Structural and metamorphic history of the Shuksan Metamorphic Suite in the Mount Watson and Gee Point areas, North Cascades, Washington

摘要

The blueschist-facies Shuksan Metamorphic Suite of the North Cascades has relict stratigraphy and sediment composition suggesting that it is of oceanic origin (broadly defined). The first metamorphic event affecting the rocks, which probably occurred on the sea floor, is a static epidote metasomatism of wide extent. The earliest penetrative structures (L₁, S₁) formed during latest Jurassic–Early Cretaceous blueschist-facies metamorphism, which is inferred to have occurred during subduction. In this event recrystallization and intense penetrative strain were closely linked. Late metamorphic isoclinal fold axes (F₂) and later crenulations (L₃) formed at high angles to the L₁ lineation. F₂ folds reflect extreme shortening perpendicular to the axis of the Shuksan Belt. Microscopic textural relations indicate that Ca-amphibole, not Na-amphibole, was stable during the development of F₂ and suggest that these structures record events related to uplift of the suite. L₁, and F₂ structures can be interpreted to indicate either that uplift was the result of a distinct tectonic event, or that metamorphism and uplift were part of a continuous tectonic process, with no change in the stress field. Final emplacement of the Shuksan Suite occurred along a fault system which at least locally is high-angle and includes zones of tectonic melange.