LINKING DEEP AND SHALLOW CRUSTAL PROCESSES IN AN EXHUMED CONTINENTAL ARC, NORTH CASCADES, WASHINGTON

The North Cascades crystalline core was a >55-km-thick Cretaceous magmatic arc that in the Eocene was marked by migmatization, magmatism, and exhumation of the deep crust coincident with subsidence and rapid deposition in nearby non-marine basins. This wide range of exposed crustal levels allows evaluation of linkages between deep and shallow-crustal processes during regional transtension. Two deeply exhumed regions are the Swakane Gneiss and migmatitic Skagit Gneiss Complex (SGC). Swakane paragneiss was exhumed from 11-12 kb in part during pervasive subhorizontal top-to-N shear, and provided detritus to an adjacent basin in the mid-Eocene. The SGC consists primarily of 76-45 Ma tonalitic orthogneiss. U-Pb dating of zircon and monazite from migmatites indicates that metamorphism, melt generation and crystallization occurred from ~71-47 Ma; the youngest dates overlap Ar/Ar hornblende and biotite dates, compatible with rapid cooling, probably in response to nearly isothermal decompression from 10 to <5 kb. Upright folds in the SGC have hinge lines parallel to gently plunging orogen-parallel (NW-SE) lineations, and are as young as 48 Ma. Gently to moderately dipping foliation, subhorizontal lineation, and constrictional domains are compatible with ductile transtension linked to dextral-normal slip on the bounding Ross Lake fault. South of the core, sediments were deposited in part at ca. 51 Ma in the Swauk basin and shortly afterward folded, and then intruded by 47 Ma Teanaway basalt dikes. Extension taken up by these dikes in >5-km-long transects ranges from 10-43%. Extension directions from Teanaway and other Eocene dikes are arc-oblique, whereas the brittle displacement field from faults is less consistent. Shallow-crustal extension from dikes is oblique (20-30⁰) to ductile flow in Skagit and Swakane rocks, implying decoupling of strain orientations in the brittle and ductile crust. Overall coupling is supported by the temporal coincidence between: basin formation, rapid deposition, and partial melting and ductile flow; and upright folding of the SGC and Swauk Formation. Arc-oblique to arc–parallel flow and brittle extension probably resulted in part from dextral shear along the plate margin and along-strike gradients in crustal thickness and thermally controlled rheology.