PLIOCENE-RECENT SHORTENING ON THE MCCALLUM CREEK FAULT: EVIDENCE FOR AN INCIPIENT THRUST SYSTEM IN THE EASTERN ALASKA RANGE

The McCallum Creek fault splays southeastward from the Denali fault in the eastern Alaska Range and thrusts Wrangellia volcanics over Late Miocene and younger alluvial strata of the McCallum foreland basin. McCallum basin strata consist of a lower mudstone and sandstone unit (~top tephra age ~5.0 Ma) and an upper conglomerate unit (~basal tephra age ~4.9 Ma). We correlate the deposition of the conglomerate unit to the initiation of thrusting on the McCallum Creek fault, based on cobbles that match lithologies in the hangingwall of the McCallum Creek fault. These strata are folded along ~E-W trending fold axes subparallel to the McCallum Creek fault.

We present balanced cross sections and forward kinematic models of folds in the McCallum basin strata, revealing that shortening the McCallum Creek area is distributed between a system of imbricate blind thrusts in the foreland and the main McCallum Creek fault. Foreland structures have absorbed at least 1 km of shortening. The McCallum Creek fault has overthrust Wrangellia basement and truncated foreland fold axes resulting in ~0.5 km of out-of-sequence shortening since the foreland structures developed. However, tilted geomorphic surfaces and seismicity imply that foreland structures are the most recently active part of this thrust system.

Based on the surface trace of the McCallum Creek fault, exposure of folds in the foreland, lack of growth strata, and a 3.7 ± 0.052 Ma (⁴⁰Ar/³⁹Ar glass) tephra age from the conglomerate, the shortening rate for the whole thrust system since 3.7 Ma is ~0.41 mm/yr. This rate is likely a minimum due to poor constraints on the shortening on the main McCallum Creek fault since 3.7 Ma and it does not account for possible additional shortening on foreland structures that are buried by glacial debris or eroded away. We speculate thrusting on the main strand of the McCallum Creek fault prior to 3.7 Ma is responsible for foreland subsidence and crustal loading before the foreland thrust system developed.

We interpret deformation in this thrust system as partitioned slip in the dextral-transpressive Denali fault system. Slip on the McCallum Creek fault results in internal shortening of the Southern Alaska Block and rapid Alaska Range exhumation. Forthcoming apatite fission track ages will determine the timing and extent of exhumation in the hangingwall.