Jean-Louis Mugnier

Thrust geometry controlled by erosion and sedimentation: A view from analogue models

Analogue sandbox models have been set up to study thrust systems formed above two decollements and affected by erosion and sedimentation. The deformation was imaged by X-ray scanner. Three erosion-sedimentation conditions were tested. For all the experiments, a duplex develops between the two decollement levels. For the experiment without erosion and without sedimentation, ramp anticlines form in the upper series. For the model with syntectonic sedimentation and without erosion, the shortening in the upper series is first characterized by fault-related folds and then by a forward propagation of the decollement that induces the development of a piggy-back basin. For the experiment with erosion and sedimentation, a “passive roof duplex” is formed. Relationships between structural style and syntectonic superficial conditions in classical thin-skinned thrust belts are similar to those found in the analogue models: the ramp anticlines of the Jura developed with low erosion and low sedimentation and the large sub-Andean thrust belt is controlled by syntectonic sedimentation, whereas the passive roof duplex of Pakistan is synchronized with intense erosion. This study outlines the crucial effect of the superficial mass-transport phenomena in the tectonic style of thrust belts.

Long-term fluvial incision rates and postglacial river relaxation time in the French Western Alps from 10Be dating of alluvial terraces with assessment of inheritance, soil development and wind ablation effects

Studying river long-profile development as a response to tectonic and climatic controls requires reliable age-dating of paleo-profile remnants preserved as river terraces. Cosmic Ray Exposure (CRE) dating often represents the only method available to date river terraces, but the interpretation of cosmogenic nuclide concentrations is complicated by pre-depositional inheritance and post-depositional disturbance of the terrace deposits through pedogenesis and surface inflation or deflation. Here, we use cosmogenic 10Be measurements to date alluvial terraces in the French western Alps, in order to estimate river incision rates and to infer river response to climatic fluctuations and tectonic forcing. We assess inheritance by constructing 10Be concentration vs. pebble depth profiles and use a Monte-Carlo technique to estimate terrace ages. We find that inheritance is negligible on all terraces, enabling us to date terraces as young as 5 ky. Terraces that predate the last glaciation experienced intense pedogenesis and wind ablation which led to significant scatter in the 10Be concentrations of surface samples. We assess these effects using a model of 10Be ingrowth and show that the oldest CRE ages of surface clasts are close to the probable terrace age. We study two catchments which have undergone varying degrees of glacial disruption. The Buech River experienced variations in runoff and sediment flux during the last glaciation, as well as occasional ice-dammings of its outlet. Its upper terraces record incision rates averaged over 190 ky of ~0.8 mm/yr, consistent with denudation rates estimated in the surrounding areas and suggesting long-term stability of river incision rates. Climatic forcing is well documented for the Drac River, which was repeatedly dammed by glaciers during cold periods. Its postglacial incision history was triggered by an 800-m drop in base level following ice-dam disappearance. Long-profile development by knickpoint propagation explains the time-lag of 2-5 ky between this base-level drop and terrace abandonment upstream, as well as subsequent peak incision rates of >6 cm/yr followed by a gradual decrease in incision rates. The present knickpoint location, ~55 km upstream from the glacial damming site, enables us to calculate a fluvial response time of 15-20 ky, controlled by knickpoint propagation rates of several meters per year, within the Drac River.

Propagation of the thrust system and erosion in the Lesser Himalaya: Geochemical and sedimentological evidence

Centre de Recherches Pe´trographiques et Ge´ochimiques, BP 20, 54501 Vandoeuvre-les-Nancy,FranceABSTRACTSedimentological and Nd isotope data of two sections of the sub-Himalaya of westernNepal are used as new constraints for understanding the erosion history of the Himalaya.Throughout the deposition of the middle and upper members of the Siwalik Group, theLesser Himalaya contribution to the total detrital input progressively increased from lessthan 20% to 40%. The increasing proportion of Lesser Himalaya sediments started at ca.10–8 Ma and is associated with a coarsening of the maximum grain size at both micro-scopic and macroscopic scales. Thin-skinned tectonics of the Lesser Himalaya thrust sys-tem would have controlled the exhumation of the Lesser Himalaya rocks and would havebegun at 12–10 Ma, taking into account the delay for denudation. Together with otherstudies, these data restrict the onset of movement on the Lesser Himalaya thrust systemto less than 3 m.y. along more than 1750 km. This short time frame implies a ratio oflateral propagation rate to shortening rate far too high for the propagation of a singlecrustal thrust; thus we suggest instead the simultaneous initiation of several thrusts aheadof the Main Central thrust at ca. 12 Ma. We suggest that a rapid rise of the TibetanPlateau at this time has transformed the Himalaya to an overcritical thrust wedge thathas propagated forward to return to a stable state. This regional rising could be the primecause of the increase of sediment influx at ca. 11 Ma around the Himalaya.Keywords: Siwalik formations, neodymium, grain size, erosion, thrust sheets, Himalaya.

