The westward lithospheric drift,its role on the subduction and transform zones surrounding Americas:Andean to cordilleran orogenic types cyclicity

We investigate the effect of the westerly rotation of the lithosphere on the active margins that surround the Americas and find good correlations between the inferred easterly-directed mantle counterflow and the main structural grain and kinematics of the Andes and Sandwich arc slabs.In the Andes,the subduction zone is shallow and with low dip,because the mantle flow sustains the slab;the subduction hinge converges relative to the upper plate and generates an uplifting doubly verging orogen.The Sandwich Arc is generated by a westerly-directed SAM(South American) plate subduction where the eastward mantle flow is steepening and retreating the subduction zone.In this context,the slab hinge is retreating relative to the upper plate,generating the backarc basin and a low bathymetry single-verging accretionary prism.In Central America,the Caribbean plate presents a more complex scenario:(a) To the East,the Antilles Arc is generated by westerly directed subduction of the SAM plate,where the eastward mantle flow is steepening and retreating the subduction zone.(b) To the West,the Middle America Trench and Arc are generated by the easterly-directed subduction of the Cocos plate,where the shallow subduction caused by eastward mantle flow in its northern segment gradually steepens to the southern segment as it is infered by the preexisting westerly-directed subduction of the Caribbean Plateau.In the frame of the westerly lithospheric flow,the subduction of a divergent active ridge plays the role of introducing a change in the oceanic/continental plate's convergence angle,such as in NAM(North American)plate with the collision with the Pacific/Farallon active ridge in the Neogene(Cordilleran orogenic type scenario).The easterly mantle drift sustains strong plate coupling along NAM,showing at Juan de Fuca easterly subducting microplate that the subduction hinge advances relative to the upper plate.This lower/upper plate convergence coupling also applies along strike to the neighbor continental strike slip fault systems where subduction was terminated(San Andreas and Queen Charlotte).The lower/upper plate convergence coupling enables the capture of the continental plate ribbons of Baja California and Yakutat terrane by the Pacific oceanic plate,transporting them along the strike slip fault systems as para-autochthonous terranes.This Cordilleran orogenic type scenario,is also recorded in SAM following the collision with the Aluk/Farallon active ridge in the Paleogene,segmenting SAM margin into the eastwardly subducting Tupac Amaru microplate intercalated between the proto-LiquineOfqui and Atacama strike slip fault systems,where subduction was terminated and para-autochthonous terranes transported.In the Neogene,the convergence of Nazca plate with respect to SAM reinstalls subduction and the present Andean orogenic type scenario.     

渤海湾中部中、新生代构造演化与潜山的形成

渤海湾盆地是华北克拉通东部的晚中—新生代断陷盆地,其东部为西太平洋活动大陆边缘,经历了多期不同性质的构造运动叠加。目前对渤海湾地区中—新生代的构造期次划分及各期次构造运动的应力状态的认识仍存在较大的争议。潜山是盆地沉积之前就已形成的基岩古地貌山,后被新地层覆盖而成,潜山内幕所保留的先存断裂及潜山与上覆盖层之间的接触关系为研究盆地构造运动提供了依据。本文以渤中19-6潜山构造为例,基于三维地震资料的精细解释、结合相干剖面及钻井资料进行系统构造解析,建立渤中19-6潜山构造演化新模型,并探讨了华北克拉通东缘的区域构造演化。研究结果表明:(1)渤中19-6潜山构造西部以一系列S-N向雁列式正断层为界,断层东侧为隆起的渤中19-6潜山构造,西侧为低洼的沙南凹陷。古潜山最早形成于晚三叠世,早白垩世形成如今以S-N向正断层为界的东隆西降的潜山构造格局;(2)渤中19-6潜山构造西部边界S-N向断裂以及上覆地层中存在的E-W向断裂为两侧大型走滑带间雁列式断裂构造,是该潜山构造储层形成的重要控制因素;(3)该潜山受华北板块与扬子板块剪刀式闭合碰撞和古太平洋板块NNW向俯冲的多重影响,中生代以来,共经历了印支早期挤压隆起、印支晚期伸展改造、燕山早期左行压扭改造、燕山中期左行伸展改造、燕山晚期左行压扭改造、喜山期右行伸展埋藏6个阶段的发育演化。     

