A NOTE ON FOLDING MECHANISMS IN THE CAPE FOLD BELT, SOUTH AFRICA

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1 137_144 Fagereng_ van Dijk copy 2012/08/03 10:42 AM Page 137 Å. FAGERENG 137 A NOTE ON FOLDING MECHANISMS IN THE CAPE FOLD BELT, SOUTH AFRICA Å. FAGERENG Department of Geological Sciences, University of Cape Town, Rondebosch 7701, South Africa ake.fagereng@uct.ac.za 2012 June Geological Society of South Africa ABSTRACT The relationship between folding and faulting in the Cape Fold Belt has been raised as an enigma. The mineral deformation mechanisms accommodating folding are integral to the relation between faults and folds. Here I discuss field and microstructural observations of folded rocks from the Laingsburg region in the Western Cape, and the underlying mineral deformation mechanisms accommodating strain in these rocks. In competent units, deformation was dominantly accomplished by flexural slip faulting and jointing; in relatively incompetent layers macroscale flow was accommodated by dissolution-precipitation creep and distributed cataclasis. Combined with previous studies suggesting deformation occurred at lowermost greenschist facies temperatures, these observations indicate that folding in the Cape Fold Belt occurred at temperatures and pressures within the normally frictional regime. Folding and thrust fault development therefore generally occurred concurrently, and partitioning between localised and distributed deformation was governed by factors such as fluid pressure conditions, strain rate, and relative viscosity. Introduction Folds are considered ductile structures, meaning that they appear macroscopically continuous and formed by a macroscopically continuous flow of rock. Such flow can occur by three different mineral deformation mechanisms: (1) cataclastic flow; (2) intracrystalline plasticity (dislocation climb and glide); and (3) diffusive mass transfer (Hadizadeh and Rutter, 1983; Knipe, 1989). Ductile flow is therefore prevalent at high temperatures (>350 to 450 C for quartzofeldspathic rocks) where crystal plastic deformation mechanisms become dominant over frictional sliding (e.g. Sibson, 1977; Brace and Kohlstedt, 1980; Scholz, 1988), but can also occur at shallow crustal levels by cataclastic flow and fluid-assisted diffusive mass transfer (e.g. Gratier and Gamond, 1990). Cataclastic flow refers to grain fragmentation and rotation, frictional sliding and associated dilatancy, and thus represents a microscopically brittle mechanism leading to macroscopically ductile flow (Sibson, 1977). Fluidassisted diffusive mass transfer refers to diffusion, through a fluid phase, of material from high to low chemical potential (generally along a normal stress gradient), and can lead to low strain rate flow at low temperature at low differential stress (e.g. Durney, 1972; Rutter and Mainprice, 1978). The factors governing deformation partitioning between folding and faulting clearly depend on the mineral deformation mechanism accommodating folding. If folding occurs by crystal plastic flow, then temperature is a critical factor controlling the brittle-ductile transition (e.g. Brace and Kohlstedt, 1980). In the case of cataclastic flow and diffusive mass transfer, however, strain rate and fluid pressure are more important controlling factors of brittle-ductile deformation partitioning (e.g. Gratier and Gamond, 1990). Based on chlorite-chloritoid thermometry, meta - morphic grade did not exceed lowermost greenschist facies (~320 C) during deformation in the Cape Fold Belt (CFB) (Frimmel et al., 2001). Hälbich and Cornell (1983) also determined a maximum grade of lowermost greenschist facies for the extent of the exposed CFB, such that the eastern extent of the belt, where the relationship between folding and faulting has been discussed extensively (see review by Booth and Shone, 2002, and references therein) also lies within this low grade of metamorphism. Folding in the exposed sections of the CFB therefore occurred within the relatively cold and shallow brittle regime in the siliciclastic rocks of the Cape and Karoo Supergroups that make up the lithology of the CFB. Thus, it is unlikely that crystal plasticity played an important part in the development of the