EOCENE MAGMATIC EVOLUTION AND EXTENSIONAL FAULTING, LITTLE WOOD RIVER RESERVOIR QUADRANGLE, BLAINE COUNTY, IDAHO

The Little Wood River Reservoir quadrangle of the southern Pioneer Mountains in south-central Idaho contains Pennsylvanian and Permian Wood River Formation deformed into upright to overturned eastward verging folds, and overlain unconformably by volcanic rocks of the Eocene Challis Volcanic Group. This volcanic group generally dips ~30° toward the east. The quadrangle is ~10 km south of the Pioneer Metamorphic Core Complex and as such, reflects Challis Group volcanism synchronous with and adjacent to active low-angle normal faulting.

The northern and southern parts of the quadrangle contain mappable dacite and andesite lava flows. From base to top, in these areas, the Challis Volcanic Group consists of local andesite flows, volcaniclastic rocks, an upper andesite flow, dacite lava and flow-domes which are difficult to differentiate, and local accumulations of rhyolitic fallout tuff. The central part of the quadrangle is bounded on the northwest by the northeast-striking, 41° northwest dipping dip-slip High Five Creek Trans-Challis normal fault. The High Five Creek fault is approximately parallel to previously documented structures in the region that are Eocene in age; this suggests that this structure was active during active low angle normal faulting in the adjacent Pioneer metamorphic core complex. This central area contains a lower dacite flow, medial undifferentiated volcaniclastic rocks that we interpret as a paleo-valley fill, and an upper dacite lava flow and volcaniclastic sequence. The quadrangle also contains northwest-striking, 30-40° dipping faults that are likely associated with Basin and Range extension and are shallower than expected.

We suggest that the High Five Creek fault accommodated upper-crustal dip-slip motion during Eocene time and is the surficial manifestation of mid-crustal low-angle ductile extension that has been previously documented in the Pioneer metamorphic core complex. The shallow to moderate dips on more recent northwest-striking faults are interpreted to result from fault rotation to progressively shallower angles with ongoing extension.