Structural characteristics off Miyagi forearc region, the Japan Trench seismogenic zone, deduced from a wide-angle reflection and refraction study

The Japan Trench subduction zone, located east of NE Japan, has regional variation in seismicity. Many large earthquakes occurred in the northern part of Japan Trench, but few in the southern part. Off Miyagi region is in the middle of the Japan Trench, where the large earthquakes (M > 7) with thrust mechanisms have occurred at an interval of about 40 years in two parts: inner trench slope and near land. A seismic experiment using 36 ocean bottom seismographs (OBS) and a 12,000 cu. in. airgun array was conducted to determine a detailed, 2D velocity structure in the forearc region off Miyagi. The depth to the Moho is 21 km, at 115 km from the trench axis, and becomes progressively deeper landward. The P-wave velocity of the mantle wedge is 7.9–8.1 km/s, which is typical velocity for uppermost mantle without large serpentinization. The dip angle of oceanic crust is increased from 5–6° near the trench axis to 23° 150 km landward from the trench axis. The P-wave velocity of the oceanic uppermost mantle is as small as 7.7 km/s. This low-velocity oceanic mantle seems to be caused by not a lateral anisotropy but some subduction process. By comparison with the seismicity off Miyagi, the subduction zone can be divided into four parts: 1) Seaward of the trench axis, the seismicity is low and normal fault-type earthquakes occur associated with the destruction of oceanic lithosphere. 2) Beneath the deformed zone landward of the trench axis, the plate boundary is characterized as a stable sliding fault plain. In case of earthquakes, this zone may be tsunamigenic. 3) Below forearc crust where P-wave velocity is almost 6 km/s and larger: this zone is the seismogenic zone below inner trench slope, which is a plate boundary between the forearc and oceanic crusts. 4) Below mantle wedge: the rupture zones of thrust large earthquakes near land (e.g. 1978 off Miyagi earthquake) are located beneath the mantle wedge. The depth of the rupture zones is 30–50 km below sea level. From the comparison, the rupture zones of large earthquakes off Miyagi are limited in two parts: plate boundary between the forearc and oceanic crusts and below mantle wedge. This limitation is a rare case for subduction zone. Although the seismogenic process beneath the mantle wedge is not fully clarified, our observation suggests the two possibilities: earthquake generation at the plate boundary overridden by the mantle wedge without serpentinization or that in the subducting slab.     

The dynamics of big mantle wedge, magma factory, and metamorphic–metasomatic factory in subduction zones

The East Asian continental margin is underlain by stagnant slabs resulting from subduction of the Pacific plate from the east and the Philippine Sea plate from the south. We classify the upper mantle in this region into three major domains: (a) metasomatic–metamorphic factory (MMF), subduction zone magma factory (SZMF), and the ‘big mantle wedge’ (BMW). Whereas the convection pattern is anticlockwise in the MMF domain, it is predominantly clockwise in the SZMF and BMW, along a cross section from the south. Here we define the MMF as a small wedge corner which is driven by the subducting Pacific plate and dominated by H₂O-rich fluids derived by dehydration reactions, and enriched in large ion lithophile elements (LILE) which cause the metasomatism. The SZMF is a zone intermediate between MMF and BMW domains and constitutes the main region of continental crust production by partial melting through wedge counter-corner flow. Large hydrous plume generated at about 200 km depth causes extensive reduction in viscosity and the smaller scale hydrous plumes between 60 km and 200 km also bring about an overall reduction in the viscosity of SZMF. More fertile and high temperature peridotites are supplied from the entrance to this domain. The domain extends obliquely to the volcanic front and then swings back to the deep mantle together with the subducting slab. The BMW occupies the major portion of upper mantle in the western Pacific and convects largely with a clockwise sense removing the eastern trench oceanward. Sporadic formation of hydrous plume at the depth of around 410 km and the curtain flow adjacent to the trench cause back arc spreading. We envisage that the heat source in BMW could be the accumulated TTG (tonalite–trondhjemite–granodiorite) crust on the bottom of the mantle transition zone. The ongoing process of transportation of granitic crust into the mantle transition zone is evident from the deep subduction of five intra-oceanic arcs on the subducting Philippine Sea plate from the south, in addition to the sediment trapped subduction by the Pacific plate and Philippine Sea plate. The dynamics of MMF, SZMF and BMW domains are controlled by the angle of subduction; a wide zone of MMF in SW Japan is caused by shallow angle subduction of the Philippine Sea plate and the markedly small MMF domain in the Mariana trench is due to the high angle subduction of Pacific plate. The domains in NE Japan and Kyushu region are intermediate between these two. During the Tertiary, a series of marginal basins were formed because of the nearly 2000 km northward shift of the subduction zone along the southern margin of Tethyan Asia, which may be related to the collision of India with Asia and the indentation. The volume of upper mantle under Asia was reduced extensively on the southern margin with a resultant oceanward trench retreat along the eastern margin of Asia, leading to the formation of a series of marginal basins. The western Pacific domain in general is characterized by double-sided subduction; from the east by the oldest Pacific plate and from the south by the oldest Indo-Australian plate. The old plates are hence hydrated extensively even in their central domains and therefore of low temperature. The cracks have allowed the transport of water into the deeper portions of the slab and these domains supply hydrous fluids even to the bottom of the upper mantle. Thus, a fluid dominated upper mantle in the western Pacific drives a number of microplates and promote the plate boundary processes.     

