedge beneath many volcanic arcs is characterized by high attenuation (1/Q), low Vp, and high Vp/Vs, all consistent with high temperatures, the presence of small degrees of melt, and perhaps the presence of aqueous fluid. However, at least two subduction zones are characterized by high 1/Q, low Vp, but low Vp/Vs, in Central Alaska and the central Andes flat-slab segment (Wagner et al., 2004 JGR). The Alaska results, presented here, are derived from inversion of receiver functions for the Broadband Experiment Across the Alaska Range (BEAAR). New stacking techniques solve for interface depths and Vp/Vs, while accounting for dipping layers. Where the slab is deeper than 80 km, the overlying mantle wedge exhibits Vp/Vs of 1.62 - 1.68. Travel-time tomography confirms the low Vp/Vs. This is lower than predicted for any common mantle mineral (1.73-1.85), and lower than commonly measured in the mantle. Calculations show that isolated pores filled with a highly-compressible fluid can lower Vp/Vs slightly below that of the surrounding matrix, but can reach these low Vp/Vs only with physically absurd parameters (e.g. 10-20% porosity containing vacuum). Of the major rock-forming minerals, only quartz has a low Vp/Vs (Poisson's ratio). Thus, we suggest that some parts of these mantle wedges contain significant quantities of quartz. High quartz, and high overall silica, is a possible consequence of extreme metasomatism, or of mechanical addition of quartz-rich crustal rocks into the mantle during previous subduction events. Estimates of the fluid flux out of subducting slabs and silica solubilities rule out the possibility of large regions of the mantle wedge being silica oversaturated by metasomatism. In contrast, mechanical mixing of crust and mantle has been observed on the meter to kilometer scale in melange zones. During Alaskan terrane accretion events, large crustal slices may have been incorporated into the mantle wedge and may represent a significant mass-flux of silica into the mantle. Past sources of silica are less clear in the Andes, but tectonic erosion may be important. Perhaps, such silica addition is a common consequence of tectonic events at subduction zones.

Abers, G. A.; Rossi, G.; Rondenay, S.; Peacock, S.; Chrsistensen, D. H.

2005-12-01

117

Trench-parallel fluid flow in subduction zones resulting from temperature differences  

Microsoft Academic Search

Differences in the thermal state of subducting crust along the trench of a subduction zone cause differences in subduction zone temperature that persist to tens of kilometers down-dip of the trench. The resulting differences in fluid viscosity, permeability, and hydraulic conductivity can lead to trench-parallel variations in fluid pressure on the plate boundary fault. Temperature differences in locations with low

Glenn A. Spinelli; Demian M. Saffer

2007-01-01

118

Links Between Faulting of the Incoming Ocean Plate and Earthquakes in the Slab of Subduction Zones  

Microsoft Academic Search

Recent multibeam bathymetry shows that bending of ocean plates at subduction zones develops a complex system of faulting. Independently, studies of slab seismicity have interpreted that most intermediate depth earthquakes occur by reactivation of faults formed during plate bending at trenches. We have studied the relation between bending faults and slab seismicity at the subduction zones of Middle America and

C. R. Ranero; A. Villasenor; J. Phipps Morgan

2002-01-01

119

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

120

Initiation of Subduction Zones: A Consequence of Lateral Compositional Buoyancy Contrast Within the Lithosphere  

Microsoft Academic Search

Subduction of oceanic lithosphere into deep mantle is one of the key aspects of plate tectonics. Pull by the subducting-slab due to its negative buoyancy is widely accepted as the major driving force for plate motion and plate tectonics. Hence, there would be no plate tectonics if there were no subduction zones. Yet how a subduction zone initiates remains poorly

Yaoling Niu; M. J. O'Hara; J. A. Pearce

2001-01-01

121

Constraints on Pore Pressure in Subduction Zones From Geotechnical Tests and Physical Properties Data  

Microsoft Academic Search

At subduction zones, as incoming sediments are either offscraped or underthrust at the trench, elevated pore pressures result from the combination of rapid loading and low permeability. Pore pressure within underthrust sediment is especially important for the mechanical strength of the plate boundary fault system, because the main décollement localizes immediately above this sediment, and at many subduction zones steps

D. M. Saffer; A. W. McKiernan

2005-01-01

122

Interplate coupling along segments of the Central America Subduction zone  

NASA Astrophysics Data System (ADS)

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

Zarifi, Zoya; Raeesi, Mohammad; Atakan, Kuvvet

2013-04-01

123

Resolution experiments for NW Pacific subduction zone tomography  

NASA Technical Reports Server (NTRS)

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

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

1989-01-01

124

Deformation across the Alaska-Aleutian Subduction Zone near Kodiak  

USGS Publications Warehouse

The Kodiak-Katmai geodetic array, nine monuments distributed along a profile trending north-northwestward across Kodiak Island and the Alaska Peninsula, was surveyed in 1993, 1995 and 1997 to determine the deformation at the Alaska-Aleutian subduction zone. Velocities on Kodiak Island measured relative to the stable North American plate decrease with distance from the Alaska-Aleutian trench (distance range 106 to 250 km), whereas no appreciable deformation was measured on the Alaska Peninsula (distances 250 to 370 km from the trench). The measured deformation is reasonably well predicted by the conventional dislocation representation of subduction with the model parameters determined independently (i.e., not simply by fitting the observations). The deformation of Kodiak Island is in striking contrast to the very minor deformation measured in the similarly situated Shumagin Islands, 450 km southwest of Kodiak along the Alaska-Aleutian trench.

Savage, J. C.; Svarc, J. L.; Prescott, W. H.

1999-01-01

125

Late Holocene tectonics and paleoseismicity, southern Cascadia subduction zone  

USGS Publications Warehouse

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

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

1992-01-01

126

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

127

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

SciTech Connect

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

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

1988-02-01

128

Rheology of serpentines, seismicity and mass transfer in subduction zone  

NASA Astrophysics Data System (ADS)

Serpentinites have a lower density and lower viscosity than "dry" ultramafic rocks and it was proposed, based on numerical simulations, that they play a major role in mantle-slab decoupling, and in downward (sink) or upward (exhumation) motion of eclogites and ultra-high pressure (UHP) rocks in subduction zones. Rheological data on antigorite, the stable variety of serpentine in subduction zones, are obtained over a P-T range of 1-4 GPa and 200-500 /deg C that cover most of its stability field. The experiments were carried out in a D-DIA apparatus installed at GSECARS on the 13-BM-D line of APS. The determined stress-strain curves were fitted to a power-law equation including both temperature and pressure dependence. The results confirm that serpentinites acts as a weak layer that allows significant mass transfer along the "serpentinized channel" and dynamic processes such as mantle slab decoupling, and mantle wedge convection. Regardless of the temperature, serpentinized mantle at the slab surface has a low viscosity that allows localizing the deformation and impeding stress build-up. It will limit the downdip propagation of large earthquakes, and allow viscous relaxation as an origin of post-seismic deformations and slow earthquakes. Models of growth and transport of a serpentinized channel using available kinetic and present rheological data explain high exhumation rates of eclogites and limited thickness of the channel at great depths (above 50 km), and slower exhumation at in a thick hydrated mantle corner at shallower depths. Such channels may be difficult to detect from sismic tomography or using guided waves because of their small thickness (less than 3 km).

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

2008-12-01

129

Carbon dioxide released from subduction zones by fluid-mediated reactions  

NASA Astrophysics Data System (ADS)

The balance between the subduction of carbonate mineral-bearing rocks into Earth's mantle and the return of CO2 to the atmosphere by volcanic and metamorphic degassing is critical to the carbon cycle. Carbon is thought to be released from subducted rocks mostly by simple devolatilization reactions. However, these reactions will also retain large amounts of carbon within the subducting slab and have difficulty in accounting for the mass of CO2 emitted from volcanic arcs. Carbon release may therefore occur via fluid-induced dissolution of calcium carbonate. Here we use carbonate ?18O and ?13C systematics, combined with analyses of rock and fluid inclusion mineralogy and geochemistry, to investigate the alteration of the exhumed Eocene Cycladic subduction complex on the Syros and Tinos islands, Greece. We find that in marble rocks adjacent to two fluid conduits that were active during subduction, the abundance of calcium carbonate drastically decreases approaching the conduits, whereas silicate minerals increase. Up to 60-90% of the CO2 was released from the rocks--far greater than expected via simple devolatilization reactions. The ?18O of the carbonate minerals is 5-10 lighter than is typical for metamorphosed carbonate rocks, implying that isotopically light oxygen was transported by fluid infiltration from the surroundings. We suggest that fluid-mediated carbonate mineral removal, accompanied by silicate mineral precipitation, provides a mechanism for the release of enormous amounts of CO2 from subduction zones.

Ague, Jay J.; Nicolescu, Stefan

2014-05-01

130

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

131

Garnet growth as a proxy for progressive dehydration in subduction zones  

NASA Astrophysics Data System (ADS)

The release of volatiles from subducting lithologies is a crucial triggering process for arc magmatism, seismicity, the growth and maturation of continents, and the global geological H2O-CO2 cycle. While numerous models have been developed to predict slab volatile release, it has proven challenging to reconstruct and test these fluid fluxes released from specific lithologies in the rock record. Here we show that the growth of garnet may be used as a proxy for progressive devolatilization at blueschist to eclogite facies conditions in subduction zones. Generally, as garnet grows in a subducting rock, fluid is produced due to metamorphic dehydration reactions. Using rigorous thermodynamic analysis, which includes the crucial effects of phase fractionation, we model the proportional relationship between garnet and water production in common lithologies (pelitic sediment and hydrated MORB) along three representative subduction geotherms. The results show that several dehydration reactions contribute to garnet growth especially within a crucial span of the subduction zone (~1.5 to 2.5 GPa) within which slab-mantle decoupling has been predicted to occur in some models and volatile fluxes may be focused. The water:garnet production ratio varies during garnet growth, constrained by the specific hydrous reactant phases that are breaking down, but the average water:garnet production ratios are surprisingly consistent regardless of composition and geotherm. Over the garnet growth interval ~400 to 700 C (and corresponding depths for each geotherm) the average production ratio for altered MORB compositions is 0.52 (wt % water per vol % garnet) in cooler geotherms (Honshu and Nicaragua) and 0.27 in hotter (Cascadia) geotherms, with predictably lower ratios if the input basalt previously experienced less hydrous alteration. Over the same interval the water production ratios are approximately 50 % lower for pelite (0.24 and 0.13, respectively). Lower temperature water release is not correlated with garnet growth (it occurs before garnet stability), and higher temperature garnet growth is not correlated with dehydration (as the system is essentially anhydrous); fortunately, the chemistry of garnet produced at these conditions is readily identifiable. Taken together, these results suggest that garnet abundance, its P-T growth span, and any constraints on its growth chronology may be used to indirectly but accurately reconstruct dehydration fluxes for natural samples in the blueschist-eclogite rock record.

Caddick, M. J.; Baxter, E. F.

2012-12-01

132

3D Lithospheric Density Structure of the Central American Subduction Zone from Gravity Data  

NASA Astrophysics Data System (ADS)

Data from the EGM2008 Combined Geopotential Model has been interpreted to construct a comprehensive three-dimensional model of the lithospheric density structure along the Central American Isthmus. This is the first time that integration of all geophysical information available for the isthmus has been undertaken. The density model is constrained by seismic velocity models, magnetotelluric cross-sections, receiver functions,and hypocenter data from local seismic networks acquired along the Middle American Subduction Zone by different institutes and projects during the last three decades. The segmentation of the crustal basement of the Caribbean Plate was modeled with separate units for the Chortis Block (2.77 Mg/m^3), the Mesquito Composite Oceanic Terrane / SiunaTerrane (3 Mg/m^3), and the Caribbean Large Igneous Province (2.90 Mg/m^3). Furthermore, first order boundary layers such as the Moho and the Cocos-Caribbean plate interface were modeled and extracted for correlation with tectonic features and dynamic processes. The Costa Rican segment has been the most widely studied along the Central American margin. Here, it is possible to review the slab geometry based on the three-dimensional density model against seismological information from local networks in greater detail. By integrating probabilistic relocated hypocenters with the density model by means of 3D visualization, a joint interpretation of the distribution of seismicity with the density units in the subducted slab was carried out.A change in the depth of intra-plate seismicity is observed reaching 220 km for the northwestern part and becoming shallower toward the southeast where it reaches a maximum depth of 75 km. The changes in the maximum depth registered for the seismicity, correlate with changes in the density structure of the subducted slab which were modeled based on the gravity response of the model. The crust of the oceanic plate was assigned an initial density of 2.80 Mg/m3, deeper than this the density changes within the slab were modeled based on published petrological calculations constrained by thermal and lithostatic pressure conditions and considering metamorphic reactions. In consequence, the initial density for the Cocos Plate crust increases to 3.15 Mg/m^3 corresponding to the downdip extent of the Wadati-Benioff seismicity and then to 3.30 Mg/m^3 for the deepest, aseismic sections of the slab where the subducted crust is interpreted as being anhydrous. The increase in density varies in depth along the subduction zone and its correlation with the terminal depth of intraslab seismicity supports the hypothesis that differences in the state of initial hydration of the oceanic lithosphere affect the depth reached by dehydration reactions.

Lücke, O. H.; Arroyo, I. G.; Linkimer, L.

2013-12-01

133

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

134

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

NASA Astrophysics Data System (ADS)

In this study, we revisit the effects of along-strike variation in slab geometry and oblique subduction on subduction zone thermal structures. Along-strike variations in slab dip cause changes in the descending rate of the slab and generate trench-parallel pressure gradients that drive trench-parallel mantle flow (e.g., Kneller and van Keken, 2007). Oblique subduction also drives trench-parallel mantle flow. In this study, we use a finite element code PGCtherm3D and examine a range of generic subduction geometries and parameters to investigate the effects of the above two factors. This exercise is part of foundational work towards developing detailed 3-D thermal models for NE Japan, Nankai, and Cascadia to better constrain their 3-D thermal structures and to understand the role of temperature in controlling metamorphic, seismogenic, and volcanic processes. The 3-D geometry of the subducting slabs in the forearc and arc regions are well delineated at these three subduction zones. Further, relatively large compilations of surface heat flow data at these subduction zones make them excellent candidates for this study. At NE Japan, a megathrust earthquake occurred on March 11, 2011; at Nankai and Cascadia, there has been a great effort to constrain the scale of the next subduction thrust earthquake for the purpose of disaster prevention. Temperature influences the slip behavior of subduction faults by (1) affecting the rheology of the interface material and (2) controlling dehydration reactions, which can lead to elevated pore fluid pressure. Beyond the depths of subduction thrust earthquakes, the thermal structure is affected strongly by the pattern of mantle wedge flow. This flow is driven by viscous coupling between the subducting slab and the overriding mantle, and it brings in hot flowing mantle into the wedge. The trench-ward (up-dip) extent of the slab-mantle coupling is thus a key factor that controls the thermal structure. Slab-mantle decoupling at shallow depths causes mantle stagnation and a cool condition, which allows serpentinization to occur, whereas coupling at greater depths drives hot flowing mantle, providing the thermal condition required for melt generation in the mantle wedge. The flowing mantle also causes rapid heating of the subducting slab and affects the occurrence of intraslab earthquakes. In the generic model calculations in the study, we also investigate the effect of local fluctuations in the depth of decoupling-coupling transition on the 3-D mantle wedge flow pattern and thermal structure. Kneller, E.A., and P.E. van Keken (2008), Effect of three-dimensional slab geometry on deformation in the mantle wedge: Implications for shear wave anisotropy, Geochem. Geophys. Geosyst., 9, Q01003, doi:10.1029/2007GC001677.

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

2013-12-01

135

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

136

Reflection Image Spectroscopy across the Andean subduction zone  

NASA Astrophysics Data System (ADS)

This paper presents new insights into the South American subduction zone from reprocessed seismic images. We applied a 3D Kirchhoff prestack depth migration scheme to data sets containing different narrow frequency ranges in order to extract additional details from seismic reflection images. This approach accounts for the effects of scattering on the seismic image, especially for structures below a heterogeneous overburden. The reflection image in such environments will differ significantly when focusing on different frequency ranges due to the frequency dependence of scattering that is likely to be present. The narrow frequency range images uncover reflectors in one frequency range that are masked in another range. Furthermore, the images enable for instance the characterization of the medium in terms of scatterer concentration and thus improve the structural interpretation. The analysis of these images might help to distinguish between small-scale structures in the high-frequency band and large-scale structures in the low-frequency band. We call this imaging approach Reflection Image Spectroscopy (RIS). We applied the RIS approach to the ANCORP'96 data set, an onshore deep seismic reflection profile across the South American Central Andes. The narrow frequency range images revealed additional details that we interpret as features directly linked to fluid migration processes in the subduction zone. Furthermore, structural details of the oceanic crust and the overlying mantle and crust are revealed. From the narrow-frequency range images we interpret that the top of the so called Nazca reflector at 70 km depth marks the upper limit of the hydrated mantle wedge, whereas the bottom of the reflector represents the top of the subducted oceanic crust. The compilation of our results with local earthquake data confirms this interpretation. Similar features observed in another deep seismic profile (PRECORP'95) support this interpretation, too. Furthermore, the RIS images show a highly reflective heterogeneous zone between the Nazca reflector at 70 km depth and a prominent mid-crustal Bright Spot (Quebrada Blanca Bright Spot) at about 30 km depth. We associate this zone with a complex network of ascending fluids or partial melts, initiated by ascending fluids released from the subducted oceanic plate. This observation links the Quebrada Bright Spot directly to the dehydrating oceanic plate.

Yoon, M.; Buske, S.; Shapiro, S. A.; Wigger, P.