大兴安岭地区上古生界变形特征及构造层划分

大兴安岭地区古生界构造变形表明,上古生界自二叠纪末以来遭受了3期构造变形改造:第一期变形为二叠纪末华北板块与佳蒙地块碰撞造成的近EW向展布的断裂和褶皱构造,强度由南向北有减弱的趋势;第二期变形为侏罗纪西太平洋板块俯冲导致的NE—NNE向左行走滑断裂和褶皱构造;第三期为NW向具有右行走滑特征的断裂构造,时间大致在晚侏罗世—早白垩世。综合区域构造、沉积岩古地理分析对比,初步将大兴安岭地区上古生界划分为早古生代、D—C1、C2—P2、P3—T14个构造层:早古生代末加里东运动之后,在D—C1期间早期以伸展为主,总体表现为北海南陆的古地理特征;早石炭世末期松嫩地块与额尔古纳—兴安地块沿嫩江—扎兰屯一线碰撞拼接;C2—P2期间总体表现为造山后伸展特征,表现为北陆南海;P3—T1时期古亚洲洋的闭合,海水退出,转为陆相。     

Subsidence and strike-slip tectonism of the upper continental slope off Manzanillo, Mexico

The direction of convergence between the Rivera and North American plates becomes progressively more oblique (in a counter-clockwise sense as measured relative to the trench-normal direction) northwestward along the Jalisco subduction zone. By analogy to other subduction zones, the forces resulting from this distribution of convergence directions are expected to produce a NW moving, fore-arc sliver and a NW–SE stretching of the fore-arc area. Also, a series of roughly arc parallel strike-slip faults may form in the fore-arc area, both onshore and offshore, as is observed in the Aleutian arc.In the Jalisco subduction zone, the Jalisco block has been proposed to represent such a fore-arc sliver. However, this proposal has encountered one major problem. Namely, right-lateral strike-slip faulting within the fore-arc sliver, and between the fore-arc sliver and the North American plate, should be observed. However, evidence for the expected right-lateral strike-slip faulting is sparse. Some evidence for right-lateral strike-slip faulting along the Jalisco block–North American plate boundary (the Tepic–Zacoalco rift system) has been reported, although some disagreement exists. Right-lateral strike-slip faulting has also been reported within the interior of the Jalisco block and in the southern Colima rift, which forms the SE boundary of the Jalisco block.Threefold, multi-channel seismic reflection data were collected in the offshore area of the Jalisco subduction zone off Manzanillo in April 2002 during the FAMEX campaign of the N/O L'Atalante. These data provide additional evidence for recent strike-slip motion within the fore-arc region of the Jalisco subduction zone. This faulting offsets right-laterally a prominent horst block within the southern Colima rift, from which we conclude that the sense of motion along the faulting is dextral. These data also provide additional evidence for recent subsidence within the area offshore of Manzanillo, as has been proposed.     

皖浙赣相邻区中生代以来构造活动及古应力场特征

皖浙赣相邻区位于中国东南大陆边缘,中生代以来经历多期强烈构造作用,是研究中生代以来构造运动的理想区域。本文以皖浙赣相邻区中生代以来的脆性断层为研究对象,通过系统的野外构造观测和运动学研究,在区内划分出了4期主要断裂构造,确定了各期断裂的运动学特征,恢复了古构造应力场。第一期构造大致发生于早侏罗世-中侏罗世末,应力场为近南北向挤压,可能与华南板块与华北板块碰撞最后阶段的挤压作用相关,第二期构造出现在早白垩世,应力场为近北东-南西向挤压,可能与太平洋板块向南-南西俯冲有关,第三期构造大致发生在晚白垩世末,应力场为北西-南东向挤压,与太平洋板块向欧亚大陆的北西向正向俯冲有关,第四期构造应力场为东西向挤压,可能与太平洋板块弧后扩张产生的向西侧向挤压及印度板块俯冲碰撞产生的向东挤出共同作用有关。     

GPS-derived strain in northwestern California: Termination of the San Andreas fault system and convergence of the Sierra Nevada–Great Valley block contribute to southern Cascadia forearc contraction

