A rifted margin origin for the crescent basalts and related rocks in the northern Coast Range Volcanic Province, Washington and British Columbia

Abstract

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.

Subduction zone and crustal dynamics of western Washington; a tectonic model for earthquake hazards evaluation

“…Velocities in mafic rocks such as garnet granulites or garnet gabbros are 7-7.3 km/s and ultramafic rocks such as mafic eclogite, dunite, and pyroxenite are 7.4-7.9 km/s (Christensen, 1979). Babcock et al (1992) find no surface evidence for ultramafic rocks in western Washington or Oregon to match with the voluminous basaltic rocks that they interpret were associated with a rifted margin, with the exception of some fairly rare cumulate layers in the basalts. However, the rift phase may have included a large ponding of ultramafic melt from the mantle, which then partially solidified and differentiated to generate the basalts seen in such volume at the surface.…”

“…The eastern, structurally lower member has compositions like those in inter-arc-basin tholeites. The upper, more effusive, member has compositions very similar to Columbia River Group basalts and Babcock (1992) places both members in a tectonic model which involves forearc rifting. However, it is clear that the section of Crescent Formation (most likely the lower member) interpreted from the V p model under the Puget Sound region has highly variable V p /V s values in the depths near its base where most earthquakes occur.…”

“…An earlier Oligocene to Miocene volcanic arc is displaced somewhat westward, but overlaps the Quaternary arc. The Coast Ranges of Oregon and Washington are composed of voluminous basaltic flows of Eocene age that apparently formed in a nearshore pull-apart system, possibly in conjunction with a leaky transform (Wells et al, 1984;Babcock et al, 1992). These basalt flows ( Figure 1) are known as the Siletz River volcanics in Oregon, Crescent Formation in Washington and Metchosin Formation on Vancouver Island.…”

Constraining fault interpretation through tomographic velocity gradients: application to northern Cascadia

“…The Metchosin Igneous Complex in southern Vancouver Island is the extreme northerly exposure of the Crescent Terrane, which includes the Crescent Formation and Coast Range Basalts of western Washington State and Oregon (Babcock et al, 1992). This complex dips approximately 30 • to the north-northeast (Massey, 1986) and is bounded to the north by the Leech River fault (Fig.…”

Large-scale coast-parallel displacements in the Cordillera: a granitic resolution to a paleomagnetic dilemma

“…Mantle plume generated¯ood basalts erupted above the hotspot at 70 Ma (the Carmacks Group; Johnston et al, 1996) and 52 Ma (the Coast Range basalts; Babcock et al, 1992), providing pinning points for measuring subsequent orogen-parallel displacements. Crust onto which the Carmacks Group erupted was subsequently displaced 2000 km north of the hotspot track (Johnston et al, 1996), consistent with anomalously low magnetic inclinations recorded by this¯ood basalt (Wynne et al, 1999).…”