Some considerations on the kinematics of chevron folds F. Bastida a, * , J. Aller a , N.C. Toimil b , R.J. Lisle b , N.C. Bobillo-Ares c a Departamento de Geologı ´a, Universidad de Oviedo, Jesu ´s Arias de Velasco s/n, 33005 Oviedo, Spain b School of Earth, Ocean and Planetary Sciences, Cardiff University, Cardiff, CF10 3YE, UK c Departamento de Matema ´ticas, Universidad de Oviedo, 33007 Oviedo, Spain Received 31 August 2006; received in revised form 26 February 2007; accepted 5 March 2007 Available online 15 March 2007 Abstract Geometrical modelling and field analysis of chevron folds suggest that these structures are a result of the combination of several kinematical mechanisms, whose magnitude and order of application vary within certain limits. A possible mechanism operating early in the fold growth is homogeneous layer shortening, whose contribution is restricted by the high competence contrast that the multilayers developing chevron folds usually exhibit. In the early stages of folding, when the curvature is small, equiareal tangential longitudinal strain (ETLS) is an essential mech- anism, since the operation of parallel tangential longitudinal strain (PTLS) or flexural flow (FF) would give rise respectively to area changes or strain in the limbs of the final fold that are too high to be geologically realistic. After folding by ETLS, probable mechanisms are PTLS and FF, which can operate in this order or simultaneously. In general, FF is necessary at the last stage of buckling, although the increment of folding due to this mechanism can be very small. High values of slip between layers and area change produced in the later stages of chevron folding can bring an end to buckling, probably at an interlimb angle value of 60e70  , and induce the onset of homogeneous strain (HS). This strain is not coaxial in many cases, with simple shear playing an important role, and gives rise to asymmetrical folds. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Structural geology; Chevron folds; Folding mechanisms; Mathematical modelling 1. Introduction Chevron folds can be defined as symmetric or slightly asymmetric angular folds that usually show an acute interlimb angle (Fig. 1). They are common in multilayers of alternating competent and incompetent rocks and their geometry, kine- matics and mechanics have been studied by many authors (de Sitter, 1956, 1958; Bayly, 1964, 1976; Ramsay, 1967, 1974; Ghosh, 1968; Chapple, 1969, 1970; Johnson, 1970; Smythe, 1971; Johnson and Honea, 1975; Dubey and Cobbold, 1977; Casey and Huggenberger, 1985; Narahara and Wiltschko, 1986; Johnson and Pfaff, 1989; Pfaff and Johnson, 1989; Tanner, 1989; Stewart and Alvarez, 1991; Fowler and Winsor, 1996; Fletcher and Pollard, 1999; Pollard and Fletcher, 2005; among others). Geometrical-kinematical studies (de Sitter, 1956, 1958; Ramsay, 1967, 1974; Ramsay and Huber, 1987; Pollard and Fletcher, 2005) are based on theoretical models that take into account the peculiarities of natural chevron folds and per- mit predictions to be made about the strain and displacements undergone by the folded rocks as well as the minor structures that can develop associated with this type of fold. Among the conclusions obtained in these studies are: e When the multilayer is made up of competent layers, the development of chevron folds involves rotation of the limbs with associated slip between layers (de Sitter, 1956, 1958; Ramsay, 1967; Pollard and Fletcher, 2005). When chevron folds occur in sequences with alternating competent and incompetent beds, the slip between layers * Corresponding author. Fax: þ34 98 510 3103. E-mail address: bastida@geol.uniovi.es (F. Bastida). 0191-8141/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsg.2007.03.010 Journal of Structural Geology 29 (2007) 1185e1200 www.elsevier.com/locate/jsg