Strain localisation within ductile shear zones beneath active faults: The Alpine Fault contrasted with the adjacent Otago fault system, New Zealand

Abstract

The Alpine Fault accommodates around 60-70% of the 37 mm/yr oblique motion between the Australian and Pacific plates in the South Island of New Zealand. Uplift on the fault over the past 5 Ma has led to the exhumation of the deep-seated mylonite zone alongside the present surface trace. Shear strain estimates in the mylonites reach 200-300 in the most highly strained rocks, and provide an integrated displacement across the zone of 60-120 km. This is consistent with the amount of displacement during the last 5 Ma, suggesting that displacement on the fault is localised within a 1-2 km wide ductile shear zone to depths of 25-30 km. Existing geodetic data, together with Late Quaternary slip rate and paleoseismic data, are consistent with the steady build-up and release of elastic strain in the upper crust driven by ductile creep within a narrow mylonite zone at depth. Faults of the Otago Fault System form a parallel array east of the Alpine Fault and accommodate c. 2 mm/yr contraction. Long periods of quiescence on individual structures suggest episodic, or "intermittently characteristic", behaviour. This is more consistent with failure on faults within an elastico-frictional upper crust above a ductile lower crust. Localisation of crustal deformation may be initiated by inherited weaknesses in the upper crust, with downward propagation of slip causing strain weakening within the ductile zone immediately beneath. Inherited structures of great length focus a greater amount of displacement and hence more rapidly develop underlying zones of ductile shear.

Quaternary shelf structures SE of the South Island, imaged by high-resolution seismic profiling

“…The termination of these structures can be accommodated by linking the motion of the blocks on the south-eastern side of the Titri Fault System and the north-eastern side of the Waihemo Fault System. These structures are presently insufficiently imaged to allow a more detailed and quantitative analysis, but this style of fault termination is consistent with several other central Otago faults such as the Taieri Ridge, Rock and Pillar, Rough Ridge and Raggedy Faults (Norris 2004;Norris & Nicolls 2004), which all have north-eastern terminations in the Waihemo Fault System. These offshore data suggest that the major tectonic boundary between the SWÁNE-striking range and basin inversion structures of Otago and the NWÁSE-striking faults of South Canterbury has an equivalent expression on the adjacent shelf where it is overlain by Quaternary sedimentation.…”

National Seismic Hazard Model for New Zealand: 2010 Update

“…Where paleoseismic data are absent, slip rates were inferred from a combination of geomorphic expression and long-term uplift rates (Bennett et al, 2005(Bennett et al, , 2006 and are within the bounds of ∼2 mm=yr total horizontal contraction across the region (Norris, 2004;Wallace et al, 2007).…”

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

“…Clearly, crustal thickening beneath the Southern Alps cannot be explained by a simple 2‐D model, driven by the component of plate motion orthogonal to the Alpine Fault. In addition, there is a lack of evidence for substantial distributed shortening of the upper crust in the Southern Alps (i.e., shortening <30 km) [see Craw , ; Turnbull , ; Forsyth et al ., , ; Norris , ; Cox and Barrell , ; Cox et al ., 2007; Landis et al ., ; Rattenbury et al ., ], which indicates that crustal thickening must be mainly in the lower crust. Gerbault et al .…”

“…The stretched rift margin “docked” with Pacific Plate crust southwest of Fiordland at ∼10 Ma, along the Puysegur subduction zone (Figures b and a). The Late Miocene initiation of block faulting, trending nearly orthogonal to the strike of the Alpine Fault, in southern and central South Island [ Norris , ], may be the tectonic expression of this in the Pacific plate.…”