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Mafic rock samples from the northern Channel Islands (western Transverse Ranges) and the Peninsular Ranges and offshore islands were dated by 40Ar/39Ar incremental heating experiments on groundmass and plagioclase concentrates (24 experiments total). Many of the ages are substantially different from published K-Ar dates. Early Miocene 40Ar/39Ar dates on two lavas from San Miguel Island agree with the paleontology control there (Saucesian) and conflict with prior late Oligocene K-Ar dates from the same units. Hypabyssal intrusions from Santa Rosa Island are around 18 Ma. The top of the Santa Cruz Island volcanic section is 16.33 ± 0.26 Ma. Anacapa ages near 16 Ma are the youngest found for the northern Channel Islands. Published 40Ar/39Ar dates from the Tranquillon Volcanics are 17.8 Ma .suggesting a widespread volcanic event between 18 and 16 Ma. We interpret this as timing the beginning of rifting of the northern borderland. The apparent age progression of volcanism from west (18 to 17 Ma) to east (16 Ma) is suggestive of a propagating rift prior to major extension and rotation of the western Transverse Ranges. The El Modeno volcanics in the Peninsular Ranges give ages of about 11 Ma, several million years younger than published K-Ar dates. The Rosarito Beach Formation comprises Miocene basalts, tuffs, and sediments that accumulated in two basins between Tijuana and Ensenada. The section spans from 15.51 + 0.14 to 16.19 ± 0.28 Ma. This formation was derived from a western (offshore) source that has since been translated away, submerged, or eroded. It is possible that part of the northern Channel Islands platform was adjacent there before rifting and rotation. Ages from San Clemente Island are younger than the Rosarito section and span 14.5 to 16.0 Ma. The combined sections on San Clemente and at Rosarito span several reversed magnetic polarity intervals. More ages from these sections could aid in calibrating the magnetic reversal time scale.

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