Transpressional deformation,strain partitioning and fold superimposition in the southern Chinese Altai,Central Asian Orogenic Belt

Transpressional deformation has played an important role in the late Paleozoic evolution of the western Central Asian Orogenic Belt (CAOB), and understanding the structural evolution of such transpressional zones is crucial for tectonic reconstructions. Here we focus on the transpressional Irtysh Shear Zone with an aim at understanding amalgamation processes between the Chinese Altai and the West/East Junggar. We mapped macroscopic fold structures in the southern Chinese Altai and analyzed their relationships with the development of the adjacent Irtysh Shear Zone. Structural observations from these macroscopic folds show evidence for four generations of folding and associated fabrics. The earlier fabric (S₁), is locally recognized in low strain areas, and is commonly isoclinally folded by F₂ folds that have an axial plane orientation parallel to the dominant fabric (S₂). S₂ is associated with a shallowly plunging stretching lineation (L₂), and defines ∼NW-SE tight-close upright macroscopic folds (F₃) with the doubly plunging geometry. F₃ folds are superimposed by ∼NNW-SSE gentle F₄ folds. The F₃ and F₄ folds are kinematically compatible with sinistral transpressional deformation along the Irtysh Shear Zone and may represent strain partitioning during deformation. The sub-parallelism of F₃ fold axis with the Irtysh Shear Zone may have resulted from strain partitioning associated with simple shear deformation along narrow mylonite zones and pure shear-dominant deformation (F₃) in fold zones. The strain partitioning may have become less efficient in the later stage of transpressional deformation, so that a fraction of transcurrent components was partitioned into F₄ folds.     

Transpressional regime in southern Arabian Shield: Insights from Wadi Yiba Area, Saudi Arabia

Detailed field-structural mapping of Neoproterozoic basement rocks exposed in the Wadi Yiba area, southern Arabian Shield, Saudi Arabia illustrates an important episode of late Neoproterozoic transpression in the southern part of the Arabian-Nubian Shield (ANS). This area is dominated by five main basement lithologies: gneisses, metavolcanics, Ablah Group (meta-clastic and marble units) and syn- and post-tectonic granitoids. These rocks were affected by three phases of deformation (D₁–D₃). D₁ formed tight to isoclinal and intrafolial folds (F₁), penetrative foliation (S₁), and mineral lineation (L₁), which resulted from early E-W (to ENE-WSW) shortening. D₂ deformation overprinted D₁ structures and was dominated by transpression and top-to-the-W (?WSW) thrusting as shortening progressed. Stretching lineation trajectories, S-C foliations, asymmetric shear fabrics and related mylonitic foliation, and flat-ramp and duplex geometries further indicate the inferred transport direction. The N- to NNW-orientation of both “in-sequence piggy-back thrusts” and axial planes of minor and major F₂ thrust-related overturned folds also indicates the same D₂ compressional stress trajectories. The Wadi Yiba Shear Zone (WYSZ) formed during D₂ deformation. It is one of several N-S trending brittle-ductile Late Neoproterozoic shear zones in the southern part of the ANS. Shear sense indicators reveal that shearing during D₂ regional-scale transpression was dextral and is consistent with the mega-scale sigmoidal patterns recognized on Landsat images. The shearing led to the formation of the WYSZ and consequent F₂ shear zone-related folds, as well as other unmappable shear zones in the deformed rocks. Emplacement of the syn-tectonic granitoids is likely to have occurred during D₂ transpression and occupied space created during thrust propagation. D₁ and D₂ structures are locally overprinted by mesoscopic- to macroscopic-scale D₃ structures (F₃ folds, and L₃ crenulation lineations and kink bands). F₃ folds are frequently open and have steep to subvertical axial planes and axes that plunge ENE to ESE. This deformation may reflect progressive convergence between East and West Gondwana.     

