OPHIOLITES AND THE OCEANIC UPPER CRUST; TWO-WAY TRAFFIC IN SCIENTIFIC CONCEPTS

Current understanding of the processes that construct and modify the oceanic upper crust has been the result of nearly 50 years interplay between concepts derived from ophiolites and those derived from active mid-ocean ridges. The Troodos ophiolite in Cyprus has played a central role in this exchange, since there the upper ophiolitic crust is preserved for over 100 km across strike, and has scarcely been deformed or heated more than a few tens of degrees since it formed in an arc environment over 90 million years ago. Early in the plate-tectonic debate, the presence of the sheeted dyke complex in Troodos, demonstrating 70 km of 100% extension, was crucial in allowing acceptance of the new ideas. Later, the recognition of the exhalative nature of the ore deposits within the extrusive unit prepared the way for the discovery of black smokers, and then the oceanic observations in turn brought new insights into the structure and activity of the hydrothermal systems that had fed the ophiolitic deposits. These insights are now being returned to the oceans. Demonstration of the narrowness of the zone of crustal construction in the oceans, and the structure of upper crust at spreading axes, led to a reassessment of the structure of the upper crust in Troodos, in turn generating new approaches to the construction of the upper crust in the oceans. This two-way traffic seems set to continue, fuelled by the contrasting types of information that can be gained from each environment. In the oceans, rates of processes can be measured, and the spatial distribution of current activity determined. In ophiolites one can walk around on cross-sections of the fully-formed crust, unhampered by the limited exposure available on the ocean floor. Because of chemical differences between even the most oceanic of ophiolitic crust and that of open ocean basins, parallels must be drawn with care; it is naturally safer to compare physical processes than petrological ones. But creative interplay between the two environments, so crucial early in the plate tectonic revolution, shows no sign of abating. This topic is not just history; it is also active science in the making.