Accretion and tectonic erosion processes revealed by the mode of occurrence and geochemistry of greenstones in the Cretaceous accretionary complexes of the Idonnappu Zone, southern central Hokkaido, Japan

The Cretaceous accretionary complexes of the Idonnappu Zone in the Urakawa area are divided into five lithological units, four of which contain greenstone bodies. The Lower Cretaceous Naizawa Complex consists of two lithologic units. The Basaltic Unit (B‐Unit) is a large‐scale tectonic slab of greenstone, consisting of depleted tholeiite similar to that of the Lower Sorachi Ophiolite (basal forearc basin ophiolite) in the Sorachi‐Yezo Belt. The Mixed Unit of Naizawa Complex (MN‐Unit) contains oceanic island‐type alkaline greenstones which occur as slab‐like bodies and faulted blocks with tectonically dismembered trench‐fill sediments. Repeated alternations of the two units in the Naizawa Complex may have been formed by the collision of seamounts with forearc ophiolitic body (Lower Sorachi Ophiolite) in the trench. The Upper Cretaceous Horobetsugawa Complex structurally underlies the Naizawa Complex in its original configuration, and it also contains greenstone bodies. Greenstones in the MH‐Unit occur as blocks and sedimentary clasts in a clastic matrix, and exhibit depleted tholeiite and oceanic‐island alkaline basalt/tholeiite chemistry. This unit is interpreted as submarine slide and debris flow deposits. Greenstones in the PT‐Unit occur at the base of several chert‐clastic successions. Most of the greenstones are severely sheared and show normal‐type mid‐ocean ridge basalt composition. The PT‐Unit greenstones are considered to have been derived from abyssal basement peeled off during accretion. The different accretion mechanism of the greenstones in the Naizawa and Horobetsugawa complexes reflects temporal changes in subduction zone conditions. Seamount accretion and tectonic erosion were dominant in the Early Cretaceous, due to highly oblique subduction of the old oceanic crust and minimal sediment supply. Whereas, thick sediments with minor mid‐ocean ridge basalt and olistostrome accreted in the Late Cretaceous, due to near‐orthogonal subduction of young oceanic crust with voluminous sediment supply.     

沿雅鲁藏布江（东段）地区推覆与反向冲断构造的地质特征及其形成机制

沿雅鲁藏布江（东段）两岸至少发育着两套断裂系统。其一是断面北倾,由北向南远距离的推覆断裂系,发育着构造窗和飞来峰。该断裂系形成在洋－陆俯冲和陆－陆碰撞两个造山阶段（１００～２６Ｍａ）;其二是断面向南陡倾,由南向北逆冲,切割了早期的由北向南的推覆断裂系的反向冲断层系。该断裂系形成于碰撞造山阶段晚期（＜２６Ｍａ）的局部反向道冲作用或造山期后的重力伸展作用。上述两套断裂系的叠加造成沿江地区构造的复杂     

Cenozoic geodynamic evolution of the Andaman–Sumatra subduction margin: Current understanding

The Andaman–Sumatra margin displays a unique set‐up of extensional subduction–accretion complexes, which are the Java Trench, a tectonic (outer arc) prism, a sliver plate, a forearc, oceanic rises, inner‐arc volcanoes, and an extensional back‐arc with active spreading. Existing knowledge is reviewed in this paper, and some new data on the surface and subsurface signatures for operative geotectonics of this margin is analyzed. Subduction‐related deformation along the trench has been operating either continuously or intermittently since the Cretaceous. The oblique subduction has initiated strike–slip motion in the northern Sumatra–Andaman sector, and has formed a sliver plate between the subduction zone and a complex, right‐lateral fault system. The sliver fault, initiated in the Eocene, extended through the outer‐arc ridge offshore from Sumatra, and continued through the Andaman Sea connecting the Sagaing Fault in the north. Dominance of regional plate dynamics over simple subduction‐related accretionary processes led to the development and evolution of sedimentary basins of widely varied tectonic character along this margin. A number of north–south‐trending dismembered ophiolite slices of Cretaceous age, occurring at different structural levels with Eocene trench‐slope sediments, were uplifted and emplaced by a series of east‐dipping thrusts to shape the outer‐arc prism. North–south and east–west strike–slip faults controlled the subsidence, resulting in the development of a forearc basins and record Oligocene to Miocene–Pliocene sedimentation within mixed siliciclastic–carbonate systems. The opening of the Andaman Sea back‐arc occurred in two phases: an early (～11 Ma) stretching and rifting, followed by spreading since 4–5 Ma. The history of inner‐arc volcanic activity in the Andaman region extends to the early Miocene, and since the Miocene arc volcanism has been associated with an evolution from felsic to basaltic composition.     