2009-07-01

137

Rheology of hydrous phases in subduction zone settings (Invited)  

NASA Astrophysics Data System (ADS)

The unique properties of hydrous phases impart important constraints on the dynamics of subduction zones. In this presentation I will provide a brief overview of our recent experimental projects on talc (Escartin, Andreani, Hirth and Evans, 2008) and serpentine (Chernak and Hirth, 2010; Kohli, Goldsby, Hirth and Tullis, 2010) and discuss the extrapolation of these results to natural settings. A number of experimental studies (and intuition instilled from our first geology labs as students) demonstrate that talc is very weak. However, closer inspection of talc deformation illustrates some surprising results. With increasing temperature within the talc stability field we found a transition from distributed to localized deformation (Escartin et al.). TEM analyses of these samples showed no evidence for dehydration. We concluded that this unique behavior occurred owing to the strong plastic anisotropy of talc and speculated that the transition to more localized deformation occurred due to differences in the temperature dependence of “friction” and “bulk rock strength” (c.f. Byerlee, 1968). Subsequently, we found similar behavior in serpentinite (Chernak and Hirth). Extrapolation of the serpentinite results to geologic conditions results in an effective viscosity similar to that predicted by published flow laws for dislocation creep of antigorite (Hilairet et al. 2007). However, the observation of localized deformation - rather than distributed ductile flow -may have important implications for the style of slip that occurs on such faults. From this context, the observation of dynamic weakening of serpentinite (Kohli et al.) may have important implications. In these room temperature experiments, the friction coefficient drops dramatically at sliding velocities greater than around 100 mm/s. Analysis of the conditions of weakening supports the flash weakening hypothesis of dynamic friction. Because the flash weakening velocity depends on the temperature of the fault, the observation of localized slip in serpentinite indicates significant weakening may occur at sub-seismic sliding velocities - providing a possible explanation for the spectrum of fault slip behaviors observed in subduction zones, as well as a weakening mechanism to further “decouple” the slab from the overlying mantle wedge.

Hirth, G.

2010-12-01

138

Phase equilibria constraints on models of subduction zone magmatism  

NASA Astrophysics Data System (ADS)

Petrologic models of subduction zone magmatism can be grouped into three broad classes: (1) predominantly slab-derived, (2) mainly mantle-derived, and (3) multi-source. Slab-derived models assume high-alumina basalt (HAB) approximates primary magma and is derived by partial fusion of the subducting slab. Such melts must, therefore, be saturated with some combination of eclogite phases, e.g. cpx, garnet, qtz, at the pressures, temperatures and water contents of magma generation. In contrast, mantle-dominated models suggest partial melting of the mantle wedge produces primary high-magnesia basalts (HMB) which fractionate to yield derivative HAB magmas. In this context, HMB melts should be saturated with a combination of peridotite phases, i.e. ol, cpx and opx, and have liquid-lines-of-descent that produce high-alumina basalts. HAB generated in this manner must be saturated with a mafic phase assemblage at the intensive conditions of fractionation. Multi-source models combine slab and mantle components in varying proportions to generate the four main lava types (HMB, HAB, high-magnesia andesites (HMA) and evolved lavas) characteristic of subduction zones. The mechanism of mass transfer from slab to wedge as well as the nature and fate of primary magmas vary considerably among these models. Because of their complexity, these models imply a wide range of phase equilibria. Although the experiments conducted on calc-alkaline lavas are limited, they place the following limitations on arc petrologic models: (1) HAB cannot be derived from HMB by crystal fractionation at the intensive conditions thus far investigated, (2) HAB could be produced by anhydrous partial fusion of eclogite at high pressure, (3) HMB liquids can be produced by peridotite partial fusion 50-60 km above the slab-mantle interface, (4) HMA cannot be primary magmas derived by partial melting of the subducted slab, but could have formed by slab melt-peridotite interaction, and (5) many evolved calc-alkaline lavas could have been formed by crystal fractionation at a range of crustal pressures.

Myers, James D.; Johnston, Dana A.

139

Melt Inclusions as Windows on Subduction Zone Processes - A Retrospective  

NASA Astrophysics Data System (ADS)

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

Sisson, T. W.

2002-12-01

140

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

141

Geodetic measurements of convergence across the New Hebrides Subduction Zone  

NASA Astrophysics Data System (ADS)

Between 1990 and 1994, geodetic measurements (GPS observations) have been conducted across the New Hebrides subduction zone where the Australia plate subducts under the New Hebrides Arc. This paper establishes convergence rate variations along the trench. In the South, at Tanna, the relative motion is oriented N244 ±4 and has a uniform rate of 11.7±0.8 cm/yr. The rate at Efaté is 10.3±0.9 cm/yr, oriented N242 ±4. Both azimuths very well compare with slip vectors of the last major earthquakes. In the North, the rate at Santo is only 3.6 ±1.2 cm/yr, oriented N253 ± 26. The difference in the convergence rates between Santo on the one hand and Efaté and Tanna in the other hand points to a right lateral shear zone between Santo and Efaté. At Santo where the plate coupling is very high, the very low convergence rate might be related to the absence of recent strong earthquakes. No significant variations are detected for the baselines within the Australia plate.

Calmant, Stéphane; Lebellegard, Pierre; Taylor, Fred; Bevis, Michael; Maillard, Didier; Récy, Jacques; Bonneau, Jocelyne

142

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

143

Geoid anomalies in the vicinity of subduction zones  

NASA Technical Reports Server (NTRS)

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

Mcadoo, D. C.

1980-01-01

144

Radiocarbon test of earthquake magnitude at the Cascadia subduction zone  

USGS Publications Warehouse

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

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

1991-01-01

145

Downgoing plate controls on overriding plate deformation in subduction zones  

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

146

Potential contributions of Seafloor Geodesy to understanding slip behavior along the Cascadia Subduction Zone  

NASA Astrophysics Data System (ADS)

We examine new methods to measure the crustal deformation along Cascadia that crosses the entire region of the submerged portion of the subduction zone from the incoming plate, the offshore continental slope and up to the sub-aerial continent. Presently, onshore geodesy has determined that the locked region lies almost entirely offshore. However, these data lack proximity and poorly resolve details of the stick-slip behavior near the deformation front and the location of the boundary from full stick to some component of stable sliding. To date GPS-Acoustic measurements have been collected infrequently using ships with special capabilities and seafloor acoustic transponders with finite life-spans. Developments underway include moored-buoys for continuous collection of GPS-Acoustic data, small autonomous platforms that use less expensive ships or no ship at all, and permanent 3-D benchmarks on the seafloor to extend the time series of seafloor position measurements for years to decades. Here we focus on the initial results from testing a wave-powered autonomous platform for collecting GPS-Acoustic data along with a numerical study of the optimum placement of seafloor sites for monitoring strain along the submerged portion of the interface. Work supported by NSF Grants OCE 11-44493 and OCE 11-30003.

Chadwell, C. D.

2012-12-01

147

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

USGS Publications Warehouse

Seismic reflection profiles shot across the Cascadia forearc show that a 5-15 km thick band of reflections, previously interpreted as a lower crustal shear zone above the subducting Juan de Fuca plate, extends into the upper mantle of the North American plate, reaching depths of at least 50 km. In the extreme western corner of the mantle wedge, these reflectors occur in rocks with P wave velocities of 6750-7000 ms-1. Elsewhere, the forearc mantle, which is probably partially serpentinized, exhibits velocities of approximately 7500 ms-1. The rocks with velocities of 6750-7000 ms-1 are anomalous with respect to the surrounding mantle, and may represent either: (1) locally high mantle serpentinization, (2) oceanic crust trapped by backstepping of the subduction zone, or (3) rocks from the lower continental crust that have been transported into the uppermost mantle by subduction erosion. The association of subparallel seismic reflectors with these anomalously low velocities favours the tectonic emplacement of crustal rocks. Copyright 2003 by the American Geophysical Union.

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

2003-01-01

148

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

NASA Astrophysics Data System (ADS)

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

Métois, Marianne; Vigny, Christophe; Socquet, Anne

2014-05-01

149

New seismic images of the Cascadia subduction zone from cruise SO108 — ORWELL  

NASA Astrophysics Data System (ADS)

In April and May 1996, a geophysical study of the Cascadia continental margin off Oregon and Washington was conducted aboard the German R/V Sonne. This cooperative experiment by GEOMAR and the USGS acquired wide-angle reflection and refraction seismic data, using ocean-bottom seismometers (OBS) and hydrophones (OBH), and multichannel seismic reflection (MCS) data. The main goal of this experiment was to investigate the internal structure and associated earthquake hazard of the Cascadia subduction zone and to image the downgoing plate. Coincident MCS and wide-angle profiles along two tracks are presented here. The plate boundary has been imaged precisely beneath the wide accretionary wedge close to shore at ca. 13 km depth. Thus, the downgoing plate dips more shallowly than previously assumed. The dip of the plate changes from 2° to 4° at the eastern boundary of the wedge on the northern profile, where approximately 3 km of sediment is entering the subduction zone. On the southern profile, where the incoming sedimentary section is about 2.2 km thick, the plate dips about 0.5° to 1.5° near the deformation front and increases to 3.5° further landwards. On both profiles, the deformation of the accretionary wedge has produced six ridges on the seafloor, three of which represent active faulting, as indicated by growth folding. The ridges are bordered by landward verging faults which reach as deep as the top of the oceanic basement. Thus the entire incoming sediment package is being accreted. At least two phases of accretion are evident, and the rocks of the older accretionary phase(s) forms the backstop for the younger phase, which started around 1.5 Ma ago. This documents that the 30 to 50 km wide frontal part of the accretionary wedge, which is characterized by landward vergent thrusts, is a Pleistocene feature which was formed in response to the high input of sediment building the fans during glacial periods. Velocities increase quite rapidly within the wedge, both landward and downward. At the toe of the deformation front, velocities are higher than 4.0 km/s, indicating extensive dewatering of deep, oceanic sediment. Further landward, considerable velocity variation is found, which indicates major breaks throughout the accretionary history.

Flueh, Ernst R.; Fisher, Michael A.; Bialas, Joerg; Childs, Jonathan R.; Klaeschen, Dirk; Kukowski, Nina; Parsons, Tom; Scholl, David W.; ten Brink, Uri; Tréhu, Anne M.; Vidal, Neus

1998-07-01

150

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

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

151

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

ERIC Educational Resources Information Center

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

Shea, James Herbert

1991-01-01

152

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

SciTech Connect

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

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

1995-11-10

153

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

154

A strong-motion database from the Peru–Chile subduction zone  

Microsoft Academic Search

Earthquake hazard along the Peru–Chile subduction zone is amongst the highest in the world. The development of a database\\u000a of subduction-zone strong-motion recordings is, therefore, of great importance for ground-motion prediction in this region.\\u000a Accelerograms recorded by the different networks operators in Peru and Chile have been compiled and processed in a uniform\\u000a manner, and information on the source parameters

Maria C. Arango; Fleur O. Strasser; Julian J. Bommer; Ruben Boroschek; Diana Comte; Hernando Tavera

2011-01-01

155

Controls on the Migration of Fluids in Subduction Zones  

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

156

Rare earth element budgets in subduction-zone fluids  

NASA Astrophysics Data System (ADS)

Subduction zone fluids play a fundamental role in the geochemical cycle of the Earth. The nature and composition of these fluids are determined by complex processes and still poorly understood. As a result of a variety of metasomatic and partial melting events, arc-related magmas display a typical trace element abundance spectrum, in which the rare earth elements' (REE) signature is an important record of petrogenetic processes. Therefore, investigating the behavior of REE in fluids at high pressure (P) and temperature (T) conditions is crucial for constraining fluid composition, as well as understanding subduction-zone processes in general. However, up to date, the experimental studies on REE solubility and speciation are limited to quite low P-T conditions (300 °C, saturated water vapor pressure) [1]. The theoretical predictions of the stability of REE complexes have been performed up to 350 °C [2] and 1000 °C, 0.5 GPa [3] by the extrapolation of thermodynamic data obtained at ambient conditions. In this study we present new experimental data on REE silicate (REE2Si2O7) solubility in aqueous quartz saturated fluids, containing various ligands, at conditions relevant for subducting slabs (600, 700, 800 °C, 2.6 GPa). The aim of the experiments was to investigate the relative effect of temperature and ligands on the solubility of REE. The experiments were conducted in an end-loaded piston-cylinder apparatus and the fluids were in situ sampled at P-T in the form of primary fluid inclusions in quartz [4]. The gold capsule was typically loaded with a chip of synthetic REE silicate (La,Nd,Gd,Dy,Er,Yb)2Si2O7, an aqueous fluid (~20 wt.%) and a piece of natural quartz. During the experiment (24-48 h) a thermal gradient along the capsule promoted intensive dissolution of quartz at the hottest part and precipitation of new quartz at the cooler part of the capsule, allowing the primary fluid inclusions to be trapped (~30-50 ?m). Rubidium and cesium were added to the fluid as the internal standards for LA-ICPMS analyses. The solubility of REE in quartz saturated water, free of additional ligands, increases more than an order of magnitude as temperature is increased from 600 °C to 800 °C. The effect of halogen ligands (Cl-, F-) on the solubility of REE was tested on experiments conducted at 800 °C. Addition of 1.5 m NaCl enhances the solubilities of all REE by a factor of 2 to 4 and induces moderate LREE/HREE fractionation; the La/Yb ratio increases by factor of 2. Unlike chlorine, the presence of fluorine ligands in the fluid (0.3 m NaF) promotes increase in HREE solubilities with almost no change in LREE solubilities compared to water, hence decreasing the La/Yb ratio by a factor of 2. The results of our experiments suggest that temperature plays an important role in mobilization of all REE by fluids. The presence of Cl- and F- ligands in the fluid shows opposing effects on the REE pattern: Cl- seems to be a more efficient ligand for LREE, while F- tends to form more stable complexes with HREE. [1] Migdisov A. A. et al., 2009, Geochim Cosmochim Acta, 73, 7087-7109 [2] Wood S. A. 1990b, Chem Geo., 82, 159-186 [3] Haas J. R. 1995, Geochim Cosmochim Acta, 59, 4329-4350 [4] Bali E. et al., 2011, Contrib Mineral Petrol, 161, 597-613

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

2012-12-01

157

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

158

Volatile transfer and recycling at UHP metamorphism; constraint from CCSD (Chinese Continental Scientific Drilling) eclogites  

NASA Astrophysics Data System (ADS)

Study of dehydration and decarbonation processes of subducting oceanic crust is important to understand the island arc volcanism and recycling of water and carbon to deep mantle. Recent UHP experiments in C-O-H fluid-bearing MORB system have revealed that phase change and fluid composition depend on oxygen fugacity (e.g. Molina and Poli, 2002; Crottini and Poli, 2004). If oxygen fugacities represented by the equilibrium NNO (Ni-NiO) or FMQ (fayalie-magnesite-quartz) are assumed to be the average condition of UHP metamorphism, then the phase assemblages of UHP rocks are expected to have graphite/diamond only, graphite/diamond +carbonates, or carbonates only depending on the bulk compositions (Poli and Fumagalli, 2004, EMU notes in miner. vol. 5). C-species are well described in Chinese UHP eclogites (e.g. Zhang and Kai, 1996). However, carbonates can be easily leached from outcrop. Therefore in the worst case, only graphite could be recognized from surface exposures although drilled core samples represent carbonates with graphite (e.g. Sanbagawa schist in Japan, Goto et al., 2000, Ann. Meet. Japan. Petrol. Miner. Mining Geol. Assoc.). From this point of view, CCSD (Chinese Continental Scientific Drilling) samples are probably the best for identification of C-species in UHP rocks. We investigated nine eclogites from various depths (170 to 2000 m). Two types of eclogite can be distinguished; dry- and phengite-eclogite. The phengite eclogite is associated with orthogneiss. Under the microscope, the dry eclogites contain relative coarse-grained (> 500 microns across) garnet, clinopyroxene and rutile with or without graphite, quartz, apatite, zircon, and pyrite. The phengite eclogites exhibit garnet, clinopyroxene, rutile, quartz, and phengite with or without graphite, pyrite, talc, apatite, zircon, and K-feldspar. Graphite is always recognized with pyrite, suggesting oxygen fugacity was low (NNO) at UHP stage. Estimated P, T conditions based on the assemblage garnet-clinopyroxene-phengite (Water and Martin, 1993; Ravna and Terry, 2004) gives P = 4 GPa, and T = 850 oC, suggesting that peak P reached graphite-diamond boundary. Temperature conditions of the eclogites exceed wet solidus in MORB composition. The solidus at 3-5 GPa is defined as follows; phengite + clinopyroxene + coesite (quartz) + fluid = melt (Schmidt et al., 2004). This reaction is evidenced by the _gmelt texturEh: K-feldspar pool containing blocky quartz, clinopyroxene and graphite. The presence of graphite suggests that the eclogites released H2O-rich (CO2-poor) fluids and melt at UHP stage (Crottini and Poli, 2004).

Okamoto, K.; Iizuka, Y.; Jahn, B.; Tzeng-Fu, Y.; Xu, Z.

