TECTONIC SIGNIFICANCE OF THE MAGIC MOUNTAIN GNEISS, NORTH CASCADES WASHINGTON

The Cascade crystalline core is composed of sedimentary and volcanic units intruded by Cretaceous plutons and regionally deformed and metamorphosed in the mid-Late Cretaceous. Considerable controversy exists over whether the tectonism occurred during thrusting or strike-slip orogenesis. This study focuses on the lithology and structure of the Magic Mountain Gneiss (MMG) in the Mt. Formidable area of the core in order to determine its origin and tectonic significance. The MMG (layered greenschists and leucogneiss) occurs as a gently SE-dipping slab overlying the Cascade River Unit (CRU, metavolcanic and metasedimentary rocks) and is also overlain by the CRU. Previous work suggests the MMG was either derived by metamorphism and thrusting of the Marblemount Metaquartz Diorite (MMQD, Triassic pluton) over the CRU (Tabor et al., 2001) or that it formed as a sill-like pluton affected by later tectonism (Dougan, 1993, Brown et al., 1994). Analysis of the MMG indicates that some greenschist layers, previously interpreted as derived from the MMQD or tectonic breccia, are instead metaconglomerate. Other greenschist protoliths include turbidites, volcaniclastic rocks, and volcanic rocks. Metaconglomerate occurs in the MMG and CRU near the contacts and throughout all levels of the MMG. Strain data from conglomerate clasts indicate low strain values with Y/Z=1.4-2.6 and X/Y=1.0-1.9. Crosscutting relations suggest the leucogneiss protolith intruded the greenschist protoliths. At the upper and lower MMG-CRU contacts leucogneiss layers become less abundant while greenschist layers increase in thickness and abundance. At least three generations of folds, typically with gently plunging hinges and steep limbs, fold the foliation of the MMG and CRU. The km-scale folds define a gently SE-dipping enveloping surface. The lack of metamorphic discordance, shown by previous workers, together with low strain, indicates the MMG-CRU contact is not a thrust. We conclude the MMG is a sill complex within the CRU. Multiphase folding of foliation in the MMG and CRU makes it unclear if the foliation is related to thrust or strike-slip tectonism. The moderate to gentle dip of the enveloping surface is unusual in this part of the Cascade core; further work is needed to determine if it is related to regional thrusting.