Aeromagnetic evidence and geologic structure, northern Olympic peninsula and Strait of Juan de Fuca, Washington

Four north-trending aeromagnetic profiles provide a means of interpreting crustal structure of the northern Olympic Peninsula. Consistent westerly structural trends, evident from geologic field relations and from the aeromagnetic data, make feasible a two-dimensional analysis of the data. A simple model which satisfies interpolated magnetic data and surface geology along a selected line consists of (1) a stratiform mass of volcanic rock of the Crescent Formation (Metchosin Formation of Clapp on Vancouver Island) with an average magnetization of 0.0015 emu/cc; these volcanic rocks are asymmetrically folded in the Clallam syncline and they are both underlain and overlain by relatively nonmagnetic sedimentary rocks; and (2) a tabular body, dipping steeply northward beneath the central part of the strait, with an average magnetization of 0.0030 emu/cc. The volcanic rocks, as modeled, are thickest (about 30,000 ft or 9,000 m) in the south limb of the Clallam syncline and thinnest (about 15,000 ft or 4,500 m) beneath the strait. The modeled tabular north-dipping rock body beneath the strait is about 20,000 ft (6,000 m) thick at depths greater than that of the base of the Crescent Formation. These characteristics of the model are consistent with a structural interpretation in which post-Crescent sedimentary rocks onlap the Crescent toward the north, and one in which the present limit of the post-Crescent Tertiary sedimentary basin extends no farther north than the exposed volcanic rocks along the south shore of the Strait of Juan de Fuca. Although evidence for the north-dipping magnetic body beneath the strait is derived entirely from the aeromagnetic data, possible counterparts are found in the gabbroic rocks that C. H. Clapp described and mapped as the Sooke intrusives on southern Vancouver Island.