2005-12-01

159

Heat-flow density across the Central Andean subduction zone  

NASA Astrophysics Data System (ADS)

New and revised heat-flow density determinations for the Central Andes in Chile and Bolivia between 60° and 75°W and between 15° and 30°S are presented. Heat flow was determined from temperature logs obtained from fourteen boreholes in northern Chile and from a large Bottom-Hole Temperature (BHT) data set in Bolivia. Thermal data were corrected for perturbations resulting from drilling, topography, and fluid circulation, and effects of porosity and temperature were considered in adjusting thermal conductivity for in-situ conditions. Thus, the 74 reliable heat-flow values available were used to interpret heat flow in the context of lithospheric conditions. The heat flow in the Andean orogen shows a strong dependence on the different tectono-morphologic units. Clusters of heat-flow values projected onto a W-E profile reflect significant changes from the Nazca Plate in the west to the Chaco Basin in the east. Heat-flow density is low (20 mW/m 2) in the area of the Coastal Cordillera and high (50-180 mW/m 2) in the active magmatic arc and the Altiplano. The back-arc region is characterized by a comparably high heat flow (80 mW/m 2) whereas the Andean foreland has a heat flow of 40 mW/m 2, which is slightly lower than the heat flow in the adjacent Brazilian Shield farther east. The Central Andean subduction zone shows a heat-flow pattern similar to the active margin of the North American continent, but differs in the subduction parameters. Consequently, this results in different lithospheric thermal conditions.

Springer, Michael; Förster, Andrea

1998-06-01

160

Permeability-porosity relationships of subduction zone sediments  

USGS Publications Warehouse

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

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

2011-01-01

161

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

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

2013-12-01

162

Sediment strength contrasts and decollement localization in Nankai subduction zone  

NASA Astrophysics Data System (ADS)

We experimentally characterized the mechanical properties of marine sediments entering the Nankai Trough subduction zone (southwestern convergent margin of Japan). The mechanical tests, based on the independent application of isotropic confining Pc and pore pressure Pp, aimed at assessing both the poroelastic compressibility as well as the resistance to compaction samples from ODP Site 1173 regularly spaced over the 250-700 meter below seafloor (mbsf) depth interval. The resistance to compaction, estimated as the irreversible uniaxial shrinkage upon application of an effective pressure in the order of twice in-situ value, shows large and non-linear variations. In a shallow domain (0-450 mbsf), samples are affected by large compaction, whereas in a deep domain (550-700 mbsf) compaction is much more limited; the transition zone between the two domains occurs at depths compatible with the actual decollement localization. Building on such experimental results, we designed a model to assess the effect of the increase in stress in a sedimentary column approaching the prism toe, which is based on the assumptions that (i) the porosity reduction associated with stress variations in the column varies strongly with depth according to our experimental results as well as drill core data, and that (ii) at least a small fraction of this porosity reduction results from horizontal compression (i.e. lateral shortening). In this modeling framework, differential compaction in response to horizontal compression between shallow and deep sections of an incoming column results in large differential displacement, i.e. shear strain in the transition zone, which may facilitate decollement propagation and therefore poses some control its localization.

Raimbourg, Hugues; Ujiie, Kohtaro; Hamano, Yozo; Saito, Saneatsu; Kinoshita, Masataka; Kopf, Achim

2010-05-01

163

Wide angle seismic refraction imaging of the Northern Sumatra Subduction Zone  

NASA Astrophysics Data System (ADS)

Two deep penetrating wide angle seismic refraction profiles were acquired during the SUMATRA-OBS cruise offshore Sumatra, to image the subsurface in the zone of maximum co-seismic slip for the great Sumatra- Andaman earthquake of December 2004. Here we present the 2-D velocity image of the subsurface obtained by traveltime tomographic inversion of the northern profile. This line was shot twice using airguns onboard R/V Marion Dufresne (~8260 cu inch) and the one onboard Western-Geco/Schlumberger vessel the Searcher (~10000 cu inch) with much denser shot spacing. The velocity image spanning a ~520 km long profile almost at orthogonal orientation to the trench, is obtained by the inversion of about ~30000 traveltime picks made on the 56 ocean bottom seismometers (OBSs) that were deployed along the profile. The velocity model depicts the rather thin (~4.5-6.0 km) Indo-Australian oceanic plate in the region, subducting underneath the Sunda plate at an angle of about 8-10°. The subducting slab is overlain by thick volume of the evolved accretionary complex for about 150 km, followed with the system of Aceh basin in forearc region and the volcanic arc domain further east. The high velocity structure just east underneath of the Aceh basin appears to be of continental origin and might be a result of the complex tectonic history of the region. On the eastern end of the line the continental crust with thickness of about 12-15 km appears to be thinned by the pull-apart extension activity of the various branches of Sumatra fault. The experiment was conducted in conjunction with the acquisition of coincident seismic reflection lines by the Western-Geco/Schlumberger's "Searcher" seismic vessel. Future work aims at obtaining a depth migrated subsurface image of the coincident seismic reflection profile WG2 using this velocity model, to further constrain the geometry and structure of this highly active subduction zone.

Chauhan, A. P.; Singh, S. C.; Carton, H.; Hananto, N. D.; Klingelhoefer, F.; Dessa, J.; Permana, H.; Graindorge, D.; Dean, S.; White, N.; Chaubey, A.; Shankar, U.; Galih, D.; Royle, G.; Aryawan, K.; Laesanpura, A.; Prihantono, J.

2007-12-01

164

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

NASA Astrophysics Data System (ADS)

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

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

2009-04-01

165

Deformation of the central Andes (15-27 deg S) derived from a flow model of subduction zones  

NASA Technical Reports Server (NTRS)

A simple viscous flow model of a subduction zone is used to calculate the deformation within continental lithosphere above a subducting slab. This formulation accounts for two forces that dominate the deformation in the overriding lithosphere: tectonic forces and buoyancy forces. Numerical solutions, obtained by using a finite element technique, are compared with observations from the central Andes (15-27 deg S). The model predicts the observed deformation pattern of extension in the forearc, compression in the Western Monocline (corresponding to magmatic activity), extension in the Altiplano, compression in the Eastern Monocline and Subandes, and no deformation in the Brazilian Shield. By comparing the calculated solutions with the large-scale tectonic observations, the forces that govern the deformation in the central Andes are evaluated. The approximately constant subduction velocity in the past 26 million years suggests that the rate of crustal shortening in the Andes has decreased with time due to the thickening of the crust.

Wdowinski, Shimon; O'Connell, Richard J.

1991-01-01

166

Talc and Chlorite 'Hybrid' Rocks in Subduction Melanges; Their Role in Fluid and Element Recycling Through Subduction Zones  

NASA Astrophysics Data System (ADS)

The fluxing of H2O, CO2 and trace elements through subduction zones is one of the most important geochemical cycles on Earth. Volatile components released during progressive metamorphism of subducting oceanic crust are thought to cause melting of the overlying mantle wedge, producing arc magmas. The petrological evolution and devolatilisation of the principle components of subducting slabs (MORB, sediments, serpentinite) are now well constrained. However, volatile and trace element flux through subduction zones remains highly contentious. The eclogite-facies rocks of northern New Caledonia represents oceanic crust that was previously subducted to depths of ~60 km. Within the terrane are high-pressure melange zones consisting of a range of mafic, metasedimentary and ultramafic lithologies that have been strongly sheared and intermixed. These melange zones have been observed on sub-metre to kilometre scales and comprise a significant proportion of the terrane. The melanges contain hybrid rock-types, such as carbonate-rich talc and chlorite schists, that are not equivalents of any typical protolith found at the Earth's surface. Using field relations, bulk-rock geochemistry and oxygen isotopes, we suggest that these hybrid rocks formed during subduction by metasomatism and mixing of serpentinites, mafic rocks and metasediments. Subducted mafic rocks and serpentinites may deliver H2O to sub-arc depths, but fluid release occurs at temperatures too low (>650 °C) to dissolve significant amounts of the slab-sourced elements that are found in arc magmas. Hybrid rock-types may comprise a significant proportion of subducting slabs and have the potential to transport large amounts of volatiles to sub-arc depths. Talc and chlorite-rich hybrid rocks will undergo almost complete dehydration at relatively high temperatures (~ 800 °C) allowing for elevated element solubility in fluids or partial melting of adjacent pelitic and mafic rocks. By contrast, carbonates in these rocks are stable to very great depths and may be important for recycling C into the deep mantle. Therefore, hybrid rock-types in subducting slabs may be critical for element and fluid recycling through subduction zones and the evolution of arc magmas.

Arculus, R. J.; Spandler, C.; Hermann, J.; Mavrogenes, J.

2005-05-01

167

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

168

Temporal variation of large intraplate earthquakes in coupled subduction zones  

NASA Astrophysics Data System (ADS)

The focal mechanisms of intraplate earthquakes within subducting lithosphere are frequently used to infer large-scale stress regimes induced by slab-pull, bending or unbending, and lateral segmentation and undulations of the slab. Numerous studies have further postulated that the intraplate activity is influenced by transitory regional stress regimes such as those associated with interplate thrust events. Temporal variations of the latter type may potentially play an important role in assessing regions of uncertain seismic potential, and possibly even in earthquake forecasting. A systematic analysis of 1130 focal mechanisms for intraplate earthquakes with mb ? 5.0 in the depth range 0-300 km is conducted for nine circum-Pacific subduction zones, all of which are known to have large interplate thrust events. The spatial and temporal relationships of the earthquakes within the subducting slab to the large thrust events in each region are appraised. The earthquake catalog assembled contains all published focal mechanisms, and is probably complete for mb ? 6.5 for the years 1963-1986. For many of the localized regions considered in detail the catalog is complete to lower thresholds of mb ? 6.0 or mb ? 5.5. This analysis provides compelling evidence for a temporal link between large interplate thrust activity and intraplate seismicity. For the seismically coupled regions considered here, outer rise compressional events have occurred prior to several large thrust events or are associated with seismic gaps, while outer rise tensional events generally only follow interplate ruptures. In the intermediate depth range, large down-dip tensional events generally precede interplate thrusts, and are often concentrated at the down-dip edge of the coupled zone. A transition to down-dip compressional stress or diminished tensional activity at intermediate depth is observed after several large thrust events (e.g., 1960 Chile, 1974 Peru, 1957 Aleutian, 1971 New Britain). These examples support the notion that the intraplate stress environment responds viscoelastically to the temporally varying interplate stress regime. Assuming that this concept is correct, the seismic potential of several seismic gaps is considered on the basis of both outer rise and intermediate depth earthquake activity.

Lay, Thorne; Astiz, Luciana; Kanamori, Hiroo; Christensen, Douglas H.

1989-04-01

169

Mélange Formation, Mantle-Wedge Diapirs And Subduction Zone Magmatism  

NASA Astrophysics Data System (ADS)

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

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

2012-12-01

170

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

NASA Astrophysics Data System (ADS)

Metamorphic basement rocks in the southern part of the Western Hindu Kush at contact with the Kabul and Helmand crustal blocks were investigated to elucidate pressure-temperature variation and relative time relations among different metamorphic rocks. The rocks are represented by Proterozoic amphibolite facies para-/orthogneisses and migmatites with low-grade Paleozoic volcano-sedimentary sequences. Major- and trace-element geochemistry from two orthogneiss bodies and geochronological data, including new SHRIMP analyses on zircon from one of these bodies shows that they are derived from granitic rocks that related to two different magmatic arcs of Triassic and Cretaceous ages. The Triassic granites are common in the Western Hindu Kush where they intrude basement units; the Cretaceous granitic belt crosses the Afghan Central blocks south of the Hindu Kush Mountains. Three different metamorphic events have been distinguished in the southern part of Western Hindu Kush. Based on an unconformity between basement units and Carboniferous cover sequences, the first two amphibolite and greenschist facies metamorphic events are Proterozoic and Pre-Carboniferous in age respectively. The third metamorphism was recognized in Triassic and Cretaceous granitic rocks near to contact with the Kabul Block. It is of Eocene age and reached medium pressure amphibolite facies conditions. This event is genetically linked to the collision of India and Eurasia which produced a series of trans-Afghan Central block magmatic arcs and crustal scale deformation.

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

2013-08-01

171

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

NASA Astrophysics Data System (ADS)

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

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

2011-12-01

172

Three-dimensional dislocation model for great earthquakes of the Cascadia Subduction Zone  

NASA Astrophysics Data System (ADS)

There have been no historical Cascadia great subduction thrust earthquakes, but there is good recent evidence that very large earthquakes have occurred in the past and that strain is building up toward a future great event. Geodetic measurements in the coastal region from northern California to southern British Columbia show vertical and horizontal deformation as expected for the strain accumulation of a locked thrust fault. The segment of the subduction thrust that is locked and may rupture in future great events has previously been estimated through two-dimensional (2-D) elastic dislocation modeling of interseismic deformation geodetic data. In this study, a general 3-D dislocation model for thrust faults has been developed that accommodates curved fault geometry and nonuniform interseismic locking or coseismic rupture. The model is based on die surface deformation due to shear faulting in an elastic half space. The 3-D model of the Cascadia subduction zone calculates the surface deformation for a locked zone or a rupture zone of variable width along the margin. The bend in the margin trend and subducting slab end effects are included. There is a downdip transition zone between interseismic completely locked and free slip portions of the fault or between coseismic full rupture and no displacement. An initial 3-D model based upon 2-D dislocation models and upon thermal constraints was adjusted to optimize the fit of the predicted interseismic surface deformation to current deformation geodetic data. The best fit model has the thrust locked along the whole margin with an average locked zone width of 60 km and a transition zone width of 60 km. The two zones lie mainly offshore beneath the continental shelf and slope. The locked and transition zone widths vary smoothly along the margin, being greater off northern Washington where the thrust dip is shallow and narrower off central Oregon. Assuming that the locked plus transition zones approximate the maximum coseismic rupture area, these widths permit aMw=9 earthquake.

Flück, P.; Hyndman, R. D.; Wang, K.

1997-01-01

173

Dehydrated fluid and seismic deformation in deep subduction zone  

NASA Astrophysics Data System (ADS)

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

Okamoto, K.

2013-12-01

174

Thermobarometric and fluid expulsion history of subduction zones  

SciTech Connect

Phanerozoic, unmetamorphosed, weathered, and altered lithotectonic complexes subjected to subduction exhibit the prograde metamorphic facies sequence: zeolite {r arrow} prehnite-pumpellyite {r arrow} glaucophane schist {r arrow} eclogite. Parageneses reflect relatively high-P trajectories, accompanied by semicontinuous devolatilization. The thermal evolution of convergent plate junctions results in early production of high-rank blueschists, high-P amphibolites, and eclogites at depth. Inclusion studies suggest that two-phase immiscible volatiles are evolved in turn during progressive metamorphism of the subducted sections. Expulsion of pore fluids and transitions from weathered and altered supracrustal rocks to zeolite facies assemblages release far more fluid than the better understood higher-grade transformations. Many blueschist parageneses (e.g., Western Alps) have been partially overprinted by later greenschist and/or epidote-amphibolite facies assemblages. Less common blueschist terranes (e.g., Franciscan belt of western California) preserve metamorphic aragonite and other high-P minerals, and lack a low-pressure overprint; physical conditions during retrogression approximately retraced the prograde path or, for early formed high-grade blocks, reflect somewhat higher pressures and lower temperatures. The ease with which volatiles are expelled from a subduction complex and migrate upward along the plate junction zone is roughly proportional to the sandstone/shale ratio: low-permeability mudstones tend to maintain P{sub fluid} values approaching lithostatic, lose strength, and deform chaotically (forming melange belts), whereas permeable sandstone-rich sections retain structural/stratigraphic coherence and fail brittlely (forming coherent terranes).

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

1990-06-10

175

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

176

Uncertainty in turbidite correlations along the Cascadia Subduction Zone  

NASA Astrophysics Data System (ADS)

An influential reconstruction of Cascadia earthquakes builds on tenous evidence for synchronous shaking. The synchroneity has been inferred by correlating deep-sea turbidites with one another (http://pubs.usgs.gov/pp/pp1661f/). The correlated deposits record turbidity currents that likely evolved from other kinds of subaqueous mass movements. The stratigraphic correlations reportedly distinguish between long ruptures and series of shorter ones; the greater the extent of turbidite correlation along the plate boundary, the longer the inferred fault rupture. The correlations have been interpreted as exact enough to show whether the initial mass movements began during the same few minutes in areas hundreds of kilometers apart. A recent review of these findings focuses on deep-sea channels that head offshore Washington (http://pubs.usgs.gov/of/2012/1043/). Two of the points reviewed: (1) Sensitivity of Cascadia Channel turbidites to the extent of fault rupture beneath its tributary canyons. The Juan de Fuca tributaries head mainly in submarine canyons offshore northern Washington, while the combined Quinault, Grays, and Willapa tributaries head offshore southern Washington. It has been assumed that flows from north and south, if initiated at the same time, were both large enough to merge at the head of Cascadia Channel and to continue together for hundreds of kilometers downstream. This so-called confluence test is often cited as evidence that 13 fault ruptures of the past 7,500-7,800 years extended along the full length of the Washington coast. But the test is confounded by uncertainties in proximal turbidite counts, flow paths, and relative sizes of flows. (2) Ability of individual sandy layers within a turbidite to mimic the series of strong-motion pulses during an earthquake. According to the reconstruction: (a) A series of ground-motion pulses during an earthquake produces a series of sediment pulses in a turbidity current. (b) The sediment pulses yield a sequence of sandy layers. (c) The sandy sequence correlates among core sites hundreds of kilometers apart along the length of the subduction zone. (d) This stratigraphic similarity, evidenced mainly by logs of density and magnetic susceptibility, enables a full-length rupture of magnitude 9 to be distinguished geologically from a series of shorter ruptures. Open questions include: Do the density and magnetic signatures of a sandy sequence have enough complexity in shape and reproducibility among adjacent cores to justify long-distance correlation of individual sandy layers? Do the initial mass movements respond less to individual pulses than to cumulative shaking, and do they commonly begin or continue after the mainshock has finished? Are the pulses of shaking likely to vary along strike, as in strong-motion records from the 2010 Maule and 2011 Tohoku earthquakes? An unknown fraction of the so-called full-length ruptures represents series of shorter ruptures, and solitary short ruptures may sometimes break the plate boundary offshore southern British Columbia and northern Washington.

