gravity models of the wairarapa region, new zealand
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Gravity models of the Wairarapa region,New ZealandS. R. Hicks a & D. J. Woodward aa DSIR , Wellington , New ZealandPublished online: 12 Jan 2012.
To cite this article: S. R. Hicks & D. J. Woodward (1978) Gravity models of the Wairaraparegion, New Zealand, New Zealand Journal of Geology and Geophysics, 21:5, 539-544, DOI:10.1080/00288306.1978.10424083
To link to this article: http://dx.doi.org/10.1080/00288306.1978.10424083
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N.Z. Journal of Geology and G.ophysics Vol. 21, No.5 (1978) : 539-44
Gravity models of the Wairarapa region, New Zealand
S. R. HICKS AND D. J. WOODWARD Geophysics Division, DSIR, Wellington, New Zealand
Two-dimensional and three-dimensional gravity models of the Mesozoic basement topography in the Wairarapa show that the major feature is the Wairarapa Trough, aligned SSW-NNE through the area. The Trough has two arms, one north of Eketahuna and the other south of Carterton. All three features are fault-angle depressions, bounded to the west by the West Wairarapa, Wellington, and Dry River Faults, respectively, and reaching their greatest depths at these faults. A structural contour map of the Mesozoic surface in the Wairarapa shows that the basement is covered by more than 3000 m of Cenozoic material.
The West Wairarapa Fault is shown to be reverse in the south, with a dip as low as 15. The other faults are apparently near-vertical.
The Wairarapa region lies in the southern part of the North Island of New Zealand. Basement rocks are high-ly faulted and folded Mesozoic greywackes and argillites (Kingma 1967) which outcrop to form the Rimutaka-Tararua Range and the Aorangi Mountains (see Fig. 1). Between these, the basement is down-faulted and covered by thick Cenozoic sedimentary rocks and unconsolidated sediments. The West Wairarapa Fault is a major fault along the eastern margin of the southern Rimutaka-Tararua Range, while the Wellington Fault runs through the range to its eastern margin in the north. Kingma (1967) showed the Dry River Fault separating Creta-ceous from Jurassic rock in the Aorangi Mountains, and has projected the fault northeast to explain the boundary between Cretaceous and Pliocene rocks.
Kingma (1967) has postulated two geological cross sections in the Wairarapa and suggested that the Meso-zoic surface may be as deep as 4 km below sea level. Heine (1964) published gravity contour maps of the middle Wairarapa, and gave suggested boundaries of "the major physiographic depressions in the basement rocks". Assuming a density contrast of o 5 Mg/ma, Heine calculated the depths of these depressions to be 12 km. He conceded this density contrast is prob-ably over-estimated and the depths so obtained are probably minimum values.
Gravity anomalies are caused by differences in density between geological bodies: in this area by differences between the Cenozoic material and the sur-rounding Mesozoic basement rocks. Hence, examination of the gravity anomalies here can indicate the thickness of the Cenozoic material. The present analysis consists of three stages. First, a series of two-dimensional gravity profiles across the area was examined. From these, a three-dimensional model was established. Finally, a struc-tural model is presented, which combines the gravity models with geological details presented by Kingma (1967). All gravity data were taken from Ferry & Doone (1974). Rock densities were obtained from Whiteford &
Received 16 August 1977, revised 2 March 1978
Lumb (1975), gIvmg an average wet density for grey-wacke and argillite of 261 Mg/m' and for surface Cenozoic sedimentary rocks of 2 14 Mg/m'.
Five gravity isostatic anomaly profiles were constructed across the Wairarapa region (Fig. 1) by projecting onto each line all gravity observations within 5 km of the line (Fig. 2). The scatter of observed anomalies is a result of the distribution of the stations over a 10 km width; stations at the same position along the line are separated by up to 10 km perpendicular to the line, and so their gravity anomalies can be different.
The density of Cenozoic sedimentary material increases with age and depth of burial; thus the density contrast u between the Cenozoic and the Mesozoic material will decrea~e with depth. The density variation with depth within the Cenozoic material was estimated from the density-depth curves summarised by Hunt (1969). For simplicity of calculation it has been assumed that den sities are uniform in horizontal slabs 500 m thick. Hence, the assumed density of the Cenozoic body relative to the surrounding Mfsozoic basement is: from 0 -0' 5 km depth, u, from o 5-1,0 km depth, u, from 1,0-1 5 km depth, u. below 1 5 km depth, u.
-047 Mg/m'; -041 Mg/m'; -0,32 Mg/m"; -0,20 Mg/m'.
The regional anomaly values were obtained by draw-ing a smooth curve through those values of isostatic anomaly measured on Mesozoic basement rock. This is the "basement value" type of regional anomaly pattern used by Cowan & Hatherton (1968) and Hunt (1969). From the size and shape of the residual anomaly pat tern, models of the underlying geological structure were set up for each of the five profiles. Faults and Mesozoic rock outcrops as shown by Kingma (1967) were used, where possible, to fix the margins of the models. Each model is two-dimensional and is assumed to be infinite in the third dimension. The gravity effects of each
540 N.Z. JOURNAL OF GEOLOGY AND GEOPHYSICS, VOL. 21, 1978
UJ r C>
6053 '''' N
~non~ , 6000
''C~Oln : oulcrop~
__ ~ ___ . __________ W~~ FIG. I-The Wairarapa region survey area, showing profi Ie lines, relevant fault-lines and Mesozoic outcrops, sim-
plified from Kingma (1%7). Lines I & V coincide with Kingma's geological cross sections AA' & BB' respec-tively. Grid is the New Zealand Map Grid.
model were calculated using the method of Talwani ef al. (1959), and the shape and size of each model was adjusted until the calculated gravity anomalies matched the observed anomalies. The final models are shown in Fig. 2. Positions along the profiles are given as distances from the western end of each profile: for Line I this is as shown in Fig. 1, and for Lines II-V the origin is the west coast (for line III the west coast of Kapiti Island). The models shown in these figures have a vertical exaggeration of 10: l. Angles of 50 and 150 are shown for comparison. Faults are indicated by dashed lines.
THREE-DIMENSIONAL GRAVITY MODEL
A three-dimensional gravity model of the Wairarapa was constructed from the two-dimensional models described above. For simplicity of calculation, it was assumed that the density contrast rr in the model was uniform: this was taken as a mean of the values of rr used for the two-dimensional models, weighted for depth, giving rr = -0,4 Mg/m'. The model is con-structed of 48 vertical triangular prisms, with variable
depths and fixed lateral boundaries. Details of the method are given by Woodward (1975) . A leastsquares iterative method used 261 gravity stations to determine the model depth at each of the 39 triangular vertices, and to determine the coefficients of a second-order poly-nomial regional gravity field. The regional gravity field is shown in Fig. 3, and a plan of the model, showing point depths, is given in Fig. 4.
THREE-DIMENSIONAL STRUCTURAL MODEL
Both the two-dimensional and the three-dimensional analyses show that the major feature on the Mesozoic surface in the Wairarapa region is the Wairarapa Trough, which includes two extensions, one t