Did the Pamir slab form by intracontinental subduction or lithospheric delamination? Clues from the deformation and thermal history

长期以来，帕米尔阿莱一直被认为是活跃的大陆内俯冲带的最佳代表。这样的带是最不为人所知的板块边界类型，因为现代的例子很少，而古代的类似物已经被后来的构造和侵蚀过程广泛覆盖。在过去的十年中，从仍然鲜为人知的帕米尔造山带出现了大量新的观测结果。提出了两种已发表的模型来解释帕米尔高原北部下具有突出中深度地震的南倾地震带。主要观点认为，下行板块代表了陆内俯冲作用，在北帕米尔和天山之间形成了约300公里的新生代辐合带。新生代早期，北帕米尔与西昆仑山毗连；现在由喀什-叶城运输系统（KYTS）抵消。该模型预测，在北帕米尔高原和西昆仑山的显著缩短和挖掘同时开始。此外，KYTS将在同一时间或更晚的时间启动-沿着其长度同步。如果在板块中出现比正常地壳密度更大的地壳，则有利于大陆内俯冲。我们建议研究新生代在帕米尔高原下俯冲的中生代大洋弧后盆地的证据。基于广泛的地球物理数据库的新工作表明，下地壳和岩石圈地幔剥离可以解释帕米尔板块。在这个模型中，帕米尔岩石圈的下部由地幔和下地壳组成，作为一个连续的板块从上地壳中剥离出来。通过剥离、变陡和向北回滚，下沉的下岩石圈形成了一个类似于俯冲产生的板块。这种强迫剥离是由印度克拉通向北楔入亚洲地壳所驱动的。由于该模型不考虑现在的北帕米尔和天山之间的实质性缩短，因此在印度-亚洲碰撞之前，北帕米尔和西昆仑山似乎不太可能形成一个连续的带。因此，KYTS应该是从南方开始向北发展的，最近到达了北帕米尔高原。如果亚洲大陆地壳厚且浮力大，那么分层模型最有效。如果这个模型是正确的，它将对地球动力学概念产生深远的影响，因为帕米尔被用作解释古代造山带大陆内俯冲的类比。这两种模型对变形的时空模式以及由此产生的挖掘和形成板块的地壳性质做出了不同的预测。我们的目标是系统地进行地质观测，并从垂直剖面收集样品进行热年代学分析，以限制北帕米尔高原、西昆仑山和KYTS的变形开始和大小，从而确定更合理的模型。

The Pamir-Alai has long been considered to represent the best example of an active intracontinental subduction zone. Such zones are the least understood type of plate boundaries because modern examples are few, and ancient analogs have been extensively overprinted by subsequent tectonic and erosion processes. During the past decade, a significant body of new observations have emerged from the still poorly-known Pamir orogen. Two published models have been proposed to explain the south-dipping seismic zone with prominent intermediate-depth earthquakes beneath the northern Pamir. The dominant view is that the downgoing slab represents intracontinental subduction which has accommodated ca. 300 km of Cenozoic convergence between the North Pamir and the Tien Shan. In the early Cenozoic, the North Pamir and the Western Kunlun Shan were contiguous; they are now offset by the Kashgar-Yecheng Transfer System (KYTS). This model predicts that significant shortening and hence exhumation in the North Pamir and the Western Kunlun Shan commenced synchronously. Furthermore, the KYTS would have initiated either at the same time or later - synchronously along its length. Intracontinental subduction is facilitated if denser than normal crust occurs in the slab. We propose to investigate evidence for a Mesozoic oceanic backarc basin subducted beneath the Pamir in Cenozoic time. New work based on an extensive geophysical database suggests that lower crustal and lithospheric mantle delamination could explain the Pamir slab. In this model, the lower part of the Pamir lithosphere, consisting of the mantle and lower crust, delaminates as a continuous sheet from the upper crust. By peeling off, steepening and rolling back northwards, the sinking lower lithosphere forms a slab resembling that produced by subduction. This forced delamination is driven by northward wedging of the Indian craton into Asian crust. Since this model does not allow for substantial shortening between the present North Pamir and the Tien Shan, it seems unlikely that the North Pamir and the Western Kunlun Shan formed a continuous belt prior to the India-Asia collision. Therefore, the KYTS should have started in the south and grown northward, reaching the North Pamir recently. The delamination model works best if Asia´s continental crust was thick and buoyant. If this model is correct, it has profound implications for geodynamical concepts, because the Pamir is used as an analogue to explain intracontinental subduction in ancient orogens. The two models make different predictions about the spatial and temporal pattern of deformation and hence exhumation and about the nature of the crust forming the slab. Our goal is to systematically make geological observations and collect samples from vertical profiles for thermochronologic analysis in order to constrain the onset and magnitude of deformation in the North Pamir, the Western Kunlun Shan, and the KYTS and thereby identify the more plausible model.