THE EXHUMATION RESPONSE TO SILETZIA COLLISION AND ASSOCIATED TECTONIC REORGANIZATION RECORDED IN THE THERMAL HISTORIES OF EOCENE PLUTONS IN THE NORTH CASCADES, WA

The Pacific Northwest was subject to widespread magmatism and rapid tectonic reorganization during the Eocene. Flat slab subduction ceased following the collision of the Siletzia oceanic plateau, initiating slab rollback and eventually slab breakoff. This work focuses on the record of rock uplift and exhumation in response to plateau collision. To better understand these processes, we applied a suite of thermochronometers to early Eocene plutons, including the Bald Mountain Pluton and the Mount Pilchuck and Granite Falls stocks, that intrude the Cretaceous to Paleogene forearc during the accretion of Siletzia. The exhumation histories of these plutons recorded by thermochronometric data will complement existing evidence for regional extension and uplift shortly after accretion recorded by faulting and sedimentation in nearby transtensional basins.

Thermochronometric ages and thermal history modeling suggest that the Mount Pilchuck stock cooled rapidly between its emplacement at 49 Ma to temperatures as low as 100 ºC ca. 45-40 Ma, followed by slow cooling to the present day. The Bald Mountain pluton shares the early thermal history of the Mount Pilchuck stock, constrained by a zircon (U-Th)/He age of 44.9 ± 1.8, but an apatite fission track central age of 18.5 ± 1.2 Ma suggests that this pluton either remained buried and cooled more slowly into the Miocene or experienced a period of reheating. The few apatite (U-Th)/He ages from the Bald Mountain pluton we have to date are highly overdispersed and therefore inconclusive. Zircon (U-Th)/He ages from the Granite Falls stock suggest rapid cooling shortly after its emplacement at ~45 Ma, but an apatite fission track central age of 38.05 ± 4.58 Ma and a apatite (U-Th)/He age of 31.4 ± 1.2 Ma indicate that it remained at temperatures around 50-80 ºC until at least the end of the Eocene. The differing post-Eocene thermal histories between the neighboring Bald Mountain Pluton and the Mount Pilchuck stock suggest local structural controls and/or other processes largely govern the more recent portions of the plutons’ cooling histories. However, the shared record of rapid Eocene cooling suggests a regional exhumation response to the Siletzia collision, consistent with regional sedimentary and structural evidence.