«By MOLATLHEGI LARTY LOSTMAN MOSEKI STUDENT NO. 208523856 Submitted in fulfillment of the academic requirements For the degree of Master of Science In ...»
Bedding (S0) and foliation (S1) define an F2 fold. Strike and dip of fold axial plane is 240°/62°. Plunge of fold 32°/055°. F2 axial planar foliation is designated S2. Photograph from SW closure of Foley structure.
Figure 3.9: Minor F2 fold in quartz-dominated schist plunging towards ENE, 60°/070°.
Strike and dip of axial surface, 250º/68º, Foley structure.
Evidence for structural overprinting/refolding is the occurrence of crinkle lineation (asymmetric folds) deforming foliation in quartz-mica schist units (Figs 3.10 and 3.11). These folds typically deform bedding and a bedding parallel foliation (S1) and are therefore F2.
Locally however there are a set of tight to open folds deforming flattened pebbles and foliation (Fig. 3.11). These folds plunge NNE and are interpreted as F3.
Figure 3.10: An elongation fabric on bedding in quartzite plunging ENE parallel to the axis of minor (F2) fold.
The minor fold is Z shaped suggesting the outcrop is from the limb left hand limb of the antiform as indicated on the sketch beside the photograph. Note the very obvious linear element going into the page and some asymmetrical folds (F 2) deforming older foliation carrying the linear element. Plunge of minor fold, 32°/055°.Strike and dip of axial surface, 240º/50º.
22.214.171.124 GEOMETRICAL ANALYSIS OF STRUCTURAL ELEMENTS The attitude of 49 foliation (S1) and bedding planes measured from the Foley synformal structure are plotted in Fig. 3.12. The stereographic projection shows two concentrations of poles that plot on a great circle signifying folding. There is an obvious orientation of poles in the SE quadrant, signifying that the bedding and foliations steeply dip towards NNW. The poles in this quadrant represent data from the fold limbs. In addition there is a set of poles which plot in the SW quadrant. These represent bedding (S0) and foliation (S1) surfaces dipping to the NE and were measured around the fold closure. Bedding is dipping ENE around the SSW closure of the Foley structure and therefore this closure structure is a synform. The data from the Foley structure indicate a fold axis plunging towards NNE. The plunge and trend of the fold axis was calculated to be 50°/006° by fitting a great circle to the poles to bedding (bedding (S0) and foliation (S1) and data.
This trend is not compatible with the plunge of minor folds measured in the field except at locality 12 (Fig 3.11), and is thus interpreted as F3. The axial surface has a strike of 244º and a dip of 55º. The structural data indicate that the Foley structure is comprised of steeply NNW dipping rocks which have been folded into an ENE trending belt. The 81 elongation lineation of the Foley structure plunges down dip and is orientated at a high angle to the fold axis (Fig 3.12).
Figure 3.12: A-B: Structural orientation data from the Foley synformal structure
A) Equal area, Lower hemisphere projections of poles to bedding (So)/foliations (S1), plunge of long axis of pebbles and plunge of measured minor folds (excluding Locality 12).
B) Contoured diagram of poles to bedding/foliation in A. C. Contoured plot of steeply NNW plunging elongation lineation defined by deformed quartz pebbles in the Foley structure (Domain 1).
82 Figure 3.13: Cross section across the Foley synformal structure. F2 axial plane strike and dip, 244°/55°. NB. Strike and dip values used were derived from plot on Figure 3.12.
3.2.2 THE GULUSHABE STRUCTURE The Gulushabe structure is made up of a folded metasedimentary sequence that forms an ENE-WSW structure. Quartzite, marble, amphibolite and a mixed metasedimentary unit characterize the sequence. Foliations (S1) dipping consistently NNW were recorded in the southeastern limb of the fold structure. Beds in the NW limb have been reoriented by folds trending NNE-SSW (F3) thus dip and strike varies (Fig.3.14). Based on the regional trend on the dissertation map, the general shape of the Gulushabe structure is attributed to 83 coaxial folds trending ENE (such that F1 and F2 fold axes plunge to the NE and a younger NNE trending F3 deformation.
The map shape of the main Gulushabe fold is an example of a Type 3 interference pattern also referred to as fish Hook fold (Fig. 3.14, Ramsay, 1962). With this type of pattern, the axial trace of the first fold (F1) is curved, and the axial trace of the second fold (F2) cuts the trace of F1 (Fig. 3.14). The different types of interference patterns depend on two angles: the angle between the first generation axial surface and the second generation slip direction and the angle between the first generation fold axis and the second generation axial surface (e.g. Ramsay, 1967, Ramsay and Huber, 1987, Twiss and Moore, 1992).
