LONG-TERM FOCUSED EXHUMATION ALONG MEGATHRUST SPLAY FAULTS AT MONTAGUE AND HINCHINBROOK ISLANDS, PRINCE WILLIAM SOUND, ALASKA

Maximum uplift during the 1964 earthquake, documented by George Plafker, occurred on southern Montague Island in southern Prince William Sound. Plafker also documented 6-9 m of reverse fault slip during the 1964 earthquake across the onshore Patton Bay and Hanning Bay faults. Recent seismic data show that the Patton Bay and Cape Cleare faults form offshore fault scarps to the southwest. This fault system, which extends farther southwest offshore and farther northeast on Hinchinbrook Island, is interpreted as a megathrust splay fault system. Splay fault systems may be a conduits for long-term (million year) subduction-related deformation, but they rarely are emergent above sea level where their long-term history can be studied. New apatite (U-Th)/He ages from along and across the splay faults on Montague and Hinchinbrook Islands generally are 1.1 to ~5 Ma and new apatite fission-track ages are ~5 to 20 Ma. The ages are youngest at the southwest end of Montague Island, coincident with maximum fault slip and the highest shoreline uplift during the 1964 earthquake. These data show that exhumation rates and magnitude increase to the southwest and that the rates increased along the splay fault system starting about 2-3 Ma. The regional pattern of ages indicate that the splay faults on Montague and Hinchinbrook Islands, though separated by only a few kilometers, accommodate kilometer-scale exhumation at million-year time scales. The remarkable coincidence between long-term rock uplift and 1964 seismogenic uplift patterns indicates that rock uplift and exhumation has been focused on the megathrust splay fault system for at least the last 2-3 M.y. Increased exhumation starting 2-3 Ma is coincident with rapid glacial erosion in southern Alaska and worldwide. We interpret the Pleistocene uplift and exhumation along the splay fault system to be induced by underplating of the rapidly eroded sediments along the subduction megathrust where the splay faults sole.