VERTICAL VARIATIONS IN FOOTWALL ROCKS OF THE NORTHERN SNAKE RANGE, NEVADA; IMPLICATIONS FOR METAMORPHIC CORE COMPLEX DEVELOPMENT

The recent finding of mid-to-lower crustal rocks (30 Km depths; Lewis et. al., 1999, GSA Bulletin, v. 111, p. 39-51) exposed in foot wall rocks of the Northern Snake Range (NSR) metamorphic core complex, Nevada, raises many questions. These rocks were emplaced within 1 km of the NSR detachment surface, which is considered to have developed at the brittle-ductile transition at depths of 10 to 15 km. This requires 15 to 20 km of uplift during Tertiary extension. Such uplift can be explained by 1) faulting and structural omission, or 2) extreme thinning accompanying extension. Each of these scenarios could accommodate different tectonic models, but each will leave a different metamorphic and microstructural fingerprint. The NSR footwall rocks record a complex metamorphic and tectonic history. Relics of early (M1) metamorphism, in the form of biotite fish, and garnet porphyroblasts, are preserved throughout a vertical section examined in this study. All of these rocks are overprinted by Tertiary extensional fabrics. Systematic variations in Tertiary metamorphic assemblages (M2) suggest nearly isothermal uplift resulting from extreme thinning of footwall rocks. Such a mechanism for emplacement of deep seated crust to levels near the brittle-ductile transition may be accommodated by ductile thinning or, more likely, by diapiric and gneiss dome formation.