Reference 1385

Abstract

The Boise front range/western Mount Bennett Hills and the OwyheeMountains define the northeastern and southwestern margins of thewestern Snake River Plain graben WSRP. respectively. Borehole dataindicate that 1 normal faults along the range fronts have displacedthe 10-11 Ma Idavada volcanics 0.32 km near Boise and 1.8-2.2 km nearthe Owyhee Mountains and western Mount Bennett Hills fronts,respectively; and 2 there are no Idavada Group rocks or eruptivecenters within the western Snake River Plain. Comparing amounts ofvertical offset with K-Ar ages of the Neogene volcanic units in theWSRP suggests that graben formation occurred in three phases over thelast 16 million years. Phase 1, between 16.6 and 14.5 Ma. is seen onlyin the Owyhee Mountain front, and involved, at most very minorfaulting. Phase 1 coincides with the formation of the McDermittVolcanic Field over the Yellowstone Hotspot. During Phase 2, activegraben faulting between 14.5 and 9 Ma accounted for 20 to 80% of thecumulative fault displacement. Silicic volcanism along both marginsoccurred only at this time, while the hotspot migrated northeastwardfrom the Owyhee-Humboldt Eruptive Center to the Picabo Volcanic Field.In Phase 3, between 9 Ma and the present, the rate of graben faultingwaned and accounted for 5-20% of cumulative displacement along bothmargins. During this time, the hotspot migrated northeastward beyondthe WSRP from the Picabo Volcanic Field to its present locationunderneath the Yellowstone Caldera. This sequence of quiescencefollowed by active and then waning rates of faulting is consistentwith a proposed model for passage of the Yellowstone Hotspot,according to which a V-shaped stress parabola precedes the upliftedregion over the hotspot. In the WSRP, the 16.6-14.5 Ma Phase 1 maycorrespond to the tectonically quiet region ahead of the hotspot'sinfluence: the 14.5-9 Ma Phase 2 may correspond to the tectonicallyactive region near the hotspot, and the 9-0 Ma Phase 3 may correspondto the region of waning tectonic activity in the stress shadow of thehotspot. This model suggests that the majority of graben faulting andsilicic volcanism near the margins occurred between 14.5 and 9 Maowing to crustal heating and weakening accompanying the passage of theYellowstone Hotspot. This crustal weakening may have reactivated anolder structural basin, creating the 50-70 km-wide WSRP.

53 samples from this reference