folds, and the underlying mineral deformation mechanism was therefore either brittle or dependent on solution transfer. The dominant deformation mechanism is important for questions on the relative timing and importance of folding and faulting in the CFB, as raised in recent review articles (Booth and Shone, 2002; Booth, 2011). Here I consider field and microstructural evidence for the deformation mechanisms active during folding in the Cape orogeny. Geological background North-verging folds are well exposed in the Laingsburg region of the Western Cape (Figure 1) in the eastern branch of the CFB. Folded rocks in this area are of the Carboniferous Witteberg Group of the Cape Supergroup, and the Permian to Early Triassic Dwyka, Ecca, and Beaufort Groups of the Karoo Supergroup. The Witteberg Group comprises sandstones and shales in similar proportions, whereas the Dwyka Group is a dominantly marine, glacial diamictite overlain by an, 2012, VOLUME PAGE doi: /gssajg

2 137_144 Fagereng_ van Dijk copy 2012/08/03 10:42 AM Page FOLDING MECHANISMS, CAPE FOLD BELT, SOUTH AFRICA Laingsburg 34 Cape Town Port Elizabeth 100 km pre-cape Cape Granite Table Mountain Bokkeveld Dwyka Ecca and Beaufort Mesozoic/Cenozoic Witteberg Figure 1. Map showing the lithostratigraphy of the Cape Fold Belt and the location of Laingsburg (after Paton et al., 2006; Tankard et al., 2009). overall upward coarsening sequence of deep marine to terrestrial deposits of the Ecca and Beaufort Groups (Tankard, 2009, and references therein). The CFB was deformed in a Carboniferous to Permian compressional event (Hälbich 1983; de Wit and Ransome, 1992) generally referred to as the Cape orogeny. As such, the Cape Supergroup was deposited prior to, and in early stages of, folding; whereas the Karoo Supergroup is inferred to represent syn-tectonic sedimentation in a foreland basin (Catuneanu et al., 1998). Deformation in the eastern branch of the CFB is northward verging, with deformation intensity decreasing northward and becoming insignificant approximately 250 km from the coast (de Wit and Ransome, 1992; Hälbich, 1993; Paton et al., 2006; Lindeque et al., 2011). Field observations Folds in the Laingsburg region are present at outcrop to regional scale. Several folds are related to faults (e.g. Figure 2a), and duplex structures similar to those described by Booth et al. (2004) are present at a range of scales. The folds are upright to steeply north verging, open to tight (Figures 2b; d), and horizontal to gently plunging (variably to the east and west, and commonly doubly plunging). Chevron folds are common in some formations (in the mudstone-dominated Prince Albert Formation in particular). An axial planar cleavage is commonly developed within phyllosilicaterich lithologies (Figure 2d), and cleavage refraction is commonly clearly observed indicating strong variability in viscosity between different layers. Fractures are common within folded sand-rich layers (Figure 2b), and more competent (generally quartz-rich) layers in mudstone sequences (Figures 2c; d). These fractures are generally near-perpendicular to bedding, and tend to concentrate near the hinge region (Figures 2b; d). The intense concentration of extension fractures in some hinges indicates significant fault curvature accommodated by macro-scale fracturing. The com - petent layers commonly preserve sedimentary structures such as cross bedding, indicating they have experienced little internal deformation. Slickenfibre-coated surfaces are in places present along bedding surfaces (Figure 3). The fibres are generally composed of quartz, and show slip directions up-dip along the bedding surface, near-perpendicular to the fold hinge line. The fault motion indicated by the slickenfibres is therefore consistent with flexural slip folding. The example in Figure 3 is from Prince Albert Formation mudstones, where slickenfibre surfaces are present along boundaries between massive quartzrich mudstone and strongly laminated clay-rich mudstone. Similar examples are widespread in the Prince Albert Formation, and other lithostratigraphic units with anisotropy caused by layers of different composition. Quartz veins are also present in places filling tensile fractures near-perpendicular to bedding.

