Stratigraphic controls on eformation patterns in fault-related folds: a detachment fold example from the Sierra Madre Oriental, northeast Mexico

Abstract

Deformation patterns are described by the style, intensity and distribution of structures throughout a volume of rock. In fault-related folds, presently observed deformation patterns are controlled by structural factors like kinematic history, environmental factors like burial history and stratigraphic factors like layer thickness and competence. To demonstrate the role of stratigraphy in controlling fold-related deformation patterns, we document the stratigraphic variability of mesostructures at one structural position on a map-scale detachment fold. Unique assemblages of faults, fractures and cleavages in certain stratigraphic intervals define a mechanical stratigraphy comprised of several units, each exhibiting a characteristic deformation response to the folding process. The varying response of each mechanical unit suggests that folding mechanisms such as orthogonal flexure, flexural slip or flexural flow, were sequentially or coevally operative in each of the units. Using the simple, theoretical strain distributions associated with each folding mechanism and the fact that each mechanical unit accommodates fold-related strain by developing a characteristic set of mesostructures, we are able to predict general deformation patterns elsewhere in the fold. The significance of this approach is that it recognizes that even in a single structure, fold kinematics and deformation response may be dramatically different in various parts of a stratigraphic section. Because of this, the quality and character of reservoir rocks in anticlinal traps may change markedly over short stratigraphic distances. Predictions of fold-related deformation based solely on fold kinematics are unlikely to resolve these variations.