Geology, Petrology, and Paleomagnetism of Eocene Basalts from the Black Hills, Washington Coast Range

Geologic mapping in the Black Hills area strongly suggests that the middle Eocene basalts of the Crescent Formation and over- lying upper Eocene and Oligocene sedimentary rocks constitute a structurally coherent terrane that is bounded by northeast- and northwest-trending faults. I interpret the Black Hills as a homocline which dips about 10° to 15° to the west. Units within the block are commonly cut by normal and reverse faults, but are not appreciably folded. Major- and trace-element geochemical analyses indicate that the Black Hills suite is co-magmatic, and is composed of hyper- sthene-normative tholeiites which were apparently derived by plagioclase — clinopyroxene — olivine ± magnetite fractionation. The suite is petrochemically similar to basalts from the upper part of the Crescent Formation of the Olympic Peninsula, and the upper flows of the lower member of the Siletz River Volcanics of coastal Oregon. An island arc origin for the Black Hills rocks, and by analogy the Crescent basalts, is not supported by field and petrochemical evidence. Discriminant plots of incompatible element data for the Black Hills rocks indicate that the suite is nearly identical to tholeiites from an oceanic island (Hawaiian-type) setting, although the lavas approach mid-ocean ridge basalt compositions as well. These intermediate incompatible-element compositions of the Black Hills rocks, along with Sr isotopic ratios. resemble those of tholeiites from Iceland and Galapagos, both of which are oceanic islands that were erupted at, or close to, the ridge crest of an active oceanic spreading center. I suggest that the Black Hills lavas, and possibly the tholeiitic and overlying alkalic flows of Eocene age in the Oregon-Washington Coast Range, may reflect generation of the seamount chain on the crest or flanks of an active or fossil spreading center. The composition of the erupting lavas probably evolved toward progressive enrichment in certain incompatible elements (i.e. Ti, Zr, Y, Nb) as the chain moved away from the spreading center axis. The Coast Range oceanic island chain was subsequently sutured to the leading edge of North America by late Eocene to early Oligocene time. Paleomagnetic study of 35 sites in the Black Hills and adjacent areas indicates that most of the lava flows have declinations of remanent magnetizations that are significantly more easterly-directed than expected for cratonic North America, both before and after application of tectonic corrections. Using the preferred procedure for tilt-correction, the mean Black Hills paleomagnetic direction is: Dec.= 16.3°, Inc.= 67.3°, ∝(95= 4.9°. A rotation of 25.9° ±15° clockwise since middle Eocene time is inferred from these data; there is no evidence of north-south translation. The Black Hills show significantly less clockwise rotation than coeval rocks in the Oregon Coast Range, such as the Siletz River Volcanics, Tyee-Flournoy sediments, and Tillamook Volcanic Series. Data from the Willapa Hills south of the study area confirm the differential rotations the Oregon and Washington coastal blocks. The paleomagnetic results suggest that the entire Coast Range terrane, extending from the Olympic Peninsula to north of the Klamath Mountains, has not been a coherent terrane since middle Eocene time. A tectonic model more consistent with available paleomagnetic data involves accretion, and independent clockwise rotation of two or more Coast Range blocks, or microplates , in response to oblique subduction of the Farallon plate beneath western North America during Paleogene time