Shear zone-related folds

Abstract

Folds in ductile shear zones are common structures that have a variety of origins. These can be pre-existing folds that become modified or folds developed during the shearing event. Among the syn-shearing folds, a second subdivision is based on the relative age of the folded surface, which can be pre-existing or newly formed during the shearing event. In each of the three categories final fold geometry and orientation show complex relationships with the kinematic frame. The final fold geometry depends on the vorticity within the shear zone, the rheology and the initial orientation of the folded surface relative to the kinematic framework. It follows that folds are complex structures, difficult to use as kinematic indicators. However, in shear zones where undeformed wall rocks with pre-shear structures are accessible and where kinematics can be well established, folds can provide a valuable natural means to understand the initiation and evolution of structures under non-coaxial regimes. We point to the need of discriminating among different plausible categories, based on the nature of the folded surface and on the inherent structural features of each category.

Introduction

Folds are common structures in many ductile shear zones. The origin of these folds is variable, as their development may predate, be contemporaneous with, or postdate the shearing event. The origin of folds in the interiors of some shear zones cannot always be determined, and they can be referred to as simply intramylonitic folds. However, many ductile shear zones display a complete transition from unsheared wall rock to highly strained domains and, in these, distinctions among different types of folds may be made. This is the case for the Cap de Creus shear zones (Eastern Pyrenees, Spain), on which this study is essentially based.

Folds and other common meso- and microstructures in shear zones (e.g. crenulations and boudins) are often asymmetric. This asymmetry is regarded as a kinematic indicator (Simpson and Schmid, 1983, White et al., 1986, Choukroune et al., 1987, Bjornerud, 1989, Hanmer and Passchier, 1991, Passchier and Trouw, 1996). However, the final geometry, symmetry and orientation of a shear-related fold are influenced by many variables other than the shear sense. In consequence, folds often have complex links with kinematics, as evidenced in shear zones where shear sense can be established from other kinematic criteria.

The aim of this work is to present in a systematic way different situations in which the geometry of folds inside a shear zone may be associated with the shearing event. These different situations are reviewed qualitatively to gain some understanding of the common problems associated with the genetic and kinematic interpretation of shear zone-related folds.

Three different basic categories can be envisaged in which the final geometry of folds becomes intimately associated with the shearing event: sheared pre-existing folds, shear-related early folds and shear-related late folds (Fig. 1). Folds overprinting shear-related structures but unrelated to the shear event, although common in nature, will not be examined in this work.

The first category arises in which shear zones cut across a country rock already containing folds. The final geometry of the folds is modified by the overprinting effect of shearing.

The second category occurs when shear zones cut across rocks containing planar fabrics (i.e. lithological layering or penetrative foliations). Such planes are deflected towards parallelism with the shear plane. Deflection of the pre-existing surface is commonly accompanied by the development of mechanical instabilities, leading to buckling structures that are subsequently modified by shear intensification (Ramsay, 1980, Skjernaa, 1980). Such folds are thus contemporaneous with the localization and development of the shear zone.

A third common category occurs when a newly formed shear zone-related foliation (e.g. a mylonitic foliation) or a stretched layer inside the shear zone becomes unstable during shearing (Carreras et al., 1977, Cobbold and Quinquis, 1980, Platt, 1983, Ghosh and Sengupta, 1984, Mies, 1991, Mandal et al., 2004). In consequence, a single fold or a train of folds nucleate inside the shear zone, and these are subsequently affected by continued shear. Although these are also syn-shear folds, their nucleation often starts after a considerable amount of deformation has taken place and, thus, they should be differentiated from the previously mentioned ones.

Combinations of the three basic categories are not only possible but common in nature. Despite the significant differences among the three categories, it is not always easy to differentiate one from another, and they are often not clearly distinguished in the literature. This difficulty appears both in distinguishing sheared pre-existing folds from shear-related early folds and also in separating this latter category from shear-related late folds. In the first case, the difficulty is greater if the deformation facies does not vary significantly with time, and a progressive shearing event initiates with the formation of cross-folds and ends with development of discrete shear zones (Sanderson, 1973, Williams, 1978, Skjernaa, 1989). This problem is documented for a variety of settings and scales, as shown by Graham (1978) when referring to folds in a shallow seated, wrench dominated domain. Iglesias and Choukroune (1980) also describe folds developed under progressive deformation associated with shear zones which cut across folded domains. In those settings where repeated folding occurs within non-discrete high strain zones or shear zones with no direct access to the undeformed wall rock and to the marginal zone (e.g. Ghosh and Sengupta, 1984, Ghosh and Sengupta, 1987), it is unrealistic to attempt to distinguish between the two categories of syn-shear folds. However, this is justified by the fact that in some settings two or three of these categories coexist and can be identified and distinguished.

In addition to these three basic categories, other complex shear-related folds can occur, such as folds and deflections in syn-tectonic veins or dykes. These other cases will be briefly treated separately from the three basic categories.