P. van der Beek

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.

Cenozoic unroofing history of the Ladakh Batholith, western Himalaya, constrained by thermochronology and numerical modelling

Abstract: The Ladakh Batholith is part of the Transhimalayan Plutonic Belt, which crops out north of the Indus Suture Zone. We propose that the exhumation history of the Ladakh Batholith is linked to the tectonic, magmatic and erosion history of the Karakoram terrane and SW Tibet. We present new multiple low-temperature thermochronometry data (zircon (U–Th)/He, apatite fission-track and apatite (U–Th)/He) to gain insight into the cooling history of the Ladakh Batholith and recognize key periods in the evolution of the region. From the Indus Valley northwards the ages decrease across the batholith for all three thermochronometers applied. A model is proposed in which magmatism in the Ladakh Batholith ceased in the Late Eocene and initial denudation was driven by topographic uplift caused by collision. Southward tilting of the batholith occurred in the Late Palaeogene. This tilting resulted in an asymmetric topography with increasing elevation to the north. Strong erosion occurred in this northern region whereas the southern margin was affected by northwards thrusting of the Indus Molasse. For the first time, clear temporal and spatial variations in exhumation rate are identified in this region, highlighting why sampling strategy is critical in documenting exhumation changes in active tectonic settings. Supplementary material: Sample co-ordinates from the Ladakh Batholith, sample preparation and fission-track age measurements of apatite grains and grain size dimensions are available at http://www.geolsoc.org.uk/SUP18350.