Atwater, B. F.

2012-12-01

177

Mantle convection and crustal tectonics in the Tethyan subduction zone  

NASA Astrophysics Data System (ADS)

Mantle convection drives plate tectonics and the size, number and thermotectonic age of plates codetermines the convection pattern. However, the degree of coupling of surface deformation and mantle flow is unclear. The use of SKS waves seismic anisotropy shows a coherence of mantle and surface deformation, but significant examples depart from this scenario. We review geological observations and present kinematic reconstructions of the Aegean and Middle East and 3D numerical models to discuss the role of asthenospheric flow in crustal deformation. At the scale of the Mediterranean backarcs, lithosphere-mantle coupling is effective below the most extended regions as shown by the alignment of SKS fast orientations and stretching directions in MCCs. In the Aegean, the directions of mantle flow, crustal stretching and GPS velocities are almost parallel, while, below the main part of the Anatolian plate, SKS fast orientations are oblique to GPS velocities. When considering the long-term geological history of the Tethyan convergent, one can conclude that asthenospheric flow has been an important player. The case of Himalaya and Tibet strongly supports a major contribution of a northward asthenospheric push, with no persistent slab that could drive India after collision, large thrust planes being then decoupling zones between deep convection and surface tectonics. The African plate repeatedly fragmented during its migration, with rifting of large pieces of continents that had then been moving northward faster than Africa (Apulia, Arabia). This also suggests a dominant role of an underlying flow at large scale, dragging and mechanically eroding plates and breaking them into fragments, then passively carried. Mantle flow thus seems to be able to carry plates toward subduction zones, break-away pieces of plates, and deform backarc upper crust where the lithosphere is the thinnest. Most numerical models of lithospheric deformation are designed such that strain is a consequence of kinematic boundary conditions (push or pull on lateral sides), and rarely account for basal stresses due to mantle flow. On the other hand, convection models often treat the lithosphere as a single-layer stagnant lid with vertically undeformable surface. There is thus a gap between convection models and lithospheric-scale geodynamic models. We test different degrees of coupling using 3-D lithospheric deformation models. Preliminary results suggest that lithosphere can be carried by asthenospheric flow, which may lead to plate fragmentation, especially if this flow is applied on a large surface and involves mantle upwelling. However, the presence of a ductile lower crust inhibits the upward transmission of stresses. A highly extended crust in a hot environment such as a backarc domain, with no lithospheric mantle and a ductile lower crust in direct contact with asthenosphere, is more prone to follow the mantle flow than a thick and stratified lithosphere.

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

2013-12-01

178

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

USGS Publications Warehouse

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

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

1993-01-01

179

Dependence of earthquake size distributions on convergence rates at subduction zones  

SciTech Connect

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

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

1994-10-01

180

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

NASA Astrophysics Data System (ADS)

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

Ujiie, K.; Yamaguchi, H.

2004-12-01

181

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

NASA Astrophysics Data System (ADS)

I propose a hypothesis that might be used to differentiate zones that produce Mw ? 9 earthquakes from zones that do not. I calculate stress drop (??) values, for Mw ? 7 thrust-type earthquakes over worldwide subduction zones, compiling the studies that obtained well-constrained slip distributions by inverting seismic, geodetic or tsunami data. Earthquakes are grouped into class 1: Mw ? 9 earthquakes, class 2: Mw < 9 earthquakes in a subduction zone segment in which at least one Mw ? 9 earthquake has occurred, and class 3: earthquakes in a subduction zone segment in which no Mw ? 9 earthquake has occurred. A total of 53 earthquakes are analyzed. The average stress drop (??bar) values of the class 1, 2, and 3 events are 4.6, 3.4 and 1.6 MPa, respectively. In individual subduction zones, ??bar values of the class 2 events are by more than twice greater than those of the class 3 events, except Kuril-Hokkaido. Based on these results, I propose a hypothesis that if ??bar is greater than 3 MPa in a subduction zone segment, this segment possibly produces Mw ? 9 earthquakes, and if ??bar is less than 2 MPa, the segment would not produce Mw ? 9 earthquakes. I examine the fault parameters obtained in this study based on the newly derived scaling relations that take into account the variation in ??. The rupture of subduction zone earthquakes, from Mw ? 9 through Mw ~ 7, can be understood on the basis of the same scale-invariant physics, if Mo is normalized by ??1.5 in the scaling relation between L and Mo. Using this relation, I estimate the maximum magnitude of an earthquake which may rupture the entire Nankai-Suruga Trough off SW Japan, and obtain Mw = 8.6-8.4.

Seno, Tetsuzo

2014-05-01

182

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

NASA Astrophysics Data System (ADS)

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

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

1990-03-01

183

Depth Distribution of The Subduction Zone Earthquakes and Devolatilization Phase Equilibria of Subducting Slab  

NASA Astrophysics Data System (ADS)

In the present study, we show a close link between the depth distribution of subduction-zone earthquakes and that of dehydration reactions in hydrated slab peridotite. The depth distribution of world subduction-zone earthquakes is known to show bimodal distribution. Frequency of mantle earthquakes is high in shallow to intermediate-depth (0-300 km) and in mantle transition zone (450-650 km). A depth range from 300 to 450 km has less number of seismicity. However, the depth distribution of hypocenters in individual subduction zone varies from area to area. We classified the world subduction-zones into three types on the basis of the mode of distribution of hypocenters with depth; type 1 is for the subduction zones having only shallow to intermediate-depth earthquakes (e.g. SW-Japan, Alaska, Kyushu); type 2 is for the subduction zones showing clear bimodal depth-distribution (e.g. northern Chile, Sunda, a part of Izu-Mariana); and, type 3 is for the subduction zones having deep earthquakes, but without obvious bimodal depth-distribution (e.g._@Tonga, NE-Japan, a part of Izu-Mariana). We constructed a phase diagram up to 30 GPa in the MgO-CaO-Al2O3-SiO2-CO2-H2O system by combination of thermodynamic calculation and the Schreinemakers analysis on the previous experimental data. In the intermediate-depth (<250 km), devolatilizations including antigorite, talc, brucite, clinochlore, and Mg-sursassite occur. In the range of 250-400 km depth, only dehydration of Mg-sursassite is possible. And dehydrations of dense hydrous Mg-silicates (phase E and superhydrous phase B) occur in the mantle transition zone. Such depth dependences of dehydration reactions are consistent to the depth distribution of earthquakes. The variations of individual seismic zones are explained by the difference of thermal structure depending on the age of subducting plate, subducting speed and angle. In previous studies, it was shown that the dehydration hypothesis explains the mesoscopic structure of intermediate-depth seismic zone, i.e. double seismic zone and organized interplane earthquakes (Omori et al. 2000, 2002). Therefore, the dehydration hypothesis should be a unique model that explains the earthquakes from intermediate-depth to mantle transition zone. And we may see the site of devolatilizations and temperature in subduction zone by the hypocenter distribution.

Omori, S.; Komabayashi, T.; Maruyama, S.

2002-12-01

184

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

185

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

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

186

Metamorphic P–T–t evolution of mafic HP granulites in the northeastern segment of the Tarim Craton (Dunhuang block): Evidence for early Paleozoic continental subduction  

NASA Astrophysics Data System (ADS)

The Dunhuang block, which constitutes the northeastern segment of the Tarim Craton, is located between the Central Asian Orogenic Belt (CAOB) to the north and the Altyn–Qilian Orogenic Belt to the south. The present study reveals that the early Paleozoic HP mafic granulites from the Dunhuang block underwent four stages of metamorphism: prograde amphibolite-facies (M1), peak high-pressure (HP) granulite-facies (M2) and two late stages of amphibolite-facies retrograde metamorphism. Based on phase equilibrium modeling, P–T conditions of the four stages of metamorphism are estimated at 720–750 °C and 11–13 kbar, 760–800 °C and 14–16 kbar, 690–720 °C and 8–8.5 kbar, and at < 6 kbar and < 640 °C respectively. Peak granulite-facies metamorphism is characterized by a low geothermal gradient of ca. 16 °C/km. Zircon U–Pb dating shows that the HP granulite-facies metamorphism occurred at ca. 431 Ma and the early retrograde amphibolite-facies overprint at ca. 403 Ma. Thus the investigated mafic rocks here reveal a clockwise P–T–t path involving burial heating before peak granulite-facies metamorphism and subsequent decompression–cooling with an uplift rate of ca. 0.8 km/Ma. This, together with a continental affinity of the HP metamorphic rocks, indicates that the Dunhuang block experienced a collisional orogenesis during the early Paleozoic.

He, Zhenyu; Zhang, Zeming; Zong, Keqing; Xiang, Hua; Klemd, R.

2014-05-01

187

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

Microsoft Academic Search

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

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

2003-01-01

188

Finite Element Modeling of Subduction Zone Stresses in the Wake of Bathymetric Feature Subduction Offshore Costa Rica  

Microsoft Academic Search

There are several cases of interaction between subducting bathymetric features and large subduction zone earthquakes. For large earthquakes in the Nankai subduction zone, it appears that subducted features have acted as barriers to rupture along the plate boundary. In other regions, such as Costa Rica and Java, subducted seamounts appear to be in regions of high slip during large plate

S. L. Bilek; C. Lithgow-Bertelloni

2003-01-01

189

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

190

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.

191

Unlocking the Secrets of the Mantle Wedge: New Insights Into Melt Generation Processes in Subduction Zones  

Microsoft Academic Search

Recent laboratory studies of the melting and crystallization behavior of mantle peridotite and subduction zone lavas have led to new insights into melting processes in island arc settings. Melting of the mantle wedge in the presence of H2O begins at much lower temperatures than previously thought. The solidus of mantle peridotite at 3 GPa is ~ 800 °C, which is

T. L. Grove

2007-01-01

192

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

193

Temporal and Spatial Correlations Between Interplate and Intraplate Subduction Zone Seismicity  

Microsoft Academic Search

Subduction zone earthquakes on the interface between subducting and overriding plate represent some of the world's most destructive natural disasters. Updip from this interface, the outer rise comprises an upwarping of the oceanic lithosphere just before it descends into the trench. Previous work established the characteristics of the stress regime within the subducting lithosphere and has suggested that a temporal

J. Polet

2004-01-01

194

TEMSPOL: a MATLAB thermal model for deep subduction zones including major phase transformations  

Microsoft Academic Search

TEMSPOL is an open MATLAB code suitable for calculating temperature and lateral anomaly of density distributions in deep subduction zones, taking into account the olivine to spinel phase transformation in a self-consistent manner. The code solves, by means of a finite difference scheme, the heat transfer equation including adiabatic heating, radioactive heat generation, latent heat associated with phase changes and

A. M. Negredo; J. L. Valera; E. Carminati

2004-01-01

195

Detection of Slow Slip Events Along the Cascadia Subduction Zone Using Plate Boundary Observatory Borehole Strainmeters  

Microsoft Academic Search

In the spring of 2005, the Plate Boundary Observatory (PBO) began installing borehole strainmeters in the Pacific Northwest. Currently there are 18 borehole strainmeters operating from Vancouver Island, Canada to Southern Oregon that are favorably located to detect slow slip along the Cascadia subduction zone. While the longest (> two weeks) subduction tremor episodes are accompanied by slow slip events,

W. McCausland; E. Roeloffs

2007-01-01

196

Seismic velocity and attenuation in Izu-Bonin subduction zone inferred from BBOBS data  

Microsoft Academic Search

Seismic velocity and attenuation in the upper mantle and the subduction slab surrounding Izu-Bonin subduction zone is measured by using the waveform data recorded by Broad Band Ocean Bottom Seismometer (=BBOBS). As a part of Stagnant Slab Project, we deployed 12 BBOBS from October 2005 to October 2006 in and around the Philippine Sea. The sites were equipped with three

A. Shito; H. Shiobara; H. Sugioka; A. Ito; H. Kawakatsu; C. Adam; T. Kanazawa

2007-01-01

197

Imaging the Western Pacific Subduction Zones Using High-resolution Radon Transforms  

Microsoft Academic Search

Underside reflections (SS, PP precursors) have been widely used in imaging seismic discontinuity structures since the early 1990s. While these waves provide a outstanding global coverage, some regions remain under- sampled, for example, much of the Western Pacific subduction zones and South America. Furthermore, the so called \\

Y. J. Gu; R. Schultz; Y. An

2007-01-01

198

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

199

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

200

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

201

Dislocation model of strain accumulation and release at a subduction zone  

Microsoft Academic Search

Strain accumulation and release at a subduction zone are attributed to stick slip on the main thrust zone and steady aseismic slip on the remainder of the plate interface. This process can be described as a superposition of steady state subduction and a repetitive cycle of slip on the main thrust zone, consisting of steady normal slip at the plate

J. C. Savage

1983-01-01

202

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

203

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

204

Behaviour of high field strength elements in subduction zones: constraints from Kamchatka–Aleutian arc lavas  

Microsoft Academic Search

Models explaining the characteristic depletion of High Field Strength Elements (HFSE) relative to elements of similar compatibility in subduction zone magmas invoke either (1) the presence of HFSE-rich minerals in the subduction regime or (2) a selectively lower mobility of HFSE during subduction metasomatism of the mantle. In order to investigate the properties of HFSE in subduction regimes closer, we

Carsten Münker; Gerhard Wörner; Gene Yogodzinski; Tatiana Churikova

2004-01-01

205

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

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

206

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

207

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

USGS Publications Warehouse

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

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

2012-01-01

208

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

209

The formation of mantle phlogopite in subduction zone hybridization  

NASA Astrophysics Data System (ADS)

Extrapolation and extension of phase equilibria in the model system KAlSiO4-Mg2SiO4-SiO2-H2O suggests that at depths greater than 100 km (deeper than amphibole stability), hybridism between cool hydrous siliceous magma, rising from subducted oceanic crust, and the hotter overlying mantle peridotite produces a series of discrete masses composed largely of phlogopite, orthopyroxene, and clinopyroxene (enriched in Jadeite). Quartz (or coesite) may occur with phlogopite in the lowest part of the masses. The heterogeneous layer thus produced above the subducted oceanic crust provides: (1) aqueous fluids expelled during hybridization and solidification, which rise to generate in overlying mantle (given suitable thermal structure) H2O-undersaturated basic magma, which is the parent of the calc-alkalic rock series erupted at the volcanic front; (2) masses of phlogopite-pyroxenites which melt when they cross a deeper, high-temperature solidus, yielding the parents of alkalic magmas erupted behind the volcanic front; and (3) blocks of phlogopite-pyroxenites which may rise diapirically for long-term residence in continental lithosphere, and later contribute to the potassium (and geochemically-related elements) involved in some of the continental magmatism with geochemistry ascribed to mantle metasomatism.

Wyllie, Peter J.; Sekine, Toshimori

1982-12-01

210

The influence of subduction zone thermal structure on arc magma chemistry: B and fluid-mobile elements  

NASA Astrophysics Data System (ADS)

It is postulated that contents of B and other highly fluid-mobile elements (HFMs) in primitive arc lavas are sensitive to thermal structures of subducting slabs (sensu lato), which are the principal sources for these elements in subduction zones. Initial slab HFM inventories may be variably depleted in proportion to progressive fluid release as a consequence of gradual warming and metamorphism of descending slabs. Also, some variation in the rate of slab heating is expected from arc to arc due to differences in subduction rate, slab age, Benioff Zone (BZ) geometry, etc. Thus, a greater fraction of the initial slab HFM inventory will be available to modify subarc mantle domains in relatively cool subduction zones, and vice versa. Nearly 100-fold variation in B-enrichment (e.g., as monitored by B/Zr or similar ratios normalized to constant B content) is observed for volcanic front (VF) basalts from arcs worldwide. B-enrichment is highly correlated with the above measurable subduction parameters and with other temperature-sensitive parameters such as `slab length' (= down-dip extent of BZ seismicity). B-enrichment is also well correlated with slab-surface temperatures (SSTs) below the VF, as determined from numerical models (Huang et al., this volume). These relations suggest that slab HFM fluxes are strongly influenced by slab thermal history, and probably controlled by stability of HFM host minerals as well as the availability of fluid transport media. Different behavior is expected for various elements depending on their fluid-solubility and/or P-T stability of relevant host phases - in which case geochemical fractionations are readily feasible and may explain some of the geochemical variability among arc magma suites. Moreover, B/Zr and similar ratios potentially can be used to infer aspects of slab thermal structure. However, care must be taken that the ratios are representative of the most primitive mafic magmas and not modified by shallow crust-level processes. For example, interaction with lower crust rocks can lower normalized B/Zr values for relatively evolved lavas.

Leeman, W. P.

