ALONG-STRIKE VARIATION IN ACCRETION, NON-ACCRETION, AND SUBDUCTION EROSION, FRANCISCAN COMPLEX CALIFORNIA

Post-subduction dextral faulting of the San Andreas fault system (SAFS) was restored to evaluate the spatial distribution of units of the Franciscan subduction complex of California that formed as a result of subduction accretion. The Franciscan exhibits significant along-strike variation in its recording of subduction-accretion processes. Most notably, two segments 830 km apart record subduction erosion associated with low-angle subduction events that took place at ca. 120 Ma (Condrey Mtn. schist) and ca. 80-70 Ma (Rand-Pelona schist and correlatives) in the north and south, respectively. The along-strike distance between the inboard tectonic windows is unaffected by restoration of San Andreas fault system slip because of the lack of dextral faulting between them. Between segments recording subduction erosion, the Franciscan records net accretion from ca.175 Ma to 12 Ma, but includes horizons recording non-accretion. None of the accreted units of the subduction complex extends the full length of the complex. One unit, correlated on the basis of its structural level in the subduction complex, lithologic characteristics, and distinctive detrital zircon age spectra, accreted at about 80-83 Ma extends 850 km (580 km without restoration of SAFS slip). The long-strike length of accretion of this unit highlights a pitfall of using detrital zircon age populations of subduction complex clastic rocks as indicators of strike-slip displacement. Whereas restoration of SAFS slip extends the along-strike length of some Franciscan units, it shortens that of others, such as the Skaggs Springs schist (330 km length unrestored, 100 km restored) and limestone-bearing Permanente terrane (260 km length unrestored, 110 km restored). Some reaches of the subduction complex include coherent schistose blueschist facies rocks, whereas others do not, and some reaches lack coherent blueschist facies rocks altogether.