STRUCTURE AND IMPLICATIONS OF EOCENE DIKE SWARMS IN THE WASHINGTON CASCADES

Eocene dike swarms of intermediate to mafic composition are widespread in the Central and North Cascades of Washington and provide information on the regional strain field during dextral transtension and possibly ridge subduction. There is a wide variation in dike orientations from swarm to swarm. In the south, the ~46-47 Ma mafic Teanaway dike swarm, which intrudes slightly older rocks of the Swauk basin, is the best studied and most voluminous of the Eocene swarms. It extends >75 km from east to west and 15 km from north to south. Abundant dikes accommodate between ~15% and ~43% extension in two transects across the swarm. Teanaway dikes strike NE, average ~16 m in thickness, and vary in strike from 044˚ in the W to 027˚ in the E. Dikes show no systematic change in thickness across or along strike in contrast to some well-studied mafic dike swarms in the N Atlantic and elsewhere.

Approximately 75 km north of the Teanaway swarm, dikes of intermediate to mafic compositions intrude metamorphic rocks and 96 to 50 Ma plutons of the crystalline core of the North Cascades. From S to N, dikes range in strike from ~035˚ near the Eocene Duncan Hill pluton, to ~005˚ near the Eocene Cooper Mountain batholith, to ~032˚ in the southern part of the Skagit Gneiss Complex, to ~092˚ in the central part of the complex. Extension directions inferred from orientations of the dike swarms range from ~275˚ to ~002˚ across the region. Extension directions are thus highly variable, but on average record broadly arc-parallel rather than arc-normal extension. This extension direction is at a significant angle to the E-W extension in core complexes to the east, probably reflecting the role of dextral strike-slip in the W. The wide variation in extension directions recorded by the dikes implies complex spatial variations of the strain field, or a rapidly changing strain field during emplacement of swarms of slightly different ages.