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