A Rouse‐based method to integrate the chemical composition of river sediments: Application to the Ganga basin

[1] The Ganga River is one of the main conveyors of sediments produced by Himalayan erosion. Determining the flux of elements transported through the system is essential to understand the dynamics of the basin. This is hampered by the chemical heterogeneity of sediments observed both in the water column and under variable hydrodynamic conditions. Using Acoustic Doppler Current Profiler (ADCP) acquisitions with sediment depth profile sampling of the Ganga in Bangladesh we build a simple model to derive the annual flux and grain size distributions of the sediments. The model shows that ca. 390 (±30) Mt of sediments are transported on average each year through the Ganga at Haring Bridge (Bangladesh). Modeled average sediment grain size parameters D 50 and D 84 are 27 (±4) and 123 (±9) mm, respectively. Grain size parameters are used to infer average chemical compositions of the sediments owing to a strong grain size chemical composition relation. The integrated sediment flux is characterized by low Al/Si and Fe/Si ratios that are close to those inferred for the Himalayan crust. This implies that only limited sequestration occurs in the Gangetic floodplain. The stored sediment flux is estimated to c.a. 10% of the initial Himalayan sediment flux by geochemical mass balance. The associated, globally averaged sedimentation rates in the floodplain are found to be ca. 0.08 mm/yr and yield average Himalayan erosion rate of ca. 0.9 mm/yr. This study stresses the need to carefully address the average composition of river sediments before solving large‐scale geochemical budgets. Citation: Lupker, M., C. France‐Lanord, J. Lave, J. Bouchez, V. Galy, F. Metivier, J. Gaillardet, B. Lartiges, and J.-L. Mugnier (2011), A Rouse‐based method to integrate the chemical composition of river sediments: Application to the Ganga basin,

Piggyback basin development above a thin-skinned thrust belt with two detachment levels as a function of interactions between tectonic and superficial mass transfer: the case of the Subandean Zone (Bolivia)

Abstract The Subandean fold and thrust belt of Bolivia is characterised by two major detachment levels and large piggyback basins. ‘Sand-box’ and numerical models have been used to study sedimentation and erosion control on thrust belt evolution and to study the retroactive effects of tectonics on piggyback development in thin-skinned thrust belts with two detachment levels. Analogue models show that surface processes play a dominant role in controlling wedge evolutions: erosion promotes fault reactivation and tectonic delamination (passive roof duplex) while sedimentation promotes forward shifting of the frontal thrust and consequently piggyback basin development. Numerical models were used to understand the development of the Subandean fold and thrust belt of Bolivia. Numerical experiments show that the simultaneity of basement tilting and high sedimentation rates promotes the formation of a stable tectonic wedge. Outer and inner faults are alternately active during the beginning of deformation, a kinematic evolution that favours the development of piggyback basins between. The step-by-step history of the thrust belt predicts that each change in tectonic location is recorded with large unconformities in basins, but these unconformities are not well preserved from progressive erosion in the final geometry.

Assessing Quaternary reactivation of the Main Central thrust zone (central Nepal Himalaya): New thermochronologic data and numerical modeling

We study the recent dynamics of the central Nepal Himalaya, focusing on possible reactivation of the footwall of the Main Central thrust, which is marked by an abrupt topographic transition. Different tectonic mechanisms, such as overthrusting of a major crustal ramp, underplating, or out-of-sequence thrusting, have been suggested to explain the morphology and exhumation patterns in this area. We present 25 new apatite fission-track ages collected along a north-south transect in central Nepal, as well as two age-elevation profiles. Ages are consistently younger than 3 Ma old in the Main Central thrust zone and increase continuously to 4–6 Ma old in the south. No jump in apatite fission-track ages is observed across the topographic transition. Apparent exhumation rates from age-elevation relationships vary from 0.46 +0.13/−0.09 km/Ma in the Palung granite south of Kathmandu to 4.4 +4.8/−1.5 km/Ma in the Main Central thrust zone; the latter rate is probably overestimated by a factor of two due to topographic effects. As shown by a new numerical model, these strongly varying exhumation rates can be explained by overthrusting of a crustal ramp, which exerts a primary control on age patterns, and do not require out-of-sequence reactivation of thrusts in the Main Central thrust zone.

