Abstract
The eastern margin of the Cretaceous Valles–San Luis Potosí carbonate platform (Hidalgo, Querétaro, and San Luis Potosí States) became activated during the formation of the Sierra Madre Oriental fold-thrust belt. A series of thrust sheets, with a minimum linear shortening of 3 to 4 km each and 10 to 12 km in total, formed where the platform edge was approximately normal to the greatest principal stress trajectories, as deduced from fold axes. A tear fault developed where the platform edge formed an acute angle with the same stress trajectories. The overthrusts cut across the competent layers of the Lower Cretaceous on tectonic ramps that dip >10° and became partly steepened by subsequent imbrication or folding. In the mechanically weak Upper Cretaceous rocks, the thrust faults are nearly parallel to bedding for several kilometres. The location of the thrusts is controlled mainly by the lithological and thickness change of the middle Cretaceous rocks from an orthotropically layered basin facies into a two to five times thicker homogeneous platform–edge assemblage. The bank margin was a zone of stress concentration. The magnitude of the horizontal tectonic components which caused the deformation may have changed by a factor of two to five across the platform margin, due to the change in cross-sectional area. Oblique and layer-parallel discrete shears and subordinate tectonic stylolites appear to have been the dominant deformational mechanisms. The rocks did not suffer any measurable penetrative ductile deformation; ooids present at the base of the Xilitla Thrust are unflattened but are marked by a closely spaced stylolitic cleavage perpendicular to bedding. The deformations are bracketed by the paleontological age of the youngest strata affected by the overthrusts (Globotruncana contusa planktonic foraminiferal zone) and by the isotopic age of a post-tectonic pluton (62.2 Ma) and are thus of late Maastrichtian/Paleocene age.
