Origin of blueschist-bearing chaotic rocks in the Franciscan Complex, San Simeon, California

Chaotic rocks exposed in sea cliffs south of San Simeon, California, consist of sub-rounded to lens-shaped fragments of graywacke, greenstone, and less abundant blueschist and chert dispersed in a matrix of argillite. This nonbedded mélange has been deformed twice. An earlier deformation, D₁, produced a strong northwest-striking, northeast-dipping foliation defined by both tectonically flattened inclusions and a parallel penetrative cleavage in argillite. Most inclusions, even of blueschist, are imperfect oblate ellipsoids or display pinch-and-swell structure and, locally, extreme necking and boudinage. Overall ductile behavior during D₁ was succeeded by the development of subparallel shear fractures that record a brittle deformation, D₂. Displacements on these fractures were generally small or negligible.

It is clear that neither D₁ nor D₂ was responsible for the original lithologic heterogeneity and chaotic fabric of the mélange. Mixing of foliated, glaucophane-lawsonite blueschist with lower-grade graywacke and greenstone to yield a nonbedded diamictite composed of variously sized clasts in a mudstone matrix must have occurred prior to D₁, probably by sedimentary processes involving submarine sliding and downslope transport of debris flows. There is no evidence that the mélange was bedded or that the blueschist inclusions were tectonically introduced among lower-grade rocks, prior to D₁. The sequence of sedimentary and tectonic events suggests that as the subduction ultimately responsible for the Franciscan Complex as a whole proceeded, blueschists in elevated parts of previously accreted material were eroded, mixed with lower-grade rock, and deposited as olistostromes that were subsequently accreted and deformed.