Did the Pamir slab form by intracontinental subduction or lithospheric delamination? Clues from the deformation and thermal history
Final Report Abstract
Our project aimed to provide evidence for or against the intracontinental subduction or delamination tectonic models proposed for the Pamir, based on the timing and magnitude of deformation. One main goal was to constrain the Cenozoic exhumation of the North Pamir using low-temperature thermochronology. This has proved surprisingly difficult for several reasons. There are only a few regions where one can access the hangingwall of the MPT in the North Pamir without expedition methods. We managed to sample most of these localities. Unfortunately, these areas are dominated by either mafic igneous units (which do not contain useful abundances of either apatite or zircon), plagiogranites (which have proved to contain no apatite and usually yield partially-reset ZHe ages), or units which are too fine-grained to yield datable material. The limited number of our own and published thermochronologic ages presently suggests broadly synchronous timing of deformation in the Pamir and West Kunlun, starting around the Late Oligocene-Early Miocene. Our structural work has resulted in new balanced cross-sections and updated shortening estimates for the Main Pamir thrust (MPT) and the External Pamir (EP) thrust belt in its footwall. Despite being higher than previous estimates, our minimum shortening of 30-75 km still falls substantially short of the ca. 300 km magnitude required to form the slab and explain the Pamir´s location in a subduction model. The EP is a thinskinned fold-and-thrust belts with several prominent décollement levels in Lower Cretaceous, Upper Cretaceous and Paleogene rocks. Restoration of the thrust stack yields a low-angle basal décollement whose projection intersects the base of the crust well south of the Pamir slab, precluding a direct connection of the slab´s updip end with the MPT. An unexpected outcome of our work is new evidence for a pre-Cenozoic arc shape of the Pamir. Our age dating and geochemical studies of plagiogranites from the north Pamir demonstrated that they belong to one suite of Carboniferous plutons associated with a subduction zone that is linked to the Oytag suture. As they have no equivalent in the Kudi suture of the West Kunlun, the allegedly once contiguous Oytag-Kudi suture is eliminated as a marker for northward displacement of the Pamir. A Carboniferous subduction zone west of the Tarim block supports our original working hypothesis that the lithosphere forming the Pamir slab is different from Tarim. In addition, we found that pillow-bearing Carboniferous rocks of oceanic affinity occur not only in the MPT´s hanging-wall but locally also in its footwall, attached to the Alai-Tien Shan foreland by overlying Permo-Mesozoic strata. This observation puts an additional limit on Cenozoic displacement on the MPT-EP system, consistent with our results from cross-section balancing. The Carboniferous suture or basin remnant probably already determined the site of a regional Mesozoic thrust fault that was in part reactivated by the Cenozoic MPT. Taken together, these observations indicate that the MPT and EP originated around 25 Ma, much less than 300 km south of their present position and already offset to the north from the West Kunlun by more than 100 km. The MPT´s location was preconditioned by a strip of Carboniferous ocean crust and at least in part by a Mesozoic (Cimmerian) thrust fault. The eastern border of the Pamir follows the western border of thick, rigid Tarim lithosphere. The formation of the Pamir slab was probably initiated by several tens of km of shortening on a trans-lithospheric MPT and its associated footwall thrust belt. This geometry corresponds to intracontinental subduction. The magnitude of underthrusting was sufficient to induce eclogite formation in mafic lower crust that is now part of the slab. The denser slab could begin to sink and roll back, its increasing length now independent of shortening in the Pamir. We thus envisage a first phase of intracontinental subduction triggering and followed by a second phase of delamination. More broadly, this implies that ancient examples of intracontinental subduction could also represent quite limited magnitudes of shortening.
Publications
- 2021, The Carboniferous Arc of the North Pamir: Lithosphere
Rembe, J., Sobel, E.R., Kley, J., Zhou, R., Thiede, R., Chen, J.
(See online at https://doi.org/10.2113/2021/6697858)
