A.W. Bally, 1983. "Tectonics of Extensional Provinces—Introduction", 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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In section 22 of this atlas we grouped seismic profiles from structural provinces that were either dominated or at least initiated by extension or, in other words, stretching of the lithosphere. All rifting phenomena in this section involve the basement. in contrast with this, section 23 illustrates examples from structural provinces in which extension is displayed in detailed portions of the sedimentary sequence.
Section 221 contains a couple of crustal sections across rifted areas. More detailed sections across rifted structures are shown in section 222. For additional seismic profiles across rifted structures the reader is also referred to Effimoff and Pinezich (1981), Arthur et al (1982), and Fisher, Patton and Holmes (1982). Brief overviews of selected aspects of rifting are contained in Bally and Snelson (1980), Bott (1980), Avedik et al (1982), Ziegler (1982), and Bott (1982). For an overview on listric normal faulting and other forms of extensional tectonics, the reader may refer to Bally et al (1981), and Wernicke and Burchfiel (1982). Finally, the books edited by Illies (1981) and Palmason (1982) contain in-depth reviews of the current understanding of the geodynamics of rifting.
Section 223 is the logical continuation of the preceding section because most passive margins of the world are initiated by a rifting event. There is general agreement today that the evolution of a passive margin is best described by the following phases:
1) A rifting phase involves stretching of the lithosphere and/or a thermal uplift of the mantle. This phase is characterized by complex horst and graben tectonics.
2) The onset of drifting involves the separation of continental lithospheric plates. Oceanic crust is for the first time emplaced and accretion by ocean spreading across mid-ocean ridges begins in the gap between attenuated continental blocks. The onset of drifting may be dated by magnetic sea floor anomalies and corresponds simply to the oldest oceanic crust adjacent to a continent. On the continental crust which has been attenuated by lithospheric stretching, the onset of drifting is often indicated by an unconformity which is called the breakup unconformity (after Falvey, 1974).
3) The main drifting phase is dominated by massive subsidence with rates of subsidence that decrease exponentially from the date of the onset of drifting. The rate of sedimentation tends to exceed the rate of subsidence resulting in an accumulation of thick prograding sedimentary sequences. These sequences are frequently separated by unconformities, which in turn, determine the sequence boundaries delineating the units that are used by Vail et al (1977), Vail and Todd (1981), and Vail and Hardenbol (1981) in their seismic stratigraphic work.
In recent years much has been published on passive or Atlantic-type margins. For recent discussions of various aspects of passive margins the reader is referred to the books edited by Burk and Drake (1974), Keen (1979), Blanchet and Montadert (1981), Bally (1981), Scrutton (1982), Watkins (in press), and Watkins et al (1979).
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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.