Tethyan evolution of Turkey: A plate tectonic approach

The Tethyan evolution of Turkey may be divided into two main phases, namely a Palaeo-Tethyan and a Neo-Tethyan, although they partly overlap in time. The Palaeo-Tethyan evolution was governed by the main south-dipping (present geographic orientation) subduction zone of Palaeo-Tethys beneath northern Turkey during the Permo-Liassic interval. During the Permian the entire present area of Turkey constituted a part of the northern margin of Gondwana-Land. A marginal basin opened above the subduction zone and disrupted this margin during the early Triassic. In this paper it is called the Karakaya marginal sea, which was already closed by earliest Jurassic times because early Jurassic sediments unconformably overlie its deformed lithologies. The present eastern Mediterranean and its easterly continuation into the Bitlis and Zagros oceans began opening mainly during the Carnian—Norian interval. This opening marked the birth of Neo-Tethys behind the Cimmerian continent which, at that time, started to separate from northern Gondwana-Land. During the early Jurassic the Cimmerian continent internally disintegrated behind the Palaeo-Tethyan arc constituting its northern margin and gave birth to the northern branch of Neo-Tethys. The northern branch of Neo-Tethys included the Intra-Pontide, Izmir—Ankara, and the Inner Tauride oceans. With the closure of Palaeo-Tethys during the medial Jurassic only two oceanic areas were left in Turkey: the multi-armed northern and the relatively simpler southern branches of Neo-Tethys. The northern branch separated the Anatolide—Tauride platform with its long appendage, the Bitlis—Pötürge fragment from Eurasia, whereas the southern one separated them from the main body of Gondwana-Land. The Intra-Pontide and the Izmir—Ankara oceans isolated a small Sakarya continent within the northern branch, which may represent an easterly continuation of the Paikon Ridge of the Vardar Zone in Macedonia. The Anatolide-Tauride platform itself constituted the easterly continuation of the Apulian platform that had remained attached to Africa through Sicily. The Neo-Tethyan oceans reached their maximum size during the early Cretaceous in Turkey and their contraction began during the early late Cretaceous. Both oceans were eliminated mainly by north-dipping subduction, beneath the Eurasian, Sakaryan, and the Anatolide- Tauride margins. Subduction beneath the Eurasian margin formed a marginal basin, the present Black Sea and its westerly prolongation into the Srednogorie province of the Balkanides, during the medial to late Cretaceous. This resulted in the isolation of a Rhodope—Pontide fragment (essentially an island arc) south of the southern margin of Eurasia. Late Cretaceous is also a time of widespread ophiolite obduction in Turkey, when the Bozkir ophiolite nappe was obducted onto the northern margin of the Anatolide—Tauride platform. Two other ophiolite nappes were emplaced onto the Bitlis—Pötürge fragment and onto the northern margin of the Arabian platform respectively. This last event occurred as a result of the collision of the Bitlis—Pötürge fragment with Arabia. Shortly after this collision during the Campanian—Maastrichtian, a subduction zone began consuming the floor of the Inner Tauride ocean just to the north of the Bitlis—Pötürge fragment producing the arc lithologies of the Yüksekova complex. During the Maastrichtian—Middle Eocene interval a marginal basin complex, the Maden and the Çüngüş basins began opening above this subduction zone, disrupting the ophiolite-laden Bitlis—Pötürge fragment. The Anatolide-Tauride platform collided with the Pontide arc system (Rhodope—Pontide fragment plus the Sakarya continent that collided with the former during the latest Cretaceous along the Intra Pontide suture) during the early to late Eocene interval. This collision resulted in the large-scale south-vergent internal imbrication of the platform that produced the far travelled nappe systems of the Taurides, and buried beneath these, the metamorphic axis of Anatolia, the Anatolides. The Maden basin closed during the early late Eocene by north-dipping subduction, synthetic to the Inner-Tauride subduction zone that had switched from south-dipping subduction beneath the Bitlis—Pötürge fragment to north dipping subduction beneath the Anatolide—Tauride platform during the later Palaeocene. Finally, the terminal collision of Arabia with Eurasia in eastern Turkey eliminated the Çüngüş basin as well and created the present tectonic regime of Turkey by pushing a considerable piece of it eastwards along the two newly-generated transform faults, namely those of North and East Anatolia. Much of the present eastern Anatolia is underlain by an extensive mélange prism that accumulated during the late Cretaceous—late Eocene interval north and east of the Bitlis—Pötürge fragment.     

