Lithospheric Structure of the Southeast Australian Lachlan Orogen along the Victorian Global Geoscience Transect

Upper-crustal elements of the ～35 km thick crust of the southern portion of the Lachlan Orogen consist of a chevron-folded and faulted turbidite package (15 to 17 km structural thickness) overlying imbricated Cambrian metabasites and cherts (～5 km structural thickness). These are intruded by both Early and Late Devonian granites and are overlain by Upper Silurian (?) marine to continental clastics (Grampians Group) in the west, and Upper Devonian-Early Carboniferous silicic volcanics and continental redbed elastics in the east. The turbidites show a general younging to the east, as well as eastward vergence apart from a local reversal (Tabberabbera zone). The region has been relatively stable since the mid-Paleozoic with apatite fission-track data recording cooling below ～100°C at 340-330 Ma in the west and 300 to 280 Ma in the east. Younger fission-track ages to the south, approaching the present coastline, reflect denudation during the opening of Bass Strait and the formation of the Cretaceous Otway and Gippsland basins. The major crustal discontinuities, the Woorndoo-Moyston and Mount Wellington fault zones, show significant Mesozoic reactivation and juxtapose regions of younger against older apatite fission-track ages. The nature of the lower crust remains unclear, but there is increasing evidence that it is not underlain by thinned Proterozoic continental crust. The Lachlan Orogen is an example of mid-Paleozoic tectonic accretion in a Southwest Pacific-style oceanic setting. Subduction-related oceanic thrusting produced the deformed and imbricated turbidite packages, and subduction-related magmatic underplating (perhaps during “rollback”) produced the large volumes of granite and volcanic rocks, and the localized high-T/low-P metamorphism.     

The Northwestern (Maghreb) boundary of the Nubia (Africa) Plate

A study of the present compressional deformation of the Northwestern (Maghreb) Nubia (Africa) margin is derived from the analysis of more than 20,000 km of seismic profiles. In the western part the compression is distributed in a large zone with on-land compression in Algeria, mainly strike-slip deformation on the Algerian margin and folds and strike-slip faulting in Eastern Spain. In the middle of the Algerian margin, around Algiers, the evidences of compression become more obvious. In this area a ridge trending N–S that is interpreted as a middle to late Miocene spreading center interacted with the transpressional margin that trends E–W. North of the location of the Boumerdes–Zemmouri earthquake the oceanic crust is deformed by blind thrusts up to 60 km from the coast. These thrusts are south dipping and with the northward dipping thrusts located onshore form a wedge that maybe a positive flower structure at a crustal scale related to the right-lateral transpression of the margin. In the eastern part of the Northwestern (Maghreb) Nubia (Africa) Deformed Belt, off eastern Algeria and Tunisia, the deformation is more intense but limited to the north by the continental slope. Large late Miocene Tortonian folds are cut by the Messinian erosional surface but the present deformation is also evident. It is suggested that the deformation with a double vergence may be followed up to the north of Sicily. After the docking (18 Ma) of the Kabylies to the Africa Plate, the crust has been thinned and the Algerian Basin opened during the middle-late Miocene with an E–W direction. From the late Miocene to the Present the margin has been rethickened by transpression and uplifted.     

Seismogenic structures along continental convergent zones: from oblique subduction to mature collision

