PACIFIC SUBMARINE PROVINCES 



523 



VERTICAL EXAGGERATION 10. 



Fig. 32.9. View of Middle American trench to northwest from Gulf of Tehuantepec. Tehunan- 

 tepec Ridge is in left foreground. Reproduced from Fisher 1961. 



Tehuantepec Ridge 



A northeast-southwest trending band of ridge and trough topography, 

 60 miles wide, separates the 10,800-1 1,400- foot sea floor outside the trench 

 off southern Mexico from the 12,600-13,200-foot Guatemala basin. This 

 zone has been traced from several hundred miles offshore to an inter- 

 section with the trench near the west side of the Gulf of Tehuantepec, 

 and has been called the Tehuantepec Ridge (Figs. 32.8 and 32.9). 



Ocean Floor and Seamounts 



The ocean floor outside the trench is fairly flat except for numerous 

 seamounts which undoubtedly are volcanic cones. The map of Fig. 32.8 

 shows the distribution of the seamounts charted by Fisher and Shor 

 (1959) and also the volcanic cones of Recent or Pleistocene age on land 

 in southern Mexico and Central America as far as the writer has been able 

 to locate them from the literature. 



The Guatemala basin, which is about 1800 feet deeper than the floor 

 north of the Tehuantepec Ridge, shoals to the southeast. It contains few 

 volcanoes whereas a row of majestic active and dormant volcanoes lies 

 opposite on land and stretches from southern Chiapas across Guatemala, 

 El Salvador, Nicaragua, and Costa Rica. Volcanism in Mexico is discussed 

 in Chapter 35. 



As far as known the distribution of volcanoes on the ocean floor south- 



west of the trench is random, although one or two rows seem apparent. 

 None of the seamounts has been recognized as beveled, so it is Dot 

 possible to infer vertical movements of the ocean floor such as in the 

 Mid-Pacific Mountains, described on following pages. 



Crustal Structure 



Three seismic refraction stations were taken along the axis of the 

 trench west of Acapulco and two along its axis off Guatemala and El 

 Salvador. Another station was shot on the shelf and one 60 miles seaward 

 of the trench off Guatemala. Upper mantle velocities appear on all lines 

 (Fisher and Shor, 1959). 



Thick sediments were found in the Tres Marias basin off Manzanillo 

 and at the shelf station off Guatemala. On a section normal to the trench 

 off Guatemala, the depth below sea level to the Mohorovicic discontinuity 

 in the trench zone is 16 kilometers, and in the shelf area 17 kilometers. 

 Below the sea floor the crust thickens from 5 to 7 to 10 to 17 kilometers 

 along this section (Fig. 32.11). 



The Mohorovicic discontinuity is deeper and the crust below the sedi- 

 ments thicker under the two southern stations than under the two central 

 trench stations. The mantle is deeper under the Tres Marias basin, where 

 thick sediments (1/2 kilometers) are found, than under the central 

 stations. 



Fig. 32.10. View of southeastern end of Middle America trench. Reproduced from Fisher 1961. 



