GSA QUATERNARY GEOLOGY AND GEOMORPHOLOGY DIVISION GLADYS W. COLE MEMORIAL RESEARCH AWARD: GLACIER CAPTURE DRIVEN BY ACTIVE NORMAL FAULT PROCESSES IN THE LOST RIVER RANGE, IDAHO

Active faulting influences geomorphic evolution by increasing relief and steepening footwall valley gradients, providing rich possibilities for stream capture. Geomorphic evidence along the central Lost River normal fault provides evidence of both stream capture and Late Pleistocene glacier capture.

The Lost River fault bounds the W edge of the Lost River Range in the northern Basin and Range. Prominent scarps and strong footwall-hanging wall geomorphic contrasts document long-term and ongoing slip. Maximum offset (2.7 m) in the M6.9 1983 Borah Peak earthquake occurred at the mouth of Rock Creek, marked by Late Pleistocene strath terraces with scarp relief ca. 2-15 m. Correlative terminal moraines (EM2-5, older to younger) lie 0.8-1.5 km upvalley of the scarp at the confluence of Rock Creek and its N Fork.

Rock Creek follows a tortuous path from Borah Peak to the fault, with three right-angle bends separating N- and W-trending sections. A high, Middle Pleistocene footwall landscape lies 250 m above lower Rock Creek. Lateral moraines (EM1) bound an abandoned, W-trending valley parallel to lower Rock Creek, and the associated outwash fans mantle the high landscape. A younger terminal moraine (EM2) lies at the head of the dry valley, mantling a prominent bend in the deep valley of modern Rock Creek. Upstream of that prominent bend, middle Rock Creek aligns with the dry valley, but below the bend, the creek trends N through a canyon cut in lower Paleozoic carbonate rocks to intersect with the N Fork Rock Creek and thence W to the fault.

We infer that Rock Creek and its glacier were captured by its N Fork during or before Late Pleistocene time. Capture occurred at the prominent bend marked by EM2, and diverted both streamflow and glacier flow from its westward path down the abandoned valley to its northward path. The rapid fault offset steepened the gradient and permitted valley capture to occur. The capture occurred prior to EM2 construction, as that moraine system both occupies the head of the dry valley and is inset within lower Rock Creek valley. The capture also created conditions for ice dams and outburst floods. Pending moraine exposure ages will provide a chronology to constrain timing of both glacier capture and fault slip.