ACCOMMODATION OF OPPOSITE VERGENCY BETWEEN TWO DETACHMENT FAULTS: INSIGHTS FROM THE THERMAL HISTORY OF THE EL JARALITO BATHOLITH (MEXICO)

The El Jaralito batholith in Central Sonora crops out in the footwall of two large-scale NW-SE detachment faults with opposite vergencies. These faults bound the two southernmost metamorphic core complexes of the Basin and Range Province and expose the largest continuous batholithic outcrop in central Sonora. To explain this structure, it was proposed that the range may be divided into three segments separated by “accommodation” zones, which were drawn as hypothetical NNE-SSW fracture zones, parallel to the direction of extension. In this model, the northern block (the Aconchi metamorphic core complex) is controlled by the northeast-dipping El Amol detachment; the central block (Sierra El Jaralito) is similar to a symmetric horst controlled by two opposite dipping high-angle normal faults; and the southern block (Sierras Puerta del Sol and Mazatán) is a metamorphic core complex controlled by the southwest-dipping Mazatán detachment fault. According to this interpretation, the three segments should act as independent blocks with distinct tilting and differential exhumation magnitude and histories. To evaluate the existence of these fracture zones, we present new petrological, structural and thermochronological (apatite fission track) data. We discuss our results along with the previously published data presenting QTQt thermal history models for the batholith. The results confirm that rapid cooling occurred from 25 to ~16 Ma along the southwest-dipping Mazatán detachment fault and reveal two distinct intervals of cooling along the northeast-dipping El Amol low-angle normal fault (~25 to 20 Ma and ~10 to ~5 Ma). Contrary to earlier hypotheses, we found no evidence of E-W strike-slip accommodation zones dissecting the range. Instead, the data indicate that both detachment faults exhibit increasing gradients of metamorphism and exhumation: from SW to NE along the El Amol detachment and NE to SW along the Mazatán detachment, respectively. Additionally, the lateral overlapping of the tips of these two detachments defines a high-relief accommodation zone, suggesting that these structures developed with a scissor-like opening geometry.