DUCTILE DEFORMATION PATTERNS IN THE LOWER PLATE OF THE NORTHERN SNAKE DETACHMENT: DEFORMATION TEMPERATURE, VORTICITY, QUARTZ C-AXIS FABRIC PATTERN, AND FINITE STRAIN STUDIES

The Snake Range, NV metamorphic core complex is composed of a low angle fault, the northern Snake Range detachment (NSRD), which juxtaposes a hanging wall of unmetamorphosed, normal faulted rocks upon a footwall of ductilely thinned and stretched metamorphic rocks. Published work (Lee et al., 1987) suggests that lower plate rocks record dominantly pure shear and relatively low strains, well preserved in the northwest, overprinted by top-SE simple shear and very high strains, now well preserved in the southeast. To test this hypothesis, we combined deformation temperature, mean vorticity number (Wm) from rotated rigid grain and oblique grain shape (OGS) measurements, quartz c-axis data, and finite strain calculations for 20 samples spanning the northern Snake Range from the Prospect Mountain quartzite, the structurally highest quartzite in the footwall. From northwest to southeast, bulging recrystallization in quartz and purely brittle deformation of feldspar grades eastward to sub-grain rotation of quartz and both ductile and brittle feldspar deformation, indicating a temperature range of ~300° to ~450° C. In thin section, top-SE kinematic indicators are ubiquitous except for the western most samples. Wm measurements from rotated feldspar porphyroclasts indicate 71-64% pure shear in the northwest decreasing to 60-57% pure shear in the southeast. Wm measurements from OGS indicate nearly 100% pure shear in the northwest decreasing to 27-16% pure shear in the southeast. Quartz c-axis fabric patterns are consistent with OGS results, with samples from the northwest yielding symmetrical cross-girdle patterns, indicating pure shear, which transition to asymmetric single-girdle patterns, indicating top-SE shear, in the southeast. Finite strain ratios (Rxz) increase from 5.8 in the northwest to ~100 in the southeast. A deformation history in the lower plate of the NSRD characterized by an initial phase of dominantly pure shear deformation superimposed on an eastward dipping sequence largely overprinted by a top-down to the SE simple shear concentrated to the southeast explains: (a) the increase in deformation temperature to the southeast, (b) pure shear deformation on the northwest that switches to simple shear on the southeast, and (c) an increase in finite strain northwest to southeast across the range.