Neoproterozoic tectonothermal evolution of the Central Eastern Desert, Egypt: a slow velocity tectonic process of core complex exhumation

Regional cooling in the course of Neoproterozoic core complex exhumation in the Central Eastern Desert of Egypt is constraint by ⁴⁰Ar/³⁹Ar ages of hornblende and muscovite from Meatiq, Sibai and Hafafit domes. The data reveal highly diachronous cooling with hornblende ages clustering around 580 Ma in the Meatiq and the Hafafit, and 623 and 606 Ma in the Sibai. These ⁴⁰Ar/³⁹Ar ages are interpreted together with previously published structural and petrological data, radiometric ages obtained from Neoproterozoic plutons, and data on sediment dynamics from the intramontane Kareim molasse basin. Early-stage low velocity exhumation was triggered by magmatism initiated at 650 Ma in the Sibai and caused early deposition of molasses sediments within rim synforms. Rapid late stage exhumation was released by combined effect of strike-slip and normal faulting, exhumed Meatiq and Hafafit domes and continued until 580 Ma. We propose a new model that adopts core complex exhumation in oblique island arc collision-zones and includes transpression combined with lateral extrusion dynamics. In this model, continuous magma generation weakened the crust leading to facilitation of lateral extrusion tectonics. Since horizontal shortening is balanced by extension, no major crustal thickening and no increase of potential energy (gravitational collapse) is necessarily involved in the process of core complex formation. Core complexes were continuously but slowly exhumed without creating a significant mountain topography.     

鲁西北西向断裂系与沂沭断裂带晚中生代演化关系及其动力学背景探讨

晚中生代是华北地块构造演化的转折时期。由出露于沂沭断裂带、鲁西地体、鲁东地体的中生代地层、岩浆,结合断裂活动年代学、区域地质等资料分析,可以将沂沭断裂带晚中生代构造演化划分出距今约160Ma、130～110Ma、90～80Ma等3个关键时期,并分别与左行压剪、左行张剪、右行压剪构造活动相对应。晚中生代沂沭断裂带与鲁西北西向断裂系间的几何学、运动学、年代学的共轭匹配关系表明,它们为特定构造动力学背景下形成的一组共轭断裂系。同时对鲁东地体晚中生代构造演化、鲁东北西向断裂系特征,以及沂沭断裂带、北西向断裂系晚中生代构造演化的动力学背景进行探讨。     

The termination of the North Anatolian Fault System (NAFS) in Eastern Turkey

ABSTRACT

The present-day tectonic framework of Turkey comprises mainly two strike-slip fault systems, namely dextral North Anatolian and sinistral East Anatolian faults. They are considered as the main cause of deformation patterns in Anatolia. These two mega shear systems meet at Kargapazar? village of Karl?ova county. The area to the east of the junction has a transpressional tectonic regime between the Eurasian and Arabian plates and is characterized, based on field observation, by a network of faults defining a typical horsetail splay structure. The horsetail splay is interpreted as marking the termination of the North Anatolian Fault System (NAFS), which continues eastward into the Varto Fault Zone (VFZ) and then dies out. The present study reveals that the VFZ is made up of two main parts, namely the principal displacement zone (PDZ) and the transpressional splay zone (TPSZ), both characterized by the right-lateral strike-slip with reverse motion. However, the area to the east of Varto is characterized dominantly by reverse-thrust faults and E–W-trending faults as shown by focal mechanism solutions. The generation of the VFZ as a transpressional termination to the NAFS can be related directly to the block movements of the Eurasian, Anatolian, and Arabian plates.     

