DEFORMATION AND INTRUSIVE PATTERNS DURING EOCENE TRANSTENSION: EASTERN PART OF THE SKAGIT GNEISS COMPLEX, NORTH CASCADES, WASHINGTON

The North Cascades crystalline core is inferred to record a shift from a dominantly transpressional to transtensional tectonic regime in the Eocene at ~ 55 Ma. The Late Cretaceous to Eocene Skagit Gneiss Complex (SGC), Eocene Ruby Creek heterogeneous plutonic belt (RCHP), and Paleogene dextral Ross Lake fault zone provide important insights into mid-crustal deformation and intrusion during the transtensional regime. The regional structure of the tonalitic to trondhjemitic SGC is dominantly antiformal with outcrop to km-scale, upright NW to more commonly SE-plunging folds. In the highway 20 corridor, the SGC maintains a gently to moderately S-SE-plunging lineation, including in the eastern part of the gneiss where foliation swings from NW-striking, to E-W-striking and dominantly S-dipping. This transition occurs in part near an extensional step-over in the Ross Lake fault zone. Fabrics become increasingly constrictional to the south in the study area and aspect ratios of deformed markers fluctuate from 2:1 to locally >10:1 (Lillian Creek metavolcanic breccia). The NW-trending, dominantly tonalitic RCHP intrudes the Ross Lake fault zone in the extensional step-over and has a 500-m-wide, gradational sheeted contact with the eastern SGC. This sheeted contact zone is characterized by numerous RCHP bodies and a series of gently to moderately S-dipping metasedimentary rafts, which separate orthogneiss sheets of the SGC. The contact marks the transition from the E-W-striking foliation of the gneiss to the NE-striking, NW-dipping foliation in the RCHP, and from well-lineated gneiss to intrusive rocks lacking a preferred lineation orientation. The intrusive style of the Skagit orthogneisses and Ruby Creek intrusions (sheets separated by rafts), constrictional fabrics, and transition of foliation orientation in the extensional step-over are interpreted to reflect regional transtension.