STRESS STATE AND FLUID-PRESSURE LEVEL ALONG THE WHIPPLE DETACHMENT FAULT, CALIFORNIA

Brittle structures in the upper 200 m of the footwall of the Whipple d etachment fault are used to characterize the state of stress under whi ch the detachment slipped. They are younger than or coeval with chlori tic brecciation, and those that record shear typically are either mode rately northeast-dipping normal structures or steep, northwest-strikin g, southwest-down structures that we interpret as conjugate shears. Fr om this, we infer that the maximum principal stress was oriented 55-de grees-80-degrees from the detachment while they were active. The angle between the conjugate faults averages about 30-degrees, indicating fa ilure in the transtensional regime. During conjugate faulting the effe ctive least principal stress was nearly equal to the tensile strength, the Whipple fault probably had a normal coefficient of static frictio n, and it slipped under hydrostatic to only moderately elevated fluid pressure. Rapid mineralization in the surroundings of major fault zone s allows transtensional failure there and permits differential stress to be higher than if cohesionless friction limits strength. This cause s resolved shear stress sufficient for slip on faults severely misorie nted relative to the maximum principal stress.