Three models are proposed for the former extent of a continuous heavily cratered plateau within the present area of fretted terrain between 270¿W and 360¿W. To the south and west, each model area limit is well constrained by the northern boundary of the unfretted heavily cratered plateau. The northern limit of each model is defined by the gradational northern limit of fretted terrain. The eastern limits are distinct for each model. Model 1 assumes a once continuous plateau was dropped to the north along NW-trending normal boundary faults in the Ismenius Lacus SE subquadrangle. Model 2 includes model area 1 and places the former limit of the plateau 400 km farther east along a trace subparallel to the present plateau margin in this subquadrangle. Model 3 includes model area 2 and uses the rim of the proposed Borealis basin in Casius SW subquadrangle [Wilhelms and Squyres, 1984> as the eastern limit of the former plateau.

The estimates for the amount of material removed vary from 1.04¿0.60¿106 km3 for model 1 through 1.53¿0.80¿106 km3 for model 2 to 1.99¿1.01¿106 km3 for model 3. This implies that this area of fretted terrain is not a significant source of sediment for the northern lowlands of Mars. ¿American Geophysical Union 1990