Denudation outpaced by crustal thickening in the eastern Tianshan

The modern high topography of the Tianshan resulted from the reactivation of a Paleozoic orogenic belt by the India/Asia collision. Today, the range exhibits tectonically active forelands and intermontane basins. Based on quantitative morphotectonic observations and age constraints derived from cosmogenic 10Be dating, single-grain post-infrared infrared stimulated luminescence (p-IR IRSL) dating and modeling of fault scarp degradation, we quantify the deformation in the Nalati and Bayanbulak intermontane basins in the central Eastern Tianshan. Our results indicate that at least 1.4 mm/yr of horizontal crustal shortening is accommodated within these two basins. This shortening represents over 15% of the 8.5 ± 0.5 mm/yr total shortening rate across the entire range at this longitude. This shortening rate implies that the Eastern Central Tianshan is thickening at a mean rate of ∼1.4 mm/yr, a rate that is significantly higher than the average denudation rate of 0.14 mm/yr derived from our cosmogenic analysis. This discrepancy suggests that the Tianshan range has not yet reached a steady-state topography and remains in a transient state of topographic growth, most likely due to limited denudation rates driven by the arid climate of Central Asia

Évolution tectonique du Tianshan oriental du Néogène à l'actuel

The Tianshan range is a 2000-km-long tectonically active mountain range. The Tianshan plays a major role in the India-Asia collision since it presently accommodates up to 40% of the total convergence between those two continents. Despite this intense deformation, the topography of the range shows series of high elevated range separated by flat intermountain basins. This range is therefore a remarkable case of example to study orogenesis processes and remains a key area to better understand how the deformation related to the India collision has been distributed through the Asian continent. Enhanced aridity since Pleistocene has moreover remarkably preserved alluvial terraces and fans which may have recorded the deformation enabling the quantification of the tectonic rates during the Quaternary period. Therefore, using Terrestrial Cosmogenic Nuclide we have dated several alluvial surfaces and analyzed their deformation using D.E.M. or high resolution DGPS measurements. Hence, in the eastern Tianshan we quantify the quaternary shortening rates across several structures located in the Northern and Southern foreland piedmonts but also within the range in the intermountain Bayanbuluk basin. Moreover, when possible we have also derived the Neogene shortening rates from geometric modeling of syntectonic strata. Our results highlight the recent acceleration of the eastern Tianshan since ~150-200 ka. We also show that a significant proportion (>15%) of the whole deformation is accommodated within the range while the remaining part is distributed between the different thin skin tectonic structure in the piedmonts. This result supports the hypothesis that the range is out of equilibrium and an in-building stage of deformationLe Tianshan est une orogène intracontinentale de près de 2000 km de long. Cette chaîne accommode jusqu'à 40% de la convergence entre l'Inde et l'Eurasie, jouant ainsi un rôle fondamental dans l'évolution de la déformation en Asie. De plus, malgré cette déformation intense et ses hauts reliefs, sa topographie reste irrégulière avec de nombreux bassins intrachaîne préservés. Elle représente ainsi un cas remarquable pour l'étude des processus orogéniques dynamiques hors équilibre et constitue une zone clé pour l'étude de la déformation en Asie. Depuis le début du Pléistocène, une aridité prononcée a permis une préservation de nombreux marqueurs morphologiques. Ces marqueurs ont localement subi des déformations d'origine tectonique. Ainsi, la datation par isotopes cosmogéniques de ces marqueurs et la quantification du raccourcissement à l'origine de leur déformation, a permis de connaître précisément les vitesses de raccourcissements quaternaires de nombreuses structures actives au travers l'ensemble du Tianshan oriental. À cela s'ajoutent ponctuellement des études des vitesses de raccourcissement depuis le Néogène, basées sur la modélisation géométrique de strates syntectoniques. Ces mesures nous ont permis de mieux contraindre les taux de déformation sur les piémonts Nord et Sud de la chaîne mais également dans le bassin intrachaîne de Bayanbuluk. Sur la base de ces observations, je propose que les vitesses observées actuellement par les mesures GPS ne soient acquises par la chaîne que depuis ~150 à 200 ka. Nous démontrons aussi qu'au minimum 15% du raccourcissement est accommodé dans les parties internes de la chaîne. Je soutiens ainsi l'hypothèse que cette chaîne reste dans un stade de croissance immature et hors-équilibr