滇西澜沧江构造带及邻区几何学、运动学和构造年代学分析

滇西澜沧江构造带自北向南沿碧螺雪山和崇山连续延伸; 按照构造几何学特点和运动学特征我们把该构造带分为3段：北段、中段和南段; 本文对各段的构造、组构、运动学及构造年代学进行了翔实研究,得到以下认识：构造带呈双变质岩带,核部为强变形高级变质岩带,两侧为强变形低级变质岩带,部分剖面几何形态似“花状”构造; 宏观和微观组构特征均指示构造带北段和中?南段存在明显的运动学差异,北段为右旋走滑剪切,中、南段为左旋走滑剪切; 同构造浅色花岗质糜棱岩中分选出白云母(北段)和黑云母(中段),进行单颗粒矿物的激光熔化40Ar?39Ar定年,结果显示,糜棱岩化造成了花岗质岩石同位素时钟的重置和部分重置; 表面年龄指示了该构造带中新世的构造变形事件; 其中,北段右旋韧性剪切作用年龄为17.8～13.4 Ma或更早,至少持续到13.4 Ma,构造带中段记录了17.9～13.1 Ma的左行韧性剪切事件; 构造变形时代表现出同时代和同期次特点。综合分析认为,位于印度与欧亚大陆斜向汇聚带东缘的澜沧江构造带,是调节印支块体陆内变形的重要变形区域,为典型的新生代剪压应变区; 与区内哀牢山-红河构造带新生代左旋走滑相对应; 剪压应变和应变分解过程中,构造带东-西向减薄作用通过韧性物质垂向挤出和沿剪切方向的挤出平衡,垂向挤出导致地壳增厚和高应变体的抬升,形成现今的地貌高位,统一的陡立面理和亚水平拉伸线理是韧性物质沿剪切方向挤出的流变学响应; 构造带南段和北段运动学差异是澜沧江构造带新生代左旋剪压应变分解的必然产物和运动学要求。     

Intracontinental thrusts and inclined transpression along eastern margin of the East Dharwar craton, India

Recent works suggest Proterozoic plate convergence along the southeastern margin of India which led to amalgamation of the high grade Eastern Ghats belt (EGB) and adjoining fold-and-thrust belts to the East Dhrawar craton. Two major thrusts namely the Vellikonda thrust at the western margin of the Nellore Schist belt (NSB) and the Maidukuru thrust at the western margin of the Nallamalai fold belt (NFB) accommodate significant upper crustal shortening, which is indicated by juxtaposition of geological terranes with distinct tectonostratigraphy, varying deformation intensity, structural styles and metamorphic grade. Kinematic analysis of structures and fabric of the fault zone rocks in these intracontinental thrust zones and the hanging wall and footwall rocks suggest spatially heterogeneous partitioning of strain into various combinations of E-W shortening, top-to-west shear on stratum parallel subhorizontal detachments or on easterly dipping thrusts, and a strike slip component. Although relatively less prominent than the other two components of the strain triangle, non-orthogonal slickenfibres associated with flexural slip folds and mylonitic foliation-stretching lineation orientation geometry within the arcuate NSB and NFB indicate left lateral strike slip subparallel to the overall N-S trend. On the whole an inclined transpression is inferred to have controlled the spatially heterogeneous development of thrust related fabric in the terrane between the Eastern Ghats belt south of the Godavari graben and the East Dharwar craton.     

Extent,kinematics and tectonic origin of the Precambrian Aswa Shear Zone in eastern Africa