3 137_144 Fagereng_ van Dijk copy 2012/08/03 10:42 AM Page 139 Å. FAGERENG 139 Microstructures Based on field observations, rocks in the CFB in the Laingsburg area can be subdivided into: (1) relatively competent rocks exhibiting little or spaced cleavage, and generally appearing fractured with little macroscopic evidence for flow; and (2) incompetent rocks with strong cleavage and macroscopically continuous appearance. Sandstones that appear folded but fractured at the macroscale (Figure 2b) also show no evidence for microscopic flow (Figure 4a). Quartz grains appear undeformed and show no evidence for dislocation creep, whereas feldspar is largely altered to sericite, but also form relatively intact grains. Microfractures are relatively rare relative to the larger fractures seen at the outcrop scale. No preferential orientation of grains is observed in any mineral in the folded sandstones. Quartz-rich mudstones of the Prince Albert Formation are generally folded in chevron folds and also relatively intensely fractured at the macro-scale (Figure 2c, d). At the micro-scale, these rocks are cut by extension veins and scattered stylolites formed perpendicular to the vein-filled fractures (Figures 4b; c). The rock surrounding the veins appears microscopically undeformed with no clear deformation or preferential orientation of grains. Phyllosilicate-rich rocks from the Prince Albert Formation, on the other hand, have a strong cleavage defined by preferential orientation of white mica (Figure 4d). Scattered grains of other minerals (mostly quartz and feldspar) are generally weakly preferentially oriented, undeformed, but commonly wrapped by phyllosilicate grains (Figure 4d). Evidence for dissolution seams and microfractures is rare in these rocks. Microstructures similar to those in phyllosilicate-rich rocks from the Prince Albert Formation are also seen in the Kweekvlei Formation of the Cape Supergroup. These rocks are strongly cleaved at the macro-scale, and microscopically show clear evidence for preferential Figure 2. Field photographs illustrating the macroscopic appearance of folds in the Laingsburg area: (a) fault-related folds in Laingsburg Formation turbidites, (b) close-up of folded turbidite (interbedded sandstone and shale) in the Laingsburg formation, note high fracture intensity in sandstone in the hinge region, (c) chevron folds in the Prince Albert Formation mudstones, (d) close-up of hinge region in Prince Albert Formation chevron fold, note strong axial planar cleavage within clay-rich layers, and bedding-normal fractures in more competent layers.

4 137_144 Fagereng_ van Dijk copy 2012/08/03 10:42 AM Page FOLDING MECHANISMS, CAPE FOLD BELT, SOUTH AFRICA Figure 3. Example of flexural slip fold in the Prince Albert Formation. Lower hemisphere, equal area, stereoplot shows bedding orientation (great circles) and hanging wall slip directions derived from slickenfibres (arrows). alignment of white mica (Figure 4e). These rocks are coarser grained than the Prince Albert Formation shales, but also do not show evidence for dislocation creep, and do not have clear evidence for dissolution-precipitation creep in the form of dissolution seams. Dissolution seams are, however, relatively abundant in folded rocks of the Dwyka Group diamictites. These rocks show a prominent cleavage at the macro-scale, generally developed in fine-grained phyllosilicate-rich matrix and wrapping around more competent clasts. Microscopically, this cleavage is seen as dark seams, which disrupt grain boundaries of adjacent clasts (Figure 4f). These dark seams are interpreted as dissolution residue and thereby provide evidence for volume loss by dissolution in the Dwyka diamictites. Overall, phyllosilicate-rich rocks from both Cape and Karoo Supergroups have a distinct cleavage formed by preferential alignment of white mica. Macroscopically this cleavage is axial planar, and thereby represents a plane perpendicular to the direction of greatest shortening. Phyllosilicate-poor rocks show little evidence for deformation at the micro-scale, are generally fractured at a range of scales, and only contain scattered dissolution seams. No evidence for dislocation creep (crystal plasticity) is observed in any rocks. Discussion and conclusions Folding of relatively incompetent layers Low temperature of deformation (<350 C) and lack of microstructural evidence for crystal plastic deformation excludes dislocation creep as an important mineral deformation mechanism in the CFB at Laingsburg. Because temperatures of deformation in excess of 350 C have not been inferred anywhere in the CFB (Hälbich and Cornell, 1983; Frimmel et al., 2001), ductile flow without dislocation creep is likely a common feature in the CFB. In the Eastern Cape, stratigraphically deeper in the CFB that the rocks exposed at Laingsburg, there also seems to be a mutually cross-cutting relationship between fold and fault structures (Booth and Shone, 2002), consistent with concurrent folding and faulting with deformation style controlled primarily by strain rate, at temperatures somewhat higher than in the rocks exposed near Laingsburg.

