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

帕米尔-阿莱一直被认为是活跃的陆内俯冲带的最佳代表。这些区域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随后的构造和侵蚀过程广泛覆盖。在过去的十年中，一个重要的机构的新的观察出现了从仍然知之甚少帕米尔造山带。两个已发表的模型已被提出来解释南倾地震带与突出的中深度地震下的北方帕米尔。主流观点认为，下行板片代表了陆内俯冲，它容纳了约10000个板块。300公里的新生代收敛北帕米尔和天山之间。新生代早期，北帕米尔高原和西昆仑山相连;现在，它们被喀什-叶城输送系统（KYTS）所抵消。该模型预测，北帕米尔和西昆仑山的显著缩短和折返是同步开始的。此外，KYTS将同时或稍后启动-沿着其长度同步。如果板片中的地壳密度大于正常地壳，则有利于陆内俯冲。我们建议调查的证据，中生代大洋弧后盆地俯冲下帕米尔在新生代的时间。 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拆沉可以解释帕米尔板块。在这个模型中，帕米尔岩石圈的下部，包括地幔和下地壳，作为一个连续的片从上地壳拆离。通过剥离、变陡和向北回滚，下沉的岩石圈下部形成了类似于俯冲作用产生的板块。印度克拉通向北楔入亚洲地壳，导致了这种强制拆沉。由于该模型不考虑现今北帕米尔和天山之间的显著缩短，北帕米尔和西昆仑山在印度-亚洲碰撞之前似乎不可能形成一个连续的带。因此，青藏高原应该是从南部开始向北发展的，最近到达了北帕米尔。如果亚洲的大陆地壳很厚而且有浮力，那么分层模型的效果最好。如果这一模型是正确的，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造山带中陆内俯冲作用的类比。 这两种模式对变形的时空模式以及由此产生的折返作用和形成板块的地壳的性质作出了不同的预测。我们的目标是系统地进行地质观测和收集样品的热年代学分析，以限制在北帕米尔，西昆仑山，和KYTS的变形的开始和规模，从而确定更合理的模式。

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.