A middle Cenozoic dike swarm and crystalline basement rocks in an area of the eastern San Gabriel Mountains, southern California, are described here for the first time. Dikes were emplaced in multiple pulses during two phases of intrusion and are exposed over a limited area (≥450 km2) of the eastern San Gabriel Mountains. The initial phase of dacites and rhyolites likely are conduits between the previously described 24-19 Ma Telegraph Peak Granodiorite and extrusive Mountain Meadows Dacite. The younger phase of primarily andesites is inferred to be related to the previously described ~13 Ma Glendora Volcanics. On the basis of coeval sedimentation, erosion of the volcanic cover was synchronous with extrusion. Remaining outcrops of volcanic rocks are limited to the northeastern San Gabriel Valley. Extension is proposed to explain tilting and uplift of the eastern San Gabriel Mountains, and exposure of the dikes. Based on texture and mineralogy, a total of thirteen dike types, 6 felsic and 7 mafic, are described. A statistical analysis is conducted on the dike orientation data gathered during this investigation. These data provide additional constraints on the timing and amount of displacement along Neogene and Quaternary faults, amount of post-mid Miocene vertical and horizontal-axis block rotation, and local stress field orientation during emplacement of the dikes. Blocks of crust in the study area were defined by orientations of dikes within them and by the presence of block-bounding faults. Dike orientation data show that the present configuration of the eastern San Gabriel Mountains can be reconstructed with little post-middle Miocene vertical-axis block rotation, in contrast to previous paleomagnetic-based models. A middle Miocene palinspastic reconstruction of the eastern San Gabriel Mountains is created using inferred displacement values of local faults and is based on the assumption that the younger, and more abundant, mafic dikes were emplaced over a short time frame (<2 m.y.). The resulting reconstruction shows that the mafic dikes were emplaced during local -N-S extension (current direction) in steeply dipping conjugate sets about a vertical plane with a -N80W strike. However, felsic dikes strike at oblique angles to the mafic dikes. Preliminary data shows that the inferred direction of extension during emplacement of the early Miocene felsic phase was between N55E and N90E. It is postulated that emplacement of early Miocene felsic dikes is related to normal or dip-slip movement along a series of northeast-trending faults, including the Evey and San Antonio Canyon faults, while middle Miocene mafic dikes are associated with later dip-slip movement along these faults, reactivation of the Vincent thrust, or early San Gabriel fault motion. The reconstruction also indicates that the northeast-trending faults (e.g., Evey, San Antonio Canyon, etc.,) were probably early extensions of the Malibu Coast fault system and that a south branch of the San Gabriel fault either has <10 km of sinestral offset, or must project south of the Puente Hills. Approximately 6 km of additional sinestral offset (pre-San Gabriel fault) is demonstrated on the Evey-San Antonio fault.