Metamorphosed oceanic gabbros provide a record of the cooling history of the lower crust near mid-ocean ridges, but the temperature range, rate, and location of subsolidus events are poorly known. We combine hornblende-plagioclase thermometry, statistical analysis, and thermal models to estimate precisely the temperature, time, distance from axis, and duration of metamorphism in East Pacific Rise gabbros from Hess Deep, ODP Hole 894G. Metamorphic hornblende and plagioclase, which formed during microfracturing and sea water penetration, equilibrated at a mean temperature of 716 +/- 8°C (90% confidence level). Comparison of the properties of the observed temperature distribution with those of model events indicates that metamorphism spanned <= 60°C. When combined with thermal models of fast-spreading centers, this implies that metamorphism was rapid (<= 6000 yr) and occurred 1-4 km off axis. Application of this approach to other gabbros will allow comparison of spatial and temporal characteristics of deformation and fluid flow in the lower oceanic crust as a function of ridge setting.