Modeling the shortening history of a fault tip fold using structural and geomorphic records of deformation

We present a methodology to derive the growth history of a fault tip fold above a basal detachment. Our approach is based on modeling the stratigraphic and geomorphic records of deformation, as well as the finite structure of the fold constrained from seismic profiles. We parameterize the spatial deformation pattern using a simple formulation of the displacement field derived from sandbox experiments. Assuming a stationary spatial pattern of deformation, we simulate the gradual warping and uplift of stratigraphic and geomorphic markers, which provides an estimate of the cumulative amounts of shortening they have recorded. This approach allows modeling of isolated terraces or growth strata. We apply this method to the study of two fault tip folds in the Tien Shan, the Yakeng and Anjihai anticlines, documenting their deformation history over the past 6–7 Myr. We show that the modern shortening rates can be estimated from the width of the fold topography provided that the sedimentation rate is known, yielding respective rates of 2.15 and 1.12 mm/yr across Yakeng and Anjihai, consistent with the deformation recorded by fluvial and alluvial terraces. This study demonstrates that the shortening rates across both folds accelerated significantly since the onset of folding. It also illustrates the usefulness of a simple geometric folding model and highlights the importance of considering local interactions between tectonic deformation, sedimentation, and erosion

Comment on “Magnetostratigraphic study of the Kuche Depression, Tarim Basin, and Cenozoic uplift of the Tian Shan Range, Western China” Baochun Huang, John D.A. Piper, Shoutao Peng, Tao Liu, Zhong Li, Qingchen Wang, Rixiang Zhu

International audienceThe recent publication of “Magnetostratigraphic study of the Kuche Depression, Tarim Basin, and Cenozoic uplift of the Tian Shan Range,Western China” by B.C. Huang, J.D.A. Piper, S.T. Peng, T. Liu, Z. Li, Q.C. Wang, R.X. Zhu [Earth Planet. Sci. Lett., 2006, doi:10.1016/j.epsl.2006.09.020] discusses the Cenozoic uplift history of the Tianshan Mountains by studying the magnetostratigraphy of Paleogene to Neogene continental sediments from two sections located in the Kuche basin at the northern edge of the Tarim basin. To support their conclusion they reinterpreted a magnetostratigraphic study of the Yaha section, which lies ~ 10 km south of their sections, we previously published [J. Charreau, S. Gilder, Y. Chen, S. Dominguez, J.-P. Avouac, S. Sen, M. Jolivet, Y. Li and W. Wang, Magnetostratigraphy of the Yaha section, Tarim Basin (China): 11 Ma acceleration in erosion and uplift of the Tianshan Mountains, Geology 34(3), 2006, 181­184.]. Here, (1) we argue that the interpretations of the sedimentation rate changes they proposed for the Kuche sections are partially invalid, (2) we disagree with their reinterpretation of the age of the Yaha section, and (3) we think that the way they interpret their AMS data is incorrect

Addendum to “Late Cenozoic magnetochronology and paleoenvironmental changes in the northern foreland basin of the Tian Shan Mountains” by Jimin Sun, Qinghai Xu, and Baochun Huang

International audienceSun et al. (2007) constructed a magnetostratigraphic record of the Kuitun River section (Xinjiang Province, China) and reinterpreted a magnetostratigraphic record from the same section previously published by Charreau et al. (2005). In this paper, we show that Sun et al. miscorrelated their column with respect to that of Charreau et al. A compatible correlation, recognized by both sides, is presented

Neogene uplift of the Tian Shan Mountains observed in the magnetic record of the Jingou River section (northwest China)

