Focused exhumation in the syntaxis of the western Chugach Mountains and Prince William Sound, Alaska

The western Chugach Mountains and Prince William Sound are located in a syntaxial bend, which lies above flat-slab subduction of the Yakutat microplate and inboard of the Yakutat collision zone of southern Alaska. The syntaxis is characterized by arcuate fault systems and steep, high topography, which suggest focused uplift and exhumation of the accretionary prism. We examined the exhumation history with low-temperature thermo-chronometry of 42 samples collected across the region. These new apatite (U-Th)/He, apatite fission-track, zircon (U-Th)/He, and zircon fission-track ages, combined with ages from surrounding regions, show a bull's-eye pattern, with the youngest ages focused on the western Chugach syntaxis. The ages have ranges of ca. 10-4 Ma, ca. 35-11 Ma, ca. 33-25 Ma, and ca. 44-27 Ma, respectively. The youngest ages are located on the south (windward) side of the Chugach Mountains and just north of the Contact fault. Sequentially higher closure temperature systems are nested across Prince William Sound in the south, the Chugach Mountains, and the Talkeetna Mountains to the north. Computed exhumation rates typically are 0.2 mm/yr across Prince William Sound, increase abruptly to similar to 0.7 mm/yr across and adjacent to the Contact fault system, and decrease to similar to 0.4 mm/yr north of the core of the Chugach Mountains. The abrupt age and exhumation rate changes centered on the Contact fault system suggest that it may be a critical structural system for facilitating rock uplift. Our data are most consistent with Yakutat flat-slab subduction starting in the Oligocene, and since then similar to 11 km of rock uplift north of the Contact fault and similar to 4-5 km of rock uplift in Prince William Sound to the south. These data are consistent with a deformation model where the western Chugach core has approached long-term exhumational steady state, though exhumation rates have probably increased in the last similar to 5 m.y. We interpret that rock uplift in the overriding wedge has been driven dominantly by underplating, with long-term vertical displacement concentrated at the southern edge of the Chugach Mountains and centered on the Contact fault system. Though our data do not unequivocally differentiate between Pliocene tectonic- or climate-related causes for increased exhumation in the last similar to 5 m.y., we interpret the increased rates to be due to increased influx of underplated sediments that are derived from erosion in the Saint Elias orogen collision zone.