Exhumation, crustal deformation, and thermal structure of the Nepal Himalaya derived from the inversion of thermochronological and thermobarometric data and modeling of the topography

Two end‐member kinematic models of crustal shortening across the Himalaya are currently debated: one assumes localized thrusting along a single major thrust fault, the Main Himalayan Thrust (MHT) with nonuniform underplating due to duplexing, and the other advocates for out‐of‐sequence (OOS) thrusting in addition to thrusting along the MHT and underplating. We assess these two models based on the modeling of thermochronological, thermometric, and thermobarometric data from the central Nepal Himalaya. We complement a data set compiled from the literature with 114 ^(40)Ar/^(39)Ar, 10 apatite fission track, and 5 zircon (U‐Th)/He thermochronological data. The data are predicted using a thermokinematic model (PECUBE), and the model parameters are constrained using an inverse approach based on the Neighborhood Algorithm. The model parameters include geometric characteristics as well as overthrusting rates, radiogenic heat production in the High Himalayan Crystalline (HHC) sequence, the age of initiation of the duplex or of out-of-sequence thrusting. Both models can provide a satisfactory fit to the inverted data. However, the model with out-of-sequence thrusting implies an unrealistic convergence rate ≥30 mm yr^(−1). The out-of-sequence thrust model can be adjusted to fit the convergence rate and the thermochronological data if the Main Central Thrust zone is assigned a constant geometry and a dip angle of about 30° and a slip rate of <1 mm yr^(−1). In the duplex model, the 20 mm yr^(−1) convergence rate is partitioned between an overthrusting rate of 5.8 ± 1.4 mm yr^(−1) and an underthrusting rate of 14.2 ± 1.8 mm yr^(−1). Modern rock uplift rates are estimated to increase from about 0.9 ± 0.31 mm yr^(−1) in the Lesser Himalaya to 3.0 ± 0.9 mm yr^(−1) at the front of the high range, 86 ± 13 km from the Main Frontal Thrust. The effective friction coefficient is estimated to be 0.07 or smaller, and the radiogenic heat production of HHC units is estimated to be 2.2 ± 0.1 µWm^(−3). The midcrustal duplex initiated at 9.8 ± 1.7 Ma, leading to an increase of uplift rate at front of the High Himalaya from 0.9 ± 0.31 to 3.05 ± 0.9 mm yr^(−1). We also run 3-D models by coupling PECUBE with a landscape evolution model (CASCADE). This modeling shows that the effect of the evolving topography can explain a fraction of the scatter observed in the data but not all of it, suggesting that lateral variations of the kinematics of crustal deformation and exhumation are likely. It has been argued that the steep physiographic transition at the foot of the Greater Himalayan Sequence indicates OOS thrusting, but our results demonstrate that the best fit duplex model derived from the thermochronological and thermobarometric data reproduces the present morphology of the Nepal Himalaya equally well

Seasonal modulation of seismicity in the Himalaya of Nepal

International audience[1] For the period 1995 –2000, the Nepal seismic network recorded 37 ± 8% fewer earthquakes in the summer than in the winter; for local magnitudes ML > 2 to ML > 4 the percentage increases from 31% to 63% respectively. We show the probability of observing this by chance is less than 1%. We find that most surface loading phenomena are either too small, or have the wrong polarity to enhance winter seismicity. We consider enhanced Coulomb failure caused by a pore-pressure increase at seismogenic depths as a possible mechanism. For this to enhance winter seismicity, however, we find that fluid diffusion following surface hydraulic loading would need to be associated with a six-month phase lag, which we consider to be possible, though unlikely. We favor instead the suppression of summer seismicity caused by stress-loading accompanying monsoon rains in the Ganges and northern India, a mechanism that is discussed in a companion article

Convergence rate across the Nepal Himalaya and interseismic coupling on the Main Himalayan Thrust: Implications for seismic hazard

