422 riSHER AND HESS [CHAP. 17 



In general, samples taken from depths shoaler than 4500 m on trench tlanks 

 are predominantly calcareous, composed of Globigerina oozes and silts ; in 

 tropical areas coralline fragments are common. The carbonate fraction is 

 mixed with volcanic debris and pyroclastics. Further downslope the sediments 

 are brown or red clays and silts together with ash and volcanic lapilli ; organic 

 remains commonly are siliceous. Surface deposits on the trench floors range 

 from lirown clays or silts to sand and fragments of volcanic glass and pumice ; 

 beneath the surface layers the sediments commonly are gray to black or dark 

 brown. Coarse graded layers, sometimes several centimeters thick, result from 

 intermittent turbidity-current deposition and from slumi^ing of material down 

 the steep walls. Occasionally such layers contain tests of shallow-water Fora- 

 minifera, attesting to turbidity-current origin and rapid burial. Bezrukov 

 (1957) reported that large fragments or galls of older laminated clays litter 

 portions of the western Pacific trench floors ; some of the laminations in the 

 fragments show distortion by sliding. He concluded that trench-floor sediments 

 may show three types of stratification: (1) a micro-lamination of the solid 

 clays, not yet explained ; (2) an alternation, ujj to several millimeters or centi- 

 meters in thickness, of clays or silts with volcanic ash or sands and gravel, 

 attributable to volcanic eruptions or to periodic stirring up, by earthquakes, 

 of pyroclastic material on the trench slopes ; (3) vertical differences, expressed 

 in colors, of the oxidation states of iron and manganese in the silts and clays. 

 This latter stratification, due to diagenetic processes, is controlled by rate of 

 accumulation of the sediments and the incorjDoration of organic material into 

 them. 



The Middle America Trench (Fig. 5), shoalest of the 20 trenches listed in 

 Table I, borders the west coast of Central America for more than 2600 km, 

 from central Mexico to Costa Rica. In morphology and setting it strikingly 

 resembles, on a smaller scale, the Peru-Chile Trench bordering the South 

 American continent (Zeigler et al., 1957; Fisher, 1958). Strongly arcuate 

 over part of their extent, nearly straight in other sections, the axes of these 

 trenches parallel closely the break in slope at the continental margin. Both are 

 associated with gravity minima, shallow and intermediate depth earthquakes 

 (deep-focus earthquakes also occur in the South American sequence) and, over 

 part of their length, vulcanism. In both trenches a shallow northern section is 

 separated from a deeper southern section by an offshore ridge intersecting the 

 trench at a large angle. The ridges may be off"slioots of the large fracture zones 

 of the eastern Pacific (Menard and Fisher, 1958; Fisher and Norris, 1960). 

 Riiegg (1960), however, considers that the ridge bisecting the Peru-Chile 

 Trench represents very recent uplift of a former land-mass that fomidered 

 before or during the Nevadan movement, prior to the formation of the trench. 



The Middle America Trench is intersected and approximately bounded on 

 the north by the Clarion Fracture Zone that has been linked to the transverse 

 volcanic province of Mexico (Menard, 1955). Topographically the northern 

 termination is rather abrupt ; farther to the northwest, the Gulf of California 

 may mark a continuation of the same seismically active zone. To the south the 



