Abstract:
The tectonic structure of the area immediately to the north of the Raukumara Peninsula, New Zealand (approximately 35º-38ºS, 177º-182º) was largely unknown prior to this study. The structure of this area is of great interest since it is, simplisticly, a region of transition between the ocean-ocean crustal plate boundary of the Tonga-Kermadec active margin, and the ocean-continent crustal plate boundary of the Hikurangi - New Zealand active margin. A 10-day cruise in the area was run by Auckland University and Geophysics Division, DSIR, in November, 1980. Detailed contour maps of the bathymetry, together with magnetic (total force) and free air gravity anomalies of the area were produced from existing and new data. In addition, seismic reflection and refraction data were collected and collated with existing data, to delineate sediment and basement structures, and to assist in modelling of those structures with pronounced gravity and magnetic anomalies. Existing seismicity data were also utilized in the delineation the regional tectonic structure. Detailed contour maps of the gravity and magnetic data confirm the divergence in strike between the Kermadec Ridge and Trench, and consequent widening of the Raukumara Plain, south of about 35ºS. Two-dimensional gravity modelling indicates the existence of a very thick sequence of low density material beneath the inner trench slope (up to 6km), as well as beneath the Raukumara Plain (4.0-4.5km). The seismic reflection records show no visible structure or bedding associated with the inner trench slope sediments. However, within the Raukumara Basin, at least four sediment sequences, with compress ionaI velocities of 1.8-4. Okm/s, can be delineated bounded by clear unconformities. These sedimentary sequences indicate sediment supply from both the volcanic arc to the northwest and the East Cape Ridge to the southeast at different times. Basement is not always identifiable on the reflection records, but has a Vp=4.5-5.Okm/s. Two-dimensional modelling of the gravity and magnetic anomalies suggests the presence of several west-dipping slabs of high density, highly magnetic material beneath the East Cape Ridge and Raukumara Basin. Smaller blocks and slabs probably exist as well beneath the low density sediments of the inner trench slope. Measured velocities of 7.1-7.4km/s may be related to these bodies. Simplified two-dimensional gravity modelling across the Raukumara Peninsula and eastward across the Hikurangi plate margin indicate continuation of the basic structure (including the high density slabs) southward along the east coast of the North Island, New Zealand. The Raukumara region appears to be a 'typical' arc / forearc basin / outer ridge / accretionary prism / trench system with slabs of oceanic crust and possibly upper mantle material incorporated in the accretionary underplating. A dredged limestone of lower Pliocene age from the East Cape Ridge probably indicates an initiation of this phase of forearc basin development since about 5Ma. A tectonic reconstruction of the Tonga - Kermadec - Hikurangi subduction system for the last 100Ma is presented, and suggests that the Raukumara area has been part of the Kermadec / Hikurangi forearc region for at least the last 20-25Ma.