Reconstructing Orogenesis Using Single-Crystal Dating

Burbank, Doug¹ (1) UCSB, Santa Barbara, CA

The advent of single-crystal dating of detrital grains in basins has enabled improved and increasingly diverse reconstructions of the orogenic history of mountain belts. Low-temperature thermochronometers, such as apatite fission-track or [U-Th]/He dating, are particularly useful for assessing variations in the spatial distribution of past erosion rates. “Lag times” between the age of cooling through the closure temperature and the time of deposition are commonly proportional to erosion rates. Changes in lag times can be used to assess thermal or exhumational steady state within the source area. For ranges with a known stratigraphy of bedrock cooling ages, erosion produces a cooling-age stratigraphy in a basin that can be inverted to define the progressive unroofing and lateral propagation of ranges.

For ancient basins, contributions from different source areas are commonly unresolvable from the detrital record alone. Studies of modern detrital systems permit quantification of the mixing that occurs among multiple source areas with diverse erosional and cooling histories. Modeling of the downstream mixing of dated grains reveals the contributions from subcatchments and emphasizes the interpretational intricacies of detrital foreland samples. Similarly, U-Pb ages of detrital zircons can be used to define discrete source areas and, when sufficiently distinctive age contrasts are present, the growth of individual mountain ranges can be discerned using detrital zircons in sedimentary basins.

Examples from the Tien Shan, Himalaya, and Tibet are used to illustrate ways to detect the growth and propagation of ranges, as well as assess variations in source-area contributions to sedimentary basins.