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

The area of study was a roughly 10 km long stretch of coast from Puketu Island to the western edge of Coopers Beach, in Northland, representing excellent Tangihua Complex shore platform exposure. Previous work suggests that the Tangihua Complex was formed during subduction at the eastern Gondwana margin during 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 through the Tangihua Complex are geochemically similar to the Mt Camel Volcanic Suite, indicating the Mt Camel predates the Tangihua Complex.

It is believed the Tangihua Complex was emplaced on top of the autochthonous basement and Mt Camel terrane during the Oligocene, moving approximately 200-250 km southwest of its original position. Structural work on the Mt Camel terrane indicates that it may have been tectonically moved, although how much is unclear. Deformation within the Mt Camel terrane is more complex than the Tangihua Complex. Due to metamorphism related to emplacement of the Northland Allochthon however, the Tangihua Complex records tectonic events not recorded in the Mt Camel.

Detailed structural mapping of several exposures was completed as well as more generalised trends identified at outcrop along the whole field area. Most large scale fractures, jointing and faulting along the entire area were consistent with two main orientations: a NE-SW suite of features bearing between 40-60 degrees, and a NW-SE suite, bearing between 300-320 degrees. Large scale diking seemed to follow the NE-SW trend; however there were frequent outliers to that trend. Data was also collected for geostatistical comparison between the three units. Detailed 10 meter long 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. A section was done through the igneous materials at Rarawa and Puketu, and sections were done through the sediments at all three locations, for a total of five sections for comparison. The relationship between the Tangihua Complex, Mt Camel terrane and the autochthonous basement is poorly understood, making such structural and statistical comparisons valuable in understanding the history of the Gondwana margin.