The paleomagnetism of middle to late middle Miocene volcanic rocks from regions of the western Transverse Ranges suggests that large amounts of subsequent clockwise tectonic rotation has occurred. We studied rocks from five sites in the western Santa Monica Mountains, Conejo Hills, and Anacapa Island. East declinations of from 64° to 81° were found, compared to expected Miocene declinations of 0°, and inclinations were too shallow by up to 21° for the present latitude. Preliminary data from Santa Cruz and San Miguel Islands suggest similar but larger rotations. Stratigraphically higher units show the least amount of rotation, which may indicate that about 60° of rotation occurred primarily during middle and late Miocene time. North-pointing directions can be restored by rotating the northern Channel Islands and Santa Monica Mountains counterclockwise about a pivot near the eastern end of the range. This restoration also changes paleocurrent directions in Eocene and Oligocene rocks from north to west, which is consistent with data from surrounding regions. Shallow remanent inclinations suggest northward movement of this block of ∼10° since early Miocene time.
- Geological Society of America