GPS-derived velocities (1993–2002) in northwestern California show that processes other than subduction are in part accountable for observed upper-plate contraction north of the Mendocino triple junction (MTJ) region. After removing the component of elastic strain accumulation due to the Cascadia subduction zone from the station velocities, two additional processes account for accumulated strain in northern California. The first is the westward convergence of the Sierra Nevada–Great Valley (SNGV) block toward the coast and the second is the north–northwest impingement of the San Andreas fault system from the south on the northern California coastal region in the vicinity of Humboldt Bay. Sierra Nevada–Great Valley block motion is northwest toward the coast, convergent with the more northerly, north–northwest San Andreas transform fault-parallel motion. In addition to the westward-converging Sierra Nevada–Great Valley block, San Andreas transform-parallel shortening also occurs in the Humboldt Bay region. Approximately 22 mm/yr of distributed Pacific–SNGV motion is observed inland of Cape Mendocino across the northern projections of the Maacama and Bartlett Springs fault zones but station velocities decrease rapidly north of Cape Mendocino. The resultant 6–10 mm/yr of San Andreas fault-parallel shortening occurs above the southern edge of the subducted Gorda plate and at the latitude of Humboldt Bay. Part of the San Andreas fault-parallel shortening may be due to the viscous coupling of the southern edge of the Gorda plate to overlying North American plate. We conclude that significant portions of the upper-plate contraction observed north of the MTJ region are not solely a result of subduction of the Gorda plate but also a consequence of impingement of the western edge of the Sierra Nevada–Great Valley block and growth of the northernmost segments of the San Andreas fault system.     

东秦岭－大别山及邻区盆－山系统演化与动力学

东秦岭－大别造山带受不同块体间的拼合碰撞及其之后的陆内变形控制,在造山带边缘和内部形成了不同的盆山系统。造山带北缘响应北秦岭与华北板块的弧陆碰撞及其之后陆内变形作用,形成了后陆逆冲与弧后前陆盆地系统。造山带南缘三叠纪至白垩纪随着扬子板块与秦岭－大别微板块沿勉略缝合带自东向西的斜向俯冲和之后的陆内旋转挤压,在扬子北缘形成了前陆逆冲与周缘前陆盆地系统。自晚侏罗世末至白垩纪造山带挤压与伸展并存,伸展自核部向边缘发展,形成造山带伸展塌陷与近东西向裂谷盆地系统。大致在中始新世之后,受中国东部环太平洋构造带东西向伸展作用和深部构造作用控制,横跨造山带形成近南北向的裂谷盆地。     

Active faulting at the Eurasian, North American and Pacific plates junction

The active faults known and inferred in the area where the major Pacific, North American and Eurasian plates come together group into two belts. One of them comprises the faults striking roughly parallel to the Pacific ocean margin. The extreme members of the belt are the longitudinal faults of islands arcs, in its oceanic flank, and the faults along the continental margins of marginal seas, in its continental flank. The available data show that all these faults move with some strike-slip component, which is always right-lateral. We suggest that characteristic right-lateral, either partially or dominantly, kinematics of the fault movements has its source in oblique convergence of the Pacific plate with continental Eurasian and North American plates. The second belt of active faults transverses the extreme northeast Asia as a continental extension of the active mid-Arctic spreading ridge. The two active fault belts do not cross but come close to each other at the northern margin of the Sea of Okhotsk marking thus the point where the Pacific, North American and Eurasian plates meet.     

Mesozoic interaction of the Kula plate and the western margin of north america

Oceanic crust west of North America at the beginning of the Jurassic belonged to the Kula plate. The development of the western margin of North America since the Jurassic reflects interaction with the Kula plate, the Kula-Farallon spreading center and the Farallon plate. The Kula plate ceased to exist in the Paleocene and later developments were caused by interaction of the Farallon plate and, subsequently, collision with the East Pacific Rise.At the beginning of the Jurassic, when spreading between North and South America began, the Kula-Farallon-Pacific triple junction moved to the north relative to North America, and the eastern end of the Kula-Farallon spreading center swept northwards along the continental margin.During the Paleocene, Kula-Pacific spreading ceased and the Kula plate fused to the Pacific plate. Throughout the Mesozoic, subduction of the Kula plate took place along the Alaskan continental margin. When the Kula plate joined the Pacific plate a new subduction zone formed along the line of the present Aleutian chain.Wrangellia and Stikinia, anomalous terrains in Alaska and northwestern Canada respectively, were emplaced by transport on the Kula plate from lower latitudes. Hypotheses which require transport of these plates in the Mesozoic from the “far reaches of the Pacific” ignore the problem of transport across either the Kula-Pacific or Kula-Farallon spreading centers. The interaction of the Kula plate and western North America throughout the Jurassic and the Cretaceous should result in emplacement of these terrains by motion oblique to the continental margin. Tethyan faunas in Stikinia must come from the western end of Tethys between North and South America, not the Indonesian region at the eastern end of Tethys.As the northeastern end of the Kula-Farallon ridge moved northward, the sense of motion changed from right lateral shear between the Kula and North American plates to collision or left lateral shear between the Farallon and North American plates. Left lateral shear along zones analogous to the Mojave-Sonora megashear may have been the means by which anomalous terrains were transported to the southeast into the gap between North and South America forming present day Central America. Such a model overcomes the overlap difficulties suffered in previous attempts to reconstruct the Mesozoic paleogeography of Central America.     

