Stop 1 - Peninsular Ranges Batholith Overview

After turning south off highway 2 at the town of El Hongo, we proceeded to stop 1, where we became acquainted with the salient points of the Peninsular Ranges Batholith.

The central and eastern portions of San Diego County contain a series of small north-northwest trending mountain ranges that are part of the larger Peninsular Ranges geomorphic province. The mountain ranges are part of the Peninsular Ranges Batholith (PRB), a complex mixture of igneous and metamorphic rocks that extends southward from Riverside, California, to about the 28th parallel in Baja California. The batholith averages nearly 100 km in width and has a north-south length of approximately 1,000 km. It is composed of discrete bodies of coarse-grained igneous rock (plutons) that vary from a few kilometers to more than 40 kilometers in diameter as well as metamorphosed remnants of the Jurassic and pre-Jurassic rocks described earlier. The igneous rocks within the PRB reflect a complex and nearly continuous history of igneous activity that began near 125 Ma and lasted until about 90 Ma. That these rocks are related to subduction processes is. not disputed. Similar rocks are found in the Salinian block of west-central California and in the great Sierra Nevada Batholith. They indicate that during Mesozoic time, an extensive magmatic arc formed the western edge of the North American continent. This arc was part of a larger system that outlined much of the eastern Pacific Basin. It extended as far north as Alaska and southward along the western edge of South America to Tierra del Fuego at the southern tip of Argentina. The Mesozoic was a time of extensive volcanic activity during which plate motions, driven by changes at mid-ocean spreading centers, were rapid and variable. Thus, the Mesozoic history of greater San Diego County reflects that of the entire western Pacific and was irrevocably tied to fluctuations in spreading rates at mid-ocean ridges hundreds of kilometers distant.

The PRB contains hundreds of individual plutons, and countless dikes and small igneous bodies that were intruded into preexisting crustal rocks such as the Julian Schist and the gneissic granitic rocks in the Cuyamaca-Laguna Mountain (CLM) belt. It has an elongate, somewhat curved shape similar to that of a volcanic arc created by the collision of two lithospheric plates and is considered to be the root zone of such a crustal structure. In fact, the PRB consists of the plutonic roots of two adjacent parallel arcs that formed sequentially in response to changes in plate interactions. The boundary between these two systems occurs just east of the Laguna Mountains and trends northwest-southeast. It divides the batholith into an older western and a younger eastern zone that have their own distinct rock types and structures. The geologic history of each zone is thus the story of a single period of plate collisions that created the separate pieces of the batholith. We view this history by examining the rock types in each zone and then reconstructing the processes that likely created them.

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