Abstract

The Black Hills, located about 15 km west of Olympia, constitute one of several large basement uplifts in the Washington Coast Range. The oldest rocks exposed in this west-dipping homocline consist of early to middle Eocene basalt flows and breccias with minor sedimentary interbeds, which are correlated with the Crescent Formation in the northern Olympic Peninsula. K/Ar geochronology of five basalt samples yields an average age of 53.1 ± 2.0 m.y. B.P., or early Eocene.

A total of 263 paleomagnetic cores were collected from 37 sites. All sites were cleaned using AF demagnetization, and 35 sites were corrected for measured and inferred tilt. The mean direction of these sites was virtually unchanged by applying tectonic corrections, although the precision was slightly improved. No flattening or steepening of inclination is apparent, although the mean declination of the 35 sites is easterly discordant with respect to the expected early Eocene declination for “stable” North America by 28.7° ± 15.4°. This discordance indicates that the Black Hills have rotated clockwise about 29° since early Eocene time.

The Black Hills basalts show significantly less rotation than Eocene rocks in the Oregon Coast Range, but they show nearly the same amount as analogous basalts in the southwestern Washington Coast Range and latest Eocene to late Oligocene volcanic rocks in the Oregon-Washington Cascade Range. Thus, the rotations of coastal Oregon and Washington, while identical in direction, differ significantly in amount, suggesting that the entire Coast Range block, from the Olympic Peninsula to north of the Klamath Mountains, has not been a coherent terrane since early Eocene time.

Several tectonic models are discussed that involve accretion and independent clockwise rotation of two or more Coast Range blocks, or “microplates,” in response to episodic periods of underthrusting and extensive right-lateral shear along the Farallon-North America plate boundary during early and middle Tertiary time. The easterly discordances of declination that are observed in almost all Tertiary rocks sampled in the Oregon-Washington Coast and Cascade Ranges reflect a composite of differential rotation during accretion of the Coast Range seamount terrane, followed soon afterward by large-scale rotation of the western margin of the Pacific Northwest in response to oblique subduction and consequent distributed shear.

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