Single Piezometric Stress Preserved in Strained Feldspar and Quartz From Intracontinental Lower Crust

Abstract The strength of the intracontinental lower crust exerts a primary control on regional‐scale deformation during continent‐continent collision. Because of the nominally anhydrous nature of much of the intracontinental lower crust, it is generally considered to be more viscous than lower crust in hydrated tectonic margins. Current stress estimates from felsic lower crust are mainly derived from deformation experiments on monomineralic aggregates and paleopiezometry on quartzites in hydrous shear zones. However, much of the intracontinental, felsic crust is composed of nominally anhydrous minerals, such as K‐feldspar, plagioclase, and quartz, all of which have contrasting viscosities. Here, we quantify paleostress in a nominally anhydrous lower crustal shear zone using subgrain‐size piezometry on quartz, plagioclase and K‐feldspar. Concentrations of titanium in quartz and concentration gradients of calcium and potassium in feldspars within neocrysts and along subgrain boundaries verify that all three phases were last deformed under eclogite‐facies metamorphic conditions (650°C–700°C). Mean subgrain sizes of 15 ± 4 μm in quartz, 15 ± 2 μm in plagioclase, and 14 ± 2 μm in K‐feldspar are all consistent with a mean differential stress of 29 ± 4 MPa and reveal minimal stress partitioning between the framework phases. The average differential stress recorded in our samples is hundreds of megapascals less than predictions for long‐term flow of rheologically dry felsic aggregates, implying that stresses supported by nominally anhydrous felsic aggregates within the Davenport shear zone are not necessarily greater than those supported by hydrated assemblages common to shear zones along plate margins over geological timescales.