ROCK UPLIFT AT THE TRANSITION FROM FLAT SLAB TO NORMAL SUBDUCTION: THE KENAI RANGE, SE ALASKA

The transition from flat slab to normal subduction is a process that can produce complex upper plate deformation. In south central Alaska, the Yakutat microplate is undergoing flat slab subduction beneath the southern margin, which transitions to normal subduction of the Pacific plate. The Kenai Range is the western limit of a 1000 km laterally continuous forearc which is being exhumed and spans the transition from flat slab to normal subduction. To better understand the uplift and tectonic driver for the Kenai Range across this unique transition, 17 new Apatite (U-Th)/He low temperature thermochronometry ages were completed. The ages in the north, range from 10-20 Ma and merge smoothly into the exhumational high in the Chugach Mountains. In the South, the ages range from 30-50 Ma which sharply transition from lower to higher ages around Seward, Alaska. These ages are relatively old in comparison to other studies in southern Alaska and do not suggest recent rapid exhumation. The landscape of the Kenai Range is different from the Chugach Range in that it is less deeply incised and the axis of maximum topography and drainage divide shifts approximately 30 km to the west around Seward. The elevation of peaks and ridges are somewhat concordant and imply that the Kenai Range is a transient landform with erosion that lags behind surface uplift. Since the Kenai Range is undergoing heavy glacial erosion the observations suggests that the surface uplift may be recent and the southern tip may be propagating to the southwest. This transition from young to older ages corresponds with the southwestern edge of the Yakutat plate. We propose that underplating of Yakutat collision-originated sediments (Surveyor Fan) may be responsible for the uplift and exhumation of the Kenai Range. The Kenai Range can be considered a nascent emergent forearc that is growing via underplating processes that are comparable to those inferred for the longer history of deformation on Kodiak Island. The forearc architecture spanning flat slab and normal subduction suggests that exhumation in the overriding plate quickly drops off away from the active collision of the Yakutat micro plate and that the uplift of the Kenai Range may be recent and driven by underplating.