THE DARRINGTON–DEVILS MOUNTAIN FAULT—A PROBABLE ACTIVE REVERSE OBLIQUE SLIP FAULT ZONE IN SKAGIT AND ISLAND COUNTIES, WASHINGTON

The Darrington–Devils Mountain fault zone (DDMFZ) has likely been locally active in the Holocene. This regional fault zone juxtaposes the Northwest Cascades System and the mélange belts and has a complex displacement history beginning in the mid-Eocene (or perhaps the mid-Cretaceous). Tertiary left-lateral strike-slip offset is well demonstrated for the DDMFZ main strand. New 7.5-minute scale mapping along most of the DDMFZ shows that antithetic right-lateral faults merge into the main strand and en echelon synthetic faults locally broaden the DDMFZ to 8 miles wide. The stratigraphy and provenance of Eocene and Oligocene sedimentary rocks indicate that the DDMFZ has a transpressional and transtensional strike-slip history with major sub-basin instability starting in the mid-Eocene.

Post-glacial DDMFZ activity has been concentrated along the main strand, with perhaps some additional offsets along nearby antithetic and synthetic segments. In the Cascade foothills, stratigraphic and geophysical data are most consistent with main strand reverse faulting, with perhaps some left-lateral strike-slip or oblique movement. Uplifted Pleistocene Olympia beds and latest Pleistocene Glacier Peak laharic deposits imply episodic, south-side-up offset in the Quaternary. An anomalously steep river gradient where Pilchuck Creek crosses the main strand also imply post-glacial offset. In the north fork Stillaguamish River valley, well located earthquake hypocenters with reverse slip focal mechanisms spatially correlate with the main strand. The down-dip hypocenter distribution suggests that the fault zone shallows into a regional décollement. On Whidbey Island, the occurrence of anomalously high ancient Skagit River fluvial-deltaic sediments (Olympia beds) exposed in a growth fold directly south of the main strand appear to be the result of DDMFZ reverse or oblique faulting and uplift.

Reinterpretations of trench data combined with new lidar images near Lake McMurray provide evidence for Holocene main strand activity. Two east–west-trending scarps visible on lidar are interpreted as a Holocene graben formed in the hanging wall directly south of the DDMFZ main strand. The trench logs show a probable tectonic offset of the glacial and nonglacial deposits where the fault scarps cross the trenches.