Tectonics,magmatism, and mineralization of Circum–Balkash–Junggar area in the Central Asian Orogenic Belt

ABSTRACT

The Circum–Balkhash–Junggar area, including mostly Kazakhstan, NW China, Russia, Kyrgyzstan, Tajikistan, Uzbekistan, and Mongolia, occupies an important tectonic position of the Central Asian Orogenic Belt (CAOB) (Figure 1). Tectonically, this vast area records the complicated geodynamic processes, among which the most prominent stages are the formation of the U-shaped Kazakhstan Orocline and its interactions with adjacent Altai (Altay), Junggar (West Junggar, Junggar Basin, and East Junggar), and Tianshan orogenic collages in the Palaeozoic, bearing large-scale mineral deposits. The formation of the Late Palaeozoic mineral deposits is related to the tectonic evolution of the Devonian and Carboniferous–Permian volcano-magmatic arcs in the region. However, the link between the metallogeny and the evolution of the volcano-magmatic arcs is not well understood and existing geodynamic models have not explained satisfactorily the mechanism of the huge metallogenic belt. Therefore, this special issue focuses on the formation of the Late Palaeozoic porphyry Cu deposits and their link to the tectonic evolution of the Devonian and Carboniferous–Permian volcano-magmatic arcs with emphasis on comparative studies across the international borders.     

鄂西弧形构造变形特征及成因机制

对川东-湘鄂西断褶带内鄂西地区的弧形构造,从构造剖面特征、叠加褶皱样式和断裂性质入手进行几何学和运动学分析。结果发现鄂西弧形构造具有多期变形特征:早期普遍为北东东向的直线型褶皱,随着弧形带扩展,在东、西两翼分别发育右行和左行的逆冲-走滑断裂,同时分别形成北北东向和北西西向的弧形褶皱。晚期弧形带中部发育北北东向构造并叠加改造了早期北东东和北西西向褶皱,同时在黄陵背斜以西还发育交切早期构造的北北西向仙女山右行走滑断裂。根据弧形带扩展的几何学-运动学分类原则,并结合前人的古地磁研究结果,推测鄂西弧形构造应属于构造弯曲形成的弯曲弧。区域滑脱层和黄陵隆起阻挡可能是控制弧形样式的主要原因。区域滑脱层控制了拆离滑脱褶皱的构造样式; 黄陵基底隆起的阻挡作用使弧形带东翼进一步弯曲变形,并导致了构造应力场方向发生改变,造成了晚期北北东向与早期北东东向构造的叠加。由此恢复的鄂西弧形构造变形过程对于揭示川东-湘鄂西断褶带构造演化具有重要的指示意义。     

扬子褶皱带侏罗纪砂岩古地磁及其褶皱带弧形弯曲的成因

为了更好地认识上扬子褶皱带和中扬子褶皱带走向差异的机制,我们对中、上扬子褶皱带过渡的关键地区重庆市万州和云阳两个地区的中、晚侏罗世砂岩进行了古地磁研究.逐步热退磁分离出两个组分,低温组分（LTC）在所有样品中均分离出来,为现代地磁场的重磁化;中侏罗世样品和万州地区的晚侏罗世样品分离出来的高温组分（HTC）也为现代地磁场的重磁化.云阳地区晚侏罗世样品分离出来的高温组分通过逐步展平褶皱检验显示：在褶皱展平至33.8%时,精度参数达到最大,相应的古地磁方向为D=19.1°,I=48.9°（α₉₅= 6.3°）,古地磁极为73.5°N,198.2°E（dp=5.5°, dm=8.3°）,与白垩纪参考古地磁极对比,此高温组分揭示云阳地区在褶皱变形的后期经历了7.7°±6.1°的顺时针旋转.结合前人的数据,我们认为中扬子褶皱带普遍存在弯山构造（orocline）,这可能与华北板块向华南板块的挤入作用有关;但是中、上扬子褶皱带过渡地区的弧形弯曲总体上不是由弯山构造形成的,很可能是在太平洋板块向北西方向俯冲的宏观板块构造背景下的应变分异作用形成的.     

