Late Neogene Vertical Tectonic Movements in Western Wairarapa, New Zealand

Abstract

Western Wairarapa is located near the western edge of the actively deforming plate boundary zone between the Australian and Pacific Plates, where it forms part of a transpressional zone associated with active strike-slip movement on the Wairarapa Fault and uplift of the Tararua Range. The Late Neogene (Late Miocene-Pliocene) history of vertical tectonic history has been deduced from a study of the sedimentary basins that have developed in this area since the beginning of the Taranaki epoch. Burial history curves have been drawn from paleobathymetric and original stratigraphic thickness data obtained from surface mapping and interpretation of a seismic reflection profile through the mapped area. Western Wairarapa was part of a fold and thrust belt on the ancient Hikurangi margin from the end of Middle Miocene time. The oldest sedimentary basins were small scale (≤10km) depressions in which sediments accumulated from ca 10My BP when marine transgression extended that far west in response to the combined influences of basement subsidence and eustatic rise of sea level. The basins were characterised by lateral facies change, thinning towards the uplifting eastern margin of westward tilting basins, and progressive migration of the deepest part of the basin to the west. During Pliocene time the basin characteristics changed to a longer wavelength fold (≥20km). At first (during the early Pliocene) the long wavelength basin was tilted to the east, but with the development of the southern portion of the East Coast Ranges from the middle of Early Pliocene time, the basin tilted to the west. During Middle to Late Pliocene time, the deepest part of the western Wairarapa basin was near the present site of the Wairarapa Fault Zone. Around 2 My B.P., there was another large scale change in basin orientation, as the westernmost part of the basin began to rise on the flanks of the growing Tararua Range. At the same time a large fold (Taratahi Syncline) developed in the axis of the Wairarapa Depression. The western Wairarapa basin complex subsided at an average rate of ca 0.33mm/yr during the Late Neogene. After ca 2My B.P., the westernmost part of the area was uplifted at an average rate of ca 1.5 mm/yr, while the adjacent syncline in the axis of the Wairarapa Depression continued (and still continues) to subside. The burial history of westernmost Wairarapa is similar to an earlier (Middle Miocene) cycle of subsidence and uplift in eastern Wairarapa. Such cycles have affected areas progressively further west through time, and are related to episodes of locking and unlocking of the plate interface near the leading edge of the subducted Pacific Plate as it moved westward across southern North Island beneath Wairarapa. The paired development of the Tararua Range and Wairarapa Depression is explained in terms of ramping of the Australian plate onto the subducted Pacific Plate. The Wairarapa Fault Zone has the appearance of a high angled positive flower structure: it is a relatively young feature which has not had a large influence on Late Neogene basin development.