Geologic, structural, and thermochronologic constraints on the tectonic evolution of the Sierra Mazatán core complex, Sonora, Mexico: New insights into metamorphic core complex formation

Abstract

The Sierra Mazatán in northwestern Mexico is the southernmost metamorphic core complex in the North American Cordillera. Large‐magnitude Tertiary extension at Sierra Mazatán involved both ductile and brittle slip along a major normal fault that presently dips 10°–15° west. Extension was polyphase and involved an early period of extension from 25 to 23 Ma followed by major slip from 21 to 16 Ma. Total slip was ≤20 km and occurred at rates of 3–4 mm/a. This extension predated the plate boundary change to transtension at ∼12 Ma and was largely decoupled from relative Pacific–North American plate motion. Numerous lines of evidence suggest that the presently low‐angle normal fault initiated at a steep dip (50°–60°) and was rotated to lower angles during slip. When corrected for this tilting, fault corrugations at Sierra Mazatán had a similar geometry to the segmentation of many active normal faults, which is compatible with their origin as primary fault features. Many aspects of the Sierra Mazatán are comparable to large active normal faults, indicating that this core complex formed owing to prolonged extension on an otherwise typical high‐angle normal fault. Therefore, core complexes need not represent a fundamentally unique mode of crustal extension.