TESTS OF THE NEWLY RECOGNIZED PEND OREILLE RIVER FOLD AND THRUST BELT IN NORTHEASTERN WASHINGTON

Northeastern WA and adjacent BC are underlain by Mesoproterozoic to Mississippian strata of Laurentia deformed by accretion of the Quesnel terrane. Based on reinterpretation of published maps, Cheney and Zieg (2004, in press) recognized seven northeasterly trending thrust faults along the border between BC and WA. They named this the Pend Oreille River fold and thrust belt (PORFT). Cheney (2010) extended the belt southeastward to 48⁰N (where it passes beneath Tertiary strata). The orientation of the thrusts (and the strata) change from northerly in the south to northeasterly in the north, defining the so-called Kootenay Arc. The thrusts verge westerly in the south to northwesterly in the north. Here we present preliminary structural data for three sites that confirm variable folding and thrusting from the southern portion of the belt.

Thirteen km northwest of Addy, WA, at least two styles of folding occur within a thinly-bedded, phyllitic limestone mapped as Cambrian-Ordovician, Metaline Formation. Early tight, reclined folds have axial planes subparallel to an S1 cleavage and bedding, and verge to the N-NW. Broad, open folds overprint the tight folds and are overprinted by an upright S2 crenulation cleavage. S1 orientations are scattered and fold axes define two groups, consistent with multiple episodes of folding.

Three km southwest of Addy, early isoclinal, recumbent, bedding-parallel folds and associated S1 cleavage occur in mostly thin bedded carbonates of the Metaline Formation. Overlying gray limestones have broad open folds and E-NE verging low-angle thrusts and tension gashes that formed after the earlier S1 fabrics.

Eight km west of Springdale, WA, open upright folds with an axial planar cleaveage (S1, steeply dipping to the SSE) occur within the dark argillites of the Mesoproterozoic Deer Trail Group. S1 related structures were then faulted and gently folded and overprinted by a sub-vertical crenulation cleavage.

In summary, early, tight, layer-parallel, recumbent folding with S1 cleavage is followed by upright open folding and an associated axial planar cleavage. A widespread crenulation cleavage overprints the older structures. Late east-directed thrusting and brittle fracturing, best expressed in carbonate rocks, likely coincided with upright folding and related fabrics.