On microboudin paleopiezometers and their applications to constrain stress variations in tectonites

摘要

Accounting for interfacial shear-induced near-end fracturing, we propose a new microboudin piezometer, tau(e) = tau/C = 0.25/(L/W), where tau is the shear flow stress of the ductile matrix (e.g., polycrystalline quartz), C is the tensile fracture strength of a columnar mineral (e.g., tourmaline), which ranges generally between E/1000 to E/250 (E is Young's modulus), tau(e) is a dimensionless parameter named 'equivalent shear strength', and L/W is the mean length/width ratio of the microboudins formed by near-end fracturing. This provides a straightforward method of estimating the magnitude of shear flow stresses responsible for ductile deformation of natural rocks. The piezometer is easy to apply because it does not require the use of TEM, SEM-EBSD or the identification of prevailing deformation mechanism (e.g., dislocation creep, diffusion creep, hardening or weakening). The samples from the Gaoligong metamorphic belt (Yunnan, China) yield the equivalent shear flow stresses of 0.145-0.424 (36-106 MPa if taking C = E/800 or 250 MPa) for felsic gneisses, mylonites and phyllonites. The earlier formed microboudins had consistently a larger mean length/width ratio than the later formed ones, suggesting that the rocks recorded a tectonic history of continuous hardening prior to the cessation of quartz plasticity.