DUCTILE DEFORMATION IN SEDIMENTARY COVER ROCKS OF THE EAST POTRILLO MOUNTAINS PULL APART BASIN – THE RECORD OF PRE-LARAMIDE OROGENESIS

In southernmost New Mexico the northwesterly trending East Potrillo Mountains contain the local record of Mesozoic orogenesis. The range is part of a block tilted to the west that exposes Mesozoic sedimentary strata disconformably above Permian carbonate beds. The northeastern range front exposes the section including basinal Late Jurassic (?) – Early Cretaceous conglomerate overlain by sandy, calcareous strata and conspicuous rudist-bearing limestone. Structural mapping reveals folds, both penecontemporaneous and tectonic, foliation, detached rock units, and zones of ductily deformed rocks characterized by simple shear. Multiple episodes of Mesozoic deformation are recognized including: 1) normal faulting (Late Jurassic) followed by deposition of conglomerate and sandy calcareous strata that record slumping and soft-sediment deformation, 2) detachment and sliding of massive Cretaceous carbonate beds that were folded and cut by thrust faults during, 3) Late Cretaceous northeast-directed contraction. Hinges of penecontemporaneous slump folds in sandy strata overlying the basal Glance (?) conglomerate, which are corrected for Tertiary tilting, trend northwesterly, suggesting southward transport of semi-lithified strata. Foliation, locally comprising S-C fabric in the Glance conglomerate and overlying clastic section, and mylonitic foliation at the base of Cretaceous carbonate units, and fold hinges and common kinematic indicators in Paleozoic strata also record south-directed tectonic transport. The ductile deformation is compatible with gravity emplacement of massive Cretaceous carbonate beds above detachments down the axis of the basin. Locally the slide masses are folded and faulted and record commonly recognized northeast-directed Laramide contraction. The coincidence of the movement directions of the rocks parallel with the presumed basin axis in response to gravity-driven deformation and the re-activation of the basin margins as steep thrusts suggests that Cordilleran orogenesis was strongly influenced by pre-existing transtensional basins.