TERTIARY FAULTING AND THE RELATIONSHIP WITH EOCENE-OLIGOCENE VOLCANISM IN WESTERN OAXACA, SOUTHERN MEXICO

Three phases of faulting related to Eocene-Oligocene volcanism can be recognized in western Oaxaca. Starting in the Eocene a continental sequence, consisting of basal conglomerates and lacustrine sediments overlain by pyroclastic and epiclastic deposits and an upper unit of intermediate lavas, accumulated in N-S trending depressions in this region. General north-south trending faults with an important vertical component accompanied subsidence in these basins that was partially contemporaneous with the initial stages of volcanism. This phase appears to be related to an extensional stage that followed compression deformation during the Laramide. One example of this extensional phase is a basin formed in the area of Tonala, although there is evidence in several other areas. Another phase of deformation in this region produced faults with a general WNW orientation that cut the entire volcanic sequence. This system shows left-lateral kinematic indicators and is considered to be associated with the displacement of the Chortis block. The effects of this phase can be observed in the curvature to the west of the Mesozoic fold axes in the Diquiyú and Teposcolula areas. The Río Salado fault is the most important example of this phase. We have also observed NE-trending conjugate faults of this system that were later reactivated as left-lateral faults. Some hypabyssal intrusives were emplaced along another system, with a N30W orientation, that corresponds to older structures that were reactivated in the Tertiary. The N-S trending system was later reactivated as a left-lateral system and dikes that cut the pyroclastic unit were emplaced along these structures. The dikes show internal deformation and striae, indicating coeval deformation and magmatism. This phase is related to a stress field imposed by the combined effect of the passage of the trench-trench-transform junction associated with the Chortis block displacement and the exhumation of the Xolapa terrane. Examples of this phase in western Oaxaca are observed in the Tamazulapan-Tepelmeme area, where N-S structures are probably related to the reactivation of the Caltepec fault, the contact between the Paleozoic Mixteco terrane and the Precambrian Oaxacan terrane.