Description:

_Journal of Structural Geology 33 (2011) 38e50_ Contents lists available at ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com locate jsg Kinematic interpretation of shearband boudins: New parameters and ratios useful in HT simple shear zones Jorge Pamplona a,*, Benedito C. Rodrigues a Centro de Investigação Geológica, Ordenamento e Valorização de Recursos, Universidade do Minho, Campus de Gualtar, 4710-057 Braga, Portugal b Centro de Geologia da Universidade do Porto, Rua do Campo Alegre, 687, 4169-007 Porto, Portugal Article info Article history: Received 30 December 2009 Received in revised form 12 October 2010 Accepted 27 October 2010 Available online 12 November 2010 Keywords: Shearband boudins Geometric parameters HT simple shear zone Kinematics Abstract Shearband boudins (asymmetric boudins showing slip along the inter-boudin surface, which is synthetic with respect to the bulk shear sense) are ubiquitous and well exposed in HT simple shear zones. The present work aims to extend the methodology of analysis of shearband boudins developed by Goscombe and Passchier (2003). Such shearband boudins represent complex objects that require an adequate methodology for unambiguous kinematic interpretation. We propose new geometric parameters (Bbs, B-t, c0, D0, d, j0) in order to describe and identify, with confidence, the kinematics of boudinage in this particular geological framework. The key-observation for kinematic interpretation in simple shear regimes is the boudin axis (Lb) that is commonly present, excluding the necessity to identify the regional stretching lineation (Lx) in the metasedimentary matrix. In monoclinic HT simple shear zones, this approach involves the use of a local displacement plane (Sx), which is always normal to the boudin axis (Lb). © 2010 Elsevier Ltd. All rights reserved. 1. Introduction The terms boudin and boudinage were used for the first time by Lohest et al. (1908) to describe sausage-like structures occurring in psammite layers of the Lower-Devonian metasedimentary series in the Mardassonn quarry, near Bastogne (Ardenne, Belgium). In fact, these original structures look like boudins, but resulted from a sequence of deformation episodes that generated “textbook examples of mullions” (cf. Sintubin and Urai, 2007). The concept of boudinage has evolved from a purely descriptive term, without any kinematic implications (e.g., Lohest et al., 1908; Wegman, 1932), passing through a kinematic approach (Cloos, 1947), to the actual kinematic definition highlighted by Price and Cosgrove (1990) and later adopted by Goscombe et al. (2004). The placement of boudins in the universe of strained bodies in a ductile matrix may be taken as the response to the following question: what enables a body to be deformed as a boudin? This requires the consideration of several concepts, such as viscosity contrast, strain and mechanical constraints, external morphological axial ratios and deformation regimes (simple shearing vs. pure shearing). * Corresponding author. Tel.: +351 253604307; fax: +351 253678206. E-mail address: jopamp@dct.uminho.pt (J. Pamplona). 0191-8141 $ e see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsg.2010.10.004 The difference in viscosity between the veins and the matrix has been cited as the major factor that determines the evolution of boudinage (Ghosh and Sengupta, 1999; Mandal et al., 2000; Bons et al., 2004; Goscombe et al., 2004; Treagus and Lan, 2004). Nevertheless, this statement is not sufficient to explain the generation of all types of boudins, and has been questioned in several studies. When structures, like foliation boudins are generated, authors like Mandal et al. (2000) admit the existence of boudin genesis at low viscosity contrast, while Arslan et al. (2008) suggest that genesis of foliation boudins may be independent of viscosity contrast. Indeed, such structures only have a certain geometric identity with boudins and were generated in a distinct way. In situations of high viscosity contrast, boudinage is controlled by the occurrence of fracturing with simultaneous shearing and extensional components, which affect the strained bodies in the ductile matrix (Mandal et al., 2000). These authors define different types of boudins for high strength ratio between individual brittle and ductile layers using the relationship between tensile stress and compressive stress acting in brittle layers. Analysis of boudinage usually considers bodies of infinite dimension. In Nature, this geometric condition cannot occur and is rarely approximated. There is a shape ratio representing the limit at which the tabular structure ceases to generate boudins and will behave instead as a solid particle in ductile matrix, described by the models of Jeffery (1922) and Eshelby (1957). Theoretically, the rotation of these bodies is locked only for very low a:c shape ratios (external morphological axis: a e short axes and c e long axes), giving rise to the possibility that boudinage will occur. Ghosh and Ramberg (1976) and Rambousek et al. (2005), discussing the influence of the a:c ratio on the rotation of elongate rigid inclusions during combined pure and simple shear, confirmed the stabilization of long axes parallel to the shear plane when the a:c ratio reaches a suitable value. As stated above, different authors at different times have different approaches to explain boudinage phenomena. From this summary remains that the three major factors that control boudinage are: the deformation regime (pure shearing vs. simple shearing), the external morphological axial ratio (shape ratio) and the viscosity contrast. A methodology for boudin analysis was recently established by Goscombe et al. (Goscombe and Passchier, 2003; Goscombe et al., 2004). Nevertheless, this methodology does not clarify all the types of boudins and usually has application only in ideal geometric cases. Therefore, the present work results from the need to better define the criteria for interpreting the kinematics of boudins, extending the analysis to the case of HT simple shear zones and some ill-defined shearband boudins. For the easiest perception of the use of boudins as kinematic indicators in simple shear zones, it is necessary to define geometric parameters that must be simple to record and combine. Frequently, the geometry of the boudins leads to an ambiguous interpretation of their kinematics. The proposed methodology seeks to overcome this limitation. The approach developed uses field data from the sector of Salgosa that presents an excellent set of outcrops of different boudins. This sector is part of the Malpica-Lamego ductile shear zone (MLDSZ), a major crustal Variscan structure in the NW of the Iberian Peninsula (Rodrigues et al., 1999; Llana-Funez and Marcos, 2001; see Section 3.1). 