| This research involved the investigation of the geometry of subsurface Snake RiverT type rhyolites near the western margin of the eastern Snake River Plain (ESRP). A recently drilled rhyolite in borehole USGST142 is tentatively correlated to the Walcott Tuff B in borehole WOT2. Another rhyolite, exposed at the surface southeast of Arco, Idaho, dips 20° south toward the ESRP, and is tentatively correlated to the uppermost PicaboTaged rhyolite found in borehole INELT1. These correlations suggest that the tilts of surface and subsurface rhyolites must shallow toward their correlative units from the margin to the center of the ESRP; the tilts of subsurface rhyolites are localized near the margin of the ESRP and northern Basin and Range.
This research also involved a kinematic analysis of northeastTstriking, smallToffset faults due east of Arco, Idaho as a basis for inferring the tectonic evolution of the western margin of the ESRP. NortheastTstriking faults record nearly pure dipTslip offset and a northwestTsoutheast extension direction. In addition, faults proximal to the ESRP record a northwestTplunging extension direction, whereas faults distal to the ESRP record a shallowly southeastTplunging extension direction. These observations suggest that the northeastTstriking faults likely formed as a result of early stages of flexure from the subsidence of the ESRP and were later rotated similarly to Mesozoic foldThinges.
Key words: Eastern Snake River Plain, Snake RiverTtype Rhyolites, Flexure, Kinematic Analysis, Eastern Snake River Plain Aquifer. |