Active range front thrusts and uplift of the Aorangi Mountains

Abstract

Seismic, gravity and late Quaternary deformation data are used to interpret the emplacement and subsequent support of the Aorangi Mountains, south-east North Island, New Zealand. The Aorangi Mountains are comprised of complexly deformed "basement" greywacke of the (Pahau) Torlesse terrane. The Mountains coincide with the southern termination of the East Coast Gravity High, a continuous feature observed from Southern Hawke Bay to Cook Strait, where it terminates abruptly against the Cook Strait Gravity Low. The maximum Bouguer gravity anomaly of the East Coast Gravity High (94 mgal) occurs over the Aorangi Mountains. A seismic refraction survey undertaken on the north-western boundary of the Aorangi Mountains delineates the depth to greywacke basement, and compressional P-wave velocity structure of the overlying sediments. Velocities of 1.9-2.5 km/s are observed for the sediments, which increase in thickness to the north-west, away from the Mountains. Basement velocities of 4.7-5.5 km/s, at depths of 0.6-1.0 km, compare well with values for Torlesse greywacke found by previous workers elsewhere in New Zealand. A gravity survey covering the Aorangi Mountains and surrounding areas consists of 161 new stations. These new data further define the known gravity anomaly over the Aorangi Mountains. Short wavelength 2-D gravity modelling of anomalies either side of the Aorangi Mountains are interpreted to show that a total relative displacement of ~1 km is accommodated on reverse faults at the range fronts. These faults agree well with those mapped geologically, increasing in dip from 45 to 80° as the range is approached. West of the Aorangi Mountains, the gravity models indicate the structural style changes from faulting to folding, with depth to basement reaching a maximum of ~2 km at the axis of the Whangaimoana Syncline (new name), about 7 km from the range front. Uplift rates for the Aorangi Mountains are calculated from deformed Holocene beach ridges and late Quaternary fluvial aggradation surfaces, indicating vertical displacement rates in the order of 1.0-2.3 m/ka. This rate is shown to be evenly distributed between uplift of the Aorangi Anticline fold axis and differential movement on the Ngapotiki Fault. Across the Ngapotiki Fault, which forms the eastern margin of the Aorangi Mountains, a loess layer containing Kawakawa Tephra mantles an aggradation surface that has been offset vertically by 24 m at a rate of 0.8-2.0 m/ka. Interpretation of gravity anomalies on a regional scale requires knowledge of the flexural response of the lithosphere to the imposed load of the Aorangi Mountains. The structures associated with the East Coast Gravity High are modelled as emplaced loads flexurally supported by the subducted Pacific Plate, with an assumed elastic thickness (Te) of 30-50 km. The East Coast Gravity High is explained by a block of Mesozoic metasediments uplifted during the compressional tectonics of the late Neogene and now supported by the relatively high strength of the subjacent Pacific Plate. The East Coast Gravity High terminates abruptly against the Cook Strait Gravity Low to the south. Regional 3-dimensional gravity analysis indicates a dextral offset of the overlying Australia Plate within Cook Strait of 160 km. This is consistent with the proposal that the present-day Cook Strait was formed by clockwise rotation of the proto-Wairau Fault. There is no requirement for deep seated mass anomalies, (i.e. tears in the subducted Pacific Plate) to explain the abrupt gravity and topographic change from the high to the low.