and others (1979) , and the metamorphic assemblage recovered from the 

 eastern base of the West Mariana Ridge at DSDP Site 453 (Scientific 

 Staff, 1978). Although higher in metamorphic grade (amphibolite vs 

 greenschist) , the tectonic deformation exhibited by the Sorol Trough 

 metamorphics would certainly not be surprising if they represent part of 

 the deformed crust and upper mantle found on the inner-wall of an 

 island arc subduction zone, exposed in the deeply rifted Sorol Trough. 



Rifting in the Sorol Trough probably ceased earlier in the east 

 than in the west, perhaps as a result of the earlier cessation of 

 spreading in the East Caroline Basin lowering subduction rates, or 

 perhaps as result of the attempted ingestion of the low-density 

 Eeuripik Rise into the subduction zone south of the Caroline Ridge as 

 proposed by Bracey (1975). Rifting continued in the west, accompanied 

 by the appearance of mid-ocean ridge type basalts in addition to the 

 continuing transitional basalt flows at the northern trough margin, 

 until collision of the West Caroline Ridge and Basin with the east- 

 ward advancing Yap-Mariana island arc system (Bracey, 1975; Vogt and 

 others, 1976) at about 24 m.y. B.P. At this time, spreading activity 

 ceased in the West Caroline Basin, as did inner-arc spreading and all 

 other magmatic activity in the Sorol Trough. 



A final pulse of West Caroline Basin activity from 14.5-12.5 m.y. 

 B.P. may have reactivated the western Sorol Trough inner-arc spread- 

 ing, and the fresh pillow basalts found near the SW margin of the 

 trough may reflect this activity. The apparent lack of sediments in 

 this area of the trough is certainly comparable to the sediment absence 



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