Évolution paléogéographique et paléotopographique du Tian Shan Chinois au Mésozoïque

The Tian Shan is an intracontinental range located in Central Asia. The structure of the range formed during the Paleozoic through the accretion of several blocks. Recently the range has been reactivated due to far-field effects of the collision between India and Asia. The topographic and tectonic evolution of the range in-between these two major relief-building phases (Late Paleozoic and Cenozoic) is still poorly understood. Two different approaches are combined in this work, in order to determine the Mesozoic topographic evolution of the area, in a qualitative way.The Late-Paleozoic range has been progressively eroded until the Upper Triassic/Lower Jurassic. Tectonic activity was relatively quiet during the Jurassic characterised by low exhumation rates. We suggest that the tectonic regime was dominated by transtension in the Tian Shan area. This period is linked to a regional peneplanation in Central Asia. At the Jurassic-Cretaceous boundary, the occurrence of alluvial fan deposits shows a reactivation of the range, though not strong enough to be recorded by low-temperature thermochronology. This period is characterised by an overall extensional tectonic regime all over Central Asia, with the opening of the Caspian Basin further west. It precedes the phase of very slow exhumation that occurs during the Lower Cretaceous. From 100 Ma and during the Upper Cretaceous, the range is progressively reactivated. We suggest that this correspond to a delayed answer of the Lhassa bloc collision (140 - 120 Ma). Around 65 - 60 Ma, a new phase of rapid exhumation attests of a stronger reactivation, localised along the major faults. This is contemporary of bloc collisions along the south-west margin of Asia, such as the Kohistan Block, the Dras arc or the Afghan Block. In the Tian Shan area, the development of calcrete features in the Paleocene suggests the end of tectonic activity, before the new reactivation in the Neogene.The combination of low temperature thermochronology on the basement rocks and facies sedimentology in the various basins indicate that while during the Mesozoic, the Palaeo-Tian Shan topography generally flattens, some small-scale tectonic events driven by far-field effects of major geodynamic processes around the edges of Asia did occur. However, those tectonic movements did not induce enough exhumation to be recorded by low temperature thermochronometers. Only the sediment record allows their detection and detailed description.Le Tian Shan est une chaîne intracontinentale d’Asie Centrale, dont la structure lithosphérique résulte de l’accrétion de divers blocs au cours du Paléozoïque. Son histoire tectonique récente est marquée par sa réactivation au Tertiaire, liée à la collision entre l’Inde et l’Asie. L’histoire topographique et tectonique de la chaine entre les deux orogenèses majeures (paléozoïque terminal et cénozoïque) reste peu contrainte. Cette thèse associe deux approches, la sédimentologie et la thermochronologie basse température, dans le but de déterminer de façon qualitative l’évolution de la topographie au cours du Mésozoïque.Le démantèlement des reliefs associés à la chaine tardi-paléozoïque s’achève au Trias supérieur. Il est suivi d’une activité tectonique très faible au Jurassique, majoritairement transtensive et caractérisée par des taux d’exhumation très faibles dans le Tian Shan. Une surface majeure de pénéplanation se développe alors en Asie Centrale. A la limite Jurassique – Crétacé, la mise en place de cônes alluviaux indique une réactivation de la chaîne, qui n’est néanmoins pas suffisante pour être enregistrée par la thermochronologie. Cette période est caractérisée par un régime généralement extensif en Asie Centrale (jusqu’au Bassin Caspien qui s’ouvre à l’est), et précède la phase d’exhumation lente, qui suit au Crétacé inférieur. La chaîne est progressivement réactivée à partir de 100 Ma et pendant le Crétacé supérieur, ce qui pourrait correspondre à un effet retardé de la collision du Bloc de Lhassa (140 - 120 Ma). Vers 65 - 60 Ma, une phase d’exhumation rapide atteste d’une réactivation plus intense et localisée le long des principales failles. Elle est sans doute liée aux collisions de blocs le long de la marge sud-ouest de l’Asie (e.g. Bloc du Kohistan, arc du Dras, Bloc Afghan). Dans la région du Tian Shan, l’activité tectonique semble totalement s’arrêter au Paléocène permettant le développement d’un niveau majeur de calcrêtes, avant la nouvelle réactivation au Néogène. Par conséquent, l’association des données de thermochronologie sur le socle avec la reconstruction des milieux de dépôt dans les différents bassins, montre que la paléo-chaîne du Tian Shan s’aplanit durant le Mésozoïque. Des évènements tectoniques de faible envergure ont lieu en Asie Centrale, induits par les principaux mouvements géodynamiques le long des bordures entourant l’Asie. Pourtant leur enregistrement est incomplet et seule la combinaison des deux approches étudiées permet de reconstituer l’évolution topographique et paléogéographique du Tian Shan

