Exhumation along the Fairweather fault, southeastern Alaska, based on low-temperature thermochronometry

The southern Alaskan syntaxis marks the spectacular junction between the >1000-km-long Pacific-North America transform margin and the Chugach-St. Elias belt, where subduction and terrane accretion drive rapid convergent deformation and rock uplift. New low-temperature thermochronometry reveals that intense orogenic deformation is not restricted to one side of the syntaxis but extends nearly 300 km south along the dextral Fairweather fault. Apatite and zircon (U-Th)/He ages as young as 0.9 and 2.0 Ma suggest maximum exhumation rates of nearly 2 mm/a in close proximity (<10 km) to the Fairweather fault and average exhumation rates of >0.5 mm/a along the entire plate margin. We estimate that long-term rock uplift accommodates ???3 mm/a of fault-normal convergence in this area. This suggests that the Fairweather fault is slightly transpressive and highly partitioned, analogous to the central San Andreas fault. This convergence only accounts for ???1/5 of the obliquity between Pacific plate motion and the continental margin, however, implying the deficit is taken up by 1-2 cm/a thrust-sinistral motion along the offshore Transition fault. Additionally, thermochronometry shows a marked increase in bedrock cooling coincident with onset of heavy glaciation, similar to what has been observed in other parts of the Pacific Northwest. The tectonically active Fairweather corridor is distinguished, however, by the magnitude of the acceleration and the depth of exhumation since Pliocene climate change. Copyright 2009 by the American Geophysical Union.