Continent‐scale strike‐slip on a low‐angle fault beneath New Zealand's Southern Alps: Implications for crustal thickening in oblique collision zones

Abstract

New Zealand's Southern Alps lie adjacent to the continent-scale dextral strike-slip Alpine Fault, on the boundary between the Pacific and Australian plates. We show with a simple 2-D model of crustal balancing that the observed crustal root and erosion (expressed as equivalent crustal shortening) is up to twice that predicted by the orthogonal plate convergence since 11 Ma, and even since 23 Ma when the Alpine Fault formed. We consider two explanations for this, involving a strong component of motion along the length of the plate-boundary zone. Geophysical data indicate that the Alpine Fault has a listric geometry, flattening at mid crustal levels, and has accommodated sideways underthrusting of Australian plate crust beneath Pacific plate crust. The geometry of the crustal root, together with plate reconstructions, requires the underthrust crust to be the hyperextended part of an asymmetric rift system which formed over 500 km farther south during the Eocene-the narrow remnant part today forms the western margin of the Campbell Plateau. At 10 Ma, the hyperextended margin underwent shallow subduction in the Puysegur subduction zone, and then was dragged over 300 km along the length of the Southern Alps beneath a low-angle (<208) section of the Alpine Fault. We speculate that prior to 10 Ma, more distributed lower crustal shortening and thickening occurred beneath the Southern Alps, accommodating southward extrusion of continental crust in the northern part of the plate boundary zone, providing a mechanism for clockwise rotation of the Hikurangi margin.

Microseismicity in Southern South Island, New Zealand: Implications for the Mechanism of Crustal Deformation Adjacent to a Major Continental Transform

“…(a) Schematic model of central South Island deformation, modified from Warren‐Smith et al () and based on discussion in Lamb et al (). In map view, the pink triangle shows region adjacent to Alpine Fault, which is underlain by thinned Eocene‐rifted Australian crust.…”

“…(b) Increased seismicity (manifest as peaks in the microseismic strain rate, blue dash‐dotted line, and moment release, dark red solid line) is observed 50–60 km southeast of the Alpine Fault surface trace, roughly corresponding with the Nevis‐Cardrona fault system (NCFS). (c) This region also corresponds with a change in dip of a proposed midcrustal detachment plane, modeled geodetically (e.g., Lamb et al, ; Lamb & Smith, ) and based on thermo‐kinematic modeling of fission track ages (Warren‐Smith et al, ). This region is interpreted as a broad proshear zone formed through concentration of vertical stresses.…”

“…Underlying colors show Qp from Eberhart‐Phillips et al (2015). Seismicity near the southern Alpine Fault is less well constrained (occurring outside the COSA network) but appears to correspond to locked portions of the fault plane modeled using campaign GPS data (black dashed plane, including motion to west of Alpine Fault not included in Lamb et al () and Wallace et al (). (d) Focal mechanisms are plotted side‐on, such that colored segments indicate motion into the page.…”

“…However, there is growing evidence in both the central Southern Alps and Southern Lakes for seismicity, seismic tremor, and low‐frequency earthquakes, extending to depths >25 km, and also into the mantle (Boese et al, ; Chamberlain et al, ; Wech et al, ), suggesting that there may be more than one brittle‐ductile transition in the lithosphere here. In the model of Lamb et al (), crust beneath the detachment may be relatively “cold” giving rise to a deeper layer of crustal and mantle seismicity such as that observed in the northern part of our study area (see profiles along X = 20 km and X = 60 km in Figure ).…”

“…Warren‐Smith, Lamb and Stern () used these data to show that slip directions of microseismicity in the central South Island exhibit a strong relationship to the relative plate convergence direction. We further this work by analyzing the microseismicity in terms of where and how plate convergence is accommodated, and the implications for longer‐term deformation, in the context of the detachment model of Lamb et al (). In particular, we aim to address the following general questions: What is the distribution of microseismicity adjacent to the southern Alpine Fault? What are the time scales of deformation for which short‐term microseismicity is indicative? What does the pattern of microseismicity tell us about the mode of continental deformation in this region? …”