Superposed folding of a blind thrust and formation of klippen: results of anisotropic magnetic susceptibility studies from the Lesser Himalaya

The rocks of the Garhwal Lesser Himalaya have undergone a weak superimposed deformation, hence linear and planar structures are either absent or poorly developed. This puts a severe limitation on application of conventional methods of finite strain determination in understanding the deformation pattern. However, the geometry, orientation, and distribution of magnetic susceptibility strain ellipses clearly reveal the effects of early and superposed deformations in the area. The orientation patterns of the ellipses also help to identify reversal of displacement along an oblique fault ramp during the superposed deformation. The Hrouda double plot reveals a combination of lateral shortening and simple shear, thereby suggesting a small translation along the klippe detachment thrust. The study has important implications for understanding the structural evolution of the Lesser Himalayan klippen, because the earlier models, in the absence of the relevant data, are based on assumptions concerning thrust displacement. The present field studies and the AMS data favour an alternate model for the structural evolution of the Lesser Himalayan klippen, that lie in the core of the Mussoorie Syncline. The model explains structural features and outcrop patterns as due to a combination of fault bifurcation, back thrusts, pop-up, and subsequent superposed deformation. The klippen lie over their roots and are described as pop-up klippen.     

Tectonic evolution of a crustal-scale oblique ramp,Hellenides thrust belt,Greece

This study investigates the tectonic evolution of the Omalos transverse zone, which served as a crustal-scale oblique ramp in the External Hellenides thrust belt on Crete island. The Omalos oblique ramp developed above an inherited Mesozoic fault zone that strikes NE–SW, oblique to the regional SSW-directed tectonic transport. During the Early Miocene–Pleistocene evolution of the thrust belt, the oblique ramp was repeatedly reactivated localizing deformation above the inherited structure. Geological and structural mapping combined with kinematic analysis of ductile and brittle structures suggest that the Omalos oblique ramp generated a local kinematic field, which deviated significantly from the regional kinematic pattern in the thrust belt. The most conspicuous feature in the tectonic evolution of the oblique ramp is a change from a ductile wrench-dominated to a brittle, primarily reverse faulting regime across the brittle–ductile transition, followed by brittle wrench deformation after the final exhumation of high-pressure (HP) rocks. Deflections of transport and compression orientations from the regional pattern are attributed to buttressing against basement-cover offsets produced by the pre-existing fault zone, to oblique ramping, and to transfer faulting, respectively. Our findings are potentially applicable to other examples of crustal-scale oblique thrust ramps in various tectonic settings.     

胶东东部中生代走滑逆冲构造带的超微构造研究

胶东东部区域上处于秦岭-大别-苏鲁造山带的东端,是中生代的走滑逆冲构造带.进行超微构造研究有利于解决岩石的变形机制问题.应用偏光显微镜和透射电子显微镜方法,进行了胶东东部走滑逆冲构造带显微构造及超微构造研究.研究认为主要走滑逆冲剪切带的石英位错亚构造以线位错为主,并常见位错环、位错弓弯、位错壁、位错列、位错网及亚晶粒等构造型式颗粒等.石英超微位错构造总体反映的是中温或低温塑性变形环境;石岛剪切带、荣成剪切带、牟平剪切带的平均古差异应力值呈现逐渐降低的趋势,跟温度呈反相关的关系.     

四川雷波马家河坝逆冲断层研究

大比例尺填图和构造解析表明,马家河坝断层是发育于地台内部的一个小型逆冲断层,可分为5个带:前缘逆冲叠瓦带、中部剪切滑动带、东缘斜向逆冲走滑带、西缘斜向正断走滑带及后缘构造变形带。喜山期持续的区域应力作用使那里沟背斜SE翼沿软弱面断裂,经递进扩展变形,最终形成逆冲推覆构造。     

Main Frontal thrust deformation and topographic growth of the Mohand Range,northwest Himalaya

