THE MEXICAN AND THE CHIAPAS FOLD AND THRUST BELTS: STRUCTURE, TIMING AND TECTONICS (Invited Presentation)

The Mexican (MFTB) and the Chiapas (CFTB) fold-thrust belts are two of the most remarkable tectonic features of Mexico as they cover important areas in eastern and central Mexico, and influenced the petroleum systems of the Gulf of Mexico. The MFTB is older, better known and much simpler than the CFTB. The MFTB resulted from an extensive history of subduction, while the formation of the CFTB was also affected by wrench tectonics.

The MFTB extends from Sonora to Oaxaca, it is NW-SE oriented, and consists of folded and reverse-faulted mesozoic and paleogene strata dominated by carbonates alternating with shale and sandstone. Middle Jurassic evaporites horizons provide local detachment surfaces, as do carbonaceous shale units. The geometry of the structures is controlled by the spatial distribution of paleogeographic elements and detachment horizons as well as the overall kinematics of the deformation which is dominated by NE-directed tectonic transport. The structural style is mostly thin-skinned in central and southern Mexico, and thick-skinned in NE-Mexico, where high-angle thrusts result from reactivation of normal faults. The timing of deformation was constrained using Ar-Ar illite dating of clay gouge generated by layer-parallel slip in flexural folds. Such ages suggest episodic pulses of deformation between 93-80, 75-64 and 55-43 Ma, which are contemporaneous to the development of a foreland basin filled with sediments progressively younger to the east. The development of this orogenic wedge occurred in a retroarc setting after the formation of the Guerrero arc, and the opening and closure of the oceanic Arperos basin in between. Periods of acceleration of the North American plate to the roughly coincide with the episodes of deformation recorded in the MFTB. Thus, subduction of the Farallon slab from Late Cretaceous to Eocene time was probably the primary driving mechanism of shortening in the MFTB.

Despite the fact that the CFTB is a much younger belt than the MFTB, it shows a more complex structure, as not only involves Mesozoic and Paleogene strata, but also Oligocene and Miocene successions in the deformation, which were shortened by folding in the Eocene, by lateral faulting in the Middle Miocene and by the Polochic fault system in more recent times. We compare the two belts and discuss similarities and differences.