TECTONIC EVOLUTION OF THE CRYSTALLINE CORE OF THE NORTH CASCADES (CASCADES CORE), WASHINGTON

The Cascades core consists of pre-Tertiary terranes, originally defined by Rowland Tabor and colleagues, which were intruded by 96- to 45 Ma arc magmas and metamorphosed largely to amphibolite facies at ~ 2- to 11 kbar. The earliest terrane juxtaposition can only be documented as pre-96 Ma (ophiolitic Ingalls and peltitic Nason), and initial juxtaposition may be as young as 48 Ma for the oceanic Napeequa and metaclastic Swakane terranes. Earliest documented structures are foliations that predate 96-91 Ma plutons and folds in schist clasts in Lower Cretaceous metaconglomerates in the eastern core. The most widespread and intense deformation, associated with major crustal thickening and metamorphism recording burial of up to 20 km, occurred between ~ 96 and 78 Ma. This deformation migrated from the margins of the core inward and has been viewed by some to record distributed strike slip, but our studies demonstrate a fold-dominated, contractional history. Recumbent to gently inclined, tight to isoclinal folds are overprinted by more upright, NW-SE-trending, mesoscopic to map-scale folds. This pattern is interrupted by several km-thick, SW-vergent reverse shear zones localized next to plutons. We interpret the overall structure as a mid-crustal, SW-vergent fold-and-thrust belt with NNE-vergent backthrusting (Windy Pass thrust) at the southern margin of the core. The total shortening from this 96-78 Ma event may reach 400 km, and long-term rates are on the order of 2.2 mm/yr. Transpression dominated from ~ 75 to 57 Ma. Magmatism and deformation were less widespread than during the contractional regime, but were more extensive than previously recognized. Subsequent deformation and magmatism were restricted mainly to the NE half of the core and were associated with NW- to NE-trending subhorizontal stretching and extensive exhumation. A transition from transpression to transtension occurred at about 57 Ma in the Ross Lake fault zone and top-to-north shear on the Dinkelman decollement helped exhume the deepest rocks of the eastern core, the footwall Swakane terrane, between ~ 72 to 45 Ma. This decollement may have large displacements as arc-type plutons are not found in the footwall in contrast to the structurally higher terranes in the Cascades core.