NEW GEOLOGIC BEDROCK MAPPING AND ANALYTICAL RESULTS FROM THE ANIAK VOLCANIC FIELD, SOUTHWESTERN ALASKA

Recent advancements in Alaskan tectonics imply that the Alaskan orocline may have developed in response to multiple tectonic processes operating in southern Alaska since 70 Ma. These processes include: strike-slip faulting in the Cordillera, convergence between North America and Eurasia, formation of the Aleutian subduction zone, and <30 Ma collision of the Yakutat block with the southern Alaska convergent margin. To understand how these tectonic processes lead to counterclockwise rotation of western Alaska and related development of the Alaskan orocline, we present geologic bedrock mapping and paleomagnetic data collected in the Aniak volcanic field in southwestern Alaska. Existing 1:250,000-scale mapping shows the Aniak volcanic field as an undivided suite of mafic lava flows and hypabyssal intrusions that yield K-Ar dates ranging from ca. 69-72 Ma. The volcanic field is located at the intersection of the right-lateral Denali fault and left-lateral-extensional Aniak fault, both of which are poorly exposed but are expressed as lineaments in the topography and regional aeromagnetic data. Our 1:24,000 scale bedrock mapping reveals three petrological distinct basalt flows, a NE-striking dextral strike-slip fault, and a NW-striking normal fault. We interpret the strike-slip fault to be a subsidiary strand of the nearby Denali fault and the normal fault to be related to extension at the intersection of the Denali and Aniak faults. Preliminary paleomagnetic work reveals the presence of magnetite as the main remanence carrier with higher coercivity in the bottom and the top basalt flows and low coercivity in the middle flow. Alternating field and thermal demagnetizations show a stable paleomagnetic record on one block sample from each of the three petrologically distinct basalt flows, constituted by a single magnetic component for most specimens. This demonstrates their high potential for determining the magnitude of vertical axis rotation in the Aniak volcanic field. The forthcoming paleomagnetic data will be the first from the Aniak volcanic field and will provide insight to the magnitude of post-70 Ma vertical axis rotation in western Alaska.