The structure of the Olympic Mountains, Washington; analysis of a subduction zone

Abstract

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-northwesttrending 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 ofthe Crescent Formation to the east resisted the eastward movement of the accreted sedimentary prism.

Dynamics of sediment subduction‐accretion at convergent margins: Short‐term modes, long‐term deformation, and tectonic implications

“…A simple explanation is an earlier P-C phase, followed by the transition to P-U-(C?). This combination accounts for the tectonic underplate, the uplift and denudational exhumation of the core of the Olympic Mountains, and the rotation of the Coast Range Terrane [Tabor and Cady, 1978;Brandon et al, 1998]. The rotation of the backstop clearly indicates a change of mode and demonstrates that the backstop ceased to act in this role when the mode changed.…”

Petrology and stratigraphy of Paleogene nonmarine sandstones, Cascade Range, Washington

“…These include, from structurally highest to lowest, the upper Triassic Haro Formation and the upper Jurassic to lower Cretaceous Spieden Formation which are mainly volcanic!astic rocks deposited in mixed shallow marine environments adjacent to a nearby volcanic source terrane (Johnson, 1979). While no suitable source terrane is known for the Haro, appropriate source rocks were available for the Spieden (Johnson, 1979 (Tabor and Cady, 1978); peripheral rocks consist predominantly of tholeiitic basalts of the early and middle Eocene Crescent Formation of Brown, Gower, and Snavely (1960) and intercalated deep-water clastic deposits overlain by upper Eocene to…”

Regional Geophysics and Structural Framework of the Vancouver Island Margin Accretionary Prism

“…However, to the south, late Eocene and younger Core rocks exposed below and seaward of the Crescent Terrane volcanics on the Olympic Peninsula (Fig. 8) probably represent the older components of this accretionary prism (e.g., Tabor and Cady, 1978;Snavely, 1987).…”