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Deformation along oblique and lateral ramps in listric normal faults: Insights from experimental models

¹ ConocoPhillips School of Geology and Geophysics, University of Oklahoma, Norman, Oklahoma 73019;shamik.bose-1{at}ou.edu
² ConocoPhillips School of Geology and Geophysics, University of Oklahoma, Norman, Oklahoma 73019;smitra{at}ou.edu

Shamik Bose is a Ph.D. student at the University of Oklahoma. He received his B.Sc. degree from the University of Calcutta (India), an M.Sc. degree from the Indian Institute of Technology, Kharagpur (India), and an M.S. degree from the University of Oklahoma. His research interests include analog modeling of natural structures in the extensional regime, primarily using wet clay and three-dimensional structural modeling.

Shankar Mitra holds the Monnett Chair in Energy Resources at the University of Oklahoma. He received his Ph.D. in geology from Johns Hopkins University in 1977. His primary interests are in structural interpretation and modeling and their application to hydrocarbon exploration and production.

Listric growth faults in passive margin settings such as the Gulf of Mexico and Niger Delta are commonly characterized by lateral and oblique ramps related to preexisting structural or stratigraphic discontinuities. Clay experiments have been used to model the geometry, orientation, density, and connectivity of secondary faults formed along lateral and oblique ramps. Extension results in the formation of an expanding set of synthetic faults tied to the fixed footwall and a corresponding set of antithetic faults tied to a moving hanging wall. Some of the synthetic fault strands eventually connect to form the master fault, whereas antithetic faults continue to develop, with progressive transfer of slip to newly formed faults. Characteristics such as fault orientation, fault density distribution, and shape, size, and distribution of connected fault clusters vary with (1) ramp offset angles, (2) structural position, and (3) total extension. In map view, secondary antithetic and synthetic faults mimic the geometry of the main fault, but the orientations of secondary faults are approximately 25–33% of the offset angle of the oblique or lateral ramps. Fault densities and connectivities are initially higher along the frontal ramps. With increasing extension, the maximum cluster size of connected faults increases dramatically in the oblique and lateral segments due to the intersection of fault sets of different orientations. These observations regarding fault orientations, densities, and connectivities provide important insights on the structural geometry and mechanisms of formation of faults as well as the configuration of fault networks for fluid flow in passive margin settings.