Coeval folding and boudinage under plane strain with the axis of no change perpendicular to the layer

Abstract

Plane-strain coaxial deformation of a competent plasticine layer embedded in an incompetent plasticine matrix was carried out to improve our understanding about the evolution of folds and boudins if the layer is oriented perpendicular to the Y-axis of the finite strain ellipsoid. The rock analogues used were Beck's green plasticine (matrix) and Beck's black plasticine (competent layer), both of which are strain-rate softening modelling materials with a stress exponent n=ca. 8. The effective viscosity g of the matrix plasticine was changed by adding different amounts of oil to the original plasticine. At a strain rate _ e of 10 À3 s À1 and a finite strain e of 10%, the effective viscosity of the matrix ranges from 1.2·10 6 to 7.2·10 6 Pa s. The effective viscosity of the competent layer has been determined as 4.2·10 7 Pa s. If the viscosity ratio is large (ca. 20) and the initial thickness of the competent layer is small, both folds and boudins develop simultaneously. Although the growth rate of the folds seems to be higher than the growth rate of the boudins, the wavelength of both structures is approximately the same as is suggested by analytical solutions. A further unexpected, but characteristic, aspect of the deformed competent layer is a significant increase in thickness, which can be used to distinguish plane-strain folds and boudins from constrictional folds and boudins.

Formation of chocolate-tablet boudins: Results from scaled analogue models

“…Layer thickening during boudinage is not uncommon. A similar weak thickening of the competent layer, as observed in the present study, has been described from viscous boudins produced experimentally under bulk plane strain with the stiff layer perpendicular to the Y-axis (Enama Mengong and Zulauf, 2006). Thickening of the boudins, with respect to initial layer thickness, can be explained by in-plane stretch of the matrix material inside the neck during advanced stages of boudinage.…”

“…The boudin aspect ratio is an all cases >>1. A non-linear relation between layer thickness and boudin width (number of boudins) and a boudin aspect ratio >>1 has been described from boudins produced experimentally in a viscous environment under plane strain and under bulk pure flattening (Enama Mengong and Zulauf, 2006;Zulauf et al, 2011b), whereas brittle boudinage results in a boudin aspect ratio of ca. 1 and in a linear relation between boudin width and layer thickness Zulauf et al, 2012).…”

Pinch and swell structures: evidence for strain localisation by brittle–viscous behaviour in the middle crust

“…Moresi and Mühlhaus, 2006), it is generally accepted that the most suitable rheological model for the middle to lower crust is thermally activated viscous flow (e.g. Molnar and Houseman, 2004). However, the assumption of viscous flow for the middle continental crust is now increasingly being questioned (Brantut et al, 2013;Bürgmann and Dresen, 2008;Karrech et al, 2011;Mancktelow, 2006Mancktelow, , 2008Mancktelow, , 2009Mancktelow and Pennacchioni, 2005;Pennacchioni and Cesare, 1997;Pennacchioni and Mancktelow, 2007;Regenauer-Lieb and Yuen, 2008).…”

Determining relative bulk viscosity of kilometre-scale crustal units using field observations and numerical modelling

“…Zuber and Parmentier, 1986). Analogue experiments have concentrated mainly on pinch and swell structures in non-Newtonian flow regimes (Neurath and Smith, 1982;Kobberger and Zulauf, 1995;Zulauf and Zulauf, 2005; Mengong and Zulauf, 2006). The refinement of numerical models has allowed A C C E P T E D M A N U S C R I P T 6 more detailed analysis of the impact of the viscosity ratio between adjacent layers in both Newtonian and non-Newtonian flow regimes (Abe and Urai, 2012;Schmalholz and Maeder, 2012;Komoróczi et al, 2013;Gardner et al, 2015;Gardner et al, 2016).…”