Structure of the Texas Orocline beneath the sedimentary cover (southeast Queensland,Australia)

The New England Orogen in eastern Australia is characterised by orogenic-scale curvatures (oroclines). The largest and most prominent curvature in this system is the Texas Orocline, but its subsurface geometry is still poorly constrained. A large component of the orocline is covered by post-oroclinal sedimentary rocks, which obscure deeper sections of the orocline and make it difficult to understand how the structure is connected to other segments of the New England Orogen. Here, we present geophysical data that elucidate the structure of the Texas Orocline below the sedimentary cover. Using 2D seismic, aeromagnetic TMI (total magnetic intensity) and Bouguer gravity datasets, in combination with outcrop and well data, we identified the depth to the New England ‘basement’ and significant faults intersecting it. We also traced the strongly contorted subsurface continuation of the Peel-Yarrol Fault System, which is characterised by local gravity and magnetic anomalies associated with isolated serpentinite outcrops. Constraints on the timing of oroclinal bending were obtained from the interpretation of seismic transects, which showed that early Permian sedimentary rocks of the Bowen Basin were deposited in a subtrough that deviates from the general north–south trend of the Bowen Basin. The subtrough is oriented approximately parallel to the western limb of the Texas Orocline, thus suggesting that the orocline formed during and/or after early Permian rifting. Our analysis indicates that initial bending occurred contemporaneously with the development of the early Permian rift basins, most likely in the backarc region of a retreating subduction zone. Subsequently, phases of strike-slip and contractional deformation have further tightened the pre-existing curvatures.     

The tectonic significance of lower Permian successions in the Texas Orocline (Eastern Australia)

The Texas Orocline is a prominent orogenic curvature that developed during the early Permian in the southern New England Orogen. Outliers preserving lower Permian sedimentary successions (Bondonga, Silver Spur, Pikedale, Terrica, Alum Rock and Ashford beds) approximately outline the oroclinal structure, but the tectonic processes responsible for the development of these basinal successions, and their relationships to the Texas Orocline, are unclear. Here we address this shortcoming by providing new U–Pb detrital and primary zircon ages from these successions, as well as detailed stratigraphic and structural data from the largest exposed succession (Bondonga beds). Field observations and U–Pb geochronological data suggest that the lower Permian successions in the Texas Orocline are remnants of a single, formerly larger basin that was deposited after ca 302 Ma. Time constraints for formation of this basin are correlative with constraints from the lower Permian Nambucca Block, which was likely deposited in response to regional back-arc extension during and/or after the development of the Texas Orocline. The conclusion that the lower Permian sedimentary basins in the Texas Orocline belong to this back-arc extensional system supports the suggestion that oroclinal bending in the New England Orogen was primarily controlled by trench retreat and associated overriding-plate extension.     

