KEY ISSUES OF THE MEXICAN FOLD AND THRUST BELT WHICH MUST BE ADDRESSED BY PROPOSED MECHANISMS OF CRETACEOUS-PALEOGENE SHORTENING
In central Mexico, the Mexican Fold-Thrust Belt (MFTB) shows a dominantly thin-skinned deformation style and an overall wedge shape tapering to the east. The deformation is partitioned between cretaceous carbonate sequences deposited on alternating platform-basin pairs. These rocks are unconformably covered by calcareous syn-tectonic clastic strata. Two different generations of shortening structures were identified within the MFTB, D1 and D2. D1 structures are penetrative on a metric to microscopic scale and accommodated far more shortening than D2 structures, which are represented by km-spaced, meter-displacement thrusts as well as broadly-folded D1 folds. Detailed structural observations of D1 structures along a well-exposed section of the MFTB document the partitioning of deformation between different lithogies at different positions along the section: Thrusting dominates in the platforms, folding in the basinal carbonates, and pressure-solution in calcareous shale (abundant in the syn-tectonic clastics). Pressure-solution is accentuated in the vicinity of major thrusts and along the basal detachment. Stratigraphic and geochronologic constraints indicate that D1 started ~91 Ma on the western side of the MFTB and ended before 65 Ma to the east. D2 must have occurred between the Eocene and Oligocene.
Further north, in the state of Coahuila, thin-skinned deformation structures affecting jurassic clastics and late cretaceous carbonates are clearly cross-cut by high-angle reverse faults involving crystalline basement rocks. This sequence of thin-skinned deformation of mesozoic rocks followed by thick-skinned deformation is also observed along various sections in the western U.S. and Canada.
Although the relationship between D2 in central Mexico and the high-angle reverse faulting in northern Mexico (and beyond) is uncertain, it is clear that there are two distinct pulses of shortening for thousands of kilometers along strike, which means that the tectonic mechanism (or combination of mechanisms) chosen to account for the regional structure must explain these detailed features.