DEFORMATION PATTERNS OF SYNTECTONIC STRATA IN THE VIZARRÓN-TAMAZUNCHALE CROSS-SECTION OF THE MEXICAN FOLD AND THRUST BELT OF CENTRAL MEXICO

The Mexican fold and thrust belt in central Mexico (between Vizarron and Tamazunchale) has excellent exposure of syntectonic deposits. These deposits are represented by the Soyatal (in the west) and the Méndez (in the east) Formations, which represent the syntectonic clastic sediments that filled foreland basins in the Late Cretaceous. The Soyaral and Méndez Formations are characterized by a thinly bedded succession of carbonate sandstone, marl and shale.

Geological studies of this structural cross-section has focused on the stratigraphy and deformation of pretectonic sedimentary units (two platforms, and two basins); however, the structures and the amount of deformation that they accommodate have not been described. This is fundamental to understanding the mechanical conditions in which the syntectonic deposits are deformed in general. In this study we investigated the structural variation and the strain distribution of the syntectonic strata, conducting systematic field observations and detailed structural analysis at various locations where these formations are exposed.

Our data show that there is a structural variation of the syntectonic deposits in the cross-section. The deposits are affected by mesoscopic folds in all areas and some inverse shear zones near the contact with pretectonic formations, specially the platforms, and near the thrust faults. The west part of the cross-section is characterized by isoclinal folding with sub-horizontal axial plane cleavage inclined to the SW (10-20°). On the platforms these deposits are affected by isoclinal folds with sub-horizontal axial plane cleavage inclined 10-35° to the SW. Toward the middle portions of the basins there are more open folds with a range of interlimb angles between 25-60°. In the case of the cleavage, this is more penetrative in the west (25-15 cleavage planes per cm) and less in the east (0-8 cleavage planes per cm). The intensity of cleavage was measured in the more competent layers (carbonate sandstones). The shortening variation within the syntectonic deposits (determined using chevron folds) is consistent with the critical-wedge theory, since there is more deformation in the western part of the cross-section ( >70- 60%) and less deformation in the east (50-40%)