The Main Frontal thrust (MFT) uplifts the Himalayan topographic front. Deciphering MFT deformation kinematics is crucial for understanding how the orogen accommodates continuing continental collision and assessing associated hazards. Here, we (a) detail newly discovered fault-zone exposures along the MFT at the Mohand Range front in northwestern India and (b) apply contemporary fault zone theory to show that the MFT is an emergent fault with a well-developed fault zone overlain by uplifted Quaternary gravels over a horizontal length of ∼700 m. Northward from the front, the fault zone grades from a central, gouge-dominated core to a hanging-wall, rock-dominated damage zone. We observed incohesive, non-foliated breccia, fault gouge, and brittle deformation microstructures within the fractured country rocks (Middle Siwaliks) and outcrop scale, non-plunging folds in the proximal hanging wall. We interpret these observations to suggest that (1) elastico-frictional (brittle) deformation processes operated in the fault zone at near surface (∼1–5 km depth) conditions and (2) the folds formed first at the propagating MFT fault tip, then were subsequently dismembered by the fault itself. Thus, we interpret the Mohand Range as a fault-propagation fold driven by an emergent MFT in contrast to the consensus view that it is a fault-bend fold. A fault-propagation fold model is more consistent with these new observations, the modern range-scale topography, and existing erosion estimates. To further evaluate our proposed structural model, we used a Boundary Element Method-based dislocation model to simulate topographic growth from excess slip at a propagating fault tip. Results show that the frontal topography could have evolved by slip along a (a) near-surface fault plane consistent with the present-day MFT location, or (b) blind MFT at ∼3 km depth farther north near the drainage divide. Comparing modelled vs. measured high resolution (∼16 cm) topographic profiles for each case provides permissible end-member scenarios of an either dynamically-evolving, high erosion, northward-migrating frontal scarp or a static, low, and symmetric, MHT-related fold, respectively. Our integrated approach is expected to deliver an improved understanding of coupled fault-generated deformation and topographic growth that may be applied more broadly across the entire Himalayan front.     

The lesser Himalayan Duplex in Sikkim: Implications for variations in Himalayan shortening

Thrust duplexes account for large fractions of the total shortening in most fold-thrust belts (FTBs). They also provide an efficient mechanism for transferring slip upward from the basal decollement and for transporting roof thrust sheets over long distances. The Lesser Himalayan duplex (LHD) plays a prominent role in the overall evolution of the Himalayan FTB and has been described from Garhwal-Kumaon to Bhutan. In Sikkim the LHD shows unique structural geometry and has been responsible for transporting crystalline thrust sheets (MCT 1 and MCT 2) farther southward than other parts of the Himalaya. Such lateral variations in LHD geometry imply variations in the kinematic history of the Lesser Himalaya and variations in shortening and shortening history along the length of the Himalayan arc, and these are reflected in observable large scale structural patterns.     

Was Himalayan normal faulting triggered by initiation of the Ramgarh–Munsiari thrust and development of the Lesser Himalayan duplex?

The Ramgarh–Munsiari thrust is a major orogen-scale fault that extends for more than 1,500 km along strike in the Himalayan fold-thrust belt. The fault can be traced along the Himalayan arc from Himachal Pradesh, India, in the west to eastern Bhutan. The fault is located within the Lesser Himalayan tectonostratigraphic zone, and it translated Paleoproterozoic Lesser Himalayan rocks more than 100 km toward the foreland. The Ramgarh–Munsiari thrust is always located in the proximal footwall of the Main Central thrust. Northern exposures (toward the hinterland) of the thrust sheet occur in the footwall of the Main Central thrust at the base of the high Himalaya, and southern exposures (toward the foreland) occur between the Main Boundary thrust and Greater Himalayan klippen. Although the metamorphic grade of rocks within the Ramgarh–Munsiari thrust sheet is not significantly different from that of Greater Himalayan rock in the hanging wall of the overlying Main Central thrust sheet, the tectonostratigraphic origin of the two different thrust sheets is markedly different. The Ramgarh–Munsiari thrust became active in early Miocene time and acted as the roof thrust for a duplex system within Lesser Himalayan rocks. The process of slip transfer from the Main Central thrust to the Ramgarh–Munsiari thrust in early Miocene time and subsequent development of the Lesser Himalayan duplex may have played a role in triggering normal faulting along the South Tibetan Detachment system.     

Kinematic vorticity technique for porphyroclasts in the metamorphic rocks: an example from the northern thrust in Wadi Mubarak belt, Eastern Desert, Egypt

New structural, metamorphic, finite strain, and kinematic vorticity data for mylonitic granitic rocks from northern thrust in Wadi Mubarak reveal a history of deformation reflecting different tectonic regimes. The vorticity analysis of porphyroclasts was determined in high temperature mylonites. The kinematic vorticity number for the mylonitic granitic samples in the northern thrust in Wadi Mubarak range from 0.66 to 0.90, and together with the strain data suggest deviations from simple shear. It is concluded that nappe stacking occurred early during the underthrusting event probably by brittle imbrication and that ductile strain was superimposed on the nappe structure during thrusting. The accumulation of ductile strain during thrusting was not by simple shear and involved a component of vertical shortening, which caused the subhorizontal foliation in the northern thrust in Wadi Mubarak and adjacent units.     