2. Geometric analysis of boudins and significance of parameters A recent boudin definition considers it as a body resulting from a process of disruption of layers, bodies or foliation planes within the rock mass as a response to extension along the enveloping surface (Goscombe et al., 2004). There are other bodies with geometric affinities with boudins but subjected to a different genetic interpretation. These are the cases of pseudo-boudins or misleading boudin-like structures referred to by Bons et al. (2004), for example mullions and sigmoidal structures. Boudins can be classified into a kinematic and geometric scheme as proposed by Goscombe et al. (2004). Table 1 presents a summary of such classification. 2.1 Definition of parameters In its simplistic form, the shearband boudin “anatomy” can be described as a parallelepiped body with four opposite sides two by two, whose edges are alternately acute and obtuse, referred herein respectively as the sharp tip (S-t) and blunt tip (B-t). The geometric analysis as proposed by Goscombe and Passchier (2003) identifies the group of parameters that are described in Table 2 and Fig. 1. The most important parameters are Lb, L, W, q, D, and some relationships established between them (L W, D W, L W vs. q and D W vs. q). In addition to the parameters highlighted as the most significant to shearband boudin analysis in Table 2, Goscombe et al. (2004) present a set of other parameters and related concepts used in boudin characterization. For the dependence with the inter-boudin surface (Sib), the concepts of slip sense (lateral displacement along Sib, with respect to bulk shear sense), vergence (relation between bulk shear sense and Sib inclination) and “drag” on Sib (sense of Table 1 Classification of boudin types (adapted from Goscombe et al., 2004). Kinematic class Boudin block geometry Boudin types Geometric description of boudin No slip along the inter-boudin surface Symmetric Orthorhombic symmetry that is distinguished by the shape of interboudin surface: straight in torn boudins (with or without vein infill) and smooth for drawn boudins Antithetic slip Slip along the inter-boudin surface Asymmetric Synthectic slip Monoclinic symmetry with an angular rhomb-shape and sharp, straight inter-boudin surface Monoclinic symmetry with rounded and even sigma shape, and curviplanar inter-boudin surface 40 J. Pamplona, B.C. Rodrigues Journal of Structural Geology 33 (2011) 38e50 Table 2 Most relevant geometric and kinematics parameters of boudins defined by Goscombe and Passchier (2003) and introduced in this work. Sb is the original surface of boudin block with host rock and represents the final external surface of boudin; Sib is the inter-boudin surface that means, either a discrete slip surface separating boudins or an imaginary surface between two adjacent boudins. References Parameters introduced by Goscombe and Passchier (2003) Parameters introduced in present work Parameters Lb L W D q c’ type-I D’ Bbs B-t d J’ Description Long axis of boudin Ключевые слова: royal society, rodrigues, shear zone, measure, monoclinic symmetry, clockwise rotation, abstract, point, s-t, major factor, basement, synthetic, tectonophysics, servios geolgicos, viscosity contrast, analysis, sharp, rodrigues journal, parallel, intersection point, deformation intensity, bbs, bulk kinematics, ?eld, geology, geol, kinematic, result, journal structural, dimensional parameter, angular relationship, broad range, fernandes, observation, shear, pamplona, comparison, boudin, min max, shearband boudins, approach, sx, bbs orientation, angle bbssn, boudin type, observation plane, simple, stretching, bbs b-t, vorticity axis, boudins, quasi-normal distribution, orientation, domino boudins, swanson, shape, elsevier, holtz, surface, sib, salgosa sector, kretz, journal, author, parameter, kinematic interpretation, symmetry, boudinage, asymmetric boudins, relative position, valid, orthogonal projection, inter-boudin surface, structure, wa, london, eshelby, mineral, segment dened, bosworth, pamplona rodrigues, zone, eld, ductile, measured, geometric, width, consecutive sharp, foliation, mandal, asymmetry, displacement, relationship, methodology, high degree, long, position, mldsz, mldsz belong, bimodal tendency, matrix, kinematics, geological, university, studied, sinistral, angular, lohest, shear sense, case table, ht, table, allochthonous complex, table goscombe, goscombe, stretching lineation, structural geology, lb, proposed parameter, boudin body, strained body, identi?cation, type-i orientation, proposed, studied body, kinematic analysis, antithetic rotation, asymmetric, plane, interpretation, lineation, goscombe passchier, rst approximation, measurement, passchier, sector, type-i surface, nearest b-t, simple shear, variscan belt, consistent determination, criterion, etchecopar, directional parameter, relative, inter-boudin, regional, ghosh, ha, shearband, rodriguez, shearband boudin, lw, type, sn, type-i, boudin train, b-t, journal structural geology, internal, geometric analysis, sense, practical difculty, data, body, bulk, blunt, acute angle, original layer, adjacent boudins, rock, rotation, axis, train, eld data, iberia, host rock, variscan, ribeiro, shear plane, ductile matrix, american, de?ned, high, foliation boudins, jeffery, angle, deformation, ratio, work, slip, bulk shear, b-t position, antithetic, structural, geophysical, abstracts