Collaborative Research: Deep Structure of the Northeast Tibetan Collision Zone- INDEPTH IV

该项目(INDEPTH IV)是先前资助的项目(INDEPTH I、II、III)的延续。INDEPTH研究的首要目标是了解大陆-大陆碰撞中发生的基本过程。深入一、二和三是中美联合研究，以了解青藏高原和喜马拉雅山脉的形成，这两个山脉共同构成了地球上活跃的大陆-大陆碰撞带的典型例子。这些研究的结果深刻地影响了在这样的构造背景下造山的流行理论。INDEPTH IV的震中位于青藏高原的东北边界。这一界线在当代一场关于大陆高原形成为碰撞造山作用的陆内反应的辩论中发挥着核心作用。该地区最近的地质研究和有限的地球物理测量表明，a)隆起是由于亚洲地壳内部的叠瓦状“堆积”，而b)亚洲大陆岩石圈正在分离并“俯冲”到西藏地幔。这些模型与主要来自高原南部和东部观察的竞争范式形成对比，后者将高原的形成归因于地壳深部的韧性流动。对于任何地表构造模型来说，高原和邻近地区之下的地幔动力学是至关重要的。北部高原下的上地幔地球物理性质被不同程度地归因于加厚岩石圈的不稳定、地幔拆离和/或与印度大陆岩石圈俯冲有关的软流圈逆流所引发的拆沉。深度四号将通过一个综合的、有重点的地球物理调查计划来测试这些假设中的关键要素，该计划涉及东北高原边界带的三个关键部分。地球物理调查将包括综合地震反射、折射和密集分布的无源地震剖面、详细的大地电磁测量和区域宽带地震台阵。调查将研究的具体特征包括：a)建议的莫霍面偏移量的存在和性质；b)高原边界主要逆冲断层的深部几何形状；c)主要逆冲和走滑断层之间的关系；d)地壳标志的结构连续性对下地壳流动的限制；e)亚洲岩石圈俯冲的存在和性质；f)高原前陆之下的岩石圈增厚模式。

This project (INDEPTH IV) is a continuation of previously funded projects (INDEPTH I, II, III). The overarching goal of the INDEPTH studies is to understand the basic processes that take place in a continent-continent collision. INDEPTH I, II, and III were joint Sino-U.S. studies to understand the formation of the Tibetan Plateau and Himalayan mountains, which together comprise Earth's type example of an active continent-continent collision zone. Results from these studies have profoundly influenced prevalent theories of mountain building in such tectonic settings. The focus of INDEPTH IV is the northeastern boundary of the Tibetan Plateau. This boundary plays a central role in a contemporary debate concerning continental plateau formation as an intracontinental response to collisional orogeny. Recent geological studies and limited geophysical measurements in this region have been cited to argue that a) uplift is due to internal imbricate "stacking" of Asian crust while b) Asian continental lithosphere is being detached and "subducted" into the Tibetan mantle. Such models contrast with a competing paradigm, derived primarily from observations in the southern and eastern portions of the Plateau, that attribute plateau formation to ductile flow in the deep crust. Critical to any model of surface tectonics is the mantle dynamics beneath the plateau and adjacent regions. Upper mantle geophysical properties beneath the northern plateau have been variously attributed to delamination triggered by instability of a thickened lithosphere, mantle lid detachment and/or asthenospheric counterflow associated with subduction of Indian continental lithosphere. INDEPTH IV will test key elements in these hypotheses with an integrated, focused program of geophysical surveys across three key segments of the NE Plateau boundary zone. Geophysical surveys will consist of integrated seismic reflection, refraction and densely spaced passive seismic profiles, a detailed magnetotelluric survey, and a regional broadband seismic array. Specific features to be investigated by the surveys include: a) the existence and nature of proposed Moho offsets; b) the deep geometry of major thrust faults at the plateau boundary; c) the relationship between major thrusts and strike slip faults; d) constraints on lower crustal flow from structural continuity of crustal markers; e) the existence and nature of subducting Asian lithosphere; and f) the mode of lithospheric thickening beneath the plateau foreland.