Coastal and Baja California paleomagnetism reconsidered

Abstract

Systematic reappraisal of paleomagnetic data from upper Mesozoic and lower Cenozoic sedimentary rocks and igneous rocks of late Mesozoic age in coastal and Baja California indicates that pluton tilt and shallowing of paleomagnetic directions by sedimentary compaction resolve observed discordances with the North American apparent polar wander (APW) path when the best available reference paleopoles are used for comparison. The hypothesis of major tectonic transport in excess of distances inherent in current models for evolution of the San Andreas transform system is unnecessary and in conflict with geologic observations. Fresh insights derived from recent data sets and improved analysis include the following. (1) The limited paleomagnetic data recording primary remanence for Upper Jurassic and Lower Cretaceous volcanic rocks in coastal California are concordant with the North American APW path. (2) The tilt of the Peninsular Ranges batholith inferred from paleobarometric data eliminates its declination discordance and reduces its inclination discordance to a level where ovals of confidence for observed and expected paleomagnetic directions overlap. (3) Key Cretaceous and Paleogene red-bed successions on the Salinian block of coastal California and in Baja California have yielded concordant paleolatitudes. (4) The discordances of different apparent paleolatitudes inferred from Cretaceous and Paleogene marine sedimentary strata lie within a range that can be attributed to compaction shallowing on the basis of both experimental information and observations from elsewhere. (5) Turbidites are more susceptible to compaction shallowing than shelf and deltaic strata. (6) The uniform rotation of paleomagnetic directions inferred previously for peninsular California is not supported by the full array of data now available from multiple localities. (7) Basement ties of the Salinian block in California and of the Baja California peninsula to the continental interior preclude any hypothesis for tectonic transport much in excess of San Andreas transform slip.