ridge- forming processes constant through time and ridge crest relief 

 "frozen" Into the topography, the same roughness would be expected to 

 persist, at least In long wavelengths, on older sea floor. The other 

 major process affecting the ridge flanks, sedimentation, would be 

 expected to affect the short wavelengths Initially (due to their lower 

 amplitude), and form a smooth transition zone, not the abrupt boundary 

 observed In the roughness pattern. Magnetic data from the area Indicate 

 that this zone falls within the Bruhnes-Matayama boundary and must 

 therefore represent crust younger than 0.7 my. Recalling that Barnard 

 (1978) found evidence of a slowing of compression rate on the continen- 

 tal margin at a time younger than 0.5 mybp In areas to the north. It Is 

 possible that this roughness feature reflects the same change In proc- 

 ess, most likely a slowing of spreading rate. Future examination of 

 other ridge axes should reveal whether this pattern is unique to the 

 Gorda Rise or present under other tectonic conditions. 



Perhaps equally interesting is the abrupt termination of this ridge 

 crest at latitude 42°20'N. The roughness values drop (as supported by 

 three tracklines) two and three roughness levels at the feature's termi- 

 nus. This disruption of the ridge crest falls along a trend which 

 encompasses President Jackson Searaounts and other seamounts to the 

 northwest, and a major (900 m) ba thyme trie deep to the southeast. The 

 break in the ridge crest trend also appears in the bathymetric chart. 



Hey (1977) developed a "propagating rift" model to describe the 

 plat6 geometries and magnetic anomaly pattern found on the Juan de Fuca 

 Ridge by the Pioneer survey. According to this model, the growing 

 spreading center propagates along strike as the dying spreading center 

 becomes inactive and is added to one of the rigid plates. As this proc- 



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