Tectonic evolution of Franciscan and related rocks in the Lassics Peaks region, northern California Coast Ranges

The Lassics outlier, a 35 km long displaced structural block engulfed by melanges of the Franciscan Central Belt, provides unique insights into the tectonic evolution of the California Coast Ranges. The internal structure of the outlier consists of three fault-bounded nappes: an upper unit consisting of relatively undeformed and unmetamorphosed Great Valley Sequence (GVS) strata, a zone of serpentinite-matrix melange of the Coast Range ophiolite (CRO), and a lower unit consisting of variably deformed and metamorphosed Eastern Belt Franciscan Complex (EBFC) strata here named the Lassics terrane. Structural analysis of the Lassics outlier requires two major episodes of faulting to produce the current terrane assemblage--an early subduction-related thrust which emplaced the CRO over the Lassics terrane (Coast Range thrust), and a subsequent low-angle normal fault which dropped high stratigraphic levels of the GVS down onto its CRO basement. The latter fault, here named the Coast Range detachment, is inferred to be responsible for the tectonic unroofing of the coherent blueschists of the EBFC, and to be causally related to the coeval Laramide Orogeny.; Metasandstones of the Lassics terrane are of anomalously low metamorphic grade compared to typical EBFC, and many of the sandstones preserve their original depositional character. Detrital modes of Lassics sandstones are similar to the coeval Stoney Creek petrofacies of the GVS. Thus, in contrast to much of the Franciscan Complex, the Lassics terrane sediments may have been deposited in proximity to the Upper Jurassic Great Valley forearc basin.; Tertiary strike-slip faults in the Lassics Peaks area are shown to be responsible for as much as 150-165 km of northward translation of the Lassics outlier. These faults are part of a complex zone of faulting which trends up the middle of the northern California Coast Ranges, bypassing the Mendocino triple junction. Analysis of ongoing geodetic strain in a profile across northern California suggests that this interior fault trend may be accommodating a significant fraction of the Pacific-North American relative plate motion.