Metamorphic and plutonic rocks within the Peninsular Ranges batholith of southern and Baja California can be divided into structural and petrographic zones parallel and symmetrical to the axis of the batholith. If the axial zone exposes deep plutonic phenomena and the lateral zones progressively shallower phenomena, it is possible to reconstruct a cross section of the area as it existed during the last phases of plutonic rock emplacement. Such a cross section can be used to demonstrate that great welts of tonalitic magma accumulated near the base of the pre-existing crust. These spawned plutons that rose as diapirs through the zone of plastic deformation to the level of fracture deformation; the diapirs then bled upward into shallow plutons and extrusive masses.

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