Late Quaternary Marine Terrace Distribution, Stratigraphy, and Deformation at Mahia Peninsula, New Zealand

Abstract

At Mahia Peninsula, northern Hawkes Bay, there is a well preserved sequence of 7 late Pleistocene marine terraces that range in age from 40 to 212 thousand years and a Holocene coastal plain that is younger than 6.5 thousand years old. The Holocene coastal plain can be subdivided into 5 sublevels. The terrace sequence is the most complete sequence known for the last 212 thousand years in eastern North Island and perhaps in all of New Zealand. The terrace sequence exists as a consequence of rapid tectonic uplift (up to 3 mm/yr) and the presence of relatively soft, easily eroded Miocene age, mudstone bedrock. A thick succession of loess and tephra beds occur above marine deposits on each of the late Pleistocene terraces and provides the basis for assigning ages to the terraces. Interpretation of loess and marine terrace formation as a record or climate fluctuation, has improved the climato-stratigraphic subdivision of the period since the Last Interglacial period. In particular, Oxygen Isotope Stage 3 is now known to be characterised by 2 warm interstadials at 59 and 40 thousand years separated by a cool stadial. These interstadial periods bracket the deposition of Rotoehu Ash, an extensive tephra marker horizon in the North Island. An age for the Rotoehu Ash of 52 ± 7 thousand years has been determined using these stratigraphic and climatic considerations. Holocene aged sequences of estuarine sediments and open-coast marine terraces record the rapid rise of sea level following the last glaciation until about 6.5 thousand years BP. Since then tectonic movements have been predominant over sea level fluctuations as the major factor in coastal evolution. Each of the 5 sublevels of the Holocene coastal plain have been sampled for radiocarbon dating and this has enabled correlation of terraces over the whole of the peninsula. The stepped morphology of the terrace sequence implies an episodic mechanism of formation. Each Holocene terrace was formed during tectonic quiescence whereas risers between terraces represent tectonic uplift, in association with nearby, large magnitude earthquakes. The Holocene terrace sequence therefore represents a paleoseismicity record. Earthquakes of Mw 7 or greater have occurred at 300, 1600, 1900, 3500 and 4500 years BP. Marine terraces are uplifted at higher rates in the east of the peninsula and they are tilted by about 1° toward the WNW. Similarly, Miocene bedrock generally dips to the WNW but at 25-30°. Late Quaternary deformation therefore represents the continuation of post-Miocene tilting. Late Quaternary uplift and tilting is related to growth of the Lachlan Anticline, the axis of which lies immediately offshore of the eastern coast of Mahia Peninsula. The anticline is a major feature of eastern North Island, extending in a NNE direction for about 150 km from southern Hawke Bay to Ariel Bank, east of Gisborne. Interpretation of a seismic profile that crosses the Lachlan Anticline to the south of the peninsula indicates a major, west-dipping reverse fault, the Lachlan fault, on the steeper eastern flank. Similair anticline/syncline pairs and west-dipping reverse faults have been observed offshore, south of Mahia. These structures extend to the Hikurangi Trough, 50-150 km east of the eastern North Island coastline. This style of deformation is a consequence of subduction of the Pacific Plate at the Hikurangi Trough and shortening imposed in the frontal wedge of the overlying Australian Plate. Contractile deformation related to subduction extends westward to Mahia Peninsula. There is a marked change in the nature of tectonic deformation along the length of the Hikurangi subduction margin. In the southern sector, which extends from Wairarapa to Hawke Bay, there is accretion at the trough, under plating or trough-fill sediment, and reverse faulting. From Hawke Bay north to East Cape, tectonic erosion may be occurring at the trough, normal faults disrupt the trough-fill sediments and the frontal wedge, and the region is undergoing extension. Onshore the style of deformation changes from a contractional environment south of Hawke Bay to an extensional environment north of Gisborne. Between these two areas there exists a 100 km-wide transition zone with elements of both regimes. This change parallels a similar change offshore. The transition zone lies above a postulated northwest-trending tear fault in the subducted plate. The extensional tectonic environment in the northern sector of the Hikurangi subduction margin may be a recent phenomenon, related to initiation of rifting in the Taupo Volcanic Zone about 4 million years ago.