| Detrital thermochronology attempts to decipher the thermotectonic history of orogens by studying the chronology of their erosional products. To recover reliable information about detrital thermochronological provenance, knowledge of the effects that post-erosional processes might have on the age distribution is required. The most elementary of these processes is associated with sampling. Because detrital thermochronology is expensive and time-consuming, an essentially infinite detrital population must be approximated by a relatively small sample. To be 95% certain that no fraction greater than 0.05 of a provenance population is missed, at least 117 grains must be dated. The resulting grain-age distribution of such a small sample is typically represented as a histogram, a visual representation of a multinomial distribution. The statistical uncertainty associated with such a rough approximation to the detrital population can be assessed by reporting simultaneous Bayesian credibility bounds for the histogram containing, for example, 95% of the posterior Dirichlet distribution. Detrital thermochronology greatly benefits by applying multiple techniques, as illustrated by combining U-Pb and fission-track dating on detrital zircon and apatite, as well as vitrinite reflectance measurements to a series of samples from the Great Valley Group (GVG) near New Idria, California. These data contain both pre- and post-depositional information. Short lag-times between the cooling ages and the timing of deposition reveal rapid exhumation and cooling of the Cretaceous Sierra Nevada batholith, the source area of the GVG. Bimodal fission track age distributions reveal a mid-Miocene heating event associated with the passage of the Mendocino triple junction and the rapid exhumation of the New Idria serpentinite dome. Apart from thermotectonic events and sampling processes, detrital thermochronological age distributions can be affected by non-uniform erosion. The cumulative distribution of detrital apatite fission track ages of a simple drainage in the northern White Mountains (California) shows that erosion is most active at low elevations, with exponentially smaller contributions from higher up in the drainage basin. |
