Analyses of structures along the Castle Mountain fault reveal the mechanical relations and relative timing of orogen-parallel strike-slip faulting to distributed wrenching and shortening and to plate motion in the southern Alaskan transpressive margin. The Castle Mountain fault is a >200-km- long, orogen-parallel, right-lateral seismogenic fault in the southern Alaskan plate margin. Exposures of the fault in the study area have been exhumed from 3−4 km depth and ∼80 °C. Moment tensor summations and stress inversions of slip data from fault networks within 400 m of the Castle Mountain fault and analyses of structures in forearc deposits 2−4 km from the fault yield maximum incremental shortening and maximum compressive stress (σ1) axes that are subhorizontal and trend ∼325°. This trend is close to the 340° North American–Pacific plate convergence direction. The inferred σ1 makes a 70°−80° angle with the ∼070°-striking Castle Mountain fault, indicating that the fault slips at a lower shear stress than predicted by laboratory rock friction experiments if hydrostatic pore pressure exists in the fault (i.e., coefficient of friction ∼0.85, Byerlee's Law). The forearc structures record coaxial strain and apparently remain in their formative orientations, showing that previously documented distributed shearing in the form of vertical-axis block rotations in the forearc ceased prior to formation of the structures in late Oligocene time. The mechanical weakness of the Castle 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 σ1 at 70°−80° to the fault. Another mechanism, such as elevated pore pressure, further weakens the fault 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 Tertiary time, the fault could not have formed a >200-km-long planar zone of low-friction clay-rich gouge or elevated pore pressure; thus, the mechanisms 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.

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