Seismic measurements in the Cook Islands, south-west Pacific Ocean

Abstract

Seismic refraction measurements were made along partly reversed profiles up to 5 km long on the volcanic islands of Rarotonga and Aitutaki (Southern Cook Group) and on the atoll of Manihiki (Northern Cook Group). In addition, the thickness of the crust was measured near Aitutaki along two reversed refraction profiles with a total length of about 120 km. On the atoll of Manihiki the coral thickness is only about 0·05 km in the centre but increases to about 0·5; km under the reef edge. The volcanic rocks beneath are dome-shaped; the velocity of these rocks increases with depth up to at least 5·7 km sec^-1. On the volcanic islands of Rarotonga and Aitutaki the basaltic rocks at the surface, which consist of a sequence of scoriaceous beds and denser flows, have a velocity of 4·05–4·35 km sec^-1; the velocity increases only slightly with depth. On Aitutaki these rocks are about 1·65 km thick along the seismic section and rest on denser volcanic rocks with a velocity of 5·95 km sec^-1. The thickness of the crust near Aitutaki is 6·4 ± 0·4 km; the velocity structure is different from that of the average South Pacific crust and the velocity of the upper mantle is significantly smaller (7·70 ± 0·05 km sec^-1). The crust is downwarped under. Aitutaki, the vertical deflection under the centre of the island being about 1·6 ± 0·4 km, which is too large to be explained by buckling of an elastic crust.