No abstract

[1] The Himalayan range is commonly presented as largely laterally uniform from west to east. However, geological structures, topography, precipitation rate, convergence rates, and low-temperature thermochronological ages all vary significantly along strike. Here, we focus on the interpretation of thermochronological data sets in terms of along-strike variations in geometry and kinematics of the main crustal detachment underlying the Himalaya: the Main Himalayan Thrust (MHT). We report new apatite fission track (AFT) ages collected along north-south transects in western and eastern central Nepal (at the latitudes of the Annapurna and Langtang massifs, respectively). AFT ages are consistently young (<3 Ma) along both N-S transects in the high-relief zone of the Higher Himalaya and increase (4 to 6 Ma) toward the south in the Lesser Himalaya. We compare our new data to published low-temperature thermochronological data sets for Nepal and the Bhutan Himalaya. We use the full data set to perform both forward and inverse thermal kinematic modeling with a modified version of the Pecube code in order to constrain potential along-strike variations in the kinematics of the Himalayan range. Our results show that lateral variations in the geometry of the MHT (in particular the presence or absence of a major crustal-scale ramp) strongly control the kinematics and exhumation history of the orogen.Citation: Robert, X., P. van der Beek, J. Braun, C. Perry, and J.-L. Mugnier (2011), Control of detachment geometry on lateral variations in exhumation rates in the Himalaya: Insights from low-temperature thermochronology and numerical modeling,

ABSTRACT

International audienceWe 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

Competing hypotheses suggest that Himalayan topography is sustained and the plate convergence is accommodated either solely along the basal décollement, the Main Himalayan thrust (MHT), or more broadly, across multiple thrust faults. In the past, structural, geomorphic, and geodetic data of the Nepalese Himalaya have been used to constrain the geometry of the MHT and its shallow frontal thrust fault, known as Main Frontal thrust (MFT). The MHT flattens at depth and connects to a hinterland mid-crustal, steeper thrust ramp, located ~100 km north of the deformation front. There, the present-day convergence across the Himalaya is mostly accommodated by slip along the MFT. Despite a general agreement that in Nepal most of the shortening is accommodated along the MHT, some researchers have suggested the occurrence of persistent out-of-sequence shortening on interior faults near the Main Central thrust (MCT). Along the northwest Himalaya, in contrast, some of these characteristics of central Nepal are missing, suggesting along-strike variation of wedge deformation and MHT fault geometry. Here we present new field observations and seven zircon (U-Th)/He (ZHe) cooling ages combined with existing low-temperature data sets. In agreement with our previous findings, we suggest that the transect of cooling age patterns across the frontal Dhauladhar Range reveals that the Main Boundary thrust (MBT) is a primary fault, which has uplifted and sustained this spectacular mountain front since at least the late Miocene. Our results suggest that the MBT forms an ~40-km-long fault ramp before it soles into the MHT, and motion along it has exhumed rocks from depth of ~8-10 km. New three-dimensional thermokinematic modeling (using Pecube finite-element code) reveals that the observed ZHe and apatite fission track cooling ages can only be explained by sustained mean MBT slip rates between ~2.6 and 3.5 mm a-1 since at least 8 Ma, which corresponds to a horizontal shortening rate of ~1.7-2.4 mm a-1. We propose that the MBT is active today, despite a lack of definitive field or seismogenic evidence, and continues to accommodate crustal shorting by out-of-sequence faulting. Assuming that present-day geodetic shorting rates (~14 ± 2 mm a-1) across the northwest Himalaya have been sustained over geologic time scales, this implies that the MBT accommodated ~15% of the total Himalayan convergence since its onset. Furthermore, our modeling results imply that the MHT is missing a hinterland mid-crustal ramp further north.

[1] Geological observations of mantle flow-driven dynamic topography are numerous, especially in the stratigraphy of sedimentary basins; on the contrary, when it leads to subaerial exposure of rocks, dynamic topography must be substantially eroded to leave a noticeable trace in the geological record. Here, we demonstrate that despite its low amplitude and long wavelength and thus very low slopes, dynamic topography is efficiently eroded by fluvial erosion, providing that drainage is strongly perturbed by the mantle flow driven surface uplift. Using simple scaling arguments, as well as a very efficient surface processes model, we show that dynamic topography erodes in direct proportion to its wavelength. We demonstrate that the recent deep erosion experienced in the Colorado Plateau and in central Patagonia is likely to be related to the passage of a wave of dynamic topography generated by mantle upwelling.

We have investigated whether low temperature thermochronological datasets can be used to constrain the rate of surface evolution during the post-orogenic phase of a mountain belt. We use a numerical method to solve the heat transport equation in the Earth's crust, including the effects of a changing, finite-amplitude topography and the resulting flexural isostatic rebound. We demonstrate that accurate estimates of the amount of relief loss can be obtained by applying a recently developed spectral method that is based on estimates of the relationship between age and surface elevation as a function of topographic wavelength. We also show that the rate at which topography decays with time following cessation of tectonic activity can be constrained from estimates of exhumation rate derived from the slope of ageelevation profiles collected across short wavelength topography. Using the Neighbourhood Algorithm to perform a thorough search through parameter space, we are able to find a tectonomorphic scenario that predicts age distributions compatible with a thermochronological dataset collected in the Dabie Shan of eastern China by Reiners et al. (American Journal of Science 2003, vol. 303, pp. 489 -518). We demonstrate that, in the Dabie Shan, the mean topographic relief has decreased by a factor of 2·5 to 4·5 during the last 60-80 Ma, while the mountain belt experienced a mean exhumation rate of 0·01 to 0·04 km Ma − − − − −1 . We confirm the conclusions of Reiners et al. that there is no need to invoke a discrete Cenozoic tectonic event to explain the observed age distribution. The thermochronological dataset can also be used to put constraints on the effective elastic thickness of the lithosphere underlying the orogen (10 to 30 km). There is, however, a trade-off between elastic thickness, mean exhumation rate and amount of topographic relief loss. The most likely scenario also predicts that the topography has decreased at a constant rate since the end of orogenic activity about 100 Ma ago.

Many islands of the eastern Indonesian Archipelago exhibit Late Cenozoic sequences of coral reef terraces. In SE Sulawesi, on the Tukang Besi and Buton archipelagos, we identified 23 islands bearing such sequences. Remote sensing imagery and field mapping combined to U/Th and 14 C dating enable to establish a chronologic framework of the reef terrace sequences from Wangi-Wangi, Buton as well as on the neighbouring, smaller islands of Ular, Siumpu and Kadatua. We identified the terraces from the last interglacial maximum (MIS 5e) at elevations lower than 20 m except on W Kadatua where it is raised at 34 ± 5 m. Such elevations yield low to moderate Upper Pleistocene uplift rates (<0.3 mm yr À1). On SE Buton Island, a sequence culminates at 650 m and includes at least 40 undated strandlines. Next to this exceptional sequence, on the Sampolawa Peninsula, 18 strandlines culminate at 430 m. Dated samples at the base of this sequence (<40 m) yield mean Middle Pleistocene uplift rates of 0.14 ± 0.09 mm yr À1. Extrapolation of these uplift rates compared to the geological setting suggests that the sequences of the Sampolawa Peninsula provide a record of sea-level high-stands for the last 3.8 ± 0.6 Ma. The sequences on SE Buton Island therefore constitute the best preserved long-lasting geomorphic record of Plio-Quaternary sea-level stands worldwide.