The imbricated frontal wedge of the central Hikurangi subduction margin is characteristic of wide (ca. 150 km), poorly drained and over pressured, low taper (~4°) thrust systems associated with a relatively smooth subducting plate, a thick trench sedimentary sequence (~3–4 km), weak basal décollement, and moderate convergence rate (~40 mm/yr). New seismic reflection and multibeam bathymetric data are used to interpret the regional tectonic structures, and to establish the geological framework for gas hydrates and fluid seeps. We discuss the stratigraphy of the subducting and accreting sequences, characterize stratigraphically the location of the interplate décollement, and describe the deformation of the upper plate thrust wedge together with its cover sequence of Miocene to Recent shelf and slope basin sediments. We identify approximately the contact between an inner foundation of deforming Late Cretaceous and Paleogene rocks, in which widespread out-of-sequence thrusting occurs, and a 65–70 km-wide outer wedge of late Cenozoic accreted turbidites. Although part of a seamount ridge is presently subducting beneath the deformation front at the widest part of the margin, the morphology of the accretionary wedge indicates that frontal accretion there has been largely uninhibited for at least 1–2 Myr. This differs from the offshore Hawkes Bay sector of the margin to the north where a substantial seamount with up to 3 km of relief has been subducted beneath the lower margin, resulting in uplift and complex deformation of the lower slope, and a narrow (10–20 km) active frontal wedge. Five areas with multiple fluid seep sites, referred to informally as Wairarapa, Uruti Ridge, Omakere Ridge, Rock Garden, and Builders Pencil, typically lie in 700–1200 m water depth on the crests of thrust-faulted, anticlinal ridges along the mid-slope. Uruti Ridge sites also lie in close proximity to the eastern end of a major strike-slip fault. Rock Garden sites lie directly above a subducting seamount. Structural permeability is inferred to be important at all levels of the thrust system. There is a clear relationship between the seeps and major seaward-vergent thrust faults, near the outer edge of the deforming Cretaceous and Paleogene inner foundation rocks. This indicates that thrust faults are primary fluid conduits and that poor permeability of the Cretaceous and Paleogene inner foundation focuses fluid flow to its outer edge. The sources of fluids expelling at active seep sites along the middle slope may include the inner parts of the thrust wedge and subducting sediments below the décollement. Within anticlinal ridges beneath the active seep sites there is a conspicuous break in the bottom simulating reflector (BSR), and commonly a seismically-resolvable shallow fault network through which fluids and gas percolate to the seafloor. No active fluid venting has yet been recognized over the frontal accretionary wedge, but the presence of a widespread BSR, an extensive protothrust zone (> 200 km by 20 km) in the Hikurangi Trough, and two unconfirmed sites of possible previous fluid expulsion, suggest that the frontal wedge could be actively dewatering. There are presently no constraints on the relative fluid flux between the frontal wedge and the active mid-slope fluid seeps. |
