STRUCTURE OF THE SWAUK BASIN AND TEANAWAY DIKE SWARM, CENTRAL CASCADES, WASHINGTON

The Swauk Basin is one of several Eocene, non-marine basins inferred to have formed during regional transtension in Washington. Arkose, mudstone, and conglomerate of the Swauk Formation are intercalated with ca. 52 Ma dacitic and andesitic rocks in the basin, are unconformably overlain by the ca. 47 Ma basaltic Teanaway Formation, and are intruded by a multitude of Teanaway dikes. Previous work suggests that the Swauk basin experienced mid-Eocene subsidence and accumulation of >8000 m of sediment, followed by folding and extension. Before our work, however, no detailed study of the structures of the basin and dikes, and in particular the magnitude of shortening and extension recorded by them, had been undertaken.

Shortening of the Swauk Formation produced open to locally tight, generally WNW trending, upright to mainly NNE vergent folds with wavelengths of 0.5 to >10 km. Detailed study of facing indicators documented a panel of ≥ 5 km of overturned section dipping ~80˚ NE in the central part of the basin. This panel probably is the limb of a large, SW-vergent, overturned fold, but rotation by an unrecognized fault cannot be precluded. Construction of several cross-sections across the Swauk basin indicates a minimum shortening of 25 to 30% (ca. 7 km shortening) by folding. Thrust faults also cut the rocks of the basin and accommodated a poorly constrained amount of shortening. Most of the faults have small or unknown displacement. Available age data suggest shortening was accomplished over 5 Myr. This folding event, which presumably resulted in basin inversion, is not well documented elsewhere in the Cascades.

After folding, the Swauk Basin underwent extension and was intruded by the ca. 47 Ma, largely basaltic to diabasic Teanaway dike swarm. Dike thicknesses range from <0.5 m to >70 m. In two strike-normal transects with a combined length of 11.6 km, more than 120 measured dikes accommodated 15-43% extension. Most of the dikes trend NE to NNE. Extension recorded by the dikes is broadly orogen parallel (298˚ to 313˚), and dike trends are at high angles to fold axes, compatible with a similar shortening direction during folding and extension. NE-striking mid-Eocene dike swarms are also present in the North Cascades and probably record significant orogen-parallel extension at ca. 45-48 Ma.