TEMPORAL AND SPATIAL PATTERN OF DEFORMATION AT THE NORTHERN END OF THE FAIRWEATHER TRANSFORM BOUNDARY, SOUTHEAST ALASKA

The temporal distribution of strain in regions where a plate boundary transitions from transform to convergence is complex. One example of such a transition is in the ongoing oblique collision zone between the Yakutat microplate and the North American Plate in Southeast Alaska. In the Yakutat Bay area, the northwest striking dextral Fairweather transform boundary terminates as it enters the St. Elias syntaxis region and bends to a west trending thrust system. The Boundary Fault and Yakutat Fault parallel this northern end of the Fairweather transform and cause deformation and facilitate exhumation of the Yakutat microplate as it is carried into the syntaxis corner.

We use low-temperature thermochronology to quantify rates of rock exhumation and the spatial progression of deformation accommodated by the faults that divide the area into three blocks. From south to north these are the Yakutat block, the Boundary block, and the North American block. We present new apatite and zircon (U-Th)/He ages from 19 bedrock samples collected along the coast of Yakutat Bay and surrounding fjords. Our new cooling ages reveal differential exhumation across the fault bounded blocks. Average exhumation rates over the past 5 Ma across the Boundary block and North American block are 3 – 5 mm/yr, accommodating rock exhumation from depths greater than 10 km north and south of the Fairweather Fault. In contrast, rocks of the Yakutat block exhumed from a shallower depth and at an average rate of 1 – 2 mm/yr since 5 Ma. This exhumation pattern changed 2 – 1 Ma from the Boundary block to the Yakutat block. While rock exhumation of the North American block in the vicinity of the Fairweather fault zone remained rapid (3 – 4 mm/yr), exhumation of the Boundary block decreased to rates of 1 – 2 mm/yr, and deformation shifted south resulting in accelerated rock exhumation of the Yakutat block to rates of 3 mm/yr. We infer that deformation resulted in continuous, rapid exhumation along the Fairweather Fault since at least 5 Ma, with a southwards propagation of reverse faulting accommodated by the Boundary and Yakutat splay faults.