2001-12-01

211

Seismic studies of the southern Cascadia subduction zone near the Mendocino triple junction  

NASA Astrophysics Data System (ADS)

Offshore northern California lies the southern Cascadia subduction zone, where oblique subduction of the Gorda plate interacts with Mendocino triple junction tectonics. Seismic images of the Gorda plate, Cascadia accretionary prism, and Eel River forearc basin record the tectonic evolution of the region. These images allow examination of the northward migrating triple junction's influence on internal deformation of the Gorda plate, accretionary processes within the prism, and tectonic history of the Eel River forearc basin. Examination of the accretionary prism shows that 90% of incoming Gorda basin sediments currently are being accreted to the margin while only 10% are being subducted. The outer forearc is experiencing little effect by the triple junction. Tectonic history of the Eel River forearc basin includes: uplift of the outer margin at ˜3 Ma, widespread outer basin erosion at ˜1 Ma, local erosion of the continental shelf at ˜500 ka, and collapse of the Klamath plateau region by ˜250 ka. The northward migrating triple junction affected the Eel River basin by ˜500 ka causing uplift, northward tilting, and erosion south of the basin. In addition, broad uplift of the southern basin forced Eel River sediment to be deposited both further north and to Dow out the Eel Canyon. Northward compression by the triple junction has deformed the southern Eel River basin to the northeast forming northwest-southeast oriented transpressional structures. The southern Gorda plate is internally deformed along northeast-southwest oriented left-lateral strike-slip faults, rotating clockwise, and fragmenting near the triple junction. Flow of the Gorda lithosphere around the rigid Pacific plate results in delamination of the Gorda crust allowing for fragments of Gorda crust to enter the slab-window and accrete to the base of the North American plate. The propagation of Gorda plate northeast-southwest deformation beneath the margin contrasts with the northwest-southeast deformation in the southern forearc requiring a decoupled Gorda-North American plate boundary, which is further demonstrated by a mixed vergent deformation front and Mohr-Coulomb analysis of the basal and topographic taper of the wedge. The decoupled Gorda-North American plate boundary has allowed for propagation of Pacific-North American related strike-slip faulting north of Cape Mendocino.

Gulick, Sean Paul Sandifer

2000-10-01

212

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

213

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

NASA Astrophysics Data System (ADS)

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

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

2007-12-01

214

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

NASA Astrophysics Data System (ADS)

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

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

2013-04-01

215

Areas of slip of recent earthquakes in the Mexican subduction zone  

NASA Astrophysics Data System (ADS)

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

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

2012-12-01

216

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

NASA Astrophysics Data System (ADS)

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

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

2006-05-01

217

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

NASA Astrophysics Data System (ADS)

Subduction zones are one of the most important components of the Earth's plate tectonic system. Knowing the detailed seismic velocity structure within and around subducting slabs is vital to understand the constitution of the slab, the cause of intermediate depth earthquakes inside the slab, the fluid distribution and recycling, and tremor occurrence [Hacker et al., 2001; Obara, 2002].Thanks to the ability of double-difference tomography [Zhang and Thurber, 2003] to resolve the fine-scale structure near the source region and the favorable seismicity distribution inside many subducting slabs, it is now possible to characterize the fine details of the velocity structure and earthquake locations inside the slab, as shown in the study of the Japan subduction zone [Zhang et al., 2004]. We further develop the double-difference tomography method in two aspects: the first improvement is to use an adaptive inversion mesh rather than a regular inversion grid and the second improvement is to determine a reliable Vp/Vs structure using various strategies rather than directly from Vp and Vs [see our abstract ``Strategies to solve for a better Vp/Vs model using P and S arrival time'' at Session T29]. The adaptive mesh seismic tomography method is based on tetrahedral diagrams and can automatically adjust the inversion mesh according to the ray distribution so that the inversion mesh nodes are denser where there are more rays and vice versa [Zhang and Thurber, 2005]. As a result, the number of inversion mesh nodes is greatly reduced compared to a regular inversion grid with comparable spatial resolution, and the tomographic system is more stable and better conditioned. This improvement is quite valuable for characterizing the fine structure of the subduction zone considering the highly uneven distribution of earthquakes within and around the subducting slab. The second improvement, to determine a reliable Vp/Vs model, lies in jointly inverting Vp, Vs, and Vp/Vs using P, S, and S-P times in a manner similar to double-difference tomography. Obtaining a reliable Vp/Vs model of the subduction zone is more helpful for understanding its mechanical and petrologic properties. Our applications of the original version of double-difference tomography to several subduction zones beneath northern Honshu, Japan, the Wellington region, New Zealand, and Alaska, United States, have shown evident velocity variations within and around the subducting slab, which likely is evidence of dehydration reactions of various hydrous minerals that are hypothesized to be responsible for intermediate depth earthquakes. We will show the new velocity models for these subduction zones by applying our advanced tomographic methods.

Zhang, H.; Thurber, C. H.

2005-12-01

218

Middle Cambrian to Late Ordovician evolution of the Appalachian margin: Foundering of a passive margin to form a subduction zone and volcanic arc  

SciTech Connect

From late Middle Cambrian to early Late Ordovician time, the Appalachian passive margin experienced a series of orogenic events culminating in the Taconic orogeny. Most of these events are generally viewed as enigmatic and isolated, but they can be viewed as a coherent tectonic sequence of events. The early stages involved broad uplifts and localized extension, especially of internal shelf and adjacent continental interiors. Later stages involved increased subsidence rates of the outer shelf, resulting in retreat of the outer margin of the carbonate platform.The beginning of volcanic activity coincides with, or immediately follows, the rapid subsidence. Onset of compressional orogenesis is often temporally separated from the initial rapid subsidence. These events can be integrated into a tectonic model in which the passive margin is converted into an active Andean margin. Early uplift and extension events represented the surface expression of the beginning of deep-seated downward mantle convection. Subsequent rapid subsidence events represented the mechanical failure of the lithosphere as the convection reaches maturity. Failure of the lithosphere resulted in a subduction zone that quickly created arc volcanism. The compressive Taconic orogenesis occurred when the arc was thrust back onto the shelf margin as the subduction zone migrated continentward in response to progressively channeled convective flow.

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

1994-03-01

219

The Hellenic Subduction System: High-Pressure Metamorphism, Exhumation, Normal Faulting, and Large-Scale Extension  

NASA Astrophysics Data System (ADS)

The Cenozoic history of the retreating Hellenic subduction system in the eastern Mediterranean involves subduction, accretion, arc magmatism, exhumation, normal faulting, and large-scale continental extension from ˜60 Mya until the Recent. Ages for high-pressure metamorphism in the central Aegean Sea region range from ˜53 Ma in the north (the Cyclades islands) to ˜25-20 Ma in the south (Crete). Younging of high-pressure metamorphism reflects the southward retreat of the Hellenic subduction zone. The shape of pressure-temperature-time paths of high-pressure rocks is remarkably similar across all tectonic units, suggesting a steady-state thermal profile of the subduction system and persistence of deformation and exhumation styles. The high-pressure metamorphic events were caused by the underthrusting of fragments of continental crust that were superimposed on slab retreat. Most of the exhumation of high-pressure units occurred in extrusion wedges during ongoing lithospheric convergence. At 23-19 Mya large-scale lithospheric extension commenced, causing metamorphic core complexes and the opening of the Aegean Sea basin. This extensional stage caused limited exhumation at the margins of the Aegean Sea but accomplished the major part of the exhumation of high-grade rocks that formed between 21 and 16 Mya in the central Aegean. The age pattern of extensional faults and contoured maps of fission-track cooling ages do not show a simple southward progression. Our review of lithologic, structural, metamorphic, and geochronologic data is consistent with a temporal link between the draping of the subducted slab over the 660-km discontinuity and the large-scale extension causing the opening of the Aegean Sea basin.

Ring, Uwe; Glodny, Johannes; Will, Thomas; Thomson, Stuart

2010-05-01

220

New constraints on subduction zone structure in northern Cascadia  

NASA Astrophysics Data System (ADS)

A detailed passive seismic experiment was carried out across southwestern British Columbia and northwestern Washington to investigate the structure of the subducting Juan de Fuca plate and mantle wedge in Cascadia and its relation to intraslab seismicity. As part of the POLARIS project, 31 three-component broad-band stations were deployed in an approximately linear array spanning southern Vancouver Island, the Gulf and San Juan Islands, Watcom county and the British Columbia lower mainland. P-wave coda from 41 teleseismic events have been employed in formal inversions for fine-scale shear-velocity structure. Our results indicate a structure very similar to that identified across a comparable profile in central Oregon. The continental Moho is evident at the eastern end of the profile near 35-km depth but disappears towards Georgia Strait/Puget Sound. A prominent low-S-velocity zone is clearly evident below southern Vancouver Island dipping eastwards through Georgia Strait/Puget Sound, and coincides with the E-reflection zone originally identified in LITHOPROBE studies. Structure below the E-layer is much less prominent and varies intermittently along the array. Based on the observations and interpretations of similar structures beneath Oregon, Alaska and South America, and its projection to mantle depths, we suggest that the low-velocity E-layer represents the dehydrating oceanic crust of the subducting Juan de Fuca plate. This interpretation is consistent with recent seismicity studies that place shallow Wadati-Benioff events within the oceanic mantle, and implies that the oceanic crust is 6-8 km shallower beneath Vancouver Island than previously assumed. As in Oregon, we interpret the diminished signature of oceanic crust below a depth of 45 km to signal the presence of eclogitization, which in turn supplies water to serpentinize the overlying forearc mantle.

Nicholson, T.; Bostock, M.; Cassidy, J. F.

2005-06-01

221

Aqueous Silicate Polymers: An Alternative to `Supercritical' Fluids as Transport Agents in Subduction Zones  

NASA Astrophysics Data System (ADS)

The chemistry of subduction-zone fluids is complicated by melt-vapor miscibility and the existence of critical end-points in rock-H2O systems. It is commonly assumed that fluids in subduction zones attain properties intermediate in composition between hydrous silicate liquid and H2O, and that such fluids possess enhanced material transport capabilities. However, the relevance of supercritical, intermediate fluids to subduction zones presents four problems. (1) Albite-H2O is typically used as an analogue system, but the favorable position of its critical curve is not representative; critical curves for polymineralic subduction-zone lithologies lie at substantially higher P. (2) Even if albite-H2O is relevant, jadeite may interfere because of its different solubility and the positive clapeyron slope of its solidus, which points to liquid-structure changes that could cause reappearance of the liquid+vapor field. (3) Critical curves are features of very H2O-rich compositions; low-porosity, H2O-poor natural systems will coexist with intermediate fluids only over a narrow PT interval. (4) Intermediate fluids are expected only over short length scales because their migration will likely result in compositional shifts via reaction and mineral precipitation in the mantle wedge. Although supercritical, intermediate fluids are probably relatively unimportant in subduction zones, they reflect a chemical process that may hold the key to understanding high- P mass transfer. Miscibility in melt-vapor systems is a consequence of polymerization of dissolved components, primarily Si ± Al, Na and Ca. This behavior yields, e.g., aqueous Si-Si, Si-Al, Si-Na-Al, and Si-Ca oxide dimers and other multimers of varying stoichiometry (silicate polymers), even in subcritical, dilute, H2O-rich vapor. Silicate polymers in subcritical aqueous solutions have been inferred from high- P mineral-solubility experiments. The abundance of these species at high P shows that the chemistry of aqueous fluids in subduction-zones differs fundamentally from the more familiar ionic solutions of the upper crust. This has important consequences for minor element transport. Measurements of Fe, phosphorous and Ti solubility reveal that dissolved concentrations rise with increased aqueous albite content at fixed P and T, with maximum enhancements exceeding 10X at melt saturation. Subcritical silicate polymerization thus permits transport of low solubility components via their substitution into sites on aqueous multimers constructed of "polymer formers" such as Na, Al, and Si, even in dilute solutions. The partitioning of elements between the bulk fluid, the polymer network, and the rock matrix likely controls the overall compositional evolution of subduction-zone fluids. Because they form over a wider PT and bulk X range, subcritical silicate polymers in dilute solutions are likely responsible for more mass transfer in subduction zones than intermediate, supercritical fluids.

Mannig, C. E.

2005-12-01

222

Crustal deformation at the Nankai subduction zone, southwest Japan, derived from GPS measurements  

Microsoft Academic Search

The interseismic strain accumulation process at the Nankai subduction zone, the plate-boundary region between the Philippine Sea and the Eurasian plates off Southwest Japan, has been revealed by GPS measurements spanning five years. Large strains characterized by a contraction of 2.2~3.4×10-7\\/yr have accumulated in a narrow region extending parallel to the strike of the plate boundary. The contraction axes lie

Takao Tabei; Taku Ozawa; Yuki Date; Kazuro Hirahara; Takehide Nakano

1996-01-01

223

Constraints on upper plate deformation in the Nicaraguan subduction zone from earthquake relocation and directivity analysis  

Microsoft Academic Search

In the Nicaraguan segment of the Central American subduction zone, bookshelf faulting has been proposed as the dominant style of Caribbean plate deformation in response to oblique subduction of the Cocos plate. A key element of this model is left-lateral motion on arc-normal strike-slip faults. On 3 August 2005, a Mw 6.3 earthquake and its extensive foreshock and aftershock sequence

S. W. French; L. M. Warren; K. M. Fischer; G. A. Abers; W. Strauch; J. M. Protti; V. Gonzalez

2010-01-01

224

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

225

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

226

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

NASA Astrophysics Data System (ADS)

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

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

2010-12-01

227

Seismic Anisotropy of Subduction Zone Minerals–Contribution of Hydrous Phases  

Microsoft Academic Search

The seismology is the most effective method to explore the structure of subduction zones to great depth. The distinguishing\\u000a feature of the mantle in the subduction regions is the presence of hydrated phases, which transport water into the Earth's\\u000a interior and release it with dramatic local consequences, triggering earthquakes and melting. The seismological detection\\u000a of these hydrous phases and geo-dynamic

David Mainprice; Benoit Ildefonse

2008-01-01

228

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

NASA Technical Reports Server (NTRS)

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

Lundgren, Paul R.; Giardini, Domenico

1992-01-01

229

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

230

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

231

Pronounced zonation of seismic anisotropy in the Western Hellenic subduction zone and its geodynamic significance  

NASA Astrophysics Data System (ADS)

Many subduction zones exhibit significant retrograde motion of their arc and trench. The observation of fast shear-wave velocities parallel to the trench in such settings has been inferred to represent trench-parallel mantle flow beneath a retreating slab. Here, we investigate this process by measuring seismic anisotropy in the shallow Aegean mantle. We carry out shear-wave splitting analysis on a dense array of seismometers across the Western Hellenic Subduction Zone, and find a pronounced zonation of anisotropy at the scale of the subduction zone. Fast SKS splitting directions subparallel to the trench-retreat direction dominate the region nearest to the trench. Fast splitting directions abruptly transition to trench-parallel above the corner of the mantle wedge, and rotate back to trench-normal over the back-arc. We argue that the trench-normal anisotropy near the trench is explained by entrainment of an asthenospheric layer beneath the shallow-dipping portion of the slab. Toward the volcanic arc this signature is overprinted by trench-parallel anisotropy in the mantle wedge, likely caused by a layer of strained serpentine immediately above the slab. Arcward steepening of the slab and horizontal divergence of mantle flow due to rollback may generate an additional component of sub-slab trench-parallel anisotropy in this region. Poloidal flow above the retreating slab is likely the dominant source of back-arc trench-normal anisotropy. We hypothesize that trench-normal anisotropy associated with significant entrainment of the asthenospheric mantle near the trench may be widespread, but only observable at shallow-dipping subduction zones where stations nearest the trench do not overlie the mantle wedge.

Olive, Jean-Arthur; Pearce, Frederick; Rondenay, Stéphane; Behn, Mark

2014-05-01

232

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

NASA Astrophysics Data System (ADS)

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

Höchner, Andreas; Babeyko, Andrey; Zamora, Natalia

2014-05-01

233

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.

234

B-type olivine fabric in the mantle wedge: Insights from high-resolution non-Newtonian subduction zone models  

Microsoft Academic Search

Several hypotheses have been proposed to explain trench-parallel shear wave splitting in the mantle wedge of subduction zones. These include 3-D flow effects, parallel melt filled cracks, and B-type olivine fabric. We predict the distribution of B-type and other fabrics with high-resolution thermal and stress models of subduction zones. A composite viscous rheology is used that incorporates wet diffusion creep,

Erik A. Kneller; Peter E. van Keken; Shun-ichiro Karato; Jeffrey Park

2005-01-01

235

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

236

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

237

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

NASA Astrophysics Data System (ADS)

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

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

2006-12-01

238

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

NASA Astrophysics Data System (ADS)

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

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

2012-04-01

239

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

NASA Astrophysics Data System (ADS)

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

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

2012-12-01

240

Imaging the Western Pacific Subduction Zones Using High-resolution Radon Transforms  

NASA Astrophysics Data System (ADS)

Underside reflections (SS, PP precursors) have been widely used in imaging seismic discontinuity structures since the early 1990s. While these waves provide a outstanding global coverage, some regions remain under- sampled, for example, much of the Western Pacific subduction zones and South America. Furthermore, the so called "cap centers" of common signal enhancement schemes do not always reflect the true centers of data, nor do they aim to reduce the effect of uneven coverage. We present results from analyzing the largest SS precursor dataset to date (from 1981-2007) along the Western Pacific subduction zones. This data set more than doubles the data used in the global study of Gu et al. (2003) in the same regions. We adopt variable cap sizes (hence variable resolutions) to equalize the data count, and apply a high-resolution Radon Transform method to simultaneously constrain the ray angle and timing of SS precursors. Our study shows strong depressions of the 660-km discontinuity along the subduction zones, and the lateral variations strongly correlate with regional seismic velocities. The lateral extent of the depressions is smaller than previously found, which we attribute to the high resolution of our new approach. The 410-km discontinuity is elevated beneath the northwestern Pacific, and the anti-correlation with the topography of the 660- km discontinuity reflect significant thermal variations. Overall, however, the correlation between the two transition zone discontinuities is poor in along the Western Pacific subduction zones. The presence of water, garnet phase transformation and the limited lateral dimension of subducted slabs could all affect the topography of these discontinuities, particularly the 410 km discontinuity. The thickness of the transition zone, however, appear to be a more robust measure of temperature: it is consistently narrower than the global average across these subduction zones, as would be expected from the olivine phase transitions. In addition to the transition zone discontinuities, our high-resolution Radon images also present clear evidence of mantle reflectors near 250 km and 850-950 km. The occurrence of the latter reflector is more common than previously proposed and could represent a potential global discontinuity. Phase transformation or chemical differentiation could contribute to this well-resolved interface.