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.     

Structure of the crust and uppermost mantle of the Yanshan Belt and adjacent regions at the northeastern boundary of the North China Craton from Rayleigh Wave Dispersion Analysis

We have used Rayleigh wave dispersion analysis and inversion to produce a high resolution S-wave velocity imaging profile of the crust and uppermost mantle structure beneath the northeastern boundary regions of the North China Craton (NCC). Using waveform data from 45 broadband NCISP stations, Rayleigh wave phase velocities were measured at periods from 10 to 48 s and utilized in subsequent inversions to solve for the S-wave velocity structure from 15 km down to 120 km depth. The inverted lower crust and uppermost mantle velocities, about 3.75 km/s and 4.3 km/s on average, are low compared with the global average. The Moho was constrained in the depth range of 30–40 km, indicating a typical crustal thickness along the profile. However, a thin lithosphere of no more than 100 km was imaged under a large part of the profile, decreasing to only ~ 60 km under the Inner Mongolian Axis (IMA) where an abnormally slow anomaly was observed below 60 km depth. The overall structural features of the study region resemble those of typical continental rift zones and are probably associated with the lithospheric reactivation and tectonic extension widespread in the eastern NCC during Mesozoic–Cenozoic time. Distinctly high velocities, up to ~ 4.6 km/s, were found immediately to the south of the IMA beneath the northern Yanshan Belt (YSB), extending down to > 100-km depth. The anomalous velocities are interpreted as the cratonic lithospheric lid of the region, which may have not been affected by the Mesozoic–Cenozoic deformation process as strongly as other regions in the eastern NCC. Based on our S-wave velocity structural image and other geophysical observations, we propose a possible lithosphere–asthenosphere interaction scenario at the northeastern boundary of the NCC. We speculate that significant undulations of the base of the lithosphere, which might have resulted from the uneven Mesozoic–Cenozoic lithospheric thinning, may induce mantle flows concentrating beneath the weak IMA zone. The relatively thick lithospheric lid in the northern YSB may serve as a tectonic barrier separating the on-craton and off-craton regions into different upper mantle convection systems at the present time.     

Seismic velocity discontinuities in the crust and uppermost mantle beneath the Kanto district, central Japan, identified from receiver function imaging and repeating earthquake activity

We constructed vertical cross-sections of depth-converted receiver function images to estimate the seismic velocity structure of the crust and uppermost mantle beneath the Kanto district, central Japan. Repeating earthquake data for the plate boundary were also used to estimate geometries of the subducting Philippine Sea plate and the subducting Pacific plate. As a result, we present images of some major seismic discontinuities. The upper boundary of the Pacific plate dips to the northwest in northern Kanto and to the west–southwest in southern Kanto with some undulations. On the other hand, the upper boundary of the Philippine Sea plate as a whole dips to the northwest. However, it is concave to the northeast in the southern Boso peninsula. We suggest that the low-velocity mantle wedge may be indicated on the top of both subducting plates. Plate thickness gradually decreases to the northeast. The northeastern end of the Philippine Sea plate is interpreted to be at depths of 45–90 km. The Moho discontinuity in the overriding plate is deeper than 25 km in the northern Kanto. It contacts the subducting Philippine Sea plate in the southwestern part near 35.8°N.     

