FIELD OBSERVATIONS ON FLUID BUDGET OF THE CASCADIA EPISODIC TREMOR AND SLIP SYSTEM, OLYMPIC PENINSULA, WASHINGTON

Fluid movement in accretionary prisms has been linked to the recently discovered episodic tremor and slip (ETS) events along subduction zones. This study focuses on the exhumed accretionary prism of the Cascadia subduction zone, where ETS events are well-documented. The exposed sandstone, shale, siltstone and minor basalt in the study location were buried to 6 – 15 km, within the depth constraints of ETS. This past summer, field work focused on observations of subduction related fluid budget as evidenced by veins, metamorphism, and pore space was conducted along an east-west transect on the Olympic Peninsula. Approximately 40 representative samples were collected near Obstruction Peak, Hurricane Ridge, Lake Mills (Elwha), and Sore Thumb (Sol Duc). Observations indicate progressively higher-grade metamorphism from west to east, in agreement with previous studies. Mudrocks show a clear progression from shale to phyllite, while sandstones generally appear equally micaceous across the transect, with the exception of one location. This location is unique in that micas are larger and other metamorphic indicators are visible in hand specimen, epidotes are visible in the rock body and veins, and this is the only location where veins make up a significant percent of the outcrop. These veins also contain quartz and calcite, usually as the primary mineral, are perpendicular to bedding and are primarily found in the coarser units. In addition to the veins, water is present in pore spaces and mineral structure. Preliminary observations indicate the veins and pore space decrease to the east, while evidence for fluid movement increases to the east. These observations will be tested and quantified by thin section analysis.