Two-phase opening of Andaman Sea: a new seismotectonic insight

High-resolution reconstruction of Benioff zone depth–dip angle trajectory for Burma–Java subduction margin between 2° and 17°N Lat. reveals two major episodes of plate geometry change expressed as abrupt deviation in subduction angle. Estimation of effective rate of subduction in different time slices (and then length of subducted slab) allowed drawing of isochrones in Ma interval through these trajectories for the time period 5–12 Ma. With these isochrones, the deformation events on the subducting Indian plate are constrained in time as of 4–5 and 11 Ma old. This well-constrained time connotation offered scope for the correlation of slab deformation events with the well-established two-phase opening history of the Andaman Sea. While the 11 Ma event recorded from southern part of the study area is correlated with early stretching and rifting phase, the 4–5 Ma event is interpreted as major forcing behind the spreading phase of the Andaman Sea. Systematic spatio-temporal evaluation of Indian plate obliquity on the Andaman Sea evolution shows its definite control on the early rifting phase, initiated towards south near northwest Sumatra. The much young spreading phase recorded towards north of 7° Lat. is possibly the result of late Miocene–Pliocene trench retreat and follow-up transcurrent movement (along Sagaing and Sumatran fault system) with NW–SE pull-apart extension.Nonconformity between plate shape and subduction margin geometry is interpreted as the causative force behind Mid-Miocene intraplate extension and tearing. Enhanced stretching in the overriding plate consequently caused active forearc subsidence, recorded all along this plate margin. Initial phase of the Andaman Sea opening presumably remains concealed in this early–middle Miocene forearc subsidence history. The late Miocene–Pliocene pull-apart opening and spreading was possibly initiated near the western part of the Mergui–Sumatra region and propagated northward in subsequent period. A temporary halt in rifting at this pull-apart stage and northeastward veering of the Andaman Sea Ridge (ASR) are related with uplifting of oceanic crust in post-middle Miocene time in form of Alcock and Sewell seamounts, lying symmetrically north and south of this spreading ridge.     

Thermal structure of the Barbados accretionary complex

Finite element modeling of the thermal structure within the Barbados subduction complex is carried out. Kinematics of the sediments inside the complex are computed from a viscous model with inhomogeneous viscosity and the effect of gravity. The model yields an uplift rate compatible with observational data. Advective heat transfer affects the heat flow across the complex. Imbricated thrust faulting further reduces the heat flow across the slope. These mechanisms predict an arcward decrease of heat flow on the lower slope, followed by an increase of heat flow approaching the ridge, little change in heat flow at the forearc basin, and a significant increase of heat flow near the volcanic are, in agreement with the existing observations.     

Moment Tensor Solutions of Earthquakes in the Indian Ocean from Surface Waves and their Tectonic Implications

—Rayleigh and Love waves generated by sixteen earthquakes which occurred in the Indian Ocean and were recorded at 13 WWSSN stations of Asia, Africa and Australia are used to determine the moment tensor solution of these earthquakes. A combination of thrust and strike-slip faulting is obtained for earthquakes occurring in the Bay of Bengal. Thrust, strike slip or normal faulting (or either of the combination) is obtained for earthquakes occurring in the Arabian Sea and the Indian Ocean. The resultant compressive and tensional stress directions are estimated from more than 300 centroid moment tensor (CMT) solution of earthquakes occurring in different parts of the Indian Ocean. The resultant compressive stress directions are changing from north-south to east-west and the resultant tensional stress directions from east-west to north-south in different parts of the Indian Ocean. The results infer the counterclockwise movement of the region (0°–33°S and 64°E–94°E), stretching from the Rodriguez triple junction to the intense deformation zone of the central Indian Ocean and the formation of a new subduction zone (island arc) beneath the intense deformation zone of the central Indian Ocean and another at the southern part of the central Indian basin. The compressive stress direction is along the ridge axis and the extensional stress manifests across the ridge axis. The north-south to northeast-south west compression and east-west to northwest-southeast extension in the Indian Ocean suggest the northward underthrusting of the Indian plate beneath the Eurasian plate and the subduction beneath the Sunda arc region in the eastern part. The focal depth of earthquakes is estimated to be shallow, varying from 4 to 20 km and increasing gradually in the age of the oceanic lithosphere with the focal depth of earthquakes in the Indian Ocean.     

