TECTONIC IMPLICATIONS OF EOCENE TEANAWAY DIKE SWARM IN THE EASTERN SWAUK BASIN, CENTRAL WASHINGTON

The Teanaway dike swarm represents the most extensive exposure, > 365 km², of mafic to intermediate Eocene dikes that widely intrude early to middle Eocene transtensional basins and older crystalline rocks of the North Cascades of central and northern Washington. These Eocene dikes provide a “snapshot” of the regional middle Eocene strain field. The Teanaway dikes intrude the thick (> 4800 m), early Eocene, non-marine Swauk basin, which is in part bounded by steep N-S to NW-SE striking faults. The basin underwent middle Eocene (~ 53-47 Ma) shortening that produced dominantly WNW- trending folds and was shortly followed by extension recorded by intrusion of the ~ 47 Ma Teanaway dike swarm. In this study, the width and orientation of 145 dikes were measured in the eastern part of the Swauk basin to evaluate extension direction and magnitude. The dikes are basaltic to diabasic and commonly contain phenocrysts of plagioclase. Dike thicknesses ranges from 0.5 to 85 m, average 14 m, and have a mean of 10 m. The thickest dikes are probably composite. The concentration of dikes is highly variable; few dikes are present in the very eastern portion of the Swauk basin. More than 85% of the dikes strike NE, with a mean orientation of N40°E. Dips are steep, with a mean of 78°. The overall orientation of the dikes indicates sub-horizontal extension towards N50°W-S50°E, suggesting that extension was sub-parallel to slightly counter-clockwise of the regional strike. This direction is broadly compatible with the results of Doran et al. (2007) for the western and central part of the Swauk basin. Their study demonstrated that the dikes trend NE to NNE and record 15-43% extension in the direction of N70°W to N57°W. Strikes of dikes in the eastern Swauk basin overlap with those measured by Doran et al. (2007), but the mean orientation of dikes in the east strike more easterly, suggesting small (~ 7°-20°) variations in the direction of the strain field. Extension directions for the entire Teanaway dike swarm are broadly sub-parallel to the stretching direction recorded by lineations in Eocene crystalline rocks of the North Cascades (Miller et al., 2007). Such orogen-parallel stretching contrasts with orogen-perpendicular extension in Eocene metamorphic core complexes to the east.