Unbalanced sediment budgets in the catchment-alluvial fan system of the Kuitun River (northern Tian Shan, China): Implications for mass-balance estimates, denudation and sedimentation rates in orogenic systems

International audienceMass balances are often used to calculate sediment fluxes in foreland basins and denudation rates in adjacent mountain ranges on intermediate to long timescales (from a few tens of thousand to a million years). Here, we study the simple Quaternary catchment-alluvial fan system of the Kuitun River, in northern Tian Shan, to discuss some ideas about sediment storage, release, and bypass in relatively short (100 km long) sediment routing systems. This study shows that the Kuitun catchment and piedmont areas clearly present evidence of a significant and temporary storage of sediments during the Pleistocene. These sediments were then excavated and delivered farther into the foreland basin during the Holocene. The difference between the volumes of materials released from the catchment and piedmont areas (5.5 ± 1.7 km3) and the volume stored in a contemporaneous fan downstream (2.6 ± 0.6 km3) indicates that the latter did not trap the whole sediment load transported by the river. The alluvial fan was bypassed by 27 to 78% of this load toward its distal alluvial plain. If this value is well estimated, it implies a major volumetric partitioning of the deposits between the fan and the alluvial plain, with a very high sedimentation rate in the fan (1.97 ± 0.52 mm*y− 1) and a much lower one downstream (0.11 ± 0.11 mm*y− 1). However, this volumetric partitioning might only occur during periods with a very specific hydrological regime such as the Holocene deglaciation. Eventually, the peculiar sediment storage and release pattern within the Kuitun catchment and piedmont areas during the Pleistocene and Holocene complicates the calculation of mean paleodenudation rates using either sediment budgets or in situ produced cosmogenic nuclides

Stable isotope characterization of pedogenic and lacustrine carbonates from the Chinese Tian Shan: constraints on the Mesozoic - Lower Cenozoic palaeo-environmental evolution

International audienceIn the Mesozoic–Cenozoic continental deposits of the Tian Shan area, two main levels containing pedogenic carbonates have been identified on both the southern and northern foothills of the range: one in the Upper Jurassic series and one in the Upper Cretaceous–Lower Palaeocene series. In order to reconstruct the palaeoenvironmental and palaeotopographic characteristics of the Tian Shan area during these two periods, we measured the oxygen and carbon isotope composition of these pedogenic carbonates (calcrete and nodules). The stable isotope compositions are homogeneous: most δ18O values are between 21 and 25‰ and most δ13C values are between −4 and −6‰. No distinction can be made between the calcrete and nodule isotopic compositions. The constancy of isotopic values across the Tian Shan is evidence of a development of these calcification features in similar palaeoenvironmental conditions. The main inference is that no significant relief existed in that area at the Cretaceous−Palaeogene boundary, implying that most of the present relief developed later, during the Cenozoic. In addition to the pedogenic carbonates, few beds of limestones interstratified in the Jurassic series of the southern foothills display oxygen and carbon isotope compositions typical of lacustrine carbonates, ruling out brackish water incursion at that period in the regio

Paleogeographic and paleotopographic evolution of the Chinese Tian Shan during the Mesozoic

