Km In Subduction Zones Research Articles

Elasticity of akimotoite under the mantle conditions: Implications for multiple discontinuities and seismic anisotropies at the depth of ∼600–750 km in subduction zones

The equation of state and elastic properties of akimotoite at simultaneously high pressures and high temperatures are obtained using first-principles calculations based on the density functional theory (DFT). The calculated results agree with the available experimental data. Combining our results with the elastic data of other minerals, we estimated the VP, VS, and density contrasts caused by the akimotoite-related transitions. The velocity contrasts between akimotoite and bridgmanite are 4.6% and 8.3% for VP and VS, respectively, which are only about half of those between majorite and akimotoite. Moreover, because both the akimotoite-bridgmanite and majorite-akimotoite transitions have broad phase boundaries, these two phase transitions may not contribute to multiple discontinuities around ∼660 km depth in subduction zones as detected by seismic studies. Instead, the decomposition of pyrope into bridgmanite and corundum, which would occur in cold subduction zones with a sharp phase boundary and a large impedance contrast due to the inhibition of the pyroxene-garnet transformation at relatively low temperatures, could be a more reasonable explanation for the discontinuity at ∼700–750 km in subduction zones. Furthermore, the transformation from high-pressure clinopyroxene to akimotoite at the depth of ∼600 km can increase the VP, VS, and density by 10.1%, 14.8%, and 9.9%, respectively, indicating that the phase transition may account for the local discontinuity at ∼600 km in some subduction zones. In addition, the anisotropies of akimotoite are significantly higher than those of other major minerals at the base of the mantle transition zone and could be the origin of the seismic anisotropies detected in some subduction zones.

Petrology and Geochemistry of the lawsonite (pseudomorph)-bearing eclogite in Yuka terrane, North Qaidam UHPM belt: An eclogite facies metamorphosed oceanic slice

Although lawsonite eclogite is predicted to be common at depth of ~45–300km in subduction zones, it is rarely exposed at Earth's surface. In this study, we identified a lawsonite (pseudomorph)-bearing eclogite in Yuka terrane, North Qaidam UHPM belt. It is mainly composed of garnet, amphibole, clinozoisite, quartz and rutile with minor omphacite relics and talc. Petrographic investigation identified abundant rectangular- or rhombic- shaped mineral aggregates composed of polycrystalline clinozoisite or polyphase clinozoisite+paragonite±phengite±quartz and clinozoisite+amphibole±quartz±Na-rich plagioclase occurring in garnet porphyroblasts. The peculiar shape, mineral assemblages and reconstructed compositions of these aggregates show that they are pseudomorphs after lawsonite. Phase equilibrium modeling predicts that the lawsonite (pseudomorph)-bearing eclogite experienced a clockwise P–T evolution with a peak stage at 570°C, 27kbar, near the quartz–coesite transition boundary. Geochemical data show that the lawsonite (pseudomorph)-bearing eclogite has a MORB-like affinity with flat Chondrite–normalized rare earth element (REE) distribution patterns. Zircons from this sample have similar δ18O values (5.38±0.10‰) with the normal mantle (5.3±0.3‰), indicating that the protolith of the lawsonite (pseudomorph)-bearing eclogite is derived directly from mantle rock without continental crust contamination. The MORB-like character and zircon O isotopic composition of the lawsonite (pseudomorph)-bearing eclogite indicate that its protolith is likely to be an oceanic crustal slice. SIMS and LA–ICP–MS zircon U–Pb dating yielded a 436–438Ma eclogite-facies metamorphic age for the lawsonite (pseudomorph)-bearing eclogite, similar to the continental subduction age of the North Qaidam UHPM belt, but did not obtain its protolith age. These data indicate that the protolith of the lawsonite (pseudomorph)-bearing eclogite may be either a piece of Proterozoic ocean that was emplaced in the active continental margin of the Qaidam block during the amalgamation of the Rodinia supercontinent, or the early Paleozoic oceanic crust in the transition zone between the Qilian Ocean and the Qaidam block.

