Investigation of arc processes: Relationships among deformation, magmatism, mountain building, and the role of crustal anisotropy in the evolution of the Peninsular Ranges Batholith, Baja California (Mexico)

Detailed structural and geochronological studies of a spectacularly exposed region of Baja California have revealed the structural evolution and links between orogenic processes at mid- to upper crustal levels of the Mesozoic Cordilleran magmatic arc system. The Jura-Cretaceous Peninsular Ranges batholith (PRB) of southern and Baja California contains one of the most remarkable examples of a crustal transition between oceanic- and continental-floored arcs in the world. Within this transition is an enigmatic belt of contractional deformation that stretches at least 800 km along the batholith and corresponds with a dramatic eastward increase in exposed crustal depths. In the Sierra San Pedro Martir of northern Baja California a spectacular ∼20 km-wide, contractional, doubly vergent fan structure occurs across the PRB basement transition. This structure formed over a period of >40 m.y. during intrusion of the batholith and consists of mylonitic, inward-dipping marginal thrust sheets that gradually change to a steeply-dipping zone of mylonite gneiss in the center. More than 15 km of differential exhumation occurred across the western side of the fan structure in Late Cretaceous time.; The mechanical effects of inherited crustal heterogeneity, including the basement transition zone in the batholith and a crustal buttress to the east, played a major role in the evolution of the fan structure. Intrusion of an extensive, sheeted plutonic complex in the center of the fan created rheological contrasts within the arc that led to partitioning of deformation for a limited period of time. Variation in crustal structure across the PRB also determined the extent and dynamics of uplift and denudation during mid- to Late Cretaceous evolution of the batholith, possibly including an Andean-style orogenic plateau across its eastern half. A portion of the western side of the batholith in this region appears to have collided with the North American margin in late Early Cretaceous time, but the effects of this event on the margin were masked by subsequent orogenesis that affected much of the southern Cordillera.