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

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 η of the matrix plasticine was changed by adding different amounts of oil to the original plasticine. At a strain rate of 10⁻³ s⁻¹ and a finite strain e of 10%, the effective viscosity of the matrix ranges from 1.2×10⁶ to 7.2×10⁶ Pa s. The effective viscosity of the competent layer has been determined as 4.2×10⁷ 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.An erratum to this article can be found at     

岩石非弹性应变恢复(ASR)地应力测试方法中柔度研究进展评述

本文通过分析国内外非弹性应变恢复(ASR)法地应力测量研究现状,并对ASR法中重要的柔度参数作了系统整理,认为该测量方法在理论上逐渐趋于成熟,在技术应用上有广阔的前景,但该地应力测量原理中,针对温度、应力环境(作用应力)对ASR岩石柔度影响的研究相对较少,还存在一些缺陷,为此提出了该地应力测试方法有待改进或补充的建议,为完善ASR法地应力测量原理指明了研究方向,对今后ASR地应力测量技术的完善和发展具有重要的意义。     

Structural arcuation in the Berwyn Hills,North Wales

New mapping has better defined the well known arc in Acadian (late Caledonian) structures centred in the Berwyn Hills. This arc, convex to the north-west and about 20 km across, is defined by the strikes of bedding and cleavage and the trends of fold hinges. In contrast, a mineral lineation within the cleavage has a north-north-westerly plunge throughout the arc. It approximates to the long (X) axes of measured strain ellipsoids. A primary origin for the arcuation is implied by this consistency, by the absence of secondary cleavages and by opposing senses of fold transection across the arc. The new data favour the hypothesis of shortening of the Lower Palaeozoic cover against and over a pre-existing basement high inferred from geophysical data and from metamorphic and sedimentary facies.