Deformation-Enhanced Element Mobility in Feldspar: A Strain Speedometer?

Naomi Barshi, Department of Earth and Planetary Sciences, McGill University
Christie Rowe, Department of Earth and Planetary Sciences, McGill University
Vincent van Hinsberg, Department of Earth and Planetary Sciences, McGill University

Strain rate is an important parameter for understanding rock deformation and estimating paleostress, but we cannot directly measure strain rate in the rock record. Strain rate can be inferred in specific cases when dateable events such as mineral growth or intrusions accompany deformation, but a generally applicable tool has not yet been developed. Previous studies on tourmaline show deformation-enhanced element mobility that could, in theory, be used to calculate a duration of deformation. Unfortunately, the required diffusion parameters are lacking for tourmaline, rendering it unusable as a tool to infer bulk rock strain rate. Feldspar is a better candidate for a paleostrain-rate tool as it dominates the rheology of Earth's crust, diffusion parameters are known across a range of pressure-temperature conditions, and its compositional zoning serves as a physical, and potentially chemical, strain marker. I will combine strain measurements along a 500-meter-long strain gradient with element mobilities measured as a function of strain in zoned plagioclase phenocrysts. The strain gradient ranges from undeformed tonalite to biotitic tonalite gneiss in the marginal unit of the San José Pluton, Peninsular Ranges Batholith, Baja California, which experienced solid-state deformation during subsequent intrusions. I expect that modeling changes in element mobilities will yield an estimated duration of deformation, which can be used to infer bulk strain rate in the rock record.