Kinematics and Ar–Ar illite age of deformation in the late Paleozoic Chicomuselo Fold-Thrust Belt (CFTB), Chiapas, Mexico and tectonic implications

The Chicomuselo fold-thrust belt (CFTB) is perhaps the less known cover orogen in Mexico. It is exposed in southernmost Mexico, Guatemala and Belize, along a fringe of about 500 km long and 50–100 km wide, on the southern edge of the Maya tectonostratigraphic terrane. The CFTB is constituted by a succession of sedimentary late Paleozoic marine rocks that include the Santa Rosa Inferior, the Santa Rosa Superior, the Grupera and the Paso Hondo formations. Based on the fossil content of the Santa Rosa Inferior and Paso Hondo formations, the age of these units was constrained from the Mississippian to the late Wolfcampian (early Permian). These successions are folded and thrusted, affecting the two more competent upper formations, whereas the two finer-grained lower units are also highly foliated. Folds, thrusts, and axial-plane foliation (S1) are more consistent with a NE-SW shortening direction and the asymmetry indicates a vergence towards the NE. Such structures are unconformably covered by red-beds of the Middle Jurassic Todos Santos Formation. No angular unconformity was found among the Paleozoic formations, as reported in previous works, which indicates that shortening deformation may have occurred in an orogenic cycle that took place between the latest Permian and Triassic. Structures related to such deformation are cut by kink bands and lateral faults (both dextral and sinistral), spaced across the belt. In order to constrain the thermal conditions of deformation in the CFTB, about 53 samples of phyllite, slate and shale were collected, and their illite crystallinity index (ICI) determined along the belt axis. Calibrated ICI's indicate that deformation occurred between low epizone to the upper diagenetic zone (Kübler index between 0.2 and 1 Δ°2θ angle), with a slow increase in crystallite size from the lower to the upper units, along and across the belt. Deformation-related illite from the Santa Rosa Inferior and Paso Hondo formations was dated with the 40Ar-39Ar method. In the first sample, mica fish of tens of microns dominate the fabric, though a finer-grained illite parallel to the axial plane of late kink bands overgrows such fabric. In the second sample, only one generation of micrometric size illite is visible under the SEM. However, the characterization and dating of several clay size fractions (0.2–0.5, 0.5–1 and 1–2 μm, in addition to 2–4 μm in the first sample) indicate that both samples contain the same two end-member generations of illite, one formed during a first event of intense shortening at 252.3 ± 8.1 Ma, and another associated to spaced faulting formed at 175.1 ± 6.5 Ma. All these data indicate that all sedimentary units within the CFTB were shortened simultaneously during an orogeny that started in the Permian-Triassic threshold and were presumably affected by Middle Jurassic faulting during the opening of the Gulf of Mexico.