Gravity survey of the Middle Wairarapa

The Eketahuna district lies within the Axial Tectonic Belt in North Island, New Zealand and it overlies a subducting Pacific slab. It was strongly deformed during the late Cenozoic. The district contains several major NNE-trending faults, some of which have significant late Quaternary dextral displacements. The dextral Wellington Fault, in the west, is active along its entire trace in the sheet district, but other major faults are active along only parts of the traces, usually in the south. Many NNE-trending faults (synthetic faults) diverge from the major faults at c. 10-30°.Between the major NNE-trending faults, Late Miocene, Pliocene, and early Quaternary strata, thicker than suggested from gravity models, are commonly tilted to the northwest at 15-25°to form fault-angle depressions. Where the major NNEtrending faults are closely spaced, the strata between them usually dip steeply southeastwards. Most cross faults trend normal to the NNE-trending faults.Within fault blocks bounded by major NNEtrending faults, large folds vary in trend from parallel to c. 40°from the major faults. Parallelism of fault/fold trends may indicate that free slip occurred along the bounding faults whereas en echelon folds may indicate that shear strain was not entirely dissipated along these boundary faults. The orientations of drag folds adjacent to some of the major NNE-trending faults indicate that vertical components of faulting occurred along many of the major faults.On the structural contour map the following faults and folds, active during the late Cenozoic and/or Quaternary, are described and named (see Appendix 1): Cliff Road, Faulkner Road, Kaitawa, Newman, Nirvana, Rongokokako, Rongomai, and East and West Taiko (faults); Arakoa, Hinemoa, Kaiparoro, Kameru, Peep-o-day, and Rongomai (folds).

Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Structure of NZMS1 N153 Eketahuna sheet district and tectonics of Northern Wairarapa, North Island, New Zealand

Neef

1981

“…If the results are combined, it can be inferred that between the conducting surface layer and its highly resistive substratum, there is a region where the resistivity increases from its surface value to 100 nm at 14·5 km, and the 4 km boundary must be due to the two layered model assumed. Heine (1960), using gravity and geological data has constructed a profile with an assumed depth of 1 km to greywacke basement in the region of our field station. MacDonald (pers.…”

Section: Discussionmentioning

confidence: 99%

The Magnetotelluric method for locating major geological features and its application in the Wairarapa

Christoffel

Linford

1968

In the magnetotelluric analysis proposed by Cagniard, a resistivity profile of a homogeneous stratified earth is determined from the ratio of the tangential components of the natural electric and magnetic field fluctuations at the surface of the Earth. Two methods of this analysis are discussed and applied to measurements made in the W airarapa. One method gives a conducting surface layer of 4 km followed by a highly resistive substratum; the other gives a discontinuity at 14 km.

“…The nature and dip of the Wairarapa Fault at various locations has been studied using gravity measurements by Gresham (1967), Hicks & Woodward (1978), Bowler (1998), andMcClymont (2000). Heine (1964), Gibbs (1969), and Hicks & Woodward (1978) have presented more widespread gravity data from across the Wairarapa region. These studies indicate that the dip of the Wairarapa Fault varies along its length and in the southern part of the region is believed to be reverse.…”

Section: Introductionmentioning

confidence: 99%

A magnetotelluric transect of the Wairarapa region, New Zealand

McLoughlin

Ingham

Whaler

et al. 2002

Magnetotelluric (MT) soundings have been made at 14 sites along a traverse of the Wairarapa region at the southwest end of the active Hikurangi margin. Joint inversion of direct current resistivity soundings and the MT data indicates that the MT data are unaffected by static-shift. Analysis of the dimensionality of the data suggests that the transverse electric (TE) mode data (derived from an electric field parallel to the strike of the margin) may be distorted by 3D effects arising from the presence of Cook Strait and the non-continuity of structure along the margin. 2D inversion of the more robust transverse magnetic (TM) mode responses has therefore been used to derive a model of electrical resistivity structure beneath the transect. This shows the existence of two depressions containing lowresistivity sediments separated by a more resistive region which is inferred to correlate with a spur of Mesozoic greywacke extending north from the Aorangi Mountains. The same features were previously identified from gravity data. The thickness of sediments adjacent to the Wairarapa Fault is of the order of 3-4 km, whereas that in the western arm of the Wairarapa trough is c. 2-3 km. The resistivity structure of the southwest end of the Hikurangi margin is more complex than the structure determined farther north along the margin.