The Fiordland seismic refraction survey, 1974-5

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Broadbent, M.; Davey, F.J. 1978 The Fiordland seismic refraction survey, 1974-5. Wellington: DSIR, Geophysics Division. Report / Geophysics Division 124 119 p.

Abstract: A seismic refraction survey has been conducted over a major positive isostatic gravity anomaly, of magnitude 1600 micro N/kg, in Fiordland, south-west New Zealand. The measurements were made to check an interpretation of the gravity observations which suggested that the crustal thickness below the positive gravity anomaly might be less than 10 km. The seismic measurements were along two principal lines. Line A, 73 km long, ran from Dusky Sound to Doubtful Sound along the crest of the anomaly, being aligned approximately north-east to south-west and parallel to the coast. Line C, aligned approximately perpendicular to the coast, passed over the positive gravity anomaly. It was about 102 km in length and it ran from 17 km offshore, south- eastwards through Doubtful Sound. The survey was conducted using both land and marine techniques. Analyses of the travel-time and distance data collected on line A indicate that rocks with seismic P-wave velocities of about 5.1, 5.7, 6.5, and 6.8 km/s have been detected. Some evidence suggests the existence of a layer with a P-wave velocity of 7.3 to 7.4 km/s. The rock with velocity 5.1 km/s occurs mainly below the south-west end of the line. The depths to the rock layers with velocities of 6.5, 6.8 and 7.3 km/s have been calculated to range between 1 and 3.6 km, 3 and 7.2 km, and to be 8.2 km below sea level respectively. The depths are in general greater towards the south-west end of the line. Seismic P velocities of 5.3 and 6.5 km/s were detected below line C. Below most of Doubtful Sound, the depths of the boundaries between rocks with velocities 5.3, 6.5 and 6.8 km/s are between 1.4 and 1.8 km, and 2.6 and 3.8 km respectively. The seismic boundaries are inferred to plunge steeply down below line C near the coastline, and to descend eastwards less steeply near the nead of Doubtful Sound. The maximum seismic velocity observed implies that the base of the crust was not detected and that the crustal thickness is likely to exceed 20 km. The results suggest that rocks with velocities normally associated with the lower parts of the crust occur below the positive gravity anomaly at an unusually shallow depth for a continental area, and for New Zealand.