The trend of F1 and F2 axial trace on Figure 3.14 complies with ENE closing minor folds but not with the N closing folds on the NW limb of the main structure. As in the Foley structure, the rocks are flattened parallel to the S1 foliation plane and elongated within the foliation. The tonalitic gneiss in the SE part of the Gulushabe structure (see dissertation map) truncates the F1 axial trace implying that its intrusion was post-F1.
Figure 3.14: A Type 3 interference fold (plan view) due to refolding of F1 axial surface trace by F2.
The N closing folds along the NW limb of the map pattern are interpreted as F 3 structures.
Generally, the elongation lineation in the Foley structure plunges steeply NW to NNW but in the Gulushabe structure the elongation lineation in pebbly quartzite unit developed on bedding planes in competent quartzite units plunge steeply N and NE (Figs 3.16 to 3.19) with an exception to Locality 188 (Figure 3.15) around the core of the Gulushabe structure where the elongation plunges NNW.
Figure 3.15: An alternation of pebbly quartzite and a non-pebbly bed defining bedding in the Gulushabe structure.
Elongation lineation is defined by deformed pebbles, angle and direction of plunge; 66°/335°. Strike/dip of foliation (S1), 243°/66°.
Figure 3.16: subvertical plunging lineation on WNW-ESE striking foliation (S1).
Strike and dip of foliation is; 300°/80°. Lineation plunges 75°/030°.
Figure 3.18: NNE trending elongation lineation on a quartzite unit defined by recrystallized quartz.
The lineation plunges towards NNE (50°/005°). Strike and dip of foliation (S1) is 270º/52º.
The biotite schist in the ENE closure of the Gulushabe structure (Locality 287) shows axial planar foliation. Felsic bands in the schist show evidence of flattening perpendicular to the foliation and elongation parallel to dip (Fig. 3.20). The foliation dips NNW and based on the position of the locality 287 in the fold closure, it is interpreted as S2.
` Figure 3.20: Well developed S2 foliation (S1) in biotite schist dipping steeply to the NW with deformed felsic material; strike and dip of foliation, 242°/68° in the closure of the Gulushabe structure.
The geometries of both small scale and map scale structures are compatible with folding along ENE-WSW trending axial surfaces. Small-scale folds in the Gulushabe structure (Fig 3.21 and 3.22) have steeply dipping axial surfaces (248º/88º) and fold axes plunging ENE (65º/050º). Both bedding (S0) and foliation (S1) are deformed.
C Figure 3.21 A, B and C: F2 minor fold structure in the quartzite from the Gulushabe structure.
Photograph B and C are the same structure captured from different views.
Figure A was captured facing NE, B captured facing SW and C captured facing SE. Axial surface is orientated ENE-WSW; 248°/88°. Plunge of fold axis 65°/068°.
88 Figure 3.22: A-B: Example of NE-SW trending small scale fold structures in quartz-mica schist. Plunge of fold 60°/248° 126.96.36.199 GEOMETRICAL ANALYSIS OF STRUCTURAL ELEMENTS The attitude of 24 foliation (S1) and bedding planes that were measured in the Gulushabe structure are plotted on Figure 3.23. The projection shows a concentration of poles in the SE quadrant and a spread of poles in the SW quadrant. Those in the SE quadrant represent NNW dipping bedding (S0) and foliation (S1) in the southeastern limb of the Gulushabe fold. Poles to bedding (S0) /foliation (S1) in the SW quadrant were measured from the fold closure.The poles plot along a great circle and gave a value for the fold axis plunge and plunge direction of 49º/014º. The axial surface has a strike and dip of 254º/53º. Bedding and foliations around the closure of Gulushabe structure (Locality 287 on dissertation map) strike 345 and dip 60 ENE indicating a fold closure is antiformal and plunging ENE. Of note is the similarity between the steep NE plunges of elongation lineation in the Gulushabe structure and those in the Gulushabe shear zone (Domain 4). A NW-SE cross section across the antiformal structure is sketched in Figure 3.24. The elongation lineations of the Gulushabe antiformal structure are steeper than the F3 axis, suggesting that the X axis of the strain ellipsoid is at an angle to the F3 fold axis indicated in Figure 3.23.
