Franciscan Complex limestone deposited at 17° South paleolatitude

Abstract

At Laytonville, California, about 230 km north-northwest of San Francisco, three blocks of pelagic limestone, each a few tens of metres long, are incorporated in the Franciscan melange. Bedding is well defined, but top indicators are lacking. Paleomagnetic study of two of the blocks (16 samples, 52 specimens) yielded directions of D: 183.9°, I: −22.6°, α95: 21.6° (block 1), and D: 229.5°, I: −31.8°, α95: 10.7° (block 2) relative to the present orientation of bedding. Inclinations were not significantly different, but the difference in declinations shows that magnetization preceded emplacement of the blocks in the melange. Study of foraminifera showed that the blocks are of Albian and Cenomanian age and were deposited during the Cretaceous Long Normal Polarity Interval. Details of the foraminiferal zonation show that both blocks are right side up. The observed inclinations imply deposition on the Farallon plate at 17° ± 7° South paleolatitude at about 96 m.y. B.P. (block 2; block 1 data agree but are less definitive). Consideration of reasonable paleolongitudes and possible times of incorporation of the blocks in the melange indicates a Farallon–North America convergence rate significantly higher than any observed today. The most commonly accepted emplacement time (latest Cretaceous) indicates convergence at about 38 cm/yr (range: 24 to 60 cm/yr). An alternate interpretation, that emplacement occurred at 30 ± 15 m.y. B.P., would indicate convergence at about 15 cm/yr (range: 9 to 24 cm/yr). The convergence rates obtained in the first case are astonishingly high, and they suggest that perhaps current ideas on the Franciscan should be re-evaluated, with consideration given to the possibility that tectonic mixing continued until the middle or late Tertiary. Even so, the Farallon–North American convergence rate was apparently higher than any convergence rate observed today. This rapid convergence coincides with the Cretaceous rapid spreading pulse, and it may be responsible for such features as the large volumes of Late Cretaceous batholiths and subduction complexes in western North America.