EXTENSION AND GRAVITY SLIDING DURING THE FILLING OF THE MESOZOIC INTRAPLATE EAST POTRILLO MOUNTAINS BASIN

In southwestern New Mexico, the East Potrillo Mountains expose Paleozoic and Mesozoic rocks that contain distinct structures that we infer to record the processes of basin formation and filling. The former basin has been inverted during Cretaceous contraction and tilted during at least one episode of Tertiary extension. The floor of the former basin consists of variably broken Paleozoic carbonate strata in which fracturing and stratal tilting related to normal faulting are most prominent. However, folds and ductile deformation, expressed as boudinage, are developed locally. The overlying Mesozoic section begins with several meters of pebble conglomerate that pass into silty and sandy strata. Normal faults cut the Paleozoic and overlying Mesozoic section. Displacement on these faults dies out within the clastic beds upsection. Apparent variations of thickness from footwall to hanging wall suggest syn-depositional faulting. Adjacent to normal faults, slumping and other indications of soft-sediment deformation are locally prominent within the clastic rocks. Localized development of gently dipping shear zones sub-parallel to bedding are dispersed throughout the unit. An abrupt contact separates the clastic unit from richly fossiliferous Cretaceous limestone. The contact is not depositional. Angular and rounded cobbles and pebbles of diverse sedimentary lithologies occur adjacent to the hummocky base of the limestone. These clasts are surrounded by platy fine-grained clastic rock, which records top-to-the-south displacement as do microstructures locally developed within the basal mylonitic limestone. Small mines are located along the contact that is interpreted as a mineralized slide surface. We interpret the relations as initially recording 1) development of a late Jurassic – early Cretaceous structural basin lowered by normal faulting and receiving sediments concurrently with fault displacements and 2) a rejuvenation during middle Cretaceous when the large slabs slid into basin from flanking areas. Asymmetric folds and thrusts indicating east-directed transport are recorded in the large fossiliferous slabs and record inversion of the basin during Cretaceous contraction.