Comparison of gravity anomaly between mature and immature intra-oceanic subduction zones in the western Pacific

The western Pacific hosts major subduction systems such as Izu–Bonin–Mariana and Tonga–Kermadec, but also less conspicuous systems such as Yap, Mussau and Hjort trenches which constitute the young, incomplete, or ultraslow-member in the evolutionary spectrum of subduction zones. We used satellite-derived gravity data to compare well-developed and immature subduction systems. It is shown that at spatial resolution > 10–20 km or so, the satellite data have accuracy comparable to ship-board gravity measurements over intra-oceanic subduction zones. In the isostatic residual gravity anomaly map, the width of non-isostatically-compensated region of the mature subduction zones is much wider than that of immature ones. More importantly, when the gravitational attraction due to seafloor is removed, a large difference exists between the mature and immature subduction zones in the overriding plate side. Mature subduction zones exhibit broad low gravity anomalies of ~ 200–250 mGal centered at distances of 150–200 km from the trench which are not found over immature subduction zones. The cause of the broad low gravity anomalies over mature subduction zones is debatable due to lack of information on the deep crust and upper mantle structure and property. We discuss the following four causes: (1) serpentinization of the upper mantle beneath the forearc; (2) presence of partial melt in the mantle wedge caused by release of volatiles from the slab, frictional heating and distributed by mantle circulation; (3) difference in density structure between the overriding and subducting plates caused by difference in age and thermal structures with and without compositional stratification between crust and mantle; and (4) anomalous thickness of the arc not explained by isostasy. Our analysis suggests that serpentinization cannot explain the observed gravity anomaly which appears ~ 150–200 km from the trench. Although the extent and distribution of partial melt within the mantle wedge remain in question, to our best estimate, partial melting contributes little (< 50 mGal) to the total negative gravity anomaly. The difference in density structure reflecting temperature difference can only explain less than half of the low gravity anomaly. The sinking of lighter crustal material produces a large negative anomaly in the forearc but its location does not match the observed gravity anomaly. It appears that one cannot explain the total difference in gravity anomaly without invoking anomalous thickness of the arc. Although we could not identify the sole or combination of factors that give rise to the low gravity anomaly in mature subduction zones, the comparison of gravity anomalies between mature and immature subduction zones is likely to provide an important constraint for understanding the evolution and structure of subduction zones as more complementary evidences become available.     

Pressure–temperature conditions of ongoing regional metamorphism beneath the Japanese Islands

We evaluate the pressure–temperature (P–T) conditions of ongoing regional metamorphism at the top of the oceanic crust of the subducted Pacific and Philippine Sea plates through a combination of phase diagrams and hypocenter distribution and based on the dehydration-induced earthquake hypothesis. The brute-force method was employed to find the best match thermal structure to link the hypocenter distribution and dehydration. The estimated thermal structure varies far from the values obtained from numerical simulation. Our estimates are consistent with the qualitative physical prediction for the variation of temperature in different subduction zones and provide quantitative constraints for the models.In northeastern Japan, the P–T path for the top of the oceanic crust turns to the high-T side at a depth of around 90 km. The depth corresponds to the location of the volcanic front and an active convection of the wedge mantle below this depth is suggested. Our computations also reveal the effect of an exceptional scenario beneath the Kanto region. The temperature in the Kanto region, where the cold lid of the Philippine Sea plate prevents heating by the return-flow of mantle wedge above, is much lower than that of northeastern Japan. The subduction of younger Philippine Sea plate leads to a higher-temperature in the oceanic crust. In the central Shikoku region, the thermal structure exhibits high-T/P nature. Heating by shear deformation can explain the high-T/P path in the depth range from 20 to 35 km. The Kyushu area shows moderate type T/P path reaching up to eclogite facies conditions. In the Kii and central Shikoku region, the thermal structure exhibits high-T/P nature. However, the absolute values for the areas seem to have problem in physical context. Our results has risen the significance of sediment subduction in the southwest Japan and requirement for further improvements in this technique including the aspect of variation of the bulk composition of the subducted material.     

