Abstract

DAVALOS-ALVAREZ, Oscar Gabriel et al. Cenozoic stratigraphy of the Tehuacan region and its relationship with the northern sector of the Oaxaca fault. Rev. mex. cienc. geol [online]. 2007, vol.24, n.2, pp.197-215. ISSN 2007-2902.

The Oaxaca fault is a Cenozoic structure located in southern Mexico. The extensional deformation related to the northern sector of this fault system formed a half-graben and a topographic depression known as the Tehuacán valley. The Cenozoic strata deposited in the valley recorded a progressive deformation phase with four pulses: p1, p2, p3, and p4. Between the Upper Cretaceous and the first Cenozoic strata exists an angular unconformity, which was associated to the Laramide orogeny. After this orogeny, the tectonic regime in the Tehuacán valley changed from shortening to extension. The extension produced brittle normal faults which were the first structures in the northern sector of the Oaxaca fault defining p1. The minimum age of p1 is constrained by the older Cenozoic strata of the valley that range in age from the early to middle Eocene. The pulse p2 occurred between the late Eocene and the early Oligocene and was produced by the propagation of faults within the system; this pulse was recorded in the Calipan ramp. A change in the regional base level is marked by a disconformity, which was associated to a pulse p3 that occurred between the late Eocene to the base of late Oligocene; during this pulse the strata of the Eocene and early Oligocene were strongly tilted. In the late Oligocene, the base level returned to the valley and the Tehuacán Formation (late Oligocene - middle Miocene) began to be deposited; this indicated the end of p3. The progressive deformation continued throughout the Miocene (p4) with the development of the youngest ramp within the fault system and the deposit of the San Isidro conglomerate (middle to late Miocene). The northern sector of the Oaxaca fault is constituted by four en échelon normal faults with a small lateral-slip component forming a left-stepped arrangement. Considering the characteristics of the litostratigraphic units, the en échelon array of the faults and the identified Cenozoic pulses of deformation, we concluded that the northern sector of the Oaxaca fault grew through relay ramps with a migration and propagation from south to the northwest.

Keywords : deformation; relay ramp; Cenozoic; Tehuacán valley; Oaxaca fault; south of Mexico.

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