Tectonic significance of low-temperature blueschist blocks in the Franciscan mélange at San Simeon, California

The blueschist-bearing Franciscan mélange exposed near San Simeon is very similar to the mélange terrane forming the Central Belt north of San Francisco. Epidote-bearing low-T blueschist blocks at San Simeon (n = 3) are the same age as high-T garnet-bearing blocks in the northern, and Diablo Range mélange terranes (~ 155–150 Ma). This age approximates the time of initiation of Franciscan subduction. During the initial stages of Franciscan subduction, part of the top volcanic sequence from the subducting oceanic plate and small volumes of overlying sediments were imbricated at the bottom of the overriding plate and metamorphosed under high-P conditions. This underplated material was variably retrograded as subduction refrigerated the forearc blocks. Actinolitic rinds formed at the contacts between blueschists and intercalated mantle under relatively static conditions as the base of the overriding plate became serpentinized. A model is proposed in which exhumation was facilitated by gravity-driven, seaward-oriented extensional thinning of the forearc prism caused by the decrease in subducting plate dip starting around 80 Ma that caused the Laramide orogeny. As this occurred, blueschist and graphite-schist blocks were plucked from the bottom of the hanging wall, incorporated into the shale- and water-rich shear zone at the plate interface (subduction channel shear zone), and exhumed during the upward flow of mélange driven by the movement of the downgoing plate while subduction was still active.

► San Simeon low-T blueschists and high-T blocks to the north formed coevally.
► Formation at bottom of hanging wall followed initiation of subduction at ~ 160 Ma.
► Dynamic underplating was followed by static recrystallization while parked at depth.
► Extensional thinning driven by fattening of the slab at 80 Ma caused exhumation.