EXTREME TOPOGRAPHIC DEVELOPMENT ALONG THE DENALI FAULT STRIKE-SLIP SYSTEM, ALASKA: WHY IS MOUNT MCKINLEY SO BIG?

By definition, motion along strike-fault systems is mainly taken up as horizontal slip, translating crustal blocks along the path of the fault. Restraining bends occur along strike-slip faults and are regions where a component of horizontal slip is partitioned into vertical slip. Topographic development along strike-slip fault systems is self limited by: a) erosion, b) the transient nature of rapid exhumation along restraining bends as crustal blocks are translated through regions of focused vertical tectonics and c) inherent mechanical inefficiency of restraining bends. Yet, we have broad (~20 km wide), high mountains like Mount McKinley (6,194 m) and neighboring Mount Foraker (5,304 m) along the Denali Fault strike-slip system in Alaska. We investigate restraining bend evolution utilizing multi-discipline approach (thermochronology, petrology, geomorphology, structural geology, seismology) at the Mount McKinley restraining bend.

Preliminary apatite fission track (AFT) cooling ages from samples on both sides of the restraining bend, range from ~8 Ma to ~2 Ma and suggest a pattern of AFT cooling ages younging to the west. This apparent east to west time progressive trend in inferred focus of exhumation is mimicked by an increase in width of the deformation front to the west. In addition, north and south side topographic development increases west towards the bends apex before decreasing further to the west. Therefore, we infer the evolution of the Mount McKinley restraining bend is the dominant control on exhumation patterns in the region, not variations in erosional forcing, basement faults, lithology, and rheology. The Peters Dome area, located north of the bend’s apex, is a region of high seismicity, young cooling ages and multiple active faults, demonstrating significant shortening that is likely accommodating the westward migration of the Mount McKinley restraining bend at a long term rate of 4-6 mm/yr. This rate is > 50% of the bend’s Holocene Denali fault slip rate, but approximates the bend migration rate predicted by kaolin analog experiments (bend migration rate ~50% of master strand slip rate). Hence crustal blocks remain in the region of focused vertical tectonics for an extended period of time, and Mount Foraker with deeply exhumed orthogneiss (~40 Ma protolith age) is likely a paleo-Mount McKinley.