Description
The north to northwest trending Peninsular Ranges batholith (PRB) crops out in southern and Baja. California. At the latitude of San Diego, California, there are several geologic discontinuities across the PRB. The principal discontinuities include a pre batholithic boundary. an I-S line. a gabbro line. a step in plutonic ages. a step in alSo values, and a magnetiteilmenite line. Geographically coincident with the discontinuities is a series of sheared metaplutonic units that define the Cuyamaca-Laguna Mountains shear zone (CLMSZ). The purpose of this investigation is to determine the kinematics of a ponion of the CLMSZ by documenting the different types of fabrics within the Harper Creek and Cuyamaca Reservoir units, which form a significant portion of the CLMSZ; to describe the mineralogic and geochemical nature of these units; to constrain the age of deformation of the Harper Creek unit; and to evaluate the role that the CLMSZ played in the formation of the prebatholithic boundary. The Harper Creek and Cuyamaca Reservoir units commonly contain a pervasive mineral foliation (Sd) that strikes approximately northwest and dips steeply to the east. An extensional lineation (Le) lies within the plane of Sd and plunges moderately to the northeast. Oriented thin sections contain a variety of kinematic indicators that include an S-C fabric, secondary shear bands (Ssb), obliquely aligned subgrains (So), offset feldspar crystals, and asymmetric feldspar crystals. These kinematic indicators indicate that this pan of the CLMSZ records an episode of normal fault displacements. The Harper Creek and Cuyamaca Reservoir units are granodioritic. Textural and mineralogical evidence indicate that they were originally igneous in origin. A zircon population from the Harper Creek unit was subdivided into six fractions based on size and magnetic properties. About 20 percent of the zircons contain a rounded core that is surrounded by a clear overgrowth. The zircons were analyzed for their U/Pb content. The resulting data plot about a regression line that intersects Concordia at 1486 +/- 91 m. y. and 166 +/- 17 m. y. The upper intercept age is interpreted to represent the average age of the inherited zircons. The lower intercept age is interpreted to represent the minimum age of crystallization of the Harper Creek unit. The age of deformation is, therefore constrained to be between the age of crystallization of the Harper Creek unit and 104 m.y. If the CLMSZ is Jurassic in age, then it is not directly related to the prebatholithic boundary, which is Cretaceous in age. On the other hand, if the CLMSZ is Cretaceous in age then, the prebatholithic boundary must, at least locally, record normal fault displacements. Extensional deformation of the Harper Creek unit implies that during the Jurassic the CLMSZ was forming within an extensional volcanic arc. A change in plate interaction during the Early Cretaceous caused the subducting lithosphere to shallow, which resulted in the formation of a contractional arc. Across the PRB, the rock types change from more mafic on the west to more felsic on the east. This change in rock types is compatible with an arc evolving from extensional to contractional.
