Figure 15 gives a graphic sunraiary of Caroline Basin and margin 

 tectonics, which together with Figure 2, should be referred to in order 

 to locate areas covered in the following discussion. 



The notable exception to the subduction marginal regime is the 

 Ayu Trough. The seismic reflection profiles of Figure 3 demonstrate 

 the sediment distribution patterns of known spreading axes as stated 

 earlier. Seismic reflection profiles shown in Hamilton (1979, his 

 Figure 141) indicate that this sediment-free area extends at least as 

 far north as 4.5 N, though attenuated in horizontal extent. 



Weissel and Anderson (1978) proposed that the Ayu Trough was an 

 active extensional feature, based on the absence of sediments from the 

 central rift and the presence of a limited number of earthquake 

 epicenters (Fig. 2) in the trough. They considered the possibility 

 that opening of the trough commenced at about 20 m.y. B.P. (based on 

 basement depths) about a pole of opening at 7°N/133°E, and proceeded 

 at a half-rate of 0.6 cm/yr. An alternative scheme of the authors 

 (based on extrapolated sedimentation rates), was that the spreading 

 began at 10 - 12 m.y. B.P. and progressed at 2 cm/yr until about 6 

 m.y. B.P. when it slowed to about 0.4 cm/yr. 



Analyses of basalts dredged from the trough by Fornari and others 

 (1979) support the spreading concept. The basalts show chemical affin- 

 ities with mid-ocean ridge basalts. 



The magnetic anomalies across the trough (Fig. 12) have very low 

 amplitudes as would be expected if they result from N-S oriented crustal 

 features formed at this low magnetic latitude, and are inconclusive in 



51 



