of the West Caroline Be sin, and at anomaly 9 (<v28.5 m.y. B.P.) time In 

 the eastern part of the East Caroline Basin. It Is not clear whether 

 this stoppage progressed at a steady rate across the basins or was an 

 abrupt stoppage of different segments of the spreading axis at progres- 

 sively later times. The lack of any significant sedimentary discon- 

 tinuities In the seismic profiles of the East Caroline Basin (Fig. 7) 

 would Indicate that In this area the stoppage progressed at a relatively 

 constant rate In contrast to the West Caroline Basin. 



In the West Caroline Basin (west of F.Z. 2) there appears to have 

 been a subsequent period of spreading Intermediate between the older 

 spreading and a final stage beginning at about anomaly 5B (f^l^.5 m.y. 

 B.P.) time. This Intermediate stage Is marked on the south (between 

 SD 2 and SD 1) by the lack of correlateable magnetic anomalies and the 

 sharp decrease In sediment thickness from south to north across a top- 

 ographic boundary (SD 2) noted above. The sediment In this southern 

 Intermediate area also show the presence of layer X (Fig. 5), which 

 indicates that the crustal age exceeds 18 m.y. B.P. 



In the northern Intermediate zone, magnetic anomalies are ten- 

 tatively identified for the interval 23.5-25.5 m.y. B.P. The age of 

 Inception of this period of spreading is unknown, the older crust and 

 the northern limb of discontinuity (SD 2) having presumably been inges- 

 ted into the subductlon zone at the northern margin of the basin. As 

 noted earlier, spreading was highly asymmetric; the northern limb mov- 

 ing much more rapidly than the southern limb. The term asymmetric 

 spreading is used rather loosely here. As pointed out by Hayes (1976) 



39 



