STRUCTURE AND TECTONIC SIGNIFICANCE OF THE NORTHEASTERN WASHINGTON FOLD AND THRUST BELT (NEWAFT)

The northeastern Washington fold and thrust belt (NEWAFT), located in Stevens and Pend Oreille counties and adjacent BC, is approximately 50 km wide by 150 km in length. From NW to SE the belt has three components, the west-vergent Pend Oreille River fold and thrust belt (PORFT), the east-vergent Colville River fold and thrust subbelt (CRFT), and the west-vergent Bald Mountain Anticline (BMA). PORFT contains eight thrusts in Neoproterozoic to Mississippian distal miogeoclinal strata (Cheney and Zeig, 2016). CRFT has three very thick sequences (Miller, 2000): strata that may be equivalent to the Mesoproterozoic Belt Supergroup, the Neoproterozoic Windermere Group, and the early Cambrian Addy Quartzite. The southeastern BMA is dominated by the Belt Supergroup, the Windermere Group and Addy Quartzite being absent.

The NEWAFT possesses an overall architecture and internal structural elements common to other thrust belts. Stratigraphic separation diagrams of the thrusts have a “peas-in-a-pod” geometry, suggesting that smaller thrust segments propagated laterally to join and form the through-going sheets of the salient. Synoptic cross sections, constructed from pseudo downplunge projects, reveal folded thrusts and forelimb imbricate stacks. These patterns suggest that thrusts initiated “in-sequence”, east to west in the PORFT and west to east in the CRFT.

The CRFT was translated eastward on the Valley thrust, over the west limb of the BMA. The Valley thrust presumably developed along the eastern margin of the Windermere/Addy basin. We infer that the CRFT and BMA subbelts are floored at depth by the same master sole thrust, perhaps atop the crystalline basement. This sole thrust may have been reutilized during Eocene extension. Removal of 32 km of Eocene extensional translation on the Newport fault (Doughty and Price, 1999) aligns the Snow Mountain anticline in the hanging wall of the Newport fault (Schroeder, 1952) with the south plunging BMA, indicating they were part of the same anticline, originally over 100 km in length. Removing a total of 68 km extension restores the CRFT and the BMA-SMA parallel to the Libby thrust belt and Purcell anticlinorium to the east. The restoration also places Cretaceous intrusions at the heart of the PORFT thrust salient, suggesting that magma intruded into the space vacated by the salient.