The crustal structure of the Peninsular Ranges batholith can be divided geophysically into two parts: (1) a western mafic part that is dense, magnetic, and characterized by relatively high seismic velocities (>6.25 km/s), low heat flow (<60 mW/m 2 ), and relatively sparse seismicity, and (2) an eastern, more felsic part that is less dense, weakly magnetic, and characterized by lower seismic velocities (<6.25 km/s), high heat flow (>60 mW/m 2 ), and abundant microseismicity. Potential-field modeling indicates that the dense, mafic part of the batholith extends to depths of at least 20 km and likely to the Moho. The magnetic anomalies of the western part of the batholith extend south beyond the spatially extensive exposures of the batholith to the tip of the Baja California peninsula, which suggests that the mafic part of the batholith projects beneath Cenozoic volcanic cover another 400 km. The linearity and undisrupted nature of the magnetic belt of anomalies suggest that the western part of the batholith has behaved as a rigid block since emplacement of the batholith. The batholith may have influenced not only the development of the Gulf of California oblique rift, but also strike-slip faulting along its northern margin, and transtensional faulting along its western margin, likely because it is thermally and mechanically more resistant to deformation than the surrounding crust.

The Lakeview Mountains pluton is a concordant teardrop-shaped pluton located at a marked deflection of the structural grain of the prepluton rocks within the northern Peninsular Ranges batholith. This dynamically emplaced 100 Ma pluton lies within the western transition zone and consists of biotite-hornblende tonalite that lacks K-feldspar. The pluton is characterized by ubiquitous schlieren that range from black hornblende-biotite rock to near-white quartz-plagioclase rock, imparting an extreme outcrop-scale mineral and chemical heterogeneity to the pluton. Geometrically, the schlieren define three structural sets; one is concordant, and the other two constitute a northeast- and northwest-oriented conjugate set. The orientation of the concordant schlieren resulted from the outward expansion of the pluton, and the orientation of the conjugate set is in response to regional stresses at the time of emplacement. Based on chemical analysis of systematically collected samples, the pluton consists of two chemically distinct parts. Initially emplaced magma formed an ellipsoidal body concordant with the regional northwest structural grain. This early-emplaced magma formed a zoned body having a relatively potassic core and a mafic outer part. Later-emplaced magma expanded the pluton to the north-northeast, deflecting the regional structural grain of the batholith, and forming the teardrop-shaped outline of the composite pluton. The later-emplaced magma was more mafic than the initial magma, producing a more mafic core and a relatively higher-potassium outer part. Variations in major and trace elements, specific gravity, magnetic susceptibility, and magnetite content, in addition to aeromagnetic and pseudogravity anomalies, all show similar patterns within the pluton. Bodies of hypersthene gabbro, large masses of melanocratic and leucocratic tonalite, and numerous potassic granitic pegmatite dikes are concentrated in the more mafic part of the pluton, and interpreted as the last to crystallize. The leucocratic and melanocratic tonalite bodies are interpreted to be late-emplaced giant schlieren. Initial 87 Sr/ 86 Sr ratios (Sr i ) have only subtle, limited systematic variation, reflecting a relatively uniform magma source. Rb/Sr ratios also are relatively constant from the early- to late-emplaced magma, indicating the absence of, or only slight, fractional crystallization from the early- to late-emplaced magma. Sr i values of the hypersthene gabbro, mafic enclaves, and granitic pegmatites are the same as the tonalite of the pluton. The deflection of the regional structural grain by outward expansion of the pluton is interpreted to be a result of dynamic emplacement of the magma. Highly attenuated mafic enclaves in a prepluton mixed granitic rock unit that partly surrounds the pluton are also interpreted to have developed in response to outward expansion during the dynamic emplacement. A comb-layered gabbro, located along the contact of the earlier-formed part of the pluton, is interpreted as an early water-rich magma emplaced in a border area of the pluton protected from primary flow and dynamic strain. The high-potassium pegmatite bodies are interpreted to have formed from residual, immiscible, water-charged fluids derived from the low-potassium tonalite magma, which are concentrated in the mafic last part of the pluton to crystallize.