Gravity models of the Wairarapa region, New Zealand

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This article was downloaded by: [Brown University Library]On: 29 October 2014, At: 18:02Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registeredoffice: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UKNew Zealand Journal of Geology andGeophysicsPublication details, including instructions for authors andsubscription information:http://www.tandfonline.com/loi/tnzg20Gravity 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:10.1080/00288306.1978.10424083To link to this article: http://dx.doi.org/10.1080/00288306.1978.10424083PLEASE SCROLL DOWN FOR ARTICLETaylor & Francis makes every effort to ensure the accuracy of all the information (theContent) contained in the publications on our platform. However, Taylor & Francis,our agents, and our licensors make no representations or warranties whatsoever as tothe accuracy, completeness, or suitability for any purpose of the Content. Any opinionsand views expressed in this publication are the opinions and views of the authors,and are not the views of or endorsed by Taylor & Francis. The accuracy of the Contentshould not be relied upon and should be independently verified with primary sourcesof information. 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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 ABSTRACT 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. INTRODUCTION 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'. GRAVITY PROFILES 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 Downloaded by [Brown University Library] at 18:02 29 October 2014 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 towards the Aorangi Mountains and one near Eketahuna. The models obtained by the two methods are similar. The differences are caused by the different assumed regional fields and by the complicated structure near Lines III and IV. The regional anomalies for the two-dimensional models were determined independently for each line, .and so, although they were each consistent Downloaded by [Brown University Library] at 18:02 29 October 2014 HICKS & WOODWARD - GRAVITY MODELS, WAIRARAPA +400 LINE I ... o IJNI kg -000 o 50 km 80 Eo m L-I --'---rh,', 0"""'-, 7r-,..j)~Oj -+---,------,-----,-1 ~ -500 ::' ~ -1000 ":' cr, +600 LINE 1Il o IJN Ikg ~700 t\oom -1000 -1500 -2000 -2500 +600 -400 Om -500 -1000 -1500 -2000 -2500 -3000 -3500 -4000 .. LINE V o 50 km 50km I I I I I I I I 90 70 +600 LINE 11 o IJN I kg -800 o 50km ~om -500 -1000 -1500 -2000 +600 LINE IV - 500 o ~~~o I -1000 -1500 -2000 Observed Isostatic Gravity Anomaly ,.--- Calculated Gravity Anomaly Regional Gravity Anomaly Fault Density contrast -0.47 Mg/m3 - 0.41 - 0.32 - O. 20 541 90 FIG. 2-Lines I-V two-dimensional gravity anomalies and geophysical models. Vertical exaggeration is 10: 1. Angles of 50 and 15 0 are shown for comparison. HICKS & WOODWARD - GRAVITY MODELS, WAIRARAPA +400 LINE I ... o IJNI kg -000 o 50 km 80 Eo m L-I --'---rh,', 0"""'-, 7r-,..j)~Oj -+---,------,-----,-1 ~ -500 ::' ~ -1000 ":' cr, +600 LINE 1Il o IJN Ikg ~700 t\oom -1000 -1500 -2000 -2500 +600 -400 Om -500 -1000 -1500 -2000 -2500 -3000 -3500 -4000 .. LINE V o 50 km 50km I I I I I I I I 90 70 +600 LINE 11 o IJN I kg -800 o 50km ~om -500 -1000 -1500 -2000 +600 LINE IV - 500 o ~~~o I -1000 -1500 -2000 Observed Isostatic Gravity Anomaly ,.--- Calculated Gravity Anomaly Regional Gravity Anomaly Fault Density contrast -0.47 Mg/m3 - 0.41 - 0.32 - O. 20 541 90 FIG. 2-Lines I-V two-dimensional gravity anomalies and geophysical models. Vertical exaggeration is 10: 1. Angles of 50 and 15 0 are shown for comparison. Downloaded by [Brown University Library] at 18:02 29 October 2014 542 N.Z. JOURNAL OF GEOLOGY AND GEOPHYSICS, VOL. 21, 1978 FIG. 3-Wairarapa region three-dimensional regional gravity anomaly map. Contour interval is 100 JLN/kg. with the local geology, they did not necessarily form a smooth regional anomaly field between the lines. The position, and hence the dip, of the West Wairarapa Fault is better determined by the two-dimensional models, whereas the general shape