STRUCTURAL ANALYSIS OF THE TANGIHUA COMPLEX, COOPERS BEACH, NORTHLAND NEW ZEALAND, WITH COMPARISONS TO THE MT. CAMEL TERRANE AND BASEMENT

The study area is a 10 km long stretch of coast from Puketu Island to the western edge of Coopers Beach, on Doubtless Bay in Northland, New Zealand, representing Tangihua Complex shore platform. Previous work suggests the Tangihua Complex was formed during subduction at the eastern Gondwana margin in the late Cretaceous as a submarine back-arc system. The Mt Camel terrane formed at the same time to the west as part of a volcanic arc system. Dikes cutting the Tangihua Complex are geochemically similar to the Mt. Camel Volcanic Suite, indicating the Mt Camel predates the Tangihua Complex.

The Tangihua Complex is believed to be emplaced on top of the autochthonous basement and Mt. Camel terrane during the Oligocene, moving about 200-250 km southwest of its original position. Structural work indicates the Mt. Camel terrane may have been tectonically moved, although how much is unclear. Due to metamorphism related to emplacement, the Tangihua Complex records tectonic events not recorded in the Mt. Camel.

The Tangihua Complex is composed mostly of basalts consisting of almost entirely plagioclase and clinopyroxene. Basalts contain secondary chlorite, calcite and prehnite. Gabbros and porphyritic rocks are also present with similar mineralogy, although the porphyry contains much more prehnite. The sedimentary rocks within the Tangihua Complex are mostly intercalated limestones and mudstones. The mudstones grade into sandstones and display detrital sedimentary texture with sub-angular moderately sorted quartz, plagioclase, opaque grains and little or no calcite.

Data was also collected for geostatistical comparison between the three units, in both sedimentary and igneous material. Detailed 10 m N-S, E-W, horizontal sections were structurally catalogued at Puketu Island for the Tangihua Complex, Hatfields Beach for the basement, and Rarawa Beach for the Mt Camel terrane. Structural mapping of several exposures was completed with generalized trends identified at outcrop scale. Most large scale fractures, joints and faults lay along two main orientations: a NE-SW suite bearing between 40°-60°, and a NW-SE suite, between 300°-320°. Large scale diking follows the NE-SW trend, however there were some outliers.