TOWARD A STRUCTURAL SYNTHESIS OF THE KOOTENAY ARC IN NORTHEASTERN WASHINGTON AND ADJACENT BRITISH COLUMBIA: RECOGNITION OF THE PEND OREILLE RIVER FOLD AND THRUST BELT FROM 49.2ºN TO 48.1ºN

Northeastern WA and adjacent BC are underlain by North American Mesoproterozoic to Mississippian unconformity-bounded sequences. The 1:250,000 maps of Stoffel et al. (1991) and Miller et al. (1999) show few regional structures, except for the general northeasterly trend of lithostratigraphic units. Structural patterns are obscured, in part, by numerous Mesozoic plutons and areas of Quaternary sediments.

The newly recognized Pend Oreille River fold and thrust belt (PORFT) extends from 49.2ºN to 48.1ºN, where it passes unconformably below Miocene basalts. Although Yates (1964, 1971) recognized elements of PORFT, the compilation of Cheney and Zieg (2004, in press) showed that along the International Border it consists of several regional, northwesterly verging thrusts. The lowest thrust, Waneta, places the North American sequences over the Quesnel terrane. Mid-Jurassic, insertion of that terrane beneath the cover sequences presumably caused the thrust faults.

Interpretation of many of the maps used by Stoffel et al. (1991) and Miller et al. (1999) allows the recognition of PORFT south of 48.8ºN. It consists of eight, northwesterly verging thrusts, the plates of which constitute the Kootenay Arc. In general, progressively younger and more deformed strata occur in progressively lower thrust plates. The original mappers recognized (and named) some segments of some of the thrusts but did not appreciate their regional extent, partly because of some stratigraphic misinterpretations. From top to bottom the names of the thrusts with precedence (and synonyms) are: Harvey (Dunn Mountain), Slate Creek, Russian Creek (Black Bluff, Haller Creek), Black Canyon, Day, Columbia (Huckleberry Range), Tillicum, and Waneta. Northeasterly trending folds occur within the thrust plates. Four previously unrecognized, 1:250,000-scale, northeasterly trending folds deform some plates. Northeasterly trending Eocene normal faults offset most of the thrusts.

Southeast of PORFT southeasterly verging thrusts are well known, of which the Lane Mountain (Helmer Creek) fault is the most extensive. With respect to these, PORFT probably is a series of back thrusts. All of the thrust plates are preserved in the structural low between the Eocene Kettle and Priest River metamorphic core complexes