Revised ages of blueschist metamorphism and the youngest pre-thrusting rocks in the San Juan Islands, Washington

Abstract

New ages of rocks in the San Juan Islands, northwest Washington, significantly change our understanding of the evolution of the San Juan Islands thrust system. Re-examination of foraminifera-bearing mudstones at Richardson on Lopez Island indicates a late Aptian (112–115 Ma), not late Albian (100 Ma) age as currently presented in the literature. The age brackets of thrusting, marked by these pre-thrusting mudstones and 84-Ma post-thrusting sedimentary rocks, span a much longer period than previously thought, diminishing controls on rates of displacement in the thrust system and the timing of regional deformation in western Washington. New 40Ar/39Ar plateau ages of phengite in blueschist-facies meta-volcanic rock, also at Richardson, are 124 ± 0.7 Ma (2σ, late Barremian). These blueschist-facies volcanic rocks are in fault contact with the fossiliferous mudstones. Therefore, the blueschist-facies metamorphism at Richardson, previously inferred to be associated with the thrusting, now appears to have occurred prior to thrusting. Further, the Ar ages demonstrate that blueschist-facies fabric formed earlier than the thrust event and is therefore not directly useful in analyzing the thrusting kinematics. The Richardson 40Ar/39Ar age is similar to isotopic ages found in the eastern San Juan Islands and in the Shuksan blueschist terrane in the northwest Cascades, and thus fits into an emerging regional age pattern of blueschist-facies metamorphism during Late Jurassic – Early Cretaceous (up to Barremian) but not late Albian – Cenomanian. If this pattern is more broadly confirmed for the San Juan Islands, all the blueschist-facies metamorphism can be regarded as having formed in subduction zones elsewhere along the continental margin rather than in the anomalous setting of an on-land thrust system, as in the San Juan Islands.