The seismogenic zone of subduction thrust faults

Abstract Subduction thrust faults generate earthquakes over a limited depth range. They are aseismic in their seaward updip portions and landward downdip of a critical point. The seaward shallow aseismic zone, commonly beneath accreted sediments, may be a consequence of unconsolidated sediments, especially stable-sliding smectite clays. Such clays are dehydrated and the fault may become seismogenic where the temperature reaches 100--150°C, that is, at a 5--15 km depth. Two factors may determine the downdip seismogenic limit. For subduction of young hot oceanic lithosphere beneath large accretionary sedimentary prisms and beneath continental crust, the transition to aseismic stable sliding is temperature controlled. The maximum temperature for seismic behavior in crustal rocks is ～ 350°C, regardless of the presence of water. In addition, great earthquake ruptures initiated at less than this temperature may propagate with decreasing slip to where the temperature is ～ 450°C. For subduction beneath thin island arc crust and beneath continental crust in some areas, the forearc mantle is reached by the thrust shallower than the 350°C temperature. The forearc upper mantle probably is aseismic because of stable-sliding serpentinite hydrated by water from the underthrusting oceanic crust and sediments. For many subduction zones the downdip seismogenic width defined by these limits is much less than previously assumed. Within the narrowly defined seismic zone, most of the convergence may occur in earthquakes. Numerical thermal models have been employed to estimate temperatures on the subduction thrust planes of four continental subduction zones. For Cascadia and Southwest Japan where very young and hot plates are subducting, the downdip seismogenic limit on the subduction thrust is thermally controlled and is shallow. For Alaska and most of Chile, the forearc mantle is reached before the critical temperature, and mantle serpentinite provides the limit. In all four regions, the seismogenic zones so defined agree with estimates of the extent of great earthquake rupture, and with the downdip extent of the interseismic locked zone.     

马尼拉俯冲带缺失中深源地震成因初探

马尼拉俯冲带是整个南海地震活动多发区,地震成因与南海的形成和构造演化关系密切.对马尼拉俯冲带地震数据和层析成像结果进行了深入分析.结果表明:马尼拉俯冲带的地震活动主要为密集的浅源地震,缺失中深源地震.进一步分析揭示:①脱水和榴辉岩的形成在南海洋壳到达软流圈前就基本停止.马尼拉俯冲带南部在较浅的深度就转变为塑性变形,并停...     

Seismic-stratigraphic framework of the forearc basin off central Sumatra,Sunda Arc

New multichannel seismic reflection data provide information on the stratigraphic framework and geologic history of the forearc basin west of central Sumatra. We recognize six seismic-stratigraphic sequences that reflect the Cenozoic history and development of the outer continental shelf and forearc basin southeast of Nias Island. These sequences indicate several episodes of uplift of the subduction complex and filling of the forearc basin.Early in the development of this margin, Paleogene slope deposits prograded onto the adjacent basin floor. Onlapping this assemblage are two units interpreted as younger Paleogene(?) trough deposits. Uplift associated with rejuvenation of subduction in the late Oligocene led to erosion of the Sumatra shelf and formation of a regional unconformity.The early Miocene was a period of significant progradation. A Miocene limestone unit partly downlaps and partly onlaps the older Paleogene deposits. It is characterized by shallow shelf and oblique progradational facies passing into basin floor facies. A buried reef zone occurs near the shelf edge. The cutting of an erosional unconformity on the shelf and slope in late Miocene/early Pliocene time culminated this episode of deposition.In the late Pliocene, a large flexure developed at the western boundary of the basin, displacing the outer-arc ridge upward relative to the basin. Over 1 km of Pliocene to Recent sediment was deposited as a wedge in the deep western portion of the basin landward of the outer-arc ridge. These deposits are characterized by flat-lying, high-amplitude, continuous reflections that overstep the late Miocene unconformity. Up to 800 m of shallow-water limestone have been deposited on the shelf since mid-Pliocene time.     

