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Book Chapter

Structure Within Oceanic Crust off the Norwegian Margin

By
J.C. Mutter
J.C. Mutter
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Published:
January 01, 1983

Abstract

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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Contents

AAPG Studies in Geology

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
A. W. Bally
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American Association of Petroleum Geologists
Volume
15
ISBN electronic:
9781629810188
Publication date:
January 01, 1983

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