HORIZONTAL CRUSTAL DEFORMATION ON THE HIKURANGI MARGIN

The first-order geodetic triangulation of New Zealand, completed between 1925 and 1949 by the Department of Lands & Survey, has been reobserved in part between 1976 and 1984 along a series of narrow belts, as a contribution to to the national Earth Deformation Programme of the Royal Society of New Zealand. This has provided a set of data of uniformly high quality, which has been analysed by the method of simultaneous reduction, to yield a continuum model of horizontal deformation for the region between Christchurch and Auckland. This spans the Hikurangi Margin, the southern extension of the Tonga-Kermadec boundary of the subducting Pacific plate that terminates at the Chatham Rise. The continuum model of deformation comprises the 16 terms that specify the horizontal derivatives of the horizontal velocity field of orders 2-4, and their error covariance matrix. Between Hawke Bay and Chatham Rise the pattern of shear-strain rate shows a consistent azimuth of the axis of maximum relative shortening of 110/290-degrees, with a maximum intensity (tensor) at the east coast of Wellington of about (0.20 +/- 0.02) mu-rad/yr, decreasing rapidly westward. This direction is consistent with that that would be predicted for the axis of maximum horizontal compression in the overriding plate for the case of isotropic frictional slip between the plates at the obliquely convergent subduction margin. North of Hawke Bay, the azimuth of the axis of maximum relative shortening swings to northeast-southwest. This confirms previous evidence of extension across the Central Volcanic Region, and also of trenchward extension in the East Cape peninsula; this effect may be due to gravity sliding of the superficial, and rapidly uplifted, Tertiary sediments (which carry the geodetic monuments). Analysis of bending in the East Cape peninsula shows axes of maximum bending rate concave to the SSE. In the northern South Island, strong bending, with concave normals to the axes of maximum bending lying in the northwest quadrant, marks the southern extremity of Pacific plate subduction, with its transition to the intense compressional mountain building of the central South Island. This pattern is not incompatible with the role of the Chatham Rise as a westward-thrusting rigid indenter. A fourth-rank invariant of the deformation model shows the inflexion of bending on the Hikurangi Margin to be of S-form.