The 40 Ar/ 39 Ar thermochronology of core complexes and other basement rocks in Sonora, Mexico: Implications for Cenozoic tectonic evolution of northwestern Mexico

Northwestern Mexico remains an important region for understanding the tectonic evolution of the North American Cordillera. However, many fundamental aspects of the geologic history of the region remain poorly known. We present new 40 Ar/ 39 Ar thermochronology of exposed basement rock from throughout Sonora, Mexico, including from metamorphic core complexes. These results provide new constraints on the Late Cretaceous to Cenozoic tectonic evolution of northwestern Mexico. Following Laramide (ca. 80–50 Ma) emplacement, most Sonoran plutons experienced rapid post‐magmatic cooling to ∼400–200°C, which we attribute to shallow level emplacement. Following this rapid cooling, plutons cooled slowly (<10°C/m.y.) from the Paleocene to the Oligocene, suggesting that the early Tertiary was a period of tectonic quiescence within Sonora during which exhumation was minor. Core complexes are the only exposed Sonoran basement that remained > 300°C until the late Oligocene. Rapid footwall cooling documents that extension at Sonoran core complexes began synchronously at ca. 25 Ma, despite the fact that these regions are separated by > 200 km. Extension at most Sonoran core complexes continued until ca. 15 Ma, although extension at the Magdalena core complex ceased earlier (ca. 21–20 Ma). Slip at each core complex was 10–30 km and occurred at average rates of 1–9 mm/yr. This major period of extension primarily occurred during subduction, indicating that relative plate motions between the Pacific and North American plates cannot be invoked as a driving force for this phase of extension.