NEW FELDSPAR PALEOPIEZOMETRY ALLOWS REVISED GRANITE RHEOLOGY AND CRUSTAL STRENGTH ESTIMATES

Feldspars are the most abundant minerals in the continental crust and hence control its rheology. Crystal-plastic deformation causes dynamic recrystallization and hence grain-size reduction, so that most feldspar deformation occurs by grain-size-sensitive creep. The stress - grain-size relationship for feldspars is therefore critical for determining feldspar rheology, yet this relationship has not been determined experimentally. A recently published experimental subgrain-size piezometer that is likely to be valid for most minerals, combined with a new theoretical relationship between dynamically recrystallized grain-size and subgrain size, yields a stress - grain-size relationship for feldspars.

A critical aspect of granite rheology is that dynamic recrystallization in feldspars is likely to reflect the stress developed in the initial microstructure, in which the feldspars form a load-bearing framework. During deformation the microstructure evolves to an interconnected weak layer geometry dominated by quartz, which deforms and recrystallizes at a lower stress for a given imposed strain rate. Rheological mixing laws accounting for these changes allow calculation of the bulk rheology of granite as a function of temperature, water content, and strain.