The Aswa Shear Zone (ASZ) is a fundamental Precambrian lithospheric structure that has been shaped by many tectonic events in eastern Africa. It separates the Saharan Metacraton in the northeast from the Northern Uganda terrane (which represents part of the Northeastern Congo block of the Congo craton) to the southwest. Nonetheless, its tectonic evolution is not fully understood. We used high-resolution airborne magnetic and radiometric data over Uganda integrated with Shuttle Radar Topography Mission (SRTM) Digital Elevation Model (DEM) in South Sudan to assess the extent, kinematics and contribute to the understanding of the tectonic origin of the ASZ. (1) Our results showed that the ASZ extends in a NW–SE direction for ~ 550 km in Uganda and South Sudan. (2) The airborne magnetic and radiometric data revealed a much wider (~ 50 km) deformation belt than the 5-10 km of the exposed surface expression of the ASZ. The deformation belt is defined by three NW-trending sinistral strike-slip shear zones bounding structural domains with magnetic fabric showing splays of secondary shear zones and shear-related folds. These folds are tighter close to the discrete shear zones with their axial traces becoming sub-parallel to the shear zones. A similar fold pattern is observed in South Sudan from the SRTM DEM. We interpreted these folds as due to ENE–WSW contraction associated with the sinistral strike-slip movement. (3) To the northeast, the magnetic patterns and radiometric signatures suggest the presence of a series of W-verging nappes indicative of strong E–W to NE–SW contraction deformation. (4) We relate the evolution of the ASZ to E–W to NE–SW Neoproterozoic oblique collision between East and West Gondwana. The deformation related to this collision was partitioned into E–W to NE–SW contraction resulting in W-verging thrusts in the east and a sinistral strike-slip movement along the NW-trending ASZ with the strain localized at the boundary between the Saharan Metacraton and the Northern Uganda terrane.     

Structures,kinematic analysis,rheological parameters and temperature-pressure estimate of the Mesozoic Xingcheng-Taili ductile shear zone in the North China Craton

The NE to ENE trending Mesozoic Xingcheng-Taili ductile shear zone of the northeastern North China Craton was shaped by three phases of deformation. Deformation phase D₁ is characterized by a steep, generally E–W striking gneissosity. It was then overprinted by deformation phase D₂ with NE-sinistral shear with K-feldspar porphyroclasts forming a subhorizontal low-angle stretching lineation on a steep foliation. During deformation phase D₃, lateral motion accommodated by ENE sinistral strike-slip shear zones dominated. Associated fabrics developed at upper greenschist metamorphic facies conditions and show the deformation characteristics of middle- to shallow crustal levels. In some parts, the older structures have been in turn overprinted by late-stage sinistral D₃ shearing. Finite strain and kinematic vorticity in all deformed granitic rocks indicate a prolate ellipsoid (L-S tectonites) near plane strain. Simple shear-dominated general shear during D₃ deformation is probably of general significance. The quartz c-axis textures indicate prism-gliding with a dominant rhomb <a> slip and basal <a> slip system formed mainly at low-middle temperatures. Mineral deformation behavior, quartz c-axis textures, quartz grain size and the Kruhl thermometer demonstrate that the ductile shear zone developed under greenschist facies metamorphic conditions at deformation temperatures ranging from 400 to 500 °C. Dislocation creep is the main deformation mechanism at a shallow crustal level. Fractal analysis showed that the boundaries of recrystallized quartz grains had statistically self-similarities. Differential stresses deduced from dynamically recrystallized quartz grain size are at around 20–39 MPa, and strain rates in the order of 10⁻¹² to 10⁻¹⁴ s⁻¹. This indicates deformation of granitic rocks in the Xingcheng-Taili ductile shear zone at low strain rates, which is consistent with most other ductile shear zones. Hornblende-plagioclase thermometer and white mica barometer indicate metamorphic conditions of medium pressures at around ca. 3–5 kbar and temperatures of 400–500 °C within greenschist facies conditions. The main D₃ deformation of the ENE-trending sinistral strike-slip ductile shearing is related to the roll-back of the subducting Pacific plate beneath the North China Craton.     

