OROGEN-PARALLEL FLOW IN THE SOUTHERN SKAGIT GNEISS COMPLEX, NORTH CASCADES, WASHINGTON

The crystalline core of the North Cascades (Cascades core) is part of a thick (≥ 55 km) Cretaceous to Eocene continental magmatic arc. The highest grade and likely most deeply exhumed part of the arc is the Skagit Gneiss Complex (SGC), which is composed of NW-striking, partially migmatitic amphibolite-facies ortho- and paragneiss. This study focuses on the southern SGC, which contains abundant 59-49 Ma orthogneisses that record regional transtension. This area is of particular interest due to the juxtaposition of the complex with the older (89 Ma) tonalitic Bearcat Ridge Orthogneiss (BRO) and Triassic metasupracrustal rocks of the Cascade River-Holden assemblage.

Foliations, lineations, and kinematic indicators in these rocks record protracted orogen-parallel flow. Lineations within the southern SGC and adjoining rock units generally trend SE and plunge gently. Foliation in these rocks is NW-striking and folded, leading to variation in dips. In general, the BRO dips SW and is strongly foliated and lineated, and probably records a longer history of deformation than orthogneisses of the SGC. The SGC dips NE and is well-foliated and lineated, with 1-10 m thick zones of higher shear strain. Microstructures demonstrate that solid-state deformation in the SGC and BRO occurred at temperatures of ≥400-450 °C. After restoration of gently plunging open folds in the SGC and BRO, kinematic indicators record dominantly top-to-the-N or -NW shear in the SGC and top-to the-S or -SE shear in the BRO. A distinct zone (~3 km wide) of 59 Ma orthogneiss in the SGC contains top-to-the-SE, melt-filled ductile shear zones that dip moderately after restoration. These ductile shear zones precede top-to-the-N shear zones in the SGC and may be contemporaneous with BRO deformation. This change in kinematics indicates a complex history of deformation within the region.

Structural and geochronologic data within the SGC imply that orogen-parallel flow occurred until a change in regional strain and the formation of the large (~50 km wavelength), steeply plunging Eocene fold that is defined by deflected foliation and lineation in the SGC. This change was preceded by a shift from top-to-the-south shear, preserved in the BRO and older parts of the SGC, to top-to-the-north shear, recorded in younger parts of the SGC.