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