Depth‐varying rupture properties of subduction zone megathrust faults

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

Nonvolcanic Tremor in the Aleutian Arc

Nonvolcanic tremors (NVT) have been observed across the Aleutian arc. The tremorlike events are observed over a large region spanning seismic networks on several volcanoes and are therefore unlikely to be related to any one volcanic complex. Although locating the events is not possible at the present time, we provide general locations based on move-out times and amplitudes across multiple networks. The majority of NVT events are recorded in regions where the subducting Pacific plate is inferred to be locked or transitioning from creeping to locked. One NVT event was recorded during a time of increased seismic and volcanic activity in the area near the Rat and Andreanof Islands. Similar simultaneous increases have been observed in the Aleutians in the past and are hypothesized to be caused by large regional stress changes, possibly caused by slow-slip events. A prominent recent NVT event was recorded on the Korovin and Great Sitkin seismic networks approximately one hour after the onset of the 12 July 2008 eruption of Okmok volcano. NVT does not appear sporadically across the Aleutian arc but seems to concentrate in a few regions. With 2500 km of subduction zone, we have an opportunity to examine these regions and compare them to regions not producing NVT in an attempt to identify factors that may play a role in its creation.

Distinctive petrological, geochemical, and geodynamic features of subduction-related magmatism

Geological-petrological and geochemical data on subduction-related magmatism (including the volumes and compositions of the corresponding magmatic series) are compared to the results of experiments and numerical simulation. The subduction zone is subdivided into five depth sectors and volcanic zones I, II, and III: 1 is the accretionary wedge that controls the geodynamic stability of subduction; 2 is the sector of dehydration and fluid filtration; 3 is the zone of eclogitization and initial partial melting in the slab above which boninite volcanic zone I is formed during early stages; 4 is the main zone of melting of the sedimentary-basite layer and the development of volcanic zone II with the predominance of andesites; and 5 is the zone of higher degree melting, above which volcanic zone III (basaltic andesite and alkali basalt) is formed. The criterion of volcanism intensity, which was obtained within the scope of the melting model, is proportional to the subduction velocity and the thickness of the melting zone, and the distance between the groups of volcanics along the subduction zone is 75–100 km, at a thickness of the melting zone of 15–20 km. The calculated isotherm of 600°C, which controls the stability of serpentine and chlorite, is not identified at depth above 150 km, and this is confirmed by the composition and P-T conditions of the high-pressure rocks (containing diamond and coesite), which were brought from depths of 150–200 km in subduction zones. Seismic sections constructed with regard for the amplitude characteristics of seismic waves show two melting zones (“wet” melting at a depth of 100–200 km and “dry” melting at a depth of 150–200 km) and a complicated thermal structure of the suprasthenospheric wedge, which can include slant magma conduits. The mineralogical and geochemical features of arc magmatic series are formed at a decisive role of an H2O-CO2 fluid and an elevated oxidation potential. The predominant buffer minerals are as follows: garnet in the slab melting zone; magnetite, Ca-pyroxene, and amphibole in intermediate magmatic chambers; and amphibole, protoenstatite-bronzite (in place of olivine), and Cr-spinel (in place of magnetite) for boninite series generated in a “hot” asthenospheric wedge at interaction with fluids or water-rich melts. Actively disputable problems are the interactions scale of melts and fluids generated in a subduction zone with a “hot” mantle wedge, the possibility of transporting water-rich minerals deep into the mantle (to depths greater than 150 km), and the evolution of the scale at which young continental crust is generated by subduction melts.

On the use of satellite altimetry to infer the earthquake rupture characteristics: application to the 2004 Sumatra event

SUMMARY Standard data and methods, such as the inversion of seismic and GPS data, have been used extensively to infer the details of the 2004 December 26 earthquake. The unprecedented large size of this event gave the opportunity to modern altimeters to provide the first clear records of a tsunami in deep ocean, therefore allowing us to study the rupture history from an independent perspective. We invert the Jason-1 and Topex–Poseidon altimetry records, considering the new constraints available on the geometry of the fault plane, and taking them into account in a 3-D rupture model. The data are corrected for the non-negligible effect of satellite motion during measurements. Our results show that the rupture propagated over the 1500 km of subduction zone initially identified by the aftershock distribution, with a magnitude of Mw= 9.1. Our solution compares well with the latitudinal distribution of slip inferred from other data sets, with a maximum of energy release north of Sumatra, and two other slip patches near the Nicobar and Andaman islands. Based on waveform comparison, we assert that the shallow portion of the megathrust offshore Banda Aceh had slip amplitudes of more than 20 m. Also, we find that significant amounts of slip (about 10 m) concentrated below the Andaman islands and did not propagate on the shallow portion of the interface. Although synthetic tests tend to show less resolution in the northern part of the rupture, this solution is compatible with the near-field data (GPS, coral heads and imagery), and would allow one to explain the apparent paradox between the large local displacements and the moderate tsunami observed locally. Finally, we demonstrate the rapidly dominating effect of propagation and slip distribution over the rupture velocity, and how it precludes the direct estimate of this latter parameter.

