«By MOLATLHEGI LARTY LOSTMAN MOSEKI STUDENT NO. 208523856 Submitted in fulfillment of the academic requirements For the degree of Master of Science In ...»
The field relationship in Figure 2.17 has been interpreted as indicating that the tonalitic gneiss is older than the megacrystic gneiss. However the discordance between the foliation in these gneisses could be due to rotation of the blocks of tonalitic gneiss during intrusion of the coarse unfoliated felsic material. In support of the preferred interpretation of Figure 2.17, Aldiss (1989) reports that locally the megacrystic granite gneiss intrudes the migmatitic gneisses which in the study area are typically tonalitic in composition.
Although field relations shows that the pink gneissic granite intrudes the tonalitic gneiss and megacrystic gneiss, the character and orientation of deformation fabrics in the pink gneissic granite is the same as that in the tonalitic and the megacystic gneisses, generally trending NE-SW (Fig. 2.18). Deformation in the SFT area predates intrusion of the pink granite but the pink granite has been deformed i.e. it is now gneissic in character and has the regional deformation fabric (S2). This suggests that the pink granite was emplaced syntectectonically (i.e. intruded during deformation). However, the presence of pink granite dykes cutting the foliation (S1) in the older megacrystic granite and tonalitic gneisses (Fig. 2.26 and 2.27) suggest that the intrusion of pink granite was progressive and continued even after deformation. The following magmatic sequence is therefore proposed; emplacement of the precursor to the tonalitic banded gneiss followed by emplacement of porphyritic (megacrystic) granite and finally intrusion of pink gneissic granite. As will be reported later, U-Pb zircon ages obtained from the tonalitic gneiss, the megacrystic granite gneiss and the pink granite gneiss are in keeping with this sequence.
70 CHAPTER 3: STRUCTURE
3.1 INTRODUCTION In this section the structural geology of the study area is documented based on observations and measurements of structural elements, both planar and linear, which are developed at the outcrop-scale. Field photographs are used to illustrate the main structural elements. Localities where the photographs were captured are indicated on the photographs. Orientation measurements are reported in terms of strike/dip for bedding, foliation and axial planes of folds, plunge angle/direction for fold axes and plunge angle/trend for lineations.
The use of minor structures to indicate the geometry of major structures is a wellestablished practice based on the supposition that minor structures identified at single outcrops are a reliable indication of the structural history across the study area. The deformation that is responsible for the formation of a foliation will normally also produces folds. Foliation is however also produced in ductile shear zones (Ramsay and Huber, 1987). When these structures are produced during the same deformation event by the same stress field, they bear a simple geometrical relationship to each other i.e. same orientation of structures. These criteria have been applied in linking different sets of minor folds and related axial planar foliation to specific phases of deformation recognized. The presence of small scale folds at outcrop and map scale is often useful during the analysis of folding. A similar geometry of small scale folds on both limbs of a large scale structure (both s-shaped or z—shaped) would imply an origin by simple shear while different but geometrically compatible fold shapes on both limbs would imply parasitic folding (development by flexure). The parasitic folds should have different shapes on the limbs of a specific larger scale fold (s or z shaped) but unfortunately the occurrence of small scale folds in the study area was limited and insufficient to confirm this relation.This chapter comprises a photographic illustration of deformation structures established from field investigations of small scale structures and concludes with a discussion of these structures in terms of regional analysis and implications. Discussions and conclusions on the structure, evolution and geological history of the study area are covered in Chapter 6.
71 The geological map of the SFT area (in envelope on back cover of the dissertation) shows the distribution of the main planar and linear structural elements. From a preliminary inspection of the variation in strikes of foliation and distribution of rock types on dissertation map, it was decided to subdivide the region into 4 domains showing different geometrical and geological characteristics. Domain 1 is dominated by ENE trending linear belt of metasedimentary (supracrustal) rocks extending from west of Foley Village to Gulushabe settlement. Based on recognition of large scale fold structures (Figs 3.1, 3.2 and dissertation map), it is divided into 2 main fold structures namely the Foley structure in the WSW and Gulushabe structure in the ENE. In the Foley structure foliation strikes ENE to NE (070°-050º) and dips NNW and NW. Two episodes of folding (F1 and F2 coaxial folding) are recognized in the metasedimentary belt.
