«By MOLATLHEGI LARTY LOSTMAN MOSEKI STUDENT NO. 208523856 Submitted in fulfillment of the academic requirements For the degree of Master of Science In ...»
The maximum elongation direction of the megacrysts is consistently parallel to the dip of the foliation but no zones of intense elongation with associated shear sense indicators related to zones of simple shear (ductile shear zones) were obvious possibly due to the lack of good outcrop in the vertical section. Evidence against simple shear includes lack of well developed elongation lineation and kinematic indicators such as asymmetrical tails to porphyroclasts in megacrystic granite gneiss and asymmetrical folds of the foliation. The evidence (asymmetrical folds) to support simple shear is restricted to a few
Figure 3.25: Plan view of shape fabric foliation (S1) in megacrystic granite gneiss NW of Tonota Village.
Strike and dip of foliation is 220°/76. Shapes of megacrysts indicate shortening NW-SE.
Figure 3.26: Well developed foliation (S1) in oblique view in megacrystic granite gneiss such that both strike and dip directions are obvious.
Strike and dip of foliation is 190º/70°.
Handle of hammer at top left corner of photograph is pointing NE and placed parallel to strike of foliation.
95 Figure 3.28: A: Well developed planar deformation fabric (foliation, S1) in megacrystic granite gneiss. Strike and dip of foliation is220º/88º. Shape of K-feldspar megacrysts indicate flattening perpendicular to the foliation and elongation parallel to the foliation surface with maximum elongation direction parallel to dip. B. Compositionally banded granite gneiss inclusion within main outcrop of megacrystic granite gneiss outcrop displaying evidence for flattening perpendicular to foliation and elongation parallel to the foliation. Elongation lineation plunges plunges 76°/310° in B.
Sketch beside photograph shows the shape fabric of the steeply plunging lineation in relation to the axes of the strain ellipsoid.
The extension direction during boudinage is down dip i.e. parallel to the WNW plunging elongation lineation defined by both deformed K-feldspar megacryst and elongation lineation on dyke surfaces (Fig. 3.29). Felsic bands are elongated parallel to the foliation (S1) and flattened perpendicular to the foliation such that they appear “augen” shaped.
Figure 3.31: Mineral elongation lineation in megacrystic granite gneiss plunging obliquely to the SW; 45º/220°.
Photograph from N of Shashe Village. Strike and dip of foliation (S1) is 210°/60°.
Figure 3.33: Elongation lineation in megacrystic granite gneiss plunging gently to the NNE: 25º/020°.
Photograph from N of Shashe Village. Strike and dip of foliation is 200º/78º.
Minor fold structures with axes trending NNE-SSW and axial surfaces dipping to the WNW have been recognized in the gneisses S and SE of the Shashe Dam; At locality 44 (along the boundary between Domain 1 and the metasedimentary belt SW of Tonota, Figure 3.34), Locality 299 (N of Tonota Village, Figure 3.35) and Locality 9 (SW of Shashe Village, Figure 3.36) and Locality 16 (N of Shashe Village, Fig 3.37). These tight to isoclinal folds deform the WNW dipping foliation (S1) in the megacrystic granite gneiss (Figs 3.34-3.38) and indicate up dip rotation to the ESE (Locality 44), down dip rotation to the ESE (Locality 299) and down dip rotation to the WNW (locality 9). The kinematics at these localities are ambiguous showing conflicting senses of rotation. As the folds deform S1 they are F2. The folds have NNE to SSW trending axial surfaces and the trend of the fold axis is parallel to the NNE strike of the main regional foliation fabric.
Figure 3.34: Minor fold plunging at 005 to 010º, with axial surface dipping WNW.
Deformed foliation (S1) dips at 58° WNW (191°/58º). Strike and dip of axial surface is 191º/58º. The minor fold is asymmetric indicating rotation to the ESE. Photograph from SW of Tonota Village. Sketches to the right of photograph show photographic interpretations of movement directions.
100 Figure 3.35: Outcrop of foliated granite gneiss interlayered with megacrystic granite gneiss at locality 299, photograph from N of Tonota Village. Foliation (S1) that is deformed dip WNW (198/60º) and kinematic indicators as highlighted suggest shear sense displacement to the ENE (down dip displacement). Sketch beside to the right shows interpretation sense of movement directions based on the photograph.
