The Mexican Fold and Thrust Belt: Structural development, timing and tectonic mechanism
Elisa Fitz-Diaz, Instituto de Geologia, UNAM
Timothy F. Lawton, Centro de Geociencias, UNAM
Gabriel Chavez-Cabello, Facultad de Cuencias de la Tierra, UANL
The Mexican Fold and Thrust Belt (MFTB) is the southernmost expression of the Cordilleran Orogenic System (COS). The overall trend of the MFTB is NW-SE, although in the Monterrey salient the trend of the MFTB changes to an E-W orientation to form a primary oroclinal fold. The MFTB consists of folded and reverse-faulted Mesozoic-Eocene strata dominated by carbonates alternating with shale and sandstone. Jurassic evaporite horizons provide detachment surfaces in some parts of the orogen, as do Upper Jurassic carbonaceous shale units. Structural geometry is controlled by the spatial distribution of paleogeographic elements, such as Jurassic extensional basins and basement blocks, and detachment horizons at varying stratigraphic levels, as well as the direction of transport, which is dominated by NE-directed tectonic transport throughout the belt. The structural style is generally thin-skinned, although high-angle faults can be observed at several localities cross-cutting the thin-skinned, shallowly-dipping faults and associated folds. The strain distribution generally satisfies critical wedge predictions, decreasing toward the foreland. Values of shortening greater than 70% are present in the hinterland of central Mexico; these decrease systematically to values <15% to the front of the range where Late Eocene onlap sequences around the Gulf of Mexico plain unconformably overlie deformed strata of the orogenic wedge. Exceptions to this pattern of regional shortening values are well documented and are related to lateral variations in mechanical properties caused by facies variations, notably massive platformal carbonates as contrasted with thinly-bedded basinal carbonates.
The timing of deformation has been constrained using Ar-Ar systematics on illite generated by layer-parallel slip in the flanks of chevron folds and are in good agreement with the age of synorogenic sedimentary successions. The results published to date suggest episodic pulses of deformation between 90-80 Ma, 75-65 Ma and 55-42 Ma. Each of these shortening events progressively affects rock units lying farther to the east. Effects of subsequent shortening are accentuated on the westernmost exposures of the thrust belt and are evident on a map scale by abrupt fold trend variations, compared to the linear axial traces of frontal folds.
The tectonic cause of the deformation in the MFTB has been widely discussed but remains enigmatic. In south-central Mexico there is evidence of accretion between the Guerrero Terrane and Mexican mainland after the closure of the Arperos-Alisitos Basin. Nevertheless, late Aptian Guerrero accretion appears to have preceded accumulation of synorogenic foreland-basin deposits by 20 m.y., and detrital zircon grains derived from Cenomanian volcanic rocks equivalent to La Posta intrusive suite (98-91 Ma) indicate that shortening took place in a retroarc setting. We therefore infer that subduction of the Farallon slab was the primary driving mechanism of shortening in the MFTB, and therefore previously posited gravitational models for deformation are untenable. However, the extent of the Guerrero Terrane alone is insufficient to account for deformation as far north as Monterrey and Chihuahua or as far south as Puebla and Oaxaca. We discuss an integrated tectonic model that considers new constraints on stratigraphy, patterns and timing of deformation and sedimentation, as well as the most recent outcomes on Cretaceous-Paleogene arc-related magmatism in the Guerrero and Sierra Madre terranes.