The Quaternary North Channel-Pitas Point Fault System in Northwest Santa Barbara Channel, California

Christopher C. Sorlien, Craig Nicholson, Richard J. Behl, Courtney J. Marshall, & James P. Kennett

Published 2014, SCEC Contribution #2081

The north margin of the rapidly-shortening, rapidly subsiding offshore and western onshore Ventura Basin is comprised of major N-dipping faults that step left. Offshore in Santa Barbara Channel, part of the shortening accommodated by the shallow Red Mountain fault is transferred farther offshore to the North Channel-Pitas Point fault system (NC-PP). We previously investigated the eastern offshore 50 km of this fault system using dense grids of industry multichannel seismic reflection (MCS) data and local grids of high resolution MCS data. Timing and rates of the observed deformation were determined by correlation to a detailed dated stratigraphy derived from piston cores that sampled seafloor outcrops back to 740 ka, as well as biostratigraphy, oxygen isotopic stratigraphy, a dated industry well-log horizon, and tephrochonology of the 639 ka Lava Creek ash. We are now continuing our interpretation of the NC-PP through its western 50 km using 2D and 3D MCS data and this previous dated seismic stratigraphy. The PP-NC is variably blind along strike, with its upper tips in places below the 1 Ma horizon and in other locations cutting up to higher levels. The blind slip is absorbed by a progressively-tilting S-dipping forelimb everywhere. Preliminary examination of cross sections through our 8 gridded, depth-converted horizons suggests that the rate of tilting has not significantly changed during the last 1 Myr, and that dips and structural relief, although variable, are not systematically greater in the east than in the west between Carpinteria and the UCSB campus at 119° 50’ W.. Farther westward, the rate of tilting, and probably the rate of offshore shortening, decreases steadily through 40 km between 120° 05’ W. and the end of the system beyond Point Conception. This trend mirrors the decrease in elevation and structural relief of the Santa Ynez Mountains above the deep fault. As in its eastern part, there is no evidence of a major change in tilt rate through the last 1 Myr in its western part. The fault bends to the northwest around Point Conception and its hanging-wall is cut by a strand of the right-lateral Hosgri fault. The NC-PP system has now been continuously mapped from onshore Ventura to west of Point Conception, a distance of ~120 km.

Sorlien, C. C., Nicholson, C., Behl, R. J., Marshall, C. J., & Kennett, J. P. (2014). The Quaternary North Channel-Pitas Point Fault System in Northwest Santa Barbara Channel, California. Eos (Transactions of AGU), 95(52), T34A-07.