http://www.sciencedirect.com/science/article/pii/S1674987113000030

The Dabashan orocline is situated in the northwestern margin of the central Yangtze block,central China.Previous studies have defined the orthogonal superposed folds growing in its central-western segment thereby confirming its two-stage tectonic evolution history.Geological mapping has revealed that more types of superposed folds have developed in the eastern segment of the orocline,which probably provides more clues for probing the structure and tectonic history of the Dabashan orocline.In this paper,based on geological mapping,structural measurements and analyses of deformation,we have identified three groups of folds with different trends （e.g.NW-,NE-and nearly E-trending folds） and three types of structural patterns of superposed folds in the eastern Dabashan foreland （e.g.syn-axial,oblique,and conjunctional superposed folds）.In combination with geochronological data,we propose that the synaxial superposed folds are due to two stages of ～N-S shortening in the west and north of the Shennongjia massif,and that oblique superposed folds have been resulted from the superposition of the NW-and NE-trending folds onto the early ～ E-W folds in the east of the Shennongjia massif in the late Jurassic to early Cretaceous.The conjunctional folds are composed of the NW-and NE-trending folds,corresponding to the regional-scale dual-orocline in the eastern Sichuan as a result of the southwestward expansion of the Dabashan foreland during late Jurassic to early Cretaceous,coeval with the northwestward propagation of the Xuefengshan foreland.Integration of the structure and geochronology of the belt shows that the Dabashan orocline is a combined deformation belt primarily experiencing a twostage tectonic evolution history in Mesozoic,initiation of the Dabashan orocline as a foreland basin along the front of the Qinling orogen in late Triassic to early Jurassic due to collisional orogeny,and the final formation of the Dabashan orocline owing to the southwestward propagation of the Qinling orogen during late Jurassic to early Cretaceous intra-continental orogeny.Our studies provide some evidences for understanding the structure and deformation of the Dabashan orocline.     

川东弧形带三维构造扩展的AFT记录

对川东弧形褶皱带北段、中段和南段的三条剖面,进行了7件样品的磷灰石裂变径迹(AFT)测试,结合前人已发表的4件样品,分析模拟了主要背斜的隆升－剥露热历史.结果表明川东弧形带主体构造变形时间为135→65 Ma,即早白垩世早期到晚白垩世晚期.进而建立并对比了三条剖面的构造变形时序,揭示出川东弧形带的三维构造扩展历史:(1) 平行于构造线走向,表现为从中心向两翼的构造扩展,弧形带中段的构造变形最早,起始时间为早白垩世早期(约135 Ma),北段和南段的变形较晚,起始时间为早白垩世晚期(约100 Ma);(2) 垂直于构造线走向,在弧形带北段和中段均表现为由东向西的构造扩展,而在弧形带南段,由于受到前缘华蓥山断裂的影响,表现为自西向东的变形时序.川东弧形带的三维构造扩展历史暗示了"弯山构造"的成因模式,以及华蓥山先存断裂对弧形构造的限制作用.     

中亚造山带东段古生代山弯构造

中亚造山带东段位于西伯利亚和华北克拉通之间，经历了多构造体系叠加和多旋回洋陆转换的复杂演化过程，目前大量研究均以构造带为核心来限定区域构造格局，但一直争议较大。本文以构造单元的构造属性及其形成过程为主线，结合区域构造带演化，重新厘定了中国东北地区基本构造格局，建立了中国东北山弯构造演化模型。研究表明，古生代时期中国东北地区的主要构造单元由两个具前寒武纪基底的古老地块——额尔古纳地块和佳木斯地块及其张广才岭陆缘弧与两个古生代增生地体——兴安增生地体和松辽增生地体组成，其间由古亚洲洋分支新林- 喜桂图洋、贺根山- 嫩江洋、龙凤山洋和索伦洋分割。早古生代，西部额尔古纳地块东南部为西太平洋型活动陆缘，发育有嘎仙- 吉峰- 环宇洋内弧和头道桥等洋岛，~500 Ma随着新林- 喜桂图洋的关闭，这些洋内弧和洋岛拼贴增生至额尔古纳地块东南缘。随后贺根山- 嫩江洋的俯冲和后撤形成了一系列沟- 弧- 盆体系，持续的俯冲导致弧陆碰撞和陆缘增生，形成兴安增生地体的主体。同时，东部佳木斯地块西侧发育有龙凤山洋的安第斯型俯冲活动陆缘，形成了张广才岭陆缘弧。伴随着各大洋的俯冲和陆缘增生，额尔古纳地块和佳木斯地块以及它们的陆缘增生带构成了一个早古生代近东西向展布的地块链。南部以锡林浩特- 龙江微地块为核心发生陆缘俯冲，形成松辽增生地体雏形。索伦洋发生双向俯冲，并通过弧陆碰撞产生陆缘增生。晚古生代，伴随着古亚洲洋的北向俯冲和后撤，早期形成的地块链逐渐发生向南弯曲。二叠纪末期—中三叠世古亚洲洋俯冲消减闭合以及西北部蒙古- 鄂霍茨克洋和东部泛大洋的俯冲挤压，导致地块链进一步弯曲，同时，早期的古老地块、增生地体、弧岩浆岩、沉积建造等发生汇聚，最终形成一个以额尔古纳地块和兴安增生地体为西翼，佳木斯地块和张广才岭陆缘弧为东翼，松辽增生地体为核心的大规模山弯构造——中国东北山弯构造。     

