GEOLOGY OF SOUTHERN MESA CENTRAL OF MEXICO: AN EXAMPLE OF THREE-DIMENSIONAL DEFORMATION IN THE OLIGOCENE

Usually, the deformation in the upper crust is considered two-dimensional, that implies null deformation in one principal strain axes. A conspicuous feature of southern Mesa Central is that the Cenozoic deformation is three-dimensional, produced by tectono-thermal activity occurred in the early Oligocene. We mapped a region in central Mexico, where the Cenozoic rocks are affected by a polymodal fault pattern. We identified six stratigraphic units: 1) The Mesozoic mafic volcanic and sedimentary marine rocks, metamorphosed to greenschist facies. 2) Clast-bearing pre-Oligocene continental deposits that lie unconformably on the Mesozoic rocks. 3) Rupelian rhyolitic lavas, lava domes and ignimbrites, of ~32-30 Ma and ~28-27 Ma. 4) Chattian volcanic rocks, dominated by non-welded pumice and ash-bearing pyroclastic flows of ~23.5 Ma. 5) Miocene basalt lava flows that coronate the volcanic stratigraphy. 6) Poorly-consolidated conglomerate and sandstone with maximum depositional age ~16.5 Ma. The main fault systems are: Villa de Reyes (NE-SW), Vergel-Cóporo (NE-SW), Matanzas (NE-SW), Santo Domingo (NW-SE) and La Quemada (NW-SE) grabens, and Los Pájaros (N-S), Santa Bárbara (E-W) and El Bajío (NW-SE) faults. Regional faults and outcrop scale crosscutting relationships are not univocal, and we cannot infer a clear chronology of faulting events. U-Pb zircon ages of lithological units allowed to stablish their peak of activity at 30-27 Ma, nearly contemporaneous with volcanism. The structural analysis shows a polymodal fault pattern with a wide dispersion of poles (n=478). The calculation of paleostress tensors was not suitable since the assumptions required were not met. From the kinematic point of view, we calculated “kinematic tensors” (Linked Bingham). The relative strain values of individual grabens and faults (e₂/e₁ less than 0.15) suggest a two-dimensional deformation state. In contrast, the eigenvalues from all fault sets (e₂/e₁ = 0.79) indicate a three-dimensional deformation. The geological mapping allows a first quantitative approach about the origin of the polymodal fault pattern located in the southern Mesa Central, which could have been generated under the same state of stress with quasi-simultaneous activity of normal fault systems during the Oligocene, producing three-dimensional deformation.