Along‐strike differences in the Southern Alps of New Zealand: Consequences of inherited variation in rheology

Abstract

[1] Long-and short-term strain variations along the Australian-Pacific plate boundary through the South Island of New Zealand, including a 300% increase in orogen width, coexistence of oblique thrusting on orthogonal structures, and variability in the locus of orogenic gold deposits, coincide with rheologically relevant geological variation. Our model investigates the consequences of thin, strong lower crust in the north and thick, weak lower crust in the south. Solution of the full 3-D mechanical equations reproduces the larger wavelength strain patterns of the orogen. A 3-D perturbation-based analytical solution leads to the identification of the sensitivity of displacement type to minor stress changes. Transition from boundary-normal thrusting to boundary-parallel thrusting occurs at the transition from strong to weak lower crust and is related to an increase in either t yz (shear stress in the yz plane) or the ratio of the coordinate normal stresses, (s yy /s xx ), where x and y are in the horizontal and z is vertical. Both mechanisms are compatible with the geologically dependent rheological variation employed in our model.

Structure of the Blue Lake Fault Zone, Otago Schist, New Zealand

“…While the regional-scale control on the location of the Blue Lake Fault Zone was the major crustal boundary between Torlesse greywacke and Otago Schist ( Fig. 1A; Upton et al 2009), on a more local scale it was the argillites and pelitic schists in the rock sequence that controlled the location (and subsequent reactivation) of specific fault strands.…”

“…The boundary between greywacke and schist also coincides with a major change in Late Cenozoic and Recent tectonic deformation as a result of the underlying crustal differences (Upton et al 2009). The Otago Schist is currently deforming into a set of northeast-trending antiformal ridges separated by synformal basins ( Fig.…”

“…1A; Upton et al 2009). The boundary is a complex structural zone that has been periodically tectonically active for more than 100 million years (Gray & Foster 2004;Mitchell et al 2009;Upton et al 2009).…”

“…1A; Upton et al 2009). The boundary is a complex structural zone that has been periodically tectonically active for more than 100 million years (Gray & Foster 2004;Mitchell et al 2009;Upton et al 2009). On a regional scale, the boundary between greywacke and schist along the north-eastern margin of the schist belt appears to be gradational, with progressive increase in metamorphic grade towards the southwest (Bishop 1972;Forsyth 2001).…”

“…1C, 1D;Lindqvist 1994;Forsyth 2001). The structural change from folded schist to faulted greywacke is linked to the lithological variations at the small to medium scale and to the major crustal discontinuity at the larger scale (Upton et al 2009). …”