Constraining Deformation in the Skymo Lake Area of the North Cascades, WA: Implications for the Rapid Exhumation of Arc Middle Crust Along Strike-Slip Fault Systems

Abstract

The Skymo Complex is a layered mafic intrusion that was emplaced at high temperature (>800 C) and low pressure (<4 kbar) into the Ross Lake Fault Zone - a major ~500 km long late Cretaceous-early Tertiary high angle shear zone in the Cordillera of western North America. To the northeast of the intrusion lies phyllites and schists of the Little Jack Terrane, and to the southwest lies upper amphibolite facies Skagit Gneiss of the North Cascades crystalline core. Previous work has determined that the Skagit Gneiss was exhumed nearly isothermally from ~30 km to <15 km in part along the high angle Ross Lake Fault Zone, but the timing of and mechanism behind this movement is poorly understood. This study exploits cross-cutting relationships around the Skymo Complex to constrain the rapid exhumation of the Skagit Gneiss along the high angle Ross Lake Fault Zone. The fabrics in the Little Jack Terrane are identified as identical to those in the Skagit Gneiss south of the study area. These fabrics may be coeval during an orogen-parallel transtentional regime in the early Tertiary, but are more likely unrelated, with the fabrics in the Little Jack Terrane representing a wide shear zone created during later dextral-normal exhumation of the Skagit Gneiss. Fabrics in the Skagit Gneiss of the present study area are rotated ~45 degrees clockwise from the gneiss south of the study area, and may imply a decoupling of upper and lower crust prior to exhumation, or drag folding during exhumation. Thermobarometric data indicates that the Skagit Gneiss was exhumed isothermally along a strand of the high angle Ross Lake Fault Zone between the gneiss and the Little Jack, and that the Skymo Complex was emplaced between the two lithologies at the end of the exhumation event. A late low angle brittle normal fault is demonstrably older than large scale exhumation, and accommodated minor vertical motion.