Emplacement of the Resurrection Peninsula ophiolite in the southern Alaska forearc during a ridge‐trench encounter

Abstract

The 57±1 Ma Resurrection Peninsula ophiolite of southern Alaska's Chugach terrane formed in a near‐trench environment as the Kula‐Farallon ridge was being subducted beneath North America. The magmatic sequence includes pillow lavas, sheeted dikes, gabbros, trondhjemites, and a poorly‐exposed ultramafic section. The lavas show mid‐ocean ridge basalt and arc‐like geochemical signatures, interpreted to reflect compositionally diverse melts derived from near‐fractional melting of a variably depleted mantle source, mixed with variable amounts of assimilated continentally‐derived flysch. A sedimentary sequence overlying the ophiolite, here named the Humpy Cove Formation of the Orca Group, preserves a continuous 3.6 plusmn;1.4 Ma record of turbidite sediments that were deposited on the ophiolite as it was transported to North America and emplaced in the Chugach accretionary prism. A transition from thinly bedded turbidites at the base of the section to thickly bedded turbidites is interpreted as a distal to proximal facies change recording the migration of the ophiolite toward the continent. Chemical trends vertically through the sedimentary sequence show an increasing biogenic component diluting terrigenous matter 950 m above ophiolitic basalt, signaling a change to more favorable biologic conditions or increased preservation of biogenic matter with proximity to the continent. Hydrothermal metals are not resolvable 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 accretionary 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 triangles, we suggest that the Resurrection Peninsula ophiolite was transported up to 500 km (461±79 km) relative to North America from the place of its formation at the Kula‐Farallon ridge prior to its emplacement into the North American margin. Since the best estimates of the amount of translation of the Chugach terrane come from paleomagnetism of the Resurrection Peninsula ophiolite, current estimates of margin‐parallel strike slip need to be reduced considerably.