Finite Strain Analysis of the Wadi Fatima Shear Zone in Western Arabia,Saudi Arabia

Neoproterozoic rocks, Oligocene to Neogene sediments and Tertiary Red Sea rift-related volcanics (Harrat) are three dominant major groups exposed in the Jeddah tectonic terrane in Western Arabia. The basement complex comprises amphibolites, schists, and older and younger granites unconformably overlain by a post-amalgamation volcanosedimentary sequence (Fatima Group) exhibiting post-accretionary thrusting and thrust-related structures. The older granites and/or the amphibolites and schists display mylonitization and shearing in some outcrops, and the observed kinematic indicators indicate dextral monoclinic symmetry along the impressive Wadi Fatima Shear Zone. Finite strain analysis of the mylonitized lithologies is used to interpret the deformation history of the Wadi Fatima Shear Zone. The measured finite strain data demonstrate that the amphibolites, schists, and older granites are mildly to moderately deformed, where XZ (axial ratios in XZ direction) vary from 2.76 to 4.22 and from 2.04 to 3.90 for the R_f/φ and Fry method respectively. The shortening axes (Z) have subvertical attitude and are associated with subhorizontal foliation. The data show oblate strain ellipsoids in the different rocks in the studied area and indication bulk flattening strain. We assume that the different rock types have similar deformation behavior. In the deformed granite, the strain data are identical in magnitude with those obtained in the Fatima Group volcanosedimentary sequence. Finite strain accumulated without any significant volume change contemporaneously with syn-accretionary transpressive structures. It is concluded that a simple-shear deformation with constant-volume plane strain exists, where displacement is strictly parallel to the shear plane. Furthermore, the contacts between various lithological units in the Wadi Fatima Shear Zone were formed under brittle to semi-ductile deformation conditions.     

新疆兴地断裂带前寒武纪构造—岩浆—变形作用特征及其年龄

本文分析了发生在塔里木北缘兴地断裂带的多期构造—岩浆演化特征。在此基础上,对三件前寒武纪火成岩样品进行了LA-ICP-MS锆石U-Pb定年,对兴地断裂带韧性变形构造进行了运动学分析。研究表明,兴地断裂带至少经历了4期地质构造演化; 其中,前寒武纪有两期。第一期发生在前南华纪,以强烈的挤压褶皱、韧性剪切变形和岩浆活动为特色; 第二期发生在南华纪—震旦纪,以广泛发育的双峰式火成岩、复式岩流和基性岩墙群为特征,伴随大规模冰川作用和地壳沉陷,对应Rodinia超大陆的裂解。LA-ICP-MS法锆石U-Pb年龄测定表明,辉长岩中的俘获锆石保留了3114±20Ma、2509±42Ma、1916±36Ma等多期古构造演化和岩浆活动信息,揭示研究区深部存在一个中-新太古代和古元古代的基底。双峰式火成岩测年数据证实,区内在820~800Ma(花岗岩脉,798±7Ma; 辉长辉绿岩墙,816±15Ma)发生过强烈的裂谷—岩浆活动。根据构造交切关系,兴地断裂带发生过两期前南华纪韧性变形;变形时代尚不清楚。运动学分析表明,第一期为朝北逆冲的推覆变形,第二期为右旋走滑变形。南华纪以来,区域变质和韧性变形微弱,为韧脆性变形。     

