Three-dimensional GPR imaging of the benmore anticline and step-over of the ostler fault, South Island, New Zealand

Because surface processes can frequently remove, mask or suppress the surface expression of faults, geophysical imaging is needed to provide information on the geometric expression of the fault in the subsurface, including dips, the presence or absence of splays and folds and the nature of the material in the fault plane. Subsurface data become especially important where a step-over occurs, however the mechanism of the step-over is not clear. The Ostler Fault is a major thrust fault in the Mackenzie Basin of the South Island of New Zealand. The geometry of the fault changes along its length, with a number of growing anticlines and step-overs forming the Ostler Fault Zone (OFZ). One of these anticlines occurs along the Benmore segment of the OFZ. A three-dimensional (3-D) ground penetrating radar (GPR) survey was carried out to supplement detailed surface topographic mapping and help characterize the nature of the OFZ at Benmore. To cover the 1 km length of the step-over, the 3-D survey was done with a coarse yet regular 50 m line spacing. The individual fault splays were continuous and could be traced from line to line. Not all splays had surface expression, whereas almost all surface features had corresponding subsurface expression. As the fault approaches the anticline and multiple splays become apparent, the main fault simply develops into one of a set of splays as the deformation becomes distributed across the wider zone. Eventually, one of the other splays becomes the primary fault strand, and the others are abandoned.