Structural evolution of the Irtysh Shear Zone (northwestern China) and implications for the amalgamation of arc systems in the Central Asian Orogenic Belt

The NW–SE Irtysh Shear Zone is a major tectonic boundary in the Central Asian Orogenic Belt (CAOB), which supposedly records the amalgamation history between the peri-Siberian orogenic system and the Kazakhstan/south Mongolia orogenic system. However, the tectonic evolution of the Irtysh Shear Zone is not fully understood. Here we present new structural and geochronological data, which together with other constraints on the timing of deformation suggests that the Irtysh Shear Zone was subjected to three phases of deformation in the late Paleozoic. D₁ is locally recognized as folded foliations in low strain areas and as an internal fabric within garnet porphyroblasts. D₂ is represented by a shallowly dipping fabric and related ∼ NW–SE stretching lineations oriented sub-parallel to the strike of the orogen. D₂ foliations are folded by ∼ NW–SE folds (F₃) that are bounded by a series of mylonite zones with evidence for sinistral/reverse kinematics. These fold and shear structures are kinematically compatible, and thus interpreted to result from a transpressional deformation phase (D₃). Two samples of mica schists yielded youngest detrital zircon peaks at ∼322 Ma, placing a maximum constraint on the timing of D₁–D₃ deformation. A ∼ NE–SW granitic dyke swarm (∼252 Ma) crosscuts D₃ fold structures and mylonitic fabrics in the central part of the shear zone, but is displaced by a mylonite zone that represents the southern boundary of the Irtysh Shear Zone. This observation indicates that the major phase of D₃ transpressional deformation took place prior to ∼252 Ma, although later phases of reactivation in the Mesozoic and Cenozoic are likely. The late Paleozoic deformation (D₁–D₃ at ∼322–252 Ma) overlaps in time with the collision between the Chinese Altai and the intra-oceanic arc system of the East Junggar. We therefore interpret that three episodes of late Paleozoic deformation represent orogenic thickening (D₁), collapse (D₂), and transpressional deformation (D₃) during the convergence between the Chinese Altai and the East Junggar. On a larger scale, late Paleozoic sinistral shearing (D₃), together with dextral shearing farther south, accommodated the eastward migration of internal segments of the western CAOB, possibly associated with the amalgamation of multiple arc systems and continental blocks during the late Paleozoic.     

Usage of strain and vorticity analyses to interpret large‐scale fold mechanisms along the Sanandaj–Sirjan HP‐LT metamorphic belt,SW Iran

3D finite strain analyses and kinematic vorticity measurements were carried out on the Loghon Anticline within the HP‐LT Sanandaj–Sirjan metamorphic belt (Neyriz area, SW Iran). Rƒ/φ and Fry methods were used on the strain markers (e.g. deformed fossils) to interpret geometric relationships between the fold axis, strain ellipsoid axes and shear zone boundaries. The results indicate the predominance of prolate strain in the anticline. Quantitative kinematic analyses show that the W_k parameter is 0. 67 ± 0. 06 (i.e. pure‐shear dominated non‐coaxial flow). This study quantitatively supports the establishment of a dextral transpressive system, which is responsible for the development of the large‐scale right‐lateral shear zones that strike sub‐parallel to the major folds. Flexural shear combined with regional dextral‐shear is suggested to be the most common mechanism of folding in this area. Copyright © 2011 John Wiley & Sons, Ltd.     

Transpression,extrusion, partitioning,and lateral escape in the middle crust: Significance of structures,fabrics, and kinematics in the Bronson Hill zone,southern New England,U.S.A.

Fabrics in the mid-crustal Bronson Hill zone of the southern New England Appalachian orogen record a range of apparent finite strains and conflicting kinematics, but structural relationships indicate coeval development. At the smallest scale of this study, shortening was accommodated in granitic orthogneiss, while transcurrent deformation was partitioned into relatively thin zones of metastratified rocks along the margins. The Monson orthogneiss can be broadly characterized by subvertical to steeply dipping S > L tectonites, subvertical to subhorizontal stretching lineations, closed to isoclinal folds, and dextral/reverse kinematics. The east-bounding Conant Brook shear zone and Greenwich syncline are characterized by steeply dipping mylonitic foliations, a range of lineations, and dextral/reverse kinematic indicators. The west-bounding Mt. Dumplin high strain zone is comprised of steeply dipping mylonites, subhorizontal lineations, and sinistral/normal kinematics. These structures reflect coeval partitioned dextral transpression, vertical extrusion, and north-directed lateral escape of the orthogneiss that was facilitated by bounding conjugate shear zones. Comparison of structural subdomains with transpressional modeling indicates vertical pseudo-monoclinic to inclined triclinic coaxial to simple shear influenced transpression. Compatibility between laterally adjacent subdomains was maintained by meso-/microscale partitioning. Absolute and relative timing constraints show that transpression was sustained from 330 Ma to 300 Ma.     

