FLUID-CATALYZED METAMORPHISM IN THE FOOTWALL OF AN ANCIENT SUBDUCTION THRUST FAULT, RING MOUNTAIN, MARIN COUNTY, CALIFORNIA

At Ring Mountain, Marin County, California, a sheet of serpentinized harzburgite is thrusted over Franciscan sandstones and cherts. Just below the thrust fault, the sedimentary rocks of the footwall display a textural and mineralogical aureole 10-15 meters thick. The aureole is characterized by a pressure solution cleavage, absent elsewhere in the footwall, and lawsonite growth along sandstone fabric. Cherts in the aureole are bleached, fractured, and densely veined with quartz.

We suggest that the aureole formed during fault activity at minimum 10 km depth, where the fault zone mélange formed a conduit for hydrous fluids escaping up-dip along the subduction thrust. Reaction rates were insufficient to activate lawsonite growth throughout the footwall sandstone, although pressure-temperature conditions were within the lawsonite stability zone. Near the fault, increased fluid activity catalyzed lawsonite nucleation and growth and the development of pressure solution fabrics, thereby producing a local zone of fault-related metamorphism.

We suggest that local-scale aureoles such as that observed at Ring Mountain may be used to infer hydrous fluid activity in an ancient fault zone, and may help preserve pressure-temperature history in low-temperature conditions otherwise imperceptible in the rock record.