A structural synthesis of the Proterozoic Arabian-Nubian Shield in Egypt

Detailed structural geological and related studies were carried out in a number of critical areas in the Proterozoic basement of eastern Egypt to resolve the structural pattern at a regional scale and to assess the general characteristics of tectonic evolution, orogeny and terrane boundaries. Following a brief account of the tectonostratigraphy and timing of the orogenic evolution, the major structural characteristics of the critical areas are presented. Collisional deformation of the terranes ended about 615-600 Ma ago. Subsequent extensional collapse probably occurred within a relatively narrow time span of about 20 Ma (575 – 595 Ma ago) over the Eastern Desert and was followed by a further period of about 50 Ma of late to post-tectonic activity. The regional structures originated mainly during post-collisional events, starting with those related to extensional collapse (molasse basin formation, normal faulting, generation of metamorphic core complexes). Subsequent NNW-SSE shortening is documented by large-scale thrusting (towards the NNW) and folding, distributed over the Eastern Desert, although with variable intensity. Thrusts are overprinted by transpression, which was localized to particular shear zones. Early transpression produced, for example, the Allaqi shear zone and final transpression is documented in the Najd and Wadi Kharit-Wadi Hodein zones. Two terrane boundaries can be defined, the Allaqi and South Hafafit Sutures, which are apparently linked by the high angle sinistral strike-slip Wadi Kharit-Wadi Hodein shear zone with a tectonic transport of about 300 km towards the W/NW. In general, the tectonic evolution shows that extensional collapse is not necessarily the final stage of orogeny, but may be followed by further compressional and transpressional tectonism. The late Pan-African high angle faults were reactivated during Red Sea tectonics both as Riedel shears and normal faults, where they were oriented favourably with respect to the actual stress regime.     

西秦岭北缘北西向断层特征和形成的构造机制及地质意义

西秦岭北缘构造带不仅发育一系列继承性多期活动或新生的近东西向断层,而且新生代地层中还发育与近东西向断层走向不一致且具有独特构造特征的北西向左旋走滑断层。这种北西向左旋走滑断层带不发育断层角砾岩、磨砾岩、碎粉岩、断层泥、摩擦镜面、擦痕线理、断层阶步等脆性断层中常见的构造现象,仅表现为地层旋转和剪切拉断形成的一定宽度的透镜化带,两条断层之间地层产状发生旋转形成了约1 km宽,平面上类似膝折构造几何形态地层扭折带。该北西向断层横切了渐新统—中新统地层,并被上新统砾岩覆盖和第四纪以来的近东西向左旋走滑断层斜切,指示了其形成于渐新世—中新世沉积地层形成之后,上新世砾岩沉积之前,即上新世早期。北西向断层带不发育脆性断层典型构造现象和断层左旋走滑作用在渐新统—中新统沉积地层中形成了类似膝折构造几何形态地层扭折带,说明其变形具有韧脆性过渡和缓慢剪切变形的特征,是西秦岭北缘一种新的断层类型。其形成机制为基底或中下地壳中大型左旋走滑韧性或韧脆性剪切带向上扩展延伸到上部沉积盖层中之结果,也就是说,新生代沉积盖层中这种北西向断层和地层扭折带是下部韧性剪切带的左旋走滑剪切在盖层中被动构造响应。这种基底或中下地壳北西向左旋韧性剪切带可能指示了上新世初期西秦岭北缘构造带深部韧性地壳物质向南东流变蠕动的构造标志,代表深部地壳缩短增厚向地壳韧性物质侧向扩展流动的转换过程。这种特殊的断层类型对理解青藏高原东北缘新生代构造变形体制转换和地壳隆升具有重要的科学意义。     

Development of quartz c-axis crossed/single girdles under simple-pure shear deformation: Results of visco-plastic self-consistent modeling

Quartz c-axis fabrics are widely used to determine the shear plane in ductile shear zones, based upon an assumption that the shear plane is perpendicular to both the central segment of quartz c-axis crossed girdle and single girdle. In this paper the development of quartz c-axis fabric under simple-pure shear deformation is simulated using the visco-plastic self-consistent (VPSC) model so as to re-examine this assumption. In the case of no or weak dynamic recrystallization, the simulated crossed girdles have a central segment perpendicular or nearly perpendicular to the maximum principal finite strain direction (X) and the XY finite strain plane, and at a variable angle relative to the imposed kinematic framework that is dependent on the modeled flow vorticity and finite strain. These crossed girdles have a symmetrical skeleton with respect to the finite strain axes, regardless of the bulk strain and the kinematic vorticity, and rotate in a way similar to the shear sense with increasing bulk strain ratio. The larger the vorticity number the more asymmetrical their legs tend to be. In the case of strong dynamic recrystallization and large bulk strain, under simple shear the crossed girdle switches into single girdles, sub-perpendicular to the shear plane, by losing the weak legs. The numerical results in our models do not confirm the above-mentioned assumption.     