of the Trough is better determined by the three-dimensional model, which used a regional anomaly field which was smoothly varying over the whole region. Figure 5 shows a structural contour map of the Ceno-zoic-Mesozoic boundary in the Wairarapa, giving depths in kilometres below sea level. This model was con-structed by combining the three-dimensional geophysical model with geological information. The model shows that the major feature on the Mesozoic surface in the Wairarapa is the Wairarapa Trough. In the south the Trough is divided by a spur of Mesozoic rock extending from the Aorangi Mountains almost to Carterton, while in the north another arm is found near Eketahuna. The margins of the Trough are defined by outcrops of Mesozoic basement. They are mostly steep and probably fault-controlled, but the eastern and southern margins in places dip at less than 10 under the Cenozoic material. Significant fault-lines (from Kingma 1967) are shown as dotted lines in Fig. 5 (see also Fig. 1). The western margin of the Trough generally coincides with the West Wairarapa Fault; this fault is steeply dipping (apparently vertical) in the north, and reverse in the south (apparent dip about 40 at Line IV and about 15 at Line V). At Line V the West Wairarapa Fault has split into three branches. The model shown in Fig. 2 assumes that the central branch is vertical and controls the model at depth, while the eastern branch is strongly reversed and controls the model for the upper 1500 m. The western edge of the eastern arm of the Trough seems to be formed by two faults, the Dry River Fault and another a few kilometres east. The deepest point of this arm is 2300 m below sea level, on the Dry River Fault. The deepest point in the main Trough is about 3200 m below sea level, at the West Wairarapa Fault. To the north, the Trough shallows to less than 500 m below sea level, although there is a local deepening to 2500 m near Masterton. Near Eketahuna, there i~ another extension to the Trough, bounded to the west by the Wellington Fault. The deepest point here seems to be about 500 m below sea level, but this arm is not well defined by this study. Both this arm and the main 542 N.Z. JOURNAL OF GEOLOGY AND GEOPHYSICS, VOL. 21, 1978 FIG. 3-Wairarapa region three-dimensional regional gravity anomaly map. Contour interval is 100 JLN/kg. with the local geology, they did not necessarily form a smooth regional anomaly field between the lines. The position, and hence the dip, of the West Wairarapa Fault is better determined by the two-dimensional models, whereas the general shape of the Trough is better determined by the three-dimensional model, which used a regional anomaly field which was smoothly varying over the whole region. Figure 5 shows a structural contour map of the Ceno-zoic-Mesozoic boundary in the Wairarapa, giving depths in kilometres below sea level. This model was con-structed by combining the three-dimensional geophysical model with geological information. The model shows that the major feature on the Mesozoic surface in the Wairarapa is the Wairarapa Trough. In the south the Trough is divided by a spur of Mesozoic rock extending from the Aorangi Mountains almost to Carterton, while in the north another arm is found near Eketahuna. The margins of the Trough are defined by outcrops of Mesozoic basement. They are mostly steep and probably fault-controlled, but the eastern and southern margins in places dip at less than 10 under the Cenozoic material. Significant fault-lines (from Kingma 1967) are shown as dotted lines in Fig. 5 (see also Fig. 1). The western margin of the Trough generally coincides with the West Wairarapa Fault; this fault is steeply dipping (apparently vertical) in the north, and reverse in the south (apparent dip about 40 at Line IV and about 15 at Line V). At Line V the West Wairarapa Fault has split into three branches. The model shown in Fig. 2 assumes that the central branch is vertical and controls the model at depth, while the eastern branch is strongly reversed and controls the model for the upper 1500 m. The western edge of the eastern arm of the Trough seems to be