We summarize seismogenic structures in four regions of active convergence, each at a different stage of the collision process, with particular emphases on unusual, deep-seated seismogenic zones that were recently discovered. Along the eastern Hellenic arc near Crete, an additional seismogenic zone seems to occur below the seismogenic portion of the interplate thrust zone—a configuration found in several other oblique subduction zones that terminate laterally against collision belts. The unusual earthquakes show lateral compression, probably reflecting convergence between the subducting lithosphere's flank and the collision zone nearby. Along oblique zones of recent collision, the equivalence between space and time reveals the transition from subduction to full collision. In particular, intense seismicity beneath western Taiwan indicates that along the incipient zone of arc–continent collision, major earthquakes occur along high-angle reverse faults that reach deep into the crust or even the uppermost mantle. The seismogenic structures are likely to be reactivated normal faults on the passive continental margin of southeastern China. Since high-angle faults are ineffective in accommodating horizontal motion, it is not surprising that in the developed portion of the central Taiwan orogen (<5 Ma), seismogenic faulting occurs mainly along moderate-dipping (20–30°) thrusts. This is probably the only well-documented case of concurrent earthquake faulting on two major thrust faults, with the second seismogenic zone reaching down to depths of 30 km. Furthermore, the dual thrusts are out-of-sequence, being active in the hinterland of the deformation front. Along the mature Himalayan collision zone, where collision initiated about 50 Ma ago, current data are insufficient to distinguish whether most earthquakes occurred along multiple, out-of-sequence thrusts or along a major ramp thrust. Intriguingly, a very active seismic zone, including a large (M_w=6.7) earthquake in 1988, occurs at depths near 50 km beneath the foreland. Such a configuration may indicate the onset of a crustal nappe, involving the entire cratonic crust. In all cases of collision discussed here, the basal decollement, a key feature in the critical taper model of mountain building, appears to be aseismic. It seems that right at the onset of collision, earthquakes reflect reactivation of high-angle faults. For mature collision belts, earthquake faulting on moderate-dipping thrust accommodates a significant portion of convergence—a process involving the bulk of crust and possibly the uppermost mantle.     

Coupling Effects on Gold Mineralization of Deep and Shallow Structures in the Northwestern Jiaodong Peninsula, Eastern China

For understanding the possible deep-seated processes and geodynamic constrains on gold mineralization, comprehensive physicochemical and geochemical studies of gold mineralization have been undertaken within the paleo-lithosphere framework during the metailogenic epoch from the northwestern part of the Jiaodong Peninsula in this paper. A general image of the paleo-crust has been remained although it has been superimposed and reformed by post-metailogenic tectonic movements. The gold ore deposits occur usually in local uplifts and gradient belts featuring a turn from steep to gentle in granite-metamorphic contact zones, relative uplifts of gradient zones of the Curier isothermal interfaces, depressions of the Moho discontinuity and areas where depth contours are cut by isotherms perpendicularly. Gold mineralization and lithogenesis are characterized by high temperature, low pressure and high strength of thermal flux. The depth of mineralization ranges from 0.8 to 4.5 km. The depth of the top interface of the granitic complex in the metallogenic epoch is about 3 km. There is a low-velocity layer （LVL） at the bottom of the upper crust with a depth close to 19.5 km, which may be a detachment belt in the crust. The appearance of the LVL indicates the existence of paleo-hyperthermal fluid or relics of molten magma chambers, which reflects partial melting within the crust during the diagenetic and metallogenic epochs and the superposition effects of strike-slip shearing of the Taulu fault zone. The subsidence of the Moho is probably attributed to the coupling process of the NW-SE continental collision between North China and the Yangtze Block and the strike-slip movement of the Tanlu fault accompanied with underplating of mantle magma in the northwestern part of the Jiaodong Peninsula. The underplating of mantle magma may result in partial melting and make granite magma transfer upwards. This is favorable for the migration of metallogenic materials from deep to shallow to be enriched to form deposits. Coupling interactions between the strike-slip of the Taulu fault, the underplating of mantle magma, partial melting within the crust, and hyperthermal fluid, etc. may be the important factors controlling the gold mineralization and spatial structures in the metailogenic system.     