Tectonic evolution of eastern Tianshan from Neogene to present

Le Tianshan est une orogène intracontinentale de près de 2000 km de long. Cette chaîne accommode jusqu'à 40% de la convergence entre l'Inde et l'Eurasie, jouant ainsi un rôle fondamental dans l'évolution de la déformation en Asie. De plus, malgré cette déformation intense et ses hauts reliefs, sa topographie reste irrégulière avec de nombreux bassins intrachaîne préservés. Elle représente ainsi un cas remarquable pour l'étude des processus orogéniques dynamiques hors équilibre et constitue une zone clé pour l'étude de la déformation en Asie. Depuis le début du Pléistocène, une aridité prononcée a permis une préservation de nombreux marqueurs morphologiques. Ces marqueurs ont localement subi des déformations d'origine tectonique. Ainsi, la datation par isotopes cosmogéniques de ces marqueurs et la quantification du raccourcissement à l'origine de leur déformation, a permis de connaître précisément les vitesses de raccourcissements quaternaires de nombreuses structures actives au travers l'ensemble du Tianshan oriental. À cela s'ajoutent ponctuellement des études des vitesses de raccourcissement depuis le Néogène, basées sur la modélisation géométrique de strates syntectoniques. Ces mesures nous ont permis de mieux contraindre les taux de déformation sur les piémonts Nord et Sud de la chaîne mais également dans le bassin intrachaîne de Bayanbuluk. Sur la base de ces observations, je propose que les vitesses observées actuellement par les mesures GPS ne soient acquises par la chaîne que depuis ~150 à 200 ka. Nous démontrons aussi qu'au minimum 15% du raccourcissement est accommodé dans les parties internes de la chaîne. Je soutiens ainsi l'hypothèse que cette chaîne reste dans un stade de croissance immature et hors-équilibreThe Tianshan range is a 2000-km-long tectonically active mountain range. The Tianshan plays a major role in the India-Asia collision since it presently accommodates up to 40% of the total convergence between those two continents. Despite this intense deformation, the topography of the range shows series of high elevated range separated by flat intermountain basins. This range is therefore a remarkable case of example to study orogenesis processes and remains a key area to better understand how the deformation related to the India collision has been distributed through the Asian continent. Enhanced aridity since Pleistocene has moreover remarkably preserved alluvial terraces and fans which may have recorded the deformation enabling the quantification of the tectonic rates during the Quaternary period. Therefore, using Terrestrial Cosmogenic Nuclide we have dated several alluvial surfaces and analyzed their deformation using D.E.M. or high resolution DGPS measurements. Hence, in the eastern Tianshan we quantify the quaternary shortening rates across several structures located in the Northern and Southern foreland piedmonts but also within the range in the intermountain Bayanbuluk basin. Moreover, when possible we have also derived the Neogene shortening rates from geometric modeling of syntectonic strata. Our results highlight the recent acceleration of the eastern Tianshan since ~150-200 ka. We also show that a significant proportion (>15%) of the whole deformation is accommodated within the range while the remaining part is distributed between the different thin skin tectonic structure in the piedmonts. This result supports the hypothesis that the range is out of equilibrium and an in-building stage of deformatio

Limited impact of Quaternary glaciations on denudation rates in Central Asia

International audienceBecause of its essential role in coupling climate and tectonics, denudation is a key parameter when constraining the history of Earth’s surface. This is particularly true at the Pliocene-Pleistocene transition, and the potential impact of the onset of Quaternary glaciations remains strongly debated. In the present study, we measured in situ cosmogenic 10Be within continuous late Cenozoic sedimentary sections that had already been dated using magnetostratigraphy. The new data were obtained from four sedimentary basins in the northern and southern Tianshan range (Central Asia). We first thoroughly discuss how in situ cosmogenic 10Be concentrations can be corrected for radioactive decay and for the contribution of postdepositional cosmogenic accumulation to derive the paleo–denudation rates. Our analysis shows that, in the four sedimentary records, the potential bias remains low enough to consider the derived denudation rates reliable. The four records, although likely influenced by local particularities due to lithological heterogeneity and local tectonics, display similar trends of continuously increasing denudation between ca. 9 Ma and the present. These rates have remained relatively high but steady since 4 Ma, ∼1.5 m.y. before the onset of the Quaternary glacial cycles. Though the rejuvenation of the Tianshan range since 11 Ma may explain most of the progressive increase (×5) in denudation, our data suggest that the Quaternary glaciations had only a limited impact on denudation in the Tianshan. Our data, however, indicate an increase in the spatial and high-frequency variability (<1 m.y.) of the denudation rates between 3 and 1 Ma. This may correspond to a transient readjustment of the landscape in response to the onset of Quaternary glacial cycles

