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Abstract

This selection of seismic profiles illustrates the structure of subduction zones along active margins. The profiles were recorded during the early 1970s as part of a marine deepwater roving program, at a time when Coastal State jurisdiction was still restricted to the continental shelf under Geneva convention rules. The goal of the program was a study of the geology of marginal basins on a global scale. The acquisition and processing parameters of these profiles, recorded some 10 years ago, were geared for rapid and economic retrieval of reconnaissance information.

Active margins are the sites of plate subduction and plate collision. Synonyms for active margins are Pacific type margins, converging margins and destructive margins. Characteristic features are oceanic trenches, island arcs, and folded mountain chains. The term "active" refers to earthquake activity. Active margins are connected with "sinks" of mantle convection. The oceanic crust is subducted; it re-enters the mantle and is eventually consumed. As a byproduct of this process, continental crust is formed. Subduction zones are generally asymmetric as one plate plunges below the other at an angle varying between 15 and 60°. On a global scale, two tectonic regimes can be distinguished, namely the island arc type and the Andean type.

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