Stress state and fluid-pressure level along the Whipple detachment fault, California

Abstract

Brittle structures in the upper 200 m of the footwall of the Whipple detachment fault are used to characterize the state of stress under which the detachment slipped. They are younger than or coeval with chloride brecciation, and those that record shear typically are either moderately northeast-dipping normal structures or steep, northwest-striking, southwest-down structures that we interpret as conjugate shears. From this, we infer that the maximum principal stress was oriented 55°-80° from the detachment while they were active. The angle between the conjugate faults averages about 30°, indicating failure in the transtensional regime. During conjugate faulting the effective least principal stress was nearly equal to the tensile strength, the Whipple fault probably had a normal coefficient of static friction, and it slipped under hydrostatic to only moderately elevated fluid pressure. Rapid mineralization in the surroundings of major fault zones allows transtensional failure there and permits differential stress to be higher than if cohesionless friction limits strength. This causes resolved shear stress sufficient for slip on faults severely misoriented relative to the maximum principal stress.