Le Tian Shan est une chaîne intracontinentale d’Asie Centrale, dont la structure lithosphérique résulte de l’accrétion de divers blocs au cours du Paléozoïque. Son histoire tectonique récente est marquée par sa réactivation au Tertiaire, liée à la collision entre l’Inde et l’Asie. L’histoire topographique et tectonique de la chaine entre les deux orogenèses majeures (paléozoïque terminal et cénozoïque) reste peu contrainte. Cette thèse associe deux approches, la sédimentologie et la thermochronologie basse température, dans le but de déterminer de façon qualitative l’évolution de la topographie au cours du Mésozoïque.Le démantèlement des reliefs associés à la chaine tardi-paléozoïque s’achève au Trias supérieur. Il est suivi d’une activité tectonique très faible au Jurassique, majoritairement transtensive et caractérisée par des taux d’exhumation très faibles dans le Tian Shan. Une surface majeure de pénéplanation se développe alors en Asie Centrale. A la limite Jurassique – Crétacé, la mise en place de cônes alluviaux indique une réactivation de la chaîne, qui n’est néanmoins pas suffisante pour être enregistrée par la thermochronologie. Cette période est caractérisée par un régime généralement extensif en Asie Centrale (jusqu’au Bassin Caspien qui s’ouvre à l’est), et précède la phase d’exhumation lente, qui suit au Crétacé inférieur. La chaîne est progressivement réactivée à partir de 100 Ma et pendant le Crétacé supérieur, ce qui pourrait correspondre à un effet retardé de la collision du Bloc de Lhassa (140 - 120 Ma). Vers 65 - 60 Ma, une phase d’exhumation rapide atteste d’une réactivation plus intense et localisée le long des principales failles. Elle est sans doute liée aux collisions de blocs le long de la marge sud-ouest de l’Asie (e.g. Bloc du Kohistan, arc du Dras, Bloc Afghan). Dans la région du Tian Shan, l’activité tectonique semble totalement s’arrêter au Paléocène permettant le développement d’un niveau majeur de calcrêtes, avant la nouvelle réactivation au Néogène. Par conséquent, l’association des données de thermochronologie sur le socle avec la reconstruction des milieux de dépôt dans les différents bassins, montre que la paléo-chaîne du Tian Shan s’aplanit durant le Mésozoïque. Des évènements tectoniques de faible envergure ont lieu en Asie Centrale, induits par les principaux mouvements géodynamiques le long des bordures entourant l’Asie. Pourtant leur enregistrement est incomplet et seule la combinaison des deux approches étudiées permet de reconstituer l’évolution topographique et paléogéographique du Tian Shan.The Tian Shan is an intracontinental range located in Central Asia. The structure of the range formed during the Paleozoic through the accretion of several blocks. Recently the range has been reactivated due to far-field effects of the collision between India and Asia. The topographic and tectonic evolution of the range in-between these two major relief-building phases (Late Paleozoic and Cenozoic) is still poorly understood. Two different approaches are combined in this work, in order to determine the Mesozoic topographic evolution of the area, in a qualitative way.The Late-Paleozoic range has been progressively eroded until the Upper Triassic/Lower Jurassic. Tectonic activity was relatively quiet during the Jurassic characterised by low exhumation rates. We suggest that the tectonic regime was dominated by transtension in the Tian Shan area. This period is linked to a regional peneplanation in Central Asia. At the Jurassic-Cretaceous boundary, the occurrence of alluvial fan deposits shows a reactivation of the range, though not strong enough to be recorded by low-temperature thermochronology. This period is characterised by an overall extensional tectonic regime all over Central Asia, with the opening of the Caspian Basin further west. It precedes the phase of very slow exhumation that occurs during the Lower Cretaceous. From 100 Ma and during the Upper Cretaceous, the range is progressively reactivated. We suggest that this correspond to a delayed answer of the Lhassa bloc collision (140 - 120 Ma). Around 65 - 60 Ma, a new phase of rapid exhumation attests of a stronger reactivation, localised along the major faults. This is contemporary of bloc collisions along the south-west margin of Asia, such as the Kohistan Block, the Dras arc or the Afghan Block. In the Tian Shan area, the development of calcrete features in the Paleocene suggests the end of tectonic activity, before the new reactivation in the Neogene.The combination of low temperature thermochronology on the basement rocks and facies sedimentology in the various basins indicate that while during the Mesozoic, the Palaeo-Tian Shan topography generally flattens, some small-scale tectonic events driven by far-field effects of major geodynamic processes around the edges of Asia did occur. However, those tectonic movements did not induce enough exhumation to be recorded by low temperature thermochronometers. Only the sediment record allows their detection and detailed description

Aridification of a Flat World: The Tectonic and Climate Evolution of Central Asia During the Jurassic.