The Tian Shan Mountains constitute central Asia's longest and highest mountain range. Understanding their Cenozoic uplift history thus bears on mountain building processes in general, and on how deformation has occurred under the influence of the India-Asia collision in particular. In order to help decipher the uplift history of the Tian Shan, we collected 970 samples for magnetostratigraphic analysis along a 4571-m-thick section at the Jingou River (Xinjiang Province, China). Stepwise alternating field and thermal demagnetization isolate a linear magnetization component that is interpreted as primary. From this component, a magnetostratigraphic column composed of 67 polarity chrons are correlated with the reference geomagnetic polarity timescale between ∼1 Ma and ∼23.6 Ma, with some uncertainty below ∼21 Ma. This correlation places precise temporal control on the Neogene stratigraphy of the southern Junggar Basin and provides evidence for two significant stepwise increases in sediment accumulation rate at ∼16–15 Ma and ∼11–10 Ma. Rock magnetic parameters also undergo important changes at ∼16–15 Ma and ∼11–10 Ma that correlate with changes in sedimentary depositional environments. Together with previous work, we conclude that growth history of the modern Tian Shan Mountains includes two pulses of uplift and erosion at ∼16–15 Ma and ∼11–10 Ma. Middle to upper Tertiary rocks around the Tian Shan record very young (<∼5 Ma) counterclockwise paleomagnetic rotations, on the order of 15° to 20°, which are interpreted as because of strain partitioning with a component of sinistral shear that localized rotations in the piedmont

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

Evolution tectonique du Tianshan au Cénozoïque liée à la collision Inde-Asie Apports de la magnétostratigraphie <br />et de la géochronologie isotopique U-Th/He.

The Tianshan range is one of the largest Asia's mountain ranges, with summits higher than 7000 m extending over an E-W distance of 2500 km. The Tianshan is the most active intracontinental range, however, its geology records a complex history of Paleozoic subduction-related processes, with a recent phase of Cenozoic reactivation induced by the India-Asia collision. The exact timing of this later reactivation is poorly constrained, which inhibits a better understanding of the evolution of continental deformation due to the indenting India. In addition, on both its northern and southern sides the Tianshan present piedmonts composed by series of fold and thrust belts with excellent surface exposures of Permian to Quaternary sediments in the adjacent Kuche and Junggar basins that present case examples of foreland basins. The history of mountain building with its related foreland sedimentation and erosion rate might be deciphered from the sediments well exposed along the piedmont.In the Neogene sediments of these basins, we carried out three magnetostratigraphic studies at the Yaha section located on the southern piedmont and the Kuitun and Jingou He sections on the northern piedmont. From stepwise demagnetization we found 18, 16, and 22 magnetic polarity chrons at the Yaha, Kuitun and Jingou He sections, which we correlated with a reference polarity time scale to date the sections between 5.2 and 12.6 Ma, 3.1 and 10.5 Ma, and 8.5 Ma and 22 Ma, respectively. At Yaha, sedimentation rate increases remarkably at ~ 11 Ma; Sedimentation rates areconstant at the Kuitun section while; the Jingou He is characterized by two important accelerations at ~15 Ma and ~11 Ma. We also performed measurements of rock magnetism to track time-transgressive changes in the sedimentary record. Several magnetic parameters, such as the anisotropy shape parameter (T) and bulk susceptibility (Κ) show obvious changes at around 21, 15 and 11 Ma on both sides of the range. Based on the changes in the rock magnetic parameters and in the sedimentation rate, we conclude that the central Tianshan Mountains likely began uplifting by 15 Ma and then underwent accelerated uplift at 11 Ma.To describe the kinematics and the kinetics of both the piedmonts and the high range we also combine structural model of the piedmont and foreland basin, with chronological constraints on the stratigraphy derived from these magnetostratigraphic sections and U-Th/He dating of detrital apatite and zircon grains to constraint : (1) the timing of development and rate of folding of the piedmont, (2) the flux history and propagation rates of sediment and (3) the erosion rates history of the high range.Le Tianshan est une des chaînes majeures d'Asie qui s'étend sur près de 2500 km de longitude avec des reliefs supérieurs à 7000 m. Il s'agit de l'orogène intracontinental le plus actif au monde issu d'une accrétion initiale au Paléozoïque suivie d'une réactivation majeure au Cénozoïque en réponse à la collision Inde-Asie. Mais l'âge exact d'initiation de la déformation cénozoïque reste mal contraint, ce qui est un obstacle important à la compréhension et à la détermination de l'évolution de la déformation en Asie. Les piémonts Nord et Sud du Tianshan sont constitués par deux remarquables prismes d'avant chaîne où les sédiments permiens au quaternaires des bassins flexuraux adjacents de Kuche et de la Junggarie sont très bien exposés. L'histoire de la construction du Tianshan peut être déterminée grâce à une étude de l'enregistrement sédimentaire facilement accessible au niveau des piémonts.Ainsi, nous avons réalisé trois études magnétostratigraphiques le long des coupes de la Kuitun et de la Jingou He au Nord et le long de celle de Yaha au sud pour lesquelles nous avons identifié respectivement 16, 22 et 18 chrons magnétiques que nous avons corrélés à l'échelle de référence donnant des âges compris entre, 3.1 et 10.5 Ma, 8.5 Ma et 22 Ma et 5.2 et 12.6 Ma respectivement. Les vitesses de sédimentations déduites de ces corrélations montrent une accélération à ~11 Ma sur la coupe de Yaha et celle de la Jingou He qui présente également une accélération antérieure à ~15 Ma alors que les vitesses restent stables depuis ~10 Ma sur la coupe Kuitun. Nous avons également réalisé des études de la minéralogie magnétique et des mesures de l'ASM qui montrent des changements importants de la susceptibilité magnétique (Κ) et du paramètre de forme (T) à 21, 15 et 11 Ma sur les deux piémonts. A partir des changements de vitesses de sédimentation, contraint par ailleurs par ceux des paramètres magnétiques, nous pensons que la surrection du Tianshan a probablement débuté vers 15 Ma puis s'est accélérée à ~11 Ma, ce qui est cohérent avec les études précédentes réalisées sur la zone suivant d'autres méthodes.A partir de modèles structuraux des piémonts et des basins de la Junggarie et de Kuche, associés aux âges magnétostratigraphiques et à des datations U-Th/He de grains d'apatites et zircons détritiques , nous déterminons également: (1) l'âge d'activation et les vitesses de déformation de certaines structures des piémonts, (2) les flux et vitesses sédimentaires et (3) l'évolution des taux d'érosion. Nous discutons et décrivons ainsi l'évolution cinétique et cinématique des piémonts et de la haute chaîne

