EARLY MIOCENE LARGE-MAGNITUDE EXTENSION IN THE SIERRA MAZATAN METAMORPHIC CORE COMPLEX, SONORA, MEXICO

The Sierra Mazatan metamorphic core complex in Sonora, Mexico is the southernmost core complex in the Cordillera. Sierra Mazatan lies within both the southern Basin and Range province as well as the Gulf Extensional province inboard of the Gulf of California. Although previous studies described the basic structure of the core complex, virtually nothing is known about the timing of rapid extension. The Sierra Mazatan core complex likely represents a significant component of the total Neogene extensional strain in Sonora. Therefore the timing of extension at Sierra Mazatan may provide insight into proposed models of extension and rifting in the Gulf region as a whole.

We conducted preliminary ⁴⁰Ar/³⁹Ar thermochronology on samples from the footwall of the Sierra Mazatan core complex to determine the timing of rapid extension. Two samples were taken from Laramide plutonic rocks from beneath the detachment fault in a transect parallel to the extension direction (WSW). A biotite sample from high on the range gave a mean plateau age of 21.1 ± 0.1 Ma whereas the K-feldspar spectrum climbs from apparent ages of 19 Ma to 23 Ma. K-feldspar diffusion modeling indicates a moderate footwall cooling rate (~20°C/m.y.) prior to 20 Ma followed by a period of rapid cooling (~80°C/m.y.) after 20 Ma. The K-feldspar spectrum from a sample locality 5 km to the west is flatter, climbing from 18 Ma to 20 Ma. Modeling of these data suggests that at least 150°C of cooling occurred from 20-18 Ma at a rapid rate.

The high rate and magnitude of footwall cooling at Sierra Mazatan are similar to those determined at other Cordilleran core complexes and are likely due to tectonic exhumation. These results suggest that rapid extensional slip on the Sierra Mazatan detachment fault occurred during the early Miocene, initiating at 20 Ma and continuing until at least 18 Ma. In the Gulf of California region, subduction at this latitude continued until ~16-12 Ma during the passage of the Rivera triple junction. Therefore, large magnitude extension in the Sierra Mazatan core complex occurred concurrently with subduction, likely in an intra-arc or back-arc setting. The timing of extension at Sierra Mazatan suggests that the formation of this core complex is more related to core complex formation within the Cordillera as whole than to late Miocene rifting in the Gulf of California.