Subduction and orogeny

Abstract

Conventional structure data from orogenic belts can be related to events of lithospheric plate subduction regardless of the great difference in depth range of the compared sets. Thrust fabrics and rotation zone fabrics are diagnostic of the two possible orientations of vergenz with respect to subduction and of events of flip, i.e., reversal in dip of the subduction zone. Thrusting synthetic (parallel) to subduction creates imbricate belts of ‘fast moving’ thrust sheets with transport fabrics. Thrusting antithetic (conjugate) to subduction creates belts of ‘slowly moving’ basement-involving folds and upthrusts. Rotation zones are fans of folds and cleavage planes created by nonaffine external rotation around subhorizontal axes. Together with antithetic thrust belts they occur as complex drag features in upper plates of subduction zones with or without evidence of arc vulcanism. Their association with retrocharriage, i.e., late orogenic synthetic thrusts, suggests that rotation must be episodic and may be caused by a decrease in dip of a subvertical subduction zone. More complex, incompletely understood structures in orogenic core zones suggest that other rotation zones may be caused by an increase in dip of the downgoing slab. Flip of a subduction zone may be caused by an imbalance of crustal densities, by a collision of two subduction zones, or by motion of plates in relation to the mantle. The surface expressions of flip are called root zone, overridden rotation zone, and rootless rotation zone.