POTENTIAL THERMAL AND RHEOLOGICAL INSTABILITY OF SERPENTINITES IN MANTLE FOREARCS: EMPLACEMENT MECHANISMS BASED ON EVIDENCE FROM CALIFORNIA MERCURY MINES AND ROAD, AND COMMERCIAL EXCAVATIONS

Many natural exposures of ultramafic rocks in California are heavily weathered and the original nature of their contact relations with their host rocks is often unclear. The USGS Strategic Minerals Program investigations of Mercury mining excavations and highway and commercial excavations in California provided abundant early evidence in fresh exposures for the emplacement by cold injection of serpentinized ultramafic rocks as sills, dikes, and plugs. This literature (1930-1972) has been largely ignored in recent investigations of how and why such mantle rocks came to travel through the crust in California. Recent observations suggest that these rocks were mobilized under metasomatic conditions where dissolution and growth processes appear to enable weak serpentinite rheology (Uno and Kirby, Lithos, 2019), but what was the source of such fluids? Geophysical observations in many subduction margins (e.g., Cascadia) indicate that such forearcs are cold and are partially serpentinized from release of water from subducting sediment and slab crust. Thus changes in plate-tectonic inputs that lead to forearc mantle warming (e.g., subduction to continental transform motion in California) can potentially lead to release of water and mobilize mantle rocks by intrusion into the crust above (Kirby et al, EPS, 2014. I review, this extensive literature and call upon recent observations at the USGS by me and my colleagues Masaoki Uno, Maddie Lewis, and Caroline Kellner of excavated serpentinite bodies in the Bay Area of California. In doing so, I provide insights into the common occurrence of serpentinites along plate boundaries, both active and former. The latter includes the Alpine-Himalayan belt representing the various stages of closure of the Tethys Oceans by subduction and subsequent collision.