THERMOCHRONOLOGY OF THE TALKEETNA MOUNTAINS OF SOUTHERN ALASKA: THE CENTER OF THE RESURRECTION-KULA TRIPLE JUNCTION CONTROVERSY?

Based on a string of younging to the east near trench plutons a model of a sweeping west to east Paleocene-Eocene triple junction across the southern Alaska margin was developed. Controversy erupted based on paleo-mag and detrital zircon work suggesting these near trench-plutons may have been translated ~1000 km along the Border Range Fault system. ~58 Ma thermochronology data from the WAR and the St. Elias Mountains inboard of the Border Range Fault provide evidence of a high geothermal gradient supporting a slab window.

The Talkeetna Mountains are a ~170 km long, ~150 km wide trench perpendicular range located between the WAR and the St. Elias Mountains. Hence, the Talkeetna Mountains a prime location to test the hypothesis of a sweeping west-east triple junction by applying apatite fission track (AFT) and ⁴⁰Ar/³⁹Ar thermochronology on granite and ⁴⁰Ar/³⁹Ar geochronology on volcanic rocks sampled across the interior Talkeetna Mountains in combination with existing data sets across southern Alaska.

Talkeetna Mountain AFT results yield onset of rapid cooling at ~60 Ma that continued to at least ~40 Ma. ⁴⁰Ar/³⁹Ar thermochronology on K-feldspar indicates partial resetting and a thermal event at ~61 Ma. ⁴⁰Ar/³⁹Ar feldspar cooling ages in the Talkeetna Mountains are within error of cooling ages in the WAR (~57 Ma). A compilation of 30 ⁴⁰Ar/³⁹Ar whole rock volcanic ages across the Talkeetna Mountains demonstrate regional initiation of magmatism at ~58 Ma. The thermochronologic data are consistent with basin analysis that document rapid accumulation of >2-km-thick successions of 60-56 Ma fluvial strata unconformably upon older granitoid plutons.

We document a synchronous widespread Paleocene-Eocene thermal event across southern Alaska north of the Border Ranges Fault with no west-east time progression in cooling ages across southern Alaska nor a west to east or south to north progression of slab window magmatism. We infer that the absence of a slab due to slab break off after the subduction of a trench-parallel aseismic (?) spreading ridge drove rapid exhumation and depleted magmatism across southern Alaska. The southern Alaska margin may have been a transform margin during this time period, in part offering a mechanism for the far translation of the Sanak-Baranof accretionary pluton suite with subduction reinitiating by ~45 Ma.