OPHIOLITES THROUGH GEOLOGICAL TIME

Ophiolites have contributed significantly to our understanding of plate boundary processes and the onset of plate tectonics in earth history, and to the formulation of plate tectonics theory. Dewey and co-workers were the first in the 1970s to use ophiolites to quantify the nature of various processes involved in mountain building. Since then, great progress has been made in documenting the origin – evolution of oceanic lithosphere through time, and the structural – compositional characteristics of modern and ancient oceanic lithosphere formed in different environments. Here, we review the distribution of world ophiolites through time –space, and the mechanisms of ophiolite formation–accretion. We also discuss the concept of “negative accretion”, tectonic erosion at convergent margins.

The oldest ophiolites in the 3.8 – 3.7 Ga Isua supra-crustal belt in Western Greenland consist of serpentinized peridotite, gabbro, MORB pillow – sheet lava flows that are covered by thick chert layers. Such dismembered ophiolites occur in other Archean terranes around the world. Accreted oceanic material in the Archean record generally includes MORB lavas overlain by chert sequences in greenstone belts and in high-grade metamorphic equivalents of granite-greenstone belts. This ubiquitous preservation of MORB lava – chert assemblages in the oldest Archean greenstone belts was due to the existence of abnormally thick MORB crust (~20 km) with thin upper mantle units (~20 km) and 200°K and higher mantle potential temperatures in the Early Archean (Komiya et al., 1999).

Dewey and Bird (1970) were first to point out the presence of ophiolites and serpentinites in orogenic belts as suture zones, representing the sites of ocean basin closure and continental collisions in the past. We re-examine this classic “suture zone” concept through a comparative analysis of Pacific- versus Collision-type orogenic belts, and the importance of ophiolites for the development – evolution of Pacific-type orogenic belts.