. The Earth beneath the sea : History . Fig. 10. Postulated mechanical situation where the crust moves into a curved trench. Movement of A'A to B'B would result in extension in the direction B'B, providing potential fissures to permit escape of magma to volcanoes at w, x, y and z. These volcanoes are in all cases on the concave side of the trench and about 180 km from the trench axis. Immediately beneath the trench, deformation is severe, and plastic flow might be expected to close the fissures. Movement on the convex side from D'D to C'C would cause compression along C'C so that open fissures
. The Earth beneath the sea : History . Fig. 10. Postulated mechanical situation where the crust moves into a curved trench. Movement of A'A to B'B would result in extension in the direction B'B, providing potential fissures to permit escape of magma to volcanoes at w, x, y and z. These volcanoes are in all cases on the concave side of the trench and about 180 km from the trench axis. Immediately beneath the trench, deformation is severe, and plastic flow might be expected to close the fissures. Movement on the convex side from D'D to C'C would cause compression along C'C so that open fissures would not form and hence volcanoes are absent. Individual sectors a, b, c, d and e might move differen- tially as portrayed by the displaced bed f'f. Strike-slip faults would bound the sectors and would tend to be sinistral toward A'B' and dextral toward AB. Note:—This figiire represents the deformation of a layer of arbitrary thickness, not necessarily the crust. propose that these trenches are formed by the continental block overriding the oceanic area or the downward-flowing limb of a convection cell. Beniofif (1949, 1955) postulated an overthrust mechanism and showed that the deep earth- quake foci lie along a plane dipping under Chile at approximately 40°. This raises a series of unresolved problems which will be briefly considered below. Before proceeding to this subject, it should be noted that the Tonga-Kermadec Trench is also a linear feature but in other respects it is related to a province having all the characteristics of an island arc. Coulomb (1945) showed that the
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