RELATIONS BETWEEN FOLDING, FRACTURES, FLUIDS, AND STRATIGRAPHY IN THE MEXICAN SIERRA MADRE ORIENTAL

In the Sierra Madre Oriental between Monterry and Saltillo, Mexico, eight to twelve Upper Jurassic through Cretaceous formations are detached and folded above the roughly 1000 m thick, evaporitic, Lower Jurassic Minas Viejas Formation. The sedimentary sequence above the detachment is comprised of a ~1300 m thick lower weak unit, a ~1200 m thick middle competent unit, and a greater than 4000 m thick upper weak unit. This weak-strong-weak, large-scale mechanical stratigraphy, coupled with the presence of a thick evaporite section, primarily controlled the folding styles and geometries in the Monterrey - Saltillo region. Folds in the area occur in regular wave trains with persistent wavelengths and amplitudes, exhibit no dominant vergence direction, and commonly extend for several tens of kilometers along strike.

In addition to these larger-scale relations between fold style and stratigraphy, smaller-scale stratigraphic variations affect the style, distribution, and intensity of mesoscopic structures that developed throughout individual map-scale structures in the fold belt. Joints, veins, cleavage, mineralized faults, and semi-ductile shear zones form to differing degrees in each part of the stratigraphic sequence, and their local characteristics are affected by stratigraphic variables such as lithology, bed thickness, and the apparent frictional strength of bed boundaries.

Stable isotopic and fluid inclusion analyses of quartz and calcite veins exposed along a ~5 km transect across the frontal detachment fold of the Sierra Madre Oriental demonstrate striking stratigraphic control on fluid migration during folding. These analyses suggest a three-part paleohydrostratigraphic system that does not directly correlate with the large-scale, three-part mechanical stratigraphy. In addition, within some paleohydrostratigraphic units, some vein types that form only in certain lithologies, exhibit unique isotopic or homogenization temperature signatures.