Island-arc volcanism commenced in the Palau Islands at least by 

 Eocene time (Dietz, 1954) and subduction may now be inactive as indi- 

 cated by the low level of seismicity (Fig. 2). 



Rocks recovered in dredging of the Palau Trench are metamorphosed 

 (zeolite and greenchist grade) basics and ultrabasics (Coleman and 

 Irwin, 1977), grossly similar to those dredged on the inner-wall of 

 the Yap Trench, with the exception that amphibolite grade metamorphism 

 appears in some of the Yap rocks (Coleman and Irwin, 1977; Hawkins and 

 Batiza, 1977). 



Hawkins and Batiza offered a somewhat complicated but plausible 

 mode of origin for the Yap arc which may be applicable to the Palau 

 arc, since many of their characteristics are the same. They proposed 

 that the introduction of the Caroline Ridge into the Yap Trench essen- 

 tially blocked subduction from the east and caused the eastward obduc- 

 tion of Phillippine Sea (Parece-Vela ?) crust and mantle material over 

 the volcanic arc due to the continuing E-W horizontal compression of 

 the colliding plates. This would account for the metamorphic mineral 

 assemblage and the foreshortened arc-trench gap. 



In the Palau arc, where the same features are found, there are 

 bathjnnetric features (seamounts; ridges) to the east of the trench 

 that may have also contributed to subduction blockage. The younger, 

 more bouyant crust (the intermediate spreading) in the western Caroline 

 Basin may also have been detrimental to subduction along the southern 

 part of the trench (Fig. 15). The equivocal nature of this hypothesis 

 as applied to the Palau Trench kinematics is stressed. 



54 



