Abstract The Chiapas Fold and Thrust Belt (CFTB) of southern Mexico underwent widespread contraction during the well-documented middle Miocene ‘Chiapanecan’ Orogeny. However, earlier phases of folding have been documented in the region and might have affected the belt before the Miocene. We carried out a stratigraphic review and a structural analysis of the belt, complemented by 40 Ar– 39 Ar dating of synorogenic illite to identify the successive pulses of deformation. Reliable radiometric ages were obtained in three folds from the southern portion of the belt, and in one thrust gouge near the front of the belt. Authigenic illite-rich samples were located in Upper Cretaceous limestone successions that experienced bed-parallel shear during folding and thrusting. They yielded ages ranging from 35 to 40 Ma in the folds and 54.5 ± 7 Ma in the thrust, documenting thin-skinned folding and thrusting in the belt mostly in Eocene time. Some of the basement faults of the belt may have been active during this phase. However, most of the pervasive lateral faults that cross-cut the regional folds in the belt were mostly active from the Miocene onwards, causing transpression and fold amplification.