5 137_144 Fagereng_ van Dijk copy 2012/08/03 10:42 AM Page 141 Å. FAGERENG 141 Figure 4. Photomicrographs illustrating microscopic deformation features in folded rocks from the Laingsburg region of the Cape Fold Belt. All samples are cut perpendicular to macroscopic axial planar cleavage. (a) sandstone from Laingsburg Formation turbidites, showing no evidence for crystal plasticity or intragranular fractures (cross-polarized light, xpl), (b) near-perpendicular quartz extension vein and scattered stylolites in Prince Albert Formation mudstone (plane-polarized light, ppl), (c) same as (b) in xpl, note vein quartz fibres near perpendicular to the vein walls, and weak preferential orientation of wall rock mica, (d) foliated clay-rich mudstone in the Prince Albert Formation, foliation expressed by preferential orientation of phyllosilicates, (e) phyllosilicate-rich shale from the Kweekvlei Formation, with a strong cleavage expressed by preferentially oriented white mica, (f) cleaved Dwyka 3c diamictite with numerous dissolution seams forming a pressure solution cleavage.

6 137_144 Fagereng_ van Dijk copy 2012/08/03 10:42 AM Page FOLDING MECHANISMS, CAPE FOLD BELT, SOUTH AFRICA Dissolution-precipitation creep can accommodate low-temperature viscous flow (e.g. Durney, 1972), but would commonly leave evidence in the form of dissolution seams of residual, undissolved material. Such seams are common in the Dwyka diamictite, but rare in other folded rocks at Laingsburg. It is therefore a significant distributed flow mechanism in some folded layers where conditions were suitable for this stress-driven fluid-assisted deformation mechanism. Units where it was important are likely those which were fluid-saturated, fine grained, and contained significant stress gradients. The Dwyka Group diamictites would be ideal as the clay-rich matrix would be fluid-rich, and the mixture of competent clasts in an incompetent matrix would lead to stress and strain gradients within diamictite layers. Dissolutionprecipitation creep may also have accommodated distributed flow in other clay-rich formations, such as in the formation of a penetrative cleavage in the Prince Albert and Kweekvlei Formations (Figures 4d; e). If dissolution has been a significant mineral flow mechanism in the incompetent, phyllosilicate-rich units, dissolution seams are not obvious petrographically, but may be detectable from silica depletion in the rocks. This would allow for a local silica source for slickenfibre surfaces, analogous to the local vein source determined by Fisher et al. (1995) in fine grained rocks of the Kodiak Complex, Alaska. Alternatively, strong axial planar cleavage in incompetent rock (Figures 2d; 4d; e) can form mechanically by cataclastic flow. Cataclastic flow as defined by Sibson (1977) involves pervasive grain fragmentation. However, Borradaile (1981) suggested that grain flow can also occur by independent particulate flow in unconsolidated, fluid saturated rocks deforming by frictional grain-boundary sliding with no modification of grain shape. Because only minor intragrain deformation is observed in rocks from the CFB (e.g. Figure 4a), any grain flow likely occurred primarily by independent particulate flow (Borradaile, 1981; Marques et al., 2010). In Karoo Supergroup rocks, which were deposited syntectonically, some mechanical reorientation of white mica to form an axial planar cleavage is likely to have occurred when the sediments were poorly cemented, leading to a cleavage defined by preferential orientation of mica with no evidence for dissolution or grain fracturing (Figure 4d). However, dissolution-precipitation creep is likely to have played a primary role in more cemented (at the time of deformation) Cape Supergroup rocks (e.g. Figure 4e), as well as a critical role in further development of cleavage in Karoo Supergroup rocks as they were compacted and cemented. Folding of relatively competent layers Competent beds commonly preserve sedimentary structures and show pervasive discontinuous deformation by faulting and fracturing. Cleavage