Origin of the early Cenozoic belt boundary thrust and Izanagi–Pacific ridge subduction in the western Pacific margin

The belt boundary thrust within the Cretaceous–Neogene accretionary complex of the Shimanto Belt, southwestern Japan, extends for more than ~ 1 000 km along the Japanese islands. A common understanding of the origin of the thrust is that it is an out of sequence thrust as a result of continuous accretion since the late Cretaceous and there is a kinematic reason for its maintaining a critically tapered wedge. The timing of the accretion gap and thrusting, however, coincides with the collision of the Paleocene–early Eocene Izanagi–Pacific spreading ridges with the trench along the western Pacific margin, which has been recently re‐hypothesized as younger than the previous assumption with respect to the Kula‐Pacific ridge subduction during the late Cretaceous. The ridge subduction hypothesis provides a consistent explanation for the cessation of magmatic activity along the continental margin and the presence of an unconformity in the forearc basin. This is not only the case in southwestern Japan, but also along the more northern Asian margin in Hokkaido, Sakhalin, and Sikhote‐Alin. This Paleocene–early Eocene ridge subduction hypothesis is also consistent with recently acquired tomographic images beneath the Asian continent. The timing of the Izanagi–Pacific ridge subduction along the western Pacific margin allows for a revision of the classic hypothesis of a great reorganization of the Pacific Plate motion between ~ 47 Ma and 42 Ma, illustrated by the bend in the Hawaii–Emperor chain, because of the change in subduction torque balance and the Oligocene–Miocene back arc spreading after the ridge subduction in the western Pacific margin.     

Geochemical characteristics of Bikou volcanic group and Sr-Nd-Pb isotopic composition: Evidence for breakup event in the north margin of Yangtze plate,Jining era

The geodynamic setting of the Bikou volcanic group is a critical question to trace the Precambrain tectonic framework and evolution for the Yangtze plate. This study has suggested that the Bikou volcanic group is composed of several residual oceanic crust units: MORB (mid-ocean ridge basalt), Alk-OIB (alkaline ocean island basalt) and Th-OIB (tholeiitic ocean island basalt) as well as subduction-related volcanic rocks. According to field observation, those distinct rocks occurred collectively in form of tectonic contact, implying that the Bikou volcanic group was an ophiolitic mélange. Coupled with geochronological data, a perished oceanic basin at the northern margin of the Yangtze block during Neoproterozoic was tested by this ophiolitic mélange. Meanwhile, the isogeochemical data suggest that the ocean occurred in the Southern Hemisphere identical to Indian, South Atlantic and South Pacific oceans in terms of their Dupal anomalies, and the original source of the rocks could be probably mixing by EMI and EMII component caused by dehydration melting of subducting oceanic crust during subduction process. On the basis of geochemical characteristics of the studied rocks, the Bikou volcanic group could imply that a partial breakup event occurred in the northern margin of Yangtze plate during the Neoproterozoic era.

     

Late Carboniferous – Middle Permian arc/forearc‐related basin in Central Asian Orogenic Belt: Insights from the petrology and geochemistry of the Shuangjing Schist in Inner Mongolia,China

The Solonker Suture Zone is thought to record the terminal evolution of the Central Asian Orogenic Belt (CAOB) in Inner Mongolia. However, two contrasting interpretations of the timing of suturing of the Solonker Suture Zone exist: (i) Permian to Early Triassic; and (ii) Middle Devonian or Late Devonian to Carboniferous. The Shuangjing Schist is exposed in the Linxi area along the Xar Moron Fault Zone, which marks the southern boundary of the Solonker Suture Zone in the eastern section of the CAOB, and thus provides insight into the timing of suturing of the Solonker Suture Zone. Detailed and systematic analysis of the petrology and geochemistry of the Shuangjing Schist shows that the Shuangjing Schist developed by greenschist facies prograde metamorphism of a volcanisedimentary rock series protolith. The volcanic parts of the Shuangjing Schist are a calc‐alkaline series with large volumes of intermediate members and subordinate acidic members. Volcanism occurred in a magmatic arc on the continental margin and was induced by subduction‐related magmatism resulting from mantle metasomatism. The sedimentary parts of the Shuangjing Schist reflect a transition from continental shelf to abyssal plain sedimentation. The formation of the Shuangjing Schist is suggested to be related to closure of an arc/forearc‐related ocean basin. The timing is constrained by a laser ablation inductively coupled plasma–mass spectrometry (LA‐ICP–MS) U–Pb magmatic zircon age of 298 ± 2 Ma from a carbonaceous biotite–plagioclase schist that was intruded by granite at 272 ± 2 Ma. In the Linxi area, southward subduction of the arc/forearc basin led to uplift, thickening, collapse, and erosion of the overriding continental crust. Collapse induced extension and widespread magmatism along the volcanic arc at the northern margin of the North China Craton. The closure of the arc/forearc‐related oceanic basin led to the formation of Late Permian to Middle Triassic collisional granites and the subsequent end of the collision of the Solonker Suture Zone.     

