Implications of upper-mantle seismicity for deformation in the continental collision zone beneath the Alpine Fault, South Island, New Zealand

We review the state of knowledge regarding lower-crustal and upper-mantle deformation in the continental collision zone beneath the Alpine Fault in the central South Island. Existing lithospheric deformation models based on a variety of geophysical observations and different interpretations of tectonic reconstructions range from intra-continental subduction to lithospheric mantle thickening. We derive independent information in the 40–80 km depth range from a new catalogue of 78 upper mantle earthquakes, after almost a decade’s observations (2006–2016). The events occur in both the Australian and Pacific plates, are clustered in distinct locations that coincide with positive magnetic anomalies and follow an inferred structural trend differing from the current plate boundary orientation. For two event clusters 13 new well-constrained focal mechanism solutions are used for stress field inversion. This yields a common, sub-horizontal maximum compressive stress orientation (120° ± 27° and 120° ± 22°) but suggests different intermediate and minimum principal stress orientations. Based on this we cannot unequivocally distinguish published mantle deformation models but we can evaluate certain model features. Strong upper-mantle anisotropy together with a VP-VS-ratio of 1.73 ± 0.08 for upper-mantle earthquakes suggests that a proposed eclogite layer is unlikely to be present at upper mantle depths. We conclude that seismicity at depths ≥40 km reflects inherited heterogeneous strength distributions in the upper mantle and delineates areas in which deformation occurs along a (structural) convergence axis in the continental collision zone.