Structure and kinematics of active faulting in the Hope-Kelly and Alpine Fault intersection zone, South Island, New Zealand

Abstract

Using lidar, photogrammetry and field studies, we investigate the structure and kinematics of the Hope, Kelly, and Alpine faults where they converge and intersect along the Australian-Pacific plate boundary in New Zealand. Approximately 50% of the reverse-dextral slip rate of the Alpine fault in the west transfers to the dextral central Hope Fault in the east via a horsetail splay zone of distributed oblique dextral-normal shear forming the Hope-Kelly fault system. The westernmost portion of the Hope fault is misoriented for dextral slip and is likely an extensional fault that accommodates the Alpine fault slip rate change across the intersection zone. As it transitions to an extensional fault, most of the strike-slip of the Hope fault is transferred southward onto Kelly fault strands via incipient, linking fault domains of coeval NE-SW and EW-striking dextral faults. The transition from the narrow zone defining the central Hope Fault to dispersed horsetail splays of the Hope-Kelly fault system is located immediately above the eastward down-dip projection of the Alpine Fault at the base of the seismogenic zone. This suggests that structural interactions among these faults within seismogenic depths control surface rupture geometries and kinematics. Coeval intermingling of surface rupture traces transecting latest Pleistocene to Holocene landforms indicates ongoing structural interactions in active faulting at the intersection of the Hope-Kelly fault splays with the Alpine fault. The transition from dextral to dextral normal kinematics on the horsetail splay accommodates differential orogenic growth through geometrically and kinematically efficient faulting that allows extension of the Alpine fault hanging wall parallel to strike, enabling the slip rate change between the central and northern Alpine fault sections.