Depths to density discontinuities beneath the Adamawa Plateau region, Central Africa, from spectral analyses of new and existing gravity data

New gravity data from the Adamawa Uplift region of Cameroon have been integrated with existing gravity data from central and western Africa to examine variations in crustal structure throughout the region. The new data reveal steep northeast-trending gradients in the Bouguer gravity anomalies that coincide with the Sanaga Fault Zone and the Foumban Shear Zone, both part of the Central African Shear Zone lying between the Adamawa Plateau and the Congo Craton. Four major density discontinuities in the lithosphere have been determined within the lithosphere beneath the Adamawa Uplift in central Cameroon using spectral analysis of gravity data: (1) 7–13 km; (2) 19–25 km; (3) 30–37 km; and (4) 75–149 km. The deepest density discontinuities determined at 75–149 km depth range agree with the presence of an anomalous low velocity upper mantle structure at these depths deduced from earlier teleseismic delay time studies and gravity forward modelling. The 30–37 km depths agree with the Moho depth of 33 km obtained from a seismic refraction experiment in the region. The intermediate depth of 20 km obtained within region D may correspond to shallower Moho depth beneath parts of the Benue and Yola Rifts where seismic refraction data indicate a crustal thickness of 23 km. The 19–20 km depths and 8–12 km depths estimated in boxes encompassing the Adamawa Plateau and Cameroon Volcanic Line may may correspond to mid-crustal density contrasts associated with volcanic intrusions, as these depths are less than depths of 25 and 13 km, respectively, in the stable Congo Craton to the south.     

Upper mantle structure of the South American continent and neighboring oceans from surface wave tomography

We present a new three-dimensional SV-wave velocity model for the upper mantle beneath South America and the surrounding oceans, built from the waveform inversion of 5850 Rayleigh wave seismograms. The dense path coverage and the use of higher modes to supplement the fundamental mode of surface waves allow us to constrain seismic heterogeneities with horizontal wavelengths of a few hundred kilometres in the uppermost 400 km of the mantle.The large scale features of our tomographic model confirm previous results from global and regional tomographic studies (e.g. the depth extent of the high velocity cratonic roots down to about 200–250 km).Several new features are highlighted in our model. Down to 100 km depth, the high velocity lid beneath the Amazonian craton is separated in two parts associated with the Guyana and Guapore shields, suggesting that the rifting episode responsible for the formation of the Amazon basin has involved a significant part of the lithosphere. Along the Andean subduction belt, the structure of the high velocity anomaly associated with the sudbduction of the Nazca plate beneath the South American plate reflects the along-strike variation in dip of the subducting plate. Slow velocities are observed down to about 100 km and 150 km at the intersection of the Carnegie and Chile ridges with the continent and are likely to represent the thermal anomalies associated with the subducted ridges. These lowered velocities might correspond to zones of weakness in the subducted plate and may have led to the formation of “slab windows” developed through unzipping of the subducted ridges; these windows might accommodate a transfer of asthenospheric mantle from the Pacific to the Atlantic ocean. From 150 to 250 km depth, the subducting Nazca plate is associated with high seismic velocities between 5°S and 37°S. We find high seismic velocities beneath the Paraná basin down to about 200 km depth, underlain by a low velocity anomaly in the depth range 200–400 km located beneath the Ponta Grossa arc at the southern tip of the basin. This high velocity anomaly is located southward of a narrow S-wave low velocity structure observed between 200 and 500–600 km depth in body wave studies, but irresolvable with our long period datasets. Both anomalies point to a model in which several, possibly diachronous, plumes have risen to the surface to generate the Paraná large igneous province (LIP).     

Crustal and upper mantle seismic structure of the Australian Plate, South Island, New Zealand

Seismic reflection and refraction data were collected west of New Zealand's South Island parallel to the Pacific–Australian Plate boundary. The obliquely convergent plate boundary is marked at the surface by the Alpine Fault, which juxtaposes continental crust of each plate. The data are used to study the crustal and uppermost mantle structure and provide a link between other seismic transects which cross the plate boundary. Arrival times of wide-angle reflected and refracted events from 13 recording stations are used to construct a 380-km long crustal velocity model. The model shows that, beneath a 2–4-km thick sedimentary veneer, the crust consists of two layers. The upper layer velocities increase from 5.4–5.9 km/s at the top of the layer to 6.3 km/s at the base of the layer. The base of the layer is mainly about 20 km deep but deepens to 25 km at its southern end. The lower layer velocities range from 6.3 to 7.1 km/s, and are commonly around 6.5 km/s at the top of the layer and 6.7 km/s at the base. Beneath the lower layer, the model has velocities of 8.2–8.5 km/s, typical of mantle material. The Mohorovicic discontinuity (Moho) therefore lies at the base of the second layer. It is at a depth of around 30 km but shallows over the south–central third of the profile to about 26 km, possibly associated with a southwest dipping detachment fault. The high, variable sub-Moho velocities of 8.2 km/s to 8.5 km/s are inferred to result from strong upper mantle anisotropy. Multichannel seismic reflection data cover about 220 km of the southern part of the modelled section. Beneath the well-layered Oligocene to recent sedimentary section, the crustal section is broadly divided into two zones, which correspond to the two layers of the velocity model. The upper layer (down to about 7–9 s two-way travel time) has few reflections. The lower layer (down to about 11 s two-way time) contains many strong, subparallel reflections. The base of this reflective zone is the Moho. Bi-vergent dipping reflective zones within this lower crustal layer are interpreted as interwedging structures common in areas of crustal shortening. These structures and the strong northeast dipping reflections beneath the Moho towards the north end of the (MCS) line are interpreted to be caused by Paleozoic north-dipping subduction and terrane collision at the margin of Gondwana. Deeper mantle reflections with variable dip are observed on the wide-angle gathers. Travel-time modelling of these events by ray-tracing through the established velocity model indicates depths of 50–110 km for these events. They show little coherence in dip and may be caused side-swipe from the adjacent crustal root under the Southern Alps or from the upper mantle density anomalies inferred from teleseismic data under the crustal root.     

