Basement geology and tectonic development of the greater New Zealand region: an interpretation from regional magnetic data

Abstract

The basement geology of New Zealand is composed of early Palaeozoic terranes of the Western Province, that are separated from late Palaeozoic–Mesozoic Eastern Province terranes by a suite of Carboniferous–Cretaceous arc-related igneous rocks (Median Tectonic Zone, MTZ). The Stokes Magnetic Anomaly System (SMAS) is associated with MTZ rocks and volcanogenic basement terranes of the Eastern Province. Offshore, it can be traced north along the northern margin of the New Caledonia Basin, and correlative Eastern Province rocks are found in New Caledonia. It can also be traced south across the Great South Basin, until a significant ENE-trending tectonic boundary is encountered on the central Campbell Plateau. This boundary is defined by linear gravity and magnetic anomalies (Campbell Magnetic Anomaly System, CMAS), and narrow fault-bounded sedimentary basins. If the sources of CMAS anomalies are correlative with those of the SMAS, then magnetic data require a ∼400 km dextral offset of basement rocks by faults along the northern margin of the CMAS prior to 80 Ma. The geometry of correlative Western Province and MTZ rocks in Marie Byrd Land supports the hypothesis that CMAS anomalies are sourced by MTZ-correlative rocks. A NNE-trending boundary on the central Challenger Plateau marks a change from high amplitude magnetic anomalies to weakly magnetic basement, and appears to represent a fundamental change in crustal character. The western Challenger Plateau and Lord Howe Rise (south of 30°S) are characterised by high amplitude magnetic and gravity anomalies with a NW-trending fabric, but the source of magnetic anomalies is unresolved. The magnetic character, combined with Cretaceous reconstruction, supports basement rock correlations with the east Lachlan Fold Belt or New England Fold Belt in Australia, rather than Western Province rocks in New Zealand. The magnetic signature of marginal ocean crust around New Zealand also offers clues into the region's tectonic history. Negative magnetic anomalies adjacent to the Campbell Plateau and Lord Howe Rise, and in the New Caledonia Basin, suggest that seafloor formation started during chron 33r (79–83 Ma). A linear positive magnetic anomaly in the outer Bounty Trough may be anomaly 33, which was isolated by a ridge-jump, and is consistent with separation of the Bollons Seamount continental fragment by ocean crust. Alternatively, it may be anomaly 34 and represent the earliest ocean crust formed between New Zealand and Marie Byrd Land. Magnetic lineations in the southern South Fiji Basin suggest that at least one ridge–ridge–ridge triple junction was active during its opening, and imply that Cenozoic ocean crust is younger in the east. At the southern edge of the South Fiji Basin, there is a significant tectonic boundary, named here the van der Linden Fault. It can be traced for ∼500 km and may have been a leaky transform during basin formation.