Convergent Wrench Fault and Positive Flower Structure, Ardmore Basin, Oklahoma
1983. "Convergent Wrench Fault and Positive Flower Structure, Ardmore Basin, Oklahoma", 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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A seismic profile from the Ardmore basin illustrates a positive flower structure and demonstrates important criteria for identifying convergent (or transpressional) wrench faults with seismic control. Positive flower structures are linear antiforms that are cut longitudinally along their apex by the upward-diverging strands of a wrench fault. Some of the fault strands have important reverse separations. These flower structures are to be distinguished from the negative flower structures discussed elsewhere in this atlas (Harding, 1983). Negative flower structures are complex, linear synforms displaced by inward-dipping strands of a wrench fault that have mostly normal separations.
In the discussion that follows, we first briefly outline the setting and the geologic history of the region in which the profile was taken. Then the general characteristics of positive flower structures and the flower structure features observed on the Ardmore basin profile are described. Lastly, the controls and mechanisms for the development of positive flower structures are discussed.
The Ardmore basin lies in the foreland of the Ouachita fold and thrust belt and within the splays of the Washita Valley wrench system. The Ardmore basin underwent initial subsidence in the latest Precambrian through Middle Cambrian as part of the southern Oklahoma aulacogen (Figure 1; Hoffman et al, 1974). The aulacogen was oriented transverse to the early Paleozoic continental margin (approximated by the Ouachita geosyncline on Figure 1) and became narrower toward the northwest. In the Late Cambrian through Pennsylvanian, basin subsidence took the form of a broad syncline.
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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.