We document geodetic strain across the Nepal Himalaya using GPS times series from 30 stations in Nepal and southern Tibet, in addition to previously published campaign GPS points and leveling data and determine the pattern of interseismic coupling on the Main Himalayan Thrust fault (MHT). The noise on the daily GPS positions is modeled as a combination of white and colored noise, in order to infer secular velocities at the stations with consistent uncertainties. We then locate the pole of rotation of the Indian plate in the ITRF 2005 reference frame at longitude = − 1.34° ± 3.31°, latitude = 51.4° ± 0.3° with an angular velocity of Ω = 0.5029 ± 0.0072°/Myr. The pattern of coupling on the MHT is computed on a fault dipping 10° to the north and whose strike roughly follows the arcuate shape of the Himalaya. The model indicates that the MHT is locked from the surface to a distance of approximately 100 km down dip, corresponding to a depth of 15 to 20 km. In map view, the transition zone between the locked portion of the MHT and the portion which is creeping at the long term slip rate seems to be at the most a few tens of kilometers wide and coincides with the belt of midcrustal microseismicity underneath the Himalaya. According to a previous study based on thermokinematic modeling of thermochronological and thermobarometric data, this transition seems to happen in a zone where the temperature reaches 350°C. The convergence between India and South Tibet proceeds at a rate of 17.8 ± 0.5 mm/yr in central and eastern Nepal and 20.5 ± 1 mm/yr in western Nepal. The moment deficit due to locking of the MHT in the interseismic period accrues at a rate of 6.6 ± 0.4 × 10^(19) Nm/yr on the MHT underneath Nepal. For comparison, the moment released by the seismicity over the past 500 years, including 14 M_W ≥ 7 earthquakes with moment magnitudes up to 8.5, amounts to only 0.9 × 10^(19) Nm/yr, indicating a large deficit of seismic slip over that period or very infrequent large slow slip events. No large slow slip event has been observed however over the 20 years covered by geodetic measurements in the Nepal Himalaya. We discuss the magnitude and return period of M > 8 earthquakes required to balance the long term slip budget on the MHT

Mountain building in Taiwan: A thermokinematic model

The Taiwan mountain belt is classically viewed as a case example of a critical wedge growing essentially by frontal accretion and therefore submitted to distributed shortening. However, a number of observations call for a significant contribution of underplating to the growth of the orogenic wedge. We propose here a new thermokinematic model of the Taiwan mountain belt reconciling existing kinematic, thermometric and thermochronological constraints. In this model, shortening across the orogen is absorbed by slip on the most frontal faults of the foothills. Crustal thickening and exhumation are sustained by underplating beneath the easternmost portion of the wedge (Tananao Complex, TC), where the uplift rate is estimated to ~6.3 mm a^(−1), and beneath the westernmost internal region of the orogen (Hsueshan Range units, HR), where the uplift rate is estimated to ~4.2 mm a^(−1). Our model suggests that the TC units experienced a synchronous evolution along strike despite the southward propagation of the collision. It also indicates that they have reached a steady state in terms of cooling ages but not in terms of peak metamorphic temperatures. Exhumation of the HR units increases northward but has not yet reached an exhumational steady state. Presently, frontal accretion accounts for less than ~10% of the incoming flux of material into the orogen, although there is indication that it was contributing substantially more (~80%) before 4 Ma. The incoming flux of material accreted beneath the TC significantly increased 1.5 Ma ago. Our results also suggest that the flux of material accreted to the orogen corresponds to the top ~7 km of the upper crust of the underthrust Chinese margin. This indicates that a significant amount (~76%) of the underthrust material has been subducted into the mantle, probably because of the increase in density associated with metamorphism. We also show that the density distribution resulting from metamorphism within the orogenic wedge explains well the topography and the gravity field. By combining available geological data on the thermal and kinematic evolution of the wedge, our study sheds new light onto mountain building processes in Taiwan and allows for reappraising the initial structural architecture of the passive margin

Investigating the kinematics of mountain building in Taiwan from the spatiotemporal evolution of the foreland basin and western foothills

