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Description
In the southernmost region of the Peninsular Ranges Batholith, prebatholithic rocks near Bahia de los Angeles comprise a steeply dipping, northwest striking package of schist and gneiss. Kilometer scale, grid mapping at 1:12,500 reveals four distinct protolith units: carbonate, volcanic+sedimentary, quartzite+conglomerate, and granite. The granite protolith, here designated the Cantera Gneiss, crops out over approximately 20 km2 as a pervasively sheared elongate body of granitic gneiss. The Cantera Gneiss has a prominent foliation and strongly developed mineral stretching lineation. The surrounding schist units have foliation and lineation fabrics concordant with the Cantera Gneiss, although the stretching lineation in these units is less prominent than in the gneiss. The sense of shear indicated by small scale structures at several localities is consistent with top to the west sense of displacement. Special attention to the measurements of the foliation and stretching lineation have provided us with a model to help determine the deformational history of the region. Petrographic analysis of samples document metamorphic mineral assemblages consistent with upper greenschist/lower amphibolite facies conditions of recrystallization. Micro structures including elongate crystal fabrics, mica fish structures and anastomosis can be seen in thin section and indicate sense of shear. Structural data were analyzed using the Stereonet 4.7a program which also confirms the continuity of the prominent fabric throughout the prebatholithic package. The structures of the metamorphic rocks in this region are consistent with major NNE-SSE compressional deformation. This shortening event is consistent with mid-Cretaceous east over west thrusting, documented along strike to the north in the Canon Calamajue region and further north in the southern San Pedro Martir region.
