Emily Warren-Smith

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

New Zealands 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 (<20°) 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.

Thermochronological evidence of a low‐angle, mid‐crustal detachment plane beneath the central South Island, New Zealand

Oblique continental convergence and uplift in the Southern Alps, New Zealand is largely accommodated by dextral transpression on the Alpine Fault. However, towards the south of the orogen the Alpine Fault becomes increasingly strike-slip, despite evidence for high exhumation rates in the Pacific plate. Here, we present 41 new apatite and zircon fission-track ages to investigate the role of the southern Alpine Fault in Pacific plate exhumation since the Miocene. Through development of a new, maximum likelihood fission-track age calculation method (to overcome extremely low (< 0.1 ppm) 238U concentrations in apatites) we estimate the width of the fully reset apatite zone (ages < 5 Ma) southeast of the southern Alpine Fault, which has been previously overestimated. Instead, this zone is ∼30 km wide, rather than 60 km. We combine our exhumation profile with thermo-kinematic modelling to impose constraints on fault kinematics and deformation history. The surface cooling age pattern can be well reproduced by exhumation along a listric reverse fault, which shallows to a low-angle (6–10°) mid-crustal detachment beneath the Southern Lakes. This structure is comparable to the listric central Alpine Fault geometry previously constrained by thermo-kinematics models to the north of our study region. We propose this detachment plane is continuous beneath a large region of central South Island and may be acting to accommodate underthrusting of Australian crust beneath the Pacific plate. This article is protected by copyright. All rights reserved.

Stress field and kinematics for diffuse microseismicity in a zone of continental transpression, South Island, New Zealand

We analyze shallow (0–20 km) microseismicity adjacent to the Alpine Fault in New Zealand, where there is oblique convergence of the Australian and Pacific plates. Focal mechanisms for 155 earthquakes (June 2012 - October 2013) are inverted to determine the orientation of the stress field. This yields a principal horizontal axis of compression, SHmax = 114° ± 10°, which cannot be explained in terms of the sum of stress from tectonic loading due to plate convergence, indicated by GPS observations, and gravitational stresses. The azimuth of slip vectors for individual focal mechanisms cluster perpendicular and parallel to the plate convergence vector. These faults, however, strike at ~45° to SHmax from the stress inversion, suggesting a very low coefficient of friction. The earthquake slip directions may be kinematically controlled, accommodating the plate convergence on a limited set of fractures, similar to the segmentation for neotectonic faulting along the Alpine Fault, which is partitioned into strike-slip and thrust segments at a 1–10 km scale. We suggest two possible controls on our calculated SHmax azimuths. Firstly, there may be a slight clockwise bias in the estimates of SHmax from earthquakes; slip may be occurring on a more limited range of fractures than assumed by the stress inversion method, although this effect is likely to be relatively small (±5°). More importantly, the components of the stress field may be relieved at different time scales during big earthquakes, resulting in a residual stress field that varies significantly (±15°) on timescales of several large earthquakes.

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

Shallow (<25km), diffuse crustal seismicity occurs in a zone up to 150km wide adjacent to the southern Alpine Fault, New Zealand, as a consequence of distributed shear and thickening in the obliquely convergent Australian-Pacific plate boundary zone. It has recently been proposed that continental convergence here is accommodated by oblique slip on a low angle detachment that underlies the region, and as such, forms a previously unrecognized mode of oblique continental convergence. We test this model using microseismicity, presenting a new, 15-month high-resolution microearthquake catalog for the Southern Lakes and northern Fiordland regions adjacent to the Alpine Fault. We determine the spatial distribution, moment release and style of microearthquakes, and show seismicity in the continental lithosphere is predominantly shallower than ~20km, in a zone up to 150km wide, but less frequent deeper microseismicity extending into the mantle, at depths of up to 100km is also observed. The geometry of the subducted oceanic Australian plate is well imaged, with a well-defined Benioff Zone to depths of ~150km. In detail, the depth of continental microseismicity shows considerable variation, with no clear link with major active surface faults, but rather represents diffuse cracking in response to the ambient stress release. The moment release rate is ~0.1% of that required to accommodate relative plate convergence, and the azimuth of the principal horizontal axis of contraction accommodated by microseismicity is 120°, 15–20° clockwise of the horizontal axis of contractional strain rate observed geodetically. Thus, short-term microseismicity, independent of knowledge of intermittent large magnitude earthquakes, may not be a good guide to the rate and orientation of long-term deformation, but is an indicator of the instantaneous state of stress and potential distribution of finite deformation. Thus, we show that both the horizontal and vertical spatial distribution of microseismicity can be explained in terms of a low angle detachment model.