Gu, Y. J.; Schultz, R.; An, Y.

2007-12-01

241

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

NASA Astrophysics Data System (ADS)

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

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

2014-06-01

242

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

NASA Astrophysics Data System (ADS)

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

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

2011-12-01

243

Three Dimensional Simulations of Strong Motions for Great Earthquakes on the Cascadia Subduction Zone  

NASA Astrophysics Data System (ADS)

Using a finite-fault rupture model, we ran a finite difference code to simulate a variety of Mw 8 and larger events on the Cascadia subduction zone using a 3D regional velocity model and two different 3D velocity models for the Seattle basin. Our results reveal the magnitude and duration of shaking that should be expected in the built environment for a megathrust event with a rupture length less than the entire length of the subduction zone. In the next step we will consider events that rupture the entire length of the subduction zone, similar in scope to the 1700 event, and compare our results to those considered for the national seismic hazard maps. In order to make predictions on the strength and duration of shaking in Cascadia due to a large megathrust event, we developed a kinematic fault rupture model based on a k-2 decay in final slip spectrum that has a scale-dependent rise time. This produces a ?-2 decay in the radiated displacement spectrum above the corner frequency, which is then modified by rupture directivity. In order to produce a k-2 decay in the final slip spectrum, we modeled the final slip as the sum of asperities with various wave numbers produced by calculating normal modes for a membrane, then shifting the phase of the standing waves to randomize the pattern. In this way, slip naturally decays towards the edges of the rupture without having to use a taper, we can produce a final slip model with any spectrum we choose, and each wave number can be assigned a unique rise time. The slip on each individual asperity initiates in time according to its closest distance to the hypocenter and the rupture velocity. The Cascadia subduction zone off the coast of northwestern United States and southwestern Canada is capable of producing megathrust earthquakes with magnitudes up to Mw 9.0 for margin-wide events and magnitudes greater than Mw 8.0 if only part of the subduction zone ruptures. The average recurrence interval for margin-wide megathrust earthquakes on the Cascadia subduction zone is about 500 years and the last margin-wide event occurred in 1700. The estimate of the recurrence interval for events that rupture only part of the subduction zone is more controversial, but the recurrence time is thought to be somewhere around 300 years. If a large event were to occur today, the cities of Portland, Oregon, Seattle, Washington, and Vancouver, British Columbia and other inhabited areas in the forearc basin and on the coast would be subjected to strong and long duration shaking and coastal areas would likely be subjected to a tsunami. Observations from the Maule, Chile and Tohoku-Oki, Japan earthquakes will be used to guide the specification of rupture time histories for the Cascadia 3D simulations. We are particularly interested in producing realistic ground motions at periods of 2 to 6 s that affect high-rise buildings. The amplitude and duration of these motions are strongly affected by the rise time of slip on the fault, the coherence of rupture propagation, and the presence of deep sedimentary basins such as the Seattle and Tacoma basins.

Delorey, A. A.; Frankel, A. D.; Stephenson, W. J.; Liu, P.

2011-12-01

244

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

245

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

NASA Astrophysics Data System (ADS)

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

Casey, J.; Dewey, J. F.

2013-12-01

246

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

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

247

In-situ formation of Indian Mantle in global subduction zones  

NASA Astrophysics Data System (ADS)

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

Nebel, Oliver; Arculus, Richard; Davies, Rhodri

2014-05-01

248

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

NASA Astrophysics Data System (ADS)

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

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

2014-03-01

249

Three-dimensional mantle circulations and lateral slab deformation in the southern Chilean subduction zone  

NASA Astrophysics Data System (ADS)

along-strike variation in plate properties is a common feature in the subduction zones. The southern Chilean subduction is a notable example where the slab age and length, and the thickness of the overriding plate, vary substantially in the trench-parallel direction. In this study the combined effects of these factors are examined using regional subduction zone models. In the models lateral slab deformation and along-arc pressure gradient in the mantle occur in response to the differential slab retrograde motion due to variations in slab buoyancy and slab strength. The lower portion of the deep segments acts as a slab edge in part, while rollback of the neighboring, shallower segments influences the toroidal motion induced by the adjacent deep segments. These factors together give rise to complex flow patterns and lead to a great extent of trench-parallel components in the mantle wedge and subslab mantle and significant upwelling in the back arc. Mantle circulations are characterized by variable length scales for both toroidal and poloidal motions extending over ~ 1500 km. Such three-dimensional circulations may lead to complicated patterns of seismic anisotropy. The upwelling may result in decompression melting to cause the extensive Patagonian plateau basalts. Our model results indicate that certain segments of the Antarctic slab reach at least 100 km depth. The trajectories of passive tracers show intricate patterns such as helical streamlines and suggest that a recent input from the adjacent subduction zone for slab-derived components of lavas from the Chilean Ridge cannot be ruled out.

Lin, Shu-Chuan

2014-04-01

250

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

251

Triggered Non-Volcanic Tremor in the Hikurangi Subduction Zone, New Zealand  

NASA Astrophysics Data System (ADS)

Recent discoveries of slow-slip events (SSE) and non-volcanic tremor (NVT) have provided insight into the relative coupling of plates at convergent margins and subsequent potential stress drop during subduction thrust earthquakes. The amount of strain accommodated through slow-slip and NVT has bearing on calculations of plate-boundary strain budgets. NVT plays an important role in the continuum of plate-boundary deformation, possibly facilitating stress transfer between aseismic and seismic deformation. Consequently, it plays an important role in earthquake hazard assessment. In most subduction zones where slow slip has been identified, NVT is present and both spatially and temporally related to the slow slip. The Hikurangi subduction zone in New Zealand and the Boso peninsula in Japan are two notable exceptions where SSEs have been identified from analysis of geodetic signals, yet NVT has previously remained undiscovered. The absence of identified NVT has created uncertainty in estimates of strain deficit, as deformation associated with NVT has the potential to inform us about smaller magnitude (or deeper) events which fall below the detection threshold of geodetic methods using continuous GPS data. In this paper, we present the first known case of NVT in New Zealand. Following the M8.8 Chilean earthquake of February 27, 2010, energy in the 2-8 Hz bandwidth was released near the Hikurangi subduction zone, New Zealand. The energy is roughly in phase with, and is modulated by passing Love waves with particle oscillation parallel to the relative plate motion direction. The energy is not apparent before the onset of the Chilean surface wave coda and does not contain significant higher-frequency signals, as might be expected from dynamic triggering of local seismicity. We use a grid search algorithm and a cross-spectral approach to locate the origin of the tremor. Preliminary locations place the NVT in the southern North Island.

Fry, B.; Chao, K.; Bannister, S. C.; Peng, Z.

2010-12-01

252

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

Microsoft Academic Search

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 (dD= -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

Simon M. Peacock

1987-01-01

253

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

NASA Astrophysics Data System (ADS)

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

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

1996-05-01

254

Seismic structure around the 660 in subduction zones and its implications  

NASA Astrophysics Data System (ADS)

Velocity structure around the 660-km seismic discontinuity in subduction zones is crucial for understanding the thermal state, chemical composition, and dynamics of the mantle. Because of limited spatial coverage of data and resolution, the complicated structure, especially in sub-slab regions, remains ambiguous. Topographic variation of the oceanward 660-km discontinuity in the Western Pacific is investigated by stacking S-to-P converted phases using large-array data. Beneath the Philippine Sea and Southern Kuril Islands, the mean depth of this discontinuity is elevated ~10 km and ~6.5 km with respect to the global average depth. Meanwhile, it is normal beneath the Sea of Japan where the subducted slab appears to be near-horizontally flattened above the 660-km discontinuity. Elsewhere, the 660-km discontinuity is ~11.5 km and ~6.4 km shallower beneath the Marianas and Tonga Islands subduction zones, respectively. This topographic variation suggests that the 660-km discontinuity beneath subducting slabs shallows as the dip of the slab increases. Using sources in the western Pacific and stations in China, 3D triplication waveform modeling of structure beneath the Southern Kuril subduction zone suggests that the high-velocity slab (+5% anomaly and ~100km thick) subducts to at least 560 km. No obvious high-velocity anomaly of the slab is observed around the 660 km depth. To match late AB and BC branch arrives at large distances, one of two possible low-velocity anomalies (-3% and ~30km thick) is necessary. The first is located beneath the subducted slab and might be due to a hot sinking anomaly emplaced at shallower depths away from the subduction zone. The second location is on the landward side of the subducted slab and above the 660-km discontinuity. We suggest this low-velocity anomaly could be generated by hot upwelling from lower mantle or small scale convection above the deep slab. Numerical modeling of mantle convection demonstrates that slab entrainment of hot sinking anomalies can explain both the topographic variation of the 660-km discontinuity oceanward of slabs and the possible low-velocity anomaly beneath the subducted slab in the Southern Kuriles.

Wang, Tao

255

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

256

Microstructural and metamorphic evolution of a high pressure granitic orthogneiss during continental subduction (Sudetes, European Variscan belt)  

NASA Astrophysics Data System (ADS)

A quantitative microstructural and metamorphic study of a naturally deformed medium- to high pressure granitic orthogneiss (Orlica-?nie?nik dome, Bohemian Massif) provides evidence of behaviour of the felsic crust during progressive burial along a subduction-type apparent thermal gradient (~10 °C/km). The granitic orthogneisses develops three distinct microstructural types, as follow: type I - augen orthogneiss, type II - banded orthogneiss and type III - mylonitic orthogneiss, each representing an evolutionary stage of a progressively deformed granite. Type I orthogneiss is composed of partially recrystallised K-feldspar porphyroclasts surrounded by wide fronts of myrmekite, fully recrystallised quartz aggregates and interconnected monomineralic layers of recrystallised plagioclase. Compositional layering in the type II orthogneiss is defined by plagioclase- and K-feldspar-rich layers, both of which show an increasing proportion of interstitial minerals, as well as deformation of recrystallised myrmekite fronts. Type III orthogneiss shows relics of quartz and K-feldspar ribbons preserved in a fine-grained polymineralic matrix. All three types have the same assemblage (quartz+plagioclase+K-feldspar+muscovite+biotite+garnet+sphene±ilmenite), but show systematic variations in the composition of muscovite and garnet from type I to type III. This is consistent with the equilibration of the three types at different positions along a prograde P-T path ranging from P<15 kbar and T<700 °C (type I orthogneiss) to P of 19-20 kbar and T>700 °C (types II and III orthogneisses). The deformation types thus do not represent evolutionary stages of a highly partitioned deformation at constant P-T conditions, but reflect progressive formation during the burial of the continental crust. The microstructures of the type I and type II orthogneisses result from the dislocation creep of quartz and K-feldspar whereas a grain boundary sliding-dominated diffusion creep regime is characteristic of the type III orthogneiss. Strain weakening related to the transition from type I to type II microstructure was enhanced by the recrystallisation of wide myrmekite fronts, and plagioclase and quartz, and further weakening and strain localization in type III orthogneiss occurred via grain boundary sliding enhanced diffusion creep. The potential role of incipient melting in strain localization is discussed.

Chopin, F.; Schulmann, K.; Štípská, P.; Martelat, J. E.; Pitra, P.; Lexa, O.; Petri, B.

2012-04-01

257

PTt path in metamorphic rocks of the Khoy region (northwest Iran) and their tectonic significance for Cretaceous Tertiary continental collision  

NASA Astrophysics Data System (ADS)

Metamorphic rocks in the Khoy region are exposed between obducted ophiolites to the southwest and sedimentary rocks of Precambrian-Paleozoic age to the northeast. The Qom formation (Oligocene-Miocene) with a basal conglomerate transgressively overlies all of these rocks. The metamorphic rocks consist of both metasediments and metabasites. The metasediments are micaschist, garnet-staurolite schist and garnet-staurolite sillimanite schist with some meta-arkose, marble and quartzite. The metabasites are metamorphosed to greenschist and amphibolite facies from a basaltic and gabbroic protolith of tholeiitic and calc-alkaline rocks. Geothermobarometry based on the equivalence of minerals stability and their paragenesis in these rocks and microprobe analyses by several different methods indicate that metamorphism occurred in a temperature range between 450 and 680 °C at 5.5 and 7.5 kb pressure. Rims of minerals reveal a considerable decrease of pressure (<2 kb) and insignificant decrease of temperature. The PTt path of this metamorphism is normal. The MFG line passes above the triple junction of Al 2SiO 5 polymorphs, and the average geothermal gradient during metamorphism was from 27 to 37 °C/km, which is more concordant with the temperature regime of collision zones. We infer that crustal thickening during post-Cretaceous (possibly Eocene) collision of the Arabian plate and the Azerbaijan-Albourz block was the main factor that caused the metamorphism in the studied area.

Azizi, H.; Moinevaziri, H.; Mohajjel, M.; Yagobpoor, A.

2006-06-01

258

The Andean subduction zone between 22 and 25°S (northern Chile): precise geometry and state of stress  

NASA Astrophysics Data System (ADS)

One year of seismicity recorded by a local network is used to obtain more precision about the geometry and the stress regime of the Andean subduction between 22 and 25°S in the northern Chile seismic gap. A sharp image of the Wadati-Benioff Zone (WBZ) is obtained down to 270 km in depth. A seismically quasi-quiescent zone is observed in the WBZ below the volcanic arc, between 150 and 210 km in depth. Hypocentres of distant intermediate depth earthquakes located with the local network are compared with worldwide seismic network hypocentres in order to evaluate the accuracy of the WBZ image at depth greater than 100-150 km in depth. No shallow microearthquakes have been observed in the continental crust but some seismic activity is likely to occur locally at the deep root of the Atacama Fault. The stress field and the characteristics of faulting along the subducted slab are investigated. Underthrusting and localized reverse faulting earthquakes define the seismically coupled plate interface from 20 to 50 km in depth (Locked zone). Downdip, intra-slab normal faulting prevails (Tensile zone), but some strike-slip faulting is observed. A transition between normal faulting with variable fault azimuth and normal faulting with nearly homogeneous NNW- to NW-oriented fault plane is found at about 80 km in depth. It is found that the stress axes ?1 and ?3 in the Locked zone are oriented in the convergence direction (75-80°E). Downdip, in the tensile zone, ?3 has a mean azimuth 60-65°E. There, the slab is hence submitted to a tensional force (slab pull) oblique relatively to the convergence. The transition between seismic underthrusting and intraplate normal faulting downdip occurs at the depth where the continental Moho encounters the Wadati-Benioff Zone, suggesting that a relationship exists between seismic coupling and the presence of continental crust at the plate interface. The pre-seismic state of this segment of the Andean subduction zone is confirmed by the occurrence of strong earthquakes located by the global network around the presumed rupture area and by the stress regime found along the Wadati-Benioff Zone.

Delouis, Bertrand; Cisternas, Armando; Dorbath, Louis; Rivera, Luis; Kausel, Edgar

1996-06-01

259

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

260

Automatic detection of low-frequency earthquakes in the Mexican subduction zone  

NASA Astrophysics Data System (ADS)

We use data from the MesoAmerican Subduction Experiment to detect and locate low-frequency earthquakes (LFEs) in the Mexican subduction zone. The analysis is based on visually-identified templates that are used to perform a network waveform correlation search that produces ~17,000 robustly detected LFEs that form 15 distinct families. Stacking a LFE family's corresponding detections results in seismograms with clear P- and S-wave arrivals; these travel times are then used to locate the sources. The locations superpose a region of permanent NVT activity (reported as a Sweet Spot by Husker et al. [2012]). The LFE family hypocenters have been located at a depth of 40 - 45 km in an area that is surrounding the upper slab-plate interface. We characterize their focal mechanisms by comparing their stacked seismograms to synthetic seismograms with varying moment tensors revealing a common low-dipping thrusting focal mechanism. To further explore the LFE activity in the Mexican subduction zone and to perform a more in-depth characterization of this phenomenon, it is necessary to greatly expand the number of LFE families. To achieve this, we are implementing an automatic detection algorithm that explores the signal coherency across the whole network.

Frank, W.; Shapiro, N. M.; Kostoglodov, V.; Husker, A. L.; Campillo, M.; Payero, J. S.; Prieto, G. A.

2013-05-01

261

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

NASA Astrophysics Data System (ADS)

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

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

2012-12-01

262

TEMSPOL: a MATLAB thermal model for deep subduction zones including major phase transformations  

NASA Astrophysics Data System (ADS)

TEMSPOL is an open MATLAB code suitable for calculating temperature and lateral anomaly of density distributions in deep subduction zones, taking into account the olivine to spinel phase transformation in a self-consistent manner. The code solves, by means of a finite difference scheme, the heat transfer equation including adiabatic heating, radioactive heat generation, latent heat associated with phase changes and frictional heating. We show, with a few simulations, that TEMSPOL can be a useful tool for researchers studying seismic velocity, stress and seismicity distribution in deep subduction zones. Deep earthquakes in subducting slabs are thought to be caused by shear instabilities associated with the olivine to spinel phase transition in metastable olivine wedges. We investigate the kinematic and thermal conditions of the subducting plate that lead to the formation of metastable olivine wedges. Moreover, TEMSPOL calculates lateral anomalies of density within subducting slabs, which can be used to evaluate buoyancy forces that determine the dynamics of subduction and the stress distribution within the slab. We use TEMSPOL to evaluate the effects of heat sources such as shear heating and latent heat release, which are neglected in commonly used thermal models of subduction. We show that neglecting these heat sources can lead to significant overestimation of the depth reached by the metastable olivine wedge.

Negredo, A. M.; Valera, J. L.; Carminati, E.

