Convergent Margins of New Zealand and Deformation Following Hikurangi Plateau Large Igneous Province Subduction.

Friday, 19 December 2014
Bryan William Davy, GNS Science-Institute of Geological and Nuclear Sciences Ltd, Lower Hutt, New Zealand
The 110 Ma subduction of the Hikurangi Plateau, 2 km shallower than at present, has uplifted and deformed the New Zealand sector of Gondwana. Jamming of this subduction near the southern limit of the South Island began c. 105 Ma leading to rotation and offset of the margin before subduction ceased a c. 100 Ma. Rotation of the offshore margin from WNW–ESE to W-E led to opening of the Great South Basin and crustal thinning between rotated crustal blocks in the Inner Bounty Trough.

Subsequent erosion of the uplifted margin has removed any uplifted volcanic arc volcanism and associated magnetic anomaly. Late Cenozoic distributed deformation throughout the South Island of New Zealand, and concentrated along the Alpine Fault, are linked to the strike slip/compressional plate boundary motion through the island. The combined Cretaceous and Neogene deformational episodes have made convergent margin recognition difficult onshore.

By examining the gravity anomaly field of the New Zealand region and in particular the horizontal gravity anomaly gradient, combined with limited crustal scale seismic reflection records available offshore, it is possible to trace some of the convergent margins of the New Zealand Late Paleozoic-Late Mesozoic Eastern province from offshore to onshore.

The Triassic-Jurassic Chatham Rise convergent margin can be traced, via two 60 km SSW left-lateral offsets, onshore into the central South Island. The first offset matches the offset Chatham Rise to the Banks Peninsula arch extending to the Canterbury foothills. Seismicity associated with the 1994 Arthurs Pass earthquake which occurred on the northern margin of the alpine segment of the Chatham Rise extension is very similar in character to the 2010 Canterbury earthquake sequence which occurred above the Canterbury Plains segment of the Chatham Rise.

The New Zealand Jurassic-Cretaceous convergent margin can be recognised in gravity, seismic reflection and bathymetry data along the northern Chatham Rise and a Permian-Triassic convergent margin can be recognised in gravity in the southern-central South Island.

The presence of the buoyant Chatham Rise crustal block has meant that Late Cenozoic faulting in the Marlborough region has all been diverted west of the Alpine Chatham Rise crustal block onto the Alpine Fault