赣东北金山金矿床构造变形特征及其区域构造意义

金山金矿产于赣东北断裂带的次级剪切带中,其成矿作用与韧性剪切带的动力变质作用密切相关。本文通过对矿区金矿体的地质特征和运动学特征的分析,认为金山金矿成矿期经历了两期韧性剪切作用,即早期由NNW向SSE的韧性推覆剪切,晚期沿NE方向的左行韧性走滑剪切,分别形成NW向超糜棱岩型矿体和NE方向石英脉型矿体,两期韧剪变形均发生在新元古代,对应于江南造山带在碰撞造山阶段和后造山伸展阶段的动力变质事件。成矿后,矿区先受到NW向的挤压应力场作用,形成NE走向的逆断层、NW走向的张性断层以及SN走向的左行压扭断层,之后转为NW向的拉张应力场,形成NE走向的正断层。金山金矿床的构造变形特征指示了赣东北断裂带活动的阶段性,赣东北地区在晋宁期经历了两期构造-岩浆-变质事件;加里东-印支期构造活动比较弱;燕山期本区构造-岩浆活动强烈,引起赣东北断裂带的再次活动。     

西准噶尔达拉布特断裂带中段晚古生代构造分析

走滑断裂构造在中亚造山带增生及演化过程的研究中扮演了重要角色,其主要构造单元均被走滑断裂带所分割。西准噶尔造山带是中亚增生型造山带的重要组成部分,达拉布特断裂是西准噶尔造山带中一条重要的走滑断裂,其复杂的构造表现吸引了大量研究者的关注。前人不仅在其构造解释上存在着走滑断层、逆冲断层或压扭性断层等诸多争议,且在其活动时代问题上也有不同的看法。本文依据在达拉布特断裂带中段开展的详细野外构造学工作,结合前人针对该地区石炭纪火山岩、浊积岩和造山后花岗岩侵入体所做的同位素年代学工作成果,对达拉布特断裂的活动性质和活动时代进行了讨论。结果确认在中二叠统沉积之前,达拉布特断裂带存在两期变形事件,分别对应于320Ma左右沿NE-SW的较深层次的左行走滑事件D1和表现为脆-韧性转换域的轴面倾向SE的褶皱作用构造事件D2。前者为主期变形事件,而后者发生在中二叠统沉积之前。本文同时报道了沿达拉布特断裂带出露右行走滑构造形迹,并讨论了其可能的成因。沿达拉布特断裂带的多期构造事件记录了西准噶尔地区造山后大规模走滑构造调整过程,是晚古生代晚期中亚各个陆块拼合后大规模陆内调整在西准噶尔造山带的具体体现。     

大别山北部糜棱岩中石英的高温应变

超高压变质岩的发现导致了人们对大别山构造格架的重新认识。最近的研究表明,超高压变质岩只产于大别山南部(Wang et al., 1992),而北部为古生代岛弧杂岩所占据,晓天-磨子潭断裂即为这些杂岩的北界(Wang et al., 1994)。晓天-磨子潭断裂作为左行走滑断裂已经得到公认,但笔者对该断裂南侧糜棱岩的研究表明,其并非左行走滑产物,而是早期变形的构造形迹。本文旨在分析这些糜棱岩中石英的显微构造,进而揭示其早期构造变形特征。     

Seismic and GPS evidence for the kinematics and the state of stress of active structures in south and south-central Tibetan Plateau

Since the middle Miocene, widely distributed N-S striking rifts or rift-depressions and NE-NW-striking strike-slip faults have developed as major structures in southern and south-central Tibet. Active structures, seismic mechanisms and directions and rates of movement determined from GPS data are different in south and south-central Tibetan Plateau. In southern Tibet, N-S striking normal faults and a few transtensional/transpressional faults are the main structures, but none of the GPS measurements show eastward movement. In south-central Tibet, NE- and NW-striking transpressional/transtensional faults and several large-scale E-W and WNW-striking dextral and sinistral strike-slip faults are the main structures, and are consistent with the movement direction indicated by the GPS data. On the other hand, the pattern of earthquake data from the south-central Tibetan Plateau, and especially from the eastern and western syntaxes, are very complicated. These structures may be produced by N-S contraction which has resulted in the N-S striking linear structures, parallel to the direction of maximum compressive stress, and conjugate NE- and NW-striking transpressional/transtensional faults on the southern side of the Qiangtang Block, and in the eastern and western syntaxes, but the deformation is not continuous from the south to the south-central Tibetan Plateau. Underthrusting of the Indian Plate under the southern Tibetan Plateau after the Middle Miocene may be the main cause of these structural features.     

