Seismic refraction measurements in the northwest Indian Ocean

Abstract

Fifteen seismic refraction profiles in the northwest Indian Ocean are described. The results suggest a connection between the linear Maldive ridge and the Deccan traps of the Indian mainland. No anomalous upper mantle velocities have been found near the crest of the Carlsberg ridge. A great thickness of material with velocity 6.03 km/sec west of the Saya de Malha bank has been interpreted as an extension of the Seychelles granitic block to the south. At two stations between the Chagos archipelago and the Seychelles bank the Mohorovicic discontinuity appears to be less than 8 km below sea level.

Site 219

“…This conclusion has been borne out by the crustal velocities of 4.8 to 6.1 km/sec and 6.7 to 7.1 km/sec found by seismic refraction studies over the Ridge at latitudes 9°N, 2°S, and 5%°S (Francis and Shor, 1966). However, only at the northernmost station, situated very close to Site 219, was an upper mantle velocity detected, and this unreversed line indicated a crustal thickness of 17.5 km.…”

“…A similar outcrop exists on profile CD, and it has been suggested by Harbison and Bassinger (1973) that together they form part of a basement ridge extending from 9°N to 20°N. The proposed site seemed to offer the best chance of sampling the acoustic basement in a region near the crest of the northern part of the Laccadive-Chagos Ridge and had the additional advantage of control from a nearby seismic refraction station (Francis and Shor, 1966). The objectives of drilling the site were thus to: 1) Determine the nature of the true igneous foundation of the ridge and date it if possible.…”

“…No deeper reflections were discernible on the airgun records so that the true depth to igneous basement is still unknown. However, an estimate may be made from the results of Francis and Shor (1966). Their Station 1 detected a 3.85 and a 5.00 km/sec refractor.…”

“…When a comparison is made with Francis and Shor's (1966) nearby seismic refraction station at 9°N, their belief that the upper refracting horizon, with a measured velocity of 3.85 km/sec, was volcanic rock appears unfounded since velocities approaching this value are shown to be present in the glauconite calcareous sandstone sequence and reach 4.0 km/sec in the cherts and limestones above this.…”

The crustal structure and subsidence history of aseismic ridges and mid-plate island chains

“…Even where refraction data are available the results show considerable variability, especially in shallow crustal structure (Figure 1). Francis and Shor (1966) reported "Moho" beneath the ChagosLaccadive Ridge at depths of about 17 km. Similar mantle depths have been reported beneath the Iceland-Faeroe Ridge (16-18 km) by Bott et al (1971) and the Nazca Ridge (16 km) by Cutler (1977).…”