Flower structures and Riedel shears at a step over zone along the Alpine Fault (New Zealand) inferred from 2-D and 3-D GPR images

High-resolution GPS and ground-penetrating radar (GPR) data are used to detect and identify hidden faults along a stretch of the transpressional Alpine Fault (South Island, New Zealand) immediately north of its junction with the Hope Fault. At this location, the Alpine Fault emerges from the basement into a sequence of variably thick late Holocene gravel deposits. Geomorphology and trenching already mapped three principal fault strands and two distinct step over zones at the study site. Our GPR images reveal numerous additional secondary fault strands throughout the region, only some of which are obvious at the surface or in the trench walls. According to the GPR data, the main fault-generated disturbance zone has a width ranging from similar to 40 to similar to 200 m. The secondary fault strands outside of the step over zones likely represent the branches of positive flower structures, whereas the faulting pattern around the step over zones is best explained in terms of linked Riedel shears. Systematic northeastward increases in the width of the main fault-generated disturbance zone and corresponding increases in principal fault-scarp height are the likely consequences of older terraces in the northeast being disrupted and offset by more earthquakes than younger terraces in the southwest. The pattern of complex faulting in this region is distinct from the system of alternating strike-slip and reverse faults characteristic of the Alpine Fault to the south and from the rather simple sequence of faults mapped to the north. GPR surveying has added new information on the distribution and nature of faulting at our study site.