The metasedimentary rocks are flanked to the NW, SE and NE by granitoid gneisses divisible into 3 domains. The NE to ENE trending (010°-054°) granitoid gneisses to the NW of the metasedimentary belt represents Domain 2. Domain 3 is represented by the ENE trends (strike values~040°-073°) of fabric to the SE of the metasedimentary belt.
Domain 4 is represented by a narrow zone of E-W to WNW striking foliation (270°°) to the NE of the metasedimentary rocks. The foliation in the granitoid rocks in the northern part of Domain 3 seems to be traced around the folds deforming the metasedimentary rocks. The granitoids form part of the fold structures deforming the metasedimentary rocks but they show no evidence for refolding. Contacts between units are concordant with the mineral layering. Gneissic foliation is the common continuous deformation foliations observed in the granitoid gneisses. The granitoid gneisses have a single foliation that is pervasive across the study area and this is regarded as (S 1), the oldest deformation fabric. S1 in the granitoid gneisses is equivalent to S2 in the metasedimentary rocks. The foliation is defined by medium–coarse grained minerals that form compositional banding with a preferred planar orientation of platy, tabular mineral grains and by parallel alignment of lenticular mineral aggregates. The compositional banding occurs at all scales from thick (metres to km scales) continuous bands that can be mapped across the entire field area to discontinuous laminae (mm to cm scales) that pinch out within individual outcrops.
Figure 3.2 ASTER image showing the NNE and ENE closing Gulushabe structure.
The dark units represent carbonate rocks, the green unit is amphibolite and the pale brown areas occupying the middle of the structure are underlain by quartzite.
Generally there is a poor development of reliable minor fold structures and associated lineations in the SFT area, especially in Domains 2 and 3. The association of lineations 73 with other structures can help in understanding the structural geometry of an area and in interpreting the conditions in which the structures formed (Ramsay and Huber, 1987;
Twiss and Moore, 1992). Because lineations are generally smaller scale structures than folds and because small-scale structures commonly reflect the geometry of large-scale structures, it is often easier to map the orientation of folds structures by mapping the orientation of lineation that can be geometrically related to the fold hinges. Considering a situation where layering/bedding is buckled by pure shear under plane strain conditions,
lineations that develop will be:
1. Fold axis, orientated parallel to the Y-axis of strain ellipsoid
2. Elongation lineation. The elongation lineation develops parallel to the X (long axis) of the strain ellipsoid, and pre-existing structures are rotated towards this axis and elongate by varying amounts depending on the amount of rotation. Under plane strain conditions the Y axis of the strain ellipsoid does not change length. There is shortening in the Z direction with equal extension in the X direction and this result in the development of an S: L fabric. The S fabric represents the axial planar cleavage and the L fabric (lineation) develops normal to the fold axis in the plane of the S fabric
3. Intersection lineation is formed by the intersection of the folded bedding or layering with the S fabric. An intersection lineation defined by a folded surface and by the axial foliation to the folds must lie parallel to the surfaces that defines them (i.e. in the plane of the layering) and plunges parallel to the fold axis if the folding is close to cylindrical
4. Slickenlines develop on the interface between layers at right angle to the fold axis when a layer is buckled by flexural slip. Slickenlines are found on bedding surfaces especially associated with flexural slip folds.
In the SFT area linear structures may be divided into mineral elongation lineation, slickenlines, lineations defined by long axis of deformed pebbles in quartzite units and lineations defined by fold axis in both the supracrustal rocks and the granitoid gneisses.
74 The lineations are treated geometrically and statistically using stereographic projection technique. A plot of linear and planar data in stereographic projections yields information about the geometry and orientation of each fold structure and orientation of deformation fabrics in each domain. The stereographic projections were generated using “GEORIENT 32V9” software on an Equal area, Lower Hemisphere nets and the contours are at 1% projection circle.