基于粒子群算法的俯冲带三维有效弹性厚度反演

岩石圈有效弹性厚度是表征岩石圈力学性质的参数,其反映了岩石圈挠曲变形的特征.本文在传统二维挠曲模型的基础上,提出了适用于俯冲及碰撞带的三维薄板挠曲模型.并发展了基于粒子群算法的俯冲带三维有效弹性厚度反演方法.该方法适用于挠曲参数存在横向差异的俯冲-碰撞带.最后利用该方法反演了马尼拉海沟处岩石圈的有效弹性厚度,结果显示:南海中央海盆岩石圈的有效弹性厚度随着距洋中脊距离的增加而增大;马尼拉海沟轴部弯矩在洋中脊两侧呈分段性变化,这表明南海俯冲板片在深部撕裂可能对浅部的挠曲形态产生影响.     

Emplacement processes of proto-arc basalt in the Izu–Bonin–Mariana arc system

Ascertaining the emplacement mechanism of oceanic basaltic lavas is important in understanding how ocean floor topography is produced and oceanic plates evolve, particularly during the early stages of crustal development of a supra-subduction zone. A detailed study of the volcanic stratigraphy at International Ocean Discovery Program (IODP) Site U1438 in the Amami Sankaku Basin, west of the Kyushu–Palau Ridge, has revealed the development of lava accretion and ridge topography on the Philippine Sea plate at about 49 Ma. Igneous basement rocks penetrated at Site U1438 are the uppermost 150 m of ~6 km-thick oceanic crust, and comprise, in a downhole direction, sheet flows (12.6 m), lobate sheet flows (61.3 m), pillow lavas (50.7 m), and thin sheet flows (25.3 m). The lowermost sheet flows are intercalated with layers of limestone and epiclastic tuff. Lithofacies analysis reveals that the lowermost sheet flows, limestone, and tuff formed on an axial rise, the pillow lavas were emplaced on a ridge slope, and the lobate sheet flows formed off ridge on an abyssal plain. The lithofacies of the basement basalt corresponds to the upper portions of fast-spreading oceanic crust, suggesting that subduction initiation was associated with intermediate to fast rates of seafloor spreading. The surface sheet flows are olivine–clinopyroxene-phyric basalt and differ from the lower basalt flows that contain phenocrysts of olivine and plagioclase, with or without clinopyroxene. The depleted chrome-spinel composition and olivine–clinopyroxene phenocryst assemblage in the surface sheet flows suggests a slight contribution of water for magma generation not present for the lower basalt flows. Considering the lithological difference between the backarc and forearc oceanic crust in the Izu–Bonin–Mariana arc, with sheet flow dominant in the former, seafloor spreading occurred faster in the later stage of subduction initiation.     

Extreme tectonic rotations within an eastern Mediterranean ophiolite (Baër-Bassit, Syria)

Palaeomagnetic results from 27 sites at five localities within the dismembered Baër-Bassit ophiolite of northern Syria are presented. The ophiolite forms part of a series of thrust sheets emplaced over Mesozoic carbonates of the Arabian platform in the middle Maastrichtian. A positive inclination-only area-wide tilt test applied to four locality mean remanences and positive fold and reversal tests from palaeohorizontal units (pillow lavas, lava flows) within one of these localities indicate that the ophiolite preserves pre-deformation magnetisations. Variable directions of remanence between localities demonstrate that the ophiolite has experienced extreme relative anticlockwise rotations on a kilometric scale. Within the most extensively sampled ophiolite massif (Bassit sheet) there is a progressive increase in rotation from north to south. The southernmost units at the lowest structural level in the imbricate thrust stack record the highest rotation (exceeding 200°). Although tectonic rotation during imbricate thrusting has been reported in a number of orogenic belts, the pattern of rotations in the Bassit sheet is difficult to explain by differential thrust sheet rotation. Instead, regional comparisons with the Hatay ophiolite of southern Turkey and the Troodos ophiolite of Cyprus suggest that a significant component of rotation may be ascribed to intraoceanic deformation of a coherent region of oceanic crust within the southern Neotethyan basin prior to ophiolite emplacement. The partially rotated Baër-Bassit ophiolite was then emplaced and structurally dismembered by thrust faulting. During the Late Tertiary the ophiolitic units were further rotated during the initiation and development of a major sinistral strike-slip fault zone, linking the Cyprus subduction zone to the Dead Sea Transform system. The extreme rotations observed in the study are therefore of composite origin, and reflect the complex development of structural fabrics within the ophiolite.     

