Roderic Bosboom

Oligocene clockwise rotations along the eastern Pamir: Tectonic and paleogeographic implications

Despite the importance of the Pamir range in controlling Asian paleoenvironments and land-sea paleogeography, its tectonic evolution remains poorly constrained in time and space, hindering its potential for understanding deep to surface processes. We provide here new constraints on vertical-axis tectonic rotations from the southwest Tarim Basin along the eastern flank of the Pamir arcuate range based on paleomagnetic results. Two well-dated Eocene to Oligocene sections, previously analyzed using biostratigraphy and magnetostratigraphy, yield consistently clockwise rotations of 21.6 ± 4.2° in 41 to 36 Ma strata then 17.1 ± 6.5° in 33 to 28 Ma strata at the Aertashi section and 14.2 ± 11.5° in 41 to 40 Ma strata at the Kezi section. Combined with a regional review of existing paleomagnetic studies, these results indicate that most of the clockwise rotations along the eastern Pamir occurred during Oligocene times and did not extend systematically and regionally into the Tarim Basin. In contrast, on the western flank of the Pamir tectonic rotations in Cretaceous to Neogene strata are regionally extensive and systematically counterclockwise throughout the Afghan-Tajik Basin. This timing and pattern of rotations is consistent with paleogeographic reconstructions of the regional sea retreat out of Central Asia and supports a two-stage kinematic model: (1) symmetric rotations of either flanks of the Pamir arcuate range until Oligocene times followed by (2) continued rotations on its western flank associated with radial thrusting and, along the eastern flank, no further rotations due to decoupled transfer slip starting in the Early Miocene.

Late Eocene palaeogeography of the proto-Paratethys Sea in Central Asia (NW China, southern Kyrgyzstan and SW Tajikistan)

Abstract The Cretaceous and Palaeogene sediments of the basins in Central Asia include the remnants of the easternmost extent of a vast shallow epicontinental sea, which extended across the Eurasian continent before it retreated westwards and eventually isolated as the Paratethys Sea. To improve understanding of its long-term palaeogeographical evolution, we complement the well-constrained chronological framework of the Tarim Basin in China with stratigraphic records of the sea retreat from the Fergana Basin and the Alai Valley Basin in southern Kyrgyzstan and the Afghan–Tajik Basin in SW Tajikistan. By lithostratigraphic analyses and identification of bivalve assemblages, this study establishes for the first time a clear and detailed regional correlation of Palaeogene marine strata across Central Asia, showing that the basins share a similar palaeogeographical evolution characterized by a long-term stepwise retreat punctuated by short-term shallow-marine incursions. Our correlation shows that the last two marine incursions recognized in the Tarim Basin can be traced westwards. The permanent disappearance of the sea from Central Asia probably occurred with limited diachroneity in the late Eocene, before the isolation of the Paratethys Sea, shifting the easternmost margin of the sea hundreds of kilometres westwards and probably significantly reducing moisture supply to the Asian interior.