Magmatic strains and foliation triple points of the Merrimac plutons, northern Sierra Nevada, California: implications for pluton emplacement and timing of subduction

Cross-cutting relationships between regional structures and post-tectonic plutons of known age are commonly used to date regional deformation in orogenic belts. In order to show that a pluton is post-tectonic one must demonstrate that the strains within the pluton are not due to regional tectonic processes. This paper reports strain measurements associated with pluton emplacement in northern Sierra Nevada. These strain data and a microstructural analysis indicate that strains within the Merrimac plutons are due to igneous not tectonic processes, which strengthens the hypothesis that the plutons were emplaced after the formation of regional structures and melanges. Radiometric ages of the Merrimac pluton, in concert with structural data, place a more reliable upper Jurassic limit on the age of the subduction in the Western Belt of northern Sierra Nevada. This paper also reports constrictional strains around plutons, and suggests that the Merrimac plutons contributed significantly to the ductile strains observed in the wall rocks.