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

International audienceDespite 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 andmagnetostratigraphy, yield consistently clockwise rotations of 21.6±4.2° in 41 to 36Ma strata then 17.1±6.5° in 33 to 28Ma strata at the Aertashi section and 14.2 ± 11.5° in 41 to 40Ma 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 TarimBasin. 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

Timing, cause and impact of the late Eocene stepwise sea retreat from the Tarim Basin (west China)

International audienceA vast shallow epicontinental sea extended across Eurasia and was well-connected to the Western Tethys before it retreated westward and became isolated as the Paratethys Sea. However, the palaeogeography and the timing of this westward retreat are too poorly constrained to determine potential wider environmental impacts, let alone understanding underlying mechanisms of the retreat such as global eustasy and tectonism associated with the Indo-Asia collision. Here, an improved chronostratigraphic and palaeogeographic framework is provided for the onset of the proto-Paratethys Sea retreat at its easternmost extent in the Tarim Basin in western China is provided. Five different third-order sea-level cycles can be recognised from the Cretaceous-Palaeogene sedimentary record in the Tarim Basin, of which the last two stepped successively westwards as the sea retreated after the maximum third incursion. New biostratigraphic data from the fourth and fifth incursions at the westernmost margin of the Tarim Basin are compared to our recent integrated bio-magneto-stratigraphic results on the fourth incursion near the palaeodepocentre in the south-western part of the basin. While the fourth incursion extended throughout the basin and retreated at ~ 41 Ma (base C18r), the last and fifth incursion is restricted to the westernmost margin and its marine deposits are assigned a latest Bartonian-early Priabonian age from ~ 38.0 to ~ 36.7 Ma (near top C17n.2n to base C16n.2n). Similar to the fourth, the fossil assemblages of the fifth incursion are indicative of shallow marine, near-shore conditions and their widespread distribution across Eurasia suggests that the marine connection to the Western Tethys was maintained. The lack of diachronicity of the fourth incursion between the studied sections across the southwest Tarim Basin suggests that the sea entered and withdrew relatively rapidly, as can be expected in the case of eustatic control on a shallow epicontinental basin. However, the westward palaeogeographic step between the fourth and fifth incursions separated by several millions of years rather suggests the combined long-term effect of tectonism, possibly associated with early uplift of the Pamir-Kunlun Shan thrust belt. The fourth and fifth regressions are time-equivalent with significant aridification steps recorded in the Asian interior, thus supporting climate modelling results showing that the stepwise sea retreat from Central Asia amplified the aridification of the Asian interior

Late Eocene sea retreat from the Tarim Basin (west China) and concomitant Asian paleoenvironmental change

International audienceThe Paleogene sediments of the southwest Tarim Basin along the West Kunlun Shan in western China include the remnants of the easternmost extent of a large epicontinental sea. This shallow sea once extended across the Eurasian continent before it retreated westward and eventually separated as the Paratethys Sea. Climate modeling results suggest that this sea retreat is an equally important forcing mechanism as the Tibetan plateau uplift in the aridification of the Asian continental interior and the intensification of the Asian monsoon system. The age and paleogeography of the retreat are poorly constrained, hindering the understanding of its cause and paleoenvironmental impacts. This study reports litho- and biostratigraphic results from two sections recording the last major regression out of the Tarim Basin that is expressed by a regional transition from marine clastics and limestones to continental red-beds. Rich micro- and macrofossil assemblages, including benthic foraminifera, ostracods, bivalves, calcareous nannofossils and organic walled dinoflagellate cysts (dinocysts), indicate a shallow, proximal and marine environment. Strong similarity to assemblages known from Central Asia and Europe confirms that surface­ocean connections extended across Eurasia from the Tarim Basin to the western Tethys during the latest Eocene. Moreover, the recovered fossil associations date the last marine sediments as earliest Priabonian in age (~37 Ma; overlap between dinoflagellate Mps Interval Zone and calcareous nannofossil Zone CP 14). The retreat of the sea from the Tarim Basin is timeequivalent with the sea level lowstand at the Bartonian­Priabonian boundary but pre-dates both the Oligocene­Miocene regional uplift of the Pamir mountains and Kunlun Shan and the major eustatic sea-level falls of the Eocene­Oligocene Transition (~34 Ma) and mid-Oligocene (~30 Ma), which are usually held responsible for the sea retreat. Furthermore, a concomitant and significant aridification step occurs at ~36.6 Ma (top of chron C17n.1n) as recorded by regional sedimentary records of the Xining Basin along the northeastern Tibetan Plateau, suggesting that the Tarim Sea served as a significant moisture contributor for the Asian interior

Step-wise change of Asian interior climate preceding the Eocene-Oligocene Transition (EOT)

