A seismic crustal section across the East Cape convergent margin, New Zealand

Crustal seismic reflection measurements were made along a 250-km profile across the Pacific-Australian plate boundary north of New Zealand. Data were recorded to 16 s two-way-time and have defined a major sedimentary basin, corresponding to the southern end of the Raukumara basin (a fore-are basin), reaching a depth of about 13 km. This basin contains a lower, generally conformable, sequence of sediments and an upper sequence dominated by several major uncomformities. The eastern part of this basin is cut by several, recently active, eastward-dipping, high-angle thrust faults which are interpreted to be caused by the recent convergence at the adjacent Pacific-Australian plate boundary. The eastern margin of the basin is marked by a sharp change in character on the seismic data. We interpret this feature as a terrane boundary coinciding with a major transcurrent fault which has juxtaposed a detached fragment of an island are (fore-are) with a thin (15 km) crust to the east against a passive margin sedimentary sequence to the west. The southern Raukumara basin is floored by Cretaceous age oceanic crust, fragments of which were obducted onto the North Island about 22 Ma ago. The area has had a complex history. A schematic model for the evolution of this margin during the past 22 Ma is developed, suggesting that the Raukaumara basin has developed as a combination of fore-are basin processes and oblique crustal extension.