Control of detachment dip on drainage development in regions of active fault-propagation folding

We have studied drainage development on active folds by using a numerical model that couples the kinematics of fault-propagation folding to a physical description of surface processes. Modeling results suggest that the dip of the underlying detachment exerts a major control on drainage development. For a nonzero detachment dip, the lateral displacement gradient sets up an axial slope behind the growing fold that diverts drainage so that the fold is shielded from rivers with large incision capacity. In this case, the characteristic fault-segment length, rather than the relative rates of fold uplift and incision, may control the spacing of transverse streams. We apply our model to the foothills of the Nepal Himalayas and suggest that striking differences in drainage patterns between central and western Nepal are controlled to a first order by variations in the dip of the underthrusting Indian plate.

Recent movements along the Main Boundary Thrust of the Himalayas: Normal faulting in an over-critical thrust wedge?

Abstract The Main Boundary Thrust (MBT) is one of the major Himalayan thrusts occurring during the Cainozoic, and it is presently incorporated within the Himalayan thrust wedge (Lesser and Outer Himalayas) displaced above the Indian lithosphere. Nonetheless the MBT shows recent normal displacement along most of its length. We suggest that the orientation of the major principal stress within the Himalayan thrust wedge deviates significantly from the horizontal and when this deviation exceeds the dip of the vectors normal to back-tilted thrusts, the normal component of displacement may act along these faults. Steep north-dipping segments of the MBT therefore show a normal component of displacement if a geometrical definition is used, but they are faults in a compressional regime where the major principal stress axis has deviated from the horizontal. Micro-structural data recorded along the Surkhet-Ghorahi segment of the MBT are consistent with a strong deviation of the state of stress. The presence of such peculiar normal faulting along the MBT is used to calibrate the mechanical characteristics of the belt considered as a Coulomb wedge. The following characteristics are suggested: (a) very poor strength contrast between basal decollement and rocks in the wedge body, (b) a high pore fluid pressure ratio (probably close to 0.8–0.9) and a higher fluid pressure ratio (close to 1.0) along the active normal faults if a high internal friction angle (close to the Byerlee value) is considered. The strong deviation in principal stress direction may have recently increased, due to a taper of the Himalayan wedge exceeding the stability boundary and may be controlled by erosion and isostatic uplift rebound of the Himalayan range.

Monitoring Temperate Glacier Displacement by Multi-Temporal TerraSAR-X Images and Continuous GPS Measurements

A new generation of space-borne SAR sensors were launched in 2006-2007 with ALOS, TerraSAR-X, COSMO-Sky-Med and RadarSat-2 satellites. The data available in different bands (L, C and X bands), with High Resolution (HR) or multi-polarization modes offer new possibilities to monitor glacier displacement and surface evolution by SAR remote sensing. In this paper, the first results obtained with TerraSAR-X HR SAR image time series acquired over the temperate glaciers of the Chamonix Mont-Blanc test site are presented. This area involves well-known temperate glaciers which have been monitored and instrumented i.e. stakes for annual displacement/ablation, GPS for surface displacement and cavitometer for basal displacement, for more than 50 years. The potential of 11-day repeated X-band HR SAR data for Alpine glacier monitoring is investigated by a combined use of in situ measurements and multi-temporal images. Interpretations of HR images, analysis of interferometric pairs and performance assessments of target/texture tracking methods for glacier motion estimation are presented. The results obtained with four time series covering the Chamonix Mont-Blanc glaciers over one year show that the phase information is rarely preserved after 11 days on such glaciers, whereas the high resolution intensity information allows the main glacier features to be observed and displacement fields on the textured areas to be derived.

Ganges basin geometry records a pre-15 Ma isostatic rebound of Himalaya

The Tertiary continental strata of the Himalayan foreland basin are subdivided into two groups, but the meaning of this subdivision was previously unclear. From the analysis of drill holes, seismic lines, dated sections, outcrops, and balanced cross sections, we find that the southward migration rate of the depositional pinch-out of the younger group is 19 ± 5 mm/yr and equals the Himalayan shortening rate. This equality shows that the flexural foreland basin development is mainly controlled by the motion of the thrust load. The long-term pinch-out migration rate was slower for the older synorogenic group. Erosion locally occurred at the end of its deposition, due to tectonic reactivation of lineaments of the Indian shield. We suggest that this change in the basin development is linked to the detachment of the subducted Indian lithosphere that decreased the slab pull and increased the mean compressive stress within the Indian plate, whereas the plate motion remained constant. The most important implication of our work is that the associated isostatic rebound could have increased the Himalayan elevation prior to 15 Ma.