印度-欧亚板块主碰撞带全地壳尺度相互作用关系研究

Crustal-scale plate interactions beneath the dominant domain in the India-Eurasia collision zone——a tectonogeophysical study

印度板块自新生代早期开始沿雅鲁藏布江缝合带向北与欧亚板块碰撞以来,印度板块的深俯冲过程一直是大家关注的焦点,而垂向上主碰撞带全地壳尺度由深及浅的相互作用关系研究程度相对较弱,主要归咎于之前缺少全地壳尺度高分辨率数据资料,从而也阻碍了对主碰撞带巨厚地壳成因机制及深部地球动力学过程的了解。在本次研究中,分别基于横穿主碰撞带中部和东部的180 km及100 km长的深反射地震剖面进行精细构造地球物理学分析,揭示了主碰撞带全地壳尺度由深及浅的相互作用关系:(1)横向上,印度板块下地壳存在北向俯冲,且俯冲前缘有限的存在于南拉萨地体南缘。上覆的南拉萨地体则出现透明反射结构和中拉萨地体统一北倾的反射结构。(2)垂向上,印度地壳主要表现下地壳俯冲、中上地壳双冲构造回返结构。南拉萨地体四分之三的地壳表现为透明反射。占据另外四分之一的上地壳顶部表现为统一的南倾结构形态;中拉萨地体则以下地壳北倾、上地壳上拱反射结构为主。三者皆在垂向上出现差异性变形。(3)主碰撞带上地壳顶部表现为统一的后展式顶板逆冲推覆构造,该逆冲推覆系统可一直从南拉萨地体北边界的洛巴堆—米拉山断裂向南越过南倾的雅江缝合带延伸至北喜马拉雅穹窿背斜北翼。结合大地电磁数据所揭示的南拉萨地体高熔体百分比区域沿俯冲印度下地壳顶边界发生的南移现象,研究结果揭示南拉萨地体巨厚地壳主要由新特提斯构造域幕式岩浆作用所形成的新生地壳物质易挤压变形引起。同时,南拉萨地体幕式岩浆作用在印度与欧亚板块的相互碰撞作用过程中发生了热量的向南运移。该过程引发北喜马拉雅构造带深熔作用的同时减弱了北喜马拉雅构造带地壳机械强度,从而使中上地壳物质的双冲构造回返主要表现为短波长背型堆垛结构,并垂向增厚了俯冲印度地壳厚度。同时,背型堆垛构造形变过程所导致的北喜马拉雅穹窿带的加速出露给主碰撞带区域上地壳顶部带来北向的构造挤压推覆作用,最终使主碰撞带区域在北喜马拉雅穹窿区域以北展现为统一的后展式顶板逆冲推覆结构构造。印度-欧亚板块主碰撞带的圈层相互关系是造成该区域巨厚地壳的关键,其上地壳顶板逆冲推覆过程亦降低了主碰撞区域的地形起伏。

Since the Early-Cenozoic onset of the Indian plate subduction beneath the Eurasian plate along the Yarlung-Zangbo suture zone(YZSZ), the subduction process has gained wide interests among geologists. However, crustal-scale vertical interactions between the two plates beneath the dominant domain in the collision zone remains unclear owing partly to the lack of high-resolution datasets, which has severely limited the understanding of the crustal thickening mechanism of the dominant collision domain and its deep geodynamic processes. In this study, fine structural interpretation of two deep seismic reflection profiles—180 km and 100 km long cutting through the middle and eastern part of the YZSZ, respectively—revealed the crustal-scale lateral and vertical contact relationships between the subducting Indian plate and the overriding Lhasa terrane.(1) Laterally, the Indian lower crust subducts northward, with limited subduction front beneath the southern margin of the Lhasa terrane(SLT) which is shown as non-reflective crust, while the Central Lhasa terrane(CLT) is north-dipping.(2) Vertically, the Indian lower crust undergoes subduction, while crustal duplexing occurs in the middle-upper crust. Nearly three quarters of the SLT crust are non-reflective crust, while the rest, the SLT upper crust, is south-dipping. The CLT crust can be divided into two domains: north-dipping lower crust and concave-downward upper crust. Differential vertical zonation is observed in all three tectonic units.(3) The upper crust of the dominant collision domain has a consistent deformation pattern, where a sequence of break-backward imbricate structures is present. This break-backward imbricate system can be traced from the Luobadui-Mila fault of the northern edge of SLT, beyond YZSZ, to the northern edge of the North Himalaya dome belt. Combing with the previous findings based on coincident magnetotelluric data on the southward migration of high conductive barrier of SLT thrusting along the main Himalayan into the northern Himalayas, we believe the episodic magmatism in the Tethyan Domain beneath SLT generated juvenile crust that is prone to anomalous thickening. Meanwhile, during India-Eurasia plate interaction, mantle-sourced magmatism in SLT—generated from northward subduction of the Neo-Tethyan oceanic slab and subsequent collision between the India and Eurasia plates—caused southward thermal migration, which induced anatexis in the northern Himalayas and weakened the crustal strength of the region. The ongoing crustal-scale duplexing therefore leads to antiformal stacking and causes crustal thickening. Rapid exhumation of the North Himalayan dome by the increasing antiformal stacking, meanwhile, exerts sudden northward compression to the overlying Tethyan Himalayan sequence, which eventually creates fault-propagation folds following a break-backward sequence in the upper crust through the whole dominant collision domain. Overall, vertical and lateral tectonic interactions within the dominant collision domain in the India-Eurasia collision zone played an important role in producing such anomalous thick crust, but the break-backward imbricates system in the upper crust lowered topographic relief in the dominant collision domain as well.