Deformation resulting from regional extension during pluton ascent and emplacement, central Sierra Nevada, California

Abstract

Solid-state foliation, lineation, small-scale folds and domainal shear zones have developed in preexisting granitic and minor metasedimentary wallrock during a combined deformation involving a regional extensional strain and ascent and emplacement of the Mt Givens pluton (MGP). Mylonite is common throughout much of the ∼1–2 km wide, 10 km long shear zone, with ultramylonite best developed near the contact with the MGP, which itself lacks significant solid-state deformation. Migmatization accompanies ultramylonite formation in the northern half of the zone, but both these features are poorly developed or absent in its southern half where the shear zone is distributed over a wider area. Strain estimates across the shear zone using microgranitic enclaves as markers show a positive gradient and an increasing ratio of simple shear/pure shear towards the MGP. Microprobe analyses on hornblende and plagioclase yield pressure and temperature estimates of ∼3.5 kb and ∼680°C respectively, during shear zone formation, at least at its late stages of development. Zircon Pb/U and ^(40)Ar/^(39)Ar ages constrain timing of the high-temperature movement on the shear zone to ∼90 Ma, essentially the age of the MGP, although movement immediately prior to that time appears likely. We speculate that a regional extensional shear zone was developing prior to the emplacement of the MGP, which, as it ascended, heated the wallrock facilitating both further strain in the zone as well as buoyant rise of the pluton along the zone. The MGP was near its critical melt fraction during the last several kilometers (?) of its ascent, and could have possessed sufficient viscosity (strength) to impose a weak shear strain on the shear zone rocks, although most of the foliation and extensional features in the zone are probably related to the regional strain field. Late-stage folding of the foliation is attributed to shouldering aside of the wallrock by the MGP during the last increment of its ascent and final emplacement.

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