Development of wrench folds along the border ranges fault system, southern Alaska, U.S.A.

Abstract

En échelon folds in the central Chugach Mountains developed during Eocene brittle dextral-slip along the Border Ranges fault system. These wrench folds deformed an incompletely lithified alluvial fan complex as well as an adjacent phyllite belt. Wrench folding of phyllite was accommodated by flexural-slip between mechanical layers > 1 m thick rather than by penetrative flexural-flow along the predominant cleavage. The folds have non-cylindrical hinges that commonly deflect into subparallelism with, and are truncated by, adjacent strike-slip faults. Arrays of subsidiary faults and syntectonic veins indicate that fold hinges developed subparallel to the direction of maximum incremental elongation ( λ 1), and that incremental λ 2 was vertical. The relationship between interlimb angle of these folds and the strike of their axial surfaces is consistent with variable clockwise rotation and tightening of these folds in a zone of heterogeneous, distributed wrench deformation. Early wrench folds trend clockwise of overprinting sets of more open folds, a relationship that suggests that the younger folds formed during the same progressive deformational event, but after the earlier set of folds had rotated clockwise, tightened and strain hardened. Megakinking attributed to strike-slip fault-bend folding resulted in local rotations of pre-existing folds. These results provide new insight into the geometry and kinematics of wrench folding, and do not support recent suggestions that the magnitude of shear stress on strikeslip faults is insufficient to induce wrench folding.