EMPLACEMENT GEOMETRY, LOCALIZED EXTENSION, AND TECTONIC DISMEMBERMENT OF THE LATE OLIGOCENE TELEGRAPH PEAK PLUTON, SAN GABRIEL MOUNTAINS, CALIFORNIA

The late Oligocene Telegraph Peak granite and related rhyolite/dacite porphyry dikes are distinctive markers that constrain the magnitude of post-middle Miocene translation along various strands of the southern San Andreas fault system. From its type locality in the eastern San Gabriel Mountains, the Telegraph Peak pluton has been displaced northwestward along the San Gabriel fault (to Cogswell Reservoir) and southeastward along the San Jacinto and San Andreas faults (to the Crafton Hills and northern Chocolate Mountains, respectively). Middle Miocene palinspastic reconstruction of these widely separated exposures permits evaluation of late Oligocene stresses that affected the basement during pluton emplacement. Subsolidus ductile deformation zones within the main granite body provide additional insight into an early Miocene extensional overprint. The reconstructed Telegraph Peak pluton defines a 50m to 300m-thick sill, emplaced into a structurally weak zone coinciding with the folded Late Cretaceous-Paleocene Vincent thrust. Dikes with northerly strikes connect the main sill to multiple sills positioned at lower and higher structural levels. Discrete normal-sense ductile shear zones with N25-45E mylonitic lineations cut the granite on the east face of Telegraph Peak. Crosscutting middle Miocene(?) andesite dikes are not deformed. Together, these field relations suggest that stress conditions in the San Gabriel-Chocolate Mountains basement evolved from EW extension during late Oligocene emplacement of the Telegraph Peak pluton to localized NE-SW extension during early Miocene time.