Bridge Zones and Stress Amplification: How Rocks Start to Deform

He Feng, UNIVERSITY OF MAINE
Christopher Gerbi, UNIVERSITY OF MAINE
Scott Johnson, UNIVERSITY OF MAINE
Alicia Cruz-Uribe, UNIVERSITY OF MAINE

Abstract

Strain localization occurs throughout the crust, in both the brittle and viscous regimes. The causes of strain localization remain under discussion. However, the rock records suggest that variations of material properties (e.g. active deformation mechanisms, crystallographic orientation, phase distribution, grain shapes, etc.) are likely to be the dominant factor for weakening. We consider microstructural aspects of this question from two perspectives. The first is observation of natural microstructures in two rocks that experienced very low macroscale strain at amphibolite and/or granulite facies conditions yet exhibit localization on the millimeter and smaller. We describe localized deformation, or bridge zones, that appear to mechanically link rheologically weak phases or domains. We combine optical and electron beam petrography with chemical mapping and electron backscatter diffraction to characterize these rheologically important domains. Rheologic calculations suggest that the small modal fraction of microstructural change represented by the bridge zones can lead to a high degree of bulk weakening. The second aspect of our work is to consider the possible role stress concentrations can play in initiating these bridge zones. Through numerical modeling, we find that the macroscale load stress is locally amplified by a factor of 1.5 quite commonly, and that amplification up to a factor of 7 is geologically reasonable. Amplification factors such as we calculate indicate that processes controlled by stress magnitude can occur when the macroscale stress is well below a given threshold. This implies that numerous geological processes at all scales may not require a high bulk stress to initiate. This local stress amplification appears more than sufficient to push local values across thresholds for, e.g., grain size reduction, recrystallization, fracture, or chemical reaction.

Session

Session 2: Rheology of the Lithosphere