Seismic reflection study across the Horowhenua coastal plain, North Island, New Zealand

Abstract

Three seismic reflection profiles and a gravity survey have been acquired along Muhunoa Road and Waiwiri stream in the Central Horowhenua region. Two seismic lines were shot with a Mini-Sosie source and 2400% fold of coverage and the third was shot using a dynamite source with 1200% fold of coverage. The gravity survey was conducted along the seismic lines with the average stations spaced at 300m intervals. The seismic lines are the first seismic surveys carried out in the region and have a combined length of 15 km. The general direction of the lines is normal to the main structural trend, and they form a cross section from the Tararua Range to the present coastline. The data was processed to industry standards with the DSIR Geophysics Division processing system. Various processing techniques were tested and described in the thesis. Minimum Entropy Deconvolution was tested on the seismic data. This process performed well on seismic data acquired with the Mini-Sosie source but performed poorly on dynamite data. Weiner Predictive Deconvolution performed acceptably for both sources. The seismic lines were interpreted by correlating reflection events and using available geological information from previous studies. The seismic interpretation was used as a model for the gravity interpretation. The two methods together gave good control on the location of the top of basement. The data interpretation reveals the depth to and structure of the buried Mesozoic basement rock and its overlying Cenozoic sediments. On the Western-most profiles, the young Cenozoic sediments show active tectonics in places, and changes of reflector dips indicating fault activity during sediment deposition. The Mesozoic-Cenozoic contact is in the form of a series of troughs and ridges more complicated than previously recognised (Figure 7.1). East of State Highway 1 the top of the basement is shallow (100m-300m in depth) and appears to be highly eroded and faulted with numerous reverse high angle faults. A deeper basement is revealed West of State Highway 1, which is separated into two sub basins by a structural high at Muhunoa. This structural high is interpreted to be the Southern continuation of the Poroutawhao High (Bekesi, 1989). It is bounded by a high angle fault to the East and gently dips to the West. Two types of faults have been interpreted:- high angle reverse faults in the East, towards the Tararua Range, and normal faults in the West. Normal faulting is an indication of tension and contradicts previous beliefs for this area. There is not enough evidence to conclude whether it is a local effect due to variations movement along strike slip faults, or a general regime. The interpretation suggests that the main fault across the region is the Levin Fault, located to the East of the Poroutawhao High. Another large fault that extends almost to the surface has been recognised on the Western-most profile.