COMPARISON OF SEVIER–LARAMIDE TECTONICS OF SONORA, MEXICO, AND SOUTHWESTERN ARIZONA–SOUTHEASTERN CALIFORNIA, UNITED STATES

A major contribution of Professor César Jacques-Ayala, made in part with his longtime collaborator Juan Carlos García y Barragán, was the discovery that several supracrustal sequences in the El Chanate–Altar area of northwest Sonora, which had previously been interpreted as Jurassic, were instead Upper Cretaceous. These rocks consist largely of detritus shed from NE-advancing thrust sheets and were themselves deformed and locally metamorphosed to greenschist facies during the latest Cretaceous. The El Chanate–Altar units represent one element of a broad belt of Sevier–Laramide tectonism in northern Sonora that extends northwestward into southwestern Arizona and southeastern California (SWAZ–SECA). Some components of the Sevier–Laramide domain of SWAZ–SECA, such as the McCoy Mountains Formation, closely resemble the El Chanate–Altar Cretaceous belt, both in sedimentary protolith and in timing of deformation and grade of metamorphism. Nonetheless, there are also significant differences between SWAZ–SECA and the El Chanate–Altar area, specifically, and Sonora, more generally: (1) The McCoy Mountains Formation is spatially associated with the higher-grade and more-highly-deformed Maria fold and thrust belt. There is no equivalent to the Maria belt in the El Chanate–Altar area. (2) Regions in which biotite ⁴⁰Ar/³⁹Ar and K-Ar cooling ages from Laramide igneous rocks substantially postdate crystallization ages are more widespread in SWAZ–SECA than Sonora. (3) A subduction complex (Orocopia Schist) that underplated North American continental crust at ca. 70 Ma is exposed in SWAZ–SECA but not Sonora. (4) Arc magmatism was completely extinguished in SWAZ–SECA prior to ca. 70 Ma, whereas magmatism swept eastward across western Sonora during the latest Cretaceous but persisted into the Paleocene and Eocene in central to eastern Sonora. Contrast (4) is consistent with previous inferences that flat subduction during the Laramide orogeny extended farther inboard in SWAZ–SECA than Sonora. This, in turn, implies that contrasts (1) to (3) are largely the result of greater reworking of the lower crust by low-angle subduction in SWAZ–SECA, although regional variations in location and amount of exhumation during mid-Cenozoic extension could also have contributed to contrasts (1) to (3).