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 = 6.5- 



