Cenozoic evolution of the Pamir plateau recorded in surrounding basins, implications on Asian climate and land-sea distribution
Abstract
The Cenozoic Pamir orogen formed in response to the India-Asia collision. Existing datasets shows that the range
grew since ca. 25 Ma, however the early Cenozoic history remains particularly enigmatic. In that peculiar period,
global climate changed from greenhouse to icehouse, the proto-Paratethys sea retreated out of Asia and continental
aridification as well as monsoons established over Asia. These environmental changes are held responsible for
major floral and faunal crises including the emergence of plant communities and the dispersion of key mammal
groups from Asia onto other continents. However, the causal relationships between these events remains to be
established because of the lack of accurate age constraints on their geological records. Here, we provide well-dated
stratigraphic records using magneto- and bio-stratigraphy from the basins surrounding the Pamir. Southeast of
the Pamir, along the Kunlun Shan into the southwestern Tarim Basin, Eocene marine deposits are continuously
overlain by 41 to 15 Ma continental redbeds themselves overlain by conglomerates in a classic foreland sequence
with upward increasing grain-size, accumulation rates and provenance proximity. However, North of the Pamir
along the southwestern Tian Shan andWest of the Pamir into the Afghan-Tadjik Basin, the entire Oligocene period
appears to be missing from the record between the last marine and the first continental sediments dated to the
Early Miocene. This supports a simple basin evolution model in response to initial Pamir indentation with Eocene
foreland basin activation in the Southeast related to the Kunlun Shan northward thrusting, followed much later
by early Miocene activation of the northern foreland basin related to the southwestern Tian Shan overthrusting.
The coeval activation of a lithospheric right-lateral strike-slip system along the Pamir/Tarim boundary may
have enabled to transfer deformation from the India-Asia collision zone to the Tian Shan and possibly the Talas
Fergana fault. This simple model suggests the following two-stage paleoenvironmental evolution: (1) Late Eocene
sea retreat linked to the onset of Pamir indentation in conjunction with global sea-level drop, decreasing CO2
levels and ice-cap formation and (2) Early Miocene closure of the Tarim Basin by northward indentation of the
Pamir plateau. This two stage evolution is consistent with the Eocene occurrence of continental aridity and Asian
Monsoons and their Early Miocene intensification.