International audienceUnderstanding the global climate change from greenhouse to icehouse conditions at the Eocene­Oligocene Transition (EOT) 34 million years ago requires climatic records from oceanic as well as continental realms of the key Late Eocene "doubthouse" period preceding this switch. Here, we report integrated stratigraphic results from well-dated Late Eocene continental mudflat to saline lake paleoenvironments of the Xining Basin (northeastern Tibetan Plateau, western China) recording regional and global change. Cyclostratigraphic analysis strongly suggests continuous dominance of the 41-kyr obliquity cycle in the whole late Eocene interval down to the base of polarity chron C18n.2n at 39 Ma with additional input of the ~100-kyr eccentricity cycle up to the base of chron C13r at ~34.7 Ma. This might imply that high-latitude climates dominated the area long before the EOT, probably related to incipient ice-volume fluctuations. Furthermore, our results reveal two paleoenvironmental deterioration steps preceding the Eocene­Oligocene Transition. The first step occurs in the top of chron C17n.1n at ~36.6 Ma. This age closely corresponds to (1) the high altitude pollen appearance in chron C16.2r at ~36.4 Ma in the same section, (2) the recently dated final retreat of the Tarim Sea in western China, and (3) a shift from precession to obliquity dominance in the Atlantic Ocean. This near co-occurrence suggests global change at this time. We hypothesize this change is related to an increase in incipient ice sheet volume leading to passing threshold conditions for the high-altitude pollen appearance and Tarim Sea retreat, finally leading to decreased moisture availability in the Xining Basin. At the second step, in the base of chron C13r at ~34.7 Ma, a substantial increase in clastic sedimentation rates is observed. This might relate to increased climate variability preceding the greenhouse to icehouse transition at the EOT that prevented landscapes to attain equilibrium configurations

Eocene sea retreat out of Asia: paleogeography, controlling mechanisms and environmental impacts

International audienceThe sediments of the Central Asian basins include the remnants of the easternmost extent of a large epicontinental sea. Before it retreated westward and eventually separated as the Paratethys Sea following the Eocene-Oligocene transition (EOT), this shallow marine sea extended across the Eurasian continent from the Mediterranean Tethys in the west to the Tarim Basin in western China in the east. However, the paleogeography and the timing of the westward retreat of the proto-Paratethys Sea are too poorly constrained to identify its proposed controlling mechanisms and paleoenvironmental impacts. The sea supposedly entered Central Asia in the Cretaceous and five third-order marine incursions have been recognized from the Cretaceous-Paleogene sedimentary record, of which the last two transgressions are documented here. We studied the sea retreat in the Tarim Basin in western China, the Alai Valley and Ferghana Basin in southern Kyrgyzstan and the Afghan-Tajik Basin in south-western Tajikistan. Integrated bio-magnetostratigraphic dating shows that the sea retreated westward from the Tarim Basin in stepwise fashion. The major fourth transgression occurred during the Lutetian, after which the sea retreated from the southwest Tarim Basin paleodepocenter at 41 Ma (base C18r). The last and fifth transgression was restricted to the westernmost margin of the Tarim basin and occurred during latest Bartonian-early Priabonian (base C17n.3n-base C16n.1n). At the level of precision of our dating, each of these marine incursions is apparently synchronous across the Tarim Basin suggesting rapid regional transgression/regression cycles in these shallow epicontinental basins with limited diachroneity. The shallow marine near-shore sediments of these last two transgressions can be convincingly correlated by litho- and biostratigraphy across Central Asia, showing for the first time that the sea may have largely retreated from Central Asia in the late Eocene. The lack of apparent diachroneity of the two last regressions and their concomitancy with the closure of the Turgai Strait in the late Lutetian, with short-term cooling events recognized in contemporary marine records, important late Eocene regressions documented in European basins (e.g. Paris Basin, Ebro Basin, and Transylvanian Basin) and with minor drops in global sea-level, suggest that the individual marine incursions may have been controlled by short-term global fluctuations in sea-level. On the other hand, the gradual continuous character of the marinecontinental transitions and the westward paleogeographic step between the fourth and fifth incursions, separated by several millions of years, rather are typical of long-term tectonic control. Tectonism is likely related to the Eocene India-Asia collision and Pamir initiation to the south in agreement with the infilling with northward paleoflow directions observed in the investigated sections. This is confirmed here by paleomagnetic data from the southwest Tarim Basin showing that clockwise rotation initiated after the fourth marine incursion, probably in response to initial symmetric radial thrusting of the Pamir Mountains. However, the presence of a major disconformity in the southwest Tarim Basin at the EOT, associated with a major global sea-level fall, confirms that the Tarim Basin remained hydrologically connected to the Mediterranean Tethys in the Eocene and had not yet been isolated by tectonic uplift of the surrounding mountains. This is in agreement with previously reported late Oligocene-early Miocene exhumation ages of the Pamir-Kunlun orogenic system and proposed kinematic models suggesting that after late Oligocene-early Miocene initiation of slip along the Kashgar-Yecheng Transfer System along the Eastern Pamir. This is consistent with the paleomagnetic data presented here showing that after the Oligocene deformation became asymmetric with ceased clockwise rotation in the Tarim Basin and continued anticlockwise rotation on the western side in the Afghan-Tajik Basin. The stepwise sea retreat and disconformity are concurrent with the documented aridification steps in the Notheastern Tibetan Plateau (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 have amplified the aridification of the Asian interior. Hence, future studies of Asian paleoenvironmental change during Eocene times also have to be interpreted in terms of fluctuations in moisture supply by the changing paleogeography of the proto-Paratethys Sea in Central Asia

