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Description
The objective of this study is to determine the basement morhology along the Rose Canyon fault zone from the San Diego River to La Jolla. Several gravity surveys by previous S.D.S.U. students were compiled into a complete Bouguer anomaly contour map. Three, two to four mile long profiles were constructed perpendicular to the Rose Canyon Fault Zone. The data were modeled using a twodimensional computer modeling program. We assumed values of 2.1 and 2.3 glee for the sedimentary rocks and 2.7 glee for the basement rock. These simple, twodimensional models show several significant geologic features related to faulting in the Mission Bay and La Jolla area. The San Diego River models show two interesting features: first, a basementlow west of the Rose Canyon Fault Zone, suggesting a continuation of the nested graben under San Diego Bay northwestward into Mission Bay. Second, up-to-thewest offset on the Pt. Loma fault. The Mission Bay models imply a northward continuation of the Point Loma Fault where it had been previously unmapped. The most interesting feature of the Mission Bay models is the gravity minimum west of the Rose Canyon Fault Zone. This was modeled as a graben having over 3000 feet of throw, and a basement depth of 5000 feet. Most of the off set on the east side of the graben appears to be on the westernmost strand of the Rose Canyon fault zone, i.e., the Mission Bay fault. Both the Mission Bay and San Diego River models suggest that the nested graben under San Diego Bay extends northwestward into Mission Bay. This depression is probably the result of transtension produced by the fact that the strike of the Rose Canyon fault zone in this area, N15W, is more northerly than the ideal N45W direction for transforms on the Pacific Plate-North American Plate boundary. The La Jolla model shows the change in the nature of the Rose Canyon fault zone from a zone of tension in Mission Bay to a zone of compression producing uplift on reverse faults dipping steeply to the southwest under Mt. Soledad. The folding and reverse faulting of Mt. Soledad is probably the result of transpression, caused by the change in the strike of the Rose Canyon fault zone to N60W, resulting in a restraining bend in the Rose Canyon fault zone.