新疆北部卡拉麦里晚古生代走滑构造及其叠加变形序次

大型走滑断裂构造是大陆地壳内部基本的构造变形样式,通常是大陆地壳形成的标志.卡拉麦里构造带是新疆东准地区构造演化研究的重要构造单元.前人的研究认为卡拉麦里构造带是板块碰撞形成的缝合带.本文结合野外考察、构造分析和年代学工作认为,该构造带主要反映了走滑构造带的特点.在遥感影像上,卡拉麦里构造带呈断续的线状延伸特征.地震剖面上,卡拉麦里断裂带主断面产状近于直立向下延伸至基底,与一般张性断层、压性逆冲断层所显示的上陡下缓的铲状特征截然不同.野外考察显示,该构造带发育密集而陡立劈理,主断面附近劈理面倾角近于直立,在相对较浅层次的地层上,劈理面成花状散开,体现花状构造的特点.卡拉麦里构造带内的石炭系、泥盆系地层以及蛇绿岩系受到强烈改造,超糜棱岩化、糜棱岩化、千枚岩化现象普遍.糜棱岩中,硅质岩透镜体拖尾指示右旋走滑特征,与同构造岩脉次级张裂面指示的结果相一致.结合前人研究资料以及地层变形证据,可以推断构造带活动时限为270～260Ma.因此,卡拉麦里构造带是一条在晚古生代-早中生代活动的右旋剪切走滑构造带,准东地区与卡拉麦里构造带相关的缝合带确认,必须以卡拉麦里走滑构造带性质的准确厘定为基础.卡拉麦里构造二叠纪时期的走滑活动性质的确定,指示新疆北部二叠纪大陆地壳已经形成,而且,新疆北部后期叠加构造变形序次研究也显示具有大区域上的共性,指示新疆北部二叠纪以来进入基本统一大陆内部构造演化阶段.     

Deformation condition determination and strain analysis: Application of microstructural and microthermometry study of the Zamanabad Shear Zone (East of Iran)

Microstructural analysis and microthermometry are useful methods for determining the deformation evolution. To address this issue, rheological behavior of quartz, feldspar and calcite in veins and host rocks during deformation, are presented in the mylonite zone of the dextral reverse Zamanabad Shear Zone (ZSZ), in northern part of Sistan Suture Zone (SSZ), in east of Iran. Microstructure evidences revealed two evolution stages of high and low temperature deformation. Quartz microstructures in the ZSZ show abundant evidences for early high-temperature plastic deformation (e.g. Bulging recrystallization (BLG)) which are as microstructures with SW directed ductile shearing in the central parts of the ZSZ. This shear zone shows progressively decreasing strain away from the central of shear zone toward the wall. High-temperature microstructures are overprinted partly or completely during shearing by the later low-temperature deformation (e.g. Pressure solution, fractures, veinlets). Microstructural observations of veins (quartz and calcite) confirms the results of microstructures in the host rock, as quartz veins occurred from peak metamorphic conditions (<400°C) and then in lower P–T conditions have been formed calcite veins (~250°C). According to microthermometric studies, two primary fluid groups are observed in quartz veins: (1) fluids trapped during peak deformation conditions, with higher-salinity, They were initially trapped at ~300–400°C, (2) smaller fluids by trapping of low-salinity inclusions at ~240–180°C that related to subsequent phases of shear zone exhumation in lower deep. Microthermometry results and microstructural analysis indicate deformation under lower greenschist facies conditions for the ZSZ, and then exhumation of the early of high-temperature rocks within regime of ductile-brittle transition to brittle.     

Growth and Deformation of the Ladakh Batholith, Northwest Himalayas: Implications for Timing of Continental Collision and Origin of Calc-Alkaline Batholiths

The calc-alkaline Ladakh batholith (NW Himalayas) was dated to constrain the timing of continental collision and subsequent deformation. Batholith growth ended when collision disrupted subduction of the Tethyan oceanic lithosphere, and thus the youngest magmatic pulse indirectly dates the collision. Both U-Pb ages on zircons from three samples of the Ladakh batholith and K-Ar from one subvolcanic dike sample were determined. Magmatic activity near Leh (the capital of Ladakh) occurred between 70 and 50 Ma, with the last major magmatic pulse crystallizing at ca. 49.8+/-0.8 Ma (2sigma). This was followed by rapid and generalized cooling to lower greenschist facies temperatures within a few million years, and minor dike intrusion took place at 46+/-1 Ma. Field observations, the lack of inherited prebatholith zircons, and other isotopic evidence suggest that the batholith is mantle derived with negligible crustal influence, that it evolved through input of fresh magma from the mantle and remelting of previously emplaced mantle magmatic rocks. The sedmimentary record indicates that collision in NW Himalaya occurred around 52-50 Ma. If this is so, the magmatic system driven by subduction of Tethys ended immediately on collision. The thermal history of one sample from within the Thanglasgo Shear Zone (TSZ) was determined by Ar-Ar method to constrain timing of batholith internal deformation. This is a wide dextral shear zone within the batholith, parallel to the dextral, N 30 degrees W-striking crustal-scale Karakoram Fault. Internal deformation of the batholith, taken up partly by this shear zone, has caused it to deviate from it regional WNW-ESE trend to parallel the Karakoram Fault. Microstructures and cooling history of a sample from the TSZ indicate that shearing took place before 22 Ma, implying that (1) the history of dextral shearing on NW-striking planes in northern Ladakh started at least 7 m.yr. before the <15 Ma Karakoram Fault, (2) shearing was responsible for deviation of the regional trend of the Ladakh batholith, and (3) dextral shearing occured within a zone apporximately 100 km wide that includes the Ladakh batholith and portions of the younger Karakoram batholith.     

