Abstract
Borehole elongations or breakouts, observed on unprocessed four-arm dipmeter logs, have been used to map in-situ stress directions throughout onshore California. Approximately 130 wells were analyzed for borehole breakouts in California, with 107 giving useful results. The wells are subvertical (most inclinations > 80/sup 0/) and range in depth from 920 to 5760 m, with 90% of the wells being deeper than 1500 m. Breakouts were observed from depths of 100-5600 m. The number of broken out intervals per well range from 3 to 38 with an average of 12. A regionally consistent stress pattern is observed with maximum horizontal compression generally oriented northeast-southwest. Breakout-determined stress directions are consistent with other stress indicators, including stress directions determined from focal mechanisms for earthquakes occurring at depths of 5-15 km. The direction of maximum compression consistently intersects the San Andreas fault at a high angle (80/sup 0/-90/sup 0/) and is approximately perpendicular to axes of young thrust-related anticlines. Heat-flow and seismic observations limit the shear stress on the San Andreas fault to 10-20 MPa, or less. The observed orientation of maximum horizontal compression (nearly perpendicular) to the San Andreas) allows generation of large regional deviatoric stresses of rock-breaking magnitudes (on the ordermore » of 100 MPa). Transpressive plate motion is decoupled into a low-stress strike-slip component and a high-stress compressive component. These observations suggest that standard concepts of transpressive wrench tectonics - which envisage drag on a high-friction fault - are wrong. The thrust structures are largely decoupled from the strike-slip fault.« less
Published Version
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