TOPOGRAPHIC DEVELOPMENT AND UPPER PLATE DEFORMATIONAL RESPONSE TO MULTIPLE CENOZOIC FLAT SLAB SUBDUCTION EVENTS: TALKEETNA MOUNTAINS ALASKA

It is known that ~10% of all convergent boundaries are undergoing flat slab subduction and that flat slab subduction can lead to mountain building, basin subsidence and basin uplift, and the migration or shutting off of arc magmatism. However, the relationship between mechanisms for flat slab subduction and upper plate controls on deformational (uplift) patterns is unclear. Southern Alaska has undergone two flat slab subduction events during the Cenozoic: Paleocene-early Eocene (~60 to ~50 Ma) subduction of an active spreading ridge and late-Oligocene (~26 Ma) to present subduction of the Yakutat microplate, a buoyant oceanic plateau. The Talkeetna Mountains of southern Alaska are a ~170 km long, trench perpendicular range that lays completely over the subducting Yakutat microplate and has also experienced slab window magmatism likely related to the ridge subduction event. We use thermochronology to test which subduction event played the primary role in the topographic development history of the mountains. In particular, we applied apatite fission track analysis to granitic samples collected along a vertical profile of Mount Sovereign (2,679 m), the highest peak in the range. Preliminary AFT results yields Cretaceous to Eocene cooling ages and an age-elevation profile presents evidence for the onset of rapid cooling during the late-Paleocene to mid-Eocene. Concurrently, the relationship between calculated Eocene exhumation rates and the HeFTy modeled cooling rates suggests that the Talkeetna Mountains region experienced a relatively high paleo-geothermal gradient (~100 ⁰C/km), which may be linked to the subduction of an active spreading ridge.