Active tectonics around the Yakutat indentor: New geomorphological constraints on the eastern Denali, Totschunda and Duke River Faults

International audienc

Geomorphic analysis of active fold growth and landscape evolution in the central Qiulitage fold belt, southern Tian Shan, China

International audienceThe Qiulitage fold-and-thrust system in the Kuqa foreland is a prominent NE-SW trending relief associated with active thrust-related folding in the southern Tian Shan. Based on open-access DEMs and field work, we carried out a detailed geomorphologic analysis of the Qiulitage topography based on drainage network characteristics and several morphometric indices. Our results suggest that the Qiulitage fold belt is composed of six morphological units, which are statistically confirmed by a principal component analysis. These morphologic structures have interacted during both vertical growth and lateral propagation of the folds, associated with different patterns of fold connection and deformation partitioning. The structural evolution of the Qiulitage fold belt has significantly influenced major rivers crossing the fold belt and minor ones developing on the fold ridges. We finally propose a morphotectonic evolution of the fold belt integrating the influence of décollement distribution, deep inherited structures, and changes in shortening amount

Major temporal variations in shortening rate absorbed along a large active fold of the southeastern Tianshan piedmont (China)

International audienceThe investigation of deformation rates on a mountain piedmont can provide key information for improving our understanding of the overall dynamics of a mountain range. Here, we estimate the shortening rate absorbed by a Quaternary emergent detachment fold on the southeastern piedmont of the Tianshan (China). Our work is primarily based on new 10Be cosmogenic exposure dating of deformed alluvial surfaces. The method we have developed combines depth profiling with sampling of surface cobbles, thereby allowing exposure time, erosion rate and inheritance to be simultaneously constrained. The exposure ages of the uppermost uplifted alluvial surfaces are around 140±17 ka140±17 ka, 130±9 ka130±9 ka and 47±9 ka47±9 ka, from west to east. A terrace lying below the 140 ka surface is dated at 65±5 ka65±5 ka. The ages of the uplifted and folded alluvial surfaces were then combined with estimates of shortening obtained using two distinct methods: (1) the excess area method, where sedimentation rates, extracted from magnetostratigraphic studies, are used to determine the amount of sedimentation after the abandonment of the river; and (2) a folding model derived from sandbox experiments. The late Pleistocene shortening rates are shown to be between 0.4±0.1 mm/yr0.4±0.1 mm/yr and 0.8±0.5 mm/yr0.8±0.5 mm/yr on the western part of the fold and 2.1±0.4 mm/yr2.1±0.4 mm/yr along its central part. The central part of the frontal Yakeng anticline therefore accommodates up to 25% of the total shortening currently absorbed across the whole Eastern Tianshan range (8 mm/yr). However, this situation seems to have prevailed for only the last 150 ka, as the shortening rate absorbed by this nascent fold was previously ten times slower. While the initiation of folding of the Yakeng anticline can be traced back to 5.5 Ma ago, the basinward migration of the active deformation front onto the Yakeng fold is a relatively recent phenomenon and appears to be diachronous from west to east, probably in relation to the tectonic activity of the folds in the hinterland