The Yenice–Gönen active fault (NW Turkey): Active tectonics and palaeoseismology

The Yenice–Gönen Fault (YGF) is one of the most important active tectonic structures in the Biga peninsula. On March 18, 1953, a destructive earthquake (M_w = 7.2) occurred on the YGF, which is considered to be a part of the southern branch of the North Anatolian Fault Zone (NAFZ). A 70 km-long dextral surface rupture formed during the Yenice–Gönen Earthquake (YGE).In this study, structural and palaeoseismological features of the YGF have been investigated. The YGF surface ruptures have been mapped and three trenches were excavated at Muratlar, Karaköy and Seyvan sites.According to the palaeoseismic interpretation and the results of ¹⁴C AMS dating, Seyvan trench shows that an earthquake of palaeoseismic age ca. 620 AD ruptured a different strand of the 1953 fault, producing rather significant surface rupture displacement, while there are indications that at least two older events occurred during the past millennia. Another set of trenches excavated near Gönen town (Muratlar village) revealed extensive liquefaction not only during the 1953 event, but also during a previous earthquake, dated at 1440 AD. The Karaköy trench shows no indications of recent reactivations.Based on the trenching results, we estimate a recurrence interval of 660 ± 160 years for large morphogenic earthquakes, creating linear surface ruptures. The maximum reported displacement during the 1953 earthquake was 4.2 m. Taking into account the palaeoseismologically determined earthquake recurrence interval and maximum displacement, slip-rate of the YGF has been calculated to be 6.3 mm/a, which is consistent with present-day velocities determined by GPS measurements. According to the geological investigations, cumulative displacement of the YGF is 2.3 km. This palaeoseismological study contributes to model the behaviour of large seismogenic faults in the Biga Peninsula.     

Comment on “207Pb–206Pb single-zircon evaporation ages of some granitoid rocks reveal continent-oceanic island arc collision during the Cretaceous geodynamic evolution of the Central Anatolian crust, Turkey” – Boztug, D., Tichomirowa, M. & Bombach, K., 2007, JAES 31, 71–86

A continent-oceanic island arc collision model was proposed as a new geodynamic scenario for the evolution of the Cretaceous Central Anatolian granitoids in the Central Anatolian crystalline complex (CACC) by Boztug et al. (2007b) [Boztug, D., Tichomirowa, M., Bombach, K., 2007b. 207Pb–206Pb single-zircon evaporation ages of some granitoid rocks reveal continent-oceanic island arc collision during the Cretaceous geodynamic evolution of the central Anatolian crust, Turkey. Journal of Asian Earth Sciences 31, 71–86]. The key aspects of this model include an intra-oceanic subduction in the Neotethyan Izmir-Ankara Ocean, formation of an island arc and its subsequent collision with the northern margin of the Tauride–Anatolide Platform. The identical scenario was initially proposed by Göncüoglu et al. (1992) [Göncüoglu, M.C., Erler, A., Toprak, V., Yalınız, K., Olgun, E., Rojay, B., 1992. Geology of the western Central Anatolian Massif, Part II: Central Areas. TPAO Report No: 3155, 76 p] . Moreover, the weighted mean values of the reported 207Pb–206Pb single-zircon evaporation ages by Boztug et al. (2007b) [Boztug, D., Tichomirowa, M., Bombach, K., 2007b. 207Pb–206Pb single-zircon evaporation ages of some granitoid rocks reveal continent-oceanic island arc collision during the Cretaceous geodynamic evolution of the central Anatolian crust: Turkey. Journal of Asian Earth Sciences 31, 71–86] from A-type granitoids in the CACC seem to be miscalculated and contrast with the field data.     

Anatolian surface wave evaluated at GEOFON Station ISP Isparta, Turkey

We have studied seismic surface waves of 255 shallow regional earthquakes recently recorded at GEOFON station ISP (Isparta, Turkey) and have selected these 52 recordings with high signal-to-noise ratio for further analysis. An attempt was made by the simultaneous use of the Rayleigh and Love surface wave data to interpret the planar crust and uppermost mantle velocity structure beneath the Anatolian plate using a differential least-square inversion technique. The shear-wave velocities near the surface show a gradational change from approximately 2.2 to 3.6 km s^− 1 in the depth range 0–10 km. The mid-crustal depth range indicating a weakly developed low velocity zone has shear-wave velocities around 3.55 km s^− 1. The Moho discontinuity characterizing the crust–mantle velocity transition appears somewhat gradual between the depth range  25–45 km. The surface waves approaching from the northern Anatolia are estimated to travel a crustal thickness of  33 km whilst those from the southwestern Anatolia and part of east Mediterranean Sea indicate a thicker crust at  37 km. The eastern Anatolia events traveled even thicker crust at  41 km. A low sub-Moho velocity is estimated at  4.27 km s^− 1, although consistent with other similar studies in the region. The current velocities are considerably slower than indicated by the Preliminary Reference Earth Model (PREM) in almost all depth ranges.     

