Abstract

The exposed structural geometry of the Sierra Madre Oriental foreland fold-thrust belt is quantified for an ∼130-km-long strip along the Moctezuma River (21°N lat., east-central Mexico). The strip extends from the belt's leading edge in the east to its most internal continuous outcrops in the west, where the Cordilleran structures disappear beneath Cenozoic volcanic rocks.

The style of deformation is controlled mainly by the geometry and lithology of two major Cretaceous carbonate banks, El Doctor and Valles-San Luis Potosi, the long dimensions of which parallel the Cordilleran structural trend. The exposed shortening is concentrated on the platform margins (5 km on the eastern edge of the El Doctor bank and 10 to 12 km on either edge of the Valles-San Luis Potosí platform) and is characterized by thrust and fold nappes. The thrust faults cut across the platform-margin carbonates on <20°-inclined tectonic ramps that were partly steepened by imbrication or by subsequent folding but pass in the overlying, mechanically weak rocks nearly parallel to bedding. The interiors of the platforms appear relatively undeformed, whereas the regions between the two platforms (Zimapán basin) and between the eastern edge of the Valles-San Luis Potosí platform and the foreland are composed of flexural slip folds resulting from decoupling along several incompetent layers. The exposed bulk shortening measures 41.4 km or 25% for the entire traverse.

Retrodeformable sections of the subsurface structure down to basement result in a bulk shortening of 90.1 km or 40%. The subsurface deformation model for the western part of the traverse is thin skinned and has geometry similar to that on the surface. The model is very speculative in the eastern part, where the areal shortening calculated for thin-skinned models is much larger than is the exposed linear shortening. Balance was attained by inferring a blind thrust on an upper detachment in the Late Cretaceous which has a shortening of 20 to 30 km. It could also be attained, however, by intrabasement decoupling in a depth of 10 to 20 km.

Deformation is brittle in the eastern part of the belt, east of the Valles-San Luis Potosí platform, whereas igneous intrusion-controlled higher ductility characterizes the western part of the foreland fold-thrust belt.

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