Cretaceous to Cenozoic sequential kinematics in the forearc–arc transition: effects of changing oblique plate convergence and the San Andreas system with implications for the La Paz fault (southern Baja California, Mexico)

We studied metasediments and mylonitic arc granitoids from the forearc–arc transition of southern Baja California, Mexico. Thin section analyses and field evidence show that metamorphism of the forearc–arc transition is of the high T/P active margin type. The heat was provided by Cretaceous arc intrusions. Field observations and thin section analyses, including the time/temperature deformation path, demonstrate that the study area was first affected by dextral, ductile shearing followed by ductile, sinistral, possibly transpressive strike-slip parallel to the magmatic arc during the Cretaceous. Both intervals are related to changing oblique plate convergence and, thus, identified as trench-linked strike-slip effects. The geometric relationship between arc-dipping foliation, stretching lineation and shear sense indicates that the arc may have been pressed onto the rocks of the study area during sinistral shearing. The sinistral interval lasted up until regional cooling (Early Cenozoic?). Because the La Paz fault is closely associated with the forearc–arc transition, it must have the same Cretaceous to Early Cenozoic kinematic history. The northern segment of the La Paz fault is a modern, brittle, strike-slip fault interpreted as a dextral synthetic fault of the San Andreas system which opened the Gulf of California (Mar de Cortés/Golfo de California). We found no evidence for Miocene Basin and Range extension.     

The Cretaceous Songliao Basin: Volcanogenic Succession, Sedimentary Sequence and Tectonic Evolution, NE China

The Songliao basin (SB) is a superposed basin with two different kinds of basin fills. The lower one is characterized by a fault-bounded volcanogenic succession comprising of intercalated volcanic, pyroclastic and epiclastic rocks. The volcanic rocks, dating from 110 Ma to 130 Ma, are of geochemically active continental margin type. Fast northward migration of the SB block occurred during the major episodes of the volcanism inferred from their paleomagnetic information. The upper one of the basin fill is dominated by non-marine sag-style sedimentary sequence of siliciclastics and minor carbonates. The basin center shifted westwards from the early to late Cretaceous revealed by the GGT seismic velocity structure suggesting dynamic change in the basin evolution. Thus, a superposed basin model is proposed. Evolution of the SB involves three periods including (1) Alptian and pre-Aptian: a retroarc basin and range system of Andes type related to Mongolia-Okhotsk collisional belt (MOCB); (2) Albian to Companian: a sag-like strike-slip basin under transtension related to oblique subduction of the Pacific plate along the eastern margin of the Eurasian plate; (3) since Maastrichtian: a tectonic inverse basin under compression related to normal subduction of the Pacific plate under the Eurasian plate, characterized by overthrust, westward migration of the depocenter and eastward uplifting of the basin margin.     

Successive Extensional Tectonic Regimes during the Mesozoic as Evidenced by Neptunian Dikes in the Pontide Magmatic Arc,Northeast Turkey