A Large Ductile Sinistral Strike-Slip Shear Zone and Its Movement Timing in the South Qilian Mountains, Western China

There is a large ductile shear zone, 2 km wide and more than 3SO km long, in the South Qilian Mountains, western China. It is composed of volcanic, granitic and calcareous mylonites. The microstructures of the ductile shear zone show nearly E-W extending subvertical foliation, horizontal and oblique stretching lineations, shearing sense from sinis-tral to oblique sinistral strike-slip from east to west, "A" type folds and abundant granitic veins. Measured lattice preferred orientations (LPOs) of the mylonitic and recrystallized quartz of the granitic mylonite in the west segment suggest a strong LPO characterized by the dominant slip systems {1010} formed at high temperature (>650℃). K-feldspar of the mylonite shows an 39Ar/40Ar high-temperature plateau age of 243.3±1.3 Ma, and biotite, 250.5±0.5 Ma, which represent the formation age of the ductile shear zone. The 39Ar/40Ar plateau ages of 169.7±0.3 Ma and 160.6±0.1 Ma and the 39Ar/40Ar isochron ages of 166.99±2.37 Ma and 160.6±0.1 Ma of biot     

From oblique accretion to transpression in the evolution of the Altaid collage: New insights from West Junggar,northwestern China

Along active margins, tectonic features that develop in response to plate convergence are strongly controlled by subduction zone geometry. In West Junggar, a segment of the giant Palaeozoic collage of Central Asia, the West Karamay Unit represents a Carboniferous accretionary complex composed of fore-arc sedimentary rocks and ophiolitic mélanges. The occurrence of quasi-synchronous upright folds and folds with vertical axes suggests that transpression plays a significant role in the tectonic evolution of the West Junggar. Latest Carboniferous (ca. 300 Ma) alkaline plutons postdate this early phase of folding, which was synchronous with accretion of the Carboniferous complex. The Permian Dalabute sinistral fault overprints Carboniferous ductile shearing and split the West Karamay Unit ca. 100 km apart. Oblique convergence may have been provoked by the buckling of the Kazakh orocline and relative rotations between its segments. Depending upon the shape of the convergence zone, either upright folds and fold with vertical axes, or alternatively, strike–slip brittle faults developed in response to strain partitioning. Sinistral brittle faulting may account for the lateral imbrication of units in the West Junggar accretionary complex.     

Interactions between serpentinite devolatilization, metasomatism and strike-slip strain localization during deep-crustal shearing in the Eastern Alps

