IS THE MEXICAN FOLD-THRUST BELT A PROGRESSIVE OR A SECONDARY OROCLINE? OR BOTH? INSIGHTS FROM THE KINEMATIC ANALYSIS OF FOLDS AND ILLITE DATING

The Mexican Fold and Thrust Belt (MFTB) is almost continuously exposed between the states of Oaxaca and Chihuahua in eastern Mexico along a distance of 2,000 km. It consists of folded and reverse-faulted Mesozoic and Lower Paleogene strata dominated by carbonates alternating with shale and sandstone. Evaporite horizons and carbonaceous shale units provide local detachment surfaces. The trend of the MFTB is mostly NW-SE, with distinct local deviations from this trend. The most noteworthy of these is the Monterrey salient, in which the trend changes to an E-W orientation to form an orocline at the front of the belt. Only one generation of Eocene km-scale detached folds are observed in the Monterrey Salient, whereas two generations of structures are observed to the south. The Monterrey orocline formed in a single progressive event. The timing of deformation in the MFTB has been well constrained using Ar-Ar systematics on illite coatings generated by layer-parallel slip in the flanks of chevron folds. On the basis of structural evidence alone-including refolded folds and fabric development – two distinct episodes of deformation can be established. However, Ar-Ar analyses indicate that deformation occurred in multiple episodes. Ages obtained so far indicate that pulses of deformation occurred between 86-80, 75-65 and 55-45 Ma. Consistent with critical wedge theory, each of these pulses progressively affected rock units further toward the east (foreland). Effects of subsequent (Paleogene?) shortening are accentuated in the westernmost exposures of the thrust belt and are evident on the map scale by broad variations in early fold axis trends, compared to the straight axes of the frontal folds. There is thus evidence for oroclines formed in two ways in the MFTB: progressive (single event) in the case of the Monterrey salient and secondary (multiple events) in the case of the western parts of the belt.