Description
Rocks of the northwestern Sierra de San Andrés of the Vizcaino Peninsula of Baja California Sur, Mexico, form a partial, dismembered ophiolite sequence. Serpentinized harzburgite with minor dunite, orthopyroxenite, and pediform and disseminated chromite comprise the ultramafic section. Ultramafic rocks of cumulate origin are absent. An interior mélange of sheared serpentinite contains exotic blocks of altered serpentinite, metavolcanic, and metasedimentary rocks which range from greenschist to probable high-grade blueschist facies. A disrupted but intercalated section of layered clinopyroxene-plagioclase ± olivine cumulates and olivine melagabbro cumulates overlie the ultramafic rocks along low-angle faults. Orthopyroxene (bronzite) is the major intercumulate phase in the olivine melagabbro. Non-cumulate uralite gabbro is exposed in isolated fault blocks adjacent to the cumulate gabbro. Hornblende-bearing plagiogranite locally intrudes the uralite gabbro, resulting in an intrusive breccia. Plagiogranite with a diabasic texture composes another fault block which may be part of a sheeted dike or sill complex. At least 400 m of microporphyritic plagioclase ± clinopyroxene pillow lava is exposed. In addition, small scattered fault blocks of plagioclase ± clinopyroxene spilite and keratophyre may represent rocks derived from a deeper level in the complex. Pillow lava is conformably overlain by tuffaceous shale, friable Buchia piochi-bearing shale, potassium feldspar-poor volcanic arenite, conglomerate and pebblestone of the Upper Jurassic Eugenia Formation. The Turonian middle member of the Valle Formation overlies the ophiolite along low- to high-angle faults of small displacement. The ophiolite complex is cut by a west-trending shear zone and by northwest-trending faults. The latter are probably related to the Cenozoic right-lateral fault system of the northern Vizcaino Peninsula. Chemical analyses of ten gabbroic and volcanic rocks exhibit low K2O content and AFM trends characteristic of other ophiolites and abyssal tholeiite. This, together with stratigraphic evidence, suggests that the ophiolite is a piece of remnant oceanic crust which formed close to the Late Jurassic continental margin. It is suggested that prior to emplacement, the ophiolite formed the trenchward part of the upper plate of a Jurassic subduction zone. As a result of water derived from dehydration of the down-going oceanic crust, serpentinite diapirs formed in the mantle above the subducted plate and rose upward to a position below the overlying oceanic plate. The emplacement of the diapirs may have produced Cretaceous and Tertiary uplifting of the Vizcaino Peninsula.
