Plate motions recorded in tectonostratigraphic terranes of the Franciscan complex and evolution of the Mendocino triple junction, northwestern California

摘要

The Mendocino triple junction area of northern California is underlain by the Coastal belt of the Franciscan Complex, flanked on the east by the Central and Eastern belts of the Franciscan Complex. The Coastal belt is further divided into three tectonostratigraphic terranes, from northeast to southwest, the Yager, Coastal, and King Range terranes. Upper Cretaceous through middle Miocene rocks included in these terranes were accreted to the North American plate margin partly during normal convergence with the Farallon plate between 49 and 25 Ma at poleward rates of 2 to 5 cm/yr (Coastal and Yager terranes), and partly during translation with the Pacific plate between 14 and 2 Ma at poleward rates of 3 to 6 cm/yr (King Range terrane). Except for sparse blocks of Upper Cretaceous limestone and basalt scraped off the Farallon plate, as well as rare blocks of blueschist of uncertain origin, the Coastal belt terranes are composed principally of Paleocene to Eocene arc- and continental-margin-derived turbiditic detritus. Midlatitudinal affinities of foraminifers and palynomorphs in the Coastal and Yager terranes, and the presence of an Eocene conglomerate containing clasts of jadeitic and lawsonitic metasandstone, radiolarian chert, and serpentinite, suggest no more than about 600 km of right-lateral postdepositional displacement of the clastic rocks. Furthermore, the presence of arc-derived turbidites with interbedded Late Cretaceous ocean-ridge or seamount basalt near Usal, Calif., suggests access of sediment derived from the American plate to the Pacific-Farallon Ridge area sometime between 82 and 69 Ma. Uplift of the California convergent margin during the middle Miocene (ca. 15 Ma) resulted in overlap of the Franciscan rocks already accreted to the margin by shelf and slope deposits. These overlap deposits, in turn, were downfaulted later in the Neogene to form the Humboldt forearc and related smaller basins. Until middle to late Miocene time, strata of the King Range terrane were deposited in a trench-slope or slope setting along the Farallon-North American plate boundary, possibly as far as 435 km south of Cape Mendocino. 13.8-Ma metalliferous hydrothermal system, as well as evidence indicating that the accompanying thermal anomaly affected the entire King Range terrane, suggests that these rocks were rifted from the California margin about 14 Ma, over a slabless window (slab gap) at the north end of the San Andreas transform, and accreted to the northeast side of the Vizcaino structural block of the Pacific plate. The King Range terrane then was translated northward with the Mendocino triple junction and obductively reaccreted to North America during the early Pleistocene. Compressional tectonics at the northwestern California margin since about 3 Ma accounts for active uplift and thrusting near Cape Mendocino, compression of earlier-formed, Neogene pullapart basins, and the absence of Coast Range volcanism north of Clear Lake. Offshore and onshore structural relations suggest that the present-day Mendocino triple junction is situated near the hamlet of Petrolia, Calif., north of the King Range terrane, rather than off shore, where it has traditionally been located. The San Andreas fault zone thus may include the east and north boundaries of the King Range terrane.