Tectonic emplacement of the Burro Mountain ultramafic body, Santa Lucia Range, California

The Burro Mountain body is a crudely equidimensional block of unusually fresh ultramafic rock. This block, along with numerous smaller and more elongate serpentinite bodies, has been emplaced in a highly sheared Franciscan terrane immediately west of the Nacimiento fault. This fault separates two major structural units: (1) on the east, a 40- by 300-mile block with basement of granitic rock, schists and gneisses, and (2) on the west, a 10- by 100-mile tract of Franciscan Formation.

Within the Burro Mountain body the primary rock types and their approximate percentages are: peridotite (65), dunite (35), and pseudopyroxenite (