The Northbrae rhyolite of Berkeley (California, USA) constrains motion of the proto-Hayward Fault

ABSTRACT

Right-lateral transform motion associated with the Pacific-North American plate boundary in the modern-day San Francisco Bay Area occurs across a series of sub-parallel fault zones. Much of this motion is accommodated east of the San Andreas Fault by the faults of the East Bay fault system. A major tool for reconstructing the spatial and temporal history of fault motion is the correlation of offset Neogene volcanic rocks. These Neogene volcanics within the California Coast Ranges formed in association with the slab gap that grew as the Mendocino Triple Junction migrated northward. Some of the volcanic centres have been variably offset by subsequent strike-slip faulting. A felsic volcanic unit exposed in Berkeley, CA, known as the Northbrae rhyolite has variably been interpreted to be one of these Neogene volcanic units or to be a Mesozoic volcanic unit associated with the Coast Range ophiolite. A new U-Pb zircon date of 11.10 $\pm$ 0.09 Ma confirms the Neogene volcanic interpretation. This date is indistinguishable from previously published Ar/Ar dates from the Burdell Mountain volcanics of the North Bay region as well as a new U-Pb zircon date of 11.07 $\pm$ 0.10 Ma. In addition to the indistinguishable ages, similarities in bulk lithology, zircon crystallization/dissolution textures, and zircon trace element geochemistry are consistent with these rhyolites being associated with the same volcanic centre. This correlation implies that 40 $\pm$ 5 km of right-lateral offset occurred to the west of the modern-day position of the Hayward-Rodgers Creek fault zone. This offset represents $\sim$20% of the total offset along the East Bay fault system. A proto-Hayward Fault with a different geometry than that of the present-day played a significant role in the evolution of the fault system. This result highlights the dynamic spatiotemporal variability of strike-slip faults along transform margins.

KEYWORDS:

• Right-lateral transform motion
• Pacific-North American plate boundary
• San Francisco Bay Area
• East Bay fault system
• Neogene volcanic rocks
• slab gap volcanism
• Mendocino Triple Junction
• strike-slip faulting
• Northbrae rhyolite
• Burdell Mountain volcanics
• U-Pb zircon dating
• Hayward Fault
• fault offset
• proto-Hayward Fault
• transform margins
• zircon rare earth elements

Acknowledgments

Conversations with Roland Bürgmann gave insight into the active tectonics of the Bay Area. N.L.S.-H. is grateful to George Brimhall for his introduction to East Bay geology and Coast Ranges volcanism in conjunction with passing along the curriculum to the UC Berkeley course EPS 101: Field Geology and Digital Mapping which has been taught continuously since 1890. The manuscript benefited from constructive reviews by Jonathan Miller and two anonymous referees. The research was supported by the UC Berkeley Ramsden Fund for undergraduate research. Mark Schmitz, Jim Crowley, and Darin Schwartz facilitated analyses at the Boise State Isotope Geology laboratory. Stanley Mertzman enabled XRF analyses at Franklin and Marshall College. We thank Yiming Zhang, Shannon Lavelle, and Leah Kahn for their assistance with fieldwork. Data and code for figure generation is available on Github (https://github.com/Swanson-Hysell-Group/Northbrae_Volcanics) and also archived on Zenodo (https://doi.org/10.5281/zenodo.10611132).

Disclosure statement

No potential conflict of interest was reported by the author(s).

Supplementary material

Supplemental data for this article can be accessed online at https://doi.org/10.1080/00206814.2024.2355620

Correction Statement

This article has been corrected with minor changes. These changes do not impact the academic content of the article.

Additional information

Funding

The research was supported by the UC Berkeley Ramsden Fund for undergraduate research.