Lateral heterogeneity in the seismic structure of the oceanic crust inferred from velocity studies in the Bay of Islands ophiolite, Newfoundland

Abstract

Received 1981 June 26; in original form 1980 October 17 Summary. In the Bay of Islands ophiolite complex of Western Newfoundland, Lower Ordovician crust and upper mantle are exposed in four massifs. Two of these, the Blow-Me-Down and North Arm massifs, contain complete or nearly complete ophiolite successions ranging from clastic sediments at the top, through pillow basalts, sheeted dykes, gabbros, and peridotites at the base. The detailed velocity structure of the North Arm massif has been reconstructed from compressional and shear wave velocity measurements to confining pressures of 6 kbar and low pore pressures for 125 cores collected from $3 sites of known stratigraphic level and is compared with the previously determined seismic structure of the Blow-Me-Down massif. Within the upper 2km of North Arm, compressional wave velocities increase from 5.0 to 7.0 km s-' and shear wave velocities increase from 2.8 to 3.8 km s-'. Velocities at this level in the Blow-Me-Down massif are higher due to the presence of pumpellyite. In both massifs, the top of layer 3 occurs within the sheeted dyke complex at a depth of approximately 1.5 km and marks the transition from greenschist to amphibolite facies metadolerites. High level gabbros and plagiogranites at depths of between 2 and 4 km are responsible for velocity inversions. In North Arm, Poisson's ratios at this depth are relatively high (0.3 1-0.34); whereas in Blow-Me-Down, abundant quartz-rich plagiogranites give rise to low Poisson's ratios (0.25-0.27). The lowest crustal unit in North Arm, which consists primarily of gabbro and amphibolite, has average compressional and shear wave velocities of 7.0 and 3.8 km s-', respectively. Within this section (4-6 km) the velocities decrease slightly with increasing depth in North Arm, but increase abruptly at 5.5 km depth in Blow-Me-Down. In the North Arm massif, the transition from crust to mantle is rather gradual due to the presence of a relatively thick mixed zone of gabbro and ultramafics.