3.2.3 DISCUSSION On the dissertation map the trend of the metasedimentary belt and the large scale folds indicated by the outcrop pattern is generally ENE or NE. The map scale folds (the Foley and Gulushabe structures) and minor folds deforming bedding (S0) and foliation (S1) plunge to the ENE and are related to NNW or NW or compression.
The outcrop pattern of the Foley fold structure closes to the SSW indicating a synform plunging ENE. Elongation lineation on the fold limbs defined by the long axes of deformed pebbles plunges NNW (Fig 3.23). It is worth noting that stereographic projections of bedding (S0) and foliation (S1) and indicate a fold plunging NNE (006º) with an axial surface striking ENE-WSW (244°). This fold axis is not compatible with the overall ENE trace of the Foley structure and the plunge of minor folds in Figure 3.21.
The overall map shape of the Gulushabe fold suggests that it is an antiform that closes to the ENE. The map pattern of the Gulushabe structure shows that it plunges ENE. Minor folds deforming bedding plunge ENE. Lineation developed in the quartzite unit plunges N to NE subparallel to bedding and probably represents slickenlines. The stereographic projection of poles to bedding (S0) and foliation (S1) in the Gulushabe structure indicates 91 a fold plunging at 49° to the NNE (014º). As in the Foley structure, the NNE plunge of this fold axis (Fig.3.23) does not correspond with the map scale folds trending ENE. This NNE plunging axis is interpreted to represent a younger (F3) deformation. A younger set of folds (F3) plunging NNE such as the one in Fig. 3.11 from the Foley structure, deform the foliation. Both the foliation (S2) and the pebbles at this outcrop are deformed indicating the crinkle lineation is post F2.The orientation of these folds is related to map scale NNE trending folds around the N closure of the Gulushabe structure, near Shashe and N of Tonota Village. These NNE trending folds reflect a third generation of folding deformation in both the Foley and Gulushabe areas. The production of these folds is attributed to WNW-ESE horizontal compression. The NNE plunge of the fold axis of the Foley structure as plotted in Figs3.12 reflects this deformation, hence the contradiction in orientation between the map scale folds and the projected folds in Domain 1.
Based on field analysis of metasedimentary rocks, the first evidence for deformation in the metasedimentary rocks is represented by ENE-WSW orientated foliation (S1) that is axial planar to folding of bedding (S0). F2 folds plunge NE to ENE parallel to F1 folds, implying that folding was coaxial. The coaxial deformation in the metasedimentary belt is attributed to NW-SE compression causing flattening perpendicular to layering. The Foley and Gulushabe structures are products of the F1/F2 coaxial folding deformation (NW-SE compression) modified by F3 deformation (WNW-ESE compression). The map pattern shows that the Foley synformal and Gulushabe antiformal structures do not lie along strike and are therefore not easily connected. Based on the stereographic projections (Figs.312 and 3.23) the deformation fabrics in the supracrustal rocks indicate folding with fold axis trending NNE-SSW which is compatible with local (F3) minor folds seen in the field but not with the NE to ENE (F1+F2) trending minor folds (Figs 3.7,3.8,3.9 and 3.10).
188.8.131.52 DEFORMATION FABRICS Domain 2 is dominated by megacrystic granite gneiss interpreted to have been formed by the deformation of porphyritic granite. Although the geometry of the foliation (S1) is consistent across Domain 2, the intensity of the foliation differs, with the fabric in the gneisses exposed SW of Shashe Dam much more intense than the rest of Domain 2. The foliation (S1) is defined by the preferred orientation of deformed K-feldspar megacrysts and mineral alignment (Fig. 3.25, 3.26 and 3.27). The foliation (S1) strikes dominantly NNE (020º) and dips moderately to steeply (44°-70°) WNW. Compositional banding is also present (Fig. 3.28 B). In the megacrystic gneiss, the shape of the K-feldspar megacrysts indicates flattening perpendicular to the foliation and elongation parallel to both dip and strike of the foliation (S1) surface (Figs 3.25, 3.26 and 3.28 A), with maximum elongation parallel to the dip of the foliation (3.28A).
The orientation of the intersection lineation on the dyke contact (as indicated in Figure
3.29 has the same orientation as the elongation defined by deformed K-feldspar megacrysts in Figure 3.28 A, implying that they are geometrically related or were developed by the same deformation event. There is an elongation lineation that lies oblique on the foliation (S1) surface and plunges moderately to gently N to NNE (Figs.
3.32 and 3.33) and locally SW (Fig. 3.31).