The continental collision zone, South Island, New Zealand: Comparison of geodynamical models and observations

Abstract

The South Island zone of oblique continent‐continent convergence occurs along a 400 km‐long section of the modern Australia‐Pacific plate boundary zone, across which about 50 km of shortening has been accommodated since about 10 Ma. The orogen comprises a central mountain range (Southern Alps) flanked on both sides by what are interpreted to be foreland basins. Two essential features that characterize the orogen are (1) the degree of denudation that accompanied deformation, and (2) a fundamental structural asymmetry. The architectural asymmetry of the orogen can be explained by plane strain, finite element models of continental convergence incorporating mantle subduction. Comparison of model and orogen polarity implies that Pacific plate mantle subducts. The models predict two crustal‐scale dipping shear zones that form above the point where the Pacific mantle subducts. The localized one more distant from the incoming plate (retro‐step‐up shear zone) corresponds to the Alpine fault, whereas its conjugate (pro‐step‐up shear zone) corresponds to the distributed strain and thrusting along the eastern margin of the mountain belt. Parameters that modify the model boundary conditions (top surface, degree of denudation; basal zone, subduction load, crust‐mantle velocity discontinuity, subduction of lower crust, mantle retreat, and distributed decrease in mantle velocity) and the internal strength of the crust (two‐layer crust with moderate coupling, temperature distribution, strain weakening) are varied in a series of numerical model calculations that establish the combination of material properties and boundary conditions that lead to different cross sectional architectures of the modeled collision zone. In turn, these are compared with observations about the South Island orogen. The calculations 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 differences in the width and two‐dimensional architecture of the orogen, simulated in the experiments by varying the model parameters, can be explained by a combination of southward increases in preconvergent crustal thickness, geothermal gradient, convergence, and potentially subduction retreat, with the added possibility of a southward decrease in the component of lower crustal subduction.

Intermediate-Depth Earthquakes in a Region of Continental Convergence: South Island, New Zealand

“…Gravitational instability can thicken the lithosphere and result in a deep, high-density body (Houseman et al, 2000). Depending on the rheology, the distributed thickening may exhibit subduction-like behavior in the uppermost mantle (Beaumont et al, 1996). One commonality among the models discussed above is that they predict a horizontal shear zone with high strain rate in the lithospheric mantle on the northwest side of the high-velocity body, broadly underlying the Alpine fault and coinciding with the mantle earthquake locations.…”

“…The Gutenberg-Richter relation that predicts greater magnitudes may be truncated at a cut-off magnitude corresponding to the maximum dimension of the brittle zone Richter, 1941, 1944 Figure 6. Schematic of deforming lithosphere and strain ellipses predicted from a qualitative compilation of the two-dimensional and three-dimensional kinemetic and dynamic models (Beaumont et al, 1996;Houseman et al, 2000;Stern et al, 2000;Gerbault et al, 2002;Pysklywec et al, 2002). Vertical to horizontal scale is 1:1.…”

Orogeny and orography: The effects of erosion on the structure of mountain belts

“…These components are imbedded into a model of orogenesis which assumes that convergent orogens form by crustal detachment and shortening above a subducting substrate consisting of the lithospheric mantle and, in some cases, parts of the lower crust [Willett et al, 1993]. This mechanical model can explain the large-scale structure of mountain belts which are formed primarily by structural thickening as occurs in subduction forearcs [Silver and Reed, 1988 [Beaumont et al, 1996b]. In its simplest form, this process can be represented by deformation of a single layer with boundary conditions as shown in Figure 2.…”

“…The tectonic model used by Beaumont et al [1992] was relatively simple compared to the models of later papers, including this one, but was adequate to demonstrate how topography develops in regions of high uplift and erosion. Other studies have continued to use the Southern Alps as the type example and have used simpler erosion models, but more sophisticated tectonic models in order to investigate the importance of erosion to patterns of exhumation, topography, and cooling rates [Beaumont et al, 1996b;Batt and Braun, 1997]. Other orogenic belts that have been the subject of similar investigations include the European Alps [Beaumont et al, 1996a; Schlunnegger and , the Pyrenees [Beaumont et al, 1999b], the Andes, [Masek et al, 1994] and the Himalayas [Jamieson et al, 1996].…”

Factors controlling the Alpine evolution of the central Pyrenees inferred from a comparison of observations and geodynamical models

“…The reference models help (1) develop the firstorder mass balance, (2) demonstrate the effect of factors introduced into the more complex models, and (3) link the models discussed in this paper to more basic styles [Beaumont and Quinlan, 1994;Beaumont et al, , 1996.…”