RESPONSE OF A THICK CONTINENTAL ARC TO CHANGING TECTONIC REGIMES: A REVIEW OF THE EVOLUTION OF THE CRYSTALLINE CORE OF THE NORTH CASCADES, WASHINGTON

The framework of the northern part of the Cascade Range is dominated by a crystalline core of mostly amphibolite-facies rocks and 96-45 Ma plutons, which represent the southern end of the Coast Mountains continental arc. The Cascades core contains a mid-Cretaceous crustal section extending from <5 to ~40 km paledepth, and was in part assembled by thrust juxtaposition of oceanic, island arc, and clastic rocks, which led to very thick (>55 km) arc crust. The deepest rocks are dominantly amphibolites and quartzites (oceanic metabasalt/metachert), succeeded upward by pelitic and psammitic schists, and a Late Jurassic ophiolite. Tonalitic plutons and thinner sheets intrude most levels of the section, and volumes of magmatic rocks, particularly sheets, increase downward. The largest magmatic flare-up in the arc was from 96-88 Ma and a second peak occurred at ~78-68 Ma; a magmatic lull started at ~60 Ma. Melt crystallization in migmatitic metapelite in the NE part of the core (Skagit Gneiss) continued from ~71-53 Ma. Coeval with the second magmatic flare-up, clastic sediments (Swakane Gneiss) as young as 81 Ma were rapidly transferred from the forearc to 35-40 km depth within the arc and subsequently metamorphosed from ~74-66 Ma during postulated low-angle subduction.

A major tectonic change to regional transtension occurred at ca. 50 Ma, probably shortly after a contractional episode caused by the collision of Siletzia, a large igneous province centered on a ridge, with the continental margin. Transtension was marked by magmatism, migmatization, subhorizontal ductile flow, and rapid exhumation of deep-crustal (8–12 kbar) rocks (Skagit and Swakane Gneisses) in parts of the core, dextral strike-slip faulting, subsidence and rapid deposition in an adjacent non-marine strike-slip basin, and intrusion of dike swarms. Magmatism was compositionally diverse, and more commonly granodioritic in contrast to the Cretaceous tonalities, and was synchronous with adakitic forearc magmatism; magmatism probably reflects slab breakoff. Eocene dextral faults arguably displaced the core from as far south as the Mojave region. A ~45 Ma pluton and lineated dikes are the youngest elements of the NW-trending North Cascades orogen, and shortly preceded initiation of a new N-S-trending subduction zone.