PHYSICAL MODELING OF MINOR FAULT POPULATIONS IN BASEMENT-INVOLVED, FAULT-RELATED FOLDS

Although the geometry, evolution and spatial statistics of minor fault populations have been widely studied in extensional tectonic settings, much less is known about minor faults that form in association with contractional structures. This research attempts to bridge that gap by using scaled physical models of minor fault development in contractional, basement-involved fault-related folds. The models consist of a homogeneous clay layer that is deformed above a homoclinally dipping reverse fault that cuts through rigid basement material. Model variables we investigate include the maximum displacement to length ratio of the fault and the length of the fold backlimb. Our goal is to provide a first order characterization of the evolving population of minor extensional faults that cut the surface of the model. Achieving this goal will improve early assessments of hydrocarbon trap viability and reservoir compartmentalization in analogous natural structures. Fault population data were collected in the form of digital photographs of the model surface. The photographs were taken every 5 mm of basement fault displacement and the imaged minor extensional faults in the surface of the clay were subsequently digitized into ArcGIS. From the resulting data we examined how minor fault length, orientation, sinuosity and spatial density varied as a function of each of our two model variables. Increasing backlimb lengths resulted in decreasing minor fault density, but did not substantially affect fault length, orientation or sinuosity. New faulting, rather than linkage and lateral propagation of existing faults, accommodates progressive strain in this situation. Increasing fault maximum displacement to length ratio resulted in strong variations in minor fault orientations and spatial density, but had little effect on fault sinuosity or length. Higher lateral displacement gradients on the basement fault resulted in minor fault populations over a larger portion of the fold, and many of these faults were oriented at a high angle to the strike of the basement fault. Our combined results suggest that the risk of trap viability is greatest in basement-involved fault-related folds with shorter backlimbs and large maximum displacement to length ratios on the underlying fault.