TECTONOSTRATIGRAPHY OF BAJA CALIFORNIA TRIASSIC OPHIOLITES; A SOUTHERN PERSPECTIVE OF ALTA CALIFORNIA OPHIOLITE TERRANES

Outstanding exposures of Upper Triassic ophiolite in two areas of the Pacific margin of the Vizcaino Peninsula include, from structural base to top, serpentinized harzburgite-dunite tectonite, cumulate (clinopyroxene + plagioclase ± olivine ± orthopyroxene) gabbro, isotropic gabbro, plagiogranite stocks and dikes, sheeted dikes, and pillow lava with interpillow carbonate rocks. The more northerly exposure is the ~3 km thick Sierra de San Andres ophiolite, which rests in low-angle structural contact on blueschist-bearing serpentinite-matrix melange and is depositionally overlain by Upper Triassic tuff, chert, and limestone and Jurassic and Lower Cretaceous volcanogenic sedimentary rocks. This ophiolite has yielded U-Pb ages of 221 and 220 Ma from plagiogranite and albitite. The southerly exposure is a ~5 km thick, southeast-striking sequence that intersects the coast between Puntas San Pablo and San Hipolito. This succession is overlain by Upper Triassic chert with pelagic limestone and breccia, followed upward by a thick succession of arc-derived Jurassic volcaniclastic sandstone. This sequence is strongly tectonized at its base and intruded by arc-related tonalitic plutons that post-date the ophiolite and yield U-Pb ages of 135-156 Ma. Geochemical data indicate both ophiolites are suprasubduction zone ophiolites formed in the forearc region of an island arc.

The Vizcaino ophiolites are dominated by extensional rather than contractional structures, lack evidence for subduction metamorphism, and are located in a structural position analogous to that of the Middle Jurassic Coast Range ophiolite above a Franciscan-like Mesozoic subduction assemblage and below a Great Valley-like Cretaceous to Eocene forearc basin sequence. The Vizcaino ophiolites, however, are Triassic rather than Middle Jurassic and are the same age as stratigraphically complex, polygenetic forearc ophiolite assemblages of the Triassic-Early Jurassic Sierra-Klamath belt. The consanguinity within the Vizcaino ophiolites distinguishes them from those rocks and suggests that they may represent fragments of a Late Triassic interarc basin not recognized in the Sierra-Klamath belt.