Erosion rates in the source region of an ancient sediment routing system: comparison of depositional volumes with thermochronometric estimates

Calculation of the total depositional volume of an ancient source-to-sink system, combined with estimates of the area of catchments acting as source regions using provenance methods, is used to evaluate average catchment erosion rates on a million year time scale. These rates are compared with values derived from thermochronological methods. Using the mid- to late Eocene (33.9–41.6 Ma) Escanilla palaeo-sediment routing system from the south–central Pyrenean orogenic wedge-top zone as an example, c. 3500 ± 300 km³ of solid particulate sediment was derived from two catchments in the south–central Pyrenees over a 7.7 myr period, equivalent to a mean erosion rate of c. 0.15–0.18 mm a⁻¹. Average exhumation rates in contributing catchments over the same time interval are estimated at 0.2–0.3 mm a⁻¹ based on apatite fission-track analysis of pebbles in proximal conglomerates, and 0.23–0.34 mm a⁻¹ from fission-track analysis of detrital apatites sampling a wider range of grain size. Sediment supply progressively increased during the mid- to late Eocene time period, at least in part driven by catchment expansion deep into the Pyrenean Axial Zone at c. 39 Ma. The consistency of the rates of catchment-averaged erosion calculated from different methods builds confidence that source areas have been connected to depositional sinks correctly.