东昆仑山南缘大型转换挤压构造带和斜向俯冲作用

东昆仑地体和巴颜喀拉－－松潘甘孜地体之间的会聚边界是一条位于东昆仑南缘的大型转换挤压构造带。研究表明该带的东段（阿尼玛卿段）和西段（东－西大滩段）构造特征不同,阿尼玛卿段的构造以印支期具往南西造山极性的逆冲叠覆岩片和新生代脆性左行走滑构造为特征,东－西大滩段是由220Ma形成的EW向韧性左行走滑剪切带及两则伴生的挤压褶皱断裂带组成,韧性变形持续至20Ma,之后表现为脆性左行走滑构造再活动。因此,东昆仑南缘大型会聚带是一条由东段的“收缩挤压”为主向西段的“转换挤压”逐渐过度的特殊复杂的构造带,它的形成与巴颜喀拉－－松潘甘孜地体往NE方向斜向俯冲于东昆仑地体之下有关。     

四川盆地西北部新生代构造变形模式讨论——对认识青藏高原东北缘新生代变形机制的意义

综合造山带内的构造热年代学及盆地内部进行的磷灰石裂变径迹研究,提出了四川盆地西北部的三个背斜（潼梓关背斜、九龙山背斜和南阳坝背斜）主要是新生代构造变形的产物。野外观察发现汉中盆地是一个第四纪的拉分盆地,其主控断层具左行走滑性质。新生代青藏高原东缘大型地块向东挤出,遭遇强硬的四川克拉通阻挡之后,沿着龙门山形成了一个右行的走滑挤压带,并且影响到邻近的四川盆地,形成北东向背斜。这期构造变形往北延伸进入米仓山,形成具有逆冲性质的北东向断层。四川盆地北面也存在向东的挤出作用,这和汉中盆地主控断层的左行走滑性质是匹配的。四川盆地北面的地块挤出影响了米仓山前缘的四川盆地,由于龙门山和米仓山构造变形的叠加,使得最东面的南阳坝背斜相对于其它两个背斜在褶皱轴上发生了偏转。     

云南白秧坪东矿区控矿断裂带构造岩的方解石组构

断裂带构造岩的方解石组构综合分析,可以推断构造活动的应力场方位,利用e双晶的C—T(C—压应力轴,T—张应力轴)图解推断古应力的方位,已为众多的地质学家在实践中引用。云南白秧坪东矿区推覆构造的华昌山断裂带和水磨房断裂带,是矿区的主要控矿构造,其构造岩的方解石及e双晶极点组构均显示点极密加环带的型式,大部分发育近岩组坐标c轴的光轴极密,代表光轴方向的压力导致沿岩组坐标ab面滑动;在C—T应力系统中,显示华昌山、水磨房断裂带晚期较强烈活动的应力场方位,主压应力方向为SE-NW向,主张应力方向为NE-SW向,断层以左旋运动为主;华昌山断裂带SE盘往NW方向逆冲,水磨房断裂带则由NW往SE方向反冲,与野外宏观构造分析吻合。     

Sustainable transpression: An examination of strain and kinematics in deforming zones with migrating boundaries

Most orogenic belts owe their development to oblique convergence and commonly have many orogen-parallel transpressional high-strain zones. To constrain the tectonic history of orogenic belts by structural and fabric analysis of rocks, it is desirable to understand quantitatively the relationship between the boundary conditions and the resulting strain distribution and kinematics in these zones. Most current models for transpression assume homogeneous deformation confined by boundaries that are fixed to material planes. This creates a strain compatibility problem at the margins of the active deforming zone and also requires that the strain rate normal to the zone boundaries increase to implausibly high values soon after the onset of oblique convergence (transpressional motion). The latter contradicts with the observation that transpressional motion can be sustained throughout an orogeny. The assumption that zone boundaries are fixed to material planes is unrealistic. The outstanding problems of current transpressional models are resolved in this paper by allowing the zone boundaries to migrate through the rock material. The consequence of zone boundary migration for the strain field and kinematics within a transpressional high-strain zone is investigated mathematically. The implications of the modeling for fabric interpretation are discussed. The modeling makes general predictions consistent with observed planar and linear fabric patterns in natural transpressional high-strain zones. It predicts that foliations in transpressional high-strain zones are subparallel to the zone boundaries regardless of variation in the imposed boundary conditions. Lineations cluster along the great circle girdle subparallel to the average foliation. The spread of the lineations may vary from point maxima to complete girdles.     