The Taiwanese range has resulted from the collision between the Luzon volcanic arc and the Chinese continental margin, which started about 6.5 Myr ago in the north, and has since propagated southward. The building of the range has been recorded in the spatiotemporal evolution of the foreland basin. We analyze this sedimentary record to place some constraints on the kinematics of crustal deformation. The flexure of the foreland under the load of the growing wedge started with a 1.5 Myr long phase of rapid subsidence and sedimentation, which has migrated southward over the last 3.5 Myr at a rate of 31 +10/−5 mm/yr, reflecting the structural evolution of the range and the growth of the topography during the oblique collision. Isopachs from the Toukoshan (~0 to 1.1 Ma) and Cholan (~1.1 to 3.3 Ma) formations, as well as the sedimentation rates retrieved from a well on the Pakuashan anticline, indicate that the foreland basement has been moving toward the center of mass of the orogen by ~45–50 mm/yr during the development of the basin. From there, we estimate the long-term shortening rate across the range to 39.5–44.5 mm/yr. By considering available data on the thrust faults of the foothills of central Taiwan, we show that most (if not all) the shortening across the range is accommodated by the most frontal structures, with little if any internal shortening within the wedge. The range growth appears therefore to have been essentially sustained by underplating rather than by frontal accretion. In addition, only the upper ~7 to 9 km of the underthrusted crust participates to the growth of the orogen. This requires that a significant amount of the Chinese passive margin crust is subducted beneath the Philippine Sea plate

Lower edge of locked Main Himalayan Thrust unzipped by the 2015 Gorkha earthquake

Large earthquakes are thought to release strain on previously locked faults. However, the details of how earthquakes are initiated, grow and terminate in relation to pre-seismically locked and creeping patches is unclear ^1-4. The 2015 Mw 7.8 Gorkha, Nepal earthquake occurred close to Kathmandu in a region where the prior pattern of fault locking is well documented ^5. Here we analyze this event using seismological records measured at teleseismic distances and Synthetic Aperture Radar imagery. We show that the earthquake originated northwest of Kathmandu within a cluster of background seismicity that fringes the bottom of the locked portion of the Main Himalayan Thrust fault (MHT). The rupture propagated eastwards for about 140 km, unzipping the lower edge of the locked portion of the fault. High-frequency seismic waves radiated continuously as the slip pulse propagated at about 2.8 km s-1 along this zone of presumably high and heterogeneous pre-¬seismic stress at the seismic-aseismic transition. Eastward unzipping of the fault resumed during the Mw 7.3 aftershock on May 12. The transfer of stress to neighbouring regions during the Gorkha earthquake should facilitate future rupture of the areas of the MHT adjacent and up-dip of the Gorkha earthquake rupture.This is the author accepted manuscript. The final version is available from Nature Publishing Group via http://dx.doi.org/10.1038/ngeo251

Linking the northern Alps with their foreland: The latest exhumation history resolved by low-temperature thermochronology

The evolution of the Central Alpine deformation front (Subalpine Molasse) and its undeformed foreland is recently debated because of their role for deciphering the late orogenic evolution of the Alps. Its latest exhumation history is poorly understood due to the lack of late Miocene to Pliocene sediments. We constrain the late Miocene to Pliocene history of this transitional zone with apatite fission track and (U-Th)/He data. We used laser ablation inductively coupled mass spectrometry for apatite fission track dating and compare this method with previously published and unpublished external detector method fission track data. Two investigated sections across tectonic slices show that the Subalpine Molasse was tectonically active after the onset of folding of the Jura Mountains. This is much younger than hitherto assumed. Thrusting occurred at 10, 8, 6–5 Ma and potentially thereafter. This is contemporaneous with reported exhumation of the External Crystalline Massifs in the central Alps. The Jura Mountains and the Subalpine Molasse used the same detachments as the External Crystalline Massifs and are therefore kinematically coupled. Estimates on the amount of shortening and thrust displacement corroborate this idea. We argue that the tectonic signal is related to active shortening during the late stage of orogenesis

Approches transversales : Des arts plastiques aux autres domaines d’apprentissages…

Professorat des collèges et lycéesCe présent mémoire s’attache, par le biais d’expériences pratiquées en cours, à rendre lisible différentes façons d’appréhender le cours d’arts plastiques sous le regard d’autres domaines d’apprentissages. Par une double approche : la transdisciplinarité et l’interdisciplinarité, nous tenterons de mettre en lumière différents modes de collaboration entre les arts plastiques et d’autres matières enseignées au collège, bref, les croisements et cohérences qui s’opèrent entre différentes disciplines

