COBBLE THERMOCHRONOLOGY AND ITS APPLICATION TO CONSTRAINING HINTERLAND EXHUMATION: LESSONS LEARNT FROM THE PYRENEES AND SOUTH-CENTRAL ALASKA

Geochronology or thermochronology on single grains from modern sediment or sandstone within a stratigraphic sequence, is often used to constrain provenance or the exhumation history of the source region. The lag-time approach, with appropriate assumptions can be used to constrain the exhumation rate of the source region. Single-grain ages determined using apatite fission-track (AFT) thermochronology usually have large uncertainties, with insufficient track-length measurements to be meaningful. Applying AFT to cobbles is a powerful alternative approach. As all grains share a common thermal history, age and track-length data (the kinetic parameter) plus Dpar, permits multi-kinetic inverse thermal modelling. This allows the cooling/exhumation histories for cobbles to be constrained, and hence if the lag-time approach to constrain exhumation rates are valid. Provenance may be better inferred based on cobble lithology, and multiple methods applied to multiple minerals may constrain provenance and/or the thermal evolution over a broader temperature interval. If cobbles are buried to temperatures within the AFT partial annealing zone, timing of basin inversion can be inferred. We demonstrate the strengths and limitations of this cobble thermochronology approach using two examples: (1) Eocene-to-Oligocene syn-tectonic continental conglomerates of the South-Central Pyrenees, 3-4 km thick. There, convergence is largely orthogonal and the cobble approach works extremely well. AFT thermochronology on cobbles from 15 Myr of strata record three episodes of cooling/exhumation in the hinterland due to progressive movement of thrust sheets, followed by burial and Late Miocene re-excavation. (2) Late Miocene strata south of the east-central Denali fault of south-central Alaska. This small transpressional foreland basin with >500 m of sediment deposited over ~3 Myr formed due to thrusting on the McCallum Creek fault. Inverse thermal models on cobbles constrain episodic cooling and exhumation, from early Cretaceous to the Late Cenozoic, but the cooling/exhumation histories are unrelated to first-cycle erosion of basement rock in the Late Miocene. Overall, tectonics in this region are dominated by strike-slip along the Denali fault system, the basins are localized and the lag-time approach is not applicable.