Evolution tectonique du Tianshan au Cénozoïque liée à la collision Inde-Asie (apports de la magnétostratigraphie et de la géochronologie isotopique U-Th/He)

Afin de mieux contraindre l évolution tectonique du Tianshan au Cénozoïque, nous avons réalisé trois études magnétostratigraphiques le long des coupes de la Kuitun et de la Jingou He située sur le piémont Nord et le long de celle de Yaha sur le piémont sud. Ainsi nous avons identifié respectivement 16, 22 et 18 chrons magnétiques que nous avons corrélés à l échelle de référence donnant des âges compris entre, 3.1 et 10.5 Ma, 8.5 Ma et 22 Ma et 5.2 et 12.6 Ma respectivement. Les vitesses de sédimentations déduites de ces corrélations montrent une accélération à ~11 Ma sur la coupe de Yaha et celle de la Jingou He qui présente également une accélération antérieure à ~15 Ma alors que les vitesses restent stables depuis ~10 Ma sur la coupe Kuitun. Nous avons également réalisé des études de la minéralogie magnétique et des mesures de l ASM qui montrent des changements importants de la susceptibilité magnétique ( ) et du paramètre de forme (T) à 21, 15 et 11 Ma sur les deux piémonts. A partir des changements de vitesses de sédimentation, contraint par ailleurs par ceux des paramètres magnétiques, nous pensons que la surrection du Tianshan a probablement débuté vers 15 Ma puis s est accélérée à ~11 Ma. A partir de modèles structuraux des piémonts et des basins flexuraux, associés aux âges magnétostratigraphiques et à des datations U-Th/He de grains détritiques , nous déterminons également: (1) l âge d activation et les vitesses de déformation de certaines structures des piémonts, (2) les flux et vitesses sédimentaires et (3) l évolution des taux d érosion. Nous discutons et décrivons ainsi l évolution cinétique et cinématique des piémonts et de la haute chaîne.ORLEANS-BU Sciences (452342104) / SudocORLEANS-ISTO (452342307) / SudocSudocFranceF

