GEOLOGIC OBSERVATIONS TOWARD RESOLVING THE PLATE BOUNDARY SLIP BUDGET ACROSS SOUTHCENTRAL AND INTERIOR ALASKA

Shortening and transpression related to the Alaska-Aleutian subduction zone is distributed over >800 km from Southcentral Alaska to the Brooks Range. In this region, geologic and geodetic rates of deformation diminish to the north and paleoseismic data for many faults are inadequate for seismic hazards assessments. Here, I present reconnaissance observations from two faults that exhibit contrasting Holocene histories, including the Pass Creek fault in the southern Alaska Range and the Tintina-Kaltag fault system north of Fairbanks. These observations are combined with existing paleoseismic studies and discussed with respect to the regional slip budget.

Along the southern margin of the Alaska Range near Chelatna Lake, the Pass Creek fault is associated with shallow seismicity consistent with southeast-directed thrusting. The fault displaces latest Wisconsin glacial moraine and outwash terrace deposits by up to ~4.3 m and has impounded lakes behind smaller scarps across inset drainages, suggesting the occurrence of multiple earthquakes. In northern Interior Alaska, the Tintina-Kaltag fault system is associated with large shutter ridges, linear valleys, and broad saddles consistent with a long history of strike slip deformation. Although the system has generated multiple moderate (~M5) earthquakes, results from a 10-km transect along the northwestern front of the Kaiyuh Mountains indicate that the fault, while associated with an unusual alignment of pingos, lacks continuous scarps in youthful deposits.

Geologic studies in Southcentral Alaska indicate that the majority of plate boundary-related shortening can be resolved along known active faults, of which the Pass Creek fault plays a small role. Geodetically measured shortening (< 3mm/yr) between the Alaska Range and the Brooks Range cannot be resolved onto individual faults. This low rate suggests long interseismic intervals for active faults in Interior Alaska and is consistent with the weak geomorphic expression of the Tintina-Kaltag fault system. I suggest that high rates of cryogenic processes also limit preservation of tectonic features and inhibit efforts to quantify paleoseismic parameters. Thus, the lack of tectonic features may not necessarily indicate a lack of tectonic activity.