Stresses and strains in a twisted subduction zone - Fiordland, New Zealand

In the Fiordland region of the South Island of New Zealand, the subducted Australian Plate steepens from south to north, becoming near-vertical below 75 km depth between Doubtful and Milford Sounds. This steepening is accompanied by a sharp change in strike of the subduction zone of about 17degrees. Here we investigate stress and strain in this twisted subduction zone using earthquakes recorded during a three-month deployment of 24 portable digital seismographs. We determine focal mechanisms of 115 earthquakes of M-L 2.5-4.5 using both first motion polarity data and amplitudes of seismogram envelopes. We also invert for the stress tensor orientation for regions within both the subducted and overlying plates, using a method which invokes the Coulomb failure criterion and considers all P wave first motions together, regardless of whether or not they are sufficient to define single event focal mechanisms. Focal mechanisms and stress inversions for earthquakes in the overlying plate indicate normal faulting shallower than 16 km and thrust faulting at greater depths. This stress and strain regime is related to the 17degrees change in strike of the subduction zone, which leads to an arching up of the subducted plate at the bend. Such arching is consistent with the exposure of lower crustal granulite facies rocks near the bend. Earthquakes in the subducted plate show predominantly normal faulting relative to the plate. Arrival time inversions for 3-D seismic velocity structure indicate that a zone of high Vp (>8.5 km s(-1) ) in the uppermost mantle of the overlying plate abuts the vertical portion of the subduction zone. Stresses and strains in the subducted plate are consistent with a model in which the sharp change in strike of the subduction zone is a consequence of the obliquely converging subducted plate having to bend around this zone. The steep dip of the subducted plate to the north appears to be a natural consequence of the minimization of membrane stress in the plate arising from the sharp bend along strike. This mechanism for steepening the plate is quite different to that of trench retreat usually used to explain steep subduction.