粤北地区乳源-潮州MT剖面深部电性结构及地学意义

为了揭示粤北地区岩石圈深部结构、深大断裂性质及花岗岩分布规律等科学问题,布设了乳源-潮州宽频带大地电磁探测剖面。由二维反演得出的电性结构,讨论了粤北地区岩石圈导电性结构特点。沿剖面存在3个花岗岩分布区,呈现不同的类型,可能代表不同的成因模式。沿剖面划分3条北东向断裂带:吉安-四会断裂、赣江断裂于韶关东形成宽度近20km的低阻区域,其间形成断陷盆地;河源-邵武断裂带,其两侧发育壳幔高导层并发育壳幔混合型花岗岩,深部电性结构复杂,可能为壳幔剧烈作用的场所;丽水-海丰断裂带,控制了燕山晚期花岗岩的分布。韶关、连平之间和龙川、丰顺之间50～150km存在2个巨大的低阻体,可能是地幔物质底侵作用的"通道";且底侵方向指向连平和龙川之间的区域,由于底侵作用力贡献,发育了一系列的壳内和上地幔高导层。粤北地区岩石圈从西向东逐渐减薄,从100余km减薄到60km,反映了太平洋板块对欧亚板块的消减作用。潮州100km深度以下的中-低阻特征,推断为太平洋板块俯冲作用留下的"洋壳"物质。     

Thermal Structure and Rheology of the Upper Mantle beneath Guangdong and Hainan Provinces, China

１．Ｉｎｔｒｏｄｕｃｔｉｏｎ　　Ｔｈｅｔｈｅｒｍａｌｓｔａｔｅａｎｄｒｈｅｏｌｏｇｙｏｆｔｈｅｕｐｐｅｒｍａｎｔｌｅａｒｅｏｆｇｒｅａｔｉｍｐｏｒｔａｎｃｅｉｎｕｎｄｅｒｓｔａｎｄｉｎｇｔｈｅｓｔｒｕｃｔｕｒｅａｎｄｄｙｎａｍｉｃｓｏｆｔｈｅｌｉｔｈｏｓｐｈｅｒｅ,ａｎｄｅｖｅｎｆｏｒｉｔｓ３ｄｉｍｅｎｓｉｏｎａｌｏｒ４ｄｉｍｅｎｓｉｏｎａｌｍａｐｐｉｎｇ（Ｏ’ＲｅｉｌｌｙａｎｄＧｒｉｆｆｉｎ,１９８５;Ｏ’Ｒｅｉｌｌｙｅｔａｌ．,１９９６;Ｘｕｅｔａｌ．１９９５;Ｘｕｅｔａｌ．,１９９…     

Slip distribution and source parameters of the 20 July 2017 Bodrum-Kos earthquake (Mw6.6) from GPS observations

Greek-Turkish boundary near the cities Kos and Bodrum has been shaken on July 20, 2017 by a Mw6.6 earthquake. The mainshock is located offshore and did not generate an on-land surface rupture. Analyzing pre- and post-earthquake continuous/survey-type static GPS observations, we investigated co-seismic surface displacements at 20 sites to characterize source parameters and slip-distribution of the mainshock. Fault plane solutions as well as co-seismic slip distribution have been acquired through the inversion of co-seismic GPS displacements modeling the event as elastic dislocations in a half space. Fault plane solution shows a southward dipping normal-type fault segment extending a depth down to ~12 km, which remains within the brittle upper crust. Results from the distributed slip inversion show that the mainshock activated a ~65 km fault section, which has three high slip patches, namely western, central and eastern patches, where the coseismic slips reach up to 13, 26, and 5 cm, respectively. This slip pattern indicates that the pre-earthquake coupling, which is storing the slip deficit, occurred on these three patches.     

