Paleoseismic analysis of the Springfield Fault, Central Canterbury

Abstract

The Springfield Fault is a northeast striking thrust fault bisecting the Russell Range at the northern end of the Malvern Hills near Springfield. The trace of the fault is best expressed by offset river terraces, as it cuts across the flood plane of the Hawkins River in the Upper Hawkins Basin. The Russell Range is composed chiefly of the basement Triassic rocks of the Torlesse terrane with Cretaceous to Cenozoic rocks occurring in a fault bounded slice and around the hinge of the hanging wall anticline of the fault. Quaternary glacial sediments occur on the plains adjacent to the Russell Range with Holocene deposits occurring in the Upper Hawkins Basin. Folding in the highly sheared basement is along NW-SE trending fold axes. The current direction of principal shortening was determined from slickenside data to be NW-SE to NE-SW. Geomorphology of the area in the Upper Hawkins Basin provided a chronology of the activity on the Springfield Fault. Offset of terrace surfaces provide evidence for the number of fault ruptures, which is at least two. An attempt to date the age of fault and river terrace scarps was made using their morphology and a diffusion model of scarp degradation. Analysis requires the diffusivity rate operating on a scarp to be known to produce an absolute age for the scarp. Only one of the scarps analyzed, a river terrace scarp produced a result. This was poorly constrained at 1000-7500 years old.