Avalonia,get bent!–Paleomagnetism from SW Iberia confirms the Greater Cantabrian Orocline

The amalgamation of Pangea formed the contorted Variscan-Alleghanian orogen,suturing Gondwana and Laurussia during the Carboniferous.From all swirls of this orogen,a double curve in Iberia stands out,the coupled Cantabrian Orocline and Central Iberian curve.The Cantabrian Orocline formed at ca.315–290 Ma subsequent to the Variscan orogeny.The formation mechanism of the Cantabrian Orocline is disputed,the most commonly proposed mechanisms include either(1)that south-westernmost Iberia would be an Avalonian(Laurussian)indenter or(2)that the stress field changed,buckling the orogen.In contrast,the geometry and kinematics of the Central Iberian curve are largely unknown.Whereas some authors defend both curvatures are genetically linked,others support they are distinct and formed at different times.Such uncertainty adds an extra layer of complexity to our understanding of the final stages of Pangea’s amalgamation.To solve these issues,we study the late Carboniferous–early Permian vertical-axis rotations of SW Iberia with paleomagnetism.Our results show up to 70
counterclockwise vertical-axis rotations during late Carboniferous times,concurring with the anticipated kinematics if SW Iberia was part of the southern limb of the Cantabrian Orocline.Our results do not allow the necessary penecontemporaneous clockwise rotations in Central Iberia to support a concomitant formation of both Cantabrian and Central Iberian curvature.The coherent rotation of both Gondwanan and Avalonian pieces of SW Iberia discards the Laurussian indenter hypothesis as a formation mechanism of the Cantabrian Orocline and confirms the Greater Cantabrian Orocline hypothesis.The Greater Cantabrian Orocline likely formed as a consequence of a change in the stress field during the late Carboniferous and extended beyond the Rheic Ocean suture affecting the margins of both Laurussia and Gondwana.     

Kinematics of orocline-parallel faults in the Texas and Coffs Harbour oroclines (eastern Australia) and the role of flexural slip during oroclinal bending

The Texas and Coffs Harbour oroclines are defined by a Z-shaped curvature in the southern New England Orogen (eastern Australia), but the geometry and kinematics of faults around these oroclines, as well as their possible role during oroclinal bending, have hitherto not been understood. Using aeromagnetic and open file seismic data, as well as field observations, the pattern, geometry and kinematics of fault systems, have been investigated. Fault traces with a strike-slip component are oriented parallel to the curved magnetic and structural fabrics of the Texas and Coffs Harbour oroclines. Our observations show evidence for sinistral or sinistral-reverse, dextral (or dextral-reverse) and normal kinematics along NW-striking faults. The dominant kinematics along NNE- and NE-striking faults is dextral or dextral-reverse. The timing of faulting is not well constrained, but the ubiquitous recognition of orocline-parallel faults may suggest that a flexural slip mechanism operated during oroclinal bending in the early–middle Permian (ca 299–265 Ma). Our observations indicate that many of the orocline-parallel faults, with strike-slip separation, were reactivated during the Mesozoic and Cenozoic, as indicated by the recognition of displaced Triassic granitoids, Mesozoic sedimentary rocks and Cenozoic basalts.