MID-EOCENE OROGENY IN WASHINGTON AND ITS NEOTECTONIC SIGNIFICANCE

Many active structures in Cascadia have an earlier, middle Eocene (c. 50-40 Ma) history that is similar to, but typically more dramatic than their Miocene-Recent history. The mid-Eocene structures can be explained by a simple model, most aspects of which have already been proposed or established based on detailed local studies:

1. Siletzia is an accreted oceanic plateau, composed of early Eocene flood basalts, not a hotspot chain or rift (Trehu et al., 1994; Pyle et al., 1997; c.f. Babcock et al, 1995);

2. accretion was broadly collisional, and triggered widespread basement involved reverse faulting and folding (e.g. England and Calon, 1991);

3. the Straight Creek fault zone began as a discrete escape structure and is unrelated to early Eocene extension;

4. west- to northwest-trending faults were partly sinistral (e.g. Miller and Patterson, 2000) and may be analagous to similar structures in the modern forearc (see Goldfinger, 1997).

Neotectonic resurgence of mid-Eocene structures is due to the oblique subduction of increasingly young and buoyant oceanic crust, (similar to underplating by an oceanic plateau). Assigning an active role to the paleo-subduction zone may explain the westward vergence of the Washington Cascade Range anticlinorium. The Straight Creek fault appears to still form the natural eastern boundary of the migrating forearc.