An anomalous structural region exists on the southwest flank of the San Julian Uplift, part of the Sierra Madre Oriental fold-thrust belt in northernmost Zacatecas, Mexico. This domain is a triply transverse feature to the Sierra Madre Oriental structural trend, and its structural evolution was previously unknown. Three separate domains, the Aneho, Puerto Los Mimbres, and Caopas domains were defined based on unique structural features and mapped geologically with intent to understand the complex structural relationships present in the transverse zone (the Caopas domain).

The Puerto Los Mimbres domain (SE of the Caopas domain) is dominated by a NE dip direction of carbonate rock bedding planes and Laramide-age contractional anticlines and synclines. The Aneho domain (NW of the Caopas domain) displays a gentle SW stratal dip. The Caopas domain consists of sub-domains that have different carbonate strata dip directions as well as complex structural relationships, including four inferred normal faults. Several structural models were tested with intent to understand the structural evolution of the San Julian Uplift. A model that places a thrust fault to the NE of the uplift and an extensional normal fault in the San Julian Uplift valley seems to best explain the structural evolution of the region. Another model involving a core complex can not be ruled out with existing data.