40Ar/39Ar data from early Middle Jurassic ophiolites (187-184 Ma) in the western Brooks Range, Alaska, indicate that detachment-related metamorphism occurred ∼20 m.y. after crystallization and ∼20 m.y. before emplacement onto the Arctic Alaska margin. Hightemperature metamorphic rocks along the basal surfaces of many ophiolites have ages that are contemporaneous with ophiolite crystallization, suggesting that detachment and thrust faulting occur while the lithosphere is young (<10 Ma) and relatively hot. From these relations it has been generally assumed that detachment and initial overthrusting of oceanic lithosphere occur near the site of generation, such as a marginal basin or mid-ocean ridge. The new data from the Brooks Range ophlolites confirm previous indications that some ophiolites have much longer intervals between crystallization, thrust-related metamorphism, and emplacement. On the basis of these new results and data from other ophiolites, we propose that ophiolites originating within large ocean basins will generally have longer crystallization-to-emplacement intervals than intervals than the more commonly recognized marginal basin-type ophiolites.

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