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青藏高原典型地区的地貌量化分析——兼对高原“夷平面”的讨论
引用本文:尹安.青藏高原典型地区的地貌量化分析——兼对高原“夷平面”的讨论[J].地学前缘,2006,13(5):0-0.
作者姓名:尹安
作者单位:美国加利福尼亚大学,地球与空间科学系和地球物理与天体物理研究所,洛杉矶,90095-1567
基金项目:美国国家科学基金;国家重点实验室基金
摘    要:尽管过去150年以来,人们对于喜马拉雅造山带有很长的一段研究历史,但是对其几何特征、运动方式、动力学演化仍然理解不深。这种情况的出现,主要是因为人们持续关注的是喜马拉雅造山带的二维构造空间特性,并将某些研究程度较高地区的地质关系向外推广到造山带其他地区。就地理、地层及构造划分而言,概念的混淆和误解在有关喜马拉雅的文章中也大量存在。为了阐明这些问题,并为那些有兴趣探究喜马拉雅造山带地质演化过程的人们提供一个新的平台,文中系统地综述了以前的基本观察。我的综述主要是强调沿走向变化的喜马拉雅地质格架在喜马拉雅剥露、变质和前陆沉积方面所起的作用。文章的主要目的是阐明占据造山带核部的大喜马拉雅结晶岩带(GHC)的侵位历史。因为喜马拉雅大部分地区是由主中央冲断层(MCT)和藏南拆离系(STD)之间的GHC所组成,所以在地图和剖面观察上确定这些一级喜马拉雅构造之间的关系是非常关键的。中喜马拉雅出露的平面模式表明,MCT具有断坪-断坡的逆断层的几何特征。南部的逆冲断坪携带了一个GHC的板片(Slab)叠置在小喜马拉雅层序之上(LHS),并形成了一个在MCT逆冲断层带之南延续100km的巨大上盘断弯褶皱。在西喜马拉雅造山带地区,东经约77°处,MCT呈现为横向逆冲断坡(Mandi倾向逆冲断坡)。在其西边,MCT将低级变质的特提斯喜马拉雅层序(THS)叠置到低级变质的小喜马拉雅之上;而在其东边,MCT将高级GHC叠置到低级LHS之上。这种沿走向变化的地层叠置和横穿MCT的变质等级表明,逆冲断层的断距向西减小,可能是由于地壳短缩总量沿着喜马拉雅造山带向西减小所致。在所有出露的地方,STD大致都沿着THS底部的同一地层面,呈现出一个长度>100km的上盘断坪。这种关系说明:STD可能沿着一个先期存在的岩石接触面,或者沿中部地壳近水平的脆性—韧性转换带而发生。虽然喜马拉雅造山带藏南拆离系的上盘都有THS发育,但是至今没有找到THS切断STD下盘的证据。这样使得估算STD的滑动距离非常困难。STD最南端地层或与MCT(即,Zanskar)相交,或者位于MCT前端1~2km的范围内(不丹),这两种可能都暗示MCT与STD在它们向南的上倾(up-dip)方向有可能结合。虽然这种几何特征在现有的模型中几乎被忽略,但对于整个喜马拉雅造山带的变形和剥露历史具有重要的指示作用。

关 键 词:喜马拉雅造山带  主中央冲断层(MCT)  藏南拆离系(STD)  被动-顶板(roof)断层  主动-顶板断层  侵蚀剥露
文章编号:1005-2321(2006)05-0416-100
收稿时间:2006-04-27
修稿时间:2006-07-20

Cenozoic tectonic evolution of the Himalayan orogen as constrained by along-strike variation of structural geometry, exhumation history, and foreland sedimentation
An YIN.Cenozoic tectonic evolution of the Himalayan orogen as constrained by along-strike variation of structural geometry, exhumation history, and foreland sedimentation[J].Earth Science Frontiers,2006,13(5):0-0.
Authors:An YIN
Abstract:Despite a long research history over the past 150 years,the geometry,kinematics,and dynamic evolution of the Himalayan orogen remain poorly understood.This is mainly due to continued emphasis on the two-dimensionality of the Himalayan orogenic architecture and extrapolation of geologic relationships from a few well-studied local areas to the rest of the orogen.Confusion and misconception are also widespread in the Himalayan literature in terms of the geographic,stratigraphic,and structural divisions.To clarify these issues and to provide a new platform for those who are interested in exploring the geologic development of this spectacular mountain belt,I systematically review the essential observations relevant to the along-strike variation of Himalayan geologic framework and its role in Himalayan exhumation,metamorphism and foreland sedimentation.A main focus of the review is to elucidate the emplacement history of the high-grade Greater Himalayan Crystalline Complex(GHC)that occupies the core of the orogen.Because the Main Central Thrust(MCT)above and the South Tibet Detachment(STD)below bound the GHC in most parts of the Himalaya,it is critical to determine the relationship between these first-order Himalayan structures in map and cross-section views.The exposed map pattern in the central Himalaya indicates that the MCT has a flat-ramp geometry.The thrust flat in the south carries a slab of the GHC over the Lesser Himalayan Sequence(LHS)and creates a large hanging wall fault-bend fold continuing N100 km south of the MCT ramp zone.In the western Himalayan orogen at the longitude ~77°E,the MCT exhibits a major lateral ramp(the Mandi ramp).West of this ramp,the MCT places the low-grade Tethyan Himalayan Sequence(THS)over the low-grade LHS,whereas east of the ramp,the MCT places the high-grade GHC over the low-grade LHS.This along-strike change in stratigraphic juxtaposition and metamorphic grade across the MCT indicates a westward decrease in its slip magnitude,possibly a result of a westward decrease in total crustal shortening along the Himalayan orogen.Everywhere exposed,the STD follows roughly the same stratigraphic horizon at the base of the THS,exhibiting a long(N100 km)hanging wall flat.This relationship suggests that the STD may have initiated along a preexisting lithologic contact or the subhorizontal brittle ductile transition zone in the middle crust.Although the STD has the THS in its hanging wall everywhere in the Himalayan orogen,no THS footwall cutoffs have been identified.This has made slip estimates of the STD exceedingly difficult.The southernmost trace of the STD either merges with the MCT(i.e.,Zanskar)or lies within 1-2 km of the MCT frontal trace(i.e.,Bhutan),suggesting that the MCT may join the STD in their up-dip directions to the south.This geometry,largely neglected by the existing models,has important implications for the deformation and exhumation history of the entire Himalayan orogen.
Keywords:Himalayan orogen  Main Central Thrust  South Tibet Detachment  passive-roof fault  active-roof fault  erosional exhumation
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