KINEMATIC INDICATORS WITHIN THE BELL PASS MÉLANGE ADJACENT TO THE WELKER PEAK AND SHUKSAN THRUSTS, SHUKSAN ARM, WASHINGTON

Shuksan Arm forms a ridge that extends for 3.5 km to the northwest from Mt. Shuksan in the Northwest Cascades Thrust system (NWCS), first defined by Misch (1966). The ridge offers outstanding exposures of the Chilliwack River terrane and the Bell Pass Mélange, separated by the Welker Peak thrust and structurally beneath the Shuksan thrust, the two structurally highest thrusts of the NWCS. The movement directions of these two faults bear on tectonic models that invoke orogen-parallel (top-northwest or north movement) vs orogen-normal (top-westward or southwestward) during the mid-late Cretaceous.

We are mapping Shuksan Arm with a focus on the Bell Pass Mélange, to investigate dominant transport directions for the Shuksan and Welker Peak thrusts. While the mélange on Shuksan Arm is currently mapped as a single unit, we find two mappable lithologies: ribbon chert structurally overlain by a banded slate. The slate’s banding is defined by alternating folia of recrystallized quartz and platy graphite. The contact is a low-angle fault marked by a variably thick zone (~60 m) of pervasively disrupted foliation. We suspect this disruption holds clues to the latest transport direction because it contains mesoscopic kinematic indicators consisting of SC fabric, asymmetric porphyroclasts, shear-bands, and slickenlines that overprint the foliation. Microscopically, similar features appear in thin-sections prepared from unoriented samples collected from the deformation zone surrounding the Welker Peak thrust. Our preliminary work indicates that similar features from oriented samples that we will collect during summer, 2022 can be used to identify the dominant transport direction of the Welker Peak and Shuksan thrusts.