Shallow magnetic inclinations in the Cretaceous Valle Group, Baja California: remagnetization, compaction, or terrane translation?

Abstract

Paleomagnetic data from Albian to Turonian sedimentary rocks on Cedros Island, Mexico (28.2° N, 115.2° W) support the interpretation that Cretaceous rocks of western Baja California have moved farther northward than the 3° of latitude assignable to Neogene oblique rifting in the Gulf of California. Averaged Cretaceous paleomagnetic results from Cedros Island support 20 ± 10° of northward displacement and 14 ± 7° of clockwise rotation with respect to cratonic North America. Positive field stability tests from the Vizcaino terrane substantiate a mid‐Cretaceous age for the high‐temperature characteristic remanent magnetization in mid‐Cretaceous strata. Therefore coincidence of characteristic magnetization directions and the expected Quaternary axial dipole direction is not due to post mid‐Cretaceous remagnetization. A slump test performed on internally coherent, intrabasinal slump blocks within a paleontologically dated olistostrome demonstrates a mid‐Cretaceous age of magnetization in the Valle Group. The in situ high‐temperature natural remanent magnetization directions markedly diverge from the expected Quaternary axial dipole, indicating that the characteristic, high‐temperature magnetization was acquired prior to intrabasinal slumping. Early acquisition of the characteristic magnetization is also supported by a regional attitude test involving three localities in coherent mid‐Cretaceous Valle Group strata. Paleomagnetic inclinations in mudstone are not different from those in sandstone, indicating that burial compaction did not bias the results toward shallow inclinations in the Vizcaino terrane.