Paleomagnetic and geologic data indicating 2500 km of northward displacement for the Salinian and related terranes, California

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Abstract
Results of a pair of paleomagnetic studies of sedimentary rocks in the Salinian terrane of westernmost California indicate a northward displacement of about 2500 km since Cretaceous time. Stratigraphic relations suggest that the Salinian terrane became amalgamated with several others by the Late Cretaceous and all underwent the same northward translation. This composite terrane (allochthon) became sutured to cratonal North America by early Tertiary time. One paleomagnetic study involved Upper Cretaceous turbidites of the Pigeon Point Formation. Fine‐grained, thin‐bedded turbidites yielded the most reliable results, whereas medium‐ to coarse‐grained sandstones are not always stably magnetized. Contorted layers resulting from soft sediment deformation usually gave directional results parallel to those of undeformed sedimentary layers, thereby implying a postdepositional remanence process. There is no indication of inclination error or other processes affecting the fidelity of the remanent magnetization. Paleomagnetic results from four separate locations in the Pigeon Point Formation gave positive fold tests and mean inclination values near 36°. A mean geomagnetic latitude calculated from these results is 21.2°±5.3°, much more southerly than the value of 46.5°±2.3° calculated for Pigeon Point using paleomagnetic pole data from Late Cretaceous rocks from cratonic North America. Declination values are not antipodal for normal and reversed polarity sites. Analysis of depositional current directions from sandstone beds of the same outcrops shows that the difference is the result of Neogene rotations of small crustal blocks adjacent to the San Gregorio fault. In a sequence of Paleocene turbidites that crop out at Point San Pedro, four sample localities yielded positive fold tests with mean inclination values near −40°. Declination values, however, were spread over a range between 230° and 320°. Comparison with depositional current directions from the same outcrops indicates clockwise rotations related to Neogene right slip on the Pilarcitos fault. The mean geomagnetic latitude calculated for the Point San Pedro sediments is 24.5°±4.0° and is more southerly than the 43.3°±2.5° latitude expected for that location. For the Salinian terrane and the west coast of cratonic North America, absolute rates of latitudinal motion were 8.5 cm/yr northward and 3 cm/yr southward, respectively.

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