The Mesozoic Peninsular Ranges batholith, part of a long-lived Cordilleran subduction orogen, is located at a critical juncture at the southwest corner of cratonal North America. The batholith is divided into northern and southern segments that differ in their evolution. In this paper, we focus on the more poorly understood southern Peninsular Ranges batholith, south of the Agua Blanca fault at ~31.5°N latitude, and we compare its evolution with the better-known northern Peninsular Ranges batholith. Adding our new insights to previous work, our present understanding of the geologic history of the Peninsular Ranges consists of the following: (1) stronger connections between the Paleozoic passive-margin rocks in the eastern Peninsular Ranges batholith and similar assemblages in Sonora, Mexico, to the east and the Sierra Nevada batholith to the north that were originally proposed by earlier workers; (2) continuity of the Triassic–Jurassic accretionary prism and forearc basin assemblage from the northern Peninsular Ranges batholith through the southern Peninsular Ranges batholith; (3) possible synchronous subduction of an ocean ridge or ridge transform along the Peninsular Ranges batholith in late Middle Jurassic time; (4) continuity of the Early Cretaceous Santiago Peak continental arc from the northern Peninsular Ranges batholith along the entire margin, including the southern Peninsular Ranges batholith; (5) development of the Alisitos oceanic arc in Jurassic and possibly Triassic time, much earlier than originally thought; and (6) removal of part of the Santiago Peak assemblage in the southern Peninsular Ranges batholith during collision of the Alisitos terrane in latest Early Cretaceous time.

The Alisitos arc segment is the southernmost and only part of the western Peninsular Ranges batholith accreted during the Cretaceous. Collision-related deformation is concentrated along the northern and eastern margins of the arc segment. While shortening within the Alisitos arc produced similar amounts of crustal thickening throughout the arc, suppression of parts of the lower crust of the Alisitos arc due to throw across the terrane-bounding faults varies substantially. Geobarometric change across the Main Mártir thrust suggests that ~15 km of additional crust was thrust onto the central Alisitos arc. Geochemical and geochronologic data from intrusive rocks of the Alisitos arc indicate arc magmatism was active before, during, and after collision. The data suggest that all Peninsular Ranges batholith intrusive rocks within the Alisitos arc were derived from a broadly similar primitive source, lacking interaction with evolved continental lithologies. Postcollisional intrusions from the central Alisitos arc adjacent to the Main Mártir thrust yield trace elemental signatures suggesting melt derivation at depths where garnet would be a stable residual phase. The spatial and temporal coincidence of these intrusions with the Main Mártir thrust suggests that the increased pressure of anatexis inferred for the depth of generation of these melts was generated by displacement on this fault. Further, close temporal and spatial characteristics, and similar geochemical characteristics between the central Alisitos arc intrusions and La Posta intrusions east of the Main Mártir thrust suggest that the Alisitos arc intrusions represent precursors to the much larger flare-up event. This observation supports models suggesting collision as a cause of magmatic flare-ups in arcs.