Ductile Deformation, Folds and Fabrics: Fabrics and Deformation Mechanisms

Chapter 2 Fabrics and deformation mechanisms 2.1 Fabrics A fabric is a particular arrangement of component features (called fabric elements) in a rock. This arrangement is usually regular: for instance the horizontal layers of sedimentary rocks are a kind of fabric, but we also refer to random fabric, which is by definition not regular. We distinguish between primary and secondary (tectonic) fabrics: the first formed with the rock and includes, flow lineation, sedimentary lam­ ination and bedding and flow foliation. Secondary fabrics are ones that have formed as a result of deformation. Another key distinction is between penetrative or con­ tinuous fabric – which is observable at all scales, as opposed to non­penetrative or spaced fabric, where there is obvious spacing between the fabric elements (eg. so­ lution cleavage seams or fractures). Finally we distinguish between planar fabrics – called foliation – and linear fabrics (lineations). 2.1.1 Foliations A foliation is the general term for any kind of planar fabric, including: bedding, lamination, flow foliation, cleavage, schistosity and gneissosity. Many foliations are products of specific tectonic environments, including a particular orientation of principal strain axes. Depending on the rock, tectonic foliations may include spaced fracture cleavage, slaty cleavage, crenulation cleavage, schistosity, gneissosity. Rocks can record many generations of fabrics, from the primary (eg. bedding) to fabrics associated with many generations of tectonic episodes. The relative timing of these is worked out using cross­cutting relations. The physical appearance of the fabrics ultimately reflects the materials, metamorphic grade and strain­rates and so careful observation and analysis of fabrics is crucial to teasing out the tectonic history of a deformed rock. Cleavage Cleavage is a secondary (tectonic) fabric formed under low T conditions which im­ parts on the rock a tendency to part or split along it. The classic example is slate, where the cleavage is so perfect that the rock is easily quarried into thin, flat slabs for pool tables and roofing materials. Disjunctive cleavage A type of spaced cleavage defined by an array of more or less parallel fabric domains (called cleavage domains). Within each fabric domain, there is typically evidence of pressure solution (hence this is often called solution cleavage or stylolitic cleavage). These domains are separated by intervals called microlithons. The spacing of the cleavage domains (and hence the width of the mi­ crolithons) is quite variable, but commonly observed on a cm­scale. Pencil cleavage This is characterized by the rock breaking into elongate, pencil­ like shards. Pencil cleavage appears to result from the intersection of two spaced cleavages, one of which forms due to the primary preferred orientation of clays (im­ parted during sedimentation and compaction), the second may form axial planar to folds or perpendicular to layer­parallel shortening. This causes detrital clays to fold and rotate, fine grained soluble materials to undergo pressure solution and new clay minerals to crystallize. When the original sedimentary parting and the new (spaced, and probably pretty weakly developed) cleavage intersect at fairly high angles, pen­ cil cleavage results. With more strain, it is possible to get the complete erasure of the original parting and production of slaty cleavage. Slaty cleavage Slaty cleavage forms by the same processes as outlined above: ro­ tation and folding of detrital clays, pressure solution and recrystallization of clay minerals. The difference is that the cleavage domains are so tightly spaced that the fabric is completely penetrative and continuous at all but high power magnification. The formation of slaty cleavage is coincident with the onset of low grade metamor­ phism, particularly the transition from smectite (a poorly ordered clay) to illite (a much more ordered clay). This transition is temperature sensitive and measurable by XRD. Phyllitic cleavage and schistosity Phyllitic cleavage is characteristic of low grade (low greenschis). It is produced by the preferred alignment of clay and mica min­ erals. At the lowest grade these will be illite, transitioning to white mica (sericite / muscovite) and chlorite at higher grades. If the cleavage is formed syndeformation­ ally, the phyllosilicate minerals grow with a strong preferred orientation. At the hand sample scale, the preferred orientation of the phyllosilicates imparts a very strong parting or cleavage, although the phyllosilicates are very fine grained and individual minerals are usually not visible without magnification. At higher grades, sericite­ chlorite assemblages are replaced with coarser muscovite and / or biotite, and the individual mica grains are visible under close inspection. At this point, the rock is re­ ferred to as a schist. Schistose rocks are usually named according to the assemblage of metamorphic minerals contained (biotite, muscovite, garnet, staurolite, kyanite, etc.). Crenulation cleavage If rocks containing an early, closely spaced cleavage (this can be slaty cleavage, phyllitic cleavage or schistocity) are shortened in a direction at a low angle to the original fabric, the older fabric folds at a very small scale. This produces