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 Sierra Calamajue study area in Baja California, Mexico, exposes a series of units in several fault-bounded blocks. Units include the Jurassic–Cretaceous Alisitos island arc, Cretaceous continental-margin volcanic arc units, Mesozoic North America–derived sedimentary rocks, and North American Paleozoic passive-margin units. Deformation and metamorphism increase eastward. Units in the west are weakly deformed and metamorphosed, while upper-greenschist- to lower-amphibolite-grade assemblages farther east are intensely deformed. Most tectonic structures are probably related to the accretion of the Alisitos arc to North America in the mid-Cretaceous, although Paleozoic units contain an older fabric that supports deformation between the latest Mississippian and Late Permian. Overall, the study area marks the transition from an accreted ocean-island arc to the North American continental margin. Results from the Sierra Calamajue study area located along the eastern side of the Alisitos arc are similar to observations in the arc-continent transition zone farther north, where deformation gradients also exist. However, along-strike variations are recognized. Deformation recorded in the study area occurred at rather shallow crustal levels. In contrast, intense ductile deformation and exhumation from midcrustal levels are described elsewhere. Furthermore, a component of sinistral transpression documented along the northern edge of the arc is not recognized along the east side. We suggest that along-strike variations in the structural style are controlled by (1) tectonic setting (i.e., angle of accretion), (2) the preexisting geometry of the continental margin, and (3) changes in geology and rheologic strength of the units involved in deformation.

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