The Kettle River Range in Ferry County, Washington, is underlain by sillimanite-grade rocks of the Tenas Mary Creek sequence. Two >800-m-thick sheets of augen gneiss occur above and below feldspathic quartzite, biotitic gneiss, and minor marble. Polyphase deformation (including mylonites and east-trending lineations) and slightly uraniferous aplitic to pegmatitic bodies are common. Cataclasis is common, and rocks of the Tenas Mary Creek sequence appear to be in tectonic contact with overlying upper Paleozoic phyllitic rocks. Fine-grained biotitic metasedimentary rocks occurring locally between the phyllitic rocks and rocks of the Tenas Mary Creek probably are older than the late Paleozoic phyllitic rocks.
Foliation and contacts in the Tenas Mary Creek sequence rarely dip >25° and define the flat-topped Kettle dome (which is >65 km long north-south, 27 km wide, and has about 3 km of structural relief). The Okanogan dome west of the Kettle dome consists primarily of orthogneisses and granitic plutons of Mesozoic(?) age. Rocks in the flat-topped Spokane dome along the Washington-Idaho border are lithologically similar to those of the Tenas Mary Creek and may be pre-Beltian.
The Sanpoil syncline between the Kettle and Okanogan domes and a syncline on the northeastern margin of the Kettle dome contain Eocene sedimentary and volcanic rocks. Because the axes and structural reliefs of the Okanogan dome, the Sanpoil syncline, and the Kettle dome are similar, the present structural relief (as opposed to the internal structure and high-grade metamorphism) of the Kettle dome probably is due to post-Eocene folding. The gently synformal Tertiary Newport fault straddling the Washington-Idaho border may be a related structural feature. Four other low-angle faults, three of which cut Tertiary rocks, occur between the domes.
The low-angle faults commonly are marked by cataclastic zones more than 100 m thick. Cataclasis occurred as the basement of batholiths and pre-Beltian(?) metamorphic rocks became decoupled from overlying Precambrian to Tertiary layered rocks. Whether this decoupling represents one or more zones of Tertiary decollement of regional extent is not yet known. Owing to post-Eocene (possibly late Miocene or younger) folding, the cataclastic zones crop out on the margins of the domes.