Evidence of decoupled deformation during Jurassic rifting and Cenozoic inversion phases in the salt-rich Corbières-Languedoc Transfer Zone (Pyreneo-Provençal orogen, France)

A detailed field study of Jurassic tectono-stratigraphic architecture of the southwestern part of the Corbières-Languedoc Transfer Zone (CLTZ, NE-SW oriented), between the Pyrenean and Provençal orogenic segments (N110 oriented) in SE France, document for the first time variations in thickness and stratigraphic geometries in relation to oblique extensional cover structures (NE-SW and N110 oriented). These structures (low-dip normal faults, salt rollers, rollovers, forced folds) formed during a Jurassic extension phase with strong decoupling between basement and cover on the Keuper pre-rift salt (Carnian-Norian evaporites). Some of these structures such as the Treilles Fault, the Valdria and the Terres Noires fold pairs, were previously interpreted as compressional and Pyrenean in origin (Late Santonian-Bartonian). Our study instead shows that these are Jurassic extensional and salt related structures, which were later affected by Pyrenean compression and Oligo-Miocene extension. Evidence of Jurassic extension is still observable in the field despite later rectivations, making these good field analogs for gently inverted extensional salt structures. During the Jurassic the interference between oblique structures above Keuper, leads to the formation of three dimensional growth strata observable at kilometric scale. Despite the mechanical decoupling effect of Keuper, we infer that their formation was linked to a strong interaction between oblique basement structures (NE-SW and N110) as the area lies during the Jurassic at the intersection between the eastern part of the Pyrenean E-W trending rift system and the NE-SW trending European margin of the Alpine Tethys rift. The decoupled deformation and the strong segmentation above and below Keuper detachement are here identified as Jurassic structural inheritence for later tectonic events, notably for the Nappe des Corbières Orientales emplaced at the end of Pyrenean orogenesis. Associated to previous works, this study highlights that the CLTZ is a key area to better understand Pyreneo-Provençal system evolution along its whole Wilson cycle and to better understand the processes that govern the formation of a salt-rich transfer zone in a strongly pre-structured crust, its multiple reactivations and the decoupling role of salt

Sulfur-isotope anomalies recorded in Antarctic ice cores as a potential proxy for tracing past ozone layer depletion events

International audienceAbstract Changes in the cosmic-ray background of the Earth can impact the ozone layer. High-energy cosmic events (e.g., Supernova, SN) or rapid changes in the Earth's magnetic field (e.g., Geomagnetic Excursion, GE) can lead to a cascade of cosmic rays. Ensuing chemical reactions can then cause thinning/destruction of the ozone layer — leading to enhanced penetration of harmful UV radiation towards the Earth's surface. However, observational evidence for such UV ‘windows’ is still lacking. Here, we conduct a pilot study and investigate this notion during two well-known events: the multiple SN event (≈10 kBP) and the Laschamp GE event (≈41 kBP). We hypothesize that ice-core-Δ33S records—originally used as volcanic fingerprints—can reveal UV-induced background-tropospheric- photochemical imprints during such events. Indeed, we find non-volcanic S-isotopic anomalies (Δ33S≠0 ‰) in background Antarctic-ice-core sulfate during GE/SN periods, thereby confirming our hypothesis. This suggests that ice-core-Δ33S records can serve as a proxy for past ozone-layer-depletion events