Controls on orogenesis along an ocean-continent margin transition in the Jura-Cretaceous Peninsular Ranges batholith

The Jura-Cretaceous Peninsular Ranges batholith (PRb) of southern and Baja California is a remarkable example of a zoned batholith containing distinct oceanic (western) and continental (eastern) basements. The transition between these basements is marked by a crustal-scale boundary along which distinct volcanosedimentary, structural, and metamorphic histories evolved during Mesozoic orogenesis. Our work across this boundary in the Sierra San Pedro Martir of Baja California, Mexico indicates that it controlled a number of processes in the PRb including magmatism, the location of forearc or intraarc basins, and the locus and extent of contractional deformation and denudation. However, our work farther north indicates that notable differences occur along strike in the character of the western arc and transition zone, and these differences are most pronounced across the modern Agua Blanca fault. This fault was also active in the Mesozoic, at which time it separated the western zone into northern and southern arc segments. In the northern half of the batholith, the western arc (Santiago Peak Volcanics) lie in depositional contact with Triassic(?)-Jurassic sediments that mostly received detritus of North American origin and show a long history of contractional deformation and late extensional overprint. We concur that this part of the batholith evolved in Jura-Cretaceous time across an inherited ocean-continent crustal join. In contrast, south of the Agua Blanca fault, the boundary between the western arc (Alisitos) and transitional zones is marked by reverse mylonite shear zones that typically correspond with inverted metamorphic gradients and sharp steps in maximum pressures and cooling histories. Here the transition zone contains basins that collected mostly volcanogenic detritus during the Late Jurassic-Early Cretaceous and preserve a history of long-lived contractional deformation including shear zone development during collision. Thus, although both northern and southern segments of the transition zone share similarities in their plutonic, structural, metamorphic, and denudaSchmidt, K.L., Wetmore, P.H., Johnson, S.E., and Paterson, S.R., 2002, Controls on orogenesis along an ocean-continent margin transition in the Jura-Cretaceous Peninsular Ranges batholith, in Barth, A., ed., Contributions to Crustal Evolution of the Southwestern United States: Boulder, Colorado, Geological Society of America Special Paper 365, p. 49–71. *E-mail: [email protected] K.L. Schmidt et al. 50 tional histories only the southern segment experienced collision. These observations indicate that the crustal-scale boundary strongly controlled subsequent evolution but that collision was not the major driving process for orogenesis. Figure 1. Map of western North America showing major Mesozoic Cordilleran batholiths and selected PaleozoicMesozoic tectonic features of the southwest North American margin discussed in text. Possible continuation of Ouachita-Marathon suture across Mexico from Stewart (1988). IDAHO PENINSULAR RANGES SIERRA NEVADA COAST RANGE ALASKA-ALEUTIAN RANGE