NEW AR-AR ILLITE AGES FROM THE CHIAPAS FOLD AND THRUST BELT IN SOUTHERN MEXICO: TECTONIC IMPLICATIONS

The Chiapas fold-thrust belt (CFTB) is a complex tectonic feature due to proximity to the diffuse triple junction between North America (NAP), Cocos (COP) and the Caribbean (CAP) plates. It is located between the subduction zone of the COP under the NAP, and the transcurrent boundary between the NAP and CAP. One complication in this tectonic configuration is the fact that the Chiapas massive, a competent lithospheric block, is located between the CFTB and the COP trench.

Two models have been proposed for the origin of the CFTB: 1) The Eocene shortening that coincided with the last stage of shortening in the Mexican Orogen (MO); and 2) The Miocene lateral faulting influenced by the activity of the Polochic-Motagua fault system (PMFS) to the south, which includes both dextral and sinistral faults. In order to decipher the complex tectonic history of the CFTB and its causes, a careful analysis of its structures and timing of deformation are of first importance.

Cross-cutting relations show that the pervasive, kilometer scale, wide folds and thrusts predate right and left lateral faults in the CFTB. Moreover, in the inverted Ixtapa pull-apart basin (in the center of the belt) dextral faults seem to predate sinistral faults, which suggest that more than two tectonic events are responsible for the construction of this belt. We dated illite from six-folds affecting Late Cretaceous limestone and shale layers from different locations spread out in the belt. Six clay-rich samples were collected from shale layers affected by bed-parallel shear on the limbs of flexural fold, from which, three grain size fractions (> 2-1, 1-0.2 and 0.2-0.05 μm) were separated and characterized with X-ray diffraction analysis. The ages from such samples were obtained with the Ar-Ar encapsulated illite method, providing an age range between 54 and 37 Ma for the authigenic illite.

These ages indicate that the first phase deformation of CFTB occurred almost at the same time as folding in the foothills of the MO in southern Oaxaca. However, they show different deformation styles and there is no clear spatial connection among them, as the Chiapas massive is in between. The exact causes for the Eocene phase of folding still remain mysterious, but subduction of the COP under NAP or deformation partitioning in a wrench tectonics scenario are two models discussed.