Structure and Metamorphism of the Magic Mountain – Johannesburg Mountain Area, North Cascades, Washington

The orogenic history of the Cascades Crystalline Core is addressed in the study of a small, central region that straddles the Entiat fault on the southwest flank of the Skagit Gneiss. This region contains an arc-derived terrane with a preserved stratigraphy of volcanic, clastic and pelitic petrofacies of the Cascade River unit on a basement of sub-arc plutonics, the Marblemount Meta-Quartz Diorite. The Magic Mountain Gneiss, derived from the Marblemount arc, is inferred to have formed as a sub-volcanic sill-like pluton that intruded the protolith of the Cascade River unit In contact with the Cascade River unit is the Napeequa unit, which contains pelitic schist, chert, mafic schist, marble and rare meta-peridotite. This unit is inferred to be a disrupted ophiolitic terrane thrust emplaced on the Cascade River unit, or a package of interlayered sediments and igneous rocks that are stratigraphically below the Cascade River unit and exposed in the core of an anticline. Metamorphic grade, pressure, and temperature northeast of the Entiat fault increase from southwest to northeast. Metamorphic isograds are parallel to and culminate in the Skagit migmatitic belt to the east. Metamorphic isograds overprint all of the macroscopic structures and lithologic contacts. From this it is inferred that major tectonism, which produced the macroscopic structures, predates and is separate from the peak metamorphic event that created the overprinting relations. Peak metamorphism was accompanied by flattening on steep northwest striking foliation, and by northwest-southeast dextral strike-slip translation. The fabric within the Magic Mountain Gneiss and at the contact between the Magic Mountain Gneiss and the Cascade River unit does not fit this trend. This fabric is parallel to the folded contact. The folding of the fabric may be a result of a northeast-southwest shortening that formed northwest-southeast-trending open folds after the regional transpressive deformation, or the fabric may represent an older event and was preserved in a low strain region due to strain partitioning. These findings indicate that syn-metamorphic deformation was transpressive and do not support a thrust-loading model for the peak metamorphism in this area of the Cascade Crystalline Core