FRANCISCAN COMPLEX STRUCTURAL ARCHITECTURE, DEPOSITIONAL AGE, AND PROVENANCE FOR WEST MT. TAMALPAIS, CA

The Franciscan Complex of western North America is considered by many to be the archetypical subduction accretionary complex, yet few geochronological studies have focused on providing constraints of the local underlying structural architecture. Preliminary results of detailed geologic mapping and detrital zircon U-Pb geochronology on the western flank of Mt. Tamalpais, CA reveal a succession of downward younging, relatively thick sandstone units representing the tectonostratigraphy. Mapping indicates that most of the sandstones constitute broken formations that can be traced laterally across the area, rather than a single sandstone unit and a widespread mélange unit that were previously mapped here.

Ten sandstone samples, consisting of either feldspathic or lithic arenite, were collected along a west to east transect from the base of Mt. Tamalpais to an elevation of approximately 2000 feet. A total of ca. 1100 detrital zircon U-Pb ages were used to constrain the maximum depositional age of these units and provide age spectra. The age spectra help to correlate the sandstone strata, evaluate provenance of the sandstones, and aid in determining the structural architecture the Mt. Tamalpais Franciscan rocks. The U-Pb ages indicate that the structurally lowest unit has a maximum depositional age ca. 87-89 Ma, while structurally higher units have maximum depositional ages ranging up to 120 Ma. One sample, collected within a sandstone olistolith in the structurally highest unit, has a maximum depositional age of ca. 124 Ma. The lower units have detrital zircon U-Pb age groups of 87-100 Ma, 105-120 Ma, 135-175 Ma, 185 to 220 Ma, 1300-1500 Ma, and 1600-1800 Ma. The primary age groups of the structurally higher samples have ages ranging from 110-170 Ma, with age peaks commonly at ca. 120 Ma and ca. 140 Ma, and very few Proterozoic and Archean ages. The detrital zircon U-Pb data are consistent with a primary source region within the Sierra Nevada Arc and associated framework terranes. The structurally lowest strata may be as old as the maximum depositional ages indicate (ca. 87-89 Ma), however the true depositional ages could be as young as early Eocene when additional sources of detrital zircon became available.