Deformation mechanisms in the frontal Lesser Himalayan Duplex in Sikkim Himalaya,India

Understanding deformation mechanisms in Himalayan rocks is a challenging proposition due to the complex nature of the deformed rocks and their genesis. Crustal deformation in the Himalayan thrust belt typically occurs in elastico-frictional (EF) or quasi-plastic (QP) regimes at depths controlled mainly by regional strain-rate and geothermal gradient. However, material property, grain-size and their progressive changes during deformation are also important controlling factors. We present evidence of EF deformation from Gondwana rocks developed during the emplacement of one of the frontal horses (Jorthang horse) in the Lesser Himalayan Duplex (LHD) structure associated with Lesser Himalayan rocks in the footwall of the Ramgarh thrust in the Rangit window near Jorthang in the Sikkim Himalaya. The rocks in the horse exhibit systematic changes in microand meso-structures from an undeformed protolith to cataclasite suggesting that it was emplaced under elastico-frictional conditions. Meso- to micro-scale shear fractures are seen developed in Gondwana sandstone and slate while intercalated fine-grained shale-coal-carbonates are deformed by cataclastic flow suggesting that material property and grain-size have played an important role in the deformation of the Jorthang horse. In contrast, the hanging wall schists and quartzites of the Ramgarh thrust exhibit quasi-plastic deformation structures. This suggests that the Jorthang horse was emplaced under shallower crustal conditions than the antiformally folded Ramgarh thrust sheet even though the Ramgarh sheet presently overlies the Jorthang horse.     

前陆冲断带非对称性变形与逆冲断层运动学指向

前陆冲断带冲断层的冲断方向一直没有得到理论解释.文中基于库伦断裂理论和造山带前陆冲断带变形的非对称性,分析了前冲断层和反冲断层的成因.变形初期将会出现两组共轭势断裂面,随后在变形非对称引起的准静力平衡条件下,两组势断裂面中所需作用力小的那组断裂面将更容易发育成冲断层,断层滑动所需作用力包括克服滑脱面摩擦力和断层面摩擦力...     

柴北缘二郎洞地区L 构造岩变形特征及其地质意义

柴北缘二郎洞地区出露包含早古生代岩石组合的达肯大坂岩群,其构造变形期次和变形时代研究接近空白, 本文首次在其中发现了面理微弱, 而拉伸线理十分发育的L 构造岩,对其详细的构造解析对认识柴北缘构造演化具有重要意义。二郎洞达肯大坂岩群存在两期构造变形作用（D₁、D₂）,D₁ 期变形表现为兼具逆冲分量的左旋走滑剪切,D₂ 期变形表现为兼具正断性质的右旋走滑剪切。D₁ 期变形产生LS 构造岩和L 构造岩。其中L 构造岩中矿物拉伸线理的倾伏向约为142°,与柴北缘造山带延伸方向一致,倾伏角一般在2°～54°。对L 构造岩以及侵入其中的未变形次安山岩进行LA-ICP-MS 锆石U-Pb定年,获得它们的结晶时间分别为457 Ma 和406 Ma,L 构造的变形时代可能介于457～406 Ma 之间。根据长石、石英等矿物的变形形式,估计L 构造岩的变形温度在380 ℃～420 ℃之间, 应形成于地下13～14 km 深处。在构造特征分析和岩相学研究的基础上,结合柴北缘早古生代岩浆活动时代构架,我们认为二郎洞地区L 和LS 构造岩的原岩可能是原特提斯洋向北俯冲增生过程中形成的早古生代弧花岗岩,由于柴达木-东昆仑板块和祁连陆块之间的斜向碰撞,导致局部地区出现挤压并沿造山带延伸方向走滑活动。在走滑剪切作用下使地壳流变层中的弧岩浆岩体向地表挤出,局部成分相对均匀的花岗质岩石,因具有较强的岩石力学性能而形成了L构造岩。     

A new kinematic evolutionary model for the growth of a duplex — an example from the Rangit duplex, Sikkim Himalaya, India

