Subsurface geometry of the San Andreas-Calaveras fault junction: influence of the Coast Range Ophiolite
Abstract
Potential-field modeling, surface geologic mapping, and relocated seismicity are used to investigate the three-dimensional structure of the San Andreas-Calaveras fault junction to gain insight into regional tectonics, fault kinematics, and seismic hazard. South of the San Francisco Bay area, the San Andreas and Hayward-Calaveras fault zones join to become a single San Andreas Fault. The fault junction, as defined in this study, represents a three-dimensional volume of crust extending from San Juan Bautista in the north to Bitterwater Valley in the south, bounded by the San Andreas Fault on the southwest and the Calaveras fault zone on the northeast. South of Hollister, the Calaveras fault zone includes the Paicines, San Benito, and Pine Rock faults. Within the junction, the San Andreas and Calaveras faults are both creeping at the surface, and strike parallel to each other for about 50 km, separated by only 2 to 6 km, but never actually merge at the surface. Geophysical evidence suggests that the San Andreas and Calaveras faults dip away from each other within the northern portion of the fault junction, bounding a triangular wedge of crust. This wedge changes shape to the south as the dips of both the San Andreas and Calaveras faults vary along strike. The main trace of the San Andreas Fault is clearly visible in cross-sections of relocated seismicity as a vertical to steeply southwest-dipping structure between 5 and 10 km depth throughout the junction. The Calaveras fault dips steeply to the northeast in the northern part of the junction. Near the intersection with the Vallecitos syncline, the dip of the Calaveras fault, as identified in relocated seismicity, shallows to 60 degrees. Northeast of the Calaveras fault, we identify a laterally extensive magnetic body 1 to 8 km below the surface that we interpret as a folded 1 to 3 km-thick tabular body of Coast Range Ophiolite at the base of the Vallecitos syncline. Potential-field modeling and relocated seismicity indicate that the southwestern edge of this magnetic body is defined by a northeast-dipping structure that we interpret as part of the Calaveras fault. The base of this magnetic slab, which is folded up along the Calaveras fault, may represent a roof thrust formed by an eastward-migrating wedge of Franciscan Complex. Fragments of Coast Range Ophiolite caught up within the San Andreas-Calaveras junction may facilitate creep and slip transfer between structures that have no apparent connection at the surface. Combined geological and geophysical results suggest that during development of the junction, the Calaveras fault preferentially followed a zone of weakness represented by the roof thrust and associated Coast Range Ophiolite. The Hayward fault occupies a similar position with respect to the Coast Range Ophiolite near San Leandro to the north.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFMGP44A..08W
- Keywords:
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- 1517 GEOMAGNETISM AND PALEOMAGNETISM Magnetic anomalies: modeling and interpretation;
- 7230 SEISMOLOGY Seismicity and tectonics;
- 8175 TECTONOPHYSICS Tectonics and landscape evolution;
- 1219 GEODESY AND GRAVITY Gravity anomalies and Earth structure