The eastern Pontide magmatic arc extends ～600 km in an E-W direction along the Black Sea coast and was disrupted by a series of fault systems trending NE-SW, NW-SE, E-W, and N-S. These fault systems are responsible for the formation of diachronous extensional basins, rift or pull-apart, in the northern, southern, and axial zones of the eastern Pontides during the Mesozoic. Successive extensional or transtensional tectonic regimes caused the abortive Liassic rift basins and the Albian and Campanian pull-apart basins with deep-spreading troughs in the southern and axial zones. Liassic, Albian, and Campanian neptunian dikes, which indicate extensional tectonic regimes, crop out within the Paleozoic granites near Kale, Gumushane, and the Malm–Lower Cretaceous platform carbonates in Amasya and Gumushane. These neptunian dikes correspond to extensional cracks that are filled and overlain by the fossiliferous red pelagic limestones. Multidirectional Liassic neptunian dikes are consistent with the general trend of the paleofaults (NE-SW, NW-SE, and E-W), and active dextral North Anatolian fault (NAF) and sinistral Northeast Anatolian fault (NEAF) systems. The Albian neptunian dikes in Amasya formed in the synthetic oblique left-lateral normal faults of the main fault zone that runs parallel to the active North Anatolian fault zone (NAFZ).

Kinematic interpretation of the Liassic and Albian neptunian dikes suggests N-S extensional stress or northward movement of the Pontides along the conjugate fracture zones parallel to the NAFZ and NEAFZ. This northward movement of the Pontides in Liassic and Albian times requires left-lateral and right-lateral slips along the conjugate NAFZ and Northeast Anatolian fault zones (NEAFZ), respectively, in contrast to the recent active tectonics that have been accommodated by N-S compressional stress. On the other hand, mutual relationships between the neptunian dikes and the associated main fault zone of Campanian age extending in an E-W direction in the Kale area, Gumushane suggest the existence of a main left-lateral transtensional wrench zone. This system might be accommodated by the counterclockwise convergence of the Turkish plate with the Afro-Arabian plate relative to the Eurasian plate, and the southward oblique subduction of Paleotethys beneath the eastern Pontide magmatic arc during the Mesozoic.     

郯庐断裂系研究的新进展——兼论东亚北东向走滑断裂系统

对郯庐断裂系研究的最新进展作了介绍。郯庐断裂是多期活动，性质多次转换的巨型断裂，最初的启动时间在晚三叠世末，与南北大陆的碰撞有着直接的关系，早期以走滑（左行）运动为主，伴随挤压和拉伸，范围限于华北地块内部，断裂纵向伸展的高峰期为白垩纪到早始新世，这一时期也是裂陷作用最强的时期，始新世以来以挤压作用为主，东亚走滑断裂系各组成断裂性质复杂，不能以中生代左行平移运动简单概括，走滑断裂系的发展和演化与同时     

郯庐断裂系研究的新进展─—兼论东亚北东向走滑断裂系统

对郯庐断裂系研究的最新进展作了介绍。郯庐断裂是多期活动、性质多次转换的巨型断裂。最初的启动时间在晚三叠世末，与南北大陆的碰撞有着直接的关系。早期以走滑（左行）运动为主，伴随挤压和拉伸，范围限于华北地块内部。断裂纵向伸展的高峰期为白垩纪到早始新世，这一时期也是裂陷作用最强的时期。始新世以来以挤压作用为主。东亚走滑断裂系各组成断裂性质复杂，不能以中生代左行平移运动简单概括。走滑断裂系的发展和演化与同时期大陆边缘所处的地球动力学环境，也就是与南北大陆的碰撞和古大洋板块的持续俯冲关系密切。新生代以来的构造事件使中生代的构造发生强烈变形变位改造。     

Paleozoic evolution of continental crust in the Beaujolais-Lyonnais area,northeastern part of the Massif Central,France

In the Beaujolais-Lyonnais area of the northeastern Massif Central accretion of continental and possibly oceanic crustal fragments occurred between Cambrian (?) and early Carboniferous time. Three distinct lithotectonic units (terranes?) have been recognized. The southern (Lyonnais-) Unit consists of medium- to high-grade metamorphics and includes eclogites; it formed in the early Paleozoic. The Brévenne-Unit to the north contains low- to medium-grade metamorphic mafic and felsic volcanics and subordinate sedimentary rocks which possibly originated during the early Paleozoic until Devonian time, in a sialic back-arc environment or along an active continental margin. The Beaujolais-Unit is represented by volcanics on the south and predominantly shallow marine clastics and carbonates on the north. It developed in a late Devonian or early Carboniferous ensialic marginal basin. The peak of metamorphism in the Lyonnais-unit (HP/HT) was reached in Silurian time. Subsequent NW-SE to E-W oriented convergence produced mylonitic foliation, structural imbrication of the Lyonnais basement rocks with the Brévenne-Unit and SE-vergent folds accompanied by low- to medium-grade metamorphism. Late Visean to Namurian N-S to NW-SE directed N-vergent thrusting produced tectonic imbrication of the metamorphic northern Brévenne-Unit with the nonmetamorphic Beaujolais-Unit. In the southern Brévenne-Unit and in the Lyonnais-Unit updoming along right-lateral high-angle normal faults was followed by emplacement of voluminous granitic plutons of crustal origin. Late Carboniferous to early Permian crustal thinning in the Beaujolais-Lyonnais area was associated with N-S trending left-lateral strike-slip faults and E-W to NE-SW trending right-lateral strike-slip faults. Basins that developed along these faults contain continental red beds.     

