Most of the work planned for legs 2 and 3 of SONNE cruise 191 is embedded in the scientific rationale of the joint research programme COMET, which aims for the investigation and detailed study of methane in the global geo-/biosystem at different cold vent systems and associated gas hydrates. The working area of the cruise is the Hikurangi accretionary complex along the east coast of New Zealand’s northern island (Fig. 1). By the choice of this working area in the SW Pacific an important region on earth, from which only sparse information exists so far, is added to the investigation of the global phenomenom of fluid venting, gas hydrates and their impact on the global methane cycle. By the geologic setting of the working area a direct coupling of hot and cold fluid venting can possibly be observed where emanating cold fluids could have a hydrothermal signature and origin. This makes it to a unique research area where direct comparisons of the investigated cold vent systems with marine and terrestial hot vents systems can be conducted. Furthermore, there are hints for the occurrence of higher hydrocarbons in the area of Hawke's Bay which may have an impact on the fluid system comparable to the Gulf of Mexico.

The geologic processes and the present appearance of New Zealand’s east coast are strongly influenced by the intense compressional tectonics during the subduction of the Pacific beneath the Australian Plate. By this process a typical accretionary prism is formed accompanied by active volcanism, hydrothermalism and seismic activity. The Hikurangi continental margin lies at the southern end of the Tonga-Kermadoc-Hikurangi Subduction Zone, along which ocean-ocean- and ocean-continent-plate kollision occures. Directly north of 36°S the pacific plate is suducted to the west underneath the oceanic lithosphere of the Kermadoc Volcanic Arc. The southern extension of this arc forms the Taupo Volcanic Zone (Fig. 2a). South of 36°S the sediments of the Hikurangi Plateau are moved beneath the continental lithosphere of New Zealand. As the subduction gets more and more oblique from north to south (60° - 20°) the subduction rates get reduced from 4.9 cm/a (37°S) to 4.0 cm/a (42°S) and turn south of 42°S into a intra-continental collision along the dextral Alpine Fault of the southern island.
The submarine area of the central Hikurangi Margin (Fig. 2b) is divided into four different regions. On land the Axial Range builds up the back stop for the incoming sediments of the accretionary prism in the east with exposed grey wackes of Trias-Jurassic basement. This continues with the Inner Forearc containing accreted sedimentary units of the Upper Creataceous until Upper Miocene which are exposed on land and can be followed submarine up to the upper continental slope. Further offshore, Pliocene and Quaterrary turbidites of the trench build up the accretionary prism down to the Proto-Deformation-Zone. The deformed sediments of the NNE-SSW streching accretionary ridges contain deposits of the formerly passive margin and overlie gray wackes of the basement.

Based on the early Miocene age of the subduction induced volcanism in the northwest of the Taupo Volcanic Zone the start of the subduction is estimated at 23-25 Ma. Essentially for the East Coast Region is the subduction of the Hikurangi Plateau, a 10-15 km thick crust of MORB-basalt of pre-tertiary age which is covered by 500 to 2000 m thick sediment. By their subduction the Pliocene to Quarterrary turbidites in the Hikurangi Trough essentially build up the rapidly growing accretionary wedge. With a slope angle of less than 4° the continental slope of the Central Hikurangi Margin is very flat. This points to a very small friction of the down-moving plate with the overlying accreted sediments infering a very high water content of the sediments. Therefore, geologists from New Zealand speak of the 'subduction of a sponge', when they talk about the sediments of the Hikurangi Margins. It is assumed that in this region sediment packages with more than 3 km thickness are trapped between the approaching seamounts and the continental slope. The rapid accretion is shown in the area off central Wairarapa, where the deformation front has been moved within the last 0.5 Ma approx. 50 km offshore. Tracks of subducted seamounts are very common along the East Coast Region and lead to increasing steepness of the sediments followed by slides.