Geological Survey Professional Paper 1033 The Structure of the Olympic Mountains, Washington—Analysis of a Subduction Zone

THE STRUCTURE OF THE OLYMPIC MOUNTAINS, WASHINGTON—ANALYSIS OF A SUBDUCTION ZONE

By R. W. TABOR and W. M. CADY

Long thin packets of structurally disrupted rocks in the eastern core of the Olympic Mountains mostly top eastward, but the overall age of the rocks decreases westward, suggesting early folding around subhorizontal axes with imbricate thrusting or imbricate thrusting alone.

Continued east-west compression overturned beds eastward, bending the packets into an arc within the horseshoe bend of the Crescent Formation, a foldlike structure that formed as the core rocks were imbricated or was extant from the original arcuate distribution of basaltic seamounts. In the western part of the eastern core, secondary structures (B elements) (mostly small-scale folds) developed parallel to the steep axis of the fold of the basaltic horseshoe: in the southern part of the core, the rock packets were sheared off beneath the basalts on the south limb of the horseshoe bend. Later shear folding on a cleavage fan oriented parallel to a north-northwest-trending subhorizontal kinematic axis rotated B elements (folds) formed earlier, producing widespread late pencil structures, in part centripetal to the basaltic horseshoe, where the late cleavage intersected earlier deformed bedding and cleavage. This late folding produced a domelike structure extended asymmetrically eastward.

The complex structure of the core rocks is consistent with current models of accretionary prisms in subduction zones and developed its form as the thick mass of volcanic rocks of the Crescent Formation to the east resisted the eastward movement of the accreted sedimentary prism.