Earth and Planetary Science Letters, 107 (1991) 523-538 523 Elsevier Science Publishers B.V., Amsterdam [DT] Sediment loading on the Western Platform of the New Zealand continent: Implications for the strength of a continental margin W.E. Holt * and T.A. Stern Department of Scientific and Industrial Research, Geology and Geophysics Dit.,ision, Box 1320, Wellington, New Zealand Received November 17, 1990; revision accepted September 13, 1991 ABSTRACT Mechanical properties of the New Zealand lithosphere are determined by forward modelling the gravity and isostatic response of a Plio-Pleistocene sediment loaded shelf margin. The Giant Foreset Formation on the Western Platform of New Zealand is a 2 km thick, 250 km long, and 100 km wide sediment package. Both the downward flexed Pliocene-Miocene boundary beneath the package and a 35-50 mGal peak-to-trough gravity anomaly over the sediment package are best matched with a flexural rigidity in the region of 5.6 × 1022-1.5 × 1023 N m (effective elastic thickness T~ = 20-28 km). The Giant Foresets are a particularly important load to model because, unlike most sediment-loaded passive margins, there is a time gap of about 70-80 Ma between rifting and subsequent deposition of the sedimentary load. Furthermore, the width of the Giant Foreset load is sufficiently small to afford a reasonable resolution of T~. 1. Introduction Our present knowledge of the strength and mechanical properties of the lithosphere depend primarily on analysing the isostatic response to loads created by volcanism, thrust mountain building, ice sheets, or sedimentation at a conti- nental margin. From analyses of the flexure ac- companying seamount loading (e.g. [1]) there has emerged a clear understanding of the rigidity of oceanic lithosphere and how rigidity increases with effective thermal age (here thermal age refers to the time since the last major thermal or oro- genic event). Rigidity can be parameterised by the effective elastic thickness, Te, which for oceanic regions generally varies between 5 and 40 km [11. Establishing an empirical relationship between T e and thermal age for the continents is proving far more difficult than for the oceans (e.g. [2]). * Now at: Department of Earth and Space Science, State University of New York at Stony Brook, Stony Brook, NY, USA Because of its varied and complicated history, and, in many cases, its much greater age, conti- nental lithosphere shows a wider range of T e values (5-130 km) [3-11]. Rigidity in continental lithosphere is, however, thought to have a first order dependence on thermal age, [12], and a plot of rigidity versus age for continents is best explained by a 250 km thick cooling plate model, in which T e is controlled by the depth to the 450 °C isotherm [13]. Karner and Watts [5], Fowler and McKenzie [14] and Watts [15] examined the lithospheric strength of sediment-loaded, Atlantic-type conti- nental margins (e.g., Northwestern Australia, Lord Howe Rise (New Zealand), and the Eastern United States). Gravity modelling of the sediment loads indicate low effective elastic thicknesses along these continental margins. In all locations sedimentation began immediately after continen- tal rifting when the lithosphere was still weak, and significant portions of the total thicknesses accumulated while the lithosphere was thermally young. In contrast, the loading problem analysed here is quite different. The Giant Foreset Forma- 0012-821X/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reserved