KINEMATIC HISTORY OF THE SOUTHERN TERMINATION OF THE LARAMIDE OROGEN, SOUTHERN NEW MEXICO AND WEST TEXAS

The kinematic history of the Laramide orogeny has been the subject of considerable debate. Some models call on transpressive deformation whereas other models suggest crustal shortening. El Paso Texas is at the southern end of the Laramide orogen where it intersects the Mexican fold belt. Therefore structures in this region may shed light on the nature of Laramide orogenesis. On the western side of the Franklin Mountains, southwest vergent asymmetric folds are present from the outcrop to map-scale within Permian limestones. The folds are upright, open, and gently southeast plunging. The asymmetric geometry of the folds suggests that they are fault propagation folds formed above high angle reverse faults in the subsurface. The geometry of the folds suggests approximately 20% shortening during northeast-southwest directed shortening. To the west of the Franklin Mountains, within the Mesilla Valley, Mt. Cristo Rey exposes and the ~45 Ma Muleros Andesite which intrudes a sequence of folded Cretaceous shale and sandstone. Fault plane solutions and the geometry of folds record northeast southwest contraction, identical to that determined from the Permian rocks in the Franklins. Mutually crosscutting relationships between faults and the intrusion suggests deformation of the Cretaceous section occurred, at least partly, as the intrusion was emplaced. Similar structures are seen to the southwest within the Sierra Juarez, Chihuahua, Mexico and to the north at the Bear Peak fault zone in the San Andres Mountains near Las Cruses, New Mexico. These relationships suggest deformation along the northern edge of the Mexican Fold Belt was spatially and temporally linked to Laramide deformation during Eocene contraction.