Neogene tectonic rotation of the Santa Ynez Range, Western Transverse Ranges, California, Suggested by paleomagnetic investigation of the Monterey Formation

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Abstract
A paleomagnetic investigation was conducted on dolomite beds in the middle and upper Miocene Monterey Formation in order to determine the timing and amount of post‐early Miocene tectonic rotation experienced by the Santa Ynez Range (part of the western Transverse Ranges province of California). Five stratigraphic sections of the Monterey Formation exposed along the coastline west of Santa Barbara were extensively sampled. The sections were correlated using tuff beds, and ages were assigned to each dolomite bed on the basis of magnetostratigraphy and diatom biostratigraphy. The dolomite beds possess stable primary remanent magnetic directions which pass fold tests and reversal tests, and show a consistent decrease in magnetic declination with decreasing age. Rocks 14.8 to 15.8 m.y. old have a mean magnetic declination of 92±7°, whereas rocks 8 to 11 m.y. old have a mean magnetic declination of 36±7°. The consistent upsection decrease in magnetic declination within these stratigraphic sections is attributed to a rapid 56±10° clockwise rotation of the entire Santa Ynez Range between 15.2±0.1 and 10±1 m.y. B.P. The mean magnetic declination of 36±7° in rocks 8 to 11 m.y. old is probably due to continued clockwise rotation of the Santa Ynez Range during the Plio‐Pleistocene. An early Miocene palinspastic reconstruction is proposed in which the area of the presently east‐west trending Santa Ynez Range is restored to an initially north‐south orientation.