Tectonics of the Longmen Shan and Adjacent Regions,Central China

The Longmen Shan region includes, from west to east, the northeastern part of the Tibetan Plateau, the Sichuan Basin, and the eastern part of the eastern Sichuan fold-and-thrust belt. In the northeast, it merges with the Micang Shan, a part of the Qinling Mountains. The Longmen Shan region can be divided into two major tectonic elements: (1) an autochthon/parautochthon, which underlies the easternmost part of the Tibetan Plateau, the Sichuan Basin, and the eastern Sichuan fold-and-thrust belt; and (2) a complex allochthon, which underlies the eastern part of the Tibetan Plateau. The allochthon was emplaced toward the southeast during Late Triassic time, and it and the western part of the autochthon/parautochthon were modified by Cenozoic deformation.

The autochthon/parautochthon was formed from the western part of the Yangtze platform and consists of a Proterozoic basement covered by a thin, incomplete succession of Late Proterozoic to Middle Triassic shallow-marine and nonmarine sedimentary rocks interrupted by Permian extension and basic magmatism in the southwest. The platform is bounded by continental margins that formed in Silurian time to the west and in Late Proterozoic time to the north. Within the southwestern part of the platform is the narrow N-trending Kungdian high, a paleogeographic unit that was positive during part of Paleozoic time and whose crest is characterized by nonmarine Upper Triassic rocks unconformably overlying Proterozoic basement.

In the western part of the Longmen Shan region, the allochthon is composed mainly of a very thick succession of strongly folded Middle and Upper Triassic Songpan Ganzi flysch. Along the eastern side and at the base of the allochthon, pre-Upper Triassic rocks crop out, forming the only exposures of the western margin of the Yangtze platform. Here, Upper Proterozoic to Ordovician, mainly shallow-marine rocks unconformably overlie Yangtze-type Proterozic basement rocks, but in Silurian time a thick section of fine-grained clastic and carbonate rocks were deposited, marking the initial subsidence of the western Yangtze platform and formation of a continental margin. Similar deep-water rocks were deposited throughout Devonian to Middle Triassic time, when Songpan Ganzi flysch deposition began. Permian conglomerate and basic volcanic rocks in the southeastern part of the allochthon indicate a second period of extension along the continental margin. Evidence suggests that the deep-water region along and west of the Yangtze continental margin was underlain mostly by thin continental crust, but its westernmost part may have contained areas underlain by oceanic crust. In the northern part of the Longmen Shan allochthon, thick Devonian to Upper Triassic shallow-water deposits of the Xue Shan platform are flanked by deep-marine rocks and the platform is interpreted to be a fragment of the Qinling continental margin transported westward during early Mesozoic transpressive tectonism.

In the Longmen Shan region, the allochthon, carrying the western part of the Yangtze continental margin and Songpan Ganzi flysch, was emplaced to the southeast above rocks of the Yangtze platform autochthon. The eastern margin of the allochthon in the northern Longmen Shan is unconformably overlapped by both Lower and Middle Jurassic strata that are continuous with rocks of the autochthon. Folded rocks of the allochthon are unconformably overlapped by Lower and Middle Jurassic rocks in rare outcrops in the northern part of the region. They also are extensively intruded by a poorly dated, generally undeformed belt, of plutons whose ages (mostly K/Ar ages) range from Late Triassic to early Cenozoic, but most of the reliable ages are early Mesozoic. All evidence indicates that the major deformation within the allochthon is Late Triassic/Early Jurassic in age (Indosinian). The eastern front of the allochthon trends southwest across the present mountain front, so it lies along the mountain front in the northeast, but is located well to the west of the present mountain front on the south.