Crustal structure of mainland China from deep seismic sounding data

Since 1958, about ninety seismic refraction/wide angle reflection profiles, with a cumulative length of more than sixty thousand kilometers, have been completed in mainland China. We summarize the results in the form of (1) a new contour map of crustal thickness, (2) fourteen representative crustal seismic velocity–depth columns for various tectonic units, and, (3) a Pn velocity map. We found a north–south-trending belt with a strong lateral gradient in crustal thickness in central China. This belt divides China into an eastern region, with a crustal thickness of 30–45 km, and a western region, with a thickness of 45–75 km. The crust in these two regions has experienced different evolutionary processes, and currently lies within distinct tectonic stress fields. Our compilation finds that there is a high-velocity (7.1–7.4 km/s) layer in the lower crust of the stable Tarim basin and Ordos plateau. However, in young orogenic belts, including parts of eastern China, the Tianshan and the Tibetan plateau, this layer is often absent. One exception is southern Tibet, where the presence of a high-velocity layer is related to the northward injection of the cold Indian plate. This high-velocity layer is absent in northern Tibet. In orogenic belts, there usually is a low-velocity layer (LVL) in the crust, but in stable regions this layer seldom exists. The Pn velocities in eastern China generally range from 7.9 to 8.1 km/s and tend to be isotropic. Pn velocities in western China are more variable, ranging from 7.7 to 8.2 km/s, and may display azimuthal anisotropy.     

Moho depth variation beneath southwestern Japan revealed from the velocity structure based on receiver function inversion

The Philippine Sea plate is subducting under the Eurasian plate beneath the Chugoku-Shikoku region, southwestern Japan. We have constructed depth contours for the continental and oceanic Mohos derived from the velocity structure based on receiver function inversion. Receiver functions were calculated using teleseismic waveforms recorded by the high-density seismograph network in southwestern Japan. In order to determine crustal velocity structure, we first improved the linearized time-domain receiver function inversion method. The continental Moho is relatively shallow ( 30 km) at the coastline of the Sea of Japan and at the Seto Inland Sea, and becomes deeper–greater than 40 km–around 35°N and 133.8°E. Near the Seto Inland Sea, a low-velocity layer of thickness 10 km lies under the continental Moho. This low-velocity layer corresponds to the subducting oceanic crust of the Philippine Sea plate. The oceanic Moho continues to descend from south to northwest and exhibits complicated ridge and valley features. The oceanic Moho runs around 25 km beneath the Pacific coast and 45 km beneath the Seto Inland Sea, and it extends to at least to 34.5°N. The depth variation of the Moho discontinuities is in good qualitative agreement with the concept of isostasy. From the configurations of both the continental and oceanic Mohos, we demonstrate that the continental lower crust and the subducting oceanic crust overlap beneath the southern and central part of Shikoku and that a mantle wedge may exist beneath the western and eastern part of Shikoku. The southern edge of the overlapping region coincides with the downdip limit of the slip area of a megathrust earthquake.     

地壳脆塑性转化带非稳态流变与震后松弛变形机制

大陆浅源地震密集分布层称为地震层,该深度处于石英脆塑性转化带,其变形除受温度控制外,地震周期各阶段变形随应变速率和应力发生变化,从间震期的稳态蠕变转化为同震破裂和震后松弛阶段非稳态蠕变。与间震期长期蠕变相关的野外塑性变形和稳态流变实验研究非常多,而与震后松弛相关的地壳深部脆塑性转化和非稳态蠕变研究非常有限,更缺少非稳态流变的本构方程。震后松弛阶段的断层滑动研究和基于GPS观测数据反演地壳形变研究都依赖于非稳态蠕变实验数据及其流变模型。本文介绍了野外断层脆塑性转化带非稳态流变和高温高压非稳态流变实验研究进展,分析震后松弛阶段断层脆塑性转化带的变形特征与变形模式,讨论了非稳态流变与脆塑性转化带强度定量化研究中存在的问题。     

Crustal architecture of central Victoria: results from the 2006 deep crustal reflection seismic survey