Elastic and inelastic triggering of earthquakes in the North Anatolian Fault zone

Deformation models used to explain the triggering mechanism often assume pure elastic behaviour for the crust and upper mantle. In reality however, the mantle and possibly the lower crust behave viscoelastically, particularly over longer time scales. Consequently, the stress field of an earthquake is in general time-dependent. In addition, if the elastic stress increase were enough to trigger a later earthquake, this triggered event should occur instantaneously and not many years after the triggering event. Hence, it is appropriate to include inelastic behaviour when analysing stress transfer and earthquake interaction.In this work, we analyse a sequence of 10 magnitude M_s > 6.5 events along the North Anatolian Fault between 1939 and 1999 to study the evolution of the regional Coulomb stress field. We investigate the triggering of these events by stress transfer, taking viscoelastic relaxation into account. We evaluate the contribution of elastic stress changes, of post-seismic viscoelastic relaxation in the lower crust and mantle, and of steady tectonic loading to the total Coulomb stress field. We analyse the evolution of stress in the region under study, as well as on the rupture surfaces of the considered events and their epicentres. We study the state of the Coulomb stress field before the 1999 İzmit and Düzce earthquakes, as well as in the Marmara Sea region.In general, the Coulomb stress failure criterion offers a plausible explanation for the location of these events. However, we show that using a purely elastic model disregards an important part of the actual stress increase/decrease. In several cases, post-seismic relaxation effects are important and greater in magnitude than the stress changes due to steady tectonic loading. Consequently, viscoelastic relaxation should be considered in any study dealing with Coulomb stress changes.According to our study, and assuming that an important part of the rupture surface must be stressed for an earthquake to occur, the most likely value for the viscosity of the lower crust or mantle in this region is 5 · 10¹⁷–10¹⁸ Pa · s. Our results cannot rule out the possibility of other time-dependent processes involved in the triggering of the 1999 Düzce event. However, the stress increase due to viscoelastic relaxation brought 22% of the 1999 Düzce rupture area over the threshold value of Δσ_c ≥ 0.01 MPa (0.1 bar), and took the whole surface closer to failure by an average of 0.2 MPa. Finally, we argue that the Marmara Sea region is currently being loaded with positive Coulomb stresses at a much faster rate than would arise exclusively from steady tectonic loading on the North Anatolian Fault.     

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.     

Crustal structure at the easternmost termination of the Variscan belt based on CELEBRATION 2000 and ALP 2002 data

The eastern margin of the Variscan belt in Europe comprises plate boundaries between continental blocks and terranes formed during different tectonic events. The crustal structure of that complicated area was studied using the data of the international refraction experiments CELEBRATION 2000 and ALP 2002. The seismic data were acquired along SW–NE oriented refraction and wide-angle reflection profiles CEL10 and ALP04 starting in the Eastern Alps, passing through the Moravo-Silesian zone of the Bohemian Massif and the Fore-Sudetic Monocline, and terminating in the TESZ in Poland. The data were interpreted by seismic tomographic inversion and by 2-D trial-and-error forward modelling of the P waves. Velocity models determine different types of the crust–mantle transition, reflecting variable crustal thickness and delimiting contacts of tectonic units in depth. In the Alpine area, few km thick LVZ with the Vp of 5.1 km s^− 1 dipping to the SW and outcropping at the surface represents the Molasse and Helvetic Flysch sediments overthrust by the Northern Calcareous Alps with higher velocities. In the Bohemian Massif, lower velocities in the range of 5.0–5.6 km s^− 1 down to a depth of 5 km might represent the SE termination of the Elbe Fault Zone. The Fore-Sudetic Monocline and the TESZ are covered by sediments with the velocities in the range of 3.6–5.5 km s^− 1 to the maximum depth of 15 km beneath the Mid-Polish Trough. The Moho in the Eastern Alps is dipping to the SW reaching the depth of 43–45 km. The lower crust at the eastern margin of the Bohemian Massif is characterized by elevated velocities and high Vp gradient, which seems to be a characteristic feature of the Moravo-Silesian. Slightly different properties in the Moravian and Silesian units might be attributed to varying distances of the profile from the Moldanubian Thrust front as well as a different type of contact of the Brunia with the Moldanubian and its northern root sector. The Moho beneath the Fore-Sudetic Monocline is the most pronounced and is interpreted as the first-order discontinuity at a depth of 30 km.     

