Fault strength and transpressional tectonics along the Castle Mountain strike-slip fault, southern Alaska

Analyses of structures along the Castle Mountain fault reveal the mechanica l relations and relative timing of orogen-parallel strike-slip faulting to distributed wrenching and shortening and to plate motion in the southern Al askan transpressive margin. The Castle Mountain fault is a > 200-km-long, o rogen-parallel, right-lateral seismogenic fault in the southern Alaskan pla te margin. Exposures of the fault in the study area have been exhumed from 3-4 km depth and similar to 80 degreesC, Moment tenser summations and stres s inversions of slip data from fault networks within 400 m of the Castle Mo untain fault and analyses of structures in forearc deposits 2-4 km from the fault yield maximum incremental shortening and maximum compressive stress (sigma (1)) axes that are subhorizontal and trend similar to 325 degrees, T his trend is close to the 340 degrees North American-Pacific plate converge nce direction. The inferred a, makes a 70 degrees -80 degrees angle with th e similar to 070 degrees -striking Castle Mountain fault, indicating that t he fault slips at a lower shear stress than predicted by laboratory rock fr iction experiments if hydrostatic pore pressure exists in the fault (i.e., coefficient of friction similar to0.85, Byerlee's Law). The forearc structu res record coaxial strain and apparently remain in their formative orientat ions, showing that previously documented distributed shearing in the form o f vertical-axis block rotations in the forearc ceased prior to formation of the structures in late Oligocene time. The mechanical weakness of the Cast le Mountain fault probably results in part from its clay-rich gouge, which averages about 43 wt% clay phases, However, the gouge composition does not account for slip in response to a, at 70 degrees -80 degrees to the fault. Another mechanism, such as elevated pore pressure, further weakens the faul t and enables it to slip right laterally in response to the small component of oblique motion across the plate boundary. Upon its inception in early T ertiary time, the fault could not have formed a > 200-km-Long planar zone o f low-friction clay-rich gouge or elevated pore pressure; thus, the mechani sms that weaken it must have developed over time. The distributed shearing in the forearc may have ended in response to the weakening and localization of dextral slip onto the Castle Mountain fault.