The Greiner shear zone in the Tauern Window, Eastern Alps, changes from a zone of distributed (dominantly sinistral) shear in supracrustal rocks to a series of narrow, gully forming dextral splays where it enters basement gneisses. Within these splays, granodiorite is transformed into quartz‐poor biotite and/or chlorite schists, reflecting hydration, removal of Si, Ca and Na, and concentration of Fe, Mg and Al. Stable isotope analyses show a prominent increase in δD and a decrease in δ¹⁸O from granodiorite into the shear zones. These changes indicate significant channelized flow of an externally derived, low‐δ¹⁸O, high‐δD fluid through the shear zones. The shear zone schists are chemically similar to blackwall zones developed around serpentinite bodies elsewhere in the Greiner zone and the stable isotope data support alteration via serpentinite‐derived fluid. Monazite in schist from one shear zone yields spot dates of 29–20 Ma, indicating that the fluid influx and switch from sinistral to dextral shear occurred at or shortly after the thermal peak of the Alpine orogeny (c. 30 Ma). We suggest that Alpine metamorphism of serpentinites released large amounts of high‐δD, low‐δ¹⁸O, Si‐undersaturated, Fe + Mg‐saturated fluids that became channelized along prior zones of weakness in the granodiorite. Infiltration of this fluid facilitated growth of chlorite and biotite, which in turn localized later dextral strain in the narrow splays via cleavage‐parallel slip. This dextral strain event can be linked to other structures that accommodated tectonic escape of major crustal blocks during dextral transpression in the Eastern Alps. This study shows that serpentinite devolatilization can play an important role in modifying both the chemistry and rheology of surrounding rocks during orogenesis.     

Transpressional imbricate thrust zones controlling gold mineralization in the Central Eastern Desert of Egypt

Strongly deformed volcaniclastic metasediments and ophiolitic slices hosting the Sukari gold mineralization display evidence of a complex structural evolution involving three main ductile deformational events (D₁–D₃). D₁ produced ENE-trending folds associated with NNW-propagating thrust slices and intrusion of the Sukari granite (689 ± 3 Ma). D₂ formed a moderately to steeply dipping, NNW-trending S₂ foliation curved to NE and developed arcuate structure constituting the Kurdeman shear zone (≤ 595 Ma) and East Sukari imbricate thrust belt. Major NE-trending F₂ folds, NW-dipping high-angle thrusts, shallow and steeply plunging mineral lineation and shear indicators recorded both subhorizontal and subvertical transport direction during D₂. D₃ (560–540 Ma) formed NNE-trending S₃ crenulation cleavage, tight F₃ folds, Sukari Thrust and West Sukari imbricate thrust. The system of NW-trending sinistral Kurdeman shear zone (lateral ramps and tear faults) and imbricate thrusts (frontal ramps) forming the actuate structure developed during SE-directed thrusting, whereas the prevailing pattern of NNE-trending dextral Sukari shear zone and imbricate thrusts forming Sukari thrust duplex developed during NE-directed tectonic shearing. Sukari granite intruded in different pluses between 689 and 540 Ma and associated with at least four phases of quartz veins with different geometry and orientation. Structural analysis of the shear fabrics indicates that the geometry of the mineralized quartz veins and alteration patterns are controlled by the regional NNW- and NE-trending conjugate zones of transpression. Gold-bearing quartz veins are located within NNW-oriented sinistral shear zones in Kurdeman gold mine area, within steeply dipping NW- and SE dipping thrusts and NE- and NS-oriented dextral and sinistral shear zones around Sukari mine area, and along E-dipping backthrusts and NW-SE and N-S fractures in Sukari granite. The high grade of gold mineralization in Sukari is mainly controlled by SE-dipping back-thrusts branched from the major NW-dipping Sukari Thrust. The gold mineralization in Sukari gold mine and neighboring areas in the Central Eastern Desert of Egypt is mainly controlled by the conjugate shear zones of the Najd Fault System and related to E-W directed shortening associated with oblique convergence between East and West Gondwana.     

Extrusion tectonics inferred from fabric study of the Guanzizhen ophiolitic mélange belt in the West Qinling orogen,Central China

The recently identified Guanzizhen ophiolitic mélange belt (GOMB) in the West Qinling orogen is regarded, by many geochemical and geochronological works, as the westward extension of the Shangdan belt in the East Qinling that separated the North and South China blocks. The GOMB trends generally NW–SE and comprises ophiolites and forearc volcanic–sedimentary complex (the Liziyuan Group). Ductile shear zones with various widths are a striking structural feature of the GOMB and studying them could contribute to a better understanding of the tectonic processes of the collisional orogeny in this area. Here we characterize the deformation style of these shear zones by combining the low-field anisotropy of magnetic susceptibility (AMS) data, microstructural and field observations. The new dataset demonstrates that the deformation of these ductile shear zones is dominated by dextral transpressional shearing under low amphibolite grade condition. Combining the dextral transpression of the GOMB and the coeval sinistral transpression of the Shangdan belt in the East Qinling, we propose a Late Triassic bilateral extrusion tectonics of the Qinling orogen in response to the collision between the Ordos and Sichuan blocks.     

