Structural evolution of the Potosí Uplift, Sierra Madre Oriental, northeastern Mexico: Insight from detailed geologic mapping

The Minas Viejas shear zone (MVSZ) is a Late Cretaceous to Paleogene-aged décollement in the Sierra Madre Oriental, Mexico. The MVSZ lies at the base of the Upper Jurassic Minas Viejas Formation, which is dominated by rheologically weak evaporite that accommodated thin-skinned deformation in the formation of the Sierra Madre Oriental (SMO) with minimal deformation to the underlying Triassic sandstone. Thin-skinned shortening above the décollement transitioned to thick-skinned shortening, resulting in exhumation of the MVSZ and development of the Potosí uplift, one of the largest thick-skinned uplifts in the orogen. Detailed geologic mapping and structural analysis provided insight into the geometry and kinematics of the Potosí uplift, and (U-Th)/He data record the timing of exhumation associated with the uplift.

Paleocene to mid-Eocene zircon (U-Th)/He cooling ages in the sandstone below the MVSZ suggest that the thick-skinned uplift was a continuation of earlier SMO shortening as opposed to a distinct tectonic event. Thick-skinned deformation involved folding of sub-décollement strata with development of a NNW-SSE-striking anticlinorium, ENE-WSW directed thrust faulting, and formation of NNE-striking extension fractures and strike-slip faults associated with barite mineralization. Folded Cretaceous strata above the décollement record an average shortening direction of N44E associated with thin-skinned deformation, whereas structures in sandstone below the MVSZ record a shortening direction of N59E associated with thick-skinned deformation. Thrust faulting and folding associated with the Potosí uplift locally modified folds within the Cretaceous overburden by decreasing tightness of a map-scale syncline and refolding a regional anticline limb into recumbent chevron folds. Normal faults and steeply-dipping dip-slip faults that juxtapose the Minas Viejas Formation against sub-décollement strata are likely outer-arc extensional features along the crest of the thick-skinned uplift. Our findings provide a new understanding on the geometry, kinematics, and timing of deformation associated with the Potosí uplift, and may help interpretations in other thick-skinned uplifts in the Sierra Madre Oriental.