THE MEXICAN FOLD-THRUST BELT: RECORD OF SUBDUCTION-RELATED DEFORMATION IN THE SOUTHERN CORDILLERAN OROGEN

The Mexican fold-thrust belt (MFTB) is a tectonic feature that extends from Sonora in northwestern Mexico to Chiapas in the southeast. It is formed mostly by folded and imbricated Mesozoic-Paleogene sedimentary strata. Middle Jurassic evaporite intervals and Upper Jurassic carbonaceous shale units provided detachment zones. Shortening style was largely controlled by the distribution of paleogeographic elements and by deformation kinematics, which is dominated by NE-directed tectonic transport. The structural style is dominantly thin-skinned in central and southern Mexico, and thick-skinned in northeastern Mexico, where high-angle rift-related normal faults reversed their kinematics during shortening. Strain distribution in the MFTB satisfies critical wedge theory predictions, decreasing toward the foreland. The timing of deformation has been well constrained using Ar-Ar dating on illite partings generated by layer-parallel slip in the flanks of flexural folds, stratigraphic relationships, (U-Th)/He zircon and U-Th/Pb monazite dating. Published data suggest that the orogen developed in episodic pulses of deformation between 95–80, 75–60 and 55­–40 Ma, all of which postdated closure of the back-arc Arperos basin near the western flank of Mexico during late Albian time.

Although the causes of shortening in the MFTB are still debated, a combination of deformation analyses, synorogenic sedimentation, and regional magmatism timing indicates that the orogenic wedge formed in a retroarc setting that post-dated thrusting of oceanic assemblages on its hinterland flank, which lay offshore of western Mexico during the Aptian. Initial shortening in the MFTB followed the closure of the Arperos basin and Guerrero superterrane accretion by ~10-15 Ma and was coeval with voluminous magmatism on the Pacific margin of Mexico and foundering of eastern carbonate platforms to form the Mexican foreland basin. Periods of acceleration of the North American plate to the west roughly coincide with the episodes of deformation recorded in the MFTB. Taken together, these observations suggest that subduction of the Farallon slab under the North American plate from Late Cretaceous to Eocene time, rather than terrane accretion, was the primary plate-tectonic mechanism of shortening in the MFTB.