Horizontal crustal deformation on the hikurangi margin

Abstract

The first-order geodetic triangulation of New Zealand, completed between 1925 and1949by the Department of Landsand 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. Thishas provided a set of data of uniformly high quality, which has been analysed bythe method of simultaneous reduction, toyield a continuum model of horizontal deformationfor 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 theChatham 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°, witha maximum intensity (tensor) atthe east coast of Wellington of about (0.20 ± 0.02) μrad/yr, decreasing rapidlywestward. This direction is consistent with that that would be predicted for the axis of maximum horizontal compressionin the overriding plate for the case of isotropic factional 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.Thisconfirms previous evidence of extension across the Central Volcanic Region, and also of trenchward extension in the EastCape 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 lyingin thenorthwest quadrant, marks the southern extremity of Pacific plate subduction, with its transition to the intense compressional mountainbuilding of the central South Island. This pattern is not incompatible with the role of the ChathamRise 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. © 1990 Taylor & Francis Group, LLC.