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.     

Integrated interpretation of satellite imagery, aeromagnetic, aeroradiometric and ground exploration data-sets to delineate favorable target zones for unconformity related uranium mineralization, Khariar basin, Central India

The Mesoproterozoic Khariar basin, to the SE of Chhattisgarh basin, comprises 1000 m thick arenaceous-argillaceous sediments. For the first time, a multidisciplinary approach has been made to the integrate interpreted satellite imagery, aero-magnetic and aero-radiometric data with available ground exploration data sets with an objective to understand structural fabric and to establish various parameters for unconformity related uranium mineralization in the environs of Khariar basin. Total Magnetic Intensity (TMI) anomaly image has been useful to mark major faults (ENE-WSW), magnetic bodies and overall basement characteristics. Combination of first vertical derivative (1^VD) and tilt derivative magnetic images brought out presence of NW-SE magnetic linear (dominant) with minor ENE-WSW and NNE-SSW trends. Basic dykes and quartz veins are the surface manifestations of NW-SE trend in basement. Radially averaged power spectrum indicates the approximate basement configuration. Enhanced Thematic Mapper satellite imagery (Landsat ETM⁺) interpretation has shown lineaments along NW-SE, NNE-SSW, ENE-WSW and ENW-WSE directions. These observations are corroborated by interpreted results of magnetic data. Analysis of both results indicate NW-SE and ENE-WSW trends as post depositional. Aero-radiometric images (U, Th, K and ternary U-Th-K) show overall radio-elemental distribution for various litho-units. Besides, Th and K images along with interpreted ETM⁺ satellite imagery (RGB: 432/752/751) are useful to map small outliers and to modify basement-sediment contact. Geochemical data from basement rocks around Khariar basin suggests the younger Bundeli granitoids and its equivalents are good source of uranium in the western margin. Presence of labile uranium is inferred from higher concentration of uranium in water samples. The Airborne gamma-ray spectrometry (AGRS) and hydro-geochemical anomalies fall along fault zones and intersection of fault zones. The western and southern margin of Khariar basin are also characterized by presence of paleosol at unconformity, which are favorable factors for unconformity type uranium mineralization. Based on the present study, part of western and southern margin emerge as potential target areas for further exploration of uranium.     

中天山北缘大型右旋走滑韧剪带研究

中天山北缘是一个近 EW向的大型右旋走滑韧剪带。宏、微观构造尺度的运动学研究表明 ,该带经历过至少二期韧性变形作用。第一期为从南向北的逆冲推覆韧剪变形 ,时代为中—晚志留世 ,以米什沟剖面为代表 ,对应于早古生代洋壳从北向南俯冲及稍后吐哈陆块朝中天山岛弧的碰撞事件。第二期为沿 EW方向的右旋走滑韧性变形 ,其构造形迹广泛分布于中天山北缘带各个地段 ;北天山石炭纪火山岩已卷入该期构造活动 ,走滑时代为晚石炭世—早二叠世 ,对应于晚石炭世塔里木与西伯利亚两大板块碰撞造山诱发的陆内变形、走滑剪切。走滑带中新生白云母 4 0 Ar/39Ar年龄为 ( 2 69± 5) Ma。剪切面理、拉伸线理、矿物韧剪构造、石英 C轴组构提供了构造运动学证据 ;地层不整合及同位素测年值提供了变形时间证据。二叠纪以后的构造事件也影响到中天山北缘带 ,但只有脆性变形形迹 ,无韧性剪切。最后对本区古生代构造演化进行了讨论     

Dike intrusion under shear stress: Effects on magnetic and vesicle fabrics in dikes from rift zones of Tenerife (Canary Islands)

