Structure and Geometry of an Exposed Active Subduction Zone Splay Fault: The Deception Creek Strand of the Patton Bay Fault, Montague Island, Alaska

The Patton Bay Fault (PBF) on Montague Island, Alaska exhibited clear evidence of surface rupture and up to 5-10 meters of displacement during the 1964 Mw 9.2 subduction megathrust earthquake, and offshore portions contributed to tsunami generation during that event. Furthermore, the PBF controls topography of the southwestern portion of the island, suggesting a protracted history of displacement over many earthquake cycles. Co-seismic splay fault activation is hypothesized to be a major source of surface displacements in megathrust earthquakes globally, but few are exposed subaerially anywhere. We investigate the outcrop to micro-scale structure of two strands of the PBF using field observations, drone-based imaging, lidar, and fault and wall rock samples to better characterize the geometry and structural history of this exposed active fault. Outcrops in the coastal intertidal zone and upland expose apparently fresh, unaltered fault gouge, damage zone, and surrounding wallrock of Orca Group marine turbidites consisting of sandstones and mudstones. The Strike Creek strand exhibited slip in the 1964 earthquake but is a zone of just a few cm thickness in exposed outcrop, with little damage zone. By contrast, the Deception Creek strand does not have documented 1964 slip, yet exposes a newly-identified meters-thick cataclastic gouge core and associated damage zone. Using structure from motion methods, we have created an orthomosaic of the area from drone imaging in order to map outcrop-scale width and extent of the gouge-dominated fault core. Fault samples of representative elements will be analyzed for microstructural features accumulated during past slip events. By mapping the outcrop scale geometry and documenting structural and petrophysical properties of the fault core, damage zone, and wallrock, we will quantify the thickness of the fault zone and its structural features, permitting comparison to observations from offshore geophysical imaging of this and other splay faults. The scope of this study from micro- to outcrop scale provides insight into the architecture of subduction splay faults and will be the framework for three-dimensional models of the seismic attributes of submarine splay faults.