Stop 3. Cuyamaca-Laguna Mountain Shear Zone
Return to Interstate 8 and proceed east approximately 15 miles to the Sunrise Highway exit. Turn left (north) and drive about 1.5 miles. Pull off the road into a wide, flat area on the northwest side of the road and examine outcrops about 100 yards to the northeast along the road. The purpose of this stop is to acquaint the trip participants with the structural and petrological characteristics of the deformed plutons in the Cuyamaca-Laguna Mountains Shear Zone (CLMSZ) and to present an overview of the structure of the PRB.
The contact between the 118 Ma Pine Valley granite (which we will examine at Stop 4) and the 156 Ma Harper Creek gneiss is just to the west in Scove Canyon. The gneiss is tonalitic to granodioritic and contains concentrically zoned andesine-oligoclase, K-feldspar, quartz, biotite, white mica, and trace amounts of tourmaline. Porphyroclasts of feldspar are clearly visible but the other minerals are largely restricted to the groundmass. The prominent foliation is a C-surface along which grain-size reduction has occurred. A stretching lineation, defined by elongate aggregates of quartz, feldspar, and mica, has an average attitude of 64º at 053. The outcrop on the northwest side of the road yielded a 156 ± 12 Ma lower--concordia intercept age and an upper--concordia intercept age of 1427 ± 29 Ma