OBLIQUE THRUST FAULTING AND FORELAND BASIN DEFORMATION IN A STRIKE-SLIP FAULT SYSTEM, EASTERN ALASKA RANGE, ALASKA

The McCallum Creek Fault in the eastern Alaska Range is a splay on the south side of the Denali Fault that juxtaposes Paleozoic and Cenozoic volcanic rocks against Neogene sedimentary strata. We present new geologic mapping and kinematic data from brittle fault fabrics for the McCallum Creek Fault and imbricate splay faults in the Gulkana Glacier area of the eastern Alaska Range.

The McCallum Creek Fault strikes WNW, sub-parallel to the Denali Fault, dips ~40º NNE, and is dominantly a reverse fault. The hangingwall of the McCallum Creek Fault contains a sub-vertical dextral fault with a minor component of reverse slip. These two faults merge and become a NW-striking high-angle dextral fault that intersects the Denali Fault near the Delta River.

The footwall of the McCallum Creek Fault is segmented by N-striking tear faults into independently deforming blocks of Neogene foreland basin strata. The dominant deformation style in these strata is south-vergent fault-propagation folding. Fault-bend folding and north-vergent folding related to back-thrusts are also present in some blocks. Kinematic data from tear faults and geometry of folds indicate that tear faulting and folding in the Neogene sediments developed coevally. Tilted glacial strata in the foreland and undeformed glacial sediments capping faults in the hinterland suggest that structures in the foreland are more recently active and deformation is propagating southward into the foreland as this fault system develops.

Apatite fission track data from ~2400 m peaks in the hangingwall of the McCallum Creek Faults show cooling at ~5 Ma. These results agree with basin analysis and tephrachronology studies demonstrating the start of coarse-clastic sedimentation into the foreland after ~5 Ma. These data suggest the McCallum Creek Fault system has been active since at least ~5 Ma. Aftershocks from the Denali 7.9 M earthquake show that it is seismically active today.

We interpret these fault kinematic data as slip partitioning within the transpressional Denali fault system. This interpretation is supported by geophysical data that show a north-dipping structure linking to the Denali Fault at depth. The dextral reverse oblique faults facilitate active exhumation in this portion of the Alaska Range and respond to plate-boundary forces in southern Alaska.