THE NORTH CASCADES: THE END OF THE OROGEN AND THE END OF OROGENY

The Coast Mountains - Cascades belt represents the exhumed middle crust of an orogen that evolved first through terrane accretion, and subsequently and more profoundly during development of a continental arc. The North Cascades is the southern end of this > 1500 km long orogen; metamorphic and plutonic rocks of N Cascades affinity do not continue beyond the southern end of the range, neither at the surface nor in the subsurface. The spatial ends of orogens may be sites for major exhumation and linked tectonic-surficial processes, such as at the E and W ends (syntaxes) of the Himalayan collisional belt. The ends of ancient orogens, however, are not commonly well-exposed, so the N Cascades provide an opportunity to examine one end of an exhumed orogen that has not been much obscured by later thermal-tectonic events. The N Cascades resemble the syntaxes in that rocks here experienced major vertical tectonic motion: migmatitic rocks were exhumed from > 30-45 km depth along near-isothermal paths. The range is broadly antiformal; the syntaxes are antiformal or domal. Syntaxes may be sites of strong links between surface processes and exhumation/decompression. This may also have been the case in the Cascades: an intriguing feature of the range is the deep gorge incised by the Skagit River across the strike of the high-grade metamorphic belt. The Skagit R has an upper segment (Ross Lake) parallel to the strike of the orogen, and a lower segment (Diablo-Newhalem) that cuts across the orogen (cf. rivers in syntaxes). The timing of incision - and therefore its relationship to tectonic-thermal events - is not known. More work needs to be done to determine rates of decompression/exhumation and their relationship to crustal melting, and the timing of major erosion/incision in relation to fluvial/glacial systems and tectonic processes.

N Cascades orogeny ended in the Eocene (45 Ma) over a broad region of the northern Cordillera that had formerly been an orogenic plateau. Two triggers for the temporal end of orogeny are: external factors (change in direction/velocity of plates) and internal factors (collapse owing to development of thick, partially molten crust). There is evidence for both in the northern Cordillera, and the vast scale of coeval orogenic termination may indicate the interaction of external and internal factors.