Emplacement of the Resurrection Peninsula ophiolite in the southern Alaskaforearc during a ridge-trench encounter

The 57 +/- 1 Ma Resurrection Peninsula ophiolite of southern Alaska's Chuga ch terrane formed in a near-trench environment as the Kula-Farallon ridge w as being subducted beneath North America. The magmatic sequence includes pi llow lavas, sheeted dikes, gabbros, trondhjemites, and a poorly-exposed ult ramafic section. The lavas show mid-ocean ridge basalt and are-like geochem ical signatures, interpreted to reflect compositionally di verse melts deri ved from near-fractional melting of a variably depleted mantle source, mixe d with variable amounts of assimilated continentally-derived flysch. A sedi mentary sequence overlying the ophiolite, here named the Humpy Cove Formati on of the Orca Group, preserves a continuous 3.6 +/- 1.4 Ma record of turbi dite sediments that were deposited on the ophiolite as it was transported t o North America and emplaced in the Chugach accretionary prism. A transitio n from thinly bedded turbidites at the base of the section to thickly bedde d turbidites is interpreted as a distal to proximal facies change recording the migration of the ophiolite toward the continent. Chemical trends verti cally through the sedimentary sequence show an increasing biogenic componen t diluting terrigenous matter 950 m above ophiolitic basalt, signaling a ch ange to more favorable biologic conditions or increased preservation of bio genic matter with proximity to the continent. Hydrothermal metals are not r esolvable in sediments interbedded with pillow lavas or in sediments above the ophiolite, suggesting that they were diluted by turbidite sedimentation . The top of the section is truncated by the Fox Island shear zone, a 1 km thick, greenschist-facies, west-over-east thrust related to the emplacement of the ophiolite into the accretionary wedge. The Fox Island shear zone is intruded by a 53.4 +/- 0.9 Ma granite, showing that the ophiolite formed, was transported to the North American continent, overthrust by a major accr etionary prism-related thrust, and intruded by granite all within 3.6 +/- 1 .4 Ma. Using these estimates for the timing of formation and emplacement of the ophiolite, with palinspastic reconstructions and plate motion vector t riangles, we suggest that the Resurrection Peninsula ophiolite was transpor ted up to 500 km (461 +/- 79 km) relative to North America from the place o f its formation at the Kula-Farallon ridge prior to its emplacement into th e North American margin. Since the best estimates of the amount of translat ion of the Chugach terrane come from paleomagnetism of the Resurrection Pen insula ophiolite, current estimates of margin-parallel strike slip need to be reduced considerably.