Late Cenozoic Vertical Crustal Movements in the Central Part of New Zealand

Abstract

The stranded shorelines at the back of marine benches are considered to represent maxima in the local shoreline displacement curves. The maxima are determined by local uplift rate and by eustatic sea-level and occurred when uplift rate first exceeded eustatic sea-level rise. The shoreline displacement maxima would have been slightly earlier than the maxima in the eustatic sea-level curve. The difference is negligible for the Last Interglacial and has been taken into account for the Present Interglacial (Holocene). The times of the eustatic maxima (mostly determined from radiometric dates outside New Zealand), the eustatic heights of the maxima, and the emergence heights of the highest marine gravels on the benches are used to infer uplift rates for the south-east corner of the North Island of New Zealand, and for that part of Barbados which provides the most important radiometric dates. For most of the places in New Zealand, and in Barbados, profiles across the marine benches are tilted seawards. Tilt is taken into account in determining the heights of the eustatic maxima in a revised eustatic sea-level curve. The Holocene part of the curve is based on radio-carbon dated emergence heights from six places – places not affected by short period (glacial) isostasy, and places where the tilt effect is thought to be negligible. The data fit together well, and Brazil is the assumed stable datum. The following age and eustatic values have been adopted for determining uplift rates: 125kyr, 00; 100kyr, -2m; 84kyr, -3m; 80kyr, -14m; 60kyr, -24m; 30kyr, -43m, 6kyr, +1m. Four benches of the Last Interglacial, and the Present Interglacial bench were identified on most parts of the south-east coast of the North Island. Inferred ages in kyrs for the back of the benches are 125, 100, 84, 80 and 6. The Last Interglacial benches were treated as a single 100 kyr-old bench by correcting them for age and eustatic sea-level differences, it being assumed that the average rate of uplift remained constant at all places. The growing folds could be traced inland by contouring the summit heights. In the coastal belt the summit height surface is twice as high as the 100 kyr surface. For the inland part the contour pattern itself provided a control, and it was found that the uplift values could not be divided by more than three if the matching pattern between the two sets of contours was to be preserved, that is if uplift rates were not to decrease with increasing summit height. Division by two, that is by assuming an age of 200 kyr, gave the best overall fit with the 100 kyr surface and was adopted in determining uplift rates for the inland area. It is thus thought that uplift was small until 200 kyr ago and then increased to the average rate for the last 100 kyr. In order to obtain a wider regional coverage, summit height contours were drawn for the mountains of the north-eastern side of the South Island. There are few elevated marine benches and the relation between height and uplift rate was assumed to be the same as in the North Island. Cook Strait lies between the North and South Islands, and over a wide stretch of the strait the uplift rate is inferred to be negative. Uplift values are obtained indirectly from a seismically determined marker horizon that is assumed to be about the same age as a 12 Myr old unconformity exposed near Cape Palliser. In the onshore area the unconformity, and strata of about 14 Myr age in the Riversdale-Flat Point Area, are thirteen times as tilted as is the 100 kyr surface. The rate of negative uplift in Cook Strait was estimated by increasing the values of the contours on the marker horizon by 1,500m so as to bring the marker horizon zero-isobase coincident with the 100 kyr zero-isobase and then reducing the slope of the marker horizon contours thirteen times. The negative uplift in Cook Strait contrasts with the positive uplift of the land on each side, and is tentatively explained by fault offsetting, in the same way as the Hanmer Depression has been explained by a dextral offset of the dextral Hope Fault. Growing folds are defined by differences in uplift values: the crest of anticlines having the highest values, and the troughs of synclines the lowest. Tilt is most on the flanks of the folds. Faults are represented by steps in the contoured uplift pattern. Rates are given as m/kyr, and tilt as degrees of arc/kyr where kyr is equal to one thousand years. Rates are compared in an integrated isobase map, and the uplift rates and flank tilt rates for the main anticlines and synclines set out in a table for the last 6 kyr, 100 kyr, and summit height surface. On the marine benches uplift rates range from 0.75m/kyr to 4.0m/kyr for the growing anticlines, and from 0.5m/kyr to 2.2m/kyr for the growing synclines. For the summit height surface uplift rates on the anticlines range from 3.0m/kyr to 4.5m/kyr. Tilt rate for the flank of the folds ranges from 0.014 0/kyr to 0.038 0/kyr, and is about 30 percent steeper on the east than on the west flanks of the anticlines. In terms of classical description there are three phases in the uplift history of the district: an epeirogenic phase (very slow subsidence without much folding) from 14Myr to 1.3Myr, a phase of tectogenesis (folding with low rate of uplift) from 1.3Myr to 0.2Myr, and an orogenic phase (folding and intense uplift) from 0.2Myr to the present day. According to plate tectonic theory the study district covers most of the width of the boundary zone between the Australian and Pacific plates. Relative horizontal displacement for the last 10Myr should be about 35m/kyr, about 25m/kyr being narrowing normal to the major faults and folds, and 25m/kyr being dextral displacement parallel to the faults and folds. Dextral displacement is outside the scope of this thesis. Narrowing, inferred from a simple hinged beam model, is unlikely to be more than 8m/kyr, and then only for the last 1.3Myr. If the plate tectonic rates are correct, then most of the narrowing must have taken place outside the study district.