A ～400 km long deep crustal reflection seismic survey was acquired in central Victoria, Australia, in 2006. It has provided information on crustal architecture across the western Lachlan Orogen and has greatly added to the understanding of the tectonic evolution. The east-dipping Moyston Fault is confirmed as the suture between the Delamerian and western Lachlan Orogens, and is shown to extend down to the Moho. The Avoca Fault, the boundary between the Stawell and Bendigo Zones, is a west-dipping listric reverse fault that intersects the Moyston Fault at a depth of about 22 km, forming a V-shaped geometry. Both the Stawell and Bendigo Zones can be divided broadly into a lower crustal region of interlayered and imbricated metavolcanic and metasedimentary rocks and an upper crustal region of tightly folded metasedimentary rocks. The Stawell Zone was probably part of a Cambrian accretionary system along the eastern Gondwanaland margin, and mafic rocks may have been partly consumed by Cambrian subduction. Much of the Early Cambrian oceanic crust beneath the Bendigo Zone was not subducted, and is preserved as a crustal-scale imbricate thrust stack. The seismic data have shown that a thin-skinned structural model appears to be valid for much of the Melbourne Zone, whereas the Stawell and Bendigo Zones have a thick-skinned structural style. Internal faults in the Stawell and Bendigo Zones are mostly west-dipping listric faults, which extend from the surface to near the base of the crust. The Heathcote Fault Zone, the boundary between the Bendigo and Melbourne Zones, extends to at least 20 km, and possibly to the Moho. A striking feature in the seismic data is the markedly different seismic character of the mid to lower crust of the Melbourne Zone. The deep seismic reflection data for the Melbourne Zone have revealed a multilayered crustal structure that supports the Selwyn Block model.     

What can strike‐slip fault spacing tell us about the plate boundary of western North America?

The spacing of parallel continental strike‐slip faults can constrain the mechanical properties of the faults and fault‐bounded crust. In the western US, evenly spaced strike‐slip fault domains are observed in the San Andreas (SA) and Walker Lane (WL) fault systems. Comparison of fault spacing (S) vs. seismogenic zone thickness (L) relationships of the SA and WL systems indicates that the SA has a higher S/L ratio (~8 vs. 1, respectively). If a stress‐shadow mechanism guides parallel fault formation, the S/L ratio should be controlled by fault strength, crustal strength, and/or regional stress. This suggests that the SA‐related strike‐slip faults are relatively weaker, with lower fault friction: 0.13–0.19 for the SA vs. 0.20 for WL. The observed mechanical differences between the San Andreas and Walker Lane fault systems may be attributed to variations in the local geology of the fault‐hosting crust and/or the regional boundary conditions (e.g. geothermal gradient or strain rate).     

Main features of crustal structure in western and southern Europe based on data of explosion seismology

During the past two decades deep seismic sounding measurements have been carried out in western and southern Europe, mainly using the refraction method. These investigations were performed partly on a national basis but as well within international cooperative programs under the sponsorship of the European Seismological Commission.

In France, a systematic study has been executed to determine the main feature of deep structures under the Central Massif and the Paris Basin. In the Forez and Margeride regions, the sub-crustal velocity is lower (7.2 km/sec) than the normal value (8.0 km/sec) observed in the adjacent areas.

The central and southern part of Western Germany is covered by an extensive network of refraction profiles. The crustal thickness varies, similarly to France, from 25 to 35 km. A great amount of deep reflection data was obtained by commercial and special reflection work. The crust beneath the Rhinegraben area shows the typical “rift system” structure with a low subcrustal velocity (7.4–7.7 km/sec).

Very intensive refraction work has been carried out in the Alpine area. The maximum crustal thickness found near the axis of the negative gravity anomaly is about 55–60 km. Furthermore, a clear lowvelocity layer at a depth between 10 and 30 km has been detected. A key position with regard to the geotectonic structure of the Alps is held by the zone of Ivrea characterized by a pronounced gravity high. From the refraction work it may be concluded that there material of the lower crust and the upper mantle (7.2–7.5 km/sec) is overlying a layer of extremely low velocity (5.0 km/sec) which is interpreted as sialic crust.