Figure 3.36: Outcrop of high strain tonalitic granite gneiss with well-developed NNE-SSW trending, WNW dipping foliation (S1), strike and dip, (190º/70º) that is deformed by reclined folds with N-S axial plane plane and a minor fold suggesting dextral shearing orientated NNE-SSW.
Minor fold at the dark arrow to the left of photo plunges, 00º/020º.
Strike and dip of axial plane is 200º/70º. A down dip sense of displacement is indicated.
Photograph from SW of Shashe Village. Sketch beside photograph is an interpretation of movement direction.
Figure 3.37: A: NNE-SSW trending small-scale open drag folds in megacrystic granite gneiss.
Strike and dip of foliation (S1) is 180°/60°. B. Section view of fairly symmetrical folds with close to upright axial surfaces at the same locality. Whereas folds in A show a top to NW movement, those in B are slightly rotated to the SW. Minor fold in A plunges
subhorizontally (03º) towards 030º. Strike and dip of axial surface in A is 210º/90º. Note:
Figure A is a plan view of minor folds in Figure B and figure 3.38.
Figure 3.38: A.
Section view of NNE-SSW trending asymmetrical folds with closely spaced (cm-scale) axial planes deforming foliation (S1) in megacrystic granite gneiss. Folds trend NNE and verge to the WNW. Pen lies parallel to fold axis. Strike and dip of fold axial planes 210°/56º. The red lines are axial planes of the asymmetric folds. Plunge of minor fold 00º/030º
1023.3.1.3 GEOMETRICAL ANALYSIS OF STRUCTURAL ELEMENTS
The attitudes of 35 foliation (S1) planes were measured in Domain 2. The stereogram in Figure 3.39 shows a general concentration of poles to foliation (S1) in the ESE quadrant of the projection signifying that the foliations (S1) are steeply dipping towards WNW and the centre of the contoured pattern corresponds to an average foliation surface. The pole to the best fit great circle gives a value for the plunge of the fold axis of 28º/017º. The strike and dip of the axial plane was calculated to be 215º/60º. The sets of data in the NE and SW quadrants (red dots) represent N and NNE (Fig 3.32 and 3.33), SW (Fig 3.31) and sub- vertical to vertically plunging elongation lineation (3.28 A, 3.29). Measured minor folds indicated in Figure 3.39 plunge NNE or almost due N.
Figure 3.39: A: Equal area, lower hemisphere projection of poles to foliation (S1),
plunge of elongation lineation and plunge of minor folds from Domain 2. B:
Contoured diagram of poles to foliation (S1) in A.
184.108.40.206 DEFORMATION FABRICS Foliation (S1) in granitoid gneisses in Domain 3 trends ENE-WSW and dips steeply to the NNW. Lineation in this domain is not well developed but where found, elongation lineation plunges gently WSW (Fig. 3.40) and ENE (Fig. 3.42 and 3.43). At locality 405 (to the west of position of Fig. 3.40) close to Foley settlement, outcrops of megacrystic granite gneiss are characterized by a strongly developed elongation lineation but weak foliation (S1) to produce a LS tectonite. Due to the strongly developed linear extension direction in these outcrops, loose blocks have an almost prolate shape (Fig. 3.41). On surfaces perpendicular to the elongation direction (X) the K-feldspar megacrysts show minimal shape change and only weakly developed shape preferred orientation (shape fabric foliation (S1), Fig. 3.42). No minor folds were found in this domain.
X Y Z Figure 3.40: A shallow plunging elongation lineation on megacrystic granite gneiss plunging WSW, and defined by quartz on a surface that is folded. Foliation (S1) strikes 252°/62°, Lineation plunges 30º/260. E of Foley. Drawing to the right shows orientation of lineation with respect to the axis of strain ellipsoid.
Figure 3.42: L-tectonite with K-feldspar showing minimal shape change in surface perpendicular to extension direction.
Elongation lineation plunges ENE; 24º/070°.
