Identifying any spreading axis. Magnetic symmetry about the trough 

 axis would be a positive test for sea-floor spreading at this location, 

 but it will probably require a detailed ( < 10 km track spacing) E-W 

 survey pattern over the trough to establish any conclusive magnetic 

 evidence. 



The gravity data of Watts and others (1978) show a small free 

 air minimum (< 25 mgal) associated with the trench axis, with flanking 

 free air highs of 100 mgal. While not inconsistent with a spreading 

 origin for the trough, these anomalies could equally well result from 

 other structural origins (graben, fracture zone, etc). 



The data presently available certainly do not negate the conten- 

 tion of Weissel and Anderson that this is a Neogene spreading area. 



The marginal area between the Ayu Trough and the Palau Trench 

 is an enigma. Although marked on Figure 15 as a possible fracture 

 zone, its origin or present characteristics are unknown. Hamilton 

 (1979) extends the "belt of rough topography" shown in his reflection 

 profiles northward to the Palau Trench itself, and it may well do so. 

 This area would then probably represent an axis of incipient or failed 

 Neogene Spreading. Its northern terminus near the proposed pole of 

 opening of the Ayu Trough may explain the limited E-W lateral extent 

 of this northern segment. 



The Palau and Yap Trenches are similar to each other, and atypical 

 of most oceanic trenches in that the distance between the axis of the 

 trench and the island-arc is only 40 - 50 km as opposed to the 150 km 

 separation at most island-arc systems (Weissel and Anderson, 1978) . 



53 



