DIVERSE DETACHMENTS IN THE EAST POTRILLO MOUNTAINS, NEW MEXICO: ANOMALOUS DEFORMATION NEAR THE EASTERN FRONT OF THE FORELAND FOLD AND THRUST BELT

The eastern front of the northwesterly trending East Potrillo Range exposes a typical Late Jurassic (?) – Early Cretaceous stratigraphy composed of conglomerate overlain by sandy, calcareous strata (Cuchillo Fm.) and conspicuous rudist-bearing limestone (Benigno (?) Fm.). The gross stratigraphy of these units is comparable to lithologic subdivisions of the Bisbee Group and basal Glance Conglomerate. The conglomerate beds contain pebbles and cobbles of chert and carbonate. The conglomerate overlies Paleozoic carbonate rocks that include well-bedded bodies as well as breccia and boulder conglomerate. Above the conglomerate, sandstone, commonly calcareous, is disrupted by penecontemporaneous slumping and normal faults with small displacements. Hundreds of meters of the sandy beds are capped by massive rudist-bearing limestone emplaced above a younger-over-older fault. This detachment and the strata above it strike northwesterly. Within the sandy units of the footwall, an abrupt structural discontinuity is recognized beneath which ubiquitous zones of ductile deformation parallel to bedding record detachment and stretching. Some clasts in the pebbly and underlying bouldery units are flattened and stretched to lenticular augen-like shapes. Within ductile shear zones, structures commonly show southward shear sense. Fold hinges, mainly in carbonate layers, trend WNW and also verge southward. Neither the emplacement of the detached, fossiliferous carbonate nor the orientation of folds and ductile shear zones recorded by the lower units suggest Late Cretaceous contraction and both deformations may be older. Brittle normal faults, parallel to the range front, cut beds of Paleozoic carbonate and overlying conglomerate. Locally, ductile shear zones, especially those along the margins of the conglomerate horizon, accommodate reactivation as normal faults. The northerly orientation and the normal sense of displacement suggest relation to Miocene extension.