印度—亚洲大陆主碰撞过程的火山作用响应

广泛发育在西藏冈底斯岩浆岩带中的林子宗火山岩及其与下伏地层间的区域性不整合 ,提供了印度—亚洲大陆碰撞 (在西藏南部 )的构造火成岩新证据。岩石学、主元素、微量元素、稀土元素及Nd Sr Pb同位素地球化学研究表明 ,林子宗火山岩早期带有较多陆缘弧火山岩特征 ,中期开始出现标志陆内岩浆活动的钾玄岩 ,晚期更多地显示了加厚陆壳条件下火山岩的特点 ,记录了由新特提斯俯冲消减末期过渡到印度—亚洲大陆碰撞的信息。系统的40 Ar/3 9Ar同位素测年确定林子宗火山岩的年龄区间为 4 0 .84～ 6 4 .4 7Ma ;其底部年龄给出了林子宗火山岩与下伏地层间不整合形成时间的最晚时限 (～ 6 5Ma)。该不整合面上、下之地层在沉积相、变形样式与变形程度上均截然不同 ,反映形成环境的重大变化 ,标志着一次重大的地质事件。根据上述事实 ,结合青藏高原岩浆活动的区域时空分布 ,及西藏南部地层、古生物与沉积研究成果 ,笔者认为印度—亚洲大陆碰撞开始于 6 5Ma左右(K/T界限时间 )。

塔里木盆地库车褶皱冲断带东段依奇克里克背斜的构造特征和形成演化

依奇克里克背斜位于塔里木盆地北缘,库车褶皱冲断带北部构造带东段。它是一个基底卷入型开阔直立背斜,是在晚海西期古隆起和印支期库车周缘前陆盆地前渊带背景下形成的一个含油气构造。背斜核部出露的最老地层是白垩系下统,向两翼方向依次出露古近系、新近系和第四系。钻井资料揭示,之下还有三叠系和侏罗系含煤碎屑岩建造,不整合于石炭系灰岩之上。中新统吉迪克组至更新统西域组是背斜的生长地层,古近系及以下地层是前生长地层。背斜位于库车褶皱冲断带根带,发育厚皮构造;向前锋方向,逐渐演变为薄皮构造。褶皱冲断带的两个主滑脱断层分别发育于侏罗系煤层和吉迪克组膏盐层。作为印度—亚洲碰撞的远程效应,背斜的变形起始于古近纪末—新近纪初,经多个变形加速期,脉式冲断,直至第四纪晚期才最终定型。

西藏南部绒布地区上白垩统宗卓组沉积相与沉积环境演化

宗卓组地层作为中生代海相沉积的重要组成部分,记载了西藏南部地区白垩纪古海洋、古气候等重要地质历史信息。通过对砂岩进行镜下观察、物源分析、粒度分析等方面的研究,笔者等对绒布地区上白垩统宗卓组的沉积相类型和沉积环境演化有了新的认识。粒度分析指示沉积时期水体搬运动力较弱、存在浊流沉积;宗卓组内3种不同类型的砂岩镜下特征、地球化学特征和锆石U-Pb年龄,指示3类砂岩的物源不同。笔者等在宗卓组共识别出陆棚相、大陆斜坡相和深海盆地3类沉积相,并对其进行了更为详细的划分。晚白垩世新特提斯洋壳向北俯冲,俯冲结束后印度—亚洲大陆发生初始碰撞。结合研究区构造背景分析,笔者等推测这是导致研究区沉积环境由半深海环境变为深海环境的原因。对研究区宗卓组地层和砂岩来源进行分析后得出:在洋壳俯冲阶段,宗卓组为被动大陆边缘沉积;在之后印度—亚洲陆陆碰撞的初始碰撞阶段,特提斯喜马拉雅被动陆缘已不复存在,并完全过渡为前陆盆地。

塔里木盆地巴楚隆起北缘的吐木休克断裂与巴东断裂

吐木休克断裂位于塔里木盆地西部,巴楚隆起和阿瓦提凹陷之间,是一条大型基底卷入型冲断构造。走向NW‐SE,呈弧形向NEE凸出;倾向巴楚隆起。根据构造变形特征,断裂自NW向SE可以划分为4段。Ⅰ和Ⅲ段为简单基底卷入型冲断构造段;Ⅱ段发育背冲断层,与主干断层呈“y”字型剖面组合关系;Ⅳ段为基底卷入型楔状构造,主冲断层顶部出现一条向巴楚隆起逆冲的反冲断层。断裂上盘发育背斜,下盘有明显的“牵引构造”,显示吐木休克断裂可能是由吐木休克背斜北翼突破形成的,是一条褶皱相关断层。吐木休克断裂形成于中新世晚期至上新世初,持续演化至第四纪。断裂带上发育的上新世末—第四纪初正断层代表印度—亚洲碰撞脉动式远程效应的一个构造间歇期。吐木休克断裂东侧的巴东断裂是巴楚隆起与塔中隆起的过渡构造带,雏形形成于奥陶纪晚期—志留纪,晚新生代复活。

青藏高原拉萨块体北缘晚白垩世竟柱山组红层古地磁学研究及其构造意义

作为亚洲大陆晚白垩世时的最南缘,拉萨块体古纬度对研究印度—亚洲大陆碰撞过程具有重要意义.本文对采自拉萨块体北缘晚白垩世竟柱山组红层35个采点415块定向样品进行了详细的古地磁研究.岩石磁学结果显示样品中主要载磁矿物为赤铁矿.通过逐步热退磁可从22个采点中获得一组稳定的高温分量方向,同沉积校正后该方向在95%和99%置信区间内均通过了McElhinny和McFadden褶皱检验,表明该方向(D_(c)=335.6°,I_(c)=31.2°,k_(c)=52.8,α95=4.3°)应为原生剩磁,其对应的古地磁极为63.3°N,329.4°E,A_(95)=3.6°.等温剩磁各向异性分析显示拉萨块体北部尼玛和班戈地区晚白垩世竟柱山组红层中可能并不存在磁倾角浅化现象.结合前人的研究成果限定了拉萨块体晚白垩世古纬度为16.6±2.1°N(参考点31.8°N,87.7°E),晚白垩世以来青藏高原北部可能发生了960±400 km的南北向构造缩短.