TOPO-OZ: Insights into the various modes of intraplate deformation in the Australian continent

As the fastest, lowest, flattest and amongst the most arid of continents, Australia preserves a unique geomorphic record of intraplate tectonic activity, evidencing at least three distinct modes of surface deformation since its rapid northward drift commenced around 43 million years ago. At long wavelengths (several 1000s km) systematic variations in the extent of Neogene marine inundation imply the continent has tilted north–down, southwest–up. At intermediate-wavelengths (several 100s km) several undulations of ~ 100–200 m amplitude have developed on the 1–10 myr timescale. At still shorter wavelengths (several 10s km), fault related motion has produced local relief at rates of up to ~ 100 m/myr over several million years. The long-wavelength, north–down tilting can be related to a dynamic topographic effect associated with Australia's northward drift from the geoid low, dynamic topography low now south of the continent to the geoid high, dynamic topography low centred above the south-east Asian and Melanesian subduction zones. The short wavelength, fault-related deformation is attributed in time to plate-wide increases in compressional stress levels as the result of distant plate boundary interactions and, in space, in part to variations in the thermal structure of the Australian lithosphere. At the intermediate wavelengths, transient, low amplitude undulations can be ascribed to either lithospheric buckling or the development of instabilities in the thermal boundary layer beneath the lithosphere. In the latter case, topographic asymmetries suggest the Australian lithosphere is moving north with respect to the mantle beneath, providing a unique attribution to the progressive alignment of seismic anisotropy and absolute plate motion observed near the base of the Australian lithosphere.     

Mantle dynamics of Western Pacific and East Asia: Insight from seismic tomography and mineral physics

Recent results of high-resolution seismic tomography and mineral physics experiments are used to study mantle dynamics of Western Pacific and East Asia. The most important processes in subduction zones are the shallow and deep slab dehydration and the convective circulation (corner flow) processes in the mantle wedge. The combination of the two processes may have caused the back-arc spreading in the Lau basin, affected the morphology of the subducting Philippine Sea slab and its seismicity under southwest Japan, and contributed to the formation of the continental rift system and intraplate volcanism in Northeast Asia, which are clearly visible in our tomographic images. Slow anomalies are also found in the mantle under the subducting Pacific slab, which may represent (a) small mantle plumes, (b) upwellings associated with the slab collapsing down to the lower mantle, or (c) sub-slab dehydration associated with deep earthquakes caused by the reactivation of large faults preserved in the slab. Combining tomographic images and earthquake hypocenters with phase diagrams in the systems of peridotite + water, we proposed a petrologic model for arc volcanism. Arc magmas are caused by the dehydration reactions of hydrated slab peridotite that supply water-rich fluids to the mantle wedge and cause partial melting of the convecting mantle wedge. A large amount of fluids can be released from hydrated MORB at depths shallower than 55 km, which move upwards to hydrate the wedge corner under the fore-arc, and never drag down to the deeper mantle along the slab surface. Slab dehydration reactions at 120 km depth are the antigorite-related 5 reactions which supply water-rich fluids for forming the volcanic front. Phase A and Mg-surssasite breakdown reactions at 200 and 300 km depths below 700 °C cause the second and third arcs, respectively. Moreover, the dehydration reactions of super-hydrous phase B, phases D and E at 500–660 km depths cause the fluid transportation to the mantle boundary layer (MBL) (410–660 km depth). The stagnant slabs extend from Japan to Beijing, China for over 1000 km long, indicating that the arc–trench system covers the entire region from the Japan trench to East Asia. We propose a big mantle wedge (BMW) model herein, where hydrous plumes originating from 410 km depth cause a series of intra-continental hot regions. Fluids derived from MBL accumulated by the double-sided subduction zones, rather than the India–Asia collision and the subsequent indentation into Asia, are the major cause for the active tectonics and mantle dynamics in this broad region.     

