420 FISHER AND HESS fcHAP. 17 



than 5000 m ; to the south it tlies out against the rise extending from Palau 

 nearly to Hahiiahera. Although only 1250 km long at depths greater than 

 6000 m, it is deeper than 8500 m for more than 800 km of its length. Three 

 basins deeper than 9500 m occur; the longest of these, near Siargao, is slightly 

 greater than 10,000 m deep. 



Where the l'liili])pine Trench is best developed, at depths greater than 

 7500 m, the west or shoreward flank is steeper, with slopes of 10-16°, than the 

 offshore flank, with general slopes of 6-8°. Shoaler than 7500 m. on the shore- 

 ward side, the slope is more gentle, generally 4-8° up to the nearshore 1000-m 

 contour. On the east flank, above 7500 m, the bottom rises at a 1-4° slope to 

 the deep Pacific sea floor. Locally, the general slope is steepened by pinnacles or 

 ridges on the trench flank. The slopes listed here are similar to those reported 

 for the Tonga Trench (Raitt et al., 1955), Kuril-Kamchatka Trench (Udintsev, 

 1955) and the Japan (Idzu-Bonin) Trench (Nasu et al., 1960). As the trench 

 shoals to north and south, the slopes of the flanks become more nearly equal. 



Bathymetric exploration of the upper slopes of the Phili])pine Trench has 

 been insufficient to establish the presence or absence there of numerous sub- 

 marine canyons and large re-entrants and depressions well described for the 

 Kuril-Kamchatka Trench (Udintsev, 1955) and the Aleutian Trench (Gibson 

 and Nichols, 1953; Gates and Gibson, 1956). These authors proposed a fault 

 origin for most features of this kind, and related submarine fault systems to 

 structural trends mapped on the adjacent islands. Udintsev, noting the change 

 in character of trench bottom and island-trench nearshore topography from 

 south to north along the Kurils, suggests that the most active trench develop- 

 ment is taking place off Kamchatka and that the frequent vulcanism of that 

 region is activated by transverse faults extending across the slope. 



Cross-sections, traced from echograms (Fig. 4), of the deepest parts of the 

 Philippine Trench clearly show a narrow flat bottom, 1-3 km wide, in this deep 

 V-shaped trench. Projecting the lower trench walls to intersection, and 

 allowing for steepening with depth, indicates a thickness of less than 300 m for 

 the sedimentary fill in this basin, and thus a very small volume of sediment in 

 this trench. This figure for fill is similar to that deduced from seismic-refraction 

 observations in the Tonga Trench (Raitt et al., 1955). The lower walls of this 

 trench are irregular, with small knolls or pinnacles, particularly on the off- 

 shore side, that dam sediments or rocks slumping from further upslope. Bare 

 rock, probably volcanic, may outcrop on such steep trench walls and pinnacles 

 (Raitt et al., 1955; Bezrukov, 1957). The small elevations shown on Sections 

 I-I' and J-J' are spurs or toes of ridges constricting the basins and ponding 

 sediments. Kiilerich (1959) notes that north of 10° 45'N, where a ridge con- 

 stricts the trench, there is a valley or depression east of the trench axis and 

 separated from it by a ridge up to 500 m high. Section A-A' shows both the 

 trench axis and this outer depression, which is interpreted as a fault trough. 



Sediments of such very deep trenches have rarely been studied critically up 

 to the present time, because of difficulties in obtaining adequate undisturbed 

 samples and in knowing exactly where in the trench the samples were collected. 



