Seismic and GPR Imaging of the Springfield Fault System, Canterbury Plains, New Zealand

The Springfield Fault System is part of a network of splays and overlapping faults that are generally hidden beneath Quaternary sediments of the Canterbury Plains on the South Island of New Zealand. These faults accommodate an important component of motion between the Pacific and Australian tectonic plates. Understanding the connection between local and broader regional tectonics is of major scientific interest. Furthermore, from a practical point of view, insight into the recent fault-related history of differential uplift and seismic hazard is of importance to the local population. Very limited exposures of the faults suggest that they are mostly west facing thrusts that have tilted (by 30°) and displaced Tertiary limestones and volcanics and Jurassic Torlesse basement. In an attempt to map faults of the Springfield Fault System, we have acquired high-resolution seismic reflection and ground-penetrating radar (GPR) data along four nearly parallel lines that range in length from 3 to 18 km. The seismic data were acquired using small explosive and hammer sources at 10 m intervals and a receiver spread comprising two hundred and forty 30-Hz geophones at 5 m intervals, yielding nominal 60-fold data with a CMP spacing of 2.5 m. The GPR data were recorded using 50 MHz antennas separated by 2 m. The processed seismic data reveal many of the features suggested by the limited surface exposures. Clearly visible tilted packets of strong reflections originate from the Tertiary limestone and volcanic sequences and the Jurassic Torlesse basement. Indirect, but convincing evidence is found for the fault strands. They are represented by narrow bands of diminished reflectivity and truncations and vertical offsets of reflections. Active folding is also suspected, given that the folded Tertiary blocks seem to deform the overlying Quaternary sediments. The GPR data should constrain possible continuations of the indicated faults and folds into the overlying Quaternary sediments. An analysis of the vertical offsets provides insights into the paleoseismicity and history of differential uplift.