3.2 DEFORMATION FABRICS IN THE METASEDIMENTARY ROCKS (DOMAIN1) 3.2.1 THE FOLEY STRUCTURE In the Foley West area (dissertation map), folded metasedimentary outcrops form an elongated shape. The metasedimentary rocks consist of quartzite and associated quartzmica schist, marble and a mixed metasedimentary unit that comprises an association of marbles and calc-silicate rocks interfolded with quartzite and amphibolites. These rocks are bound by K-feldspar megacrystic granite to the NW and SE. Deformation in the quartzite-quartz-mica schist unit in the Foley structure is characterized by a NE to ENE striking bedding (S0) and bedding parallel foliation (S1) that dips predominantly to the NW and NNW. Foliation (S1) can be traced around tight to open fold closures (F2) and is defined by an alternation of quartz and micas. The geometries of both small scale and map scale structures are compatible with folding along NE trending axial surfaces. Smallscale and map-scale folds show SE vergence indicating SE directed rotation and displacement. Trend lines on the geological map define fold structures closing to the SSW. Exposures of quartzite are characterized by elongation lineation or slickenlines on bedding surfaces that plunge parallel to dip.
220.127.116.11 FOLIATION AND LINEATION
Lineations are best developed in the quartzite unit but show a poor development in the mixed metasedimentary unit. In units of quartzite with pebble beds or pebble-bearing quartz-mica schist (e.g. localities 11, 12, 23 and 40 on the accompanying large scale geological map), deformed pebbles define a shape fabric. The pebbles have 2 axes of extension and 1 axis of shortening: pebbles are extended parallel to the fabric in cross 75 section, extended parallel to the fabric in plan and shortened perpendicular to the fabric in both cross-section and plan (Fig. 3.3 - 3.6). The geometry of this fabric is always the same; the maximum elongation direction (X-strain axis) plunges down dip typically NNW with subordinate elongation parallel to the ENE strike of the S1 foliation. This NNW plunging pebble elongation suggests the Z axis was close to horizontal. This pebble elongation is considered to reflect the end result of F1 and F2 folding produced by oblate strain. According to (Ramsay and Huber, 1987) if subjected to flattening, a sphere is deformed into a pancake-shaped (oblate) ellipsoid. If subjected to constriction, it becomes a cigar-shaped (prolate) ellipsoid and if subjected to simple shear, it becomes an ellipsoid with axes inclined relative to the shear plane and no deformation parallel to the Y axis under plane strain conditions.
Figure 3.3: Section view of steeply NNW dipping quartzite outcrop.
The maximum elongation defined by the long axis of pebbles plunges 80º/340°. Strike and dip of foliation is 250°/88°.
Figure 3.5: ENE-WSW striking foliation dipping NNW in pebble-bearing quartz-mica schist in the Foley structure.
Elongation lineation plunges down dip towards NNW. Most of the flattened pebbles are obvious on the strike section. Plunge and trend of maximum elongation is 66°/340°. Strike and dip of foliation is 250°/88°.
77 Z Y X Figure 3.6: Outcrop of deformed quartz pebble bed in mica schist unit. Photograph was captured looking down plunge of elongation lineation defined by pebbles. Dip of foliation and plunge of lineation is to the NNW. Plunge of lineation, 62°/329°. Strike and dip of foliation is 239°/62°. X and Y axes of strain are identified on deformed pebbles. Sketch to the right shows X, Y and Z axis of strain ellipsoid in relation to the shape of the deformed pebble.
18.104.22.168 FOLD STRUCTURES
Small scale folds related to the ENE trending large scale folds obvious from the geological map are linked to NNW-SSE compression. The small scale folds generally plunge to the NE to ENE and axial surfaces strike ENE-WSW and dip to the NNW. The folds deform bedding (So) and foliation (S1) in the quartzite unit (Fig. 3.7 to 3.10) and are therefore F2. At locality 33, a sub horizontal lineation is developed parallel to ENEWSW trending fold axis (Fig. 3.7). The trend of the fold axis is parallel to the strike of the main regional ENE-WSW foliation fabric.
A folded quartzite unit (Fig. 3.8) displays an axial planar foliation fabric denoted S 2 occurring at high angle to bedding (S0) in fold closures. The primary surface (bedding, S0) and the secondary surface (foliation, S1) defines the fold and the fold is F2.
Figure 3.8: Profile view of an ENE plunging F2 minor fold in the quartzite unit.