DEFORMATION RESULTING FROM REGIONAL EXTENSION DURING PLUTON ASCENT AND EMPLACEMENT, CENTRAL SIERRA-NEVADA, CALIFORNIA

Solid-state foliation, lineation, small-scale folds and domainal shear zones have developed in pre-existing granitic and minor metasedimenta ry wallrock during a combined deformation involving a regional extensi onal strain and ascent and emplacement of the Mt Givens pluton (MGP). Mylonite is common throughout much of the approximately 1-2 km wide, 1 0 km long shear zone, with ultramylonite best developed near the conta ct with the MGP, which itself lacks significant solid-state deformatio n. Migmatization accompanies ultramylonite formation in the northern h alf of the zone, but both these features are poorly developed or absen t in its southern half where the shear zone is distributed over a wide r area. Strain estimates across the shear zone using microgranitic enc laves as markers show a positive gradient and an increasing ratio of s imple shear/pure shear towards the MGP. Microprobe analyses on hornble nde and plagioclase yield pressure and temperature estimates of approx imately 3.5 kb and approximately 680-degrees-C respectively, during sh ear zone formation, at least at its late stages of development. Zircon Pb/U and Ar-40/Ar-39 ages constrain timing of the high-temperature mo vement on the shear zone to approximately 90 Ma, essentially the age o f the MGP, although movement immediately prior to that time appears li kely. We speculate that a regional extensional shear zone was developi ng prior to the emplacement of the MGP, which, as it ascended, heated the wallrock facilitating both further strain in the zone as well as b uoyant rise of the pluton along the zone. The MGP was near its critica l melt fraction during the last several kilometers (?) of its ascent, and could have possessed sufficient viscosity (strength) to impose a w eak shear strain on the shear zone rocks, although most of the foliati on and extensional features in the zone are probably related to the re gional strain field. Late-stage folding of the foliation is attributed to shouldering aside of the wallrock by the MGP during the last incre ment of its ascent and final emplacement.