A periodic shear-heating mechanism for intermediate-depth earthquakes in the mantle

Intermediate-depth earthquakes, at depths of 50-300 km in subduction zones, occur below the brittle-ductile transition, where high pressures render frictional failure unlikely. Their location approximately coincides with 600 to 800 degrees C isotherms in thermal models, suggesting a thermally activated mechanism for their origin. Some earthquakes may occur by frictional failure owing to high pore pressure that might result from metamorphic dehydration. Because some intermediate-depth earthquakes occur approximately 30 to 50 km below the palaeo-sea floor, however, the hydrous minerals required for the dehydration mechanism may not be present. Here we present an alternative mechanism to explain such earthquakes, involving the onset of highly localized viscous creep in pre-existing, fine-grained shear zones. Our numerical model uses olivine flow laws for a fine-grained, viscous shear zone in a coarse-grained, elastic half space, with initial temperatures from 600-800 degrees C and background strain rates of 10(-12) to 10(-15) s(-1). When shear heating becomes important, strain rate and temperature increase rapidly to over 1 s(-1) and 1,400 degrees C. The stress then drops dramatically, followed by low strain rates and cooling. Continued far-field deformation produces a quasi-periodic series of such instabilities.

Why is lawsonite eclogite so rare? Metamorphism and preservation of lawsonite eclogite, Sivrihisar, Turkey

A rare occurrence of lawsonite eclogite crops out in a belt of high-pressure rocks in the Sivrihisar Massif, Turkey. Although lawsonite eclogite is predicted to be common at depths of ∼45–300 km in subduction zones, lawsonite seldom survives exhumation. In the Sivrihisar Massif, lawsonite eclogite occurs as 5-cm-long to 3.5-m-long pods in lawsonite blueschist and blueschist facies quartzite and marble. We have identified >70 eclogite pods within an ∼14 km 2 area; most of them are lawsonite bearing. Phase diagrams calculated for Sivrihisar eclogite bulk compositions indicate metamorphic conditions of 21– 24 kbar, ∼422–580 °C. High-pressure minerals (omphacite, lawsonite) are synkinematic with respect to the main fabric, and therefore preserve chemical and structural features of high-pressure subduction metamorphism. The presence of both pristine and retrogressed lawsonite eclogite within meters of each other suggests that lawsonite eclogite preservation is related to rapid exhumation and other factors. For example, the effects of exhumation-related deformation ± fluid infiltration may locally overprint some rocks, producing clinozoisite and/or epidote, whereas rocks that escape these effects retain primary lawsonite.

Regional variations in the diffusion of triggered seismicity

We determine the spatiotemporal characteristics of interearthquake triggering in the International Seismological Centre catalogue on regional and global scales. We pose a null hypothesis of spatially clustered, temporally random seismicity, and determine a residual pair correlation function for triggered events against this background. We compare results from the eastern Mediterranean, 25 Flinn‐Engdahl seismic regions, and the global data set. The null hypothesis cannot be rejected for distances greater than ∼150 km, providing an upper limit to triggering distances that can be distinguished from temporally uncorrelated seismicity in the stacked data at present. Correlation lengths L and mean distances between triggered events 〈r〉 are on the order of 10–50 km, but can be as high as 100 km in subduction zones. These values are not strongly affected by magnitude threshold, but are comparable to seismogenic thicknesses, implying a strong thermal control on correlation lengths. The temporal evolution of L and 〈r〉 is well fitted by a power law, with an exponent H ∼ 0.1 ± 0.05. This is much lower than the value H = 0.5 expected for Gaussian diffusion in a homogenous medium. We observe clear regional variations in L, 〈r〉 and H that appear to depend on tectonic setting. A detectable transition to a more rapid diffusion regime occurs in some cases at times greater than 100–200 days, possibly due to viscoelastic processes in the ductile lower crust.

