CONSTRAINTS ON RIFT-RELATED EXHUMATION IN THE FLANKS OF THE WESTERN SNAKE RIVER PLAIN, IDAHO, FROM APATITE (U-TH)/HE THERMOCHRONOMETRY

The western Snake River Plain (WSRP) in southwest Idaho has been characterized as an intracontinental rift basin but differs markedly in orientation and style from neighboring Basin and Range rifts. Most WSRP faulting was active between ~11-9 Ma, but extension broadly overlapped with nearby Neogene volcanism, such as Columbia River flood basalt volcanism (~16 Ma) to the northwest and the Yellowstone hotspot-related Bruneau-Jarbidge volcanic center (~12-10 Ma) to the southeast. However, there are relatively little data to constrain what processes controlled WSRP extension. This study presents preliminary results of apatite (U-Th)/He thermochronometry (AHe) from Cretaceous granodiorite and Eocene Challis granitoid bedrock of the Owyhee Mountains on the SW flank and the Boise Mountains on the NE flank of the WSRP to quantify the pattern and timing of exhumation due to rifting.

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.