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The AAPG/Datapages Combined Publications Database
AAPG Bulletin
Abstract
AAPG Bulletin, V.
Fault
Families
and
Fault
Welds in the Northern Gulf of Mexico
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: [email protected]
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.
ABSTRACT
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. Groups of geometrically classified
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.
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.
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.
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