FRACTURE-CONTROLLED FLUID MIGRATION IN A DETACHMENT FOLD: AN EXAMPLE FROM THE MEXICAN SIERRA MADRE ORIENTAL

When there are no major faults present, as in the case of detachment folds, fracture networks play an important role in controlling fluid migration in folded structures. In such cases, spatial and temporal variations in the timing and connectivity of fracture networks that develop during folding may create a complex migration history comprised of multiple, discrete fluid migration events. This study uses mapping, structural, and geochemical analyses of vein and host rock samples to examine the pattern and timing of fracture-related fluid migration events during the evolution of a map-scale detachment fold. We studied the Nuncios Fold Complex (NFC), a gently plunging detachment fold that is the frontal structure of the Monterrey Salient in the Sierra Madre Oriental, Mexico. In this region a ~3000 m thick succession of Mesozoic rocks is detached above an evaporitic section, and folded into a series of anticlines and synclines. The structural geometry of the fold complex is determined by the stratigraphic sequence in which a middle, competent carbonate section is encased in thick, weak siliciclastic sequences. An important part of the fluid-migration network analysis lays in the understanding of the 3-D geometry of the folded complex. A GIS database that compiles together mapping, structural data, thickness changes, and geochemical results was used to characterize the fracture network connectivity and the possible sources and timing of fluid migration with respect to the evolution of the NFC. Throughout the fold complex, we recognized a systematic, unique development of mesostructures in specific stratigraphic intervals. This pattern of mesostructural development is consistent in different structural positions in the NFC. Veins present in each stratigraphic interval were subjected to various geochemical analyses to constrain the nature of fluids that migrated through the fracture network. Our structural and geochemical analyses delineate three major paleohydrostratigraphic units and suggest a general sequence of fluid migration coeval with folding. Migration began with fluid traveling along bedding-parallel surfaces, continued with flow along cross-fold fractures in later folding stages, and ended with fluid migration through mesoscopic stratabound faults in the latest stages of deformation.