STRUCTURES AT THE TERMINATION OF A MAJOR CONTRACTIONAL BELT: NORTH CASCADES, WASHINGTON

The southern end of the crystalline core of the Cascades marks the termination of a Cretaceous, SW-vergent contractional belt that extends > 1000 km along the inboard margin of the Insular superterrane and formed during arc plutonism. Segments of this orogen have been considered as a classic fold-and-thrust belt, whereas structures in a 65-km-long swath from high-P (~ 10-11 kbar), amphibolite-facies rocks to nearly unmetamorphosed strata define a different hinterland-foreland transition in the Cascades. Outcrop to map-scale structures in the southern Cascades core mainly record nearly coaxial superposition of upright folds on tight to isoclinal folds. A few late, steep, SW-vergent reverse shear zones localized next to plutons account for less shortening than folding and cleavage formation. Overall, shortening in the metamorphic hinterland was probably dominated by pure shear. The boundary between the Cascades core and more external rocks is the Windy Pass thrust, a folded N-vergent backthrust with a minimum displacement of 14 km. In its hangingwall, the Jurassic Ingalls Ophiolite grades over 11 km from amphibolite facies in the N to prehnite-pumpellyite facies in the S. Steep cleavage formed by diffusive mass transfer weakens to the S where contacts and beds define open to tight, gently to moderately plunging folds that have steep N-dipping axial surfaces and weak axial-planar fabrics, and range up to 1 km in wavelength. Steep N-dipping reverse faults separate different “stratigraphic” levels of the ophiolite and reactivated Jurassic strike-slip faults formed in an oceanic fracture zone.

Folding and faulting initiated by 96 Ma, and ended by 91 Ma in the low-grade rocks. Shortening continued after 88 Ma in the Cascades core, migrating NE toward more internal parts of the orogen. Similar hinterland migration occurred in British Columbia. The absence of gently dipping thrust sheets in the external part of the southern Cascades, however, contrasts sharply with relationships farther N, probably reflecting reactivation of steep faults in the ophiolite. Another important difference is the dominance of folding and absence of a major thrust stack in the Cascades core. This difference in style may reflect the position of the Cascades rocks at the southern termination of the Insular superterrane and contractional belt.