TECTONIC SIGNIFICANCE OF DUCTILE STRUCTURES IN THE CHELAN MOUNTAINS TERRANE, NORTH CASCADES, WA

We investigate the metamorphism and deformation of the Chelan Mountains (CM) terrane in the NW Cascades to understand the relative roles of different proposed mechanisms of Late Cretaceous crustal thickening and the relationship to arc magmatism and plate boundary processes. Field observations confirmed the stratigraphy of a Permian to Triassic arc recognized by previous workers, including, from bottom to top: Marblemount Meta-Quartz Diorite (MMQD), Cascade River unit (CRu), consisting of metavolcanics overlain by metaconglomerate and metapelite, and Magic Mountain Gneiss (a Permian to Jurassic (?) sill complex). The first deformation (D1) is preserved as a strong foliation (S1) and weak lineation formed at greenschist to amphibolite facies in all units. The second deformation (D2) folded this fabric into map scale to outcrop scale, tight to isoclinal, rounded, parallel folds. The orientation of these folds varies from recumbent to upright due to later refolding. Recumbent F2 folds verge NW and SE. D3 is preserved as moderately to steeply dipping mylonitic foliation (S3) with moderately SE plunging stretching lineations along the contact of the CRu and the MMQD. Sense of shear indicators show top to the NW and dextral shearing during D3. Although the order of D2 and D3 is not certain, D3 shearing appears to be later because it is lower grade and F2 folds do not fold mylonitic fabric. D4 is preserved as small-scale open to close, rounded, parallel, upright folds that plunge NW to SE and a large-scale SE-plunging anticlinorium.

The following sequence of events is interpreted to reflect plate-boundary kinematics during Cretaceous time based on this and previous studies. D1 may have occurred during accretion of the CM terrane, with unclear kinematics. The vergence of F2 folds may reflect NW or SE translation and orogen-parallel motion. The dextral, NW-vergent mylonitic fabric reflects orogen-parallel shear similar to D2. The orientation of F4 folds suggests orogen-normal subduction. The more open folding of D4, compared to the large-scale isoclinal recumbent F2 folds (recognized by overturned stratigraphy), indicate that D4 accommodated less shortening than D2. In-progress geochronology will constrain protolith and deformation ages.