Three years ago, a systematic study of crustal structure of the Italian peninsula has been started. Reversed profiles were observed on Sicily, in Calabria, and in Puglia. On Sicily, the structure is very complicated; the crust of the western part looks like a transition between a continental and oceanic structure whereas the eastern side shows a continental-type crust. In Calabria and Puglia, the crustal thickness has been determined to be about 25–35 km.     

Two Phases of Cenozoic Deformation in Northeastern Tibet: Thrusting Followed by Strike-Slip Faulting

TWO PHASES OF CENOZOIC DEFORMATION IN NORTHEASTERN TIBET: THRUSTING FOLLOWED BY STRIKE-SLIP FAULTING     

The Surface Rupture Zone and Coseismic Deformation Produced by the Yutian Ms7.3 Earthquake of 21 March 2008, Xinjiang

On 21 March 2008, a Ms7.3 earthquake occurred at Quickbird, Yutian County, Xinjiang. We attempt to reveal the features of the causative fault of this shock and its coseismic deformation field. Our work is based on analysis and interpretation to high-resolution satellite images as well as differential interferometric synthetic aperture radar (D-InSAR) data from the satellite Envisat SAR, coupled with seismicity, focal mechanism solutions and active tectonics in this region. The result shows that the 40?km-long, nearly NS trending surface rupture zone by this event lies on a range-front alluvial platform in Qira County. It is characterized by distinct linear traces and simple structure with 1–3-m-wide individual seams and maximum 6.5?m width of a collapse fracture. Along the rupture zone many secondary fractures and fault-bounded blocks are seen, exhibiting remarkable extension. The coseismic deformation affected a large area 100×100?km2. D-InSAR analysis indicates that the interferometric deformation field is dominated by extensional faulting with a small strike-slip component. Along the causative fault, the western wall fell down and the eastern wall, that is the active unit, rose up, both with westerly vergence. Because of the big deformation gradients near the seismogenic fault, no interference fringes are seen on images, and what can be determined is a vertical displacement 70?cm or more between the two fault walls. According to the epicenter and differential occurrence times from the National Earthquake Information Center, China Earthquake Network Center, Harvard and USGS, it is suggested that the seismic fault ruptured from north to south.     

Seismic imaging of the transitional crust across the northeastern margin of the South China Sea

Crustal structure across the passive continental margin of the northeastern South China Sea (SCS) is presented based on a deep seismic survey cooperated between Taiwan and China in August 2001. Reflection data collected from a 48-hydrophone streamer and the vertical component of refraction/reflection data recorded at 11 ocean-bottom seismometers along a NW–SE profile are integrated to image the upper (1.6–2.4 km/s), lower (2.5–2.9 km/s), and compacted (3–4.5 km/s) sediment, the upper (4.5–5.5 km/s), middle (5.5–6.5 km/s) and lower (6.5–7.5 km/s) crystalline crust successively. The velocity model shows that the thickness (0.5–3 km) and the basement of the compacted sediment are strongly varied due to intrusion of the magma and igneous rocks after seafloor spreading of the SCS. Furthermore, several volcanoes and igneous rocks in the upper/middle crust (7–10 km thick) and a high velocity layer (0–5 km thick) in the lower crust of the model are identified as the ocean–continent transition (OCT) below the lower slope in the northeastern margin of the SCS. A thin continent NW of the OCT and a thick oceanic crust SE of the OCT in the continental margin of the northeastern SCS are also imaged, but these transitional crusts cannot be classified as the OCT due to their crustal thickness and the limited amount of the volcano, the magma and the high velocity layer. The extended continent, next to the gravity low and a sag zone extended from the SW Taiwan Basin, may have resulted from subduction of the Eurasian Plate beneath the Manila Trench whereas the thick oceanic crust may have been due to the excess volcanism and the late magmatic underplating in the oceanic crust after seafloor spreading of the SCS.     

