Whatever the present-day nature of Ayu Trough-Palau Trench tec- 

 tonics, I follow Bracey (1975), in that these features earlier were 

 directly related to the Palau-Kyushu Ridge system, and acted as a trans- 

 form fault that separated the Tertiary Philippine Sea-Caroline Basin 

 spreading axes, and subsequently became a westward-dipping subduction 

 zone (Uyeda and Ben Avraham, 1972). 



Separating the Parece-Vela Basin and the NW Caroline Basin is a 

 trench-like feature (Fig. 2) marked on Figure 15 as a possible inactive 

 subduction zone. As proposed by Bracey (1975) this feature may have a 

 varied structural history: (1) A subduction zone during the earlier 

 period of West Caroline Basin formation (from prior to 36 m.y. B.P. 

 until 27 m.y. B.P.). (2) A fracture zone, with right-lateral sense 

 of offset, as the Yap arc moved westward from the Palau-Kyushu Ridge 

 (Karig, 1971) during the period 25 - 20 m.y. B.P. (3) Possible reacti- 

 vation of subduction during the final phase of West Caroline Basin 

 spreading (12.5 - 14.5 m.y. B.P.). 



A N-S seismic reflection profile shown in Hamilton (1979, his 

 Fig. 139) crosses the eastern end of this feature at 137°E, and exhib- 

 its definite trench-like morphology. Hamilton considered the feature 

 at this location to be a SW extension of the Yap Trench (his Fig. 

 136), which it may well be, but Figure 2 shows that the trench-like 

 morphology extends westward to the Palau Trench, and it is hard to 

 imagine that this E-W feature is a continuation of the N-S Yap Trench. 



Extending eastward from this feature are a series of morphologic 

 features considered to be the remnants of an extinct northward-dipping 



55 



