Newfoundland Ophiolites and the Geology of the Oceanic Layer

Abstract

THE significance and origin of ophiolites is a matter of interest to geologists. Many investigators interpret ophiolites as slices of oceanic crust which have been technically emplaced in orogenic belts^1–7. Moores and Vine⁵, Dewey and Bird⁶, and Coleman⁷ have proposed just such an origin for ophiolites as geographically diverse as Cyprus, Newfoundland and the Western United States. If ophiolites are representative of the geology of the oceanic crust, then they provide direct access to samples of oceanic crust and the underlying upper mantle—rocks at present inaccessible except for the fortuitous catch in dredge hauls located on tectonic escarpments of the ocean floor. Although we accepted the proposition that ophiolites, if well preserved and not altered by events unrelated to lithosphere accretion, are representative of oceanic crust, we wished first to test this proposition by observing whether representative ophiolite samples had measured velocities compatible with the seismic layering defined for oceanic crust. If the velocities of ophiolite samples were compatible with the range of seismic velocities recorded for oceanic crust, then we hoped to use the ophiolite measurements to understand better the details of the seismic layering of oceanic crust. Here we summarise the acoustic velocity data obtained from metadolerites, gabbros and cumulate gabbros from the Mings Bight ophiolite assemblage⁶ located along the northern coast of Newfoundland. A more detailed discussion of these ophiolite samples (J. J. P., in preparation) will give a thorough presentation of petrography, sample density and measured compressional wave velocity of each sample at pressure intervals from 1 atm to 7 kbar.