TOPOGRAPHIC DEVELOPMENT OF THE TALKEETNA MOUNTAINS ALASKA: THE RELATIONSHIP TO THE CASTLE MOUNT FAULT AND TWO CENOZOIC FLAT SLAB SUBDUCTION EVENTS

The history of vertical tectonics along the Castle Mountain Fault (CMF) has not been well studied. The sub-vertical CMF, which has a documented history of horizontal slip during the Cenozoic (~130 km of total Cretaceous-Tertiary displacement, Trop et al., 2005), extends ~200 km along the southern border of the Talkeetna Mountains. This spatial relationship suggests that slip along the CMF may be linked to the topographic development of this mountain range. However, the strike of the CMF is relatively uniform for the entirety of its length and therefore lacks any dramatic geometric complexities. These attributes make the Talkeetna Mountains a perfect location to study how slip is transferred into a vertical component along a strike-slip fault with no obvious geometric complexities.

Additionally, 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 Oligocene (~29 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. Thermochronology results from a ~50 km north-south sampling transect across the Castle Mountain Fault are forth coming to test the role this rheological weakness has played in the topographic development history of the Talkeetna Mountains.