found at the younger West Caroline spreading zone, and may indicate 

 coeval activity and age. 



Whether the limited contemporary seismic activity within the 

 Sorol Trough represents left-lateral transform motion as proposed by 

 Weissel and Anderson (1978) , or is simply the result of normal fault- 

 ing caused by isostatic adjustments in the relatively young trough 

 awaits earthquake mechanism determinations. 



Weissel and Anderson present convincing evidence (seismic reflection; 

 earthquake mechanism) for eastward directed overthrusting of the Caroline 

 Plate over the Pacific Plate in a broad zone extending N35°W from the 

 northern end of the Mussau Trough at 3.5 N, to the northern limit of 

 their data at 6°N. This area is characterized by rough ridge-trough 

 bathymetry (Fig. 2). They attribute this zone to NE-SW horizontal 

 compression about a Caroline-Pacific pole of rotation at 13 N/144 E, 

 with an angular velocity of about 7°/10 m.y. There is presently no 

 information as to how far north this deformed zone may extend. The 

 bathymetry (Fig. 2), together with the scattered earthquake epicenters 

 (Bracey and Andrews, 1974, their Fig. 2) gives some indication of 

 tectonic activity along a broad zone extending northward to the southern 

 Mariana Trench, a presently inactive (south of 12°N) feature character- 

 ized by normal faulting in the island arc to the west (Bracey and Ogden, 

 (1972). 



There is some indication that the Oligocene magnetic lineations 

 may extend northward beyond the eastern limit of the West Mariana Ridge 



61 



