The seismic velocity structure of a traverse through the Bay of Islands Ophiolite Complex, Newfoundland, An exposure of oceanic crust and upper mantle
Although the ophiolites are widely recognized as segments of oceanic crust emplaced on land, direct correlation between the ophiolites and the oceanic crust has proven difficult owing to the near absence of common criteria on which to base a comparison; the ophiolites are defined petrologically, while the oceanic crust is defined largely in terms of seismic structure. To bridge this gap the seismic velocity structure of a traverse through the Blow-Me-Down massif of the Bay of Islands ophiolite complex, Newfoundland, has been reconstructed in detail from values of compressional (Vp) and shear (Vs) wave velocity measured in the laboratory under conditions of hydrostatic confining pressure and water saturation thought to approximate conditions in the oceanic crust through oriented samples collected from 60 closely spaced sites of known stratigraphic level. The velocity structure thus determined is indistinguishable from that of normal oceanic crust: The uppermost velocity unit in the massif consists of 0.5 km of prehnite-pumpellyite facies metabasalt with Vp ≤ 5.70 and Vs ≤ 3.10 km/s, underlain by 0.8 km of greenschist facies pillow basalts and brecciated dikes with Vp ≤ 6.20 and Vs ≤ 3.35 km/s. Between 1.3 and 6.4 km, in a thick unit composed of metadolerite sheeted dikes underlain by coarse-grained metagabbro grading downward through pyroxene and troctolitic olivine gabbro, Vp and Vs increase from 6.75 and 3.75 km/s near the top to 7.40 and 3.90 km/s near the base, respectively. This increase is gradational, except at 5.3 km, where a step increase in Vp to 7.40 km/s marks an increase in olivine content. A sharp velocity inversion in Vp, caused by quartz-rich late differentiates, is found in the upper levels of this unit between 2.8 and 3.3 km. The deepest level of the complex, composed of ultramafics, is characterized by values of Vp and Vs of 8.4 and 4.9 km/s, respectively. A comparison of the seismic velocity structure and petrology of the traverse across the Blow-Me-Down massif with oceanic seismic structure suggests that at many sites in the ocean basins, (1) layer 2 consists of prehnite-pumpellyite and greenschist facies pillow basalts and brecciated dikes, (2) the layer 2-3 boundary separates greenschist facies metabasalts and brecciated dikes at the base of layer 2 from epidote-amphibolite facies sheeted dikes at the top of layer 3, (3) layer 3 consists of metadolerite sheeted dikes underlain by metagabbro, pyroxene gabbro, and troctolitic olivine gabbro, (4) the 7.4-km/s basal layer observed in sonobuoy studies consists of interlayered olivine gabbro, troctolite, and plagioclase peridotite, (5) the Mohorovičić discontinuity represents a relatively sharp transition from gabbro to dunite and peridotite, and (6) pronounced, but laterally discontinuous, velocity inversions may be present at the base of layer 2 below the relatively high velocity prehnite-pumpellyite facies metabasalt level and at intermediate levels in layer 3 in association with late differentiates.