Surface deformation of the Lake Heron fault, mid-Canterbury, New Zealand

Abstract:

The surface expressions of continental reverse faults are rarely characterized in detail due to their positions in high relief, high denudation landscapes. Here, we present an analysis of the reverse-sense Lake Heron fault, using a high-resolution Digital Surface Model (DSM) and field mapping, where it deforms late Pleistocene-Holocene fans and terraces in mid-Canterbury, New Zealand. Results show that deformation is expressed as multiple discrete fault traces and two folds at the surface. Analysis of more than 50 topographic profiles across the multiple fault scarps obtained from the DSM and differential GPS demonstrate variation in fault geometry and net slip across the terrace and fan sequence. The average net slip for the Lake Heron Fault area ranges from 0.2 m to 3.2 m in individual scarp, and the cumulative net slip measured across the terraces and fan sequence varies from 2.5 m to 25 m. The maximum deformation width is more than 396 m in the terraces older than the fan deposit, where the Lake Heron fault has accommodated the most slip. Around 55% of the vertical deformation, is associated with folding in the fan deposit while around 70% of vertical deformation is linked to folding in the terraces which are older than the fan deposit. In one area, a crestal graben has developed in the hanging wall due to bending moment faulting. Trishear fold models reliably reproduce surface deformation of the terrace sequence using fault geometries inferred from field data. A late Quaternary slip rate of around c. 1 mm/yr is estimated for the Lake Heron fault assuming Last Glacial Maximum ages for terrace and fan surfaces. The research shows the number of faults increases as the deformation width decreases, which has important implications for seismic hazard analysis of the reverse-sense faults offsetting terraces and fan.