Characterization of a Strike-Slip, Serpentine-Rich Fault System in the Twin Sisters Ultramafic Massif, Washington

The Twin Sisters ultramafic massif, Washington, USA, consists dominantly of ultramafic rocks with a regionally consistent foliation (~165, 70°E) characterized by high-temperature (mantle) fabrics. This foliation is crosscut by both a series of EW-trending serpentine faults and locally a major ENE-trending (070, 80°W) system of serpentine faults. The ENE-trending fault system locally crosscuts the EW-trending faults suggesting it is younger. We have mapped structures in a 500 m long segment of the ENE-trending fault system, corresponding to ~150 m of vertical relief. The fault system consists of a series of subparallel fault zones offset to the right or left and ranging from a few meters to 10 m apart. Each fault zone is 1-3 m thick, typically with a pair of discrete, nearly parallel, bounding faults that are coated by either mm-thick serpentine slickenfibers or cm- thick layers of foliated serpentine. Steps on slickenfibers and structures in the foliated serpentine are consistent with sinistral, strike-slip movement. The bounding faults are linked by anastomozing splay faults and bands of foliated serpentine that separate lenticular masses of fine-grained, foliated serpentine and lens-shaped lithons. Some lithons retain mineral composition and fabric comparable to that observed in the main body of the ultramafic away from the fault zone. Others are heavily serpentinized but retain evidence for the high temperature foliation. Of particular interest is the occurrence of dunite mylonites, not typical in the main body of the ultramafic, locally within and adjacent to the deformation zone branches. The serpentine faults are consistent with low to moderate temperature and lower pressure deformation typical of mid- to upper-crustal conditions. These structures are likely to have formed during the emplacement of the Twin Sisters ultramafic body. The occurrence of dunite mylonite only in the vicinity of this fault suggests that either (1) the serpentine-dominated deformation initiated along a zone of pre-existing higher-temperature shear, or (2) a deformation zone that initiated during the high-temperature deformation of the Twin Sisters massif continued to accrue slip as the body cooled.