CONSTRAINTS ON RIFT-RELATED EXHUMATION IN THE FLANKS OF THE WESTERN SNAKE RIVER PLAIN, IDAHO, FROM APATITE (U-TH)/HE THERMOCHRONOMETRY
New AHe dates are mostly reproducible and mean dates range from 16-49 Ma. All of the cooling dates from the Owyhee Mountains and most dates from the Boise Mountains are significantly older than rifting. The youngest dates (~15-20 Ma) are from the Challis intrusives in the Boise Mountains. These older dates indicate that most exhumation occurred well before rifting and Neogene volcanism. Given the temperature sensitivity of AHe (~40-90 ºC), the older dates place limits on the amount of exhumation related to rifting in this region to less than 2-3 km, depending on the geothermal gradient. This limit further differentiates the WSRP from Basin and Range structures which show substantial exhumation during extension.
The AHe datasets indicate predominantly Eocene-Oligocene exhumation in the flanks of the WSRP rift, thus limiting the extent of exhumation that occurred during late Miocene rifting. This study provides insights into different styles of intracontinental rifting that have operated in western North America during Cenozoic time.