The Late Triassic deformation is characterized by upright to overturned folded and refolded Triassic flysch, with generally NW-trending axial traces in the western part of the region. Folds and thrust faults curve to the north when traced to the east, so that along the eastern front of the allochthon structures trend northeast, involve pre-Triassic rocks, and parallel the eastern boundary of the allochthon. The curvature of structural trends is interpreted as forming part of a left-lateral transpressive boundary developed during emplacement of the allochthon. Regionally, the Longmen Shan lies along a NE-trending transpressive margin of the Yangtze platform within a broad zone of generally N-S shortening. North of the Longmen Shan region, northward subduction led to collision of the South and North China continental fragments along the Qinling Mountains, but northwest of the Longmen Shan region, subduction led to shortening within the Songpan Ganzi flysch basin, forming a detached fold-and-thrust belt. South of the Longmen Shan region, the flysch basin is bounded by the Shaluli Shan/Chola Shan arc—an originally Sfacing arc that reversed polarity in Late Triassic time, leading to shortening along the southern margin of the Songpan Ganzi flysch belt. Shortening within the flysch belt was oblique to the Yangtze continental margin such that the allochthon in the Longmen Shan region was emplaced within a left-lateral transpressive environment. Possible clockwise rotation of the Yangtze platform (part of the South China continental fragment) also may have contributed to left-lateral transpression with SE-directed shortening. During left-lateral transpression, the Xue Shan platform was displaced southwestward from the Qinling orogen and incorporated into the Longmen Shan allochthon. Westward movement of the platform caused complex refolding in the northern part of the Longmen Shan region.

Emplacement of the allochthon flexurally loaded the western part of the Yangtze platform autochthon, forming a Late Triassic foredeep. Foredeep deposition, often involving thick conglomerate units derived from the west, continued from Middle Jurassic into Cretaceous time, although evidence for deformation of this age in the allochthon is generally lacking.

Folding in the eastern Sichuan fold-and-thrust belt along the eastern side of the Sichuan Basin can be dated as Late Jurassic or Early Cretaceous in age, but only in areas 100 km east of the westernmost folds. Folding and thrusting was related to convergent activity far to the east along the eastern margin of South China. The westernmost folds trend southwest and merge to the south with folds and locally form refolded folds that involve Upper Cretaceous and lower Cenozoic rocks. The boundary between Cenozoic and late Mesozoic folding on the eastern and southern margins of the Sichuan Basin remains poorly determined.

The present mountainous eastern margin of the Tibetan Plateau in the Longmen Shan region is a consequence of Cenozoic deformation. It rises within 100 km from 500–600 m in the Sichuan Basin to peaks in the west reaching 5500 m and 7500 m in the north and south, respectively. West of these high peaks is the eastern part of the Tibetan Plateau, an area of low relief at an elevations of about 4000 m.

Cenozoic deformation can be demonstrated in the autochthon of the southern Longmen Shan, where the stratigraphic sequence is without an angular unconformity from Paleozoic to Eocene or Oligocene time. During Cenozoic deformation, the western part of the Yangtze platform (part of the autochthon for Late Triassic deformation) was deformed into a N- to NE-trending foldandthrust belt. In its eastern part the fold-thrust belt is detached near the base of the platform succession and affects rocks within and along the western and southern margin of the Sichuan Basin, but to the west and south the detachment is within Proterozoic basement rocks. The westernmost structures of the fold-thrust belt form a belt of exposed basement massifs. During the middle and later part of the Cenozoic deformation, strike-slip faulting became important; the fold-thrust belt became partly right-lateral transpressive in the central and northeastern Longmen Shan. The southern part of the fold-thrust belt has a more complex evolution. Early Nto NE-trending folds and thrust faults are deformed by NW-trending basementinvolved folds and thrust faults that intersect with the NE-trending right-lateral strike-slip faults. Youngest structures in this southern area are dominated by left-lateral transpression related to movement on the Xianshuihe fault system.

