STRUCTURAL HISTORY OF A SEGMENT OF THE PENINSULAR RANGES BATHOLITHS (PRB), SOUTHERN CALIFORNIA: NEW EVIDENCE FOR PRE-130 MA AND POST- TO SYN-116 MA DEFORMATION

Along ~N33^oW we have mapped a 1:12000 scale segment of the PRB. Within this area, the Jurassic Harper Creek gneiss structurally bounds a central region, ~2 km wide, subdivided into W- and E-belts. The W-belt includes the Julian Schist and the Lucky 5 pluton. Laser ablation ICP-MS U-Pb analyses of 30 zircon grains yielded a crystallization age of 130.3 ± 2.6 Ma for the Lucky 5. In the W-belt, S1, a transposed, NS to NW trending, coarsely to finely crystalline, spaced to continuous foliation, is folded about upright tight to isoclinal moderately plunging F1 macro and mesoscopic folds that are truncated by the Lucky 5 pluton. S1 is defined by aligned alternating lenses composed of biotite ± andalusite ± sillimanite ± cordierite, and quartz + plagioclase ± K-feldspar. The W-belt is subdivided further into 4 discreet structural domains, each bounded by shear zones that post-date F1 and S1. The eastern limit of the W-belt is the Sunrise Highway fault, a weakly to moderately developed mylonitic zone along the eastern side of the Lucky 5 pluton and its extensions to the S and N. Sandwiched between the Sunrise Highway fault and the eastern Harper Creek gneiss is the E-belt. The E-belt is composed of phyllonitic Julian Schist, intruded by the protomylonitic Oriflamme Canyon pluton. U-Pb analyses of 27 zircon grains from the Oriflamme Canyon unit yielded a crystallization age of 116.1 ± 2.4 Ma. The Julian Schist in the E-belt generally lacks macro and mesoscopic folds, and, is instead, characterized by S2, a platy NW striking steeply NE dipping foliation. S2 can be traced eastward into the Oriflamme Canyon pluton where it is represented by mylonitic foliation and west verging shear bands dipping ~30-40^o eastward. Our data suggest that the common NW trending foliation in wall rocks of the Cretaceous PRB is due to at least 2 discreet periods of range perpendicular shortening. S1 and associated transposed F1folds in the Julian Schist are older than ~130 Ma, and therefore represent pre-early batholithic deformation likely associated with development of the Cuyamaca Laguna Mountain shear zone during the Late Jurassic or earliest Cretaceous. In contrast, the Oriflamme Canyon pluton was intruded during NE-SW shortening ~116 Ma. This latter phase of shortening may be related to orthogonal subduction between the Farallon and North American plates 110-120 Ma.