PALEOMAGNETIC CONSTRAINTS ON THE PACE OF VOLCANIC ACTIVITY IN THE CLEAR LAKE VOLCANIC FIELD, CALIFORNIA

The youngest California Coast Range volcanic activity is focused within the Clear Lake Volcanic Field (CLVF), where eruptions began ~3 Myr and lasted until ~10 kyr. Existing age models for volcanism in CLVF suggest episodes of higher-volume extrusion and relative quiescence. However, existing geochronology (⁴⁰Ar-³⁹Ar, U-Pb) struggles to parse the tempo of eruption—which can be rapid even on human timescales—during periods of focused extrusion. Paleomagnetism can reveal volcanic episodes on these short timescales because of the rapid pace of geomagnetic field changes. These changes are recorded within lavas that cooled at different times, creating an interpretable record of ‘paleosecular variation’.

The oldest episode of volcanism at CLVF (~2.8-1.3 Ma), is characterized by mafic lavas. Consistent with new ⁴⁰Ar/³⁹Ar geochronology, paleomagnetic data show an extended interval of eruption, with multiple polarities and scattered paleomagnetic directions. Eruption of spatiotemporal clusters of geochemically-evolved lavas occurred next in the CLVF (~1.3 to 0.3 Ma). Paleomagnetic data from the lavas around Mount Hannah (~Jaramillo subchron) suggest extrusion of at least 7 dacite units within the same geomagnetic polarity transition, an estimated window of less than 5 kyr. The composite domes and flows of Mount Konocti, which represent the largest volume accumulation of related lavas in the CLVF, were erupted later in this episode. Five dacite map units on Mount Konocti—including two recently dated by ⁴⁰Ar/³⁹Ar —share an analytically-indistinguishable magnetization direction, suggesting an eruption duration of 100s of years rather than 1000s for a large volume of the mountain. The youngest volcanic episode (0.28–0.01 Ma), principally basaltic andesite eruptions in the northern reach of the field, has scattered paleomagnetic directions, suggesting distribution in time, akin in style and composition to the earliest episode.

Paleomagnetism demonstrates with high temporal resolution how different periods of eruption at CLVF have vastly different eruptive behavior. Occasions of felsic volcanism can be concentrated in space, and very concentrated in time, contrasting with the spatiotemporally-distributed mafic volcanism.