THE ROLE OF PRE-EXISTING STRUCTURES ON THE INBOARD UPPER PLATE RESPONSE TO FLAT SLAB SUBDUCTION OF A BUOYANT HIGH: YAKUTAT MICROPLATE, SOUTHERN ALASKA

The Yakutat microplate is currently undergoing flat slab subduction under southern Alaska. We present new and published data from southern Alaska that relates to the initial timing of Yakutat flat slab subduction and suggest that inboard deformation is focused along pre-existing structures.

We infer the initiation of Yakutat microplate flat slab subduction was underway by ~25 Ma based on coeval events across a wide region: 1) cessation of magmatism along the western Alaska Range and Denali Fault at ~25 Ma to ~28 Ma, 2) the start of Wrangell arc magmatism 3) metamorphism along the eastern Denali Fault at ~25 Ma, 4) basin subsidence along the Denali Fault system at ~29 Ma, 5) the initiation of persistent rapid exhumation along the length of the 800 km long Alaska Range by ~25 Ma, 6) Nenana River drainage reorganization at ~20 Ma, 7) the presence of Oligocene-Miocene unconformities in the Cook Inlet, Susitna, Matanuska, and Nenana basins, and 8) reactivation of contractional structures in the Brooks Range. Hence, ~30 Ma to ~25 Ma was likely a transitional time marking the initial coupling between the Yakutat microplate and the over-riding southern Alaska plate boundary zone.

The subducting Yakutat slab extends well under southern Alaska, including the trench perpendicular ~ 170 km long Talkeetna Mountains. These mountains are bound to south by the active Castle Mountain Fault (CMF) and bisected by the Talkeetna Thrust Fault. This region provides a perfect location to investigate if the inboard deformational response to the Yakutat flat slab is only focused along pre-existing structures. While, constraints on the extent of Neogene deformation are sparse, but thermochronology results suggest limited Neogene unroofing in the glaciated Talkeetna Mountains: Eocene apatite fission track (AFT) ages; early Miocene apatite (U-Th)/He ages away from the CMF. A synthesis study of the neighboring Susitna basin also indicates limited Neogene regional denudation. An AFT study along the CMF, yielded Oligocene cooling ages with a possible period of rapid cooling starting at ~10 Ma. Further work is planned, but overall results indicate that a significant inboard deformation response to the Yakutat flat slab subduction is mainly associated with a pre-existing structure, with maximum Neogene exhumation occurring along the Denali fault.