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.     

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.     

A very hydrous mantle under the western Pacific region: Implications for formation of marginal basins and style of Archean plate tectonics

The western Pacific region has been refrigerated by the subducting cold oceanic plates since 450 Ma. However, the region is also characterized by the presence of many oceanic microplates less than 1300 km across, as well as active magmatism; the Philippine Sea plate is representative. We have compiled and examined petrochemical characters of drilled basalts of DSDP from the Philippine Sea plate, and conclude that the source mantle for oceanic basalts is rich in water ca. 0.2 wt.%, and is 50–60 °C lower than that for MORB. The extensive melting is due to the high water content in the source mantle.It is well known that some marginal basins apparently have greater depths than the major oceans. We calculated the age–depth correlation based on a model of transient half-space cooling at given parameters of temperatures of mantle and surface, 1280 and 0 °C, and the thermal diffusivity, 1 mm² s^− 1. The calculation shows the correlation of age-residual depth from a mid-oceanic ridge is 367 for the Philippine Sea, consistent with the bathymetric data. Moreover, the mid-oceanic ridge may be relatively deep because this region is underlain by the cooler mantle.Addition of water to the mantle peridotite lowers the solidus temperature and viscosity. Melting experiments of hydrous peridotite show that addition of 0.2 wt.% H₂O content lowers the solidus temperature by 150 °C. As a result, the mantle under the region may practically correspond to a ca. 90 °C hotter mantle than normal MORB-source mantle in terms of magmatism and rheology. Numerical simulation for a hotter mantle suggests that many small plates should be formed because of extensive heat release by active magmatism, consistent with many microplates in this region. The presence of many oceanic microplates may be analogous to Archean plate tectonics, characterized by a hotter mantle.     

Influence of the precollisional stage on subduction dynamics and the buried crust thermal state: Insights from numerical simulations

At continental subduction initiation, the continental crust buoyancy may induce, first, a convergence slowdown, and second, a compressive stress increase that could lead to the forearc lithosphere rupture. Both processes could influence the slab surface P–T conditions, favoring on one side crust partial melting or on the opposite the formation of ultra-high pressure/low temperature (UHP-LT) mineral. We quantify these two effects by performing numerical simulations of subduction. Water transfers are computed as a function of slab dehydration/overlying mantle hydration reactions, and a strength decrease is imposed for hydrated mantle rocks. The model starts with an old oceanic plate ( 100 Ma) subducting for 145.5 Myr with a 5 cm/yr convergence rate. The arc lithosphere is thermally thinned between 100 km and 310 km away from the trench, due to small-scale convection occuring in the water-saturated mantle wedge. We test the influence of convergence slowdown by carrying on subduction with a decreased convergence rate (≤ 2 cm/yr). Surprisingly, the subduction slowdown yields not only a strong slab warming at great depth (> 80 km), but also a significant cooling of the forearc lithosphere at shallower depth. The convergence slowdown increases the subducted crust temperature at 90 km depth to 705 ± 62 °C, depending on the convergence rate reduction, and might thus favor the oceanic crust partial melting in presence of water. For subduction velocities ≤ 1 cm/yr, slab breakoff is triggered 20–32 Myr after slowdown onset, due to a drastic slab thermal weakening in the vicinity of the interplate plane base. At last, the rupture of the weakened forearc is simulated by imposing in the thinnest part of the overlying lithosphere a dipping weakness plane. For convergence with rates ≥ 1 cm/yr, the thinned forearc first shortens, then starts subducting along the slab surface. The forearc lithosphere subduction stops the slab surface warming by hot asthenosphere corner flow, and decreases in a first stage the slab surface temperature to 630 ± 20 °C at 80 km depth, in agreement with P–T range inferred from natural records of UHP-LT metamorphism. The subducted crust temperature is further reduced to 405 ± 10 °C for the crust directly buried below the subducting forearc. Such a cold thermal state at great depth has never been sampled in collision zones, suggesting that forearc subduction might not be always required to explain UHP-LT metamorphsim.     

