Expansion Fault—Gulf of Mexico
D.C. Johnson, R.G. Fifer, K.A. McQuillan, 1983. "Expansion Fault—Gulf of Mexico", Seismic Expression of Structural Styles: A Picture and Work Atlas. Volume 1–The Layered Earth, Volume 2–Tectonics Of Extensional Provinces, & Volume 3–Tectonics Of Compressional Provinces, A. W. Bally
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The prototype regional expansion fault depicted on the seismic profile is located offshore Texas in sediments of Tertiary and Quaternary age. The seismic profile A-A' illustrates expansion and stress-related structures in the stratigraphic intervals adjacent to the expansion fault. Trending northwest to southeast, the seismic profile is oriented perpendicular to the fault plane in order to obtain the optimum quality data.
The interpretation of the regional expansion fault is based on an analysis of the refractions, diffractions, and reflections. The refractions from strata terminations and the parabolic dispersion of diffractions at the fault plane act to reduce the coherency of the data. The resulting reduction of coherent reflectors creates a cross-sectional graphic representation of the fault plane on the seismic profile.
Secondary faulting associated with expansion faults consists of parallel and antithetic faults. Parallel faults form at acute angles to the expansion fault plane, the result of sediment loading stress parallel to the expansion fault plane. Antithetic faults result from the arcuate stress perpendicular to the expansion fault plane. The antithetic fault forms at an oblique angle to the expansion fault plane, commonly intersecting and terminating into parallel faults.
Expansion faults are associated with areas of highly localized sedimentation rates. The massive centralized loading creates a gravity imbalance resulting in the initial offset of strata. Continuous loading over geologic time causes increased displacement and strata expansion on the hanging wall of the expansion fault. These conditions are common to shelf areas with high energy depositional regimes where the sediment infusion tremendously exceeds the erosional forces. It should be noted that in this prototype example the paleoshelf is a massive shale ridge whose strata interface delineates the gliding plane of the expansion fault.
Structural configuration adjacent to the expansion fault is a result of variable sedimentation rates. The strata interval whose structural configuration is a rollover anticline represents a period of high sedimentation. in contrast the strata draping onto the expansion fault is indicative of lower levels of sedimentation. Thus the cyclic nature of the structure adjacent to the expansion fault reflects the depositional regimes present at each strata interval.
The interaction of massive localized sedimentation upon the in situ shale ridge creates a dynamic fault plane of vertical and translational movement. Gravitational imbalance in conjunction with the shale ridge's plastic deformation results in a flexible fault plane whose angle of dip changes with depth. The resulting fault reflects both a classical normal fault, and also a gliding plane with translational movement in an almost horizontal plane.
Of interest to exploration geologists are the rollover anticlines into the fault whose structural configuration forms the ideal trapping mechanism for hydrocarbons. Formation of the structural traps is contemporaneous with deposition and, thus in place, able to trap hydrocarbons upon their generation and migration from the basin.
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Seismic Expression of Structural Styles: A Picture and Work Atlas. Volume 1–The Layered Earth, Volume 2–Tectonics Of Extensional Provinces, & Volume 3–Tectonics Of Compressional Provinces
Until a few decades ago, structural and regional geology were traditionally the preserve of field geologists. They usually mapped areas of outcropping deformed rocks and supplemented their work by laboratory studies of rock deformation and by theoretical work. Structural geology became tied to the geology of uplifts, folded belts, and underground mines, all of which were accessible to direct observation. Since World War II we have witnessed a tremendous development of geophysics in oceanography and in petroleum geology. Academic geophysicists in oceanography led their geological colleagues into modern plate tectonics and industry geophysicists developed reflection seismology into a superb structural mapping tool that penetrated the subsurface.
Today we are facing a situation where instruction and textbooks in structural geology are almost entirely dedicated to rock deformation, analytical techniques in detailed field geology and summaries of plate tectonics. Illustrations based on reflection seismic profiles are virtually absent in textbooks of structural geology. These texts illustrate only the parts of the proverbial elephant, together with some conjecture, but without ever offering a glimpse of the whole elephant.
Some of the reason cited for the relative scarcity of published reflection profiles are: 1) the confidentiality of exploration data; 2) difficulties in the photographic reduction and reproduction of seismic profiles for a book format; 3) the two-dimensional nature of vertical reflection profiles; and 4) the obvious distortions in reflection profiles that are typically recorded in time.
The AAPG leadership felt that it was time to attempt to correct the situation and to produce this picture and work atlas. The first volumes, of what may become a series of volumes, are addressing an audience that includes: petroleum geologists concerned with structural interpretations; exploration companies that provide in-house training; the AAPG continuing education program; and academic colleagues interested in updating their curricula in structural geology by inclusion of reflection profiles from the “real world” in their teaching.
The atlas is not meant to be a textbook in reflection seismology (instead we listed some at the end of this introduction) nor a text in structural and/or regional geology. Our intent is simply to provide a teaching tool.