Based on age and numerous geophysical, geochemical, and petrological discontinuities, the Peninsular Ranges batholith (PRb ), southern California, can be divided into western and eastern zones. Within the northern part of the PRb the eastern edge of the western zone contains the northwest-striking CLMSZ, which is characterized a by westward-verging eastward-dipping mylonitic to mylonitic gneissic fabric. Field and microscopic evidence, along with previously published data, suggests two periods of deformation. An early contractional deformation (Dl) occurred at -118 to -115 Ma, and was soon followed by extensional deformation (D2) producing the Scove Canyon segment (SCs) of the CLMSZ. The SCs of the CLMSZ can be traced to the southwest into the western edge of the Cuyamaca Reservoir gneiss (CRg). Along its northwestern edge the CRg occurs as segmented sheet-like bodies against the ~ 118 Ma Pine Valley granodiorite. Geochemical and petrographical data indicate the CRg to be a tonalite that probably formed within a volcanic arc setting where local contamination of mantle melts with sialic material may have occurred. The foliation (S2) produced by D2 becomes strongly developed toward the CRg's southwestern edge where it occurs in association with well developed S-C mylonite. S2 has an average attitude of N25W /59NE. Within the plane of S2 is a well developed stretching lineation (L2) trending 66/NSSE. Shear bands (C2) strike N19W /-80NE. Sigma-type porphroclasts with asymetric tails indicate an east-sidedown sense of displacement. The S-C mylonites can be traced into the ~ 105 Ma Las Bancas tonalite, but do not cross cut the ~94 Ma La Posta pluton. These data imply that the S-C mylonite developed sometime between ~ 105 to ~ 94 Ma. Though the tectonic mechanism responsible for the extension in the SCs of the CLMSZ is not well understood, extension may best be explained by the gravitationalcollapse model. In this model, east-dipping subduction produces copious amounts of magma which rose into the crust. Eventually the inflated crust became gravitationally unstable, and collapsed on itself. As material spread outward from the collapsing central portion extensional mylonite formed, thus reflecting the apparent subvertical orientation of the maximum principal stress direction. INTRODUCTION The Peninsular Ranges batholith, southern California, can be divided into western and eastern zones (Silver and Chappell, 1988). This division is based on age and numerous geophysical, geochemical, and petrological discontinuities along a narrow belt between the two zones (Fig. 1). Ages of the two zones vary with the western zone ranging from ~ 140 to 105 Ma and the eastern zone ranging from 105 to -80 Ma (Silver and Chappell, 1988). Del 180 values, along with the spatial position of I and 1-S granites suggest that the western zone is largely ensimatic and that the eastern zone is largely ensialic (Gastil et al., 1978; Todd et al., 1988).