The extent of Cenozoic deformation within the area underlain by the early Mesozoic allochthon remains unknown, because of the absence of rocks of the appropriate age to date Cenozoic deformation. Klippen of the allochthon were emplaced above the Cenozoic fold-andthrust belt in the central part of the eastern Longmen Shan, indicating that the allochthon was at least partly reactivated during Cenozoic time. Only in the Min Shan in the northern part of the allochthon is Cenozoic deformation demonstrated along two active zones of E-W shortening and associated left-slip. These structures trend obliquely across early Mesozoic structures and are probably related to shortening transferred from a major zone of active left-slip faulting that trends through the western Qinling Mountains. Active deformation is along the left-slip transpressive NW-trending Xianshuihe fault zone in the south, right-slip transpression along several major NE-trending faults in the central and northeastern Longmen Shan, and E-W shortening with minor left-slip movement along the Min Jiang and Huya fault zones in the north.

Our estimates of Cenozoic shortening along the eastern margin of the Tibetan Plateau appear to be inadequate to account for the thick crust and high elevation of the plateau. We suggest here that the thick crust and high elevation is caused by lateral flow of the middle and lower crust eastward from the central part of the plateau and only minor crustal shortening in the upper crust. Upper crustal structure is largely controlled in the Longmen Shan region by older crustal anisotropics; thus shortening and eastward movement of upper crustal material is characterized by irregular deformation localized along older structural boundaries.     

Late Quaternary activity and dextral strike-slip movement on the Karakax Fault Zone, northwest Tibet

Field observations and interpretations of satellite images reveal that the westernmost segment of the Altyn Tagh Fault (called Karakax Fault Zone) striking WNW located in the northwestern margin of the Tibetan Plateau has distinctive geomorphic and tectonic features indicative of right-lateral strike-slip fault in the Late Quaternary. South-flowing gullies and N–S-trending ridges are systematically deflected and offset by up to ~ 1250 m, and Late Pleistocene–Holocene alluvial fans and small gullies that incise south-sloping fans record dextral offset up to ~ 150 m along the fault zone. Fault scarps developed on alluvial fans vary in height from 1 to 24 m. Riedel composite fabrics of foliated cataclastic rocks including cataclasite and fault gouge developed in the shear zone indicate a principal right-lateral shear sense with a thrust component. Based on offset Late Quaternary alluvial fans, ¹⁴C ages and composite fabrics of cataclastic fault rocks, it is inferred that the average right-lateral strike-slip rate along the Karakax Fault Zone is ~ 9 mm/a in the Late Quaternary, with a vertical component of ~ 2 mm/a, and that a M 7.5 morphogenic earthquake occurred along this fault in 1902. We suggest that right-lateral slip in the Late Quaternary along the WNW-trending Karakax Fault Zone is caused by escape tectonics that accommodate north–south shortening of the western Tibetan Plateau due to ongoing northward penetration of the Indian plate into the Eurasian plate.     

Cretaceous deformation history of the middle Tan-Lu fault zone in Shandong Province, eastern China

Based on field analysis of fault-slip data from different rock units of the Cretaceous basins along the middle part of the Tan-Lu fault zone (Shandong Province, eastern China), we document polyphase tectonic stress fields and address the changes in sense of motion of the Tan-Lu fault zone during the Cretaceous. The Cretaceous deformation history of the Tan-Lu fault zone can be divided into four main stages. The first stage, during the earliest Cretaceous, was dominated by N-S extension responsible for the formation of the Jiaolai basin. We interpret this extension to be related to dextral strike-slip pull-apart opening guided by the Tan-Lu fault zone. The second stage, during the middle Early Cretaceous, was overwhelmingly rift-dominated and characterized by widespread silicic to intermediate volcanism, normal faulting and basin subsidence. It was at this stage that the Tan-Lu-parallel Yi-Shu Rift was initiated by E-W to WNW-ESE extension. The tectonic regime then changed during the late Early Cretaceous to NW-SE-oriented transpression, causing inversion of the Early Cretaceous rift basin and sinistral slip along the Tan-Lu fault zone. During the Late Cretaceous, dextral activation of the Tan-Lu fault zone resulted in pull-apart opening of the Zhucheng basin, which was subsequently deformed by NE-SW compression. This deformation chronology of the Tan-Lu fault zone and the associated Cretaceous basins allow us to constrain the regional kinematic models as related to subduction along the eastern margin of Asia, or related to collision in the Tibet region.     

试论中国南北大陆的碰撞造山带

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渤海海域侏罗—白垩纪沉积与构造演化

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