Late Quaternary faults and their relationship to tectonism in the Olympic Peninsula, Washington

Late Quaternary movement on three reverse faults and the presence of one possible strike-slip fault have been recognized in the southeastern part of the Olympic Peninsula of Washington. Movement along all four faults occurred during or after a pre-Fraser glaciation, and the last movement on the Saddle Mountain East fault appears to have occurred about 1,240 to 1,235 yr B.P.

Displacements along these faults correlate well with other workers' recent geophysical analyses of tectonic events in the adjacent Puget Lowland. The Dow Mountain and Cushman Valley (?) faults each approximate an orthogonal plane derived by focal-mechanism solutions for two groups of recent shallow-focus earthquakes in the Puget Lowland. The Saddle Mountain faults do not correlate with recent earthquake data, but the Saddle Mountain East fault, when plotted with its orthogonal plane, is compatible with the modern regional stress field determined by other workers.

The Saddle Mountain faults are believed to be Holocene features developed within an older northeast-trending zone of fracturing. This older zone possibly developed during a late stage of complex folding and doming of the Olympic Mountains, before the development of the northwest-southeast compressive stress system that has characterized this region during the Holocene.