Numerical model of the effect of serpentinites on the exhumation of eclogitic rocks: insights from the Monviso ophiolitic massif (Western Alps)

The intimate association of serpentinite mélange with eclogites worldwide suggests that serpentinites may have played an important role in the exhumation of eclogites. Serpentinites may have formed by fluid percolation in the mantle wedge or by underplating of oceanic serpentinites. We have tested numerically the role of serpentinites in the exhumation of eclogites using physical parameters obtained in the Monviso massif of the Western Alps. Our calculation shows that the buoyancy of serpentinized peridotites in dense anhydrous peridotites may contribute to the exhumation of eclogitic blocks. Small eclogitic blocks (<km 3) may be exhumed in completely hydrated mantle wedge, but it is not common considering the absence of totally serpentinized peridotites in mantle wedges. The exhumation of large eclogitic blocks (>50 km 3) requires a viscosity of the serpentinite wedge to be between 10 20 and 10 21 Pa and a density difference between the serpentinite wedge and the surrounding mantle on the order of 100–400 kg m −3. This condition is satisfied by only partially hydrated peridotites containing 10–50% serpentinites. Such serpentinized peridotites are expected to form at a depth ranging from 20 to 60 km in subduction zones by dehydration of subducting plates. Our conclusion is supported by the common occurrences of partially hydrated peridotites in close association with high-pressure to ultra-high-pressure metamorphic rocks.

Beryllium and boron in subduction zone minerals: An ion microprobe study

Secondary ion mass spectrometry was used to measure the distribution of Be, B, Rb, Sr, Y, Nb, Ba, and Ce among minerals in rocks from the Franciscan Complex and Catalina Schist paleosubduction zone terranes. The rocks examined represent a wide range of metamorphic pressure-temperature conditions (6–15 kb, 300–750°C) and include metasomatized and migmatitic samples. White mica is the primary host of Be (2.5 ppm) and B (107 ppm) in the rocks examined (average concentrations). Amphibole (Be 0.7, B 15 ppm), lawsonite (Be 0.9, B 37 ppm), and biotite (Be 0.9, B 25 ppm) may also contain significant amounts of both Be and B. Omphacite may host Be (4.1 ppm in veins, 0.7 ppm groundmass) and chlorite is a minor host of B (22 ppm). Minerals in these subduction zone metamorphic rocks that lack measurable Be or B include albite, apatite, calcite, clinozoisite, epidote, garnet, ilmenite, sphene, stilpnomelane, plagioclase, quartz, rutile, and zoisite. White mica, biotite, and stilpnomelane are the major mineral hosts of Rb (90, 300, and 30 ppm, respectively) and Ba (2100, 1000, and 4700 ppm). Epidote and lawsonite are the major host minerals of Sr (600 and 300 ppm) and Ce (12 and 15 ppm). Garnet, epidote, lawsonite, and sphene host Y (100, 30, 40, and 60 ppm, respectively). Sphene (77 ppm, 1 analysis), ilmenite, and rutile host Nb. Evidence of veining and metasomatism is strongly correlated with Be and/or B enrichment of white mica, omphacite, amphibole, and lawsonite at the P-T conditions examined. Both Be and B appear to be transported in aqueous fluids at depths of 20–50 km in subduction zones. The transfer of Be, B, and other elements from the subducted slab into the mantle wedge can be accomplished by aqueous fluids. Transfer of the elements hosted by white mica could result directly from the breakdown of white mica through pressure-dependent dehydration reactions or, alternatively, the Be and B in white mica could be partitioned into infiltrating fluids produced at deeper levels in the slab and subsequently transported into the region of are magma genesis.

Geometry and mechanisms of fluid flow at 15 to 45 kilometer depths in an Early Cretaceous Accretionary Complex

The Catalina Schist (southern California), contains variably metamorphosed mafic, sedimentary, and ultramafic rocks that were accreted at depths of 15 to 45 km during early Cretaceous subduction. Fluid flow in the Catalina Schist was concentrated along fractures and shear zones, as indicated by the textures and abundance of veins and evidence in melange zones for metasomatism and homogenization of stable isotope compositions. Slab‐parallel orientations of major melange zones with enhanced permeability may dictate that the majority of fluid released by devolatilization at depths &gt;15 km in subduction zones moves updip toward the seafloor. Such transport could contribute to fluid budgets in shallower parts of accretionary wedges and facilitate large‐scale mass and energy transfer in forearc regions.