STRUCTURAL AND TECTONIC HISTORY OF THE MT. FORMIDABLE REGION, NORTH CASCADES, WASHINGTON

Deformation events associated with crustal thickening are preserved in the Mt. Formidable region of the North Cascades, a Cretaceous orogen and continental arc at the southern end of the Coast Plutonic Complex. The kinematics and timing of deformation in this area during burial, magmatism and metamorphism are the focus of this study.

The study area consists of supracrustal units and associated metaplutonic rocks that are part of a Triassic arc sequence. Rock units include the sub-arc Marblemount Meta-Quartz Diorite, the supra-arc metaclastic and metavolcanic Cascade River unit, interlayered greenschist and leucogneiss of the Magic Mountain Gneiss (MMG) and the ocean-floor Napeequa Schist.

New results indicate that rocks in the Mt. Formidable region experienced at least three generations of ductile deformation and two phases of metamorphism. F1 passive shear folds are isoclinal; the axial-planar, regional S1 foliation formed as a result of upper-greenschist to lower-amphibolite facies metamorphism (M1). D2 formed tight to isoclinal F2 passive shear folds and a local, incipient crenulation foliation (S2) at greenschist facies metamorphic conditions (M2). Restored F2 fold orientations are scattered, but suggest that D2 involved moderately steep shortening and possibly NE or SW shearing. Open to close F3 folds occur in Type I and II fold interference patterns with F2 folds. D3 is characterized by NE-SW shortening and flexural shear folding; D3 did not result in new fabric development or metamorphism. Variation of F3 axial planes suggests post-D3 deformation by a map-scale F4 structure.

New isotopic dates from igneous intrusions in the MMG provide important age constraints on D1 deformation. A U/Pb zircon age of a foliated, pre- D1 orthogneiss sill is 151+/-5 Ma. A late to post-D1 pegmatite dike, which is folded by F2, yields a U/Pb zircon age of 137+/-4 Ma. These ages bracket D1 deformation and regional metamorphism between the latest Jurassic to earliest Cretaceous. Dates from this study are considerably older than previous estimates for early deformation in this portion of the Cascades which has been inferred as mid to Late Cretaceous. Correlation of Mt. Formidable region rocks to similar Triassic arc units immediately north in British Columbia suggests that D1 may record a major deformational event related to accretion.