Perils of petrotectonic modeling: A view from southern Sonora, Mexico

We present major and trace element geochemical data from well-dated Oligocene-Miocene volcanic sections in southern Sonora, Mexico (28°, 110°) that span the transition from subduction to oblique extension in the proto-Gulf of California. The region of northwest Mexico including Sonora experienced this abrupt change in tectonic setting ca. 15-12.5 Ma. Syn- and post-subduction calc-alkaline volcanic rocks erupted in Sonora all have similar major and trace element signatures indicative of a continental arc setting. Post-subduction rocks in Sonora document only a subtle change in the behavior of high field strength, large ion lithophile and rare earth elements and a decrease in pre-eruptive water-content. The protracted geochemical changes in Sonora suggest it took at least 4 m.y. to erase the subduction signature from the sub-arc mantle here. Therefore Sonoran geochemistry is not consistent with the predictions of petrotectonic models and contrasts with central and southern Baja where a rapid and distinct change in the composition of lavas erupted is reported synchronous with the tectonic transition. This study exemplifies the need for caution when interpreting geochemical data for tectonic information in the absence of plate reconstructions.