A RIFTED MARGIN ORIGIN FOR THE CRESCENT BASALTS AND RELATED ROCKS IN THE NORTHERN COAST RANGE VOLCANIC PROVINCE, WASHINGTON AND BRITISH-COLUMBIA

The remarkable early to middle Eocene volcanic sequence of the Crescent Formation exposed on the Olympic Peninsula consists predominantly of tholeiitic to minor transitional alkaline basalts with sparse sedimentary interbeds. A composite section measured in the vicinity of the Dosewallips River includes 8.4 km of pillowed to massive submarine basalts overlain by 7.8 km of subaerial flows. An upper limit of about 48 Ma on the age of the Crescent basalts is indicated by faunal assemblages in sediments interbedded with the uppermost flows in the sequence and a circa 50 Ma 40Ar/39Ar age on a leucogabbro from the presumably correlative Bremerton Igneous Complex. Stratigraphically controlled samples collected from throughout the Crescent basalt sequence show that two distinctly different chemical types exist. The lower part of the sequence originated from a relatively depleted mantle course resembling normal (N) to enriched (E)-MORB. The upper flows have a chemistry resembling E-MORB to oceanic island tholeiites. This difference could be due to either variable metasomatism of a single source domain, or influx of a separate enriched-mantle source component during the extrusion of the upper part of the sequence. Paleomagnetic measurements indicate that the Crescent basalts have not been significantly rotated, nor translated northwards since their extrusion. Paleotectonic reconstructions show that formation of the Crescent basalts and the Coast Range volcanic province as a whole coincided with a marked increase in the velocity of oblique convergence of the Kula plate with North America at about 60 Ma. Other geologic, geochemical, and paleomagnetic data are consistent with the interpretation that extrusion occurred in a basin or series of basins formed by a rift system along the continental margin of North America. Rifting might have been initiated by the influence of a hotspot, an increase in the rate of oblique convergence, or the kinematic effects of the Kula-Farallon ridge as it migrated along the margin. If extrusion is related to the passage of the triple junction, then the Coast Ranges can be considered to be an important tectonic marker for early to middle Eocene plate reconstructions.