The Lesser Himalayan duplex (LHD) is a prominent structure through much of the Lesser Himalayan fold–thrust belt. In the Darjeeling - Sikkim Himalaya a component of the LHD is exposed in the Rangit window as the Rangit duplex (RD). The RD consists of ten horses of the upper Lesser Himalayan Sequence (Gondwana, Buxa, Upper Daling). The duplex varies from hinterland-dipping in the north, through an antiformal stack in the middle to foreland-dipping in the south. The Ramgarh thrust (RT) is the roof thrust and, based on a balanced cross-section, the Main Himalayan Sole thrust is the floor thrust at a depth of ~ 10 km and with a dip of ~ 3.5° N.Retrodeformation suggests that the RD initiated as a foreland-dipping duplex with the Early Ramgarh thrust as the roof thrust and the RT as the floor thrust. The RT became the roof thrust during continued duplexing by a combination of footwall imbrication and concurrent RT reactivation. This kinematic history best explains the large translation of the overlying MCT sheets. The restoration suggests that RD shortening is ~ 125 km, and the original Gondwana basin extended ~ 142 km northward of its present northernmost exposures within the window.     

米仓山构造带的应变与涡度分析

米仓山构造带东西向的断裂逆冲兼左旋走滑,西段的韧性变形较强,东段脆性为主。北东向三个主断裂带由北而南逆冲兼左行剪切,早期可能发生脆韧性变形,后期叠加了脆性变形。前震旦系基底岩系变形特征主要表现为透入性流变,碎斑结构和糜棱结构发育,镶嵌构造、S-C组构、带状构造、眼球构造为主,局部偶见"δ"和"σ"旋转碎斑以及矿物鱼。石英颗粒以亚颗粒旋转动态重结晶为主。显微特征反映岩石变形温度相当于绿片岩相。利用Fry法测定石英颗粒三维应变应变强度集中在1.35～1.60之间,显示出从北到南逐渐增强的趋势。Flinn指数K和Nadei-Hossack图解均表明应变类型为近似平面应变的拉长型。运动学涡度分析表明米仓山应变以简单剪切变形作用为主,具有由南向北递增趋势。     

Flow vorticity in Zhangbaling transpressional attachment zone,SE China

Plagioclase porphyroclasts with well-preserved idiomorphic shapes and zoning, and showing limited clast interaction, are ubiquitous in the flat-lying Zhangbaling schist that is exposed east of the Tan-Lu fault in southeast China. Plagioclase porphyroclasts define rigid particles whose distribution be related to the kinematic vorticity of the schist using the methods of porphyroclast hyperbolic distribution (PHD) and modified Rf/ϕ (Passchier/Wallis plot). The kinematic vorticity numbers calculated from this approach range from Wk = 0.72 to 0.82, increasing progressively from south to north along the Zhangbaling belt. Such a Wk distribution indicates that the Zhangbaling schist was deformed uniformly under simple-shear dominated general shear, and that the Zhangbaling ductile crust experienced relatively even crustal thinning deformation. The subhorizontal Zhangbaling schist is considered a mid-crustal attachment zone that coupled the rigid upper crust to a subvertical, wrench shear zone in the lower crust.     

斜向逆冲断层相关褶皱的正演模型与实例分析

斜向逆冲作用在自然界普遍存在,研究斜向逆冲断层相关褶皱的构造几何学特征,识别断层相关褶皱是否存在斜向逆冲有重要意义。文章采用Trishear 4.5、Gocad以及Trishear3D软件构建一系列不同滑移量的断层转折褶皱和断层传播褶皱的二维正演剖面,通过连接一系列不同排列方式的二维剖面建立了三种不同逆冲滑移方向的断层转折褶皱和断层传播褶皱的假三维模型,通过不同假三维模型的比较分析来探讨斜向逆冲断层相关褶皱的构造几何学特征。研究发现,斜向逆冲断层相关褶皱区别于正向逆冲断层相关褶皱的特征主要有两点：① 正向逆冲断层相关褶皱层面等高线图上的最高点与后翼等高线中点的连线以及水平切面上的核心点与后翼中点的连线方向均与断层走向垂直,而斜向逆冲断层相关褶皱的最高点以及核心点与后翼中点的连线方向均与断层走向斜交,并且最高点与后翼等高线中点的连线方向或者核心点与后翼中点的连线方向均与逆冲滑移方向一致;② 在褶皱平行断层走向纵剖面上,正向逆冲断层相关褶皱各个层面最高点的连线是直立的,而斜向逆冲断层相关褶皱各个层面最高点的连线发生倾斜。通过这两个特征可以判别褶皱是否存在斜向逆冲以及逆冲的方向。将模型分析结果运用到四川盆地西南部三维地震勘探资料所覆盖的邛西背斜和大兴西背斜的实例中。研究结果表明,两个背斜均存在右旋斜向逆冲,逆冲方向与各自断层走向的夹角均为70°左右,邛西背斜和大兴西背斜的逆冲方向分别是NE79°和NE77°左右,这与龙门山南段晚上新世以来的主应力方向以及反演的汶川地震最大主应力方向一致。     

