SHORTENING DEFORMATION IN THE TAMPICO-MISANTLA BASIN, ALONG THE EASTERN BOUNDARY OF THE MEXICAN FOLD-THRUST BELT (CENTRAL-EASTERN MEXICO)

The Tampico-Misantla Basin (TMB) is a Cretaceous paleogeographic unit located between the Valles-San Luis Potosí carbonate platform (VSLPP) to the west and the coastal plain of the Gulf of Mexico to the east. Its stratigraphy is characterized by a sequence of Middle to Late Jurassic terrigenous strata, Early Cretaceous thinly-bedded limestone, and Late Cretaceous calcareous turbidite deposits. The VSLPP is characterized by a 1500 m thick sequence of massive reef limestones. The rocks of the TMB experienced thin-skinned shortening during the Laramide Orogeny. The mechanical contrast between the TMB strata and the carbonate platform resulted in a partitioning of deformation, with high strain in the former, with meter-scale, tight, recumbent folds with an ENE verging asymmetry, and low strain in the latter, with broad, km-scale, upright folds. The configuration of the deformed TMB is consistent with that of an orogenic wedge developed at the leading edge of the VSLPP.

We have recognized spatial variability in the distribution of deformation mechanisms within the orogenic wedge defined by the TMB. The mechanisms include faulting, folding and solution transfer, with the relative contributions to overall strain by each mechanism being assessed quantitatively.

About 60 determinations of minimum shortening were made using mesoscopic structures, with the following results: A shortening of about 72-75% in an ENE-WSW direction is accommodated by folds at the boundary between the TMB and the VSLPP. In the center of the wedge, the fold-accommodated shortening drops to 45-60%, and in the distal portion of the TMB it drops further to 19-32%. These data are in agreement with the predictions of the orogenic wedge model, in which strain decreases from the rear to the front of the wedge. This wedge is a subsidiary part of the broad fold-thrust belt, and totally within the cover sequence.