Three-dimensional P- and S-wave velocity structures beneath the Japan Islands obtained by high-density seismic stations by seismic tomography

We construct fine-scale 3D P- and S-wave velocity structures of the crust and upper mantle beneath the whole Japan Islands with a unified resolution, where the Pacific (PAC) and Philippine Sea (PHS) plates subduct beneath the Eurasian (EUR) plate. We can detect the low-velocity (low-V) oceanic crust of the PAC and PHS plates at their uppermost part beneath almost all the Japan Islands. The depth limit of the imaged oceanic crust varies with the regions. High-V_P/V_S zones are widely distributed in the lower crust especially beneath the volcanic front, and the high strain rate zones are located at the edge of the extremely high-V_P/V_S zone; however, V_P/V_S at the top of the mantle wedge is not so high. Beneath northern Japan, we can image the high-V subducting PAC plate using the tomographic method without any assumption of velocity discontinuities. We also imaged the heterogeneous structure in the PAC plate, such as the low-V zone considered as the old seamount or the highly seismic zone within the double seismic zone where the seismic fault ruptured by the earthquake connects the upper and lower layer of the double seismic zone. Beneath central Japan, thrust-type small repeating earthquakes occur at the boundary between the EUR and PHS plates and are located at the upper part of the low-V layer that is considered to be the oceanic crust of the PHS plate. In addition to the low-V oceanic crust, the subducting high-V PAC plate is clearly imaged to depths of approximately 250 km and the subducting high-V PHS zone to depths of approximately 180 km is considered to be the PHS plate. Beneath southwestern Japan, the iso-depth lines of the Moho discontinuity in the PHS plate derived by the receiver function method divide the upper low-V layer and lower high-V layer of our model at depths of 30–50 km. Beneath Kyushu, the steeply subducting PHS plate is clearly imaged to depths of approximately 250 km with high velocities. The high-V_P/V_S zone is considered as the lower crust of the EUR plate or the oceanic crust of the PHS plate at depths of 25–35 km and the partially serpentinized mantle wedge of the EUR plate at depths of 30–45 km beneath southwestern Japan. The deep low-frequency nonvolcanic tremors occur at all parts of the high-V_P/V_S zone—within the zone, the seaward side, and the landward side where the PHS plate encounters the mantle wedge of the EUR plate. We prove that we can objectively obtain the fine-scale 3D structure with simple constraints such as only 1D initial velocity model with no velocity discontinuity.     

Imag(in)ing the continental lithosphere

This paper is primarily concerned with seismically imaging details in the mantle at an intermediate scale length between the large scales of regional and global tomography and the small scales of reflection profiles and outcrops. This range is roughly 0.1–1 km < a < 10–10² km, where a is the scale. We consider the implications of several models for mantle evolution in a convecting mantle, and possible scales present in the non-convecting tectosphere. Reflection seismic evidence shows that the structures preserved from continental accretion within and at the margins of the Archean cratons are subduction related, and we use subduction as an analog for scales left by past events. In modern orogenic belts we expect to find subduction structures, small scale upper mantle convection structures, and basalt extraction structures. We examine some of the scales that are likely formed by orogenic processes.We also examine the seismic velocity and density contrasts expected between various upper mantle constituents, including fertile upper mantle, depleted upper mantle, normal and eclogitized oceanic crust, and fertile mantle with and without partial melt. This leads directly to predicting the size of seismic signals that can be produced by specular conversion, and scattering from layers and objects with these contrasts.We introduce an imaging scheme that makes use of scattered waves in teleseismic receiver functions to make a depth migrated image of a pseudo-scattering coefficient. Image resolution is theoretically at least an order of magnitude better than traveltime tomography. We apply the imaging scheme to three data sets from 1) the Kaapvaal craton, 2) the Cheyenne Belt, a Paleoproterozoic suture between a protocontinent and an island arc, and 3) the Jemez Lineament, a series of aligned modern volcanic structures at the site of a Proterozoic suture zone. The Kaapvaal image, although not defining a unique base of the tectosphere, shows complicated “layered” events in the region defined as the base of the tectosphere in tomography images. The image of the transition zone discontinuities beneath the Kaapvaal craton is remarkable for clarity. The migrated receiver function image of the upper mantle beneath the Cheyenne belt is complicated, more so than the tomography image, and may indicate limitations in the receiver function imaging system. In contrast the Jemez Lineament image shows large-amplitude negative-polarity layered events beneath the Moho to depths of 120 km, that we interpret as melt-containing sills in the upper mantle. These sills presumably feed the Quaternary–Neogene regional basaltic volcanic field.     