Déformation de l'Himalaya du Népal

La microsismicité népalaise présente de fortes variations latérales contrôlées par la topographie. Ce contrôle nous amène à estimer une valeur de la contrainte régionale puis les variations de contraintes de Coulomb présentes en profondeur. Le modèle de déformation intersismique qui en est déduit suggère qu'au premier ordre le grand axe de l'ellipsoïde de déformation tourne avec l'arc présentant des azimuts similaires aux glissements induits par les séismes et semblables à ceux des linéations du moyen pays himalayen. Mais, en quels termes le modèle du cycle sismique peut être extrapolé pour fabriquer le long terme? Pour répondre à cette question, nous nous sommes focalisés sur l'étude du moyen pays himalayen, large domaine accrété à la chaîne. Cette région a atteint des températures comprises entre moins de 300ʿC et 550ʿC. Le gradient de température apparent inverse mis en évidence est très élevé, compris entre 20 et 50ʿC/km, impliquant tout le moyen pays supérieur. L'histoire de son exhumation, tout comme son existence même, sont incompatibles avec une extrapolation pure et simple du modèle cinématique Holocène. La rampe mi-crustale du système chevauchant himalayen doit migrer, permettant ainsi une accrétion par sous-placage ductile. Les variations latérales du système sont reliées aux volumes de moyen pays sous-plaqués mais aussi à l'entretien de la localisation de la fenêtre d'accrétion. De nouvelles contraintes géo-thermochronologiques nous permettent d'évoquer un modèle de déformation long terme. Celui-ci présente un chevauchement de milieu chaud sur milieu froid associé à de l'advection des isothermes par érosion, des déformations de foot et hangingwall et du sous-placage, engendrant un diachronisme d'exhumation et un cisaillement suffisant pour expliquer les forts gradients inverses de température. Ce modèle d'évolution à déformation de foot/hangingwall et sous placage permet de réconcilier les approches aux différentes échelles abordées.Lateral variations of nepalese microseismicity are controlled by the topography. This control allows us to determine a regional stress field and calculate the Coulomb stress variations at depth. Our modeling suggests that the azimuth of horizontal shortening varies along the arc with the azimuths of the seismic slip and the lesser himalayan lineations. But, can we build the himalayas by extrapolating in the past present kinematics of the deformation? To address this question we have studied the lesser Himalayas accreted to the Himalayan range. Their finite thermal structure shows peak temperatures ranging between 300 and 550ʿC describing strong inverse temperature gradients from 20 to 50ʿC/km. Their existence and location cannot be suitable with the Holocene kinematic model but suggest that the midcrustal ramp of the main Himalayan Thrust might migrate allowing a ductile underplating. The lateral variations of the lesser Himalayas geometry can be therefore linked to the evolution of the accretionnary window. New geo-thermochronological data showing exhumation diachronisms add strong constraints to a long term deformation model. This model presents a thrusting of hot on cold medium associated with isotherm advections by erosion, deformation of foot and hangingwall and underplating, shearing, leading to the observed thermal structure and timing of exhumation. The kinematics of this theoretical accretionnary model involving underplating of the lesser himalayas is suitable with the short term models reconciling both scale descriptions.ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF

Thermal metamorphism in the lesser Himalaya of Nepal determined from Raman spectroscopy of carbonaceous material

The determination of metamorphic conditions is critical to the understanding of the formation of mountain belts. However, all collisional mountain belts contain large volumes of accreted sediments generally lacking metamorphic index minerals and are therefore not amenable to conventional petrologic investigations. By contrast, these units are often rich in carbonaceous material, making it possible to determine thermal metamorphism through Raman spectroscopy of carbonaceous material (RSCM method), a technique that has been recently calibrated [Beyssac et al., J. Metamorph. Geol. 20 (2002) 859–871]. The Lesser Himalaya (LH) is one of these problematic cases with a very poor mineralogy, but a key structural position within the Himalayan system that makes LH considered as diagnostic of the overall thermal behaviour of the orogen. This work demonstrates the performance of the RSCM technique and shows that this technique might thus be used to detect inter-sample variations as small as ~10–15 °C, but absolute temperatures can only be determined to ±50 °C due to the uncertainty on the calibration. This study reveals that the LH has undergone a large-scale thermal metamorphism, with temperature decreasing progressively from about 540 °C at the top to less than 330 °C within the deepest exhumed structural levels