A theoretical model predicting how anisotropy of magnetic susceptibility (AMS) and vesicle fabrics are modified by shear stress resolved on the dike walls prior to the final cooling of magma is developed for vertical dikes. The resulting fabrics are asymmetric with respect to initial fabrics assumed to be symmetric. Application of this model together with collected data on magma flow direction, dike propagation direction and mechanism, and shear sense, allow us to interpret dike fabrics in terms of shear resolved on the dike walls during intrusion (en echelon arrangement, offsetting, and dike curvature). The interpretation of AMS and vesicle fabrics of the margins of four dikes shows a reasonable agreement with the proposed theoretical models, suggesting that asymmetric fabrics can be used to infer magma flow and may provide valuable information on the shear resolved on the dike walls during intrusion.     

广东省河台金矿区糜棱岩锆石LA-ICP-MS U-Pb年龄及其地质意义

河台金矿区位于广东省高要市境内,普遍认为其是典型的韧性剪切带型金矿。本次对采自韧性剪切带中的糜棱岩和初糜棱岩样品进行了LA-ICP-MS锆石U-Pb定年,以约束剪切带的活动时代,并为金矿化事件提供依据。利用糜棱岩中的热液锆石约束韧性剪切带的变质变形时间,获得两组年龄:糜棱岩中热液锆石增生边加权平均年龄约为240Ma,代表左旋运动的年龄;初糜棱岩中热液锆石加权平均年龄约为204Ma,代表右旋运动的年龄。因此,矿区在印支期可能经历过两期剪切变形,并且这两期变形事件在整个华南都是普遍存在的。结合前人对矿化时间的研究,河台金矿的成矿时间(燕山期)要晚于韧性剪切带的形成时间(印支期)。样品中继承性锆石(糜棱岩中核部锆石)年龄显示云开群最晚沉积时间为早古生代早期,而并非前寒武地层。另外,本次研究还测试了锆石的Lu-Hf同位素,实验表明,可以根据热液锆石与继承性锆石的Lu-Hf同位素关系,判定热液锆石的形成条件。云开群的成岩物质主要来自1.8Ga地壳增生事件所形成的地壳物质发生重熔而产生的岩浆岩。     

钦杭带南段庞西垌-金山银金矿田控矿构造分析及找矿指示

钦杭成矿带南段的庞西垌-金山银金矿田位于粤东与桂西交界处,包括庞西垌、金山、中苏、竹根坡、高村等银金矿床及一系列银金矿点,是20世纪80年代规划的十大银矿基地之一。庞西垌地区经历了多期的构造-岩浆活动,韧性剪切带和断裂构造尤其发育。印支期以来,可识别出3期主要构造:第一期为右行韧性剪切,前人利用糜棱岩中白云母测得时代为221 Ma;第二期为左行韧脆性剪切,北东向的庞西垌-金山断裂带反映最为突出,切过燕山晚期花岗岩;第三期为脆性走滑。银金矿体及矿石结构、构造主要受第二期左行韧脆性剪切带控制。应用里德尔剪切体系对控矿构造的分布特征分析显示,银金矿脉主要赋存于里德尔体系的D、R和T裂隙中;D裂隙是最重要的赋矿构造,热液蚀变岩型和石英脉型矿体呈透镜状左行斜列于主断裂面中,沿走向往南西侧伏;R和T裂隙主要发育梳状含金石英脉。R裂隙呈密集剪破裂产出,与主断裂面夹角15°~35°;T裂隙呈雁列状等距分布,与主断裂面夹角约为45°。这些矿脉就位体系,可以为该区进一步找矿作参考。     

Along-strike shear-sense reversal in the Vals-Scaradra Shear Zone at the front of the Adula Nappe (Central Alps,Switzerland)