Miocene to Holocene deformation rates in the southern Tianshan piedmont, China

International audienceThe Tianshan is one of the largest and highest mountain belts in central Asia and it accommodates an important part of the total India/Eurasia convergence. It is therefore critical to better quantify the deformation across this range, especially across its piedmonts that presently absorb most of the shortening. The deformation across the southern Tianshan piedmont is distributed over several thrusts and folds belts including the Qiulitagh and the Yakeng anticlines, which are identified as a fault-bend-fold and a detachment fold respectively. Across these two folds, we document the shortening rate from the Mio-Pliocene to the Pleistocene using two different approaches. First, we used a kinematic fold model based on kink band migration, in order to reproduce bed by bed the pre-growth and growth-strata of the Qiulitagh anticline. We focus our analysis on two different sections where seismic profiles and detailed surface structural data provide strong constrains on the geometry of the fold. The first section is located in the southern flank of the fold along the Kuqa River. In the bottom part of this section the depositional ages have been constrained by the magnetostratigraphic study of Sun et al. (2009), whereas the upper conglomeratic part remained undated. For this upper part, we have collected several new samples to better constrain the sediments ages using burial cosmogenic dating (10Be/26Al). The second section is located in the northern flank of the fold along the Yaha River where the depositional ages have also been constrained in detail from magnetostratigraphy (Charreau et al., 2009). Together, these two sections should provide new and high resolution (100ka) by cosmogenic (10Be) exposure method

Miocene to Holocene deformation rates in the southern Tianshan piedmont, China

International audienceThe Tianshan is one of the largest and highest mountain belts in central Asia and it accommodates an important part of the total India/Eurasia convergence. It is therefore critical to better quantify the deformation across this range, especially across its piedmonts that presently absorb most of the shortening. The deformation across the southern Tianshan piedmont is distributed over several thrusts and folds belts including the Qiulitagh and the Yakeng anticlines, which are identified as a fault-bend-fold and a detachment fold respectively. Across these two folds, we document the shortening rate from the Mio-Pliocene to the Pleistocene using two different approaches. First, we used a kinematic fold model based on kink band migration, in order to reproduce bed by bed the pre-growth and growth-strata of the Qiulitagh anticline. We focus our analysis on two different sections where seismic profiles and detailed surface structural data provide strong constrains on the geometry of the fold. The first section is located in the southern flank of the fold along the Kuqa River. In the bottom part of this section the depositional ages have been constrained by the magnetostratigraphic study of Sun et al. (2009), whereas the upper conglomeratic part remained undated. For this upper part, we have collected several new samples to better constrain the sediments ages using burial cosmogenic dating (10Be/26Al). The second section is located in the northern flank of the fold along the Yaha River where the depositional ages have also been constrained in detail from magnetostratigraphy (Charreau et al., 2009). Together, these two sections should provide new and high resolution (100ka) by cosmogenic (10Be) exposure method

Taux de déformation du Miocène à l'Holocène sur le piémont Sud du Tian Shan, Chine

International audienceLe Tianshan est une chaîne majeure située en Asie qui accommode une grande partie de la convergence entre les plaques indienne et eurasiatique. Il est donc important de quantifier dans le temps les taux de déformation à travers cette chaîne. Ses piedmonts sont des zones clés qui focalisent une partie importante du raccourcissement. Sur le piedmont Sud on observe plusieurs plis et chevauchements dont le pli de rampe de Quilitage. Nous avons ainsi estimé l'histoire du raccourcissement à travers cette structure depuis le Miocène. Pour ce faire nous réalisons une modélisation cinématique du plissement qui s'appuie sur des données sismiques et des mesures structurales en surface qui contraignent précisément la géométrie du pli. Elle permet d'identifier nettement les sédiments syn-tectoniques et de quantifier, horizon par horizon, le raccourcissement nécessaire pour chacun à travers la structure étudiée. Cette approche a donc été appliquée sur trois sections pourvues de données suffisantes. Sur chacune, les âges de dépôts ont été contraints par des données magnétostratigraphiques publiées, et complétés, le cas échéant, par des âges d'enfouissement cosmogéniques 26Al/10Be que nous avons réalisés. La répartition du raccourcissement moyen semblerait différente selon les coupes étudiées avec deux épisodes de raccourcissement comprenant une accélération après 1 Ma à l'ouest et une vitesse plus constante depuis 5 Ma sur la partie plus à l'est. Ces variations rendent potentiellement compte de la répartition du raccourcissement entre les différentes structures et des variations au sein-même de l'anticlinal. En marge de cette étude, sur la coupe centrale, nous avons également réalisé des datations 26Al/10Be de la base de la formation conglomératique de Xiyu. Ces âges, moyennant une restauration de la déformation, permettent d'estimer la vitesse de progradation sédimentaire dans le bassin d'avant-pays. Celle-ci permet d'apporter de nouvelles contraintes sur les vitesses de raccourcissement à travers le piedmont