The kinematics of northern South Island, New Zealand, determined from geologic strain rates

Abstract

Relative motions within the distributed plate boundary zone of northern South Island, New Zealand, are determined through an inversion of geologic strain rate estimates. The Quaternary fault slip rate estimates define the shear strain rates, and rock uplift rates provide information on the horizontal divergence rates. An erosion rate to rock uplift rate ratio along with a crustal compensation factor is estimated in order to convert rock uplift rates to horizontal divergence rates. Because of the uncertainty in erosion rates, horizontal divergence rates are given a large standard error of . The three horizontal strain rate components obtained from these data completely define the horizontal velocity gradient tensor. Strain rate distributions are matched with spline polynomial functions, which can be constrained to behave rigidly within specified regions, such as the Pacific or Australian plates. Inversion of the strain rate distribution, assuming uniform erosion rates across the northern South Island, yields a velocity field that has small differences in both magnitude (10% larger) and direction with the NUVEL‐1A plate motion model between Pacific and Australian plates. A revised strain rate data set, obtained from a variable erosion model in which erosion rates are a linear function of the log of the average annual rainfall magnitudes, yields a velocity field with expected directions that are indistinguishable from the NUVEL‐1A plate motion model between Pacific and Australian plates, but velocity magnitudes are still 10–15% higher than the plate motion model. Therefore the average values of slip rate on strike‐slip faults in Marlborough, required by the NUVEL‐1A plate motion model, are typically close to the low end of the published range of slip values for those structures. The major strike‐slip structures within the Marlborough region are accommodating 80–100% of the total plate motion between Australia and Pacific plates on northern South Island; as much as 20% of the relative motion could be accommodated by shear generated by folding or thrust faulting with a single orientation between the major structures. The magnitude of plate motion contributed by the major faults indicates that the distributed geodetic shear strains measured across northern South Island, and discussed by Bibby [1981] and Walcott [1984], are not a form of irrecoverable strain but rather a feature of the strain field that will eventually be released primarily as concentrated slip on the strike‐slip structures within the region. Strain modeling indicates that the earthquake moment release over the last 150 years within the northwest Nelson province, which is west of the Marlborough region, is at least half an order of magnitude higher than the long‐term rate of moment release. Overall, the northwest Nelson region is playing only a minor role in taking up the long‐term plate motion.