THERMOCHRONOLOGY OF THE TALKEETNA MOUNTAINS OF SOUTHERN ALASKA: CENOZOIC TOPOGRAPHIC DEVELOPMENT HISTORY

It is known that Cenozoic plate boundary processes have played a significant role in southern Alaska topographic development. It is not clear how these plate boundary events have influenced the development of the Talkeetna Mountains of South Central Alaska. The Talkeetna Mountains overlay the subducted portion of the Yakutat microplate and have also experienced slab window magmatism inferred to be from the subduction of an active spreading ridge. These attributes make the Talkeetna Mountains a prime location to test the topographic development response to Cenozoic plate boundary events.

We use apatite fission track (AFT) and ⁴⁰Ar/³⁹Ar thermochronology on granite and volcanic rocks sampled across a vast portion of the Talkeetna Mountains to document regional cooling and exhumation patterns. AFT ages show rapid cooling from ~60-44 Ma and indicate less than ~5 km of exhumation since ~44 Ma, suggesting there has not been significant exhumation related to Yakutat microplate subduction. The Talkeetna Mountain thermochronological data are consistent with field observations and geochronology from the region that document rapid accumulation of >2 km thick succession of 60-56 Ma fluvial strata unconformably upon ~70 Ma and older granitoid plutons. ⁴⁰Ar/³⁹Ar thermochronology on K-feldspar from a low elevation sample indicates a thermal event at ~61 Ma. ⁴⁰Ar/³⁹Ar whole rock cooling ages are ~52-42 Ma. Our overall data sets are consistent with an inferred late Paleocene-early Eocene slab window event.

⁴⁰Ar/³⁹Ar cooling ages in the Talkeetna Mountains are the same as cooling ages in the Western Alaska Range (WAR) and are consistent with the ridge subduction model. However, there is no west-east time progression in cooling ages between these two ranges as would be expected with the current model of an active spreading ridge sweeping from west-east across the subduction margin. Talkeetna Mountains and WAR cooling ages suggest a widespread thermal event synchronous across southern Alaska north of the Border Ranges Fault that persisted for millions of years during Paleocene-Eocene times. We infer that the absence of a slab due to slab break off or the subduction of a trench-parallel active spreading ridge drove exhumation and rock cooling across southern Alaska and played the primary role in the development of the Talkeetna Mountains.