CONSTRAINING THE EXHUMATION HISTORY ALONG INFERRED FAULTS IN THE WESTERN CHUGACH MOUNTAINS, ALASKA

The southern margin of Alaska is an accretionary complex that is actively being deformed and exhumed in response to the low angle subduction of the Yakutat microplate. Strain associated with subduction of the buoyant Yakutat microplate resulted in widespread deformation within the accretionary prism and may be focused in the western Chugach Mountains and Prince William Sound. Determining whether exhumation is uniform or focused along subsidiary faults in the western Chugach Mountains and Prince William Sound is important to understanding how strain is accommodated throughout the south-central Alaskan margin. The accretionary prism is bound to the north by the Border Ranges Fault Zone, which has acted as a backstop to exhumation. Several subsidiary faults, including the Contact fault, appear to have deformed the heavily glaciated, and locally high, Chugach Mountains south of the Border Ranges Fault Zone. The locations and significance of subsidiary faults in the Chugach Mountains are poorly constrained by traditional methods as they intersect monotonous greywacke packages and become untraceable beneath glaciers and fiords. Within Prince William Sound, College and Harriman Fiords are parallel to the NE-SW trend of the Contact fault and dissect the extreme relief of the Chugach Mountains farther northeast, which suggests their locations are fault controlled. Here we use apatite (U-Th)/He ages [AHe] to constrain the exhumation history across College and Harriman Fiords. Initial AHe ages across College Fiord show little variance from 3.8-4.9 Ma to 4.2-4.7 Ma on the south and north, respectively. Concordant ages indicate that exhumation has been relatively uniform ca. ~4 Ma along College Fiord. Additional samples, however, from Harriman Fiord and the northern terminus of College Fiord will refine this interpretation. Significant differences in AHe ages are displayed across the Contact fault ranging from ~5 Ma on the north side to ~ 10 Ma on the south side. These results indicate that the Contact fault may have acted as a major rock uplift and exhumational boundary for the last 5 Ma, whereas potential faults along Harriman and College Fiords have accommodated significantly less differential exhumation during this same time.