2D model of the crust and uppermost mantle along rift profile, Siberian craton

A two-dimensional model of the crust and uppermost mantle for the western Siberian craton and the adjoining areas of the Pur-Gedan basin to the north and Baikal Rift zone to the south is determined from travel time data from recordings of 30 chemical explosions and three nuclear explosions along the RIFT deep seismic sounding profile. This velocity model shows strong lateral variations in the crust and sub-Moho structure both within the craton and between the craton and the surrounding region. The Pur-Gedan basin has a 15-km thick, low-velocity sediment layer overlying a 25-km thick, high-velocity crystalline crustal layer. A paleo-rift zone with a graben-like structure in the basement and a high-velocity crustal intrusion or mantle upward exists beneath the southern part of the Pur-Gedan basin. The sedimentary layer is thin or non-existent and there is a velocity reversal in the upper crust beneath the Yenisey Zone. The Siberian craton has nearly uniform crustal thickness of 40–43 km but the average velocity in the lower crust in the north is higher (6.8–6.9 km/s) than in the south (6.6 km/s). The crust beneath the Baikal Rift zone is 35 km thick and has an average crustal velocity similar to that observed beneath the southern part of craton. The uppermost mantle velocity varies from 8.0 to 8.1 km/s beneath the young West Siberian platform and Baikal Rift zone to 8.1–8.5 km/s beneath the Siberian craton. Anomalous high Pn velocities (8.4–8.5 km/s) are observed beneath the western Tunguss basin in the northern part of the craton and beneath the southern part of the Siberian craton, but lower Pn velocities (8.1 km/s) are observed beneath the Low Angara basin in the central part of the craton. At about 100 km depth beneath the craton, there is a velocity inversion with a strong reflecting interface at its base. Some reflectors are also distinguished within the upper mantle at depth between 230 and 350 km.     

Local thickening of the Cascadia forearc crust and the origin of seismic reflectors in the uppermost mantle

Seismic reflection profiles from three different surveys of the Cascadia forearc are interpreted using P wave velocities and relocated hypocentres, which were both derived from the first arrival travel time inversion of wide-angle seismic data and local earthquakes. The subduction decollement, which is characterized beneath the continental shelf by a reflection of 0.5 s duration, can be traced landward into a large duplex structure in the lower forearc crust near southern Vancouver Island. Beneath Vancouver Island, the roof thrust of the duplex is revealed by a 5–12 km thick zone, identified previously as the E reflectors, and the floor thrust is defined by a short duration reflection from a < 2-km-thick interface at the top of the subducting plate. We show that another zone of reflectors exists east of Vancouver Island that is approximately 8 km thick, and identified as the D reflectors. These overlie the E reflectors; together the two zones define the landward part of the duplex. The combined zones reach depths as great as 50 km. The duplex structure extends for more than 120 km perpendicular to the margin, has an along-strike extent of 80 km, and at depths between 30 km and 50 km the duplex structure correlates with a region of anomalously deep seismicity, where velocities are less than 7000 m s^− 1. We suggest that these relatively low velocities indicate the presence of either crustal rocks from the oceanic plate that have been underplated to the continent or crustal rocks from the forearc that have been transported downward by subduction erosion. The absence of seismicity from within the E reflectors implies that they are significantly weaker than the overlying crust, and the reflectors may be a zone of active ductile shear. In contrast, seismicity in parts of the D reflectors can be interpreted to mean that ductile shearing no longer occurs in the landward part of the duplex. Merging of the D and E reflectors at 42–46 km depth creates reflectivity in the uppermost mantle with a vertical thickness of at least 15 km. We suggest that pervasive reflectivity in the upper mantle elsewhere beneath Puget Sound and the Strait of Georgia arises from similar shear zones.     

Stress field in the Ryukyu–Kyushu Wadati–Benioff zone by inversion of earthquake focal mechanisms

The study addresses the space distribution of the stress field in the Kyushu–Ryukyu Wadati–Benioff zone based on homogeneous data of earthquake focal mechanisms and the inverse technique by Gephart and Forsyth [J. Geophys. Res. 89 (1984) 9305]. The used data set consists of 148 Harvard CMT solutions and 22 earthquake focal mechanisms listed in previous studies. The stress field parameters are determined for 0–40, 41–100 and h>100 km depth ranges. The top 100-km layer of the Wadati–Benioff zone (WBZ) is characterized by strike normal maximum compression σ₁ and steeper than the slab minimum compression σ₃, the last indicating for unbalanced slab pull force. The Tokara channel ‘divides’ the subduction into two parts of different stress regime at depth greater than 100 km. To the south of the channel the slab is under slab parallel σ₁ and slab normal σ₃ while its northern part, beneath Kyushu, is under slab parallel extension and slab normal compression. The results of recent studies on the regional velocity structure and geochemistry of the volcanic lava indicate that the most plausible reason for the observed stress field difference below 100 km in the northern and rest part of the arc is the presence of hot low viscosity upper mantle west of Kyushu.The results of this study indicate that the forces involved in the contemporary subduction dynamics in the Ryukyu–Kyushu Wadati–Benioff zone are related to the convergence between the Philippine Sea Plate and the Eurasian plate, the trench suction force, slab pull, the slab anchor force and, in the southern-central part of the arc, mantle resistance.     

