. The Earth beneath the sea : History. Ocean bottom; Marine geophysics. SECT. 2] TRENCHES 423 trench dies out against the northeast-trending Cocos Ridge. The Tehiiantepec Ridge intersects the trench near 15°N, 95°W (Menard and Fisher, 1958); at this boundary the depth of the trench, the toi^ography outside the trench, the depth to the Mohorovicic discontinuity and the vulcanism related to the trench are changed completely (Fisher, 1961). Northwest of the Gulf of Tehuantepec, dejiths along the trench axis vary abruptly over short distances. Small steps or terraces result from ponding of sedimen
. The Earth beneath the sea : History. Ocean bottom; Marine geophysics. SECT. 2] TRENCHES 423 trench dies out against the northeast-trending Cocos Ridge. The Tehiiantepec Ridge intersects the trench near 15°N, 95°W (Menard and Fisher, 1958); at this boundary the depth of the trench, the toi^ography outside the trench, the depth to the Mohorovicic discontinuity and the vulcanism related to the trench are changed completely (Fisher, 1961). Northwest of the Gulf of Tehuantepec, dejiths along the trench axis vary abruptly over short distances. Small steps or terraces result from ponding of sediments; locally submarine hills constrict or even replace the usual trench floor (Fig. 6, A-A', B-B'). However, northern cross-sections characteristically. KILOMETERS 5772 VERTICAL EXAGGERATION 16 X Fig. 6. Cross-sections of the Middle America Trench traced from continuous-sounding records. Sounding scales corrected after Matthews (1939). (See Fig. 5 for location of the sections.) show the trench floor to be flat or gently basined; within such basins the bottom commonly rises 10-20 m toward the land and seaward flanks, with sharp breaks in slope at the basin edges. Seismic-refraction measurements (Shor and Fisher, 1961) and projection of flank slopes indicate | to 1| km of sediments in several of the basins between Islas Tres Marias and Acapulco. In this segment of the trench, lying close to a coast composed dominantly of metamorphic and intrusive rocks, trench sediments are gray to greenish brown clays and silts. The fine sand layers occasionally present here in trench-floor cores are attributed to frequent slumping or turbidity-current transport down the steep flanks. Seismic-refraction measurements along the trench axis be- tween Islas Tres Marias and Acapulco indicate that beneath the thick sediments and a probable basement (volcanic?) layer, the main crustal layer (Fp = Please note that these images are extracted from scanned page images that may have been digitally en
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