The Microboudin Method: a New Paleostress Analysis

The microboudin method has been recently established as a new paleopiezometer. The method is successfully applied to monomineralic metamorphic tectonites such as metacherts and marbles, which include the microboudinage structures of columnar minerals (tourmaline, epidote etc.). The absolute magnitude of the far- field paleodifferential stress σ0 is estimated from the two parameters: the dimensionless stress parameter λ, and the extensional fracture strength S0* of micrometer-scale columnar mineral grains. λ is derived from the mechanical model for microboudinage based on the shear-lag model and the Weibull theory. The value of S0* for tourmaline and epidote were obtained from directly measured flexural strength by considering the size and shape of these minerals and influence of time on fracturing. The values of σ0 estimated by this method for samples from Aksu (China), Eskisehir (Turkey), Wadi Tayin (Sultanate of Oman), Asemi (Japan) Greenbushes (Australia), and Sausar (India) are in the range of 1-15 MPa. These σ0-values correspond to the differential stress at mid-crustal levels (> 10 km).