J.C. Mutter, 1983. "Structure Within Oceanic Crust off the Norwegian Margin", 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 illustrations show a Lamont-Doherty multichannel seismic reflection profile obtained on the Outer Voring Plateau off the Norwegian margin. in this region, previous investigations had shown that oceanic basement could be recognized on low-power reflection profiling as the typically observed "acoustic basement," and that this basement shallows beneath the Plateau to depths as shallow as 1,400 m (4,593 ft). Reflection profiling with large sound sources and multichannel arrays has shown that the "acoustic basement" comprises a complex structural unit composed of reflectors dipping seaward, frequently exhibiting a wedge-shaped configuration with individual reflectors having upwardly-convex shapes. Similar data from the outer parts of other passive margins show remarkably similar reflector arrangements (Hinz, 1981).
Deep sea drilling into the uppermost part of the Norwegian Margin sequences recovered basalts of an age consistent with the earliest stages of sea-floor spreading (Talwani and Udintsev, 1976). The level at which basalts were recovered is the "acoustic basement" which is indicated in the illustration by the bold symbol. Basalts have a shallow to subaerial eruption environment and the overlying sediments are of shallow water clastic origin.
Velocities within the dipping sequences generally reach more than 5.0 km/sec (3.1 mi/sec) only a few hundred msecs into the basement.
A model has been developed to account for these sequences that draws on comparisons with volcanic sequences observed in Eastern Iceland (Mutter, Talwani, and Stoffa, 1982). We believe that the sequences formed at a subaerially exposed spreading center which was active during the first few million years of sea-floor spreading. The accompanying figure illustrates the model. in a subaerial environment, the outflow length of lavas may become very large and exert a load on the underlying crust (Bodvarsson and Walker, 1964; Palmason, 1973, 1980). If the outflow remains broad in comparison with the distribution of dyke injection the load causes a regular tilting of lavas toward the spreading center. We believe that a few million years after spreading commenced the accretion center fell below sea level, the lava outflow reduced dramatically, and the more typically chaotic oceanic crustal structure was developed.
The seismic profiles show a progression from a very regular basement structure comprising dipping reflectors on the upper part of the Outer Voring Plateau to more irregular structure on its outer flank which we believe to result from the change from subaerial to subaqueous eruption environment at the spreading center during the earliest phase of crustal accretion.
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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.