On the Jurassic tectonic evolution of California

The Jurassic Cordilleran margin of California experienced a prolonged period of subduction, Andean-style volcano-plutonism, terrane accretion, basaltic forearc volcanism and sedimentation, and contractional deformation; much of this history is recorded in the Sierra Nevada Foothills. These rocks also host the world-class, orogenic gold deposits of the Mother Lode belt.; The Late Triassic-Early Jurassic Person Blanco Arc comprises Jasper Point Formation lavas and cherts, basaltic volcanic rocks of the Person Blanco Formation, and coeval diorites of the Don Pedro intrusive suite. This package formed via supra-subduction zone volcanism in an extensional or transtensional forearc setting. Geochemical data suggest formation via small amounts (5-7.5%) of hydrous melting of depleted mantle source rocks.; Detrital zircon provenance studies of the Mariposa Formation suggest that the paleo-drainage divide migrated trenchward with time. Accordingly, the dominant sediment source shifted from material derived from the Klamath Mountains and Nevadan miogeocline with only modest input from the Sierran arc, to a source dominated by arc material. The zircon data also suggest that the onset of clastic deposition began circa 153-151 Ma.; 40Ar/39Ar radiometric dates obtained from mariposite samples demonstrate that Au-mineralization in the Mother Lode belt of the Sierra Nevada Foothills started as early as 152 Ma, and continued until at least 122 Ma. It is proposed that Au-mineralization occurred as a result of heating lower crustal material beneath the Mother Lode belt in response to an increase in slab-dip angle and oceanward migration of the saw tooth thermal structure.; Statistically rigorous confidence intervals calculated from analyses of basaltic volcanic samples obtained from large geochemical databases demonstrate that published binary and ternary discrimination diagrams seldom correctly classify samples from mid ocean ridges, island arcs, and ocean islands with better than 60% accuracy. A new probabilistic method for geochemical discrimination is developed. The new method is N-dimensional and uses a priori data to construct probability distribution functions from which a posteriori probabilities are generated. Tests of the new method demonstrate single analysis classification success rates for volcanic rocks from island arcs to be 83%, from ocean islands to be 75%, and from mid-ocean ridges to be 76%.