Localization and Mechanical Instabilities at Different Crustal Levels

Structural geologists commonly focus on shear zones (strain localization) as a record of the movement history in rocks. The assumption that shear zones are bounded by rigid walls, similar to upper-crustal faults, is implicit in this approach. However, the walls of the localization zones are not rigid, even in the upper crust. Rather, deformation can be divided into high strain (shear zones or faults) and low strain domains, although the lower strain domains that may record a different style of deformation (e.g., distributed faulting or folding) than the high strain zones. This partitioning occurs on a variety of scales, from the mm- to km-scale. This effect can be quantified and results from lithological heterogeneity, which itself depends on tectonic setting and metamorphic grade (i.e., which minerals are present). As general rules: 1) The high strain domains will preferentially record the noncoaxial component of a general shear; and 2) Less competent material will preferentially segregate into the high strain domains. Mechanical instabilities (i.e., folding, boudinage) are idealized models of rock deformation that address the initiation of heterogeneous strain, and strain localization, in deforming materials. Most mechanical instabilities result when adjacent materials do not deform at the same rate, which results from lithological heterogeneity that characterizes the crust. As most geological materials are lithologically heterogeneous and/or anisotropic at many scales, the onset of deformation inevitably leads to the formation of mechanical instabilities in a deforming zone. Naturally deformed rocks at all lithospheric levels contain evidence that mechanical instabilities were produced during deformation. For interpretation of seismic records, mechanical instabilities are important because: 1) They occur at all scales; 2) They provide a method of interpreting seismic images; 3) They characterize the internal configuration of deformed, lithologically-heterogeneous rocks; and 4) They provide a record of relative material behavior (rheology).