FOCUSED ROCK UPLIFT RELATED TO FLAT-SLAB SUBDUCTION IN SOUTHERN ALASKA: INBOARD, OUTBOARD, AND IN BETWEEN

Collision and flat-slab subduction of the Yakutat microplate has caused uplift and deformation of rocks in a SE to NW swath across >600 km of southern Alaska, but the styles and causes of deformation and uplift vary markedly along the swath. In SE Alaska and adjacent Canada, apatite (U-Th)/He (AHe) ages of ~0.5 Ma and detrital zircon fission-track peak ages of ~3 Ma record very rapid rock uplift. This uplift is related to focusing of deformation around transpressional bends at the outboard collision front. At the NW edge of the swath, 1 to 6 Ma AHe and apatite fission-track (AFT) ages in the Alaska Range record a complicated history of rapid rock uplift. Several factors contribute to the deformation history of this inboard region including localized transpressional deformation along the dextral Denali fault, changes in Yakutat microplate thickness, impingement of thicker upper plate crust across the Denali fault system, and changes in plate motion.

The western Chugach Mountains (CM) and Prince William Sound (PWS) areas lie at a critical location between the inboard and outboard regions. AHe ages are ~15 Ma in the PWS, but decrease northward to 3-4 Ma in the core of the western CM. AFT ages similarly decrease from 30 Ma in the PWS to 6 Ma in the CM core. Collectively, these ages record a bull’s eye pattern of exhumation that is concentrated at a syntaxial bend in the regional topographic and structural grain and between the terrane-bounding Contact and Border Ranges fault systems. The rapid rock uplift was probably caused by Yakutat underplating, which was focused into the syntaxial corner above the shallowly subducting plate. About 100 km south on Montague Island in the PWS, and well outside the CM exhumation bull’s eye, anomalous AHE and AFT ages of 1.3 and 4.4 Ma indicate very rapid rock uplift. Rapid rock uplift here is probably related to thin-skinned thrusting and may represent the westward extension of the St. Elias and Bagley fault systems thought to be responsible for rapid exhumation farther east. The Prince William Sound and western Chugach Mountains therefore are key locations, between the inboard Alaska Range and outboard St. Elias areas, in understanding the array of deformation processes related to flat-slab subduction in southern Alaska.

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Session No. 230

T9. Tectonic Development of the Northern North American Cordillera

Wednesday, 12 October 2011: 8:00 AM-12:00 PM

Room 200DE (Minneapolis Convention Center)

Geological Society of America Abstracts with Programs. Vol. 43, No. 5, p.553

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