2008 Joint Meeting of The Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, Gulf Coast Association of Geological Societies with the Gulf Coast Section of SEPM

Paper No. 4
Presentation Time: 8:50 AM

A Thermochronologic Transect of Basement Rocks across Central Sonora, Mexico: Implications for Laramide Tectonics and Cenozoic Extension

WONG, Martin S., Geology Department, Colgate University, 13 Oak Drive, Hamilton, NY 13346 and GANS, Phillip B., Department of Earth Science, University of California, Santa Barbara, Santa Barbara, CA 93106-9630, mswong@colgate.edu

Northwestern Mexico remains an important region for understanding the tectonic evolution of the North American Cordillera as a whole. However, many fundamental aspects of the geologic evolution of the region remain poorly known. We present 40Ar/39Ar thermochronology of basement from an east-west transect across central Sonora, Mexico including from metamorphic core complexes. These results provide insight into the broad–scale thermal history of Sonoran basement and constrain the Late Cretaceous to Cenozoic tectonic evolution of the region.

Most plutonic basement across Sonora was emplaced during the Late Cretaceous to early Tertiary and represent igneous activity during Laramide time (ca. 80–50 Ma). Following emplacement, most plutons experienced rapid post-magmatic cooling at high temperatures (700-450 °C) followed by protracted cooling (<20 °C/m.y.) throughout the Paleocene–Eocene and even into the early Oligocene. The lack of significant thermal events during this time may reflect a period of extended tectonic quiescence across Sonora and suggest that significant Laramide deformation is largely absent in this region. This conclusion is supported by the broad concordance of Late Cretaceous/early Tertiary sediments with Oligo-Miocene deposits in many parts of Sonora, including Santa Rosa in eastern Sonora and the Suaqui Grande and Sierra Mazatan regions of central Sonora.

Metamorphic core complexes in Sonora represent the only exposed basement that remained hotter than 300 °C until the Oligo-Miocene. Sonoran core complexes all experienced rapid footwall cooling related to tectonic exhumation that began during the late Oligocene (ca. 25 Ma) and continued until the early-middle Miocene (ca. 15 Ma). This major phase of extension began and mainly occurred during subduction and cannot be linked to post-12 Ma transtensive Pacific-North American plate motions. Total estimated slip at Sonoran core complexes was 6–25 km and occurred at average rates of 1–7 mm/yr based on these data.