Some petrological, geochemical and geochronological observations of the Peninsular Ranges Batholith near the international border of the U.S.A. and Mexico
The Peninsular Ranges batholith of southern California, U.S.A., and Baja California, Mexico, is a major segment of the great chain of late Mesozoic batholiths found on continental margins around the Pacific basin. The structural, petrological and geochemical character of this batholith seems to reflect the details of its particular tectonic setting on the southwestern margin of North America. The evolution of the batholith in time and space is inferred to have been controlled by the nature and rates of convergent interactions which occurred between the America plate to the east and unidentified oceanic plates to the west (possibly, but not necessarily, including the Kula and Farallon plates). A long episode of subduction with intermittent transform faulting in the Cenozoic has removed direct evidence for the age, relative motion and dimensions of these plates. The magmatic arc represented by the Peninsular Ranges batholith and its associated volcanic superstructure can provide much useful information about the nature of regional plate tectonics during the late Mesozoic. Field, petrological, geochemical and geochronological studies by a number of workers have revealed remarkable regularities in the architecture of the batholith which now provide significant constraints on interpretations of the plate interactions. More directly, these regularities provide valuable guides to the nature of the petrological processes and source regions from which the batholith was derived. In this article, observations made in a transverse 70 km strip of the batholith, 130 km long ENE, along the international border near San Diego, California, and Tijuana, Baja California (Figure 1) are organized to illustrate the nature of the transverse asymmetry of the batholith as it has been emerging from continuing field and laboratory studies. Although the data density for many parameters in this traverse region is less than optimal, the patterns are consistent with much larger bodies of observations from the entire northern 600 km of the batholith, except perhaps for the San Jacinto block, which exhibits some unique features. The latter terrane, which lies between the San Jacinto and San Andreas faults in the NE portion of the batholith, is therefore excluded from the following discussion.
Additional Information© 1979 San Diego State University. The authors wish to acknowledge the scientific collaboration, the assistance and the advice of the Institute of Geology, Ing. Diego Cordoba, director, National Autonomous University of Mexico, Mexico City, for portions of this work conducted in Baja California del Norte and Sonora, Mexico. The largest portions of this work were supported by grants from the U.S. National Science Foundation. The initial assistance came from a Penrose Research Grant from the Geological Society of America in which the senior author shared. Robert Hill gave valuable editorial assistance. Mrs. Victoria Kroll and Hrs. Rosemary Humphreys prepared the manuscript.