DEFORMED FLUVIAL TERRACES AT ROOT CREEK: IMPLICATIONS FOR DEFORMATION ALONG THE SOUTH-EASTERN EXTENT OF THE LITTLE SALMON FAULT ZONE

The Little Salmon fault (LSf) is an active, upper plate fault at the southern end of the Cascadia Subduction Zone (CSZ). The LSf extends on-shore as the primary northwest striking thrust fault of the southern CSZ fold and thrust belt. Along its eastern extent, the fault strikes more east-west and is mapped as both truncating against and offsetting the Coast Range Thrust. Research suggests the south eastern end of the fault is located near the transition of CSZ related contraction and dextral shear of the northerly migrating San Andreas fault system. This transition creates complex structural relations and uncertainty in the mapping of the south-eastern end of the LSf.

Recent investigations in the Root Creek area help further constrain the activity and kinematics of the south eastern termination of the LSf. Recent tectonic activity is manifest as uplifted flights of youthful fluvial terraces, a folded and faulted strath, and fault preserved coseismic colluvium. Fault striae demonstrate deformation of a youthful strath along northwest striking right oblique faults. Stratigraphic evidence suggests the most recent faulting event offset the active channel and backfilled river gravels and debris flow deposits against the scarp. A large, deep-seated landslide complex upstream is the likely source of preserved colluvium.

We map the deformation at the mouth of Root Creek extending southeast along the Root Creek drainage as a series of northwest striking lineaments and faults. The zone of faults and lineaments is south and parallel to the mapped trace of the LSf. We propose that faulting along Root Creek is part of a broad zone of deformation at the southeastern end of the LSf. This zone of deformation links CSZ related contraction to the northerly migrating dextral shear of the San Andreas fault system.