Applications of ancient cosmic-ray exposures: Theory, techniques and limitations

This study is a review of terrestrial cosmogenic nuclides (TCN) applications based on the specific circumstance of ancient exposure records, i.e. formations that have been exposed in the past before being rapidly and completely shielded by burial. The potentialities offered by ancient exposures are theoretically able to address a wide range of geosciences issues, such as (i) paleoaltimetry, (ii) determination of paleoerosion rates (both at the local and catchment scales) and (iii) estimation of lava flow emission rates. Based on accumulated TCN, the methodology required by these applications must however rely on precise geological targets (e.g. superimposed lava flows, sequestered sediments) which must meet certain conditions (e.g. erosion negligible, recent exposure negligible) and on which several independent measurements can be performed (e.g. independent estimate of the ancient exposure duration). Under favorable conditions, a careful evaluation of the analytical limitations and a rigorous sampling strategy may nevertheless allow a useful precision to be reached, from several kyr up to several Myr time scales. Thus, challenging issues might be assessed by these methods, as for example, (i) uplift or subsidence histories, (ii) the forcing effect of climatic variations on erosion, or (iii) the setting time of huge magmatic provinces.