Initiation Timing of the Jiamusi–Yitong Fault Zone in NE China

The NE–striking Jiamusi–Yitong fault zone(JYFZ) is the most important branch in the northern segment of the Tancheng–Lujiang fault zone. The precise shearing time of its large–scale sinistral strike–slip has yet to determined and must be constrained. Detailed field investigations and comprehensive analyses show that strike–slip faults or ductile shear belts exist as origination structures along the western region of Yitong Graben. The strike of the shear belts trend to the NE–SW with steep mylonitic foliation. The zircon U–Pb dating result for the granite was 264.1±1 Ma in the ductile shear belt of the JYFZ. The microstructural observation(rotated feldspar porphyroclasts, S–C fabrics, and quartz c–axis fabrics, etc.) demonstrated the sinistral shearing of the ductile shear zones. Moreover, the recrystallized quartz types show a transitional stage of the subgrain rotation toward the recrystallization of the grain boundary migration(SR–GBM). Therefore, we suggest that the metamorphic grade of the shear zone in the ductile shear zones should have reached high greenschist facies conditions, and the deformation temperatures should approximately 450–500°C, which is obviously higher than the blocking temperature of muscovite(300–400°C). Hence, the ~(40)Ar/~(39)Ar isochron age of muscovite from ductile shear zones should be a cooling age(162.7±1 Ma). We infer that the sinistral strike–slipping event at the JYFZ occurred in the late Jurassic period, and it was further inferred from the ages of the main geological events in this region that the second sinistral strike–slip age of the Tancheng–Lujiang fault zone occurred during the period of tectonic movements in the Circum–Pacific tectonic domain. This discovery also indicates the age of the Tancheng–Lujiang fault zone that stretches to northeastern China. The initiation of the JYFZ in the late Jurassic is related to the speed and direction of oblique subduction of the west Pacific Plate under the Eurasian continent and is responsible for collision during the Jurassic period.     

准噶尔西北缘斜向挤压构造与走滑断裂

古板块构造研究表明古生代以来,准噶尔地块以斜向方式拼合到阿尔泰（Altaids）造山带构造域内,导致准噶尔西北缘一直处在斜向挤压构造背景之下,并发育高角度逆冲推覆构造带以及横向走滑断裂。地面地质调查、遥感卫星影像解译、大地电磁测深剖面和地震剖面构造解释资料综合研究表明,准噶尔西北缘边界断裂为达拉布特左行走滑断裂,西北缘逆冲推覆构造带为基底卷入的高角度逆冲断裂褶皱带; 在与逆冲带走向相垂直方向,发育有北西向横向走滑断裂,这些断裂为同一斜向挤压构造背景下形成的同期构造,形成时间约在晚二叠到侏罗纪之间。大地电磁测深和地震剖面解释表明,达拉布特走滑断裂控制了西北缘高角度逆冲断裂的分布与性质。西北缘二维和三维地震剖面解释表明,横向走滑断裂样式为正花状构造或者负花状构造,同时具有向南东或北西方向逆冲和拉伸的特征。横向走滑断裂为西北缘逆冲构造南北方向分段的主要断裂,并控制了西北缘中生代地层的沉积。西北缘构造是形成于主边界断裂的斜向挤压作用,而基底卷入逆冲断裂则属非纯挤压形成的逆冲构造。     

Transposition of structures in the Neoproterozoic Kaoko Belt (NW Namibia) and their absolute timing

