EOCENE TECTONIC EVOLUTION OF THE NORTH CASCADES

The North Cascades crystalline core (Cascades core) is a > 60 km thick, 96- to 45 Ma continental magmatic arc. Waning magmatism in this arc occurred during regional dextral Eocene transtension, broadly coeval ridge subduction, and rapid basin subsidence, and overlapped with rapid exhumation of partially migmatitic Skagit Gneiss Complex (SGC). Exhumation of the complex, the highest-grade and possibly deepest part of the arc, is marked by nearly isothermal decompression from ~ 10 to < 5 kb. In the northern part of the SGC, initially gently dipping ductile shear zones typically, but not invariably, record top-to-NNW shear that occurred after the peak of metamorphism. Lineation patterns are complex, but overall indicate broadly orogen-parallel stretching. The SGC is separated from lower-grade rocks on the east by the Eocene dextral-normal Ross Lake fault zone, and the locally preserved upper boundary is the top-to-N Ruby Mountain shear zone. This shear zone may be part of an openly folded regional structure that includes a major top-to-N decollement that facilitated roughly coeval exhumation of the high-P Swakane Gneiss to the south (Paterson et al., 2004). This structure may be the roof a channel that records orogen-parallel to –oblique, mid- to deep-crustal flow during transtension. Eocene magmatism in the North Cascades was highly variable in composition and ranges from mafic dike swarms and a layered complex, to calc-alkaline granodiorite, to A-type granite. Nd isotopic values are also more variable than for older components of the arc (Matzel, 2004). Our mapping suggests that > 90% of the southern and central SGC is leucocratic tonalite orthogneiss. U-Pb dating is compatible with many of the orthogneiss protoliths having ca. 60- to 48 Ma crystallization ages, and the SGC thus represents the focus of Eocene magmatism. Eocene dike swarms also intrude the Cascades core and oldest Eocene sediments. Extension directions from the swarms vary substantially, but average WNW-ESE and are at a significant angle to ductile stretching in high-grade rocks; this implies decoupling of shallow and deeper crust. Overall, the Eocene tectonics of the North Cascades is marked by subhorizontal crustal flow, complex strain fields, diverse magmatism, and rapid basin subsidence that in part reflect major changes in the nearby plate boundary.