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Subduction of the Yakutat microplate in southern Alaska, either directly or indirectly, has had a more profound effect on deformation in southern and central Alaska than any other geologic event in the late Tertiary. Subduction underplating and related tectonics are probably responsible for deformation leading to hydrocarbon traps in the Cook Inlet and perhaps in the North Slope area. The last five or so years have seen escalated study of the geologic and geophysical aspects of Yakutat subduction, including: (1) study of deformation and tectonic processes associated with oblique subduction in the St. Elias orogen; (2) seismicity and tomographic studies to provide constraints on shallow subduction; and (3) study of recent motion of theYakutat microplate using geodetic data. Even though the main Yakutat collision zone is being heavily studied, relatively little work has been done to evaluate deformation of the accretionary complex above the subducted Yakutat microplate in the Prince William Sound and western Chugach Mountains.

The Prince William Sound and western Chugach Mountains (PWS-CM) form an ~100x100 km syntaxial region. This region consists of the highest and most glaciated mountains of the western Chugach Mountains and displays major geomorphic features, such as glacial valleys and trends of mountains, that curve from E-W trends to SW trends toward the west. The deformation style changes from dominantly strike-slip to the east and southeast to dominantly dip-slip contraction farther west. However, the nature of the contractional deformation is poorly constrained because (1) rocks that are potentially offset by large faults lack markers that allow determination of offset magnitude and (2) much of the structural evidence is covered by glaciers. The PWS-CM syntaxial region lies ~20-30 km above the interface of the subducting Yakutat microplate and the North American plate and is located west of complicated strike-slip deformation in the St. Elias area. The Prince William Sound and western Chugach Mountains are located where strain might be focused due to subduction above the shallowly dipping, relatively buoyant Yakutat microplate.

Our principal hypothesis for this work is that the PWS-CM syntaxial region is the location of rapid material removal by exhumation/erosion to balance out the accretionary flux brought in with shallow subduction of the Yakutat microplate. To test this hypothesis, we are using low-temperature thermochronometry and field study of key areas of the Prince William Sound and western Chugach Mountains. Several other low-temperature thermochronometry studies show that apatite fission-track and apatite (U-Th)/He ages are all older than about 15 Ma all around the periphery of the PWS-CM syntaxial. These studies suggest that exhumation of this region may be relatively slow and is not responsible for removing significant accreted material during Yakutat subduction. We are attacking the main syntaxial region where we believe the effect of focused erosion might be the greatest and low-temperature thermochronometry ages will be much younger than around the periphery areas.

Work to date includes two seasons of field work to sample some of the critical areas of the PWS-CM region and processing of most of the collected samples. In late summer 2008, two students and I spent three weeks sampling at sea level in the Prince William Sound and at the toe of three major glaciers on the north side of the western Chugach Mountains. Bedrock samples of granite, felsic dikes, and sandstone were collected via boat access in the fiords of the Prince William Sound. These samples will be used to evaluate the exhumation history in a N-S- and plate-transport direction to test the hypothesis of younging of ages toward the syntaxial core. The glacial outwash samples will be used to evaluate whether the mountains that are crossed by the glaciers contain different populations of ages – the glaciers essentially act as conveyor belts that sample much of the elevation range of the glaciated mountains. Young age populations would indicate that there has been relatively recent and rapid exhumation and will help to focus future sampling strategies. All of these samples have been processed to extract the minerals apatite and zircon for thermochronology work. In the second field season, two additional students and I collected more samples from the Prince William Sound based on mineral separation results from year 1. We focused on two sampling themes: sampling across potential major faults and collecting glacial outwash from glaciers draining the southern part of the Chugach Mountains of the study area. We also collected one elevation transect to evaluate exhumation rate in the southern Chugach. These samples are being processed now.

We have some preliminary results from our first season sample collections. Preliminary (U-Th)/He ages from sea level are much younger than around the periphery of the syntaxial region. Single-grain ages as young as 2 Ma have been determined for our northernmost sandstone samples. Overall, (U-Th)/He ages tend to decrease northward and into the syntaxial region, with ages as > 10 Ma in our southern samples. These initial results are starting to bear out our initial hypothesis that exhumation is focused in the syntaxial region. This deformation must be accounted for in future models of structural reconstruction for hydrocarbon generating basins in southern Alaska. We have also finished the first round of zircon fission-track dating on the glacial deposits from the north side of the range. Three samples from three major glaciers were evaluated to give age peaks of 36-42 Ma and 50-60 Ma on two of the samples; these age peaks correspond to major heating events associated with near trench plutonism. The third sample gives similar peaks, but also contains a younger robust peak of ~3 Ma; this young age probably reflects young volcanism from the Wrangell Mountains, but we cannot yet rule out the possibility of these young ages being related to very young and rapid exhumation.

Thus far, this work has funded research for one graduate student and three undergraduates and has led to the publication of four abstracts at national and international meetings.