Eocene dike orientations across the Washington Cascades in response to a major strike-slip faulting episode and ridge-trench interaction

Abstract The northern Cascade Mountains in Washington (USA) preserve an exceptional shallow to mid-crustal record of Eocene transtension marked by dextral strike-slip faulting, intrusion dike swarms and plutons, rapid non-marine sedimentation, ductile flow cooling parts the North Cascades crystalline core. Transtension occurred during ridge-trench interaction with formation a slab window, rollback break-off shortly after collision Siletzia oceanic plateau at ca. 50 Ma. Dike intruded ?1250 km2 region between 49.3 Ma 44.9 Ma, orientations more than 1500 measured dikes coupled geochronologic data provide important snapshots regional strain field. mafic Teanaway are southernmost most voluminous swarms. They strike NE (mean = 036°) average ~15 m thickness. To north, rhyolitic basaltic overlap spatially 49.3–46.5 mainly granodioritic but they typically predate nearby plutons 500 k.y. five six domains range from 010° 058°; W-NW–to NW–striking characterize one domain found lesser amounts few other domains. Overall, mean for all is 035°, extension direction (305°–125°) oblique (~320°) orogen. Extension diking reached ~45% >7-km-long transect through swarm ranged ~5% locally ~79% shorter transects across swarms, which corresponds minimum ~12 km extension. dominant NE-striking compatible motion on N- NW-striking (~355–320°) faults. Some W–NW- were arguably influenced pre-existing faults, shear fractures, foliations, potentially where both NE- W-NW–striking present, switch principal stress axes induced emplacement. Alternatively, may reflect younger field, as 49.3–47.5 U-Pb zircon ages older those dated NW-trending dikes. In scenario, interval when reflected arc-normal Diking ended magmatism migrated into N-S–trending belt west core that marks initiation ancestral arc.