Linking Tarim Basin sea retreat (west China) and Asian aridification in the late Eocene

International audienceThe Tarim Basin in western China formed the easternmost margin of a shallow epicontinental seathat extended across Eurasia and was well connected to the western Tethys during the Paleogene.Climate modelling studies suggest that the westward retreat of this sea from Central Asia may havebeen as important as the Tibetan Plateau uplift in forcing aridification and monsoon intensificationin the Asian continental interior due to the redistribution of the land-sea thermal contrast. However,testing of this hypothesis is hindered by poor constraints on the timing and precise palaeogeographicdynamics of the retreat. Here, we present an improved integrated bio- and magnetostratigraphicchronological framework of the previously studied marine to continental transition in the southwestTarim Basin along the Pamir and West Kunlun Shan, allowing us to better constrain its timing,cause and palaeoenvironmental impact. The sea retreat is assigned a latest Lutetian–earliest Bartonianage (ca. 41 Ma; correlation of the last marine sediments to calcareous nannofossil Zone CP14and correlation of the first continental red beds to the base of magnetochron C18r). Higher up in thecontinental deposits, a major hiatus includes the Eocene–Oligocene transition (ca. 34 Ma). This suggeststhe Tarim Basin was hydrologically connected to the Tethyan marine Realm until at least theearliest Oligocene and had not yet been closed by uplift of the Pamir–Kunlun orogenic system. Thewestward sea retreat at ca. 41 Ma and the disconformity at the Eocene–Oligocene transition are bothtime-equivalent with reported Asian aridification steps, suggesting that, consistent with climatemodelling results, the sea acted as an important moisture source for the Asian continental interior

Late Eocene paleogeography of the Proto-Paratethys Sea in Central Asia (NW China, S Kyrgyzstan and SW Tajikistan)

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Magnetostratigraphic record of the early evolution of the southwestern Tian Shan foreland basin (Ulugqat area), interactions with Pamir indentation and India–Asia collision

International audienceThe Tian Shan range is an inherited intracontinental structure reactivated by the far-field effects of the India–Asia collision. A growing body of thermochronology and magnetostratigraphy datasets shows that the range grew through several tectonic pulses since ~ 25 Ma, however the early Cenozoic history remains poorly constrained. The time-lag between the Eocene India–Asia collision and the Miocene onset of Tian Shan exhumation is particularly enigmatic. This peculiar period is potentially recorded along the southwestern Tian Shan piedmont. There, late Eocene marine deposits of the proto-Paratethys epicontinental sea transition to continental foreland basin sediments of unknown age were recently dated. We provide magnetostratigraphic dating of these continental sediments from the 1700-m-thick Mine section integrated with previously published detrital apatite fission track and U/Pb zircon ages. The most likely correlation to the geomagnetic polarity time scale indicates an age span from 20.8 to 13.3 Ma with a marked increase in accumulation rates at 19–18 Ma. This implies that the entire Oligocene period is missing between the last marine and first continental sediments, as suggested by previous southwestern Tian Shan results. This differs from the southwestern Tarim basin where Eocene marine deposits are continuously overlain by late Eocene–Oligocene continental sediments. This supports a simple evolution model of the western Tarim basin with Eocene–Oligocene foreland basin activation to the south related to northward thrusting of the Kunlun Shan, followed by early Miocene activation of northern foreland basin related to overthrusting of the south Tian Shan. Our data also support southward propagation of the Tian Shan piedmont from 20 to 18 Ma that may relate to motion on the Talas Fergana Fault. The coeval activation of a major right-lateral strike-slip system allowing indentation of the Pamir Salient into the Tarim basin, suggests far-field deformation from the India–Asia collision zone affected the Tian Shan and the Talas Fergana fault by early Miocene