2004-04-01

263

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

NASA Astrophysics Data System (ADS)

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

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

2014-06-01

264

Subduction zones: Not relevant to present-day problems of waste disposal  

USGS Publications Warehouse

SUBDUCTION zones are considered to be sites of disposal for vast areas of the Earth's surface1, while new surface is generated simultaneously at rise crests2. Bostrom and Sherif3 suggest that the world's industrial and domestic waste be dumped into subduction zones at deep sea trenches to allow nature to complete the recycling process at geologically rapid rates of 5 to 10 cm/yr. They also point out that trenches are often sites of rapid rates of deposition and suggest that the dumped wastes would, speaking geologically, soon be buried. Francis4 suggests that canisters of toxic chemical and radioactive wastes could be dumped onto trench sediments and be expected to sink at rates of 20 m/yr, assuming that the mass of turbidites in the trench fill often spontaneously liquefies on shaking by earthquakes. The assumption is based on the supposed lack of evidence for deformed sediment in trenches. I will argue that the suggestion of Bostrom and Sherif3 is not useful for the next few dozen generations of human populations and will point out observational evidence to show that Francis's4 assumption is incorrectly founded. ?? 1972 Nature Publishing Group.

Silver, E. A.

1972-01-01

265

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

266

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

267

Electrical conductivity of fluids and hydrous phases at subduction zone conditions  

NASA Astrophysics Data System (ADS)

Based on geophysical observations, the mantle wedge of subduction zones are characterized by low seismic velocities and high electrical conductivities. It is known that the subducted hydrous phases dehydrate and release fluids. These fluids interact with the overlying mantle wedge and rehydrates part of mantle wedge stabilizing hydrous phases such as serpentinite. In order to explain the high electrical conductivity and assess the role of fluids and hydrous phases, we have measured the electrical conductivity of natural serpentinites at pressure (2 GPa) and temperatures (up to 1000 K) relevant to mantle wedge conditions. Among the two natural serpentinite sample investigated, one contained antigorite and the other contained chrysotile. We used transmission electron microscopy (TEM) to distinguish the various polytypes of serpentine. The measured contuctivity of serpentinite (10 -4 S/m) is higher than anhydrous olivine (10 -6 S/m) by two orders of magnidute at 1000 K. Upon dehydration, serpentine releases fluids. The conductivity of the released fluid was also measured in our experiment. Fluids have a very high conductivity of the order of 10 -2 -10 0 S/m range and can easily account for the observed high conductivities in several subduction zone settings.

Manthilake, G. M.; Mookherjee, M.; Miyajima, N.

2012-12-01

268

Locking of the Chile subduction zone controlled by fluid pressure before the 2010 earthquake  

NASA Astrophysics Data System (ADS)

Constraints on the potential size and recurrence time of strong subduction-zone earthquakes come from the degree of locking between the down-going and overriding plates, in the period between large earthquakes. In many cases, this interseismic locking degree correlates with slip during large earthquakes or is attributed to variations in fluid content at the plate interface. Here we use geodetic and seismological data to explore the links between pore-fluid pressure and locking patterns at the subduction interface ruptured during the magnitude 8.8 Chile earthquake in 2010. High-resolution three-dimensional seismic tomography reveals variations in the ratio of seismic P- to S-wave velocities (Vp/Vs) along the length of the subduction-zone interface. High Vp/Vs domains, interpreted as zones of elevated pore-fluid pressure, correlate spatially with parts of the plate interface that are poorly locked and slip aseismically. In contrast, low Vp/Vs domains, interpreted as zones of lower pore-fluid pressure, correlate with locked parts of the plate interface, where unstable slip and earthquakes occur. Variations in pore-fluid pressure are caused by the subduction and dehydration of a hydrothermally altered oceanic fracture zone. We conclude that variations in pore-fluid pressure at the plate interface control the degree of interseismic locking and therefore the slip distribution of large earthquake ruptures.

Moreno, Marcos; Haberland, Christian; Oncken, Onno; Rietbrock, Andreas; Angiboust, Samuel; Heidbach, Oliver

2014-04-01

269

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

SciTech Connect

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

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

1992-07-01

270

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

NASA Astrophysics Data System (ADS)

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

Mann, W. P.

2012-12-01

271

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

272

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

273

Deformation and metamorphism at the eastern border of the Tenda Massif (NE Corsica): a record of subduction and exhumation of continental crust  

NASA Astrophysics Data System (ADS)

A major detachment fault related to the post-orogenic extensional process and linked with rifting and drifting stages of the Liguro-Provençal and Northern Thyrrhenian sea, is described in recent literature for the eastern border of the Tenda Massif, North East Corsica, Western Mediterranean. New field mapping, meso/microstructural analyses and petrological investigations have been carried out in this area. Along the eastern border of the Tenda Massif, continental and oceanic units with epidote-blueschist facies metamorphic peak conditions are juxtaposed. After their coupling in a deeper part of the Alpine Corsica subduction wedge, the two units were deformed together by greenschist facies folding and shearing. Structural geometries and metamorphic records of the studied units, the regional tectonic settings and the available thermochronological data disclaim recent interpretations and suggest only a post-orogenic extensional reactivation. The main greenschist facies fabric and structures were related to syn-orogenic processes in response to intra-wedge deformation and strain delocalization during exhumation in a continental subduction setting.

Molli, G.; Tribuzio, R.; Marquer, D.

2006-10-01

274

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

NASA Astrophysics Data System (ADS)

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

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

2014-02-01

275

Magmatic Ultramafic Rock in Sulu Ultrahigh Pressure Metamorphic Belt: Depleted Oxygen isotope Evidence From Main Hole of Chinese Continental Scientific Drilling Project  

NASA Astrophysics Data System (ADS)

There are a lot of ultramafic bodies in Sulu ultrahigh pressure metamorphic belt and the research on the origin of these rock bodies has long been a hot topic. To identify if there is the ultramafic rocks that experienced shallow crustal process before subduction in this terrane, this study carried out oxygen extraction from mineral separates olivine, garnet and clinopyroxene of three peridotite samples between the depths 603.20~683.53 m of Chinese Continental Scientific Drilling (CCSD) bore hole using CO2 laser fluorination system and isotope analysis by mass spectrometer. The results are as follows. ? 18O values of olivine are 3.31‰3.82‰, garnet 4.03‰4.10‰, and clinopyroxene 2.10‰ in average with larger error. The ?18O values of these mineral separates are comprehensively lower than typical mantle values. Combined with other evidences, it is indicated that the low?18O values of ultramafic rock in shallow crust level was got from cold meteoric water by interaction between it and the rock body and experienced afterwards subduction, UHPM, retrogressive metamorphism during exhumation etc together with wall-rocks. The?18O discrepancies between minerals are 0.394‰0.791‰?grt-olv, 1.933‰?grt-cpx, -1.538‰?cpx-olv which demonstrate oxygen isotope partial equilibria between minerals and reflect isotope diffusion during retrogressive metamorphism. This is the first case that the ultramafic rocks possess 18O depleted character found in Sulu terrane. This result bears even more important continental dynamic meaning and proves that Sulu terrane is the eastward extension of Dabie terrane in respect of ultramafic rocks.

Li, T.; Rumble, D.; Yang, J.

2005-12-01

276

Continental aggregation, subduction initiation, and plume generation  

NASA Astrophysics Data System (ADS)

Several processes unfold during the supercontinent cycle, more than one of which might result in an elevation in subcontinental mantle temperatures through the generation of mantle plumes. Paleogeographic plate reconstructions have indicated that sub-continental mantle upwellings appear below large continents that are extensively ringed by subduction zones. Moreover, several numerical simulations of supercontinent formation and dispersal attribute the genesis of sub-continental plumes to the generation of subduction zones on the edges of the supercontinent, rather than resulting from continental insulation. However, the role of the location of downwellings in producing a return-flow upwelling, and on increasing sub-continental mantle temperatures, is not fully understood. In this mantle convection study, we examine the evolution of mantle dynamics after supercontinent accretion over a subduction zone (analogous to the formation of Pangea) for a range of continental coverage. We present 2D and 3D Cartesian geometry mantle convection simulations, featuring geotherm- and pressure-dependent viscosity with thermally and mechanically distinct oceanic and continental plates. Through changing the size of the continent we are able to analyze the factors involved in the generation of mantle plumes in purely thermal convection. Furthermore, we change the upper and lower mantle viscosity to determine their relation to plume formation in vigorous mantle convection simulations. Elevated sub-continental temperatures are analyzed in relation to continental coverage to further understand the influence of continental tectonics on the thermal evolution of the mantle.

Heron, P. J.; Lowman, J. P.

2013-12-01

277

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

278

Variations in the mode of great earthquake rupture along the central Peru subduction zone  

SciTech Connect

The historic record for the central Peru subduction zone suggests significant variations in the earthquake size during the last 400 years. During this century there have been four great underthrusting earthquakes along the central Peru seismic zone. From the north to south these are the 17 October 1966 (M{sub w} = 8.1), 24 May 1940 (M {approximately} 8), 3 October 1974 (M{sub 2} = 8.1), and 24 August 1942 (M {approximately} 8.2) earthquakes. Modified Mercalli intensity data and tsunami observations for the earthquakes in this century are compared with the 29 October 1746 and 20 October 1687 earthquakes. The 1746 earthquake had maximum intensity values between 9{degree} and 13{degree}S while the 1687 event had maximum values between 12{degree} and 14{degree}S suggesting that the two events failed different segments of the subduction zone. The authors find that the 1746 event occurred along the segment that includes both 1940 and 1966 earthquakes. The size of the 1746 event is estimated to M{sub w} {approximately} 8.8 based on the ratio of near-field tsunami heights for the 1746 and 1966 earthquakes. The 1687 earthquake probably ruptured the 1974 segment as well as the adjacent segment to the south where there is at present a gap between the 1942 and 1974 rupture zones. The size of the 1687 event is estimated to be M{sub w} {approximately} 8.7 based on both far-field and near-field tsunami height ratios of the 1687 and 1974 events. Both 1746 and 1687 earthquakes appear to be much larger than the events of this century. In contrast to the simple, single asperity nature of the 20th century earthquakes, these older and larger events may represent multiple-asperity ruptures along the Peru subduction zone. Hence, variations in the mode of earthquake rupture from cycle to cycle along the central Peru seismic zone may explain the significant difference in earthquake size during the last 400 years.

Beck, S.L. (Lawrence Livermore National Lab., CA (USA)); Nishenko, S.P.

1990-10-01

279

3-D Prestack Depth Imaging of the Nankai Subduction Zone off Shikoku Island, SW Japan  

NASA Astrophysics Data System (ADS)

The Nankai Trough subduction zone off southwest Japan is one of the best-suited convergent plate margins for studying large interpolate subduction-zone earthquakes as well as the formation of accretionary prisms. At this margin, the Philippine Sea Plate (PSP) is subducting beneath the Eurasian Plate (EP) to the NNW. The plate convergence rate is estimated to be 4 - 5 cm/yr. Large thrust earthquakes have repeatedly occurred along the Nankai subduction zone with a recurrence interval of 100-200 years. The most recent interplate earthquake was the Nankai earthquake (Mw=8.2), which occurred in 1946 off the Kii Peninsula. In order to figure out seismic structure and stratigraphy of the Nankai accretionary wedge off Cape Muroto of Shikoku Island, southwest Japan, we have conducted three-dimensional (3-D) multichannel seismic (MCS) reflection survey using R/V Ewing in 1999. We acquired the MCS data on 81 separate lines with 100 m line spacing, each 80 km long, producing 8 X 80 km 3-D seismic volume. To obtain the 3-D prestack depth migration images, we constructed and updated a 3-D interval velocity model using the CDP bin gathers for which preconditioning processings including amplitude recovery, deconvolution, and multiple suppression were applied. Miocene to Pliocene Shikoku Basin sediments underthrusts the overlying accretionary prism along a decollement as the PSP subducts beneath the EP. The oceanic crust of the subducting PSP is traceable over the entire inlines. Several imbricate thrust faults are observed in the overlying accretionary wedge. The decollement steps down on the top of subducting oceanic crust around ~30 km landward from the deformation front. We recognize several sigmoid, landward dipping out-of-sequence thrust (OOST) faults in the landward thick wedge package. Most of the OOSTs are apparently developed from the subducting oceanic basement to the seafloor in the forearc region, cutting both underthrust sediments and the overriding accretionary prism. In this paper, we will show and discuss recent results of the 3-D prestack depth migration, visualization, and seismic structural/stratigraphic interpretation.

Park, J.; Tsuru, T.; Sato, S.; Kaneda, Y.

2005-12-01

280

Sharp Permeability Transitions due to Shallow Diagenesis of Subduction Zone Sediments  

NASA Astrophysics Data System (ADS)

The permeability of hemipelagic sediments is an important factor in fluid flow in subduction zones and can be affected by porosity changes and cementation-dissolution processes acting during diagenesis. Anomalously high porosities have been observed in cores from the Shikoku Basin sediments approaching the Nankai Trough subduction zone. These high porosities have been attributed to the presence of minor amounts of amorphous silica cement that strengthen the sediment and inhibit consolidation. The porosity rapidly drops from 66-68% to 54-56% at a diagenetic boundary where the amorphous silica cement dissolves. Although the anomalous porosity profiles at Nankai have received attention, the magnitude of the corresponding permeability change has not been addressed. In this study, permeability profiles were constructed using permeability-porosity relationships from previous studies, to estimate the magnitude and rate of permeability changes with depth. The predicted permeability profiles for the Nankai Trough sediment cores indicate that permeability drops by almost one order of magnitude across the diagenetic boundary. This abrupt drop in permeability has the potential to facilitate significant changes in pore fluid pressures and thus to influence the deformation of the sediment onto the accretionary prism. At the Costa Rica subduction zone, results vary with location. Site U1414 offshore the Osa Peninsula shows porosities stable at 69% above 145 mbsf and then decrease to 54% over a 40 m interval. A porosity drop of that magnitude is predicted to correlate to an order of magnitude permeability decrease. In contrast, porosity profiles from Site 1039 offshore the Nicoya Peninsula and Site U1381 offshore the Osa Peninsula show anomalously high porosities but no sharp drop. It is likely that sediments do not cross the diagenetic boundary due to the extremely low (<10°C/km) thermal gradient at Site 1039 and the thin (<100 m) sediment cover at Site U1381. At these locations, the porosity loss and permeability reduction may occur after the sediment is subducted and contribute to high pore pressures at the plate boundary.

James, S.; Screaton, E.

2013-12-01

281

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

282

Interferometric Redatuming and Imaging of Low Frequency Earthquakes for Fine-Scale Subduction Zone Structure  

NASA Astrophysics Data System (ADS)

We investigate the application of interferometric redatuming of seismic waveforms from low frequency earthquakes (LFE's) for fine-scale subduction zone structure. In this approach, seismic waveform data from two LFE sources recorded by an array at the Earth's surface are interferometrically redatumed to replace one of the sources at depth in the Earth by a virtual receiver. With many LFE sources along the top of a subducting plate, virtual source-receiver gathers can be constructed along the top of the plate boundary. Similar, but more involved approaches can also be applied to Wadati-Benioff seismic events which possess a wider depth distribution within the subducting plate. The construction of virtual shot-receiver profiles in the sub-surface has the advantage of effectively removing the distortion effects of the shallow structure above the plate and so affords the potential of providing more detailed images of the subduction zone structure itself. Here we perform initial numerical experiments for LFE sources and stations along a linear profile similar to that found in northern Cascadia. We first redatum synthetic P waveforms of LFE sources for a layered subduction zone structure including a dipping low velocity zone (LVZ) layer. The synthetic waveforms recorded at the surface array from a number of LFE sources are then redatumed to obtain a series of virtual common-source gathers along the top of the plate boundary, which can then be used for fine-scale imaging and velocity analysis of the plate itself. A second series of numerical experiments are performed using P waveforms of surface data from LFE sources to obtain virtual zero-offset profiles redatumed to the top of the plate structure. In these experiments a laterally varying lower boundary of the LVZ structure is imaged by the virtual zero-offset profiles at depth. In order to apply the redatuming approach to three-component seismic data recorded by LFE sources, an elastic formulation is required, however for this case the virtual receiver responses are no longer displacement but rather those of moment-tensor sensors. Here we investigate an asymptotic, elastic redatuming approach that explicitly identifies wave-mode components in the analysis.

Nowack, R. L.; Bostock, M. G.

2012-12-01

283

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

NASA Astrophysics Data System (ADS)

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

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

2011-04-01

284

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

285

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

Microsoft Academic Search

(1) Arc volcanism is intimately linked to mineral dehydration reactions in the subducting oceanic mantle, crust, and sediments. The location of slab dehydration reactions depends strongly on the temperature and pressure conditions at the top of the subducting plate and hence on the detailed thermal structure of subduction zones. A particularly important physical property of subduction zone thermal models is

Peter E. van Keken; Boris Kiefer; Simon M. Peacock

2002-01-01

286

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

Microsoft Academic Search

Arc volcanism is intimately linked to mineral dehydration reactions in the subducting oceanic mantle, crust, and sediments. The location of slab dehydration reactions depends strongly on the temperature and pressure conditions at the top of the subducting plate and hence on the detailed thermal structure of subduction zones. A particularly important physical property of subduction zone thermal models is the

Peter E. van Keken; Boris Kiefer; Simon M. Peacock

2002-01-01

287

Metamorphic density controls on early-stage subduction dynamics  

NASA Astrophysics Data System (ADS)

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

Duesterhoeft, Erik; Oberhänsli, Roland; Bousquet, Romain

2013-04-01

288

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

USGS Publications Warehouse

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

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

2008-01-01

289

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

USGS Publications Warehouse

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

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

2009-01-01

290

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

NASA Astrophysics Data System (ADS)

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

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

2012-12-01

291

High-resolution image of the North Chilean subduction zone: seismicity, reflectivity and fluids  

NASA Astrophysics Data System (ADS)

We obtained high-precision locations for 5250 earthquakes in the Iquique segment of the northern Chilean subduction zone from two temporary local seismic networks around 21°S. A double seismic zone in the downgoing Nazca slab can be clearly identified. One band of seismicity is located at the plate interface and a second one 20-25 km deeper in the oceanic mantle. It can be traced updip to uncommonly shallow levels of 50 km. A combined interpretation of seismicity and reflectivity along the seismic ANCORP'96 experiment suggests the prevalence of fluid processes in the subducted oceanic crust as well as in the uppermost 20 km of the mantle. Crustal seismicity is pervasive below the Coastal Cordillera. Beneath the Precordillera, the lower bound of crustal seismicity delineates a sharp west-dipping boundary down to 20 km depth, consistent with earlier findings indicating a rheological boundary.

