Sea Beam Survey of an Active Strike-Slip Fault : The San Clemente Fault in the California Continental Borderland

The San Clemente fault, located in [Spiess and Tyce, 1973] subbottom profiler, magnethe California Continental Borderland, is an tometer, and side-scan sonar records, and ocean active, northwest trending, right-lateral, wrench surface total magnetic field intensity measurefault. Sea Beam data are used to map the major ments, were used to assist in the interpretations tectonic landforms associated with active submarine of the seafloor structure. faulting in detail unavailable using conventional Bathymetric maps were used to infer that the echo-sounding or seismic reflection data. In the ridge and basin topography of the California area between North San Clemente Basin and Fortymile Continental Borderland is structurally controlled Bank, the major late Cenozoic faults are delineated [Shepard and Emery, 1941]. It was suggested that by alignments of numerous tectonic landforms, major northwest trending faults, similar to those including scarps, linear trenches, benches, and observed on land nearby, are delineated by long, sags. Character and spatial patterns of these linear, steep escarpments which bound the offshore landforms are consistent with dextral wrench ridge and island blocks. For example, the San faulting, although vertical offsets may be substanClemente fault zone is delineated by the prominent tial locally. The main trace of the San Clemente escarpment along the eastern flank of San Clemente fault cuts a straight path directly across the Island (Figures 1 and 2a). More recent oceanorugged topography of the region, evidence of a graphic studies used seismic reflection profiles to steeply dipping fault surface. Basins or sags map offshore geologic structure and confirm the located at each right step in the en echelon interpretations made from bathymetry [e.g., Moore, pattern of faults are manifestations of pull-apart 1969; Ridlon, 1969; Vedder et al., 1974]. basin development in a right-slip fault zone. In areas of rugged seafloor topography, detailed Seismic reflection profiles show offset reflectors mapping of geologic structure using seismic reflecand a graben in late Quaternary turbidires of the tion data is difficult because of the relatively Navy Fan, where the fault zone follows a more wide beam width of the acoustic systems. Subbottom northerly trend. Modern tectonic activity along features and, in many instances, the bottom reflecthe San Clemente fault zone is demonstrated by tion itself directly beneath the ship may not be numerous earthquakes with epicenters located along recorded or may be obscured by acoustic reflections the faultWs trend. The average strike of the San from adjacent topography. Narrow-beam Sea Beam Clemente fault is parallel to the predicted data provide detailed mapping of the surficial Pacific-North American relative plate motion vector expression of major tectonic features in such at this location. Therefore we conclude that the areas. San Clemente fault zone is a part of the broad Pacific-North American transform plate boundary and that the southern California region may be considered as a broad shear zone.


faulting
in detail unavailable using conventional Bathymetric maps were used to infer that the echo-sounding or seismic reflection data.

In the ridge and basin topography of the California area between North San Clemente Basin and Fortymile
Continental Borderland is structurally controlled faulting, although vertical offsets may be substan-Clemente fault zone is delineated by the prominent tial locally.
The main trace of the San Clemente escarpment along the eastern flank of San Clemente fault cuts a straight path directly across the Island (Figures 1 and 2a). More recent oceanorugged topography of the region, evidence of a graphic studies used seismic reflection profiles to steeply dipping fault surface. Basins or sags map offshore geologic structure and confirm the located at each right     Figures 2b and 5). This escarpment is broken by a 1-km-wide terrace which lies below Southwest Peak (Figures 2b, 5, and  6). The scarp below the terrace is very steep; it has an average slope gradient of about 50% (26.5 ø )

FaULTS
for its 500-m height, and locally, Sea Beam contours show slope gradients reaching 100% (45ø; Figure 6). This lower scarp is continuous for a distance exceeding 15 km. We have mapped the San  Figures 5 and 6). This linear ridge jog along the inferred traces of the San Clemente would be a continuation of the fault block and fault (Figures 2b and 5).
Its structure does not therefore points to a lateral-slip mechanism.

Legg et al.: Sea Beam Survey of an Active Strike-Slip Fault
A second explanation for the formation of the across the San Isidro fault zone show that the terrace or hillside valley is backtilting of a bulge along the southern margin of this other basin block downfaulted from Fortymile Bank.  Figures 2b and 5). These scarps form the northbasin, is to the northwest of the terrace, i.e., east flank of Navy Basin (NB). Navy Basin is Northwest Basin (Figures  2b and 5 Figures 2b and 6).

1979]. Such seafloor lithology would be less
The lateral offset mechanism proposed above also likely to support or maintain scarps as steep as implies that Shutter Ridge is a shutteridge, i.e., those to the northwest along the bedrock flanks of it was tectonically transported into its present Fortymile Bank. The scarps along Navy Basin are position by right slip along the San Clemente inferred to be tectonic because of their linearity. fault.

Impingement of Shutter Ridge against Additional evidence supporting this interpretation Fortymile Bank could break off a tectonic sliver.
is provided by the seismic profiles (Figure 9 Figures 2b and 5). A profiles [Legg, 1985] show that the northeastern linear trough, steep-sided but broader than the fault dies out near Navy Fan; the San Salvador trench, is aligned in a more northerly trend Knoll trace steepens as it continues southeastward, between the northwest end of the trench and the through the sediments of Navy Fan and toward the southeast end of the filled hillside valley or San Isidro fault (Figure 1). The overall trend of terrace described above. This linear trough is the San Clemente fault in Navy Basin is about 5 ø reminiscent of microtopography (horst and graben?) more northerly than the other major segments observed along subaerial fault zones [Keller et described above (Figures 5 and 6). Extension al., 1982]. Its north-south orientation is consis-associated with a 5 ø divergent segment of a dextral tent with east-west extension in a right-slip fault wrench fault could account for the subsidence. zone.
Legg [1985] proposes that Navy Basin is a long, Triangular Basin, the site of another major narrow pull-apart basin located between two major, right step in the generally linear trend of the San right-stepping, en echelon strands of the San Clemente fault, is also inferred to be a pull-apart Clemente fault. Thus the surface expression of basin.
The San Clemente fault continues along the divergent wrench faulting is manifest here as a straight northeast margin of Triangular Basin, graben or pull-apart basin. offset in places, and marked by subparallel and right-stepping, en echelon, southwest facing scarps Character of Faulting of low height (Figures 2b and 5).
After stepping to the right, the fault follows a linear trough out In well-defined wrench fault zones during of the southeast corner of Triangular Basin. Small advanced stages of deformation, Tchalenko [1970] scarps and low mounds that bound the more irregular observed that the displacement became concentrated southern margin of Triangular Basin (Figures 2b and on a narrow,