1. nietxnorphic Ceol., 1996, 14, 15-28 How useful are ‘millipede’ and other similar porphyroblast microstructures for determining synmetamorphic deformation histories? S. E. JOHNSON’ AND T. H. BELL’ ‘School of Earth Scienc-es,Macquarie University, Sydney, New South Wales, 2 7 09, Australia ’Department of Earth Sciences, James Cook University, Townsville, Queensland, 48 1 1, Australia ABSTRACT Oppositely concave microfolds (OCMs) in and adjacent to porphyroblasts can be classified into five nongenetic types. Type 1 OCMs are found in sections through porphyroblasts with spiral-shaped inclusion trails cut parallel to the spiral axes, and commonly show closed foliation loops. Type 2 OCMs, commonly referred to as ‘millipede’ microstructure, are highly symmetrical, the foliation folded into OCMs being approximately perpendicular to the overprinting foliation. Type 3 OCMs are similar to Type 2, but are asymmetrical, the foliation folded into OCMs being variably oblique to the overprinting foliation. Type 4 OCMs are highly asymmctrical, only one foliation is present, and this foliation is parallel to the local shear plane. Type 5 OCMs result from porphyroblast growth over a microfold interference pattern. Types 1 and 2 are commonly interpreted as indicating highly noncoaxial and highly coaxial bulk deformation paths, respectively, during porphyroblast growth. However, theoretically they can form by any deformation path intermediate between bulk coaxial shortening and bulk simple shearing. Given particular initial foliation orientation and timing of porphyroblast growth, Type 3 OCMs can also form during these intermcdiatc deformation paths, and are commonly found in the same rocks as Type 2 OCMs. Type 4 OCMs may indicate highly noncoaxial deformation during porphyroblast growth, but may be difficult to distinguish from Type 3 OCMs. Thus, Types 1-3 (and possibly 4) reflect the finite strain state, giving no information about the rotational component of the deformation(s) responsible for their formation. Furthermore, there is a lack of unequivocal independent evidence for the degree of noncoaxiality of deformation(s) during the growth of porphyroblasts containing OCMs. Type 2 OCMs that occur independently of porphyroblasts or other rigid objects might indicate highly coaxial bulk shortening, but there is a lack of supporting physical or computer modelling. It is possible that microstructures in the matrix around OCMs formed during highly noncoaxial and highly coaxial deformation histories might have specific characteristics that allow them to be distinguished from one another. However, determining degrees of noncoaxiality from rock fabrics is a major, long- standing problem in structural geology. Key words: crenulation clcavage; dcformation history; millipede microstructure; oppositely concave microfolds; porphyroblast growth; porphyroblast rotation; spiral-shaped inclusion trails; strain states. INTRODUCTION A characteristic of some porphyroblast microstructures is that inclusion trails and/or their continuation as matrix foliations form microfolds that are outwardly concave in opposite directions (e.g. Fig. 1). Such oppositely concave microfolds (OCMs) have been identified in and adjacent to several different types of porphyroblasts from various orogenic settings (e.g. Rosenfeld. 1970; Schoneveld, 1979; Bell & Rubcnach, 1980; Bateman, 1985; Bell et al., 1986; Bell & Johnson, 1989; Vernon, 1988, 1989; Reinhardt & Rubenach, 1989; Hayward, 1992; Johnson, 1993a,b; Vernon et al., 1993; Passchier & Speck, 1994; Aerden, 1995), and have generated considerable discussion among meta- morphic and structural geologists (e.g. Bell, 1981; Bell et id., 1986. 1992; Vernon, 1989; Passchier et al., 1992; Gray & Busa, 1994; Johnson & Moore, 1996). This paper examines the formation of OCMs within and adjacent to porphyroblasts, classifying them into five nongenetic types. Two of these types (1 & 2) are of particular interest because they regularly appear in the literature. Type 1 OCMs are found in sections parallel to spiral axes through porphyroblasts with spiral-shaped inclusion trails (e.g. Powell & Treagus, 1967; Rosenfeld, 1970; Schoneveld, 1979; Johnson, 1993a,b; Gray & Busa, 1994). They are commonly considered to form during single deformation events that are highly noncoaxial (e.g. Schoneveld, 1979; Brunel, 1986; Gray & Busa, 1994), and have played a fundamental role in developing specific models of porphyroblast growth during rotation relative to an 15