similar to the sediment thickness vs. age diagram developed by Vogt 

 and others (1969, their Fig. 3) to identify spreading discontinuities. 



There is no indication in the magnetic data of changes in mode or 

 direction of spreading, although these possibilities cannot be excluded 

 in the southern intermediate area where no anomalies have been iden- 

 tified. There is also no definite indication of a spreading axis 

 shift, although as stated earlier this possibility cannot be dis- 

 counted, nor is there any positive evidence of change of spreading mode. 



Within the possibilities offered, I favor spreading cessation and 

 reactivation to account for the two discountinuities, even though this 

 appears to be the rarest form of discontinuity (Vogt and others, 1969). 

 The fact that the younger spreading axis and the older sea floor have 

 similar spreading half-rates of 6 - 7 cm/yr would indicate that there 

 was no radical change in spreading rates during these widely separated 

 (18 m.y.) times, and although asymmetrical, the intermediate spreading 

 rate was also of the same magnitude. 



The sediment isopachs (Fig. 13) lend some credence to the above 

 interpretations, particularly as to the location of certain fracture 

 zones. The narrowing of the thin sediment cover area enclosed by the 

 0.2 sec contours toward the ENE from about 5°N/135°E to 6°N/140°E 

 indicates an age increase toward the ENE. Note also that sediment 

 thicknesses 0.4 sec in this area do not extend westward of 138°E. 

 Particularly striking is the "nosing" of the 0.4 sec contour to the 

 south between 135°E and 140°E, exactly as predicted by the magnetic 

 anomaly age offset between F.Z. 2 and F.Z. 3. 



42 



