CASTLE MOUNTAIN FAULT SOUTHCENTRAL ALASKA: OBSERVATIONS ON SLIP PARTITIONING FROM LIDAR AND PALEOSEISMIC TRENCHING

In the forearc of the Aleutian subduction zone, oblique convergence between the Pacific and North American Plates is responsible for counterclockwise rotation of southcentral Alaska. A component of this deformation is accommodated by strain partitioning between fault-cored anticlines in Cook Inlet and active faulting along the Castle Mountain fault among other structures. Our recent field and lidar-based geomorphic observations along the Castle Mountain fault indicate the presence of hanging-wall grabens, south-facing scarps, and folded surfaces, consistent with high angle reverse faulting. Left-stepping en echelon scarps suggest a component of right-lateral deformation. Here we present preliminary paleoseismic observations from two trench excavations across the Castle Mountain fault. These results will be combined with our previous lidar observations in an effort to assess the dominant sense of slip.

We excavated two trenches across an ~1 m high south-facing scarp that cuts flood deposits inset into late Elmendorf (14-15 ka) grounding line moraines in the Little Susitna River valley. Both trenches exposed interbedded layers of well sorted sand and poorly sorted pebble cobble gravels that are cut by a 1-2 m wide fault zone. Stratigraphic and structural relations exposed in the trench provide evidence for at least two earthquakes in latest Pleistocene/Holocene time. The most recent event is associated with several anastomosing fault traces that break the surface and form the scarp. A fault trace south of the main topographic scarp is buried by a subtle soil and overbank sand deposit providing evidence for the penultimate earthquake. Drag folding, discreet vertical offsets, stratigraphic juxtapositions and thickness changes are consistent with both vertical and lateral deformation. Thus, we infer that the Castle Mountain fault is a high angle oblique reverse fault. Ongoing topographic analyses will further refine the relative lateral component of slip.