REVERSE/THRUST FAULTS IN AND AROUND SUSITNA BASIN, ALASKA – GEOPHYSICAL EVIDENCE AND STRUCTURAL IMPLICATIONS

The Susitna basin forms an enigmatic topographic gap between the Western Alaska Range and Talkeetna Mountains in south-central Alaska. Although geomorphically contiguous with the upper Cook Inlet forearc basin, Susitna basin is structurally separated by the margin-parallel Castle Mountain fault. With the exception of low-lying Susitna basin, the region north of the Castle Mountain fault consists of high topography, with multiple peaks above 3,000 meters in the adjacent Tordrillo Mountains. Many geologic interpretations show normal faults defining the margins of the Susitna basin (e.g., Kirschner, 1994). However, analysis of new and existing geophysical data (the primary demonstration is from gravity data) shows predominantly reverse fault geometries in the region, a result previously proposed (e.g., Hackett, 1978) but subsequently forgotten. A 50-mGal gravity low culminating about 15 km northeast of the Mount Susitna/Beluga Mountain topographic front corresponds to the >4-km-thick Cenozoic sedimentary section in the Susitna basin known from drilling, seismic, and previous gravity investigations. The gravity gradient from this low to the flanking bedrock high to the southwest spans a horizontal distance of about 40 km, centered roughly at the toe of the mountain front. The location and shape of the gravity gradient rules out the interpretation of the mountain front as a normal fault dipping toward the basin. Instead, the gravity gradient is best explained by a southwest-dipping reverse/thrust fault, with the leading edge projecting several km beneath the shallow basin cover northeast of the mountain front. Geometric models constrained by geophysical data suggest that igneous rocks in the hanging wall were transported tens of kilometers northeast, with an underthrust panel of lower density sedimentary rocks in the footwall beneath. The interpretation of the Mount Susitna/Beluga Mountain fault as a southwest-dipping reverse structure is consistent with the previously recognized need to explain discrepancies in lateral movement between the Denali and Castle Mountain strike-slip fault systems; the basin-bounding thrust faults might represent part of a tectonic transfer zone that, together with the previously proposed Broad Pass fault, accommodates this differential motion.