Pull-down basin in the central part of Japan due to subduction-induced mantle flow

The driving force for the basin subsiding against isostatic balance in and around Lake Biwa in the Kinki district, Japan is discussed. The lake region is characterized by strong negative Bouguer anomalies, especially by a steep horizontal gradient zone of gravity anomaly running along the western margin of the lake. The large negative anomaly (>50 mgal) cannot be explained by low-density sediments beneath it. A down-warping structure extending to the Moho depth should be taken into account. This conjecture has been strongly supported by a short-period receiver function imaging, which shows a clear offset of about 8 km for the Moho discontinuity under the steep gravity gradient zone.A question arises as to what is the driving force to create such a large down-warping structure. We consider that the subduction of the shallow-dipping slab under the region (Philippine Sea Slab) may cause crustal deformation by dragging the viscous mantle downward. In order to verify this model, we simulated the induced mantle flow due to the subduction of the Philippine Sea Slab and the pressure distribution on the crust–mantle boundary. This numerical experiment showed that the induced flow makes a strong negative pressure zone under the lake region if the slab has a vertical offset along the direction of subduction. This offset of the slab is consistent with plate models deduced from hypocentral distributions and Sp phases of the deep-focus earthquakes.     

Crustal contributions to arc magmatism in the Andes of Central Chile

Fifteen andesite-dacite stratovolcanoes on the volcanic front of a single segment of the Andean arc show along-arc changes in isotopic and elemental ratios that demonstrate large crustal contributions to magma genesis. All 15 centers lie 90 km above the Benioff zone and 280±20 km from the trench axis. Rate and geometry of subduction and composition and age of subducted sediments and seafloor are nearly constant along the segment. Nonetheless, from S to N along the volcanic front (at 57.5% SiO₂) K₂O rises from 1.1 to 2.4 wt %, Ba from 300 to 600 ppm, and Ce from 25 to 50 ppm, whereas FeO^*/MgO declines from >2.5 to 1.4. Ce/Yb and Hf/Lu triple northward, in part reflecting suppression of HREE enrichment by deep-crustal garnet. Rb, Cs, Th, and U contents all rise markedly from S to N, but Rb/Cs values double northward — opposite to prediction were the regional alkali enrichment controlled by sediment subduction. K/Rb drops steeply and scatters greatly within many (biotite-free) andesitic suites. Wide diversity in Zr/Hf, Zr/Rb, Ba/Ta, and Ba/La within and among neighboring suites (which lack zircon and alkali feldspar) largely reflects local variability of intracrustal (not slab or mantle) contributions. Pb-isotope data define a limited range that straddles the Stacey-Kramers line, is bracketed by values of local basement rocks, in part plots above the field of Nazca plate sediment, and shows no indication of a steep (mantle+sedimentary) Pb mixing trend. ⁸⁷Sr/⁸⁶Sr values rise northward from 0.7036 to 0.7057, and ¹⁴³Nd/¹⁴⁴Nd values drop from 0.5129 to 0.5125. A northward climb in basal elevation of volcanic-front edifices from 1350 m to 4500 m elevation coincides with a Bougueranomaly gradient from –95 to –295 mgal, interpreted to indicate thickening of the crust from 30–35 km to 50–60 km. Complementary to the thickening crust, the mantle wedge beneath the front thins northward from about 60 km to 30–40 km (as slab depth is constant). The thick northern crust contains an abundance of Paleozoic and Triassic rocks, whereas the proportion of younger arc-intrusive basement increases southward. Primitive basalts are unknown anywhere along the arc. Base-level isotopic and chemical values for each volcano are established by blending of subcrustal and deep-crustal magmas in zones of melting, assimilation, storage and homogenization (MASH) at the mantle-crust transition. Scavenging of mid-to upper-crustal silicic-alkalic melts and intracrustal AFC (prominent at the largest center) can subsequently modify ascending magmas, but the base-level geochemical signature at each center reflects the depth of its MASH zone and the age, composition, and proportional contribution of the lowermost crust.     