Three structural profiles across the Coastal Terrane, the Boundary Igneous Complex and the Orogen Core have been studied in the Kaoko Belt of northwestern Namibia. The oldest known Si fabric is inherited from an older tectono-metamorphic event. It occurs in the Coastal Terrane only and the extent of its reworking increases from south to north. The S1 foliation reactivates or folds Si fabric in the Coastal Terrane and appears as an early planar fabric in granitoids of the Boundary Igneous Complex and migmatites of the Orogen Core domain. Superimposed subvertical S2 fabric corresponds to axial plane cleavage of upright close to isoclinal folds and the extent of its development also increases from south to north. Active migration of partial melt during S2 development in the Orogen Core dates the onset of this deformation at ~550?Ma. Distribution of F2 fold axes and L2 stretching lineations suggests pure shear?Cdominated deformation associated with development of N?CS trending S2 cleavage preserved in the central profile, followed by sinistral simple shear?Cdominated deformation on newly developed NW?CSE trending pervasive cleavage in the northern part of the area. Such spatial variation in the deformation record is attributed to the irregular shape of the Congo Craton indenter that is reflected by heterogeneous development of the S2 cleavage front in the Coastal Terrane and the Boundary Igneous Complex. Common orientation of L1 and L2 stretching lineations and solid-state reworking on both S1 and S2 planes suggest single event of sinistral transpression since 550?Ma with strain partitioning into domains of oblique thrusting (reactivated S1) and transcurrent sinistral shearing (S2 and S3). Such succession of deformation structures suggests that major subvertical shear zones in the Kaoko Belt do not correspond to early crustal discontinuities, but rather reflect late strain localization during cooling.     

Structural pattern and strain history of a superposed fold system in the Precambrian of Central Rajasthan,India. II. Strain history

Analysis of shapes of folds, together with other structures such as axial plane foliation boudinage, mullions and cross joints, show that the F₁ folds in the ‘main Raialo syncline’ were formed by buckling, and were subsequently modified by flattening normal to the axial planes and lengthening along the axis. The apparent buckle shortening of the F₁ folds generally ranges between 70 and 80%. The folds were formed by simple shear (giving place to pure shear at certain stages) in an almost north-south direction on subhorizontal beds. Progressive deformation in the later stage of F₁ folding resulted in gentle upright folding of F₁ axial planes on F₁′ axes slightly oblique to F₁. The F₂ folds, whose average shortening ranges from 20 to 30%, were also formed by buckling caused by horizontal compression in a nearly northwest-southeast direction. This folding was preceded and followed in some instances by homogeneous strain, as deduced fro mthe shapes of the F₂ folds and the nature of variation of the F₁ lineations. The F₃ conjugate structures developed when the maximum compressive strain was vertical and the intermediate compressive strain northwest-southeast, almost normal to the subvertical F₂ axial planes. The increase in the amplitude of the F₂ folds in the last phase of F₂ folding in certain zones resulted in an excess of vertical load, which dissipated with the formation of the F₃ structures. In the last stage of movement (F₄) the maximum compressive strain became horizontal along the strike of F₂ axial planes, whereas the minimum compressive strain was normal to them. The F₄ structures, therefore, point to a longitudinal shortening with reference to large scale F₂ folding.     

Transpression

Transpression is considered as a wrench or transcurrent shear accompanied by horizontal shortening across, and vertical lengthening along, the shear plane. A model for the strain in transpression is derived, from which the shape and orientation of the finite strain ellipsoid, and the stretch and rotation of lines can be determined. Shortening across the zone of transpression leads to oblate finite strain ellipsoids (k<1).By considering the superposition of small increments of strain various model deformation paths are computed. These are used to interpret the development of structures, such as en-échelon folds, in transpression zones. The incremental strain ellipsoid allows prediction of the orientation of the principal stresses and hence brittle structures within such zones. The model is also applied to bends and terminations of shear zones and used to interpret the observed patterns of folds and fractures in these.