秦岭商丹断裂带的构造样式与变形分析

商丹断裂带作为扬子板块与华北板块的缝合带,在秦岭造山带中占据重要的地位。对其进行精细的构造研究,是了解扬子板块与华北板块相互作用过程的关键。研究工作在商丹断裂带东段展开,对该带进行了详细的野外观察、显微构造观察、糜棱岩化过程中温度的估算、有限应变测量、差异应力的计算以及糜棱岩化过程中运动学涡度的计算。变形温压条件指示糜棱岩形成于中-高绿片岩相到低角闪岩相变质条件。差异应力计算结果指示商丹带具备大型构造动力特征。涡度分析表明该断裂带的变形以纯剪切作用占据主导作用,但包含有简单剪切的分量,从而证明了扬子板块与华北板块之间具有以碰撞挤压为主的斜向汇聚方式。     

Thermal structure of active thrust belts

Abstract In order to study the thermal structure of active thrust belts, we have developed a numerical model of conductive heat transfer between thrust sheets during deformation. Our finite difference approach alternates small, instantaneous increments of displacement and isotherm translation with conductive relaxation of perturbed isotherms. In each step, conduction occurs for a length of time equal to the displacement increment divided by the thrust velocity. Computer simulations demonstrate that conductive heat transfer is significant during deformation and that temperatures in hanging-wall rocks decrease while temperatures in foot-wall rocks increase over distances of up to 10 km from the thrust surface. When the effects of internal heat production are also calculated, heating of foot-wall rocks exceeds cooling of hanging-wall rocks. Rocks located between two thrusts may experience a complicated temperature–time path of early heating followed by cooling. These models help to explain the rapid metamorphism of rocks in the Taconian thrust belt in the northern Appalachians of New England soon after deposition of the youngest sediments.     

Strain analysis and vorticity of flow in the Northern Sardinian Variscan Belt: Recognition of a partitioned oblique deformation event

A field example of strain partitioning has been analysed along the Nurra–Asinara transect of the NW Sardinian Variscan chain (Italy). The section in the Nurra–Asinara area is in a continuous sequence of tectono-metamorphic complexes made of low- to high-grade metamorphic rocks affected by a polyphase tectonic history. The principal fabric of the area is controlled by a D2 progressive deformation phase in which the strain is partitioned into folds and shear zone domains. The D2 stretching lineation and shear sense show a clear change from south to north. The principal meso- and micro-structures, vorticity gauges and a quantitative kinematic analysis of local strain suggest that the D2 kinematic history could be envisaged as an oblique heterogeneous deformation similar to the transpressive systems described in ancient and modern settings elsewhere. Using a simple kinematic model we also propose that both a transpressive system followed by “thrusting” or a partitioned transpressive system could be responsible for the fabric distribution and strain accumulation described in the study transect.     

柴达木盆地北缘赛什腾-锡铁山左行逆冲断裂及地质意义

本文在对赛什腾-锡铁山斜冲断裂构造重点地段详细构造解析的基础上,结合沉积、地球物理资料对该斜冲断裂构造的几何学、运动学及时代进行了研究,探讨了断裂形成与区域地质背景的关系,提出柴达木盆地北缘的赛什腾、绿梁山、锡铁山是向南斜向逆覆于新第三纪沉积岩之上的无根推覆体,并认为该断裂的形成与喜马拉雅造山带陆内俯冲远程效应相关。      

Energy balance for large thrust sheets and fault-bend folds

Thrust sheet movement over ramps requires energy because of the frictional resistance and deformation within the fault zone, fault-bend folding at the base and top of the ramp, and changes in the gravitational potential energy because of uplift. To model the energy usage, a kinematic model of a foreland thrust sheet is constructed assuming: (1) the ramp is planar and the flats are parallel to bedding; (2) the fault-bend folds are concentric; (3) thickness is preserved for beds that enter the folds parallel to the basal thrust fault and (4) cross-sectional area is preserved for rocks deformed by folding. Equations for the work done within the fault zone, and during uplift and fault-bend folding are derived by combining the kinematic model with stresses that increase in proportion to depth. The relative amounts of energy consumed by friction along the fault, uplift and fault-bend folding are estimated to be 2.7:1.0.25 for a ramp angle of 30°. The energy balance for the movement of large thrust sheets thus depends principally upon friction in the fault zone and changes in the gravitational energy.