Apatite fission-track thermochronology of Laramide plutonic rocks in northwestern Mexico: Distinguishing Basin and Range extension versus Gulf of California rifting

This study presents thermochronological results from two Laramide plutons of central and eastern Sonora, Mexico. In central Sonora, the Hermosillo batholith forms a ∼ 150 km long N-S range, between Hermosillo and Santa Ana. Close to Hermosillo, in the Cerro Bachoco, apatite fission track ages range between 14.7 and 8.1 Ma, with mean track length characteristics of rapid cooling across the partial annealing zone. The Sierra La Madera pluton is located in eastern Sonora, close to the western escarpment of the Sierra Madre Occidental. Rocks of Sierra La Madera yielded apatite fission track ages between 26.5 and 18.4 Ma, with moderately rapid cooling through the partial annealing zone, which is also suggested by the thermal history modeling. We interpret the Late Miocene rapid cooling of Cerro Bachoco rocks as a result of tectonic exhumation triggered by the opening of the Gulf of California, and the Late Oligocene-Early Miocene cooling of the Sierra La Madera pluton as a consequence of tectonic exhumation and erosion during the Basin and Range extension. Laramide plutons of Sonora display distinct structural and thermal responses depending on the extensional province in which they are located, and on the geological framework, particularly the thickness of Cenozoic volcanic and sedimentary rocks overlying the intrusive rocks. Along the coast of Sonora, apatite fission track ages related to Basin and Range and Gulf of California opening coexist.