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SEDIMENTARY LANDSCAPE AND PALEO-LANDSCAPES OF AN ACTIVEENDORHEIC FORELAND BASIN: THE JUNGGAR BASIN (XINJIANG, CHINA)

National audienceThe sedimentary landscape and dynamics of foreland basins, which are shaped by the interplaysbetween tectonics, climate, sedimentation and erosion processes, are key elements toreconstruct the evolution of orogenic systems. Therefore, a detailed understanding of the relationshipsbetween the foreland landscapes and these processes is necessary to improve ourknowledge on the sedimentary record in compressive regions. In this study, we focus on thecharacterization of the present-day and palaeo-landscapes of an endorheic foreland basin wheredeformation, sedimentation and erosion are still active and easily observable: the Junggar Basinlocated in Central Asia. The interest of this basin resides in outstanding outcrops of its sedimentaryseries and structures, numerous surface and subsurface data (satellite images, digitaltopography, seismic profiles and drilling well data), and marked continental paleo-geographicchanges through the Cenozoic. The methodology consists in coupling different approaches (geomorphology,sedimentology, sequential stratigraphy and structural geology) to characterize thesepaleo-geographic changes across space and through time. As a starting point, we drew a morphosedimentologicalmap of the present-day landscape from the surface data to describe the currentdrainage organization and associated sedimentary environments in the basin. Then, we estab- lishedseveral paleo-geographic maps from the surface and subsurface data to reconstruct the spatiotemporallandscape evolution since 65 Ma. In the light of previous quantifications of its controlparameters (substratum deformation and sediment supplies), this evolution provides new qualitativeand quantitative constraints on landscape architecture and dynamics in continental forelandcontexts

Palaeogeographic and palaeotopographicevolution of the chinese tian shanduring the mesozoic : A sedimentologicalsynthesis

International audienceThe Tian Shan range occupies the southwestern edge of the CentralAsian Orogenic Belt. Our aim is to reconstruct the pre-Tertiary historyof relief building in the Chinese Tian Shan. We use the sedimentary facies,sediment provenances and paleocurrent directions of the 6 exposedMesozoic sections in the northern and southern piedmonts as well as inintra-mountain basins. The correlation of these 6 sections allows us topropose some palaeogeographic maps for the middle Jurassic, the upperJurassic - lower Cretaceous transition, the upper Cretaceous and the lateCretaceous - Paleogene transition.The dismantling of reliefs associated to the late-paleozoic tectonic phaseis largely completed by upper Triassic. The Jurassic is characterised bya low tectonic activity and results in a general planation phase of theTian Shan area. The upper Jurassic - lower Cretaceous transition is characterizedby a huge conglomeratic event at the scale of all the northernfoothills, associated with some eolian deposits implying a arid climateduring this period. We propose that the alluvial fans are resulting fromthe erosion of relatively small and local reliefs, caused by normal faultsactivity, in contradiction with the existing hypothesis supporting compressionaltectonic and reactivation of the range during the Upper Jurassic(Hendrix et al., 1992). However it fits to the interpretation oflow thermochronology data. During the Upper Cretaceous, the widespreadoccurrence of alluvial fans indicates the ongoing erosion of a localpositive topography in the Tian Shan area. A significant late Lower- early Upper Cretaceous unconformity is observable in the S-Junggar,N-Tarim and Turfan Basins. It is confirmed by low temperature thermochronologydata that show a « major » late Lower Cretaceous unroofingevent near Kuqa on the southern foothills (Dumitru et al., 2001) andsome late Lower to early Upper Cretaceous exhumation ages within therange (Jolivet et al., 2010). This marks the onset of a new exhumationphase that goes on during the Upper Cretaceous and seems to stop aftera peak of activity in the late Upper Cretaceous. Following the UpperCretaceous inversion episode, a tectonic quiet period characterises thelate Upper Cretaceous - Early Paleogene and enables the formation of awidespread calcrete in both piemont of the range

New Apatite Fission-Track Ages of the Western Kuqa Depression: Implications for the Mesozoic–Cenozoic Tectonic Evolution of South Tianshan, Xinjiang

