TECTONIC HISTORY OF SOUTH CENTRAL ALASKA AS RECORDED BY THE SUSITNA AND COOK INLET SEDIMENTARY BASINS

Cook Inlet and Susitna basin are sedimentary basins located over the northern part of the Alaska-Aleutian subduction zone, separated from each other by the right-lateral Castle Mountain fault. The region has been subject to complex tectonic events including flat slab subduction, possible spreading ridge subduction, and oroclinal bending, the effects of which are recorded in the faults and folds of these basins. Cook Inlet faults and folds are relatively well mapped via previous studies of seismic, well, magnetic and outcrop data. These studies showed NNE-striking anticlines and synclines that are mostly parallel to modern seismicity trends and are hypothesized to be related to wrench tectonics (Haeussler et al., 2000). In contrast, Susitna basin is less well understood. We combined aeromagnetic data collected 2000-2012 with industry seismic and well data in order to discern key structures. A layer of deformed Paleocene mafic volcanics generates measurable aeromagnetic anomalies that are aligned with structural highs imaged with seismic data. Comparisons with seismic and well data suggest NE-oriented folds and faults that were later cut by N-striking reverse faults. Also, in a ~25x25 km area near Yenlo Hills, aeromagnetic data suggest that structures were locally rotated counterclockwise by up to 20°, perhaps an effect of escape tectonics. The orientation of the NE-striking folds and faults is close to that of modern seismicity to the north, where flat slab subduction of the Yakutat microplate is believed to be occurring. We hypothesize that the NE-striking structures are a response to shortening associated with flat slab subduction. The cause of the N-S structures is more enigmatic, but may be related to later stages of flat slab subduction, aspects of escape tectonics, or other perturbations in the local stress field.