The Cuyamaca Laguna Mountains shear zone (CLMSZ), a regionally developed 5- to 20-km wide zone of ductile deformation, extends some 50 km through the eastcentral Peninsular Ranges. The Harper Creek gneiss and the Oriflamme Canyon unit are NW-striking and NE-dipping sheet-like plutons that comprise the westcentral portion of the CLMSZ in and around Garnet Mountain. Prior to this study, these units had not been studied in detail. The Harper Creek gneiss is a petrologically and texturally heterogeneous mylonitic orthogneiss derived from granodiorite. In contrast, the Oriflamme Canyon unit contains inclusions of the Harper Creek gneiss and is a protomylonite derived from tonalite/granodiorite. Two small volume metagranite units transect the mylonitic gneiss fabric characteristic of the Harper Creek unit. On an AFM diagram the three units mapped in the Garnet Mountain area display calc-alkaline trends. Trace-element tectonic discrimination diagrams indicate a magmatic arc setting for the Harper Creek gneiss, metagranite units, and Oriflamme Canyon unit. U-Pb zircon data from the Harper Creek gneiss indicate an approximate crystallization age of 132 ± 26 Ma. Previous investigators established a U-Pb zircon age of 119 ± 1 Ma for the Oriflamme Canyon unit. The two metagranites have not been dated. Field and petrological relationships suggest that the Oriflamme Canyon unit was emplacement during development of Sl, a prominent northwest striking and steeply northeast dipping mylonitic foliation that is pervasive throughout the Oriflamme Canyon unit. Lying in the plane of Sl is a well developed stretching lineation (L1) which plunges steeply to the northeast. Sl and L1 can be traced from the Oriflamme Canyon unit into and through the Harper Creek gneiss, the two metagranite units, and the Julian Schist. Textural studies suggest that Sl formed during dynamic recrystallization at metamorphic grades of at least epidote-amphibolite facies. Metamorphic mineral assemblages of pelitic inclusions within the Harper Creek gneiss suggest average crustal depths between 7 and 14 km. Dextral sense (Cd) shear bands, as viewed looking in a northward direction, are widely spaced in the Harper Creek gneiss and the two metagranite units. In contrast, sinistral sense (Cs) shear bands are prominent in the Oriflamme Canyon unit and Julian Schist where they define a major reverse-sense westward verging shear zone. Cd has not been observed to deform Cs or visa versa. These relationships are interpreted to indicate that Cd developed primarily in a hanging wall position during the development of the reverse- sense shear zone. Such an interpretation yields an internally consistent geometry and kinematic pattern for Cd, Cs, Ll, and Sl. New data presented here thus suggest that the CLMSZ in the Garnet Mountain area represents an Early Cretaceous (approximately 119 + 1 Ma) deep-seated northwest-striking contractional mylonite belt that formed within a Early Cretaceous calc-alkaline magmatic arc developed on sialic crust. Previous workers have suggested two end-member models to explain the origin of the CLMSZ. These two models are referred to as the arc-continent collisional and extensional-collapse models. Data presented in this thesis are consistent with the collisional model.