一个韧性转换挤压带的变形分解作用：以阿尔金断裂带东段为例

变形构造研究显示阿尔金划分成具有近水平拉伸线理的韧性左行走滑变形域和具有陡角度倾伏拉伸线理的收缩变形域。在ＳＳ与ＰＳ之间的过渡区域还发现中等角度倾伏的拉伸线理。     

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.     

中国阿尔泰造山带南缘额尔齐斯断裂带的构造变形及意义

额尔齐斯断裂是中亚造山带中的一条重要深大断裂,对于额尔齐斯断裂运动性质一直有着走滑断层、逆冲断层和压扭性断层等不同看法.本文在中国阿尔泰造山带南缘开展了详细的构造地质学工作,研究结果表明,额尔齐斯断裂及其次级断裂组成额尔齐斯断裂带.额尔齐斯断裂带在中国境内是一条宽约20 ～ 40km,长约400km,经受不同程度构造作用的强应变带,剪切作用影响范围遍布整个中国阿尔泰造山带南缘.额尔齐斯断裂带经历了左行走滑和右行走滑两个阶段.结合前人有关韧性剪切带成因型金矿、同构造岩体侵位与变形关系及对变质岩石40Ar/39Ar年代学研究,本文认为额尔齐斯断裂带的左行走滑构造形成于早二叠世(283～275Ma).早二叠世之后,额尔齐斯断裂带叠加了右行走滑事件,其活动时限可能为晚二叠世(260 ～ 245 Ma),其规模远远小于前期的左行走滑构造.额尔齐斯断裂带走滑活动性质的确定,为二叠纪北疆及整个中亚造山带造山后调整过程中不同的构造方式提供了佐证.     

阿尔金盆地群构造类型与演化

阿尔金走滑断裂带主要由阿尔金主干断层与几条近于平行的左行走滑断层和斜交断层组成,其空间分布规律可用左行走滑简单剪切模型解释。     

雪峰山西侧贵州地区中生代构造特征及其演化

贵州中生代变形主要发生在燕山期,发育三幕褶皱变形、两幕逆冲和三幕走滑。根据区域对比、卷入褶皱的地层和褶皱间的叠加关系,判断三期褶皱的形成顺序依次为近东西向、北东向和南北向,时限在J_3—K_2之间。逆冲推覆构造主要由向北西或西逆冲的近南北向逆冲断层组成,大体与南北向褶皱同时形成;自雪峰构造带西缘向西,依次划分出根部带、中部带和前锋带。但是,在根部带识别出两幕逆冲推覆,其它两带各识别出一幕。走滑断层也有3个方向:东西向、北东向和近南北向。东西向走滑断层呈现出右行压扭的运动学特征,而大多数北东向走滑断层是左行张扭性质的。依据各个方向断层间的切割和限制关系,推测东西向走滑断层最早形成,其次是南北向逆冲断层,北东向走滑断层最晚活动。这些断裂和褶皱特征,总体表现出贵州多重多种复合联合的构造特征,最后,探讨了本区的构造成因模式。     

Shear structures in the Serra do Azeite shear zone,southeastern Brazil: Transtensional deformation during regional transpression in the central Mantiqueira province (Ribeira belt)

The Serra do Azeite shear zone (SASZ) is a northeast-trending regional structure in southeastern Brazil. Kinematic analysis carried out in the SASZ suggests a ductile sinistral transtensional regime in amphibolite facies conditions. Constrictional, flattened, and simple-shear strain domains occur in mylonites with stretching lineation plunging to the east–southeast. Kinematic indicators suggest oblique top–down-to-the-east–southeast and sinistral strike-slip components along variably oriented shear planes. Results of the simple geometrical construction applied to the shear-zone pattern, coupled with field data and kinematic analysis, suggest that sinistral transtensional shearing resulted from east–northeast-directed crustal extension and sinistral strike-slip displacement, accompanied by north–northeast/south–southwest contraction and vertical thinning. The K/Ar and Ar/Ar cooling ages match the proposed interval for crustal extension in the central Mantiqueira province (0.6–0.58/0.57 Ga) based on ages of alkaline granitoids and volcanic rocks. These data indicate types-I and -S granite magmatism, as well as metamorphism and dextral transpressional deformation along the Ribeira belt. Therefore, we interpret the transtensional regime as a result of southwest-directed lateral extrusion and uplift crustal slices (overall oblique extrusion) during an orogenic-scale partitioned transpressional regime. Our results suggest this regime was coeval with a phase of regional stretching subparallel to the Ribeira belt, which would explain the coexistence of extensional and compressional structures during overall plate convergence.