QM8 Hawkes Bay
The Hawke's Bay geological map covers 18 800 km2 of the central and eastern North Island. In the west it includes the southern part of Lake Taupo, active volcanoes of the Tongariro National Park and hill country of the Manawatu region. The area is bisected by the Ruahine, Kaweka, Kaimanawa and Ahimanawa ranges, which form part of the North Island axial ranges. East of the ranges, the map area includes the extensive Heretaunga, Ruataniwha and Takapau plains, hill country, and a coastline that stretches from the Mohaka River mouth in the north to the rugged shore east of Dannevirke in the south. Offshore, the continental shelf falls gently eastward for 50-150 km towards the Hikurangi Trough, where the sea floor is deformed into submarine highs and depressions as a result of the Pacific Plate subducting beneath the Australian Plate.The oldest rocks in the map area belong to the Late Jurassic to Early Cretaceous Torlesse (composite) terrane, and are exposed in the axial ranges. From west to east they consist of the Kaweka terrane, the Esk Head belt, the Pahau terrane and the Pahaoa Group. These units were accreted onto the eastern edge of Gondwana along a west-dipping subduction zone. The boundary between the Torlesse (composite) terrane and the older Waipapa (composite) terrane within the map area is concealed beneath volcanic rocks of the Tongariro National Park. Late Early Cretaceous to Oligocene rocks crop out only east of the axial ranges. Olistostromes, sandstone and mudstone of the Mangapurupuru Group were deposited during the transition from subduction to an extensional tectonic regime in the late Early Cretaceous to early Late Cretaceous. Farther east, the Glenburn Formation was deposited in a submarine fan setting. Deposition of coarse-grained sediments gradually decreased from the Late Cretaceous through the Paleogene, with the mudstone-dominated Tinui and Mangatu groups deposited in a passive margin tectonic setting.
The development of a new subduction system between the Pacific and Australian plates is marked in the Early Miocene Tolaga Group rocks by increased sedimentation rates, deposition of olistostromes and higher sandstone content, as well as localised uplift, facies changes and formation of sub-basins. In eastern areas, Late Cretaceous to Paleogene rocks were thrust over Early Miocene rocks. In the Late Miocene, the localised sub-basins further subsided and thick successions of deep water mudstone and turbidites were deposited. Shallow marine sandstone, limestone and conglomerate were deposited around the emerging Kaimanawa Mountains and Kaweka Range in the central North Island during the latest Miocene-earliest Pliocene.
In western areas, sandstone, mudstone and limestone of the Middle to Late Miocene Whangamomona Group were deposited in a continental shelf-slope system. In the east, uplift in the areas of the Hawke's Bay coastal hills and axial ranges had, by Earl y Pliocene time, created a narrow (150 MW of power output, although exploitation of this resource is unlikely in the near future as it is currently a popular tourist attraction.
The Tongariro Power Scheme generates 4% of New Zealand's electricity production. The Hawke's Bay map area is vulnerable to seismic, tsunami, landslide and volcanic hazards. The M7.8 1931 Hawke's Bay Earthquake has, to date, claimed the most lives in the country's history of earthquakes. Active faults are widespread throughout the map area. The active Mount Ruapehu volcano erupted in 1995/96, causing major disruption to air travel. The most recent eruption from Mount Ruapehu was in September 2007. Lahars from Crater Lake, the most recent of which occurred in March 2007 down the Whangaehu River valley, pose a risk to downstream infrastructure. Coastal erosion and tsuna mi are significant hazards in coastal areas, and landslides occur commonly throughout the map area, particularly in elevated land underlain by Miocene and Pliocene mudstone.