Mantle flow-induced crustal thinning in the area between the easternmost part of the Anatolian plate and the Arabian Foreland (E Turkey) deduced from the geological and geophysical data

Eastern Anatolia consisting of an amalgamation of fragments of oceanic and continental lithosphere is a current active intercontinental contractional zone that is still being squeezed and shortened between the Arabian and Eurasian plates. This collisional and contractional zone is being accompanied by the tectonic escape of most of the Anatolian plate to the west by major strike-slip faulting on the right-lateral North Anatolian Transform Fault Zone (NATFZ) and left-lateral East Anatolian Transform Fault Zone (EATFZ) which meet at Karlıova forming an east-pointing cusp. The present-day crust in the area between the easternmost part of the Anatolian plate and the Arabian Foreland gets thinner from north (ca 44 km) to south (ca 36 km) relative to its eastern (EAHP) and western sides (central Anatolian region). This thinner crustal area is characterized by shallow CPD (12–16 km), very low Pn velocities (< 7.8 km/s) and high Sn attenuation which indicate partially molten to eroded mantle lid or occurrence of asthenospheric mantle beneath the crust. Northernmost margin of the Arabian Foreland in the south of the Bitlis–Pötürge metamorphic gap area is represented by moderate CPD (16–18 km) relative to its eastern and western sides, and low Pn velocities (8 km/s). We infer from the geophysical data that the lithospheric mantle gets thinner towards the Bitlis–Pötürge metamorphic gap area in the northern margin of the Arabian Foreland which has been most probably caused by mechanical removal of the lithospheric mantle during mantle invasion to the north following the slab breakoff beneath the Bitlis–Pötürge Suture Zone. Mantle flow-driven rapid extrusion and counterclockwise rotation of the Anatolian plate gave rise to stretching and hence crustal thinning in the area between the easternmost part of the Anatolian plate and the Arabian Foreland which is currently dominated by wrench tectonics.     

Middle Miocene to present plate tectonic history of the southern Central American Volcanic Arc

New mid Miocene to present plate tectonic reconstructions of the southern Central American Volcanic Arc (CAVA) reveal that the inception of Cocos Ridge subduction began no earlier than 3 Ma, and possibly as late as 2 Ma. The Cocos Ridge has been displaced from the Malpelo Ridge to the southeast since 9 Ma along the Panama Fracture Zone (PFZ) system. Ambiguous PFZ and Coiba Fracture Zone (CFZ) interaction since 9 Ma precludes conclusively establishing the age of initial Cocos Ridge subduction. Detailed reconstructions based on magnetic anomalies offshore reveal several other variations in subduction parameters beneath southern Central America that preceded subduction of the Cocos Ridge, including southeastward migration of the Nazca–Cocos–Caribbean triple junction along the Middle America Trench (MAT) from 12 Ma to present, and subduction of ≤2 km high scarps both parallel and perpendicular to the trench from 6 to 1 Ma.The timing of changes in subduction processes has commonly been determined by (and correlated with) geologic changes in the upper plate. However, reliable ⁴⁰Ar/³⁹Ar dating of these events has become available only recently [Abstr. Programs-Geol. Soc. Am. (2002)]. These new dates better constrain the magmatic and structural history of southern Costa Rica. Observations from this data set include: a gap in the volcanic record from 11 to 6 Ma, which coincides temporally with emplacement of most plutons in southern Costa Rica, normal arc volcanism ceased after 3.5 Ma in southern Costa Rica, and Pliocene (mostly 1.5 Ma) adakite volcanism was widely distributed from central Panama to southern Costa Rica (though volumetrically insignificant).This new data reveals that many geologic phenomena, commonly attributed to subduction and underplating of the buoyant Cocos Ridge, in fact precede inception of Cocos Ridge subduction and seem to correlate more favorably in time with earlier tectonic events. Adakite volcanic activity corresponds in space and time with the subduction of a large scarp associated with a tectonic boundary off southern Panama. Regional unconformities and an 11–6 Ma gap in arc volcanism match temporally with oblique subduction of the Nazca plate beneath central and southern Costa Rica. Cessation of volcanic activity, low-temperature cooling of plutons in the Cordillera de Talamanca (CT), and rapid increases in sedimentation in the fore-arc and back-arc basins coincide with passage of the Nazca–Cocos–Caribbean triple junction and initiation of subduction of “rough” crust associated with Cocos–Nazca rifting 3.5 Ma, closely followed by initial subduction of the Cocos Ridge 2–3 Ma. None of the aforementioned geologic events occurred at a time that would allow for underplating by the Cocos Ridge. Rather they are probably related to complex interactions with subduction of complicated plates offshore. All of the aforementioned events indicate that the southern Central American subduction system has been in flux since at least 12 Ma.     

Deep slab subduction and dehydration and their geodynamic consequences: Evidence from seismology and mineral physics

