SECT. 2] 



TRENCHES 



431 



and plastic flow in the region where the mantle and crust are bending down- 

 ward into the descending limb of the convection cell might prevent direct 

 vertical rise of the fluids. 



The above discussion treats largely with the curved trenches flanking island 

 arcs. The question might be raised whether the linear trenches, such as those 

 off Chile or middle America, have the same structure. One might reasonably 



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 