From incipient island arc to doubly‐vergent orogen: A review of geodynamic models and sedimentary basin‐fills of southern Central America

Southern Central America is a Late Mesozoic/Cenozoic island arc that evolved in response to the subduction of the Farallón Plate beneath the Caribbean Plate in the Late Cretaceous and, from the Oligocene, the Cocos and Nazca Plates. Southern Central America is one of the best studied convergent margins in the world. The aim of this paper is to review the sedimentary and structural evolution of arc‐related sedimentary basins in southern Central America, and to show how the arc developed from a pre‐extensional intra‐oceanic island arc into a doubly‐vergent, subduction orogen. The Cenozoic sedimentary history of southern Central America is placed into the plate tectonic context of existing Caribbean Plate models. From regional basin analysis, the evolution of the southern Central American island arc is subdivided into three phases: (i) non‐extensional stage during the Campanian; (ii) extensional phase during the Maastrichtian‐Oligocene with rapid basin subsidence and deposition of arc‐related, clastic sediments; and (iii) doubly‐vergent, compressional arc phase along the 280 km long southern Costa Rican arc segment related to either oblique subduction of the Nazca plate, west‐to‐east passage of the Nazca–Cocos–Caribbean triple junction, or the subduction of rough oceanic crust of the Cocos Plate. The Pleistocene subduction of the Cocos Ridge contributed to the contraction but was not the primary driver. The architecture of the arc‐related sedimentary basin‐fills has been controlled by four factors: (i) subsidence caused by tectonic mechanisms, linked to the angle and morphology of the incoming plate, as shown by the fact that subduction of aseismic ridges and slab segments with rough crust were important drivers for subduction erosion, controlling the shape of forearc and trench‐slope basins, the lifespan of sedimentary basins, and the subsidence and uplift patterns; (ii) subsidence caused by slab rollback and resulting trench retreat; (iii) eustatic sea‐level changes; and (iv) sediment dispersal systems.     

Geodynamic significance of ophiolites within the Calabrian Arc

Abstract Slices of oceanic lithosphere belonging to the neo‐Tethys realm crop out discontinuously in the northern Calabrian Arc, Southern Apennines. They consist of high‐pressure–low‐temperature metamorphic ophiolitic sequences formed from metaultramafics, metabasites and alternating metapelites, metarenites, marbles and calcschist. Ophiolites occupy an intermediate position in the northern Calabrian Arc nappe pile, situated between overlying Hercynian continental crust and the underlying Apenninic limestone units. In the literature, these ophiolitic sequences are subdivided into several tectonometamorphic units. Geochemical characteristics indicate that metabasites were derived from subalkaline basalts with tholeiitic affinity (transitional mid‐oceanic ridge basalt type), and a harzburgitic‐lherzolitic protolith is suggested for the serpentinites. The pressure–temperature‐deformation paths of the metabasites from different outcrops display similar features: (i) the prograde segment follows a typical Alpine geothermal gradient up to a metamorphic climax at 350°C and 0.9 GPa and crystallization of the high‐pressure mineral assemblage occurs along a pervasive foliation developed during a compressive tectonic event; and (ii) the retrogression path can be subdivided in two segments, the first is characterized by nearly isothermal decompression to approximately 400°C and 0.3 GPa and the second follows a cooling trajectory. During low‐pressure conditions, a second deformation event produces millimetric to decametric scale asymmetric folds that describe west‐verging major structures. The third deformation event is characterized by brittle extensional structures. The tectonometamorphic evolution of the ophiolitic sequences from the different outcrops is similar. Both thermobarometric modeling and tectonic history indicate that the studied rocks underwent Alpine subduction and exhumation processes as tectonic slices inside a west‐verging accretionary wedge. The subduction of oceanic lithosphere was towards the present east; therefore, the Hercynian continental crust, overthrusted on the ophiolitic accretionary wedge after the neo‐Tethys closure, was part of the African paleomargin or a continental microplate between Africa and Europe.     

