Department of the Interior, U.S. Geological Survey
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Thrust sheets that underlie the Greenville quadrangle in northeastern Georgia and in northwestern South Carolina have been selectively metamorphosed and deformed during several Paleozoic prograde metamorphic events.
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.
he Great Lakes tectonic zone (GLTZ) is a Late Archean crustal boundary (paleosuture) at least 1,200 kilometers long that juxtaposes a Late Archean greenstone-granite terrane (Wawa subprovince of Superior province) on the north and an Early to Late Archean gneiss terrane (Minnesota River Valley subprovince) on the south. Recent mapping of an exposed seg- ment in the Marquette, Michigan, area provides new data on the vergence of the structure. These data necessitate reexami- nation of the COCORP seismic-reflection profiling in central Minnesota, which has been the principal basis for past views on the vergence of the GLTZ. In the Marquette area, the GLTZ is a northwest-striking mylonite zone about 2.3 kilometers wide that is superposed on previously deformed rocks of both Archean terranes. Shear zone walls strike N. 55-60 W., and foliation in mylonite within the GLTZ strikes (average) N. 70 W. and dips 75 SW. A stretching lineation plunges 42 in a S. 43 E. direction. Hinges of tight to open (sheath?) folds of both Z- and S-symmetries plunge parallel to the lineation. The attitude of the lineation (line of tectonic transport and X finite strain axis), together with asymmetric kinematic indicators, indicates that collision at this locality was oblique; the collision resulted in dextral- thrust shear along the boundary, northwestward vergence, and overriding of the greenstone-granite terrane by the gneiss terrane. In contrast, the seismic-reflection profiling in central Minnesota has been interpreted by several investigators to indicate that the GLTZ is a shallowly north dipping (=30) structure, which implies southward vergence on a north-dipping subduction zone. We suggest, alternatively, that the shallow- dipping reflectors in the seismic profiles indicate lithologic contacts related to recumbent and gently inclined folds (Db1s), perhaps enhanced by ductile deformation zones, and that the Morris fault is indeed the GLTZ. The Morris fault strikes
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