Geology of the Lick Observatory Quadrangle, California

Abstract

The Lick Observatory 7.5-minute quadrangle exposes evidence of geologic events that range from subduction of Mesozoic Franciscan Complex, through accumulation of marine Miocene porcellanite and clastics, to the development of the San Andreas fault system and deformation within it. The active Calaveras fault zone, with its linear valleys and subparallel strike-slip strands, transects the quadrangle and, northwest of San Filipe Valley, joins and incorporates the older Madrone Springs fault. The topography has formed in the past 1 to 2 million years and rises northeastward from the East Evergreen range-front thrust, across the Calaveras and several inferred mountain-building faults, to the 1280 m crest of Mt. Hamilton. The stratigraphy includes coherent, variously schistose metagraywacke of the late Mesozoic Franciscan Complex; discordant zones of melange of sheared shale and blocks that include blueschist and eclogite; serpentine that may represent the Coast Range Ophiolite; relatively undeformed sandstone, shale, and conglomerate of the late Mesozoic Great Valley sequence; marine Miocene Claremont Porcellanite, mudstone, and Briones Sandstone; and deformed nonmarine gravels of the Pleistocene and Pliocene Santa Clara Formation. The Franciscan sandstones are complexly deformed and discordantly transected by tectonically emplaced melange zones; a local chert mass marks the remnant of a discordantly overlying thrust sheet. Southwest of the Calaveras zone, folded Miocene rocks are faulted over the more strongly deformed Great Valley sequence. Those rocks, in turn, are thrust over small windows of Franciscan rock, and the entire mountain mass is thrust over Santa Clara gravels at the foot of the range. These latter structures postdate the 3.5 Ma imposition of compression across the plate margin suggested by plate tectonic reconstructions.