The Adula Nappe in the Central Alps comprises pre-Mesozoic basement and minor Mesozoic sediments, overprinted by Paleogene eclogite-facies metamorphism. Peak pressures increase southward from ca. 1.2 GPa to values over 3 GPa, which is interpreted to reflect exhumation from a south-dipping subduction zone. The over- and underlying nappes experienced much lower Alpine pressures. To the north, the Adula Nappe ends in a lobe surrounded by Mesozoic metasediments. The external shape of the lobe is simple but the internal structure highly complicated. The frontal boundary of the nappe represents a discontinuity in metamorphic peak temperatures, between higher T in the Adula Nappe and lower T outside. A shear zone with steeply dipping foliation and shallowly-plunging, WSW-ENE oriented, i.e. orogen-parallel stretching lineation overprinted the northernmost part of the Adula Nappe and the adjacent Mesozoic metasediments (Vals-Scaradra Shear Zone). It formed during the local Leis deformation phase. The shear sense in the Vals-Scaradra Shear Zone changes along strike; from sinistral in the W to dextral in the E. Quartz textures also vary along strike. In the W, they indicate sinistral shearing with a component of coaxial (flattening) strain. A texture from the middle part of the shear zone is symmetric and indicates coaxial flattening. Textures from the eastern part show strong, single c-axis maxima indicating dextral shearing. These relations reflect complex flow within the Adula Nappe during a late stage of its exhumation. The structures and reconstructed flow field indicate that the Adula basement protruded upward and northward into the surrounding metasediments, spread laterally, and expelled the metasediments in front towards west and east.     

鲁西青邑韧性剪切带运动学涡度及剪切作用类型

青邑韧性剪切带是晚太古代末期发育在鲁西前寒武纪基底花岗岩中一条规模较大的韧性剪切带。剪切带NW走向,面理直立,线理水平,剪切标志反映右行剪切。石英光轴法求得运动学涡度在0．96～0．99之间变化,极摩尔圆法求得糜棱岩化岩石运动学涡度为0．91,初糜棱岩运动学涡度为0．87,糜棱岩运动学涡度为0．81,超糜棱岩运动学涡度为0．60。运动学涡度表明,剪切带剪切作用类型为一般剪切,变形初期以单剪为主,随应变的增大,运动学涡度值逐渐减小,变形的纯剪分量不断增加,最后以纯剪为主。剪切作用类型及三维参照变形分析表明,青邑韧性剪切带属加长一变宽类型的一般剪切带并且在Y轴方向上有所增长。韧性剪切在太古代末期克拉通化过程中具有加厚陆壳的作用。     

甘肃北山晚古生代花岗岩体内基性岩墙群弧形展布的成因及其构造意义

在甘肃北山一个东西向展布、椭圆状的晚古生代花岗岩体内主要发育着3期岩墙群,其中中期基性岩墙群最密集,呈现出弧形展布的特征,在该侵入体北部呈北北东向趋势,向南逐渐偏转为北北西向。关于中期岩墙群弧形展布的原因有两种可能的解释,即:纵弯褶皱和应力场扰动。野外观察不支持第一种解释,因为在岩墙或围岩内都没有观察到与褶皱有关的变形构造。本文利用Poly3D软件模拟了近东西向隐伏断层活动对上覆应力场的扰动。计算结果表明,当北北西—南南东向的区域性拉张叠加上沿着隐伏断层发生不协调的右行走滑时,水平大主压应力迹线在隐伏断层上方出现类似的弧形展布。这意味着稍早的花岗岩侵入体冷凝冷却并没有完全焊接地壳内一些大尺度断层,地壳断块的相对独立活动可以造成沿着这些断层发生与区域变形场不协调的右行走向滑动,进而形成岩墙群弧形展布所需要的地应力场。     

Palaeozoic late collisional strike-slip deformations in Tianshan and Altay, Eastern Xinjiang, NW China

ABSTRACT In Central Asia, thrusts and shear zones resulting from Palaeozoic accretional events were reworked by E–W-trending ductile strike-slip faults during late Palaeozoic–early Mesozoic time. In the Tianshan range, microstructures and quartz C-axis fabrics show a main dextral shearing associated with sinistral localized shear zones. The relationship between these conjugate structures indicates a NNW–SSE-trending bulk shortening. In the Chinese Altay mountains, the existence of δ-type microstructures in an important sinistral mylonitic zone infers high rates of deformation. This shear zone is bordered by a late dextral ductile fault synchronous with a granite emplacement. Field evidence and datings from the literature provide chronological constraints. In the late Carboniferous, the sinistral mylonitic deformation took place in the Erqishi–Irtysh shear zone in the northeastern part of Xinjiang and in Kazakhstan. During the Early Permian, a regional dextral event occurred in the Tianshan range and under the whole of northern Xinjiang.     

