Seismic Velocities Measured in Rocks of the Troodos Igneous Complex

Abstract

Summary The Troodos igneous complex of Cyprus is an ophiolite body composed of ultramafics, gabbros, dolerites and pillow lavas. Seismic velocities measured in these rocks were expected to be similar to- those in the layers under the deep ocean. Seismic experiments made in the field in individual rock types revealed only low velocities typical of deep sea layer 2. We suggest that the low velocities are due to high porosity in the shattered rocks. Geophysical field work was carried out during March 1971, in order to measure the seismic velocities at outcrop of the rocks of the Troodos igneous complex. The aim of the experiment was to compare the velocities obtained with those of the oceanic crust already established by marine seismic refraction work. Financial support for the expedition came from I.B.M. (U.K.) Ltd grants to the University of Cambridge. A preliminary account of the results has already appeared (Matthews etal. 1971). The Troodos Massif is believed to be a fragment of Mesozoic oceanic crust which has been thrust up on land during the northerly movement of the African plate towards Europe (Gass & Masson-Smith 1963; Gass 1968). The Massif occupies the central and south-eastern part of the island (Fig. 1); the ultramafics form the domed central circular outcrop around the summit of Mt Olympus at 6403ft, and are pierced by a serpentinite diapir. The rocks belong to an igneous suite characteristic of ophiolite complexes, and comparable petrologically with rocks dredged from the deep ocean floor (Moores & Vine 1971). The plutonic members range from dunites, harzburgites and pyroxenites to olivine gabbros, gabbros and quartz diorite and are ringed by the Sheeted Complex which is a swarm of steeply dipping altered basic dykes, without intervening screens, possibly formed by sea floor spreading. The Pillow Lavas occupy the outer margin of the oval outcrop pattern, and are divided into Upper Pillow Lavas with few dykes and Lower Pillow Lavas with abundant basic dykes, separated by an unconformity recognizable in only a few places. Moores and Vine classify the rocks into 3 units: Pillow Lavas, Sheeted Complex and Plutonic Complex, and these are used for the purposes of the following discussion rather than the more complicated scheme of the Cyprus Geological Survey. They suggest that a correlation may be made with rocks of the oceanic crust and upper mantle. Experimental work was carried out after careful site selection, using the geological maps of the Survey memoirs (see Appendix), and the detailed field knowledge of Drs I. G. Gass and F. J. Vine. With three exceptions all the lines were shot using the