南秦岭下地壳组成及岩石圈的拆离俯冲作用

根据新提供的Pb同位素组成及岩石地球化学研究成果，本文进一步证实了位于北秦岭北界的明港地区发育的早中生代安山玄武质火山角砾岩岩筒所携带的下地壳捕虏体属于南秦岭。所恢复的南秦岭下地壳剖面自下而上为：底侵成因的变辉长岩-基性麻粒岩(其中含有榴辉岩及辉石岩的透镜体)-酸性麻粒岩。秦岭造山带总体的岩石因模型为：南秦岭(扬子块体)向北拆离俯冲，北秦岭地壳向华北仰冲，华北岩石因呈楔状插入秦岭造山带，拆离面约在中、下地壳之间。南秦岭俯冲岩片延伸的范围在平面上有可能达到400km。     

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.     

On the role of subducting oceanic plateaus in the development of shallow flat subduction

Oceanic plateaus, aseismic ridges or seamount chains all have a thickened crust and their subduction has been proposed as a possible mechanism to explain the occurrence of flat subduction and related absence of arc magmatism below Peru, Central Chile and at the Nankai Trough (Japan). Their extra compositional buoyancy could prohibit the slab from sinking into the mantle. With a numerical thermochemical convection model, we simulated the subduction of an oceanic lithosphere that contains an oceanic crustal plateau of 18-km thickness. With a systematic variation, we examined the required physical parameters to obtain shallow flat subduction. Metastability of the basaltic crust in the eclogite stability field is of crucial importance for the slab to remain buoyant throughout the subduction process. In a 44-Ma-old subducting plate, basalt must be able to survive a temperature of 600–700 °C to keep the plate buoyant sufficiently long to cause a flat-slab segment. We found that the maximum yield stress in the slab must be limited to about 600 MPa to allow for the necessary bending to the horizontal. Young slabs show flat subduction for larger parameter ranges than old slabs, since they are less gravitationally unstable and show less resistance against bending. Hydrous weakening of the mantle wedge area and lowermost continent are required to allow for the necessary deformation of a change in subduction style from steep to flat. The maximum flat slab extent is about 300 km, which is sufficient to explain the observed shallow flat subduction near the Nankai Trough (Japan). However, additional mechanisms, such as active overthrusting by an overriding continental plate, need to be invoked to explain the flat-slab segments up to 500 km long below Peru and Central Chile.     

Crustal structure of the continental margin of Korea in the East Sea (Japan Sea) from deep seismic sounding data: evidence for rifting affected by the hotter than normal mantle

Despite the various opening models of the southwestern part of the East Sea (Japan Sea) between the Korean Peninsula and the Japan Arc, the continental margin of the Korean Peninsula remains unknown in crustal structure. As a result, continental rifting and subsequent seafloor spreading processes to explain the opening of the East Sea have not been adequately addressed. We investigated crustal and sedimentary velocity structures across the Korean margin into the adjacent Ulleung Basin from multichannel seismic (MCS) reflection and ocean bottom seismometer (OBS) data. The Ulleung Basin shows crustal velocity structure typical of oceanic although its crustal thickness of about 10 km is greater than normal. The continental margin documents rapid transition from continental to oceanic crust, exhibiting a remarkable decrease in crustal thickness accompanied by shallowing of Moho over a distance of about 50 km. The crustal model of the margin is characterized by a high-velocity (up to 7.4 km/s) lower crustal (HVLC) layer that is thicker than 10 km under the slope base and pinches out seawards. The HVLC layer is interpreted as magmatic underplating emplaced during continental rifting in response to high upper mantle temperature. The acoustic basement of the slope base shows an igneous stratigraphy developed by massive volcanic eruption. These features suggest that the evolution of the Korean margin can be explained by the processes occurring at volcanic rifted margins. Global earthquake tomography supports our interpretation by defining the abnormally hot upper mantle across the Korean margin and in the Ulleung Basin.