a characteristic "wrinkled" appearance, somewhat like the baffles in an accordion. When a very regularly closely spaced cleavage is crenulated, the spac­ ing of the small folds is very uniform, and fold hinges and fold limbs line up. This, and possibly pressure solution or shear thinning in the limbs defines a new folia­ tion, which is called a crenulation cleavage. Crenulation cleavage can be symmetric or asymmetric. Eventually, pressure solution (which will tend to transport material from the limbs to the hinges) and shear of the limbs can lead to an almost complete obliteration of the early fabric. This process is called transposition, and the new fabric is called a transposition foliation. gneissic layering . Gneissic layering is a coarse (mm to tens of cm scale) composi­ tional layering in metamorphic rocks. This compositional banding can be produced a variety of different ways. Primary layering is inherited from the original sedimen­ tary layering and compositional differences of a sedimentary protolith. Transposi­ tion layering is produced by isoclinal folding and shear of fold limbs of the original layering. Compositional layering can also be produced from metamorphic differen­ tiation, or the injection of melts either along the layering or rotated through shear into the foliation. Foliations and folds Planar fabrics can form in a shear zone, but often form in a specific geometrical relationship to a series of folds. Specifically, in folded rocks, planar fabrics are com­ monly parallel to the axial plane of the fold. You might find it useful to think of exam­ ples where this might not be the case (consider an interbedded quartzite and shale being folded). Foliations in folded terrains can be extremely useful: if they are truly parallel (or close to parallel) to the axial plane, the intersection of the layer being folded and the foliation will result in a line that is parallel to the fold axis. Moreover, there is a specific geometrical relationship between bedding and axial planar folia­ tion that allows you to infer the position of fold closures. These are key relationships that are the bread and butter of field geology in folded terrains. See TM, section 13.5. In areas of multiple deformations, you can have synformal anticlines and antiformal synclines and so on. The key to working this out is to be able to establish “younging” (or “way up”), and the geometrical relationship between the limb of a structure and an axial planar fabric. Once a foliation is formed, it can be modified by further deformation. Some processes: rotation during shear (see fig 14.4 for different mechanisms of rotation), small scale folding (crenulation), recrystallisation. See TM chapters 13 and 14 for material on the formation and modification of planar and linear fabrics (they are short chapters). The preservation of earlier generations of fabrics within porphy­ roblasts (eg. garnet) and within low strain zones (the hinges of later folds) is ex­ ploited by geologists who then label them S1, S2, S3 etc... Of particular importance is the concept of transposition foliation. Such a fabric is a composite fabric: mul­ tiple generations of planar features are found to be parallel. The common case is to find compositional layering (e.g. bedding, called S0) parallel to a tectonic fabric. How does this happen? (Chapter 13). 2.1.2 Lineations A lineation is the generic term for a linear fabric in a rock. An intersection lineation is formed by the intersection of two planes. Typically, an intersection lineation is formed by the intersection of a layer being folded and a cleavage parallel to the axial plane of that fold. In this case, the intersection lineation will form parallel to the fold hinge, so observation of the orientation of the intersection lineation can yield very useful information about the geometry of the folds in a region. A mineral lineation is defined by the preferred orientation of minerals, especially minerals that have a well­defined long axis, such as amphiboles. Since the orientation of metamorphic minerals often reflects the state of strain, it is very common to find minerals ori­ ented parallel to a principal strain axis especially parallel to fold hinges. Stretching lineations are defined by the orientation of parts of the rock that have been strained such that they have a well­defined long axis. For example, cobbles in a conglomerate that have suffered strain will often be observed to a well­defined preferred orienta­ tion of their long axis. At a smaller scale, individual minerals – such as quartz grains – can also be stretched, and form a lineation in the rock. Stretching lineations are usually interpretable as being parallel to the maximum principal strain axis. While many lineations are parallel to either regional fold axis orientations or the maximum extension direction, this is not always the case. A boudin line lineation is defined by the stretching of a relatively competent layer in a ductile rock. Instead of homogeneously stretching, this layer is likely to break into swells and necks. With further stretching the pinch and swell structure gives way to a broken line of typically elliptical fragments, called boudins due to their supposed resemblance to sausages. The alignment of the boudins defines a linear fabric element, but this lineation is, in fact, perpendicular to the stretching direction.