The attitude of 16 foliation (S1) planes and 2 elongation lineation measured in Domain 3 from the area SE of the metasedimentary belt is plotted in Figure 3.43. The diagram shows 2 distinct maxima with one situated in the SSE quadrant indicating foliation (S1) dipping to the NNW. The 5 poles to foliation in the NE quadrant reflect readings from the northern part of Domain 3 (Localities 327 and 328) where foliation strikes ~155 and dips WSW. The cause of this change is unknown but may reflect folding of the foliation (S1) in the megacrystic gneiss around the Gulushabe structure or deformation associated with the Gulushabe shear zone. The fold axis indicated on the plot plunges 42º towards 280º, a direction not recognised in the field. The strike and dip of the associated axial surface is 223º/47º (Fig. 3.43). The L-tectonite developed between Foley village and Mankha Hill (see dissertation map) trends at about 070-250 and plunges to the ENE and SSW.
Figure 3.43 A: Equal area, lower hemisphere projections of poles to foliation (S1) and measured plunge of lineation (L-tectonite) from Domain 3.
B: contoured plot of A.
220.127.116.11 DEFORMATION FABRICS Domain 4 is characterized by an E-W to WNW trending pervasive foliation (S1) best developed in gneissic outcrops (grey biotite gneiss, grey megacrystic granite gneiss, megacrystic granite gneiss and banded gneiss) occurring in an area dominated by megacrystic tonalitic gneiss. These rocks occur in a narrow intensely deformed zone in the northeastern part of the study area. The foliation (S1) dips steeply N to NE (Fig. 3.44 to 3.48). In this domain, grey megacrystic tonalitic gneiss is deformed leading to formation of augen gneiss and mylonitic gneiss. The high strain rocks are present in discontinuous layers. Sheared amphibolites, carbonates and tonalitic gneiss (Fig. 3.45) are associated with the mylonitic gneiss to the NE of Tonota (locality 413). Elongation lineation defined by streaks of deformed K-feldspar and quartz plunges N and NNE (3.47 and 3.48).
Fig. 3.44 A and B: Grey slightly megacrystic granite gneiss with well-developed foliation (S1) in Domain 4. B occurs interlayered with A. Strike and dip of foliation, 312°/75º.
Figure 3.46: Sub vertical foliation (S1) ° in foliated grey granite gneiss unit.
Strike and dip of foliation is 323º/88º. Shape of feldspar crystals at left side of outcrop suggests ENE-WSW shortening.
A down dip elongation lineation plunging steeply to the NNE is common and is strongly developed in localised areas (Fig. 3.48).
Figure 3.48: Good example of an upright elongation lineation plunging NNE, 80º/005º.
pen is placed parallel to the plunge of elongation lineation. Photograph captured facing south. Locality 161, N of Tonota. Strike/dip of foliation 323°/86°.
18.104.22.168 FOLD STRUCTURES Two generations of minor folds with orthogonal axial surfaces were recognised. These
are designated Set 1 and 2. The two generation of folds are represented by:
Set 1. Folds plunging sub-horizontally to the NW with an axial planes dipping moderately to the NE (Fig. 3.49). These may be interpreted as having formed by top to SW movement during NE-SW horizontal compression.
Set 2. Folds plunging at 60º to the NE with a steep axial plane trending NE-SW (Fig 3.50). These may be interpreted as having formed by NW-SE horizontal compression.
The minor folds are present in the form of S, Z and M folds (Fig. 3.49 and 3.50).
Figure 3.49: Subhorizontally NW plunging (Set 1) folds deforming foliation (S1) in grey megacrystic tonalitic gneiss.
The photograph is a NE-SW section. Folds verge to the SW.
Axial plane dips moderately to the NE. strike and dip of axial plane; 312°/40°. Plunge of fold axis 03°/310°.The folds formed by top-to-SW movementduring NE-SW horizontal compression. Locality N of the Gulushabe structure.
22.214.171.124 GEOMETRICAL ANALYSIS OF STRUCTURAL ELEMENTS A stereoplot of poles to shear foliation (S1) and lineation data from Domain 4 is given in Figure 3.51. Poles to foliation (S1) define a best fit great circle indicating a fold plunging at 60° towards 027º. The strike and dip of axial plane is 312º/60º. Of note is the similarity between the contoured plot of lineation in Domain 4 (Fig 3.51 C) and that of the Gulushabe structure (Fig.3.23). The fold axis in Figures 3.49 and 3.50 are plotted on Figure 3.51 A.
3.4 MINOR SHEAR ZONES Across Domains 2 and 3 and 4, shearing along centimetre scale ductile shear zones has dragged the pre-existing fabric and layering into small-scale folds trending NE-SW (Figs 3.52 to 3.56).