ridge axis can be made from 12.5 m.y. B.P. to about 14.5 m.y. B.P. 

 (anomaly 5B) . This region corresponds to the inner sediment discon- 

 tinuity (SD 1) previously noted in the seismic data. 



A second spreading sequence, intermediate between the older spread- 

 ing to the south and this later spreading, is proposed in the inter- 

 mediate sediment thickness region between the older, thicker sedimentary 

 cover to the south and the younger spreading region (between SD 1 and 

 SD 2). This sequence is tentatively identified as the period beginning 

 prior to anomaly 7 (■?26 m.y.) and persisting to anomaly 6C time ("^23. 5 

 m.y.). 



It must be emphasized here that model identifications for these 

 short spreading segments are extremely tenuous. The absence of seismic 

 layer X in the younger sequence limits its age to < 18 m.y. B.P., which 

 agrees quite well with the age of the model sequence shown, and the 

 model spreading rate agrees with earlier rates, with the exception of 

 evidence shown by the model comparisons of asjnnmietrical rates on either 

 side of the axis discussed below. 



The intermediate spreading region between SD 1 and SD 2 has identi- 

 fiable magnetic anomalies only to the north of the axis, and indicates 

 highly asymmetric spreading during this intermediate period. Anomalies 

 cannot be identified in the southern intermediate area, possibly because 

 spreading was too slow to generate recognizable anomalies. SD 2 is not 

 found on the northern side of the axis, having presumably been ingested 

 into the subduction zone at the north side of the basin. 



26 



