de Rica-Gorda Plate. Carson (1977) and Barnard (1978) have both exam- 

 ined this convergent margin In areas to the north off the coast of 

 Washington, and both present seismic cross-sections of this area. Pre- 

 sumably, similar processes are at work along the Oregon margin. Barnard 

 (1978) Infers a change of compression rate from 2.3 cm/year before 0.5 

 mybp to a present rate of 0.7 cm/year. Deformation of Cascadla Basin 

 sediments Is progressing westward, making the deepest areas of the mar- 

 gin also the most recently deformed. This westward progression of 

 deformation process Is e]q>ressed In the bathymetry as a downslope 

 Increase In surface roughness. Barnard (1978) classifies the slope ter- 

 rain Into an upper slope extending to a depth of about 1500 m, and an 

 accretlonary "borderlands" complex of en-enchelon, anticlinal ridges. 

 Between these ridges are sedlment-fllled basins. These physiographic 

 divisions, derived by qualitative observation of the geological struc- 

 ture of the region, correspond closely to the roughness model generated 

 by quantitative methods. 



The Gorda Rise Is one of the more active areas o*f the world sea 

 floor, and a corresponding complexly Is evident In the derived pattern 

 of bottom roughness provinces. Atwater and Mudle (1973) reviewed the 

 tectonic history of the area. Additional history of spreading rate and 

 spreading direction changes can be found In Elvers et al. (1973). Much 

 of this tectonic history may be peripheral to this study because the 

 sea floor affected Is now burled beneath the Tufts Abyssal Plain and 

 Gorda Deep-Sea Fan sediments. 



One Interesting feature of the bottom roughness chart presented In 

 Figure 5-7 Is the very rough ridge crest which terminates abruptly on 

 either flank. The full width of the feature Is about 25 km. Were 



65 