Bloch, W.; Kummerow, J.; Salazar, P.; Wigger, P.; Shapiro, S. A.

2014-06-01

292

Unlocking the Secrets of the Mantle Wedge: New Insights Into Melt Generation Processes in Subduction Zones  

NASA Astrophysics Data System (ADS)

Recent laboratory studies of the melting and crystallization behavior of mantle peridotite and subduction zone lavas have led to new insights into melting processes in island arc settings. Melting of the mantle wedge in the presence of H2O begins at much lower temperatures than previously thought. The solidus of mantle peridotite at 3 GPa is ~ 800 °C, which is 200 °C below previous estimates. At pressures greater than 2.4 GPa chlorite becomes a stable phase on the solidus and it remains stable until ~ 3.5 GPa. Therefore, melting over this pressure range occurs in the presence of chlorite, which contains ~ 12 wt. % H2O. Chlorite stabilized on the peridotite solidus by slab-derived H2O may be the ultimate source of H2O for subduction zone magmatism. Thus, chlorite could transport large amounts of H2O into the descending mantle wedge to depths where it can participate in melting to generate hydrous arc magmas. Our ability to identify primitive mantle melts at subduction zones has led to the following observations. 1) Primitive mantle melts show evidence of final equilibration at shallow depths near the mantle - crust boundary. 2) They contain variable amounts of dissolved H2O (up to 6 wt. %). 3) They record variable extents of melting (up to > 25 wt. %). To produce melts with such variable characteristics requires more than one melting process and requires consideration of a new type of melting called hydrous flux melting. Flux melting occurs when the H2O - rich melt initially produced on the solidus near the base of the mantle wedge ascends and continuously reacts with overlying hotter, shallower mantle. The mantle melts and magmatic H2O content is constantly diluted as the melt ascends and reacts with shallower, hotter mantle. Anhydrous mantle melts are also found in close temporal and spatial proximity to hydrous flux melts. These melts are extracted at similar depths near the top of the mantle wedge when mantle is advected up and into the wedge corner and melted by adiabatic decompression. In light of these new insights into the chemical processes that lead to melt generation in subduction zones, further study of the influence of mantle dynamics and physical processes on melting is crucial. Variations in mantle permeability near the base of the wedge may exercise important controls on the access of fluids and/or melts to the overlying wedge. The presence of chlorite in the wedge may also influence rheological properties and seismicity in the vicinity of the slab - wedge interface. Improved knowledge of rheology and permeability will help us to develop more robust models of mantle flow and temperature distribution in the mantle wedge. These are crucial for refining melting models. By combining evidence from petrology, geochemistry and geophysics the mysteries that attend the generation of melt in the mantle wedge can be resolved.

Grove, T. L.

2007-05-01

293

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

NASA Astrophysics Data System (ADS)

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

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

2012-12-01

294

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.

295

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

NASA Astrophysics Data System (ADS)

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

Faccenda, Manuele

2014-05-01

296

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

NASA Astrophysics Data System (ADS)

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

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

2003-12-01

297

Dense GPS Array Observations Across the Nankai Subduction Zone, Southwest Japan  

NASA Astrophysics Data System (ADS)

Interseismic deformation in an oblique subduction zone is a mixture of short-term crustal shortening in the direction of plate convergence and permanent margin-parallel movement of a forearc block. We have deployed two dense GPS traverse arrays across the southwest Japan arc to better illustrate strain partitioning in the Nankai subduction zone. In 1998 we constructed the first array that composed of 22 stations along a 200km-long arc-normal line. The second array with 15 stations was constructed in 2002 nearly parallel to and 120km west of the first one. Both arrays cross the Median Tectonic Line (MTL), the arc-parallel strike-slip fault system dividing the forearc block from the rest of the overriding plate. More than 100 crustal velocities from these arrays and the nationwide continuous GPS arrays are used for inversion analysis to estimate back slip vectors on plate interface, a rate of margin-parallel forearc movement, and slip deficits on the upper fault zone of the MTL. Plate interface and MTL fault plane are reproduced by multi-rectangular segments and inversions are conducted for various dip-angles of the MTL. The optimum model shows that strong plate coupling causes a margin-parallel forearc movement at a rate of 3mm/yr together with a crustal shortening of 0.3-0.4 micro strain/yr in the direction of plate convergence. Slip deficits on the MTL are nearly equivalent to the rate of margin-parallel forearc movement in the opposite direction, showing a full locking of the upper fault zone of the MTL. Moreover inversion result favors northward dipping fault segments of the MTL throughout from east to west than the vertical. The rate of margin-parallel forearc movement is consistent with the geological slip rate of the MTL in the late Quaternary.

Tabei, T.; Miyazaki, S.; Hashimoto, M.; Matsushima, T.; Kato, T.; Kato, S.

2004-12-01

298

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

NASA Astrophysics Data System (ADS)

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

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

2011-08-01

299

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

300

Interseismic deformation along the Philippine Fault system and Manila subduction zone  

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

301

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

PubMed

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

Moreno, Marcos; Rosenau, Matthias; Oncken, Onno

2010-09-01

302

Dislocation model of strain accumulation and release at a subduction zone  

SciTech Connect

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

Savage, J.C.

1983-06-10

303

Coseismic subduction zone strain-release as a constraint for slab dynamics  

NASA Astrophysics Data System (ADS)

Slab rheology and the lateral viscosity variations in the ambient mantle in subduction zones are key controls on upper mantle dynamics but still poorly constrained. Mantle flow modeling puts seismic strain-rates into the context of regional subduction dynamics and global mantle circulation, and our goal is to arrive at a globally consistent model of plate motion and slab deformation. We analyze the co-seismic strain-rates that are recorded by the global centroid moment tensor (gCMT) catalog in conjunction with the temperature anomaly distribution and slab morphology that can be inferred from seismic tomography. Our analysis of gCMTs confirms the global patterns that were established by Isacks and Molnar (1971) where intermediate depth in- slab extension merges into deep in-slab compression in some, but not all subduction zones. The large increase of seismic data over the last tens of years allows spatial refinements of Kostrov summations. Those show ubiquitous, second-order complexity on regional scales, perhaps due to effects such as slab tears. We find that no simple kinematic model can provide an adequate description of this diversity in strain patterns. Our fluid dynamical modeling builds on the 2-D analysis of Vassiliou & Hager (1988) and regional 3-D studies by Billen & Gurnis (2003); we strive to explore both regional and global fluid-dynamical effects such as slab deflection and folding at 660~km, effects of a higher viscosity lower mantle and weak mantle wedge, and the role of depth-dependent slab strength. A better understanding of the interaction of the large-scale flow with subducting slabs as imaged by seismicity motivates improved 3-D models. Those can provide background information for what slab deformation states are predicted based on global plate dynamics, and what component of strain-release may be indicative of regional, possibly transient tectonic effects.

Alpert, L. A.; Ghosh, A.; Becker, T.; Bailey, I.; Miller, M. S.

2008-12-01

304

Spherical shell tectonics: on the buckling of the lithosphere at subduction zones  

NASA Astrophysics Data System (ADS)

The buckling phenomena of the subducting lithosphere due to the sphericity of the earth are studied by a nonlinear finite element method (FEM), and the effects of geometrical nonlinearity are examined. The subduction of the lithosphere is modelled by a hemispherical shell squeezed at its circumferential edge. In scaling the geometry of the subducting lithosphere, two parameters are employed; one is the thickness of the hemispherical shell (i.e., the thickness of the lithosphere) and the other is the length of its deformable portion (i.e., the length of the descending slab). To simulate the bending of the lithosphere at the trench, uniform inward load is applied to the free circumferential edge of the deformable portion, which corresponds to the leading edge of the subducting lithosphere. At the opposite edge of the deformable portion (i.e., at the trench), a built-in boundary condition is imposed; no displacements and no rotations are allowed. With a load greater than some critical value, the deformable portion of the shell buckles and undulates azimuthally. The investigation of this buckling phenomenon can be summarized as follows: (1) Buckling with a shorter wavelength is associated with a thinner and shorter shell. Mechanical irregularities simulating lithospheric tearing or seamount collision do not alter the buckling wavelength very much. After appropriate scaling, the lengths of subduction zones coincide with the buckling wavelength calculated by F.E.M. (2) Sharp cusps pointing outward from the shell appear during the postbuckling undulation without any irregularities in shell property and in the applied load. The shape of this cuspidal undulation is quite similar to that actually observed at an arc-arc junction. (3) Buckling can take place in the stress environment likely to exist in the earth. These results strongly suggest that the arcuate shapes of subduction zones are in fact generated by buckling of the subducting lithosphere.

Yamaoka, Koshun

1988-04-01

305

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

NASA Astrophysics Data System (ADS)

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

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

2012-12-01

306

Continental Collision, Oceanic Subduction and Related Geodynamical Processes: a Numerical Modeling Perspective  

NASA Astrophysics Data System (ADS)

Collisional and subduction zones are sites on the Earth surface where two continents collide or an oceanic plate descends into the mantle, respectively. Aside these gross scale phenomena, a wide variety of other small scale geological processes take place at the same time. The study of these processes is crucial for the understanding of the subduction/collision history and for the interpretation of geophysical data that, in turn, give informations about the present day structure of these areas. In the recent years, coupled petrological-thermomechanical numerical models have been used to study processes occurring at collision/subduction settings. We use I2VIS code with realistic visco-plastic rheologies (Gerya and Yuen, 2003) to model subduction zones characterized by an early oceanic subduction followed by continental collision. Collision between two continents includes mountain chains building, continental crust recycling in the mantle and exhumation of HP-UHP metamorphic rocks. In case of plate decoupling, asthenospheric mantle wedges between the continents triggering the retreating and delamination of the converging continental plate. In order to investigate hydration and subsequent dehydration of the slab, we also performed 2D numerical models of a spontaneously bending oceanic plate using I2ELVIS code that account for visco-elasto-plastic rheologies (Gerya and Yuen, 2007). At the outer rise, bending-related slab faulting occurs and provides a pathway for water percolation in the slab. Faults generally deep trenchward, but antithetic faults are also common. As the slab subducts, serpentinized faults acquire a sub-vertical position; on the other hand, pressure and temperature increase so that hydrous phases become unstable and elevated pore fluid pressure build up allowing to brittle deformation at big depths. Results are consistent with intermediate-depth earthquakes distribution and confirm the now well accepted theory that dehydration of the slab as the main trigger mechanism for earthquakes with deep hypocenters. We also found that the anisotropy measured above subduction zones seems to be related to the spatial distribution of the serpentinized faults in the slab.

Faccenda, M.; Gerya, T.; Burlini, L.; Chakraborty, S.

2008-05-01

307

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

NASA Astrophysics Data System (ADS)

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

Massonne, H.; Willner, A. P.

2007-12-01

308

Potential links between porphyry copper deposits and exhumed metamorphic basement complexes in northern Chile  

NASA Astrophysics Data System (ADS)

Porphyry copper deposits (PCDs) are typically associated with magmatic arcs in compressional subduction zone settings where thickened crust and fractionated calc-alkaline magmas produce favourable conditions for copper mineralisation. A classic example is the Eocene-Oligocene PCD belt of Chile, the world's leading copper producing country. In other parts of the world, older late Cretaceous to early Tertiary PCDs are found in regions of former subduction-related magmatism that have undergone subsequent post-orogenic crustal extension, such as the Basin and Range province of western North America, and the Eurasian Balkan-Carpathian-Dinaride belt. In the Basin and Range there is a striking correlation between the location of many PCDs and exhumed metamorphic core complexes (isolated remnants of the middle to lower crust exhumed during extensional normal faulting). This close spatial relationship raises questions about the links between the two. For example, are their exhumation histories related? Could the presence of impermeable metamorphic rocks at depth affect and localise mineralising fluids? In Chile there appears to be a similar spatial relationship between PCDs and isolated outcrops of exhumed metamorphic basement. In northern Chile, isolated exposures of high-grade metamorphic gneisses and amphibolites are thought to be exhumed remnants of the pre-subduction Proterozoic-Paleozoic continental margin of Gondwana [2], although little is known about when they were exhumed and by what mechanism. For example, the Limón Verde metamorphic complex, exhumed from a depth of ca. 50 km, is situated adjacent to Chuquicamata, the largest open pit copper mine in the world. In northernmost Chile, another metamorphic exposure, the Belén complex, sits close to the Dos Hermanos PCD, a small deposit that is not actively mined. Comprising garnet-bearing gneisses and amphibolites, the Belén is thought to have been exhumed from a depth of ca. 25 km, but when and how is unclear [3]. We present new mapping, structural analysis, and geochronologic data from both the Limón Verde and the Belén metamorphic complexes and explore the relationship between these isolated outcrops and PCD formation in northern Chile. References: [1] Seedorff et al., 2008, Root Zones of Porphyry Systems: Extending the Porphyry Model to Depth, Ec. Geol., 103, 939-956. [2] Hervé et al., 2007, Metamorphic and Plutonic basement complexes, in: The Geology of Chile, Geol. Soc. Lond., 5-19. [3] Wörner et al., 2000, Precambrian and Early Paleozoic evolution of the Andean basement at Belen (northern Chile) and Cerro Uyarani (western Bolivia Altiplano), 13, 717-737.

Cooper, Frances; Docherty, Alistair; Perkins, Rebecca

2014-05-01

309

Comment on 'Seismic coupling along the Chilean subduction zone' by B. W. Tichelaar and L. R. Ruff  

NASA Astrophysics Data System (ADS)

The Chilean subduction zone was studied by Tichelaar and Ruff (1991) as part of a worldwide project to map the depth extent and the width of the seismogenic interplate contact in subduction regimes. They conclude that there is a change in the maximum depth of seismogenic coupling in the Chilean subduction zone at 20 deg S. They argue that to the south of this latitude the seismically coupled zone in Chile extends to depths of 48 to 53 km. Their critics discuss locally recorded data using permanent and temporary networks in Chile which suggest that the depth of the coupled zone does not vary along the Chilean subduction zone. Different results from earthquakes with magnitudes greater than 6.0 vs data from microseismicity studies are presented. Tichelaar and Ruff reply that, if the new Taltal earthquake in Suarez and Comte (1993) and the bootstrap error estimates of earthquake depths in Tichelaar and Ruff are taken into account, and if the Chilean subduction zone is divided into six or fewer segments, they agree with the 'modified' conclusion of Suarez and Comte that the maximum depth extent of the seismogenic plate interface extends down to between 40 and 50 km along Chile.

Suarez, Gerardo; Comte, Diana

1993-09-01

310

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

311

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

NASA Technical Reports Server (NTRS)

Energy release in earthquakes is discussed. Dynamic energy from source time function, a simplified procedure for modeling deep focus events, static energy estimates, near source energy studies, and energy and magnitude are addressed. Subduction zone seismicity and stress in slabs are also discussed.

Vassiliou, M. S.

1983-01-01

312

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

313

Detailed structure and sharpness of upper mantle discontinuities in the Tonga subduction zone from regional broadband arrays  

Microsoft Academic Search

Recordings of deep Tonga earthquakes from two arrays of 12 broadband seismographs each in the Fiji and Tonga islands are stacked and searched for reflections and conversions from upper mantle discontinuities in the Tonga subduction zone. The arrays operated as part of the Seismic Arrays in Fiji and Tonga (SAFT) experiment from July 2001 to August 2002. In comparison with

Rigobert Tibi; Douglas A. Wiens

2005-01-01

314

Trace element signature of subduction-zone fluids, melts and supercritical liquids at 120-180km depth  

Microsoft Academic Search

Fluids and melts liberated from subducting oceanic crust recycle lithophile elements back into the mantle wedge, facilitate melting and ultimately lead to prolific subduction-zone arc volcanism. The nature and composition of the mobile phases generated in the subducting slab at high pressures have, however, remained largely unknown. Here we report direct LA-ICPMS measurements of the composition of fluids and melts

Ronit Kessel; Max W. Schmidt; Peter Ulmer; Thomas Pettke

2005-01-01

315

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

316

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

Microsoft Academic Search

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

Robert McCaffrey

1997-01-01

317

Inverse Modeling of Surface Deformation Data to Determine the Depth of Coupling at the Cascadia Subduction Zone  

Microsoft Academic Search

Understanding the extent of seismic coupling along the Cascadia subduction zone is important in assessing earthquake hazards in the Pacific Northwest of the United States and southwestern Canada. Recently developed models suggest that surface deformation can be the result of seismic coupling deeper on the subduction interface than previously accepted models indicate. Models also indicate that the depth of coupling

D. Verdonck

2003-01-01

318

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

319

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

SciTech Connect

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

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

1995-11-10