THE CHARACTER OF DEFORMATION WITHIN AND NEAR THE CRESCENT FORMATION, OLYMPIC PENINSULA, WA, USA

The Olympic Peninsula of NW Washington exposes the long-lived orogen of the Cascadia Subduction Zone. They are defined at the map scale by the horseshoe-shaped exposure of “peripheral” basaltic and turbidite units [i.e., Crescent Fm. and Blue Mt. Unit (BMU)] of an accreted island plateau separated by the Hurricane Ridge Fault (HRF) from accretionary metasediments to the west. The “peripheral rocks” and the metasedimentary rocks just west of the HRF [i.e., Needles-Grey Wolf Lithic Assemblage (NGW)] preserve the earliest stages of deformation (Tabor & Cady, 1978), and this on-going study aims to define the character and relative timing of deformation in these understudied rocks.

The Dosewallips, Hamma Hamma, and Dungeness Rivers and Lake Cushman transects expose different parts of the Crescent Fm., BMU, and NGW that together create a composite picture of deformation from west to east. Metasediments within the vicinity of the HRF at the Hamma Hamma (i.e., NGW) and Lake Cushman (i.e., BMU) sites show northwest plunging folds at the outcrop scale. The folds and foliation have been rotated, steepened, and cut in places by a more shallowly dipping secondary shear foliation. Local folds in the metasedimentary matrix of pillow basalts in the Crescent Fm. are roughly consistent with the fold orientations west of the HRF, and the dominant fracture groups in the pillows are consistent with a conjugate set forming in the same state of stress as the northwest plunging folds. In the Dosewallips and Dungeness sites, the BMU contains locally sheared subaqueous pyroclastic material. The contact between the BMU and Crescent Fm. is defined by the Ori Fault, which has a damage zone of fractured and comminuted material leading to a ~ 4m wide fault core. The Crescent Fm. is dominantly fractured, with significant variability in the orientation of the fracture populations across strike, up section. The map pattern shows regional folding of the Crescent Fm. parallel to the orogen, and variation in fracture data may be consistent with this folding. A localized zone of cataclasite and pseudotachylyte ~1.6 km from the base of the Crescent Fm. suggests that faulting is important for strain accommodation in the basalt. Small scale faults are mutually overprinting with mineralization, indicating that fluids played a role in the development and evolution of faults.