New insights on Franciscan Complex geology, architecture, depositional age, and provenance for the western Mt. Tamalpais area, Marin County, California

ABSTRACT

New geologic mapping of the Franciscan Complex in the western Mt. Tamalpais area northwest of San Francisco, California, reveals a tectonostratigraphic stack of accretionary units that young westward and structurally downward. Serpentinite-matrix mélange at the top of the section, beneath a possible basal fragment of Coast Range Ophiolite, and a structurally low black-rock-bearing, mudrock-matrix Ocean Plate Stratigraphy (OPS) mélange located near the base of the section, bracket lithic and feldspathic metasandstone-dominated accretionary units. The ages of the metasedimentary accretionary units are constrained by U-Pb age dating of detrital zircons. Maximum depositional ages fall between 124 Ma and 88 Ma. Sediment provenance is dominantly in the Sierra Nevada arc to the east, but minor numbers of detrital zircons were derived from Paleozoic and Precambrian sources. The provenance age profiles (cumulative distribution plots) reveal changing age components of zircon populations over time, reflecting the history of unroofing in the Sierra Nevada arc. An unusual post-accretion thrust fault and atypical greenschist facies metamorphism uncharacteristic of the Franciscan Complex, plus probable Late Cenozoic faults, complicate the character of the geology of the Mt. Tamalpais area.

Graphical Abstract

KEYWORDS:

• Franciscan
• detrital zircon
• accretionary units
• Mt. Tamalpais
• geologic map
• Sierra Nevada unroofing

Highlights

• New geologic mapping of the Franciscan Complex, northwest of San Francisco, California, reveals a tectonostratigraphic stack of accretionary units that young westward and structurally downward.

• The ages of the metasedimentary accretionary units are constrained by U-Pb age dates of detrital zircons that fall between 124 Ma and 88 Ma.

• The map area exhibits an unusual post-accretion thrust fault and greenschist facies metamorphism uncharacteristic of the Franciscan Complex.

Acknowledgments

We would like to thank the Department of Geology at Sonoma State University for financial and logistical support. We also acknowledge George Gehrels, Mark Pecha, and other members of the University of Arizona Laserchron facility for hosting our research team and for use of its facilities. Thanks to Sonoma State students Victoria Simoneau, Justin Casaus, Joe Adams and Eli Messinger for assistance in detrital zircon sample collection, preparation, analyses, and data presentation. Thanks also to California State Parks for approving permits associated with the collection of detrital zircon samples. We thank Gary Ernst, Bob Stern, and Alan Chapman for critical analyses of this paper that were helpful in improving our presentation.

Disclosure statement

No potential conflict of interest was reported by the authors.

Supplementary material

Supplemental data for this article can be accessed here.