Kinematics of wrench and divergent‐wrench deformation along a central part of the Border Ranges Fault System, Northern Chugach Mountains, Alaska

The Border Ranges fault system (BRFS) bounds the inboard edge of the subduction‐accretion complex of southern Alaska. In Eocene time a central segment of this fault system was reactivated as a zone of dextral wrench‐ and oblique‐slip faulting having a cumulative strike‐slip offset of at least several tens of kilometers, but probably less than 100 km. Early wrench folds are upright, trend at less than 45° to the strike of adjacent faults and developed with fold axes oriented subparallel to the axis of maximum incremental stretch λ1. These en echelon folds rotated and tightened with progressive deformation and then were overprinted by younger wrench folds that trend at about 60° to adjacent throughgoing faults. The latter folds are interpreted as forming during a late increment of distributed wrench deformation within the BRFS that included a component of extension (divergence) orthogonal to the mean strike of the fault system. A sharp releasing bend in exposures of a strike‐slip fault originally at >4 km depth today coincides with a narrow pull‐apart graben bounded by oblique‐normal faults that dip toward the basin. Widening of this pull‐apart graben by brittle faulting and dike intrusion accommodated less than 2 km of strike‐slip and was a late‐stage phenomenon, possibly occurring at supracrustal levels. Prior to formation of this graben during a period of predominantly ductile deformation at deeper structural levels, wrench‐folded rocks on one side of the nonplanar fault were translated around the releasing bend without significant faulting or loss of coherence. Kinematically, the earlier deformation was accomplished by fault‐bend folding and rotation of a relatively deformable block as it passed through a system of upright megakinks. Such a ductile mechanism of fault block translation around a strike‐slip bend may be typical of intermediate levels of the crust beneath pull‐apart grabens and may be transitional downward into heterogeneous laminar flow occuring along curved segments of ductile shear zones. Some degree of fault‐bend folding of strike‐slip fault blocks around releasing bends may be one reason why the amount of extension measured across natural pull‐apart basins is commonly observed to be less than the amount of strike‐slip along their master faults.