Sheath fold formation around slip surfaces

Abstract

Terra Nova, 24, 417–421, 2012 Abstract Sheath folds are associated with shear zones and have been used as high strain indicators. We present results from a three‐dimensional analytical study showing that the flow perturbation around a planar weakness acting as a slip surface, in simple shear leads to the formation of sheath folds at low shear strain. Closed traces of marker layers are exhibited in cross‐sections cut perpendicular to the shear direction in the vicinity of the slip surface tip. In cross‐sections cut parallel to the shear direction and normal to the shear plane, flanking folds are observed. Our model captures the first‐order observations of natural sheath folds, such as diversity of shapes and multiple eye‐structures. Natural equivalents of the planar weaknesses that we model include fractures, veins and rheologically weak layers. Flow perturbation around these zones of weaknesses is proposed as an alternative mechanism of sheath fold formation, which emphasizes the coeval operation of brittle and ductile processes in shear zones.

Sheath fold development in monoclinic shear zones

“…Reber et al. () showed that closed contours with R yz <3 are difficult to generate in simple‐shear deformation, even if the initial orientation and aspect ratio of the slip surface are varied. Alsop and Holdsworth () suggested that R yz <3 is generally obtained in constrictional deformation.…”

“…We study a three‐dimensional mechanical model of sheath fold formation around a pre‐existing, initially circular slip surface embedded in a homogeneous, isotropic, linear viscous matrix (Exner & Dabrowski, ; Reber et al., ). We obtain the velocity field using the external Eshelby solution (Eshelby, ), which is modified for the case of an incompressible viscous matrix and an elliptical and inviscid inclusion.…”

“…For detailed analysis, we select the SY-TX, S0-T0 and SZ-TY models, which correspond to the constrictional, plane strain and flattening strain regimes discussed by Alsop and Holdsworth (2006). Figure 2a shows the fold shapes at c = 15, using _ around the eye structure form a double vergent structure (Alsop & Holdsworth, 2004), which is also referred to as an anvil shape (Mies, 1993) or an omega shape pattern (Reber et al, 2012). In the Y-Z sections intercepting the crack, the structure resembles bivergent flanking structures, a similarity which is also manifested in the contour depression that develops above the crack.…”

“…Sheath folds can develop in pure shear flow (e.g., Ghosh & Sengupta, 1984); however, due to their common occurrence in high strain shear zones, their formation is often associated with simple-shear-dominated flow regimes (Cobbold & Quinquis, 1980). Passive amplification of the original layer perturbations (Cobbold & Quinquis, 1980;Holdsworth, 1990), and flow perturbation due to the presence of rigid inclusions (Marques & Cobbold, 1995), basement corrugations (Cobbold & Quinquis, 1980) or slip surfaces (Reber, Dabrowski & Schmid, 2012) have been suggested among various mechanisms of sheath fold formation in simple shear.…”

Physical Experiments of Tectonic Deformation and Processes: Building a Strong Community

“…(Reber et al, 2013). (C) Cross section from an analytical model investigating the effect of total shear strain (Reber et al, 2012). experimental community requires coordinated and structured growth in the numbers of researchers involved in physical experiments as well as instructors engaging students with classroom experiments.…”