Description
The Peninsular Ranges batholith ( PRB) is separated into western and eastern plutonic zones. Western zone plutons range in age from ~ 140 to ~ l 05 Ma and vary in composition from gabbro to granodiorite. In contrast, eastern zone plutons range from~ l 05 to ~80 Ma and vary from tonalite to granite. The Cuyamaca-Laguna Mountains shear zone (CLMSZ) lies along the eastern edge of the western zone. Two tectonic models have been proposed for the PRB. One model predicts Early Cretaceous eastward-dipping, westward-verging thrust structures as a result of arccontinent collison. In contrast, ~50 million year old thrust structures overprinted by ~ 3 0 million year old extensional structures are predicted by the gravitaional collapse model. A portion of the ~ l 18 Ma Pine Valley granodiorite is exposed in the Scove Canyon segment of the CLMSZ. Point-count data indicate a mean granodioritic composition, with a range from monzogranite to tonalite. Major and trace elemental data indicate an alumino-cafemic associaton within a volcanic arc setting. A period of ductile deformation affected the Pine Valley granodiorite between ~ 105 and ~94 Ma. This period of deformation produced an S-C mylonitic fabric. S-surfaces have an average attitude ofN25W, 68NE. Within the plane of Sis a stretching lineation with an average attitude of 67@N53E. C-surfaces strike sub-parallel to S but dip more steeply (~84NE), and consistetnly deflect Ssu1faces in a dextral or normal sense. Neither of the two tectonic models described above are supported by the results of this study. However, processes associated with the gravitational collapse of a magmatic welt may have some significance if timing relationships are modified. Much work remains to be done in the CLMSZ before we can begin to understand the dynamics of its development.
