Abstract
The Cretaceous and Paleogene sediments of the Central Asian basins include remnants of the easternmost extent of a large epicontinental sea. Before this sea retreated westward and eventually separated as the Paratethys Sea, it extended across Eurasia from the Mediterranean Tethys to the Tarim Basin in western China. Climate models
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have shown that by diminished moisture supply, the disappearance of the sea from Central Asia may have caused continental aridification in Asia. This thesis provides the required chronological framework of both the aridification and the sea retreat to test this causal link during Eocene times. A reference for the Asian aridification is provided by the well-dated Eocene to Oligocene Xining Basin record along the north-western margin of the Tibetan Plateau. Here the record has been extended downwards into middle Eocene lacustrine mudstone-gypsum successions and accurately constrained in age by magnetostratigraphy. Sedimentologic and pollen analyses on the precisely dated record reveal several aridification steps of which the most prominent occurred concomitant with rapid cooling directly following the Middle Eocene Climatic Optimum (MECO) at ~40.0 Ma (base C18n.2n). The sea retreat has been studied in the Tarim Basin (western China), the Ferghana Basin (southern Kyrgyzstan) and the Afghan-Tajik Basin (south-western Tajikistan). The sea supposedly entered Central Asia in the Cretaceous and withdrew after five third-order marine incursions, of which the last two have been documented here. Integrated bio-magnetostratigraphic dating in the Tarim Basin shows the sea retreated stepwise from the southwest depression in the latest Lutetian at ~41 Ma (base C18r) and from the westernmost margin in the latest Bartonian-early Priabonian (near top C17n.2n-top C16n.2n). The shallow marine near-shore sediments of these last two incursions can be convincingly correlated by litho- and biostratigraphy across Central Asia, showing for the first time that the sea had probably largely retreated from Central Asia in the late Eocene. A major disconformity across the EOT in southwest Tarim confirms that Central Asia remained hydrologically connected to the Mediterranean Tethys in the Eocene and had not yet been isolated by tectonic uplift of the surrounding Pamir Mountains. This paleogeographic evolution is supported by paleomagnetic data from the southwest Tarim Basin showing that clockwise rotation took place after the fourth marine incursion, but was negligible after the Oligocene. Comparison to regional paleomagnetic data shows this is consistent with initial symmetric radial thrusting of the Pamir in response to the Eocene Indo-Asia collision, followed in the Miocene by continued anticlockwise rotation on its western margin and ceased clockwise rotations on its eastern margin subjected to the onset of a lithospheric strike-slip system. The stepwise sea retreat and disconformity are concurrent with documented aridification steps in the Xining Basin at ~41 Ma (C19n-C18r), ~37.1 Ma (top C17.1n) and the EOT at ~33.9 Ma (top C13r), suggesting that the sea retreat and aridification in Asia were indirectly paced by global climate deterioration in the Eocene through eustatic level changes affecting the Proto-Paratethys Sea. In line with climate modelling results, the sea retreat may thus have amplified the aridification of Asia’s interior.
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