PROTRACTED BROADLY OROGEN-PARALLEL STRETCHING IN AN ARC DURING SUBDUCTION, NORTH CASCADES, WASHINGTON

The North Cascades represent a continental magmatic arc, which reached crustal thicknesses of > 55 km and records a protracted period of shortening, transpression, and late transtension during a ca. 100-45 Ma history of subduction. The southern part of the arc cooled rapidly to < 300°C by 80 Ma during crustal shortening, whereas the NE part of the arc experienced a much longer period of magmatism, metamorphism, and deformation; exhumation in the NE culminated between ca. 50-45 Ma during regional transtension. Extension directions inferred from widespread subhorizontal stretching lineations suggest that flow oblique to parallel to the arc occurred at least late during Cretaceous crustal thickening and Eocene thinning. The record of crustal flow is preserved in oceanic- and arc-derived meta-supracrustal rocks and orthogneisses that were metamorphosed to amphibolite facies and range up to 12 kb. The partially migmatitic, deepest crustal levels are preserved beneath openly folded, regional subhorizontal shear zones and experienced pervasive non-coaxial flow that predated the open folding. Restoration of folds indicates dominantly top-to-north shear on subhorizontal surfaces, which probably initiated at ca. 68 Ma (Paterson et al., 2004) and continued until ~ 47 Ma. This shear also overlapped dextral strike slip near the eastern margin of the arc.

We interpret arc-oblique to –parallel extension to result from gradients in crustal thickness and thermally controlled rheology. Crustal thickness of the North Cascades arc likely decreased from N to S. The Cretaceous fold-and-thrust belt that encompasses the arc and flanking units was driven by underthrusting from the west of the rigid Insular superterrane, and this superterrane and the contractional belt appear to terminate at the south end of the arc. Other along-strike variations in arc thickness and displacement are compatible with the NW plunge between the North Cascades and lower-grade rocks of the southern Coast Mountains. During at least part of the orogen-oblique to -parallel flow, the mid- to deep crust flowed southward from the region of 65-45 Ma magmatism and partial melting. We speculate that this crustal flow is broadly similar to the modern central Andes where orogen-parallel, lower-crustal flow has been inferred between the Puna Plateau and Altiplano.