RAPID EMPLACEMENT OF THE OMAN OPHIOLITE - THERMAL AND GEOCHRONOLOGICAL CONSTRAINTS

In understanding of ophiolite emplacement requires knowledge of the el apsed time between igneous crystallization and intraoceanic thrusting, and the rate and duration of that thrusting. Hornblende Ar-40/Ar-39 a ges demonstrate that the igneous oceanic crust in Oman crystallized an d cooled to similar to 825 K in 1-2 m.y. Hornblende ages from metamorp hic rocks and from cross-cutting dikes require that the basal metamorp hic thrust fault beneath the ophiolite also cooled below similar to 82 5 K in 1-2 m.y. Motion along the sole thrust accounted for 200 km of d isplacement at a rate of 100-200 mm/yr. On the basis of age relationsh ips and thermal considerations, we favor a two-stage model for the ini tial stages of Samail ophiolite emplacement: intraoceanic thrusting ov er < 2-m.y.-old lithosphere at 150 km/m.y. parallel to a spreading rid ge for 1-2 m.y., followed by equally rapid and brief thrusting over co ld and old lithosphere. Preservation of the Samail ophiolite is a dire ct result of its young age and positive buoyancy at the time of ocean closure, and we propose that all ophiolites that originated near sprea ding centers and were emplaced onto continents were young at the time of intraoceanic thrusting.