Abstract: An Early to Mid-Eocene Large Igneous Province in the Oregon-Washington Coast Ranges

R. S. Babcock, C. A. Suczek, D. C. Engebretson

A conservative estimate of the volume of surface and subsurface exposures of the Crescent (Metchosin) basalts in Western Washington and southern Vancouver Island is 300,000 km³. This is considerably greater than the far better known Columbia River basalts; indeed, one of the largest igneous provinces in the world can be delineated by considering the volume of the adjacent and coeval volcanics of the Oregon Coast Ranges.

In the Olympic Mountains, the Crescent basalts occur as a distinct tectono-stratigrpahic package, with the volcanics sandwiched between the basal Blue Mountain unit of terrigenous turbidites and an overlying section of deep to shallow marine sedimentary rocks of the Tofino-Fuca basin. Our interpretation is that the basal sediments were deposited during the initial stages of subsidence of a series of rift basins that formed along the continental margin of North America during an early Eocene transition from a compressional to a trans-extensional tectonic regime. As rifting continued, mantle upwelling and decompression partial melting produced voluminous tholeiitic volcanics that built numerous island complexes, with thicknesses as much as 10-15 km. ⁴⁰Ar/³⁹Ar datin shows that this period of volcanism extended from at least 57 to 45 Ma in the Olympics. As volcanism waned, both thermal and tectonic subsidence resulted in deposition of thick sequences of marine sediments that overlie the basalts.

Tectonic conditions during the early to mid-Eocene that may be related to continental margin rifting and extrusion of such an immense volume of basalts could have involved any or all of the following: (1) northward transit of the Kula-Farallon-North American triple junction along the Oregon-Washington coast; (2) a significant increase in the velocity and obliquity of subduction of the Kula and Farallon plates; (3) a southerly shift of the absolute motion of the overriding North American plate; (4) a 15-20° difference in the vectors of the Kula and Farallon plates, which would have created a large "slab window" beneath North America and differential subduction traction at the triple junction; and (5) possible initiation of a hot spot (Yellowstone?).

AAPG Search and Discovery Article #90958©1995 AAPG Pacific Section Meeting, San Francisco, California