424 FISHER AND HESS [CHAP. 17 



().!» km sec) lias a thickness about the same as at normal Pacific basin stations. 

 Apparently, this portion of the Middle America Trench is relatively inactive at 

 present ; faulting and vulcanism seaward of the trench are more significant 

 tlian the trench-forming process. If so, this trench may soon, in a geological 

 sense, resemble the minor, strikingly flat-bottomed, sediment traps that 

 border the west coast of central Baja California (Fig. Sf ; Fisher and Revelle, 

 1955). 



To the southeast of the intersection \\ith the Tehuantepec Ridge, the axial 

 depth of the trench increases abruptly, then shoals slowly toward the Cocos 

 Ridge. This portion of the trench has the more usual V-shaped cross-section 

 (Fig. 6, C-C, D-D') and a line of recently active volcanoes parallels its trend. 

 However, the volume of trench-floor sediments is small. Coarse land-derived 

 material is deposited on the broad shelf or upper slope ; trench-floor deposits 

 are fine-grained, with recognizable ash layers and higher proportions of volcanic 

 glass. Seismic-refraction measurements in the deep part of the trench, off 

 Guatemala (Fig. 8e) and El Salvador, indicate that here the main crustal layer 

 is considerably thicker than it is seaward of the trench, although it thins 

 some^^•]lat to the outer portion of the continental shelf, where it is overlain by 

 a thick section of sedimentary and basement (volcanic?) rock. Off Guatemala 

 the crustal thickness and thickening at the axis of this minor, relatively 

 shallow trench are nearly as great as that of the very active, deep Tonga Trench 

 (Fig. 8c). 



The linked trenches south of New Britain, southeast of the Solomon Islands 

 and west of the New Hebrides (Fig. 7) represent a series concave toward the 

 Pacific Ocean. This being the only present example of island arcs and trenches 

 concave away from a continental area is the interesting example contrar}^ to 

 the general rule. The whole of this region is active seismically and evidence for 

 Recent or sub-Recent vulcanism is common along the islands adjacent to the 

 concave side of the trenches. Earthquake foci of moderate depth are common 

 below the concave side of the arcs. The area is not well known and the bathy- 

 metry shown in Fig. 7 is based on the w^artime data collected by the U.S.S. Cape 

 Johnson, which was used to check such sounding lines and old charts as were 

 available in the U.S. Hydrographic Office. The trench recently reported by 

 Vitiaz (Udintsev, 1958) has been added. 



One notable feature of the relatively small trenches in this area is that there 

 is a distinct tendency to shoal or disappear off the middle of the arcs. In both 

 the case of the Solomon Islands and the New Hebrides the trenches are well 

 developed at either extremity but disappear off" the centers of the arcs. A 

 somewhat similar tendency may be noted in the case of the Marianas Trench. 



The Cape Johnson- Vitiaz trench system is unique and has no known counter- 

 part in other island arcs. If the ordinary trench off the convex side of an island 

 arc represents the down-buckling of the crust, the Cape Johnson-Vitiaz Trench 

 may represent some sort of strike-sli]) fault adjustment between adjacent blocks. 

 This has been suggested by R. W. Fairbridge {in lift., 1960). 



The morphological details of the trenches paralleling the island arcs are 



