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

帕米尔-阿莱一直被认为是大陆内活跃俯冲带的最佳代表。这种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随后的构造和侵蚀过程广泛覆盖。在过去的十年里，在仍然鲜为人知的帕米尔造山带出现了大量新的观测结果。已经提出了两个已发表的模型来解释帕米尔北部下方存在显著中深地震的南倾地震带。主流观点认为，下沉的板块代表陆内俯冲，在帕米尔北部和天山之间容纳了大约300公里的新生代会聚。早在新生代，帕米尔北部和西昆仑山是毗连的，现在被喀什-叶城转换系统(KYTS)抵消。该模型预测，帕米尔北部和西昆仑山的显著缩短和折返是同步开始的。此外，KYTS将在同一时间或更晚启动--沿着其长度同步。如果板块中存在密度高于正常地壳的地壳，则有利于陆内俯冲。我们建议调查一个中生代大洋弧后盆地在新生代时俯冲到帕米尔之下的证据。基于一个广泛的地球物理数据库的新工作表明，较低的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，由地幔和下地壳组成，从上地壳剥离为一个连续的片状。通过剥离、陡化和向北卷曲，下沉的下部岩石圈形成了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致。由于这种模式不允许现今帕米尔北部和天山之间的大幅缩短，因此在印度-亚洲碰撞之前，帕米尔北部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，最近到达帕米尔群岛北部。如果亚洲的S大陆地壳较厚且浮力较大，则分层模型的效果最好。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，因为帕米尔高原被用作解释古代造山带中的陆内俯冲的类比。这两个模型对变形和折返的时空模式以及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，以限制帕米尔北部、西昆仑山和KYTS的变形开始和规模，从而确定更可信的模式。

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.