THE KING RANGE TERRANE OF THE FRANCISCAN COMPLEX, CAPE MENDOCINO REGION, NORTHERN CALIFORNIA: PRODUCT OF MIOCENE AND YOUNGER INTERACTION OF EVOLVING, MULTI-STRAND MENDOCINO TRANSFORM WITH PACIFIC AND NORTH AMERICAN PLATES

In the Mendocino triple junction area of northern California, the accretion history of the Franciscan Complex King Range terrane (KRT), a highly deformed block of Miocene–Eocene turbidites and oceanic basalt at the northeast corner of the Pacific Plate (PP), has been enigmatic. During northward propagation of the San Andreas transform and coeval eastward extension of the Mendocino transform (MT) in the Miocene, a large segment of North American Plate (NAP) was transferred to the PP as the Vizcaino block. At the same time, rocks of the KRT were assembled along the MT, and later obducted as a northeast corner of the PP onto the NAP ≤2 Ma.

⁴⁰Ar/³⁹Ar ages and geochemistry of basaltic flows and intrusives from Mendocino Ridge east of ~127.5°, show that the ridge was built and deformed by dextral slip in the MT zone between ~23 and 10.7 Ma. Sampled basaltic rocks, including those from the KRT on land, are high in FeO and TiO₂ relative to MgO, consistent with eruption near or at a ridge-transform intersection (RTI). Analysis of bathymetry and magnetic lineaments suggest a MT setting similar to the Siqueiros Fracture Zone and East Pacific Rise near 8°N. Geochemistry and K-Ar dates from KRT basalts and micropaleontologic ages (diatoms, radiolarians) from interspersed middle Miocene ribbon chert indicate eruption ~16 Ma. These rocks then were overlain by continent-derived turbidites and complexly folded and sheared at the accretionary margin, followed or accompanied by thermal overprinting ~14 Ma (K-Ar+vitrinite data).

We propose that volcanics represented in the KRT were erupted in an intra-transform spreading segment of MT, coeval with chemically similar 18-11 Ma basalts found along the Mendocino Ridge, while the northward-migrating Mendocino Triple junction was abreast of Central California. Continent-derived Miocene–Eocene sediments of the KRT were tectonically juxtaposed with the KRT basalt as this Mendocino intra-transform segment encountered the NAP margin. The KRT assemblage of RTI basalt + ribbon chert + terrigenous sediments was further deformed during its subsequent obduction onto the continental margin.