AAPG Bulletin, V. 83 (1999), No. 9, P. 1454-1484.

Salt-Related Fault Families and Fault Welds in the Northern Gulf of Mexico¹

Mark G. Rowan,² Martin P. A. Jackson,³ and Bruce D. Trudgill⁴

©Copyright 1999. The American Association of Petroleum Geologists. All rights reserved.
¹Manuscript received February 9, 1998; revised manuscript received January 25, 1999; final acceptance January 29, 1999.
²Rowan Structural Consulting, 1633 D 4th St., Boulder, Colorado 80302; e-mail: mgrowan@uswest.net
³Applied Geodynamics Laboratory, Bureau of Economic Geology, University of Texas at Austin, Austin, Texas 78713.
⁴Department of Geological Science and EMARC, University of Colorado, Boulder, Colorado 80309.

We thank Bruno Vendeville and Dan Schultz-Ela for their contributions and Ken McClay, Jim Handschy, and Jake Hossack for critical reviews of the manuscript. Within industry, many individuals, too numerous to list here, provided stimulating and helpful discussions. Diamond Geophysical (Davis Ratcliff), Western Geophysical (Denby Auble), Halliburton (Bob Graebner), and Geco-Prakla (Jerry Watson) donated seismic data, and Landmark and CogniSeis provided software. Rowan and Trudgill were funded by the EMARC Gulf of Mexico Industrial Consortium at the University of Colorado, consisting of Agip, Amerada-Hess, Amoco, Anadarko, BHP, BP, Burlington, Canadian Oxy, CNG, Conoco, Enterprise, Exxon, Marathon, Maxus, Mobil, Occidental, Phillips, Shell, Texaco, Union Pacific, Unocal, and Vastar. Jackson was funded by the Applied Geodynamics Laboratory consortium at the Bureau of Economic Geology, which comprises Agip, Amoco, Anadarko, BHP, BP, Burlington, Chevron, Conoco/DuPont, Exxon, Marathon, Norsk-Hydro, PanCanadian, Petroleo Brasileiro, Phillips, Saga, Shell, Statoil, Texaco, Total Minatome, and Vastar. We also thank Paul Weimer and EMARC for paying the cost of color figures. 

ABSTRACT

Salt-related faults and fault welds in the northern Gulf of Mexico are classified based on the three-dimensional geometry of the faults or welds, deformed strata, and associated salt. Kinematic or genetic criteria are not used in the classification. Only documented fault styles are considered; those styles produced by experimental or numerical modeling, but not yet observed in the Gulf, are not included.

Extensional faults comprising symmetric arrays include peripheral faults, which occur at the landward margin of the original salt basin; crestal faults, which are growth faults rooted in reactive diapirs; and keystone faults, which occur at the crests of anticlines. Asymmetric arrays of normal faults are grouped according to the dominant dip direction. Faults 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 decollement that merges into a salt layer. Counterregional faults are landward-dipping asymmetric arrays that link cylindrical, basinward-leaning salt stocks. Asymmetric arrays with variable dip direction include flap faults, whose footwalls comprise diapirs with uplifted and rotated roof strata, and rollover faults, which occur at the hinges of monoclinal folds. Two families of contractional faults are described: toe thrusts, which are basinward-vergent thrusts that ramp up from a salt or shale decollement, and break thrusts, which are high-angle reverse faults that cut 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 are those 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 and upwardly concave; thrust welds are landward-dipping surfaces that separate repeated stratigraphic sections; and wrench welds are steep and strike parallel to the regional dip direction.

Groups of geometrically classified fault families and fault welds are kinematically and genetically linked to each other 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 fault families 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 also can be soft linked. We illustrate five associations of linked fault systems that are directly related to five types of salt systems: autochthonous salt, stepped counterregional, roho, salt-stock canopy, and salt nappe.