Rock magnetic characterization and paleomagnetic directional analysis of Isla San Pedro Nolasco dikes, Gulf of California, Mexico

Isla San Pedro Nolasco is located in the central-eastern part of the Gulf of California, formed by late Miocene (ca. 9–11 Ma) hypabyssal rocks, between the Sonora coast and oblique-divergent Pacific-North America plate boundary. Continental blocks exposed as islands in the Gulf of California host key geological evidence for understanding the evolution of the plate boundary. We present a detailed rock magnetic study of 31 independently oriented paleomagnetic cores from late Miocene dikes collected from a southeastern bay of the island. Opaque microscopy and rock magnetic properties suggest that magnetic mineralogy is mainly Ti-poor titanomagnetite or magnetite with limited maghemite and rarely titanohematite. Magnetization has two components. The primary and more stable is of medium to high temperature (300–560 °C) or high fields (up to 100 mT). The primary thermal remanent magnetization (TRM) and the oxide mineral textures observed suggest that cores from these dikes provide high reliability for paleomagnetic analysis. The mean paleomagnetic direction is as follows: Dec = 354.86°, Inc = 46.2°, α95 = 5.2, k = 44. Comparison of these results to the 10 Ma paleomagnetic pole for North America indicates that Isla San Pedro Nolasco has experienced no vertical-axis rotation but has displaced ~ 100 km to the northwest, similar to independent estimates for fault offset along with this structural system from the northern Gulf of California and regional tectonic reconstructions. These results, supported by detailed geological studies, indicate that the island was detached from mainland Mexico by transtensional faulting in the late Miocene Gulf of California shear zone that also formed the adjacent Yaqui pull-apart basin.