OVERVIEW OF THE EXHUMATION PATTERN IN SOUTHEAST ALASKA

Many thermochronological studies have been conducted in southeast Alaska to reveal the geodynamic processes of the ongoing Yakutat–North American collision zone, and how glacial erosion interacts with active tectonics. The extensive glaciation of the region challenges thermochronology and consequently a large variety of methods and sampling strategies have been conducted, which give a limited and/or biased view of the exhumation history. We review all existing low-temperature thermochronology together with new data and give an integrated view of the overall exhumation patterns in southeast Alaska.

Exhumation is most intense at the Yakutat plate corner, the St. Elias syntaxis. In this area crustal strain is concentrated and localized deep-seated crustal flow occurs as a result of strong coupling between erosion and tectonics. The observations at the St. Elias syntaxis are comparable to those at the corner regions of the India-Asia collision, which have been described as a tectonic ‘aneurysm’. Rapid exhumation from shallower crustal levels occurs at the southern Chugach–St. Elias Range within the fold-and thrust belt, and in near-fault regions of the Fairweather Range. Similar to other active mountain ranges, rapid erosion generally coincides with the regions of highest precipitation, whereby the northern areas are dryer and are eroded less. New data from the Wrangell Range, north of the Chugach–St. Elias, show that the last major cooling event was 125-100 Ma, and rock uplift and limited exhumation (1-3 km) occurred recently. The Wrangellia terrane is suggested to form the deformational back stop and has acted as such since Mesozoic time, whereas uplift and exhumation of the Chugach terrane rocks began well before the Yakutat collision in the Early Miocene.