PALEOSEISMIC TRENCHING REVEALS SPATIALLY-VARIABLE HOLOCENE SLIP BEHAVIOR ON THE LEECH RIVER FAULT

A first-order goal in understanding strain partitioning between the subduction interface and the overriding crust is to constrain permanent forearc deformation. In the northern Cascadia forearc on southern Vancouver Island, prior paleoseismic study of the E-W striking Leech River fault in 2016 reveals three Holocene, surface rupturing, dip-slip earthquakes. However, Holocene right-lateral fault motion is recorded in bathymetric and paleoseismic studies on the Devils Mountain fault, the likely eastward continuation of the Leech River fault. To constrain the along strike kinematics of the Leech River fault, we present data from two new paleoseismic trenches, located 7 km E of the prior trench site, across an uphill-facing topographic scarp identified from lidar. Trench 1 exposes a vertical, anastomosing fault zone that cross-cuts a stratigraphic sequence consisting of lodgement till overlain by meltout till. Comparison of the orientations of elongate lodgement till clasts in the fault zone with orientations beyond the fault zone provides evidence for strike-slip faulting. North-side-up motion across the fault zone has folded lodgement till into an anticline that has a fold hinge parallel to both the main fault zone and the topographic scarp. Within the anticline, a second vertical fault zone that splays off the main fault zone at an angle of ~ 45° shows both extension and 1 m of vertical offset. We interpret the folding and extensional/dip-slip faulting to represent the formation of compressional mole track features and related tension cracks, which are common in strike-slip surface ruptures. The geometry of the fault zone and stratigraphy exposed within Trench 1 suggests that the south-facing scarp formed during a single? right-lateral surface rupture since deglaciation (ca. 15 ka). Trench 2, located 625 m NW of Trench 1, exposed left-lateral faulting of bedrock but showed no evidence for Holocene fault rupture. The slip sense observed in Trench 1 along with the right-lateral faulting documented along strike to the east, indicate the Leech River fault has accommodated Holocene right-lateral shear along its eastern extent, while in contrast the 2016 trenches to the west record only dip-slip events. This observation suggests spatial and/or temporal variations in kinematics along fault strike within the Holocene.