Fracture zone subduction and reactivation across the Puysegur ridge/trench system, southern New Zealand

Abstract

A two‐dimentional kinematic model for lithospheric thrusting that considers the flexural interaction between the underriding and overriding plates was used to assess the mechanical implications of subducting trench‐parallel fracture zones on the topography and free‐air gravity anomalies of the Puysegur ridge/trench system. Fracture zones in the underriding plate are simulated by vertical discontinuities across which bending and shearing stresses cannot be supported. The implication of reactivating fracture zones on the outer trench slope is to suppress the flexural bulge, creating topographic relief between the fracture zone and the trench, and reducing the trench gravity anomaly. In the studied region the trench subparallel L'Atalante fracture zone accounts for the lack of a flexural bulge, strong outer trench slope, and gravity gradients. The southward decrease of the trench fracture zone distance results in a decrease of the trench gravity anomaly. The effect of subducting and vertically reactivating fracture zones within the underriding plate is to segment the load of the overriding plate, creating a trough located above the underthrust fracture zone. The vertical offset and gravity modifications are functions of the fracture zone position relative to the trench. This model explains the 3500‐m‐deep troughs and the 1700‐m subsidence observed at the Puysegur ridge. We speculate that reactivating underthrust fracture zones can facilitate strike slip faults development within the upper plate in case of oblique subduction and are conducive to the formation and dispersion of terranes at convergent margins. This process can account for basal tectonic erosion.