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Late miocene silicic subvolcanic plumbing system related to oblique rifting in the Pacific-North American plate boundary, Sonora, Mexico: geodynamic implication in a regional context

ABSTRACT Rocks from subvolcanic silicic magmatism (SSM) are exposed intermittently over a wide area along the Sonora coast, NW Mexico. This region is characterized by syn-rift volcanism related to the development of the Pacific-North America transform plate boundary during the Late Miocene (~12.5-~6 Ma). The ca. 11 Ma U-Pb age obtained from the SSM in the study area makes this magmatism part of the first stages of this tectonic event. Variations in magnetic mineralogy and textural relationships of the SSM suggest that they are a hypabyssal suite emplaced at distinctive depths. The anisotropy of magnetic susceptibility used for the petrofabric analysis reveals that, generally, the magnetic lineation is subparallel to the magnetic foliation and both are subhorizontal, suggesting a lateral emplacement. Palaeomagnetic analysis of 31 sites on the intrusive bodies indicates clockwise vertical axis rotations up to 90° related to dextral strike-slip faults. The fieldwork observations, magnetic fabric orientation, and the distribution of the SSM suggest that the magmas were propagated in a transtensional setting ascending probably through tension gashes from a deep main magma body, and emplaced as sheet intrusions in the syn-rift volcanic sequence. We propose that the vertical axis rotations of the SSM bodies are part of a unique transtensional-type deformation system of the oblique-divergent plate boundary, which it gradually changed. The SSM rocks are distinguished from other plutonic rocks forming most of the basement of NW Mexico by their age, distribution, emplacement environment, and deformation style. Finally, this study provides compelling evidence of igneous plumbing system within rift zones and a valuable contribution to the history of magmatism and tectonic control related to the development of the oblique rifting in the Gulf of California plate boundary.