TWO DEFORMATIONAL EVENTS IN THE MEXICAN FOLD-THRUST BELT MFTB OF CENTRAL MEXICO

The MFTB is the southern termination of the Rocky Mountain fold-thrust belt (RMFTB). In the SW of the USA, the RMFTB has a wideness about 1000 km, and is formed by two discrete orogenic structures: a narrow thinly-skinned Late Cretaceous fold-thrust belt, located along its western margin, the Sevier Orogen, and a younger thickly-skinned orogenic belt to the east, the Laramide Orogen. In northern Mexico, the MFTB has a wideness of 400 km and presents Late Cretaceous thinly-skinned structures cut by thickly-skinned structures. In contrast, in central Mexico the MFTB reduces its wideness to 150 km, no thickly-skinned structures have been reported in this area. In the westernmost outcrops of the MFTB, exposed in the Toliman-San Joaquin area of Queretaro State in central Mexico, two different sedimentary sequences with at least two deformation events were recognized. To the west, in the Toliman area, the stratigraphy is characterized by a basal unit of quartzite and slate, unconformably covered by Late Jurassic volcaniclastic turbidite deposits, Early Cretaceous calcareous debris flow deposits and thinly-bedded calcarenite strata. The oldest unit experienced at least three shortening events. The oldest is characterized by a foliation, S1, penetrative at the grain scale. This is folded on the mesoscopic scale. All earlier structures are cut by low-angle thrust faults with a direction of transport to the NE. The younger stratigraphic units present two shortening events: the first characterized by isoclinal folds that were later refolded with a vergence to the NE. These units overlie Cretaceous carbonates of El Doctor Platform and the Zimapan Basin. In thinly-bedded marl of the Zimapan Basin, evidence of two deformational events (stretched and later folded veins and beds) was also recognized. Because the last two shortening events affect Cretaceous rocks, they could be the expression of the Sevier and Laramide orogenies in the studied area. This is corroborated with the age of thrust faults.