formed by two faults, the Dry River Fault and another a few kilometres east. The deepest point of this arm is 2300 m below sea level, on the Dry River Fault. The deepest point in the main Trough is about 3200 m below sea level, at the West Wairarapa Fault. To the north, the Trough shallows to less than 500 m below sea level, although there is a local deepening to 2500 m near Masterton. Near Eketahuna, there i~ another extension to the Trough, bounded to the west by the Wellington Fault. The deepest point here seems to be about 500 m below sea level, but this arm is not well defined by this study. Both this arm and the main Downloaded by [Brown University Library] at 18:02 29 October 2014 HICKS & WOODWARD - GRAVI'IY MODELS, WAIRARAPA 543 6050 6000 "' .. ', "eG "05 5950 ....... .05 .12 Depth below sea-level (km) G Ground ~evel o o '" N FIG. 4-Three-dimensional geophysical model of Wairarapa region, showing point depths (below sea level) to the Cenozoic-Mesozoic boundary. Density contrast is -04 Mgjm'. The dotted triangles show the lateral boun-daries of the vertical triangular prisms used in the analysis. Trough seem to deepen north of the survey area. The southern margin of the Wairarapa Trough is not defined, but offshore work by Magellan Petroleum N.z. Ltd (Ghani 1971) suggests that the feature continues south-wards. CONCLUSIONS Interpretation of the published gravity anomalies in the Wairarapa region has shown that the Wairarapa Trough is a fault-angle depression, with up to 3200 m of Cenozoic material overlying the Mesozoic surface east of the West Wairarapa Fault. An arm of the trough north of Eketahuna is bounded by the Wellington Fault, and another arm south of Carterton is bounded by the Dry River Fault. The structural boundaries south of Carter ton do not fully coincide with the faults shown by Kingma (1907), but elsewhere the structure is very similar to that postulated by Kingma on geological evidence. The general structure also agrees with that shown by Heine (1964); differences in detail and in depth are probably a result of more data being avail-able, of the choice of a lower density contrast between Mesozoic and Cenozoic material, and of the develop-ment of more rigorous interpretation methods. ACKNOWLEDGMENTS We wish to thank Trevor Hunt for his many useful comments and suggestions. REFERENCES COWAN, MARGARET; HATHERTON, T. 1968: Gravity surveys in Wellington and Hutt Valley. N.Z. Journal of Geology & Geophysics 11 (1): 1-15. FERRY, LYNLEY M.; DOONE, ANNE 1974: Sheet 12, Wellington. "Gravity Map of New Zealand 1 : 250 000". N.z. Dept of Scientific an~ Industrial Research, Wellington. GHANI, M. A. 1971: "Submarine geology of part of Cook Strait". Magellan Petroleum N.Z. Ltd. Open file. N.2. Geological Survey, Petroleum Report 574. Downloaded by [Brown University Library] at 18:02 29 October 2014 544 N.Z. JOURNAL OF GEOLOGY AND GEOPHYSICS, VOL. 21, 1978 ~ N' o o co FIG. 5-Three-dimensional model of the Wairarapa region basement structure, showing depth (below sea level) to the Cenozoic-Mesozoic boundary. Contour interval is o 5 km. Dotted lines represent faults shown in Fig. 1. HEINE, R. W. 1964: Gravity survey in the middle Wai-rarapa. N.Z. Journal of Geology & Geophysics 7 (1): 185-91. HUNT, T. M. 1969: Gravity survey of the lower Awa-tere district, Marlborough, New Zealand. N.Z. Jour-nal of Geology & Geophysics 12 (4): 633-42. KINGMA, J. T. 1967: Sheet 12, Wellington. "Geological Map of N.Z. 1 : 250 000". N.Z. Dept of Scientific and Industrial Research, Wellington. TALWANI, M.; WORZEL, J. 1.; LANDIS MAN, M. 1959: Rapid gravity computations for two-dimensional bodies with application to the Mendocino sub-marine fracture zone. Journal of Geophysical Research 64: 49-59. WHITEFORD, CHRISTINE, M.; LUMB, J. T. 1975: A cata-logue of physical properties of rocks, vol. 2: listing by geographic location. Geophysics Division Report 106. WOODWARD, D. J. 1975: The gravitational attraction of vertical triangular prisms. Geophysical Prospecting 23: 526-32. Downloaded by [Brown University Library] at 18:02 29 October 2014

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