U.S. Geological Survey Bulletin

Deformation Quadrangle, 1n the Stensgar Mountain Stevens County, Washington By James G. Evans Abstract Most deformation of the Middle and Late Proterozoic (Deer Trail and Windermere Groups) and Lower Cambrian (Addy Quartzite and Old Dominion Limestone) rocks in the Stensgar Mountain quadrangle occurred during the Mesozoic (pre-Late Jurassic, possibly Early Jurassic or Triassic), in con- nection with duplex thrusting. The principal deformation occurred in stages that generally involved: (1) thrusting, (2) penetrative dynamothermal metamorphism in the greenschist facies, and (3) renewed thrusting. The initial thrusting may have included formation of the duplex fault zone, moderate tilting of the sedimentary and volcanic rocks, and possibly low-grade metamorphism. The dynamothermal metamorphism resulted in development of a slaty cleavage that dips steeply west, as well as numerous minor and a few large folds that plunge at low to moderate angles, generally north. The folds have axial planes parallel to cleavage. Clasts in conglomerates were flattened parallel to cleavage, and their long axes were aligned north-northeastward, subparallel to fold axes. This extension direction parallels the trend of the Kootenay arc, a relation not typical of orogenic belts. The dynamothermal metamorphism included coaxial compressive pulses separated by periods of stress relaxation. The penetra- tive deformation could have been accompanied by slip on preexisting faults, including a large strike-slip component for the roof (Stensgar Mountain thrust) and floor (Lane Mountain thrust) thrusts of the duplex fault zone. Later movements along these roof and floor thrusts and connecting splays are suggested by nonfolded traces of the faults and the faulted, dynamothermally metamorphosed cataclasite adjacent to the Lane Mountain thrust. The penetrative deformation that affected the Stensgar Mountain quadrangle also affected the rest of northeastern Washington and southeastern British Columbia; it may have been the result of oblique convergence during Mesozoic subduction.