Plastic plate tectonics in New Zealand: from the Gondwana margin to the present

Abstract

Plate Tectonic reconstructions of southeast Gondwanaland have always faced the problem of large scale continental deformation in New Zealand. In this thesis I shall present a reconstruction of New Zealand which combines modern insights on plastic deformation with the classical theorem of plate tectonics. In order to do so I have first digitized geological maps of Antarctica, Eastern Australia and New Zealand and attempted an improved Australia-Antarctica-New Zealand fit using the classical spherical rigid plate approximation. This was done in order to get an estimate of the magnitude of plate deformation required for such a first order fit and without constraining the details of the deformation process. The reconstruction requires a minimum of 800-900 km slip on a transform through New Zealand. Assuming about 500 km discrete slip on the Alpine Fault there must be at least 300-400 km of slip accommodated by plastic deformation. Another estimate which, I feel, gives a maximum value of diffuse deformation can be obtained by restoring - after removing slip on the Alpine Fault - the Z bend in major structural units of New Zealand to a straight line. About 500 km of slip by plastic bending can be inferred from such an approach. In order to derive the details of plastic deformation I identified geological markers to be used for modelling deformation in New Zealand. The most important markers are: i) a more or less rectilinear belt of late Palaeozoic to Mesozoic terranes; ii) a pattern of Cretaceous sedimentary basins following the trend of the Tasman Rift; iii) a rectangular block faulting pattern formed during the same episode; iv) Neogene volcanic trends in the North Island. These markers constrain the bending deformation of New Zealand to be post-Eocene. Plastic plate tectonic reconstructions can be refined by theoretical considerations. Using a compilation of rheological parameters of the lithosphere, it is shown that a horizontal viscoelastic bending model can only explain up to 30% of the observed vertical axis folding of the New Zealand plate boundary. New Zealand folds through large scale plate convergence of the Pacific and the Australian plates. Viscoelastic bending models are able to explain about 160 km of plate convergence. The remaining convergence of the order of 250 to 350 km can be explained by plastic collapse of the New Zealand plate boundary in the area where bending moments are largest. Through application of these theoretical constraints and by use of the apparent deformation of the markers, the following plastic plate tectonic history can be inferred. From about 40 Ma to 25 Ma the initial impact of the Pacific plate into the New Zealand plate boundary zone was absorbed by viscoelastic bending as indicated by the present day pattern of Cretaceous rift basins. Viscoelastic bending affected the southern part of the North Island and the entire South Island. At the end of the Oligocene (25Ma) bending stresses culminated and the Alpine Fault developed along a plastic hinge line by a plastic collapse mechanism. Subsequently, a regime of asymmetrical bending evolved where only the North Island was bent around in a viscous manner. The South Island bend was enhanced by rigid body rotation and horizontal shortening in the southwest. Viscous bending continued in the North Island during the Miocene until, at the end of the Miocene (5Ma) due to continuously increasing bending stresses, a second plastic hinge line was induced about 100 km to the north of the Taupo Volcanic Zone. This process is known as "plastic collapse", because it results in a dramatic acceleration of bending and locally enhanced dissipation of heat. The plastic hinge line migrated south and at present causes lithosphere necking in the Taupo Volcanic Zone and lithosphere buckling in the Wanganui Basin. Assessment of the amount of heat dissipated by plastic work inside the Taupo Volcanic Zone suggests that large volumes of rhyolitic volcanics are produced by dissipation of mechanical energy of the deforming ductile lower crust which has been preheated by active arc volcanism. A similar approach is presented for the energy balance of the indentation problem applied to the Himalayas and the European Alps. Although plastic slip line fields in these examples do not reach the high strain energy density as encountered in the plastic hinge line in the Taupo Volcanic Zone a considerable amount of energy is dissipated along slip lines in the European Alps and the Himalayas. The contribution of calculated heat flow induced by plastic deformation and to be added to the background heat flow is in the European Alps about 45 mW/m2, in the Himalayas about 67 mW/m2, but reaches in the Taupo Volcanic Zone an anomalously high value of about 300 mW/m2.