Combined uranium series and ~(10)Be cosmogenic exposure dating of surface abandonment: A case study from the ?lgiy strike-slip fault in western Mongolia

摘要

Time-averaged fault slip-rates can be established by reliably dating the abandonment of an alluvial deposit that has been displaced by Quaternary movement along a cross-cutting fault. Unfortunately, many Quaternary dating techniques are hindered by uncertainties inherent to individual geochronometers. Such uncertainties can be minimised by combining multiple independent techniques. In this study, we combine ~(10)Be exposure dating of boulder tops and U-series dating of layered pedogenic carbonate cements accumulated on the underside of clasts from two separate alluvial surfaces. These surfaces are both displaced by the active ?lgiy strike-slip fault in the Mongolian Altay Mountains. We date individual layers of pedogenic carbonate, and for the first time apply a Bayesian statistical analysis to the results to develop a history of carbonate accumulation. Our approach to the U-series dating provides an age of initiation of carbonate cement formation and avoids the problem of averaging contributions from younger layers within the carbonate. The U-series ages make it possible to distinguish ~(10)Be samples that have anomalously young exposure ages and have hence been subject to the effects of postdepositional erosion or exhumation. The combination of ~(10)Be and U-series dating methods provides better constrained age estimates than using either method in isolation and allows us to bracket the abandonment ages of the two surfaces as 18.0-28.1 kyr and 38.4-76.4 kyr. Our ages, combined with measurements of the displacement of the surfaces, yield a right-lateral slip-rate for the ?lgiy fault of 0.3-1.3 mm yr~(-1), showing that it is a relatively important structure within the active tectonics of Mongolia and that it constitutes a substantial hazard to local populations.