Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
Luncheon presentation, Southeastern Geophysical Society, New Orleans, Louisiana, January 11, 2001
Salt-Related Fault Families and Fault Welds in the Northern Gulf of Mexico
Mark G. Rowan, 1 Martin P. A. Jackson, 2 and Bruce D. Trudgill 3
We classify salt-related faults and fault welds in the northern Gulf of Mexico on the basis of the three-dimensional geometry of the faults, welds, deformed strata, and associated salt bodies. Kinematic or genetic criteria are not used in the classification, although we comment on these aspects when helpful. Only documented fault styles are considered; those produced by experimental or numerical modeling but not yet observed in the Gulf are excluded.
Extensional faults comprising symmetric arrays include peripheral faults at the landward margin of the original salt basin; crestal faults, which are growth faults rooted in reactive diapirs; and keystone faults at the crests of anticlines. Asymmetric arrays of normal faults are grouped according to their dominant dip direction. Those that dip primarily basinward include roller faults, which are listric growth faults that sole into a subhorizontal salt layer; ramp faults, which extend upward from the landward margin of bulb-shaped salt stocks; and shale-detachment faults, which sole into a shale décollement that merges laterally into a salt layer. Counterregional faults are landward-dipping, asymmetric arrays linking cylindrical, basinward-leaning salt stocks. Asymmetric arrays with variable dip direction include flap faults, whose footwalls comprise diapirs overlain by uplifted and rotated roof strata, and rollover faults at the hinges of monoclinal folds. Two families of contractional faults exist: toe thrusts, which are basinward-vergent thrusts that ramp up from a salt or shale décollement, and break thrusts, which are high-angle reverse faults cutting one or both limbs of detachment folds. Fault arrays that strike parallel to the regional dip direction are termed lateral faults. Six types of fault welds are defined: primary welds form at the autochthonous level; roho welds are subhorizontal, allochthonous welds into which roller faults detach; counterregional welds comprise both subhorizontal and landward-dipping segments beneath growth monoclines; bowl welds are elliptical welds that concave upward; thrust welds are landward-dipping surfaces that separate repeated stratigraphic sections; and wrench welds are steep welds that strike parallel to the regional dip.
Groups of geometrically classified fault families and fault welds are kinematically and genetically linked to one another and to associated salt bodies and welds. Linked fault systems can contain extensional, contractional, and strike-slip components. Extensional fault families are formed by basinward translation, subsidence into salt, or folding. Those that accommodate basinward translation are balanced by salt extrusion or contractional fault families. Strike-slip fault families commonly provide hard links, although various fault components can also be soft linked. Five associations of linked fault systems are directly related to five types of salt systems: autochthonous salt, stepped counterregional, roho, salt-stock canopy, and salt nappe.
1 Rowan Structural Consulting, 1633 D 4th Street, Boulder, Colorado 80302; formerly at the Department of Geological Science and EMARC, University of Colorado, Boulder, Colorado 80309; e-mail: email@example.com.
2 Bureau of Economic Geology, The University of Texas at Austin, Box X, University Station, Austin, Texas 78713; e-mail: firstname.lastname@example.org.
3 T. H. Huxley School, Imperial College, Royal School of Mines Building, Prince Consort Road, London SW7 2BP, United Kingdom; formerly at the Department of Geological Science and EMARC, University of Colorado, Boulder, Colorado 80309; e-mail: email@example.com.