Abstract
The central Cascade Range of Washington State has become a testing ground for theories surrounding the exhumation of deep-seated arcs, generation of both arc and flood-basalt volcanism, strike-slip faulting that translated crustal blocks along the Cordilleran margin, and development of fault-bounded basins. A central existing hypothesis is that the region underwent either regional extension or transtension during the Eocene. Both the extension and transtension models derive from the interpretation that clastic Eocene formations were deposited syntectonically in local basins. Geologic mapping and structural analyses presented here support an alternative hypothesis: that these formations are preserved in regional synclines, not in separate depositional basins. The type area for the Eocene history is the so-called Chiwaukum graben or Chumstick basin, here renamed the Chiwaukum Structural Low. The southwestern boundary of the Chiwaukum Structural Low includes post-depositional, northwest-striking reverse faults with adjacent northwest-striking folds. The reverse faults place the regionally extensive and arkosic, early Eocene Swauk Formation over arkosic, mid-Eocene strata that have previously been called the Chumstick Formation. Cataclastic structures provide independent evidence for the reverse faulting. Elsewhere, 39–42 Ma rocks unconformably overlie the folds. The reverse faults and fold hinges are cut by northerly striking strike-slip faults, which likely are of late Eocene age. The Eocene folds and faults were reactivated by deformation of the Miocene Columbia River Basalt Group, the gentler folding of which largely defines the regional map pattern around the Chiwaukum Structural Low. Instead of an extensional or transtensional history, the Eocene-to-Recent history of the Central Cascade region is characterized by multiple periods of folding and reverse faulting alternating with periods of strike-slip faulting.
