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Quaternary Slip and Geometry of the Red Mountain and Pitas
Point-North Channel Faults, California
Institute for Crustal Studies, University of California, Santa Barbara, CA 93016
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A structure contour map of an approximately 1.8 Ma horizon was created
for onshore Ventura basin using existing cross sections based on surface
geology and well data. This map was extended offshore into the eastern
Santa Barbara Channel using industry seismic reflection data, well data,
and sea floor geology. The N-dipping Pitas Point fault and the North Channel
fault are interleaved or stacked along the same E-W trend, while the south
branch of the offshore Red Mountain fault is a separate parallel structure
6 km to the north. Fault plane reflections from these faults were observed
on 3D and 2-D seismic lines and faults were also detected by repeated sections
and abrupt dip changes in wells. Shallow thrusts, folds, and south dipping
reverse faults occur in the hanging wall and the footwall of the Pitas Point-
North Channel fault.
The Red Mountain fault splays into two main branches near the Carpinteria coast.
The northern branch decreases in displacement to the west and continues
as far west as offshore Goleta where it may die into folding. The south branch
decreases in displacement to the west and dies out in a syncline south of
Santa Barbara. Shortening across the Pitas Point-North Channel fault and hanging
wall structures increases westward from near Pitas Point to a maximum south
of Santa Barbara. Thus as displacement on the Red Mountain fault is decreasing
westward the displacement on the Pitas Point-N. Channel fault is increasing.
Displacement on one fault is compensating for the other. The Pitas Point
and Red Mountain faults may merge with depth, detaching the intervening upper
crustal block. Abrupt changes in shortening and fault-fold style along the
strike of the Pitas Point-North Channel fault occur across NE-SW cross faults.
Vertical axis block rotation of the segmented detached blocks can transfer
slip from one fault to the other. Shortening of the 1.8 Ma horizon due to fault
slip and folding across the Pitas Point-North Channel trend locally exceeds 5 km
south of Santa Barbara, and exceeds 1 km across the south Branch of the Red
Mountain fault south of Carpinteria. Ongoing unfolding and restoration of the
digital map will allow slip, block rotation, and shortening to be quantified,
and the structural-kinematic interpretation to be validated.
Kamerling, M. J., and Sorlien, C. C, 1999, Quaternary slip and geometry of the Red Mountain and Pitas Point-North Channel faults, California, Supplement to EOS, (Trans. AGU), v. 80, P. F1003 |