THE TIMING AND DISTRIBUTION OF MAFIC LAVA ERUPTIONS DURING THE FIRST TWO MILLION YEARS OF THE CLEAR LAKE VOLCANIC FIELD IN NORTHERN CALIFORNIA

Ridge subduction and the northward migration of the Mendocino triple junction began roughly thirty million years ago along the coast of California. This setting gave rise to a slab-free window and generated seven volcanic centers hosted primarily within the California Coast Ranges. Of these discontinuous Miocene to Holocene volcanic centers, the Clear Lake volcanic field (CLVF) is the youngest and northmost. The CLVF is located within the broader San Andreas transform fault system and is bounded by major through going right-lateral systems. Geochronology indicates volcanic activity at the CLVF spanned from ~2 Ma until ~9 ka with several periods of hiatus. This protracted eruption history, an active geothermal reservoir, elevated regional heat flow, frequent seismicity, and proximity to the densely populated Bay Area all contribute to classification of the CLVF as a high threat volcanic system in California. As such, a comprehensive understanding of the CLVF eruption history is critical to assessing the potential for future eruptions and the accompanying threats to populations, infrastructure, and resources.

Although there was significant effort to establish the CLVF eruption history in the late 1970’s, these legacy datasets require updating with modern methods in order to fill critical gaps in our understanding of the timing and tempo of volcanism. Further, the duration of eruptive activity versus repose is not comprehensively constrained. To mitigate this gap, we present new groundmass Ar-Ar dates on primitive CLVF rocks erupted during the first two million years of field activity. These data suggest that eruption began earlier than previously thought, around 2.8 Ma instead of around 2 Ma, and that these eruptions were concentrated over the next million+ years in a sub-linear NW-SE trending band located southeast of the younger, more voluminous and more evolved portion of the volcanic field. Our new dates suggest contemporaneous eruption of some Sonoma volcanic field and CLVF rocks, a long-lived eruption history of relatively mafic magmas, and no evidence of prolonged eruption hiatus during this protracted period.