EXHUMATION OF A THICK CONTINENTAL MAGMATIC ARC IN THE NW CORDILLERA (USA)

The Cretaceous to Paleogene core of the North Cascades represents a thick (> 60 km) continental magmatic arc that is proposed to have formed the western margin of a broad orogenic plateau analogous to the Altiplano. Major crustal thickening occurred at ~ 95-85 Ma. The Eocene demise of this arc coincided with the onset of transtension and broadly coeval ridge subduction at ~ 50 Ma. Eocene exhumation of high-grade rocks temporally overlapped with extensive diking, brittle dextral-normal faulting, and rapid deposition in fault-bounded basins flanking the arc.

Eocene metamorphism and deformation were focused in two domains of deep crust that cooled rapidly at 45-50 Ma. Nearly isothermal decompression from ~ 10 to < 5 kb is recorded in migmatitic Skagit Gneiss exposed in the core of a regional, orogen-parallel (NW-SE) antiform. The gneiss displays tight to isoclinal, recumbent folds of foliation that are nearly coaxially refolded by upright, more open folds; axes and lineation plunge gently NW-SE to NNE- SSW. Initially gently dipping mesoscopic ductile shear zones mainly yield top-to-NNW shear, and largely formed during decompression. The Skagit Gneiss is separated from low-grade rocks on the E by an Eocene dextral-normal fault zone. A < 1 km thick, strongly deformed, gently dipping transitional zone from migmatite, through non-migmatitic gneiss to medium-grade rock marks the upper boundary on Ruby Mountain. The other deeply exhumed domain, the 11-12 kbar Swakane Gneiss, lies south of the Skagit in another antiform. It is bounded above by a decollement that excised considerable crust. Available age data are compatible with top-to-N motion on this structure occurring during deformation in the Ruby Mountain high strain zone. We speculate that these structures may represent the top of an orogenic channel marked by orogen-parallel to -oblique flow, which occurred during regional strike slip and extension. Dike swarms broadly coeval with the high strain zones resulted in ~ 25% extension in one well-studied, > 125 km2 domain. WNW-ESE extension from the dike swarms contrasts with more N-S ductile stretching in high-grade rocks. The upper crust thus may have been decoupled from deep crust during exhumation, perhaps by the inferred orogenic channel. The exhumation of this arc may be a useful analog for younger, less deeply exhumed arcs.