FRANCISCAN MÉLANGES: COHERENT BLOCKS IN A LOW-DENSITY, DUCTILE MATRIX

The Franciscan Complex comprises the largely sedimentary basement of the California Coast Ranges. This trench deposit underwent a series of superimposed tectonic events since the end of Jurassic time, involving accretion, high-pressure (HP) recrystallization, buoyancy + wedge-driven exhumation, and transcurrent slip. Processes reflect plate convergence, transpressive-orthogonal subduction, and transpressive-transtensive offset. Besides stratigraphically intact strata, the Franciscan displays widespread mélanges of four main types: diapiric serpentinite intrusions, sedimentary olistostromes, broken formations, and tectonic block-in-matrix units. In the northern Coast Ranges, mélanges are especially prevalent in the Central belt, but do occur in the Eastern and Coastal belts. Diapirs show upward, buoyant flow relative to wall rocks, but some appear to have involved wedge-driven thrusting. Many serpentinite diapirs and tectonic mélanges contain exotic metamafic inclusions rimmed by actinolite-chlorite reaction rinds. Olistostromes include gravity slump blocks + conglomeratic lenses; petrologically similar to larger slide blocks, pebble layers document a surficial, sedimentary origin, as does the presence of volcanic arc clasts. Broken formations grade from intact stratal continuity to disrupted units; they only contain cognate boudins of rocks present in the ductile matrix. Some tectonic mélanges are simply intensely disaggregated broken formations, and include rock types of the stratigraphic host. Other tectonic mélanges carry exotic HP blocks of diverse lithologies, generally reflecting higher pressures than attended recrystallization of the low-density matrix. The four mélange types formed through diverse convergent plate-tectonic processes. Many were subjected to a multi-stage overprint; most are strongly deformed, obscuring original textures + structures. Broken formations are the most common disrupted units. In aggregate, mélanges reflect the operation of contrasting processes that attest to plate-tectonic evolution of the Franciscan Complex. Strong deformation accompanied oceanic plate underflow, but also took place during coeval HP metamorphism and surfaceward return of accretionary packets, then transitioned to long-sustained, chiefly dextral slip.