In Chapter 3, where we discussed subduction and the initiation and development of trenches, we only considered the development of such features in a two-dimensional plane, defined by a vertical line and the direction of plate motion. It was shown that trenches are most easily initiated near the junction between oceanic and continental lithospheres. Once such trenches are formed, their subsequent migration will usually be restricted and controlled by continental migration. However, quite a number of these trenches, which are closely related to the oceanic/continental junction, exhibit a well-defined arcuate form (Figure 7.1). In this chapter, therefore, we shall first briefly describe the geometry of such large, curved subduction features, which often have radii of curvature of over 2000 km, and shall outline the mechanisms which have been proposed to explain the development of these features.
However, we are not primarily concerned here with the mode of arc development, which may exhibit large or irregular radii of curvature; but shall concentrate on those examples in which the arcs form almost perfect semicircles and have radii of less than about 600 km. These features, especially ones which occur in an oceanic context, are few in number and, notwithstanding their intrinsic interest, have received little comment. These trenches and/ or associated island arcs, which are often quite remote from continental masses, are either almost perfectly semicircular in plan, or they exhibit a portion of a circular arc, which may be as small as 60°. Only five such features which exhibit an almost exact circular arc, with a spread from 60° to 180°, are identified. These features cannot readily be explained in terms of normal plate tectonics mechanisms. To date, in the few papers that deal with their development, these features have been discussed in terms of kinematics, i.e. how (without reference to force and mass) the positions of the various elements changed with time. As far as we are aware, no mechanistically based theory has been put forward to explain the initiation of these relatively small arcs.
The initiation of each of these five structures is shown to be associated with an event which gives rise to an abrupt change in direction of plate movement and/or a change in rate of plate motion. We have already seen that such changes in track, which are associated with major continental flood basalts (CFBs) and oceanic plateau basalts (OPBs), may be attributed to major impacts. Hence, based on the evidence of the change in direction in track and speed, and their well-defined arcuate morphology, we present a model for their initiation and development that requires them to be the result of major meteoric impacts.
However, we shall first briefly discuss the conventional explanations regarding the development of the larger arcuate features.
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