Controls on Bending-Related Faulting Offshore of the Alaska Peninsula

Bathymetric profiles have demonstrated that incoming plates experience extensive normal faulting as they bend and subduct at the trench axis, however the relative importance of pre-existing (usually abyssal hill) faults, plate curvature and other factors in controlling the extent and style of bending-related faulting remain debated. The subduction zone off the Alaska Peninsula is an ideal place to investigate controls on outer-rise faulting. Pre-existing abyssal-hill fabric in the region, inferred to be parallel to magnetic anomalies, show a range of east-west oriented trends west of a remnant triple junction at 158oW and north-south oriented trends to the east. The orientation of the trench itself varies smoothly over this region. Previous studies with limited data suggest that the style and magnitude of bending faulting also varies along this subduction zone. We analyze new multibeam bathymetry data collected as a part of the Alaska Amphibious Community Seismic Experiment (AACSE) to characterize faulting and constrain controls on its formation. We observe substantial variation along-strike in the number, orientations and throws of faults. Bending-related faults west of 156ºW generally have strikes that vary between the trend of the pre-existing fabric (80º-90º) and the trench (60º-75º). Faults in a narrow zone near ~160ºW exhibit the largest variability in length (10-40 km), maximum offset (30-300 m), and dip direction (trenchward and seaward). We observe an abrupt termination of E-W trending bending faults at ~156ºW; to the east, observed structures are north-south trending and follow the trend of pre-existing structures and are thus likely to be relict abyssal-hill faults rather than active bending faults. The observed cumulative faulting decreases to the east across the study area. Curvature of the slab also varies along strike, increasing from a minimum bending radius of ~400 km in the west to ~600 km in the east. Comparisons between trends of outer-rise faults and magnetic anomalies imply that orientations of faults are strongly influenced by pre-existing structures. However, the westward increase in the total amount of faulting appears to be best explained by a westward increase in slab curvature.