Seamount Subduction Dynamics and Long‐Term Evolution of the Franciscan Active Margin

Abstract

The Snow Mountain Volcanic Complex (SMVC; northern California, USA) is a well‐preserved example of a coherently‐exhumed subducted seamount. This study reappraises the genesis and evolution of this complex and surrounding units through detailed field, petro‐structural and geochronological analyses. This work demonstrates that the SMVC (a) erupted at ∼166 Ma as a hotspot volcano on the Farallon Plate, (b) entered the Franciscan subduction trench at ∼118 Ma, and (c) was subsequently subducted to a depth of ∼20 km (within the seismogenic zone), as shown by local blueschist‐facies assemblages formed at 0.6 GPa, 240°C. Transient subduction interfaces are preserved above, within, and below the SMVC, making it an exceptional target to study seamount subduction dynamics. Like other seamounts, the subduction‐related deformation was mainly accommodated along kilometer‐scale internal thrust zones lubricated by serpentinite/metasediments, and within centimeter‐thick crack‐seal veins recording pulsed fluid flow near peak metamorphism. No unequivocal proof of seismic activity was found. The integration of other seamounts (some potentially belonging to a former seamount chain) in the Franciscan Complex suggests that exhumed seamounts are more abundant than previously thought. Moreover, pressure‐temperature‐time estimates of subduction metamorphism for the surrounding units, combined with previous work constrain the thermal maturation of the subduction zone through time and the in‐sequence emplacement of the SMVC. Rapid changes in age of the subducted oceanic plate when subducted additionally hint to the subduction of large‐offset transform faults on the former Farallon plate. Such a process might have been linked to changes in accretion dynamics and magmatic flare‐ups in the arc.