Late Tertiary deformation in Cannington Basin, South Canterbury, New Zealand: Evidence from seismic and gravity data

A maximum subsidence and shortening rate of 0.2 +/- 0.05 mm/yr is calculated for the Pliocene-Pleistocene section of Cannington Basin, South Canterbury, New Zealand. This estimate is based largely on geophysical data. Strong, coherent seismic reflections down to 1.0 s two-way travel time were recorded within the basin. Seismic velocities within the basin range from 1.9 to 4.4 km/s, and seismic refraction data show a depth of c. 1200 m to a basement with a velocity of 5.6 +/- 0.1 km/s. Within the top 600 m of the sedimentary section, seismic velocities are less than or equal to 2.3 km/s, which we interpret to be the Pliocene-Pleistocene section. Residual gravity anomalies within the basin are as low as -20 mgal and are asymmetrically disposed to the eastern side of the basin against a sharp topographic scarp-the Brothers Fault. About 300 m of topographic relief is present on this 30-40 km long scarp. We propose that the Brothers Fault has been active from the Pliocene until the present as a reverse fault with dip inferred to vary between 80 degrees and 50 degrees. This interpretation is based on the steepness of the residual gravity anomalies across the Brothers Fault scarp, the seismically interpreted thickness of Pliocene-Pleistocene sediments across the fault, and the observations of tilting of late Pliocene sediments on the hills that flank the eastern margin of Cannington Basin.