LATE QUATERNARY TECTONIC ACTIVITY ASSOCIATED WITH THE GORE RANGE (BLUE RIVER) FRONTAL FAULT, CENTRAL COLORADO – THE SEARCH FOR THE SMOKING GUN

The Gore Range frontal fault (GRFF) is a 45-km-long, northwest-striking normal fault that parallels the Blue River along the eastern base of the heavily forested Gore Range. The fault is postulated to represent the northernmost extent of the Rio Grande rift in Colorado; however, both the activity and the age of the most recent tectonic displacement associated with the fault have been debated for over 40 years. While many scarps and other features are present along the fault and could suggest recent activity, most of the previously indentified features have been attributed to late Quaternary mass-wasting processes. From our initial review and field work in 2007, it was clear that while landsliding is extremely common along the range front, the possibility of recent tectonic activity could not be fully evaluated due to the thick tree cover and the difficult access along much of the range front. In order to reduce the large uncertainties associated with the seismic source characterization of the GRFF, Reclamation obtained airborne light detection and ranging (LiDAR) imagery for the entire frontal fault zone. Based on our initial review of the LiDAR imagery and the limited field studies conducted to date, 2- to 10-m-high scarps that appear to be the result of recurrent late(?) Quaternary activity are present at several localities along the 45-km-long eastern margin of the Gore Range, from about Lake Dillon on the south to possibly Spring Creek on the north. This conclusion is a fundamental change from previous studies, and many unanswered questions remain. These include: what is the age of the most recent surface rupturing event; is the GRFF segmented; and what is the slip rate and/or recurrence interval?