CONTRASTING INTERPRETATIONS OF FRACTURING AT THE SANT CORNELI-BOIXOLS ANTICLINE: HOW DO WE DISTINGUISH FOLD-RELATED FRACTURES?

Fractures are a crucial part of understanding the folding process because they record the strain history before, during, and after folding. Since the publication of Stearns’s (1967) conceptual model of fold-related fracturing, numerous researchers have attempted to classify fractures associated with folding on the basis of the fracture mode and spatial association of fractures with folded bedding. The conceptual models are widely utilized because no diagnostic criteria links a fracture to the folding process; only the classification and organization of multiple fractures and strain indicators can be used to interpret a fracture set as “fold-related.” We illustrate some of the inherent issues with interpreting fold-related fractures using a review of fracture interpretations at the Sant Corneli-Boixols anticline. The anticline is a plunging thrust-related fold in the Spanish Pyrenees that offers one of the best opportunities to document the history of fracturing in the context of folding. The structure is well exposed in three dimensions and has excellent exposures that are well suited to field mapping of fractures. Perhaps more importantly, the fold history, which consists of an early rift phase followed by salt-related basin inversion, is constrained by syn-tectonic strata with fracture mineralization linking fractures to specific depositional and structural events. However, even in such a well constrained setting, the fracture sets observed in the field can be interpreted either as fold-related fractures or as pre-folding fractures. Shackleton and others (2011) interpreted several fracture sets as fold-related fractures based on their alignment with respect to local bed strike rather than their correlation in orientation. Tavani and others (2011) studied fracture sets in the adjacent Boixols anticline and found very few fracture sets to be strictly “fold-related”, instead interpreting many of the same fracture sets as reactivated pre-folding fractures. This presentation reviews the two interpretations of the fracture system to illustrate the difficulties and pitfalls of interpreting fold-related fracture sets. Finally, we introduce ongoing petrographic and isotopic analyses aimed at providing an independent verification of associated fracture sets at Sant Corneli anticline.