PARTITIONED DEFORMATION AND THRUST FAULTING IN NORTHERN PRINCE WILLIAM SOUND, ALASKA, CONSTRAINED BY APATITE (U-TH)/HE DATING

Southern Alaska is characterized by widespread deformation associated with shallow (6°) subduction of the Yakutat microplate. The western Chugach Mountains rise abruptly from Prince William Sound to over 4 km at Mt. Marcus Baker between the Border Ranges and Contact faults to form a syntaxial bend. Extensive glaciation carved steep valleys and fiords with the same trend as major structures. Recent studies show a 50% northward increase in exhumation rates across the Contact fault and concentrically into the core of the range. Faulting evident in the trend of fiords and glaciers may accommodate contractional strain throughout the accretionary complex on subsidiary structures between the Border Ranges and Contact faults. We used apatite (U-Th)/He ages (AHe) collected across College and Harriman Fiords and from a 1000 m elevation transect on Mt. Muir adjacent to Harriman Fiord to determine background exhumation rates in northern Prince William Sound as well as potential fault locations that are poorly constrained due to monotonous lithology. AHe ages across southern College and Harriman Fiords are nearly indistinguishable, whereas ages across northern College Fiord are ~5.2 and 3.8 Ma on the north and south sides respectively, suggesting a south-dipping thrust fault with 1.4 km of throw. The elevation transect shows two parallel age profiles and a zigzag pattern of younger ages at higher elevations, suggesting that two thrust faults displaced samples with similar cooling histories about 375 m. Extrapolation of age-depth profiles from ages of ~ 5 Ma to closure temperature depth indicate a minimum exhumation rate of 540 m/Ma for the last 5 Myr. South-dipping thrust faults roughly parallel to bedding on the Mt. Muir transect have relatively small displacements and are most easily seen cutting felsic dikes. The varied scale of faulting interpreted from the geomorphology and AHe age-elevation data indicates that rock uplift and related deformation between the Border Ranges and Contact faults in the northern Prince William Sound and Chugach Mountains syntaxial region is broadly distributed and associated with relatively small-scale structures.