of NW-SE crustal weakness, or along the northern flank where the thinner 

 crust would be easier to breach? 



To answer my own somewhat rhetorical question, I think that any 

 transform boundary (leaky or otherwise) that may now exist in the Sorol 

 Trough followed a pre-existing zone of weakness. I follow the proposal 

 of Bracey and Andrews (1974) that the Sorol Trough is a remnant inner- 

 arc basin. The basin was created during rapid northward subduction of 

 Caroline Basin crust beneath an existing island arc in late Oligocene 

 (27-24 m.y. B.P.?). 



Rift formation was accompanied by voluminous outpouring of tran- 

 sitional basalts along the opening rift. These basalt flows may have 

 covered earlier crust and sediments, perhaps extending as far north as 

 the present-day Mariana Trench, as may be indicated by the low heat 

 flow values (0.6-0.9 HFU) found at the 3 locations in this area 

 (Hamilton, 1979), which would indicate a much older crust than the 

 Oligocene basalts presently found there indicate (see for example: 

 Sclater and others, 1976), and are in sharp contrast to the 2.11 HFU 

 thermal regime of the Oligocene Caroline Basin crust. 



The thickness of layer 2 crust found on the Caroline Ridge, noted 

 earlier, is comparable to the 6 km thickness found at the West Mariana 

 Ridge (Bibee and others, 1979), a region of tectonic activity analogous 

 to that proposed here. Whether there are chemical similarities between 

 the West Mariana Ridge basalts and those found here is unknown. 



There are, however, affinities between the metamorphic rocks 

 dredged from the southern escarpments of the Sorol Trough by Fornari 



59 



