READING THE RECORD OF OROGENESIS THROUGH DETRITAL FISSION-TRACK THERMOCHRONOLOGY

Orogenic belts deliver a record of their long-term evolution to flanking sedimentary basins, and in many settings these sedimentary basins retain the only record of long-term orogenesis. Reading this record from sediments depends first on understanding the evolution of the sedimentary basin and then on extracting thermochronological information from the strata. Reassembling the thermochronological information from sediments involves a few simplifying assumptions about the sediment transport, depositional setting, and diagenetic and thermal evolution of the basin. These simplifying assumptions place some practical limitations on the method. Here we highlight some important aspects of detrital thermochronology using several examples. The geodynamic evolution of the orogenic belt and the material flux through the orogen affects sediment delivery off the orogenic belt. An excellent example is the Southern Alps in New Zealand. Extremely high exhumation rates on the west side of the orogen are clearly reflected in the sediment shed westward, but the east side is essentially transparent to orogenic exhumation in that sediment has virtually no thermochronologic record of the ongoing orogenic event. In most orogenic settings, the young population of grain ages is of primary consideration because these rapidly cooled grains record the exhumation of the fastest-evolving part of the orogenic belt. In many settings, however, interpretation and quantification of this age component is complicated by the presence of volcanic grains that have young cooling ages unrelated to orogenic exhumation. Double dating (FT and U/Pb) is a powerful emerging method whereby these grains can be identified, as seen in two examples from the US Cordillera.