马尼拉俯冲带北段增生楔前缘构造变形和精细结构

马尼拉俯冲带是南海的东部边界,记录了南海形成演化的关键信息,同时也是地震和海啸多发区域.本文利用过马尼拉俯冲带北段的高分辨率多道地震剖面,分析了研究区内海盆和海沟的沉积特征,精细刻画了区内增生楔前缘的构造变形、结构以及岩浆活动特征.研究区内增生楔下陆坡部分由盲冲断层、构造楔和叠瓦逆冲断层构成,逆冲断层归并于一条位于下中新统的滑脱面上,滑脱面向海方向的展布明显受到增生楔之下埋藏海山和基底隆起的影响;上陆坡的反射特征则因变形强烈和岩浆作用而难以识别;岩浆活动开始于晚中新世末期并持续至第四纪.马尼拉俯冲带北段增生楔的形成时间早于16.5 Ma,并通过前展式逆冲向南海方向扩展;马尼拉俯冲带的初始形成时间可能在晚渐新世,而此时南海海盆扩张仍在持续.南海东北缘19°N-21°N区域为南海北部陆坡向海盆的延伸,高度减薄的陆壳的俯冲造成马尼拉海沟北段几何形态明显地向东凹进.     

An ophiolitic tectonic melange first discovered in Huashan area, south margin of Qinling Orogenic Belt, and its tectonic implications

On the basis of the synthetic studies of geology and geochemistry, an ophiolitic tectonic melange waa discovered in Sanligang-Sanyang area, the western part of Xiangfan-Guangji fault, the south margin of the Qinling Orogenic Belt. It is composed of different tectonic blocks with different lithological features and ages, mainly including the Huashan ophiolite blocks, Xiaofu Island-arc volcanic blocks, pelagic sediments, fore-arc volcanic-sedimentary system, and the massif of the basement and the covering strata of the Yangtze Block. These massifs were emplaced in the western part of Xiangfan-Guangji fault, the boundary between the Qinling Orogenic Belt and Yangtze Block, contacting each other by a shear zone or chaotic matrix. The characteristics of geochemistry indicate that the bash of the Huashan ophiolite are similar to mid-oceanic ridge basalts (MORB) formed in an initial oceanic baain, and the Xiaofu volcanic rocks are formed in a tectonic setting of island arc. The ophiolitic tectonic melange is the fragments of subduction wedge, which implies that there has been an oceanic basin between Qinling Block and Yangtze Block. Project supported by the National Natural Science Foundation of China (Grant Nos. 49773187, 49732080)     

Serpentinites and serpentinites: Variety of origins and emplacement mechanisms of serpentinite bodies in the California Cordillera

Most serpentinitized peridotite in orogenic belts is derived from oceanic lithosphere, but the emplacement mechanisms of these rocks vary greatly, as illustrated by the nature of these rock bodies and their contacts. The diverse emplacement mechanisms have important implications for connecting ophiolitic rock occurrences to large‐scale orogenic processes. In the California Cordillera, the largest bodies of ultramafic rocks are parts of ophiolite sheets, such as the Coast Range ophiolite (CRO), that were part of the upper plate of an oceanic subduction system. Such units differ from smaller bodies within subduction complexes such as the Franciscan Complex that were transferred from the subducting plate to the subduction complex during accretion. Some intra‐subduction complex ultramafic rocks occur as nearly block‐free sheets within the Franciscan Complex, and as a part of mafic–ultramafic imbricates or broken formations within the Shoo Fly Complex of the northern Sierra Nevada. Franciscan Complex serpentinite also occurs as sedimentary serpentinite mélange that was partly subducted after deposition in the trench via submarine sliding. Such mélanges include blocks that record older and higher grade metamorphism than the matrix. Sedimentary serpentinite mélange that includes high‐pressure metamorphic blocks is also found in the basal Great Valley Group forearc basin deposits depositionally overlie the CRO. Distinguishing the different serpentinite origins is difficult in the California Cordillera even though a terminal continental collision did not affect this orogenic belt. In more typical orogenic belts with greater post‐subduction disruption, distinction between the types of serpentinite occurrences presents a greater challenge.