Validation of tectonic reconstructions by crustal volume balance: New Zealand through the Cenozoic

Abstract

We describe a procedure that allows us to quantitatively test hypotheses about the tectonic evolution of a region, including areas for which there is little or no geologic record, and apply it to the Pacific-Australian plate boundary in the New Zealand region since 65 Ma. The derived models are a first attempt to quantify the effects of the development of the modern plate boundary on four major components of the tectonic evolution of New Zealand, i.e., sedimentation, paleogeography, growth of crustal root, and nonrigid deformation. The technique balances total rock volume through time using estimates of these factors. Although the solutions are non-unique, the uncertainties in each factor can be estimated and used to define the models. The plate reconstructions we present are minor modifications of earlier work, developed to test whether those proposed reconstructions are self-consistent and consistent with present-day crustal structure and sediment distribution. They are digital and geographically referenced, and they have the advantage that new data can be readily incorporated and new concepts tested against several criteria. The model of the evolution of New Zealand9s crustal root is new and incorporates a previously unpublished compilation. The models show that rock redistribution is accounted for by erosion and sedimentation during the period 65–25 Ma. The Pacific-Australian plate boundary began propagating through New Zealand ca. 45 Ma. In these models, development of the boundary has no effect on nonrigid deformation until ca. 25 Ma. That time marked the start of a significant increase in land area, and of the nonrigid deformation of a large region in the center of the New Zealand continent. Since ca. 20 Ma, ∼575,000 km 3 of sediments were deposited in basins around New Zealand, and a crustal root with a volume of ∼2,200,000 km 3 is interpreted to have developed beneath the landmass. A large area in the center of Late Cretaceous and Tertiary reconstructions of the New Zealand region, inferred from outcrop, well, and seismic reflection data, deformed, eroded, and was eventually amalgamated within the continental crust of New Zealand, a redistribution of ∼3,080,000 km 3 of rock (at 0% porosity).