Foliations and shear sense: A modern approach to an old problem

Abstract

Argument about shear on foliations began in the mid 19 th century and continues to the present day. It results from varying interpretations of what takes place during the development of different types of foliations ranging from slaty cleavages through differentiated crenulation cleavages, schistosity and gneissosity to mylonites. Computer modelling, quantitative microstructural work and monazite dating have provided a unique solution through access to the history of foliation development preserved by porphyroblasts. All foliations involve shear in their development and most can be used to derive a shear sense. The shear sense obtained is consistent between foliation types and accords with recent computer modelling of these structures preserved within porphyroblasts relative to those in the matrix. The asymmetry of curving foliation into a locally developing new one allows determination of the shear sense along the latter foliation in most rocks. The problem of shear on fold limbs and parallelism of foliation and the flattening plane of the strain ellipse is resolved through the partitioning of shearing and shortening components of deformation into zones that anastomose around ellipsoidal domains lying parallel to the XY plane. Conflicts in shear sense occur if multiple reuse or reactivation of foliations is not recognized and allowed for but are readily resolved if taken into account.

The importance of stress percolation patterns in rocks and other polycrystalline materials

“…Therefore, examining the role of stress patterning in shear localization in polycrystalline materials may prove fruitful. The magnitude of stress concentrations in the stress patterns and the regularity with which they occur may well yield to statistical descriptions, as they do in granular materials23 and should lead to a better understanding of phenomena that are tied to stress concentrations, such as the development of stylolites, slatey cleavage and compositional banding in rocks2425, as well as help explain how these rhythmic features initially arise from unpatterened rocks such as shale and granite.…”

Structural Evolution of a 1.6 Ga Orogeny Related to the Final Assembly of the Supercontinent Nuna: Coupling of Episodic and Progressive Deformation

“…Structural analysis provides a relative chronology of superimposed generations of structures and deformation events that can be construed to be either progressive or episodic in a region. Numerous studies have previously speculated that superimposition of structural elements can occur during a single deformation event (Bell, 1978; Bell & Hobbs, 2010; Tobisch & Paterson, 1988; Williams & Zwart, 1977). The conventional approach used to distinguish between progressive and episodic deformation is the application of traditional structural overprinting criteria such as fabric reorientation, kinematics of successive structures, and strain partitioning (Holdsworth, 1990).…”

“…Previous structural studies and their interpretations are presented and discussed below and summarized and compared with this study in Table 1. Many of earlier studies focused on the Robertson River area in the central domain of the GTI, where heterogeneous deformation at different scales allowed the preservation of successive stages of foliation development in adjacent rocks (Bell, 1981, 1986; Bell et al, 1986; Bell & Hobbs, 2010; Bell & Rubenach, 1980, 1983; Reinhardt & Rubenach, 1989). In the Robertson River area, the prograde growth of index minerals was interpreted as progressive, during successive stages of development of the locally dominant foliation (Bell & Rubenach, 1983).…”

Proceedings of the 25th Himalaya-Karakoram-Tibet Workshop

“…Jiang and Williams, 2004;Jessell and others, 2009), non-rotation (e.g. Aerden and Sayab, 2008;Bell and Hobbs, 2010) or little relative rotation (e.g. Johnson, 2009).…”