MOUNTAIN RANGE SEGMENTATION: AN EXAMPLE FROM THE WESTERN TRANSVERSE RANGES, SOUTHERN CALIFORNIA

The Santa Ynez Mountains of southern California have a distinct crest along 110km of the range’s 130km length, organized into five mountain segments which are geomorphic expressions of differential tectonic rates accommodated by structure. The range provides evidence that linear mountain ranges, as with the faults that forms them, are segmented. As segment length increases, they may overlap or collide to create accommodating structural features such as cross-faults or folds. From west to east, mountain segment #1 is 25km long, with mean elevation of 400m, and an average height:width ratio of 0.06. Mountain segment #2 is 15km long, with mean elevation of 775m, and an average height:width ratio of 0.09. Mountain segment #3 is 25km long, with mean elevation of 1100m, and an average height:width ratio of 0.10. Mountain segment #4 is 25km long, with mean elevation of 1100m and an average height:width ratio of 0.14. Mountain segment #5 is 20km long, with mean elevation of 1250m, and an average height:width ratio of 0.14. Boundary zones between mountain segments correspond with one of the four major structures splaying into the range from the Santa Ynez fault, a major left-reverse structure that partially controls uplift of the range. The easterly-increasing height:width ratio supports the hypothesis that relief is a primary tectonic signal recorded by the landscape. Increasing eastern elevation implies increased shortening in that direction, which is consistent with the hypothesis of clockwise rotation of the western Transverse Ranges, and shortening produced by the big bend of the San Andreas fault. Hypsometric analysis of basins on the south flank of the range yields integral values between 0.38 and 0.58, which are statistically equivalent both among and within mountain segments. Hypsometric equivalence may be explained by: 1 The topography of the five segments formed at about the same time period; 2) tectonic perturbations are slow and their effects are beyond the resolution of the hypsometric method; 3) tectonic processes have been ceased for long enough that basin hypsometry has established an equilibrium.