Geol 456-556 Field Trip May 21-23, 2010 Accreted Terranes of Northern Cordillera This Field Trip: Insular Terrane Insular Terrane Western Domain Northwest Cascades System (NWCS) Nappes of Pz Mz eugeosynclinal strata EK blueschist Mostly shallow metamorphic dips Darrington-Devils Mountain Fault Zone Western and Eastern melange belts (WEMB): Little metamorphosed Mesozoic marine rocks, lenses of Pz limestone, faut bounded Mz plutons (gabbro to tonalite) Bedding and cleavage steep Straight Creek Fault Zone Crystalline Core Lower greenschist ‐> Amphibolite facies schist, gneiss, migmatite, plutons Wenatchee block recrystallization stopped by 75 Ma Entiat Fault Eocene Chiwaukum Graben fluvial, lacustrine Chelan Block Deformati0n and recrystallization until eT Methow Domain Methow Block: Little-metamorphosed Mz strata. East verging thrusts and folds Ross Lake Fault Zone: Rx of Methow block and Chelan block Oceanic greenstone, chert, argillite of Pz->Jr Hozameen group Cross Section of North Cascades Western Domain of North Cascades P-T conditions of Skagit Gneiss Complex Ross Lake fault system Methow Terrane Okanogan Batholith of Intermontane Terrane Pipestone Canyon Formation Stratigraphy of Southern Can Cordillera SuperTerranes and faults Eocene Features of North Cascades In this region: accreted terranes are allochthonous tectonostratigraphic terranes juxtaposed along regional fault systems and intruded by Jurassic to Tertiary plutons Grouped into 2 superterranes separated by major zone of mK to T contractional deformation and plutonism: Intermont ane superterrane (Stikinia, Cache Creek, Quesnellia terranes) Insular superterrane- Wrangellia and Alexander terranes Focus on the Cretaceous-> Tertiary development of Cordillera Controversy: estimates of timing and magnitude of terrane displacement along margin: -Large Translation: paleomagnetic data. Paleomagnetic data from stratified and plutonic rocks from Insular and Intermontain suggest large northward translations between 90 and 55 Ma Intermontane: 1100 ±600 km between 70 and 55 Ma Insular : 3000±500 km between 80-55 Ma -Small Translation: geologic data. Total translation: < 1000 km since mK. Terrane amalgamation at or slightly south of current location by mK Insular : < 400 km dextral translation based on structural and stratigraphic field observations, stratigraphic correlations, and lab analyses .