GRAVITY AND MAGNETIC MODELING OF THE HIKURANGI PLATEAU

Gravity modelling of the Hikurangi Plateau, east of North Island, New Zealand, indicates the crust thickens from about 10 km in the north to about 15 km adjacent to the Chatham Rise. It appears to thicken by a lesser amount to the west, towards the Hikurangi Trough. The Chatham R ise crustal thickness is 23-26 km, thinning to the south towards the B ounty Trough and east of the Chatham Islands. The gravity models indic ate the eastern margin of the Hikurangi Plateau intersects the Chatham Rise at about 176-degrees-W. The area north of the Chatham Rise and e ast of 176-degrees-W could be either part of the Hikurangi Plateau cru stal block, or oceanic crust thickened by sedimentary deposits. Postul ated segmentation of the down-going slab beneath the North Island is n ot reflected in variations in crustal structure east of the trench. Ma gnetic modelling confirms the interpretation from seismic reflection d ata of widespread volcanism much of which may date from plateau format ion.