辽吉裂谷带北缘连山关韧性剪切带的构造属性和运动学分析

韧性剪切带在岩石圈地壳中广泛存在,蕴含应力、应变和温压等环境参数,是构造解析、流变学和成因机制研究的重要对象.辽吉裂谷带位于辽宁东部-吉林南部,是华北克拉通重要的古元古代活动带之一.连山关岩体地处辽吉裂谷带北缘,经历复杂变质变形作用,岩体南缘发育NWW向右行走滑韧性剪切带.糜棱岩显微结构观测分析揭示,剪切带内糜棱岩以S、SL构造岩为主,总体呈压扁型应变.运动学涡度值为0.91~0.97,均大于0.75,指示简单剪切为主的变形特征.糜棱岩中云母显示塑性拉长,石英动态重结晶明显,以膨凸重结晶作用为主.EBSD分析结果表明,石英发育中低温菱面组构,对应变形温度450~550℃,暗示糜棱岩形成于低绿片岩相-低角闪岩相.结合前人研究成果,我们认为连山关韧性剪切带可能起源于早元古代晚期.连山关岩体先后经历早期隆起造成的伸展-滑脱作用和晚期与上覆辽河群共同经历的南北向挤压,从而在岩体南缘形成陡倾的右行韧性剪切带.      

扬子克拉通黄陵背斜南部～860 Ma岩墙的成因及地质意义

首次报道了扬子克拉通黄陵背斜南部由辉绿岩脉、花岗闪长岩脉和正长花岗岩脉组成的新元古代早期小型岩墙.辉绿岩脉和花岗闪长岩脉的锆石U-Pb定年结果分别为856±6.4 Ma和860±6.0 Ma.全岩主量和微量元素数据显示岩墙中辉绿岩具有高Pb含量,且Ba/Nb和La/Nb比值变化较大,而花岗闪长岩脉和正长花岗岩脉样品的Y和Nb含量较低,结合继承性锆石年龄分布特征,暗示岩墙形成过程中遭受了不同程度的年轻岛弧地壳物质混染.黄陵南部~860 Ma小型岩墙的发现,表明扬子克拉通在早于该时期已进入后碰撞的伸展构造环境.综合区域已有资料,认为扬子克拉通前寒武纪基底的裂解在时空上均表现出较大差异.      

The Sierra Ballena Shear Zone in the southernmost Dom Feliciano Belt (Uruguay): evolution, kinematics, and deformation conditions

The Sierra Ballena Shear Zone (SBSZ) is part of a high-strain transcurrent system that divides the Neoproterozoic Dom Feliciano Belt of South America into two different domains. The basement on both sides of the SBSZ shows a deformation stage preceding that of the transcurrent deformation recognized as a high temperature mylonitic foliation associated with migmatization. Grain boundary migration and fluid-assisted grain boundary diffusion enhanced by partial melting were the main deformation mechanisms associated with this foliation. Age estimate of this episode is >658 Ma. The second stage corresponds to the start of transpressional deformation and the nucleation and development of the SBSZ. During this stage, pure shear dominates the deformation, and is characterized by the development of conjugate dextral and sinistral shear zones and the emplacement of syntectonic granites. This event dates to 658–600 Ma based on the age of these intrusions. The third stage was a second transpressional event at about 586 to <560 Ma that was associated with the emplacement of porphyry dikes and granites that show evidence of flattening. Deformation in the SBSZ took place, during the late stages, under regional low-grade conditions, as indicated by the metamorphic paragenesis in the supracrustals of the country rocks. Granitic mylonites show plastic deformation of quartz and brittle behavior of feldspar. A transition from magmatic to solid-state microstructures is also frequently observed in syntectonic granites. Mylonitic porphyries and quartz mylonites resulted from the deformation of alkaline porphyries and quartz veins emplaced in the shear zone. Quartz veins reflect the release of silica associated with the breakdown of feldspar to white mica during the evolution of the granitic mylonites to phyllonites, which resulted in shear zone weakening. Quartz microstructures characteristic of the transition between regime 2 and regime 3, grain boundary migration and incipient recrystallization in feldspar indicate deformation under lower amphibolite to upper greenschist conditions (550–400°C). On the other hand, the mylonitic porphyries display evidence of feldspar recrystallization suggesting magmatic or high-T solid-state deformation during cooling of the dikes.     

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。