We present new pieces of evidence from seismology and mineral physics for the existence of low-velocity zones in the deep part of the upper mantle wedge and the mantle transition zone that are caused by fluids from the deep subduction and deep dehydration of the Pacific and Philippine Sea slabs under western Pacific and East Asia. The Pacific slab is subducting beneath the Japan Islands and Japan Sea with intermediate-depth and deep earthquakes down to 600 km depth under the East Asia margin, and the slab becomes stagnant in the mantle transition zone under East China. The western edge of the stagnant Pacific slab is roughly coincident with the NE–SW Daxing'Anling-Taihangshan gravity lineament located west of Beijing, approximately 2000 km away from the Japan Trench. The upper mantle above the stagnant slab under East Asia forms a big mantle wedge (BMW). Corner flow in the BMW and deep slab dehydration may have caused asthenospheric upwelling, lithospheric thinning, continental rift systems, and intraplate volcanism in Northeast Asia. The Philippine Sea slab has subducted down to the mantle transition zone depth under Western Japan and Ryukyu back-arc, though the seismicity within the slab occurs only down to 200–300 km depths. Combining with the corner flow in the mantle wedge, deep dehydration of the subducting Pacific slab has affected the morphology of the subducting Philippine Sea slab and its seismicity under Southwest Japan. Slow anomalies are also found in the mantle under the subducting Pacific slab, which may represent small mantle plumes, or hot upwelling associated with the deep slab subduction. Slab dehydration may also take place after a continental plate subducts into the mantle.     

Double-planed deep seismic zone and anomalous structure in the upper mantle beneath northeastern Honshu (Japan)

Seismicity located by using the most recent data obtained from the high-gain seismograph network of Tohoku University shows that the deep seismic zone beneath northeastern Honshu, Japan, is composed of two thin planes which are parallel to each other and are 30–40 km apart. Focal mechanisms derived from the earthquakes in the upper plane are reverse-faulting, or, some of them, down-dip compression. As a contrast, those in the lower plane are down-dip extension. The location of the upper boundary of the descending lithospheric slab, inferred from the arrival-time difference between ScS and ScSp waves and from the travel-time anomaly of intermediate-depth earthquakes observed at the small-scale seismic array, coincides exactly with the upper plane of the double-planed deep seismic zone. Anelasticity (1/Q) structure of the upper mantle consists of three distinct zones: a high-Q (Qs− 1500) inclined lithospheric slab, an intennediate-Q (Qs−350) land-side mantle between the Pacific coast and the volcanic front, and a low-Q (Qs − 100) land-side mantle between the volcanic front and the coast of the Japan Sea.The evidence obtained here provides valuable information as to the definition of the type of mechanism producing the plate motion beneath island arcs.     

Seismic structure and origin of active intraplate volcanoes in Northeast Asia

Three-dimensional P-wave velocity structure beneath the Changbai and other intraplate volcanic areas in Northeast Asia is determined by inverting 1378 high-quality P-wave arrival times from 186 teleseismic events recorded by 61 broadband seismic stations. Low-velocity (low-V) anomalies are revealed beneath the Changbai, Longgan, Xianjindao volcanoes. High-velocity (high-V) anomalies are found in the mantle transition zone, where deep-focus earthquakes under Hunchun occur at depths of 500–600 km. The high-V anomaly reflects the deep subduction of the Pacific slab under NE Asia which may have contributed to the formation of the Changbai, Longgang, Xianjindao and Jingpohu intraplate volcanoes. A low-V anomaly is also revealed in the mantle transition zone, which may have a close relationship with the occurrence of deep earthquakes under the Hunchun area. Our results support the Big Mantle Wedge (BMW) model by Zhao et al. [Zhao, D., Lei, J., Tang, Y., 2004. Origin of the Changbai volcano in northeast China: evidence from seismic tomography, Chin. Sci. Bull. 49, 1401–1408; Zhao, D., Maruyama, S., Omori, S., 2007. Mantle dynamics of western Pacific and East Asia: insight from seismic tomography and mineral physics. Gondwana Res. 11, 120–131.] who proposed that the intraplate volcanoes in NE Asia are caused by the back-arc magmatism associated with the deep dehydration process of the subducting slab and convective circulation process in the BMW above the stagnant Pacific slab.     

Lithospheric gravitational instability beneath the Southeast Carpathians

The Southeast corner of the Carpathians, known as the Vrancea region, is characterised by a cluster of strong seismicity to depths of about 200 km. The peculiar features of this seismicity make it a region of high geophysical interest. In this study we calculate the seismic strain-rate tensors for the period 1967–2007, and describe the variation of strain-rate with depth. The observed results are compared with strain-rates predicted by numerical experiments. We explore a new dynamical model for this region based on the idea of viscous flow of the lithospheric mantle permitting the development of local continental mantle downwelling beneath Vrancea, due to a Rayleigh–Taylor instability that has developed since the cessation of subduction at 11 Ma. The model simulations use a Lagrangean frame 3D finite-element algorithm solving the equations of conservation of mass and momentum for a spatially varying viscous creeping flow. The finite deformation calculations of the gravitational instability of the continental lithosphere demonstrate that the Rayleigh–Taylor mechanism can explain the present distribution of deformation within the downwelling lithosphere, both in terms of stress localisation and amplitude of strain-rates. The spatial extent of the high stress zone that corresponds to the seismically active zone is realistically represented when we assume that viscosity decreases by at least an order of magnitude across the lithosphere. The mantle downwelling is balanced by lithospheric thinning in an adjacent area which would correspond to the Transylvanian Basin. Crustal thickening is predicted above the downwelling structure and thinning beneath the basin.