SOUTHERN CORDILLERA OPHIOLITE ASSOCIATIONS, NORTHERN CORDILLERA SUPERTERRANE ACCRETION, AND DEEP MANTLE SEISMIC WAVE SPEED ANOMALIES-AN INTEGRATED SYNTHESIS

This synthesis recognizes distinct S. Cordillera ophiolite associations and their links to the accretion of two N. Cordillera arc terranes: the Intermontane (ST1) and Insular (ST2) Superterranes. ST1 was a long-lived Pz-early Mz arc system built on rifted ribbons of the Cordilleran passive margin and trapped Panthalassa lithosphere. It accreted to North America in Permo-Triassic through Jurassic time. ST2 consists of early Mz arc segments built on an amalgam of Neoproterozoic-Pz arcs. It accreted to outer ST1 in Late Jurassic-Early Cretaceous time. S. Cordillera ophiolite associations are: 1) fragments of Pz Panthalassa abyssal lithosphere (PzPan) dispersed as basement within ST1 and its marginal basin, and as transform-subduction accretion assemblages along the central Klamaths and Sierran metamorphic framework; 2) Late Middle Jurassic inter-arc basin lithosphere (Josephine basin-JB) that rifted through PzPan that was previously accreted to southern ST1; and 3) Middle Jurassic Coast Range ophiolite (CRO) interpreted as remnants of an elongate basin that opened northwards adjacent to the southern Cordillera, and then was trapped as S. Cordilleran forearc by Franciscan neo-subduction. The ST1-PzPan linkage entailed trapping of abyssal lithosphere during the initiation of the ST1 subduction-arc system, and the subsequent shearing off and southward dispersal of the SW tip of ST1 by a major Panthalassa boundary transform that also truncated the SW Cordilleran passive margin in the late Pz. The later stage tectonics of this regime also drove closure of the ST1 marginal basin initiating ST1 accretion. ST2-CRO linkages entailed the progressive oblique (dextral) collision and escape of ST1 in the southern Cordillera as the CRO basin opened in its wake. As CRO rifting progressed northwards an inner oblique rift opened through the oblique collision deformed western Sierra-Klamath PzPan floored forearc, which progressively opened the JB system northward. Final ophiolite basin closure and oblique collision were temporally reversed to sinistral sense during North America’s APW J2 cusp. Aged Panthalassa lithosphere consumed between the obliquely colliding ST2 and accreted ST1 is a plausible source for hypothetical pre-Farallon plate mantle anomalies imaged beneath the central North American plate.