$USGT A gently northwestward-dipping mylonitic carapace on the order of 1 km thick dominates the west flank of the Ruby Mountain. The base of the carapace dies out downward into higher grade, coarser-grained, nonmylonitic rocks of the metamorphic infrastructure. Rock units within the carapace are attenuated to as little as 5 percent of original thickness. The mylonitic zone is superposed on the Lamoille Canyon nappe, for it attenuates both the upper and lower limbs of the nappe. Mylonitic foliation dips northwestward, and generally exhibits strong stretching lineation trending west-northwestward. The sillimanite isograd approximates the boundary between mylonitic carapace, exhibiting lineations striking west-northwest, and the higher-grade infrastructure, exhibiting a nearly orthogonal azimuth of folds and lineations. A top-to-thewest shear sense for the mylonitic zone is indicated by S-C fabrics (Lister and Snoke, 1984; Snoke and Lush, 1984) and is also suggested by the azimuth of drag sense determined by slip-line analysis of diversely oriented disharmonic folds (Howard, 1968), as indicated on this map. Hacker and others (1990) and Erskine and others (1993) analyzed microstructures in the mylonites. Mylonitization began before intrusion of the Oligocene biotite monzogranite unit, and some biotite monzogranite (as well as older rocks) contains the west-trending mylonitic lineation (Wright and Snoke, 1993). Biotite K-Ar and 40Ar/39Ar cooling ages as low as 20 Ma suggest the lower age limit for mylonitic deformation (Kistler and others, 1981; Dallmeyer and others, 1986; MacCready and others, 1997).
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