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.     

Surface rupture of the Cariaco July 09, 1997 earthquake on the El Pilar fault, northeastern Venezuela

This paper discusses the surface rupture of the Cariaco July 09, 1997 Ms 6.8 earthquake in northeastern Venezuela – located at 10.545°N and 63.515°W and about 10 km deep. The field reconnaissance of the ground breaks confirms that this event took place on the ENE–WSW trending onshore portion of the dextral El Pilar fault (between the Gulfs of Cariaco and Paria), which is part of the major wrenching system within the Caribbean–South America plate boundary zone. Dextral slip along this fault was further supported by the structural style of this rupture (en echelon right-lateral R shears connected by mole tracks at restraining stepovers) and by larger geometric complexities (pop-ups at Las Manoas and Guarapiche), as well as by the focal mechanism solutions determined for the event by several authors. This 1997 surface ruptre comprised two distinct sections, from west to east: (a) a main very conspicuous, continuous, 30-km-long, rather straight, 075°N-trending alignment of en echelon surface breaks, with a rather constant, purely dextral coseismic slip of about 25  cm, but reaching a maximum value of 40 cm slightly northwest of Pantoño; and (b) a secondary discontinuous, 10-km-long, boomerang-shaped rupture, with a maximum coseismic slip of 20 cm at Guarapiche. The onshore extent of the surface rupture totalled 36 km, but may continue westward underwater, as suggested by the very shallow aftershock seismicity. This aftershock activity also clearly defined the steep north dip of the fault plane along the western rupture, suggesting tectonic inheritance on this major fault.From many locals' accounts, the rupture seems to have propagated from Pantoño to the west (highly asymmetric bidirectionality). This suggests that earthquake nucleation happened at or near the Casanay–Guarapiche restraining bend and rupture quickly propagated westward, allowing only a small fraction to progress eastwards beyond the bend. Additionally, the large fraction of after-slip (or creep) released is to be related to such restraining bend, which seems to have partly locked slip during rupture.     

Analysis of airborne magnetic and gravity anomalies of peninsular shield, India integrated with seismic and magnetotelluric results and gravity anomalies of Madagascar, Sri Lanka and East Antarctica

Modelling of gravity and airborne magnetic data integrated with seismic studies suggest that the linear gravity and magnetic anomalies associated with Moyar Bhavani Shear Zone (MBSZ) and Palghat Cauvery Shear Zone (PCSZ) are caused by high density and high susceptibility rocks in upper crust which may represent mafic lower crustal rocks. This along with thick crust (44–45 km) under the Southern Granulite Terrain (SGT) indicates collision of Dharwar craton towards north and SGT towards south with N–S directed compression during 2.6–2.5 Ga. This collision may be related to contemporary collision northwards between Eastern Madagascar–Western Dharwar Craton (WDC) and Eastern Dharwar Craton (EDC). Arcuate shaped N and S-verging thrusts, MBSZ-Mettur Shear and PCSZ-Gangavalli Shear, respectively across Cauvery Shear zone system (CSZ) in SGT also suggest that the WDC, EDC and SGT might have collided almost simultaneously during 2.6–2.5 Ga due to NW–SE directed compressional forces with CSZ as central core complex in plate tectonics paradigm preserving rocks of oceanic affinity. Gravity anomalies of schist belts of WDC suggest marginal and intra arc basin setting.The gravity highs of EGFB along east coast of India and regional gravity low over East Antarctica are attributed to thrusted high-density lower crustal/upper mantle rocks at a depth of 5–6 km along W-verging thrust, which is supported by high seismic velocity and crustal thickening, respectively. It may represent a collision zone at about 1.0 Ga between India and East Antarctica. Paired gravity anomalies in the central part of Sri Lanka related to high density intrusives under western margin of Highland Complex and crustal thickening (40 km) along eastern margin of Highland Complex with several arc type magmatic rocks of about 1.0 Ga in Vijayan Complex towards the east may represent collision between them with W-verging thrust as in case of EGFB. The gravity high of Sri Lanka in the central part falls in line with that of EGFB, in case it is fitted in Gulf of Mannar and may represent the extension of this orogeny in Sri Lanka.