International audienceThe Mesozoic–Cenozoic uplift history of South Tianshan has been reconstructed in many ways using thermochronological analyses for the rocks from the eastern Kuqa Depression. The main difference in the reconstructions concerns the existence and importance of Early Cretaceous and Paleogene tectonic activities, but the existence of a Cenozoic differential uplift in the Kuqa Depression remains enigmatic. Here, we present new apatite fission-track ages obtained for 12 sandstone samples from the well-exposed Early Triassic to Quaternary sequence of the Kapushaliang section in the western Kuqa Depression. The results reveal that there were four pulses of tectonic exhumation, which occurred during the Early Cretaceous (peak ages of 112 and 105 Ma), Late Cretaceous (peak age of 67 Ma), Paleocene–Eocene (peak ages at 60, 53, and 36 Ma), and early Oligocene to late Miocene (central ages spanning 30–11 Ma and peak ages of 23 and 14 Ma), respectively. A review of geochronological and geological evidence from both the western and eastern Kuqa Depression is shown as follows. (1) The major exhumation of South TiansShan during the Early Cretaceous was possibly associated with docking of the Lhasa block with the southern margin of the Eurasian plate. (2) The Late Cretaceous uplift of the range occurred diachronically due to the far-field effects of the Kohistan-Dras Arc and Lhasa block accretion. (3) The Paleogene uplift in South Tianshan initially corresponded to the far-field effects of the India–Eurasia collision. (4) The rapid exhumation in late Cenozoic was driven by the continuous far-field effects of the collision between India and Eurasia plates. The apatite fission-track ages of 14–11 Ma suggest that late Cenozoic exhumation in the western Kuqa Depression prevailed during the middle to late Miocene, markedly later than the late Oligocene to early Miocene activity in the eastern segment. It can be hypothesized that a possible differential uplift in time occurred in the Kuqa Depression during the late Cenozoic

How did it look before it start? The Mesozoic tectonic evolution of Tian Shan

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Assessing the Mesozoic paleorelief of the NE Tian Shan (China): constraints from sedimentological marker strata.

International audienceThe Tian Shan is one of the main ranges of Central Asia Orogenic Belt. The Tertiary deformation is driven by the India-Asia collision stress field (e.g., [1], [2]). However, the deformation appears localised along inherited tectonic structures formed during the Palaeozoic - Early Mesozoic history of the range (e.g., [3]). Some of these structures have been reactivated during the early stages of the India-Asia collision which diverge from the model of northward propagation of the Tertiary deformation through time (e.g., [4]). Our aim is to reconstruct the pre-Tertiary history of relief building in the Chinese Tian Shan. We use the sedimentary facies, depositional environments, sediment provenances and palaeocurrent directions of the exposed Mesozoic sections in the northern piedmont and inside the range. This will be used to understand the long-term reactivation pattern of the main inherited structures and to assess their influence on the localisation and propagation of the deformation. We first established a nearly complete reference section spanning from the Upper Triassic to the Palaeogene, in the northern piedmont of the Tian shan range. This section, localised between the Ningjia and Manas rivers, west of Urumqi was previously studied by Hendrix et al. [5] and is thus well documented. We then studied two other sections, one in Wusu (100 km to the West) and one along the Toutun River (100 km to the East). Those sections, mostly covering the Jurassic and Cretaceous are used to assess potential lateral variations in the sedimentary facies and depositional environments along the actual piedmont. Finally, we investigated the Mesozoic sections (Jurassic and Cretaceous ?) exposed within the intramontane Yili and Bayanbulak basins and compared them with the results obtained on the northern piedmont. This comparison should allow assessing the continuity or disconnection between the internal basins and the Junggar basin. Preliminary results along the reference section show five main phases. The Upper Triassic - Lower Jurassic is marked by conglomeratic deposits indicative of the destruction of the Permian topography. During the Middle Jurassic sandstone deposits intercalated with coal indicate a quieter environment. Then, the thick (up to 400 m) conglomerates of the Upper Jurassic - Early Cretaceous Kalazha Formation may indicate either a reactivation of relief building or a strong climatic change. During Lower Cretaceous, lake deposits attest of a quiet period, followed during Upper Cretaceous by deposition of new conglomerates probably indicating renew topography. Lateral facies variations are observed along the northern piedmont throughout the whole time period. For example in the Toutun River section the Cretaceous series are only represented by lake deposits with a few conglomerate beds at the base of the series. These main phases are also partially recognized in the intramontane basins however with some local variations in facies and occurrence. This suggests that some of these phases correspond to regional events, while others may be more local such as the formation of topographic barrier between different basins