GEOCHRONOLOGICAL FRAMEWORK FOR THE CENOZOIC HISTORY OF THE SOUTHERN ALASKA RANGE FOLD AND THRUST BELT

Oblique convergence along the arcuate dextral Denali Fault (DFS) is mainly accommodated by both vertical slip on the master strand of the Fault and fold and thrust belts that splay off the DF. Diffuse deformation (basin subsidence, folding, and rock uplift) along the northern side of the DFS has been well studied by past research efforts on the transpressive Tanana Basin and the Northern Alaska Range Fold and Thrust Belt. The kinematics of Cenozoic deformation along the broad zone south of the DF has received less attention.

We integrate new and existing geochronology, structural mapping, basin analysis, and geochemistry studies to better understand both the evolution of the southern Alaska Range Fold and Thrust Belt. We focus on the McCallum-Slate Creek Fault system, which branches off the DF in the Delta River valley. Eocene volcanics depositionally overlie Eocene sedimentary strata. Research is on going on the correlation between possible slip along the DFS and both Eocene subsidence and magmatism. Dacite clasts collected from Pliocene strata were dated to ~27 Ma (⁴⁰Ar/³⁹Ar biotite) matching an age peak in detrital zircons from the same basin deposits. There is no known bedrock volcanics of this age along the proximal DFS, providing a potential piercing point.

AFT and AHe ages demonstrate initiation of rapid exhumation at ~5 Ma and possible southward propagating basement uplift by ~3 Ma. New tephrachronology, geological mapping, and basin analysis constrain both initiation of thrusting, rapid subsidence, and basin inversion at ~5 Ma of the McCallum Basin confirming the conclusions of past research efforts. After ~4 Ma, the fold and thrust belt propagated further into the basin. Modern geomorphic indicators (terraces, deformed glacial till) and seismicity indicate the fold and thrust belt is active and propagating to the south.

Propagation of the deformation front into the foreland of the McCallum-Slate Creek Fault is possibly reflecting translation of the crustal block into the Mount Hayes restraining bend. Overall faults of the Southern Alaska Fold and Thrust Belt vary in orientation from sub-parallel to the DF (e.g. McCallum Creek thrust) to those that splay off the DF (e.g. the SW striking Susitna Glacier Fault) as the tectonic regime changes from east to west: from strike-slip to transpressional, to more contractional.