EFFECTS OF ACTIVE FOLDING AND REVERSE FAULTING ON STREAM CHANNEL EVOLUTION: SANTA BARBARA FOLD BELT, CALIFORNIA

The Santa Barbara Fold Belt (SBFB) is an area of active crustal shortening comprised of east-west and southeast-northwest oriented reverse faults and folds. Changes in channel position, growth and incision are the result of active fold growth and surface uplift. Lateral stream diversion from fold and fault growth varies from 6.7 km to 0.4 km. Rates of stream incision in response to active uplift are 0.4 ± 0.1 m/ka to 0.6 ± 0.1 m/ka, this is about one-half the rate of uplift determined from dated marine terraces.

Uplift of Mission Ridge is 0.8 ± 0.2 m/ka and minimum uplift of the Mesa fault is 0.21 – 0.35 m/ka. Timing of stream diversions range from late Pleistocene to Holocene based on the age and location of incised alluvial fans and marine terraces. The minimum rate of lateral fold propagation is 6 m/ka.

Two sets of reverse faults and folds are present in the SBFB. One set is roughly east to west and truncates a more northwest to southeast set. The hypothesis that east to west folds are younger is suggested by the crosscutting relationship and is tested using geomorphic indices of active tectonics including: valley width to height ratios (Vf), mountain front sinuosity (Smf) and drainage densities (Dd). East to west trending structures have mean values of Vf, Msf, and Dd of 2.7, 1.2, and 2.2 km/km2, respectively while southeast to northwest trending structures have mean values of 5.0, 1.5, and 3.7 km/km2, respectively. These values support the hypothesis that the east to west folds are younger and more active. Clockwise rotation of the Western Transverse Ranges is probably the process that produced the two sets of structures. With rotation the older set that started out east to west eventually became northwest to southeast, and more difficult to maintain with the north to south contractional (shortening) associated with the Big Bend of the San Andreas fault 80 km to the north. The second set of younger structures formed with a more favorable (east to west) orientation to the shortening.