TECTONIC REGIMES OF ASBESTOS FORMATION IN CALIFORNIA

Fibrous/acicular/asbestiform varieties of serpentine and amphibole are found through out California, primarily in localized regions where the tectonic regime history of various rock types provided favorable conditions for asbestos to occur. While concerns over asbestiform amphiboles dominate the discussion of asbestos in California, chrysotile serpentine asbestos is more common, and found in large bodies of ultra-mafic rock in the Coast Ranges and Sierra Nevada foothills. The pioneering work of the Deep Sea Drilling Program, coupled with on land field work, has revealed the tectonic regimes and rock types necessary to form asbestos primarily include: 1) oceanic spreading centers - with serpentinization of exposed hot mantle peridotite and uralitization of extruded basalts and intruded gabbros, 2) subduction zones - with faulting and shearing of pre-existing serpentine, additional serpentinization of mantle peridotite, and high pressure/low temperature metamorphism of basaltic and gabbroic rocks, 3) fore-arc exhumation zones with shearing of diapiric serpentine, 4) low grade regional or intrusion related metamorphic zones with uralitization of basaltic and gabbroic rocks, and growth of fibrous amphiboles in meta-sedimentary rocks, and finally 5) active fault zones where previously existing serpentine is sheared and/or intruded along fault traces.