MESOZOIC CORDILLERAN TECTONICS: PRE-FRANCISCAN SUBDUCTION INITIATION AND TERMINATION EVENTS, AND EPISODIC PROCESSES DURING CONTINUOUS FRANCISCAN SUBDUCTION

Mesozoic tectonic history that affected rocks now in north-central California can be divided into a period associated with long-lived east-dipping Franciscan subduction, and an earlier history that may have had multiple subduction initiation and termination events in a comparatively short amount of time, probably involving flips of subduction polarity. Limited exposures of high-T, high-P metamafic rocks, associated with ultramafic belts, may mark inception of subduction events in the Sierra. Multiple events may be associated with a single ultramafic belt. The highest-grade metamorphic rocks in the Franciscan Complex record subduction initiation in a sediment-starved nascent arc at about 160-170 Ma, after which subduction continued without break for more than 150 m.y., with events that include the following: (1) Varying age of the subducting oceanic crust, starting with subduction of young (<5 to 10 Ma) arc crust for the first ~10 m.y. then subduction of midocean ridge crust thereafter, which was 15- 20 m.y. at at 120 Ma, and about 100 m.y. at about 95 Ma, before becoming progressively younger toward subduction termination in the late Cenozoic; (2) Northward translation of the Coast Range ophiolite (CRO), the sheet of oceanic lithosphere that tectonically overlies the Franciscan, from the time of subduction initiation to about 145 Ma when deposition of the forearc basin deposits of the Great Valley Group (GVG) began; (3) High-P Franciscan metamorphism from about 165 to 85 Ma in exposed rocks; (4) Mostly non-accretion or subduction erosion from subduction initiation to about 120 Ma; (5) Shortening may have affected the GVG at about 125-135 Ma; (6) Episodic accretion of clastic sedimentary rocks (trench sediments) with a limited episode as early as 144 Ma, but mostly beginning at 120 Ma; (7) Exhumation of many of the intact blueschist-facies sheets of the Franciscan to crustal levels near that of the CRO and GVG from 100 to 70 Ma; (8) Accretion of some coherent Franciscan volcanic units at about 95 Ma; (9) Widespread east-vergent thrusting within the GVG starting at about 70 Ma, approximately coeval with the beginning of voluminous clastic sediment accretion represented by the Franciscan Coastal Belt. The timing of these events allow evaluation of linkages to more inboard tectonic events.