THE CONTINENTAL COLLISION ZONE, SOUTH-ISLAND, NEW-ZEALAND - COMPARISON OF GEODYNAMICAL MODELS AND OBSERVATIONS

The South Island zone of oblique continent-continent convergence occur s along a 400 km-long section of the modern Australia-Pacific plate bo undary zone, across which about 50 km of shortening has been accommoda ted since about 10 Ma. The orogen comprises a central mountain range ( Southern Alps) flanked on both sides by what are interpreted to be for eland basins, Two essential features that characterize the orogen are (1) the degree of denudation that accompanied deformation, and (2) a f undamental structural asymmetry. The architectural asymmetry of the or ogen can be explained by plane strain, finite element models of contin ental convergence incorporating mantle subduction. Comparison of model and orogen polarity implies that Pacific plate mantle subducts. The m odels predict two crustal-scale dipping shear zones that form above th e point where the Pacific mantle subducts. The localized one more dist ant from the incoming plate (retro-step-up shear zone) corresponds to the Alpine fault, whereas its conjugate (pro-step-up shear zone) corre sponds to the distributed strain and thrusting along the eastern margi n of the mountain belt. Parameters that modify the model boundary cond itions (top surface, degree of denudation; basal zone, subduction load , crust-mantle velocity discontinuity, subduction of lower crust, mant le retreat, and distributed decrease in mantle velocity) and the inter nal strength of the crust (two-layer crust with moderate coupling, tem perature distribution, strain weakening) are varied in a series of num erical model calculations that establish the combination of material p roperties and boundary conditions that lead to different cross section al architectures of the modeled collision zone. In turn, these are com pared with observations about the South Island orogen. The calculation s show how the style and extent of deformation across the whole orogen depend on the rheological properties of the crustal layer and on the balance between its internal strength and the combined effects of the boundary and gravitational stresses. North to south along-strike diffe rences in the width and two-dimensional architecture of the orogen, si mulated in the experiments by varying the model parameters, can be exp lained by a combination of southward increases in preconvergent crusta l thickness, geothermal gradient, convergence, and potentially subduct ion retreat, with the added possibility of a southward decrease in the component of lower crustal subduction.