Constraining the timing of the India-Asia continental collision by the sedimentary record

Placing precise constraints on the timing of the India-Asia continental collision is essential to understand the successive geological and geomorphological evolution of the orogenic belt as well as the uplift mechanism of the Tibetan Plateau and their effects on climate,environment and life.Based on the extensive study of the sedimentary record on both sides of the Yarlung-Zangbo suture zone in Tibet,we review here the present state of knowledge on the timing of collision onset,discuss its possible diachroneity along strike,and reconstruct the early structural and topographic evolution of the Himalayan collided range.We define continent-continent collision as the moment when the oceanic crust is completely consumed at one point where the two continental margins come into contact.We use two methods to constrain the timing of collision onset:(1) dating the provenance change from Indian to Asian recorded by deep-water turbidites near the suture zone,and(2) dating the age of unconformities on both sides of the suture zone.The first method allowed us to constrain precisely collision onset as middle Palaeocene(59±l Ma).Marine sedimentation persisted in the collisional zone for another 20-25 Ma locally in southern Tibet,and molassic-type deposition in the Indian foreland basin did not begin until another 10-15 Ma later.Available sedimentary evidence failed to firmly document any significant diachroneity of collision onset from the central Himalaya to the western Himalaya and Pakistan so far.Based on the Cenozoic stratigraphic record of the Tibetan Himalaya,four distinct stages can be identified in the early evolution of the Himalayan orogen:(1) middle Palaeocene-early Eocene earliest Eohimalayan stage(from 59 to 52 Ma):collision onset and filling of the deep-water trough along the suture zone while carbonate platform sedimentation persisted on the inner Indian margin;(2) early-middle Eocene early Eohimalayan stage(from 52 to 41 or 35 Ma):filling of intervening seaways and cessation of marine sedimentation;(3) late Eocene-Oligocene late Eohimalayan stage(from 41 to 25 Ma):huge gap in the sedimentary record both in the collision zone and in the Indian foreland;and(4) late Oligocene-early Miocene early Neohimalayan stage(from 26 to 17 Ma):rapid Himalayan growth and onset of molasse-type sedimentation in the Indian foreland basin.

The influence of Cretaceous paleolatitude variation of the Tethyan Himalaya on the India-Asia collision pattern

Identifying when, where, and how India and Asia collided is a prerequisite to better understand the evolution of the Himalayan-Tibetan Plateau. Whereas with essentially the same published paleomagnetic data, a large range of different India-Asia collision models have been proposed in the literature. Based upon the premise of a northwards-moving Indian plate during the Cretaceous times, we analyze the significant variations in relative paleolatitude produced by a nearly 90° counterclockwise(CCW)rotation of the plate itself during the Cretaceous. Interestingly, recent studies proposed a dual-collision process with a Greater India basin or post-Neo-Tethyan ocean for the India-Asia collision, mainly in the light of divergent Cretaceous paleolatitude differences of the Tethyan Himalaya between the observed values and expected ones computed from the apparent polar wander path of the Indian plate. However, we find that these varied paleolatitude differences are mainly resulted from a nearly 90° CCW rotation of a rigid/quasi-rigid Greater Indian plate during the Cretaceous. On the other hand, when the Indian craton and Tethyan Himalaya moved as two individual blocks rather than a united rigid/quasi-rigid Greater Indian plate before the India-Asia collision, current available Cretaceous paleomagnetic data permit only multiple paleogeographic solutions for the tectonic relationship between the Indian plate and the Tethyan Himalayan terrane. We therefore argue that the tectonic relationship between the Indian plate and the Tethyan Himalayan terrane cannot be uniquely constrained by current paleomagnetic data in the absence of sufficient geological evidence, and the so-called Greater India basin model is just one of the ideal scenarios.

Processes of initial collision and suturing between India and Asia

The initial collision between Indian and Asian continents marked the starting point for transformation of land-sea thermal contrast,uplift of the Tibet-Himalaya orogen,and climate change in Asia.In this paper,we review the published literatures from the past 30 years in order to draw consensus on the processes of initial collision and suturing that took place between the Indian and Asian plates.Following a comparison of the different methods that have been used to constrain the initial timing of collision,we propose that the tectono-sedimentary response in the peripheral foreland basin provides the most sensitive index of this event,and that paleomagnetism presents independent evidence as an alternative,reliable,and quantitative research method.In contrast to previous studies that have suggested collision between India and Asia started in Pakistan between ca.55 Ma and50 Ma and progressively closed eastwards,more recent researches have indicated that this major event first occurred in the center of the Yarlung Tsangpo suture zone(YTSZ) between ca.65 Ma and 63 Ma and then spreading both eastwards and westwards.While continental collision is a complicated process,including the processes of deformation,sedimentation,metamorphism,and magmatism,different researchers have tended to define the nature of this event based on their own understanding,an intuitive bias that has meant that its initial timing has remained controversial for decades.Here,we recommend the use of reconstructions of each geological event within the orogenic evolution sequence as this will allow interpretation of collision timing on the basis of multidisciplinary methods.