Geological structure of Wairarapa Valley, New Zealand, from seismic reflection profiling

Abstract

Abstract Three Mini-Sosie™ seismic reflection profiles across part of southern Wairarapa, North Island, with a combined length of 22 km roughly normal to the main structural trend, reveal shallow crustal structure down to a depth of 1.5 km in upper Cenozoic strata hidden below upper Quaternary gravel deposits. Our stratigraphic correlation of the seismic data is based on the closest mapped outcrops, as there are no deep drill-holes. (™ Trademark of Societe Nationale Elf Aquitane). A profile across the Huangarua Valley, 5 km southeast of Martinborough (Line 101,3 km long) defines an asymmetrical syncline verging southeastward and formed during the last one million years, as previously described from geological outcrop. Profiles across the Wairarapa Plain, intersecting Highway 2 at a point 3.5 km southwest of Masterton (Lines 201 and 202, combined length c. 19 km) reveal the following structures south-eastward from the Alfredton Fault at the north-western edge of the Wairarapa Plain. 1. Chester Anticline formed in upper Miocene-basal Pliocene strata, buried under the north-west flank of the Taratahi Syncline (see below). 2. Taratahi Syncline, 7 km wide, its north-west flank unconformably overlying Chester Anticline, its bottom flat and nearly level for 5 km, and its south-eastern flank rising in a concentric curve to the adjacent Peter Cooper Anticline. 3. Peter Cooper Anticline and on its south-east side the complementary Fern Hill Syncline, both concentric folds, and strongly developed in Upper Miocene and Pliocene strata, but weakly developed in Quaternary strata. We have mapped in outcrop two growing structures at the south-east side of the Wairarapa Valley (south-east end of Line 201) as the active Gladstone Anticline and Huangarua Syncline, which are well displayed in the profile. Subsidence of the region commenced near the beginning of Late Miocene time, c. 10 million years bp. There was a brief folding event in latest Miocene and/or earliest Pliocene time. There was little tectonic activity other than regional subsidence during most of Pliocene and early Quaternary time. An increase of the folding rate of nearly an order of magnitude commenced in the middle of the Quaternary, c. 1 million years ago, and continues at the present day.