Estratigrafía cenozoica de la región de Tehuacán y su relación con el sector norte de la falla de Oaxaca

Abstract

The Oaxaca fault is a Cenozoic structure located in southern Mexico. The extensional deformation\nrelated to the northern sector of this fault system formed a half-graben and a topographic depression known\nas the Tehuacán valley. The Cenozoic strata deposited in the valley recorded a progressive deformation\nphase with four pulses: p1, p2, p3, and p4. Between the Upper Cretaceous and the fi rst Cenozoic strata\nexists an angular unconformity, which was associated to the Laramide orogeny. After this orogeny, the\ntectonic regime in the Tehuacán valley changed from shortening to extension. The extension produced\nbrittle normal faults which were the fi rst structures in the northern sector of the Oaxaca fault defi ning\np1. The minimum age of p1 is constrained by the older Cenozoic strata of the valley that range in age\nfrom the early to middle Eocene. The pulse p2 occurred between the late Eocene and the early Oligocene\nand was produced by the propagation of faults within the system; this pulse was recorded in the Calipan\nramp. A change in the regional base level is marked by a disconformity, which was associated to a pulse\np3 that occurred between the late Eocene to the base of late Oligocene; during this pulse the strata of the\nEocene and early Oligocene were strongly tilted. In the late Oligocene, the base level returned to the valley\nand the Tehuacán Formation (late Oligocene ¿ middle Miocene) began to be deposited; this indicated\nthe end of p3. The progressive deformation continued throughout the Miocene (p4) with the development\nof the youngest ramp within the fault system and the deposit of the San Isidro conglomerate (middle to\nlate Miocene). The northern sector of the Oaxaca fault is constituted by four en échelon normal faults\nwith a small lateral-slip component forming a left-stepped arrangement. Considering the characteristics\nof the litostratigraphic units, the en échelon array of the faults and the identifi ed Cenozoic pulses of\ndeformation, we concluded that the northern sector of the Oaxaca fault grew through relay ramps with\na migration and propagation from south to the northwest.