The San Marcos fault (SMF) cuts across Coahuila state along a NW-SE trend. This long-lived structure is part of a Middle Jurassic normal fault system, with a Late Jurassic reactivation, which constrained the deposition of Early Cretaceous red beds and carbonate deposits and was reactivated as reverse fault during Paleogene. The SMF coincides with the northwestern boundary of the Coahuila platform, with its best exposure at Las Palomas village. The Cupido Formation thrust over pelagic limestone, in a locality which was studied in detail and informally referred as Cerro La Bruja beds (CCLB). This unit correlates with the Eagle Ford Formation (EFF) in the Sabinas Basin to the north and with the Indidura Formation in the Parras Basin, to the south, all of Turonian age. The CCLB are involved in kilometer scale folding in both basins. However, these rocks are sub-horizontal in the Coahuila platform, where they were shortened only 400 m in the closest position to the SMF, showing chevron folds within a deformation wedge limited by a basal fault separating folded rocks from unfolded rocks underneath. Near the fault plane and along the thicker portion of the wedge, folds are tighter compared to the furthermost portion. Based on a detailed fold shortening analysis in the CCLB, a horizontal shortening of about 500 meters was estimated for these beds. If we assume that only one folding generation is present in the CCLB and that this shortening event was the result of the SMF activity, we propose a new method to estimate the minimum displacement on the frontal fault, which considers folds of CCLB and the fault plane inclination. A minimum displacement of 1300 m is calculated along the fault plane, showing consistency with estimations of at least 3000 m suggested by previous authors for the entire fault system. Fault displacement estimates have always been a challenge in structural analysis, and that is the reason why we consider that our new method has potential to become a valuable tool in fault kinematic analysis.

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