LATE QUATERNARY ACTIVITY OF THE CANYON FERRY FAULT, MONTANA

The 48-km-long Canyon Ferry fault bounds the west side of the Big Belt Mountains, approximately 30 km east of Helena, Montana. The fault is a major, down-to-the-west structure bounding the northern Townsend Basin. Although the fault has significant late Cenozoic displacement, like many faults in Montana the late Quaternary activity of the fault is poorly documented. Based on aerial photograph interpretation and reconnaissance surficial geologic mapping, the late Quaternary Canyon Ferry fault can be characterized by rupture lengths of at least 40 km. The possibility also exists that the Toston fault, located immediately south of the Canyon Ferry fault, may be part of this system which would indicate that total rupture lengths of over 60 km may be possible.

A paleoseismic trench excavated at the G/T Ranch near the central portion of the Canyon Ferry fault provides important information on the slip rate, recurrence, and slip per event for the fault. Age data are from eleven infrared stimulated luminescence (IRSL) analyses on fine-grained deposits (primarily loess) collected from the trench. At the trench site, total dip-slip displacement of approximately 9 m occurred over a 55 kyr period between about 68 ka and 13 ka. These data indicate a long term late Quaternary slip rate of 0.16 mm/yr (0.13-0.2 mm/yr) for the fault. Interestingly, based on about 5 m of dip slip in the last 21 kyr, the rate is 0.24 mm/yr (0.2-0.29 mm/yr). More importantly, stratigraphic relationships and the numerical ages provide strong evidence for seismic clustering of events. At least two, and probably three, surface-rupturing events occurred between about 21 ka and 13 ka indicating short term rates of 0.54 mm/yr (0.35-0.91 mm/yr). No surface rupturing events have occurred since about 13 ka. Thus, recurrence intervals for the Canyon Ferry fault could be as long or longer than 13 kyr or as short as a few thousand years.