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 calc ulated for the Pliocene-Pleistocene section of Cannington Basin, South Canterbury, New Zealand. This estimate is based largely on geophysica l data. Strong, coherent seismic reflections down to 1.0 s two-way tra vel 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 ar e less than or equal to 2.3 km/s, which we interpret to be the Pliocen e-Pleistocene section. Residual gravity anomalies within the basin are as low as -20 mgal and are asymmetrically disposed to the eastern sid e of the basin against a sharp topographic scarp-the Brothers Fault. A bout 300 m of topographic relief is present on this 30-40 km long scar p. We propose that the Brothers Fault has been active from the Pliocen e until the present as a reverse fault with dip inferred to vary betwe en 80 degrees and 50 degrees. This interpretation is based on the stee pness of the residual gravity anomalies across the Brothers Fault scar p, the seismically interpreted thickness of Pliocene-Pleistocene sedim ents across the fault, and the observations of tilting of late Pliocen e sediments on the hills that flank the eastern margin of Cannington B asin.