is either weak or not present, as expected in phyllosilicatepoor rocks. Evidence for flexural slip along bedding surfaces (Figure 3) indicates that frictional sliding along pre-existing planes was important, as was beddingnormal fracturing of competent layers, particularly near fold hinges (Figures 2b; d). Thus, localised brittle deformation appears dominant over distributed cataclastic flow. Overall, these observations and inferences indicate that frictional mechanisms, dominated by shear slip on bedding planes, and tensile fracturing of competent layers, were the predominant mechanisms during folding of competent layers within Karoo and Cape Supergroup rocks in the Cape orogeny. It is not unusual that bending of sedimentary beds by bedding normal jointing, and associated sliding on bedding surfaces, is interpreted to accommodate substantial horizontal shortening by folding in the brittle regime (e.g. Chapple and Spang, 1974; Cooke et al., 2000; Ismat and Mitra, 2005) as documented in similar rocks and conditions to the CFB in the Sheep Mountain Anticline of the Laramide orogen (Bellahsen et al., 2006; Sanz et al., 2008; Savage et al., 2010). Concurrent brittle and ductile mechanisms It is clear that in the Laingsburg area, competent units were folded by brittle mechanisms, at the same time as less competent layers, where dissolution-precipitation creep was a preferred mechanism, deformed by low-temperature viscous flow. Variation in strain accommodation between competent and incompetent lithologies, from tensile fracturing and flexural slip in competent units to distributed dissolution-precipitation creep and cataclastic flow in incompetent units, is not uncommon in fold-and-thrust belts, and dependence of deformation mechanism on mechanical stratigraphy is commonly concluded (e.g. Fischer and Jackson, 1999; Sanz et al., 2008). In a recent review paper, Booth (2011) states the relationship between ductile and brittle deformation as an enigma in the structural understanding of the CFB. The observations presented here, however, indicate that folding in the CFB was largely a brittle process assisted by dissolution-precipitation creep in relatively incompetent stratigraphic units, and therefore likely happened under the same conditions as slip along thrust faults. Major décollements have been inferred at depth under parts of the CFB (Stankiewicz et al., 2007; Lindeque et al., 2011), and slip along such major, low angle thrusts would likely be accompanied by folding and duplex faulting of overlying rocks, as imaged geophysically above décollements in active accretionary prisms (e.g. Westbrook et al., 1988; Moore et al., 2007; Barnes et al., 2010) and associated with exhumed and modelled orogenic thrust faults (e.g. Suppe, 1983; Fischer and Jackson, 1999; Savage and Cooke, 2004). The interpretation of ductile structures in the CFB must therefore be made with attention to underlying mineral deformation mechanisms, and relation to faulting considered in terms of faulting and folding being localised and distributed expressions of brittle

7 137_144 Fagereng_ van Dijk copy 2012/08/03 10:42 AM Page 143 Å. FAGERENG 143 displacement under equal pressure-temperature conditions. Partitioning between folding and faulting may be controlled by several other parameters, such as fluid pressure distribution, lithology, strain rate, consolidation state, and the presence or absence of pre-existing weak planes on which deformation can localise (e.g. Sibson, 1977; Maltman, 1994; Marques et al., 2010; Fagereng and Toy, 2011). There is also likely significant variation in fold mechanism with time, during progressive fold tightening (e.g. Tavarnelli, 1997; Ismat and Mitra, 2005). The rule of thumb suggested here is that variations in strain rate, governed by whether pressure solution creep can accommodate low strain rate flow, leads to dominantly viscous deformation of fine grained, phyllosilicate-rich, layers, and brittle deformation of coarser grained, phyllosilicate-poor units. 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