A. M. HOPGOOD Department of Geology, University of St. Andrews, Fife, Scotland

Structural Reorientation as Evidence of Basement

Warping Associated with Rift Faulting in Uganda

ABSTRACT phic rocks exposed throughout the area. To study the subsequent effect of movements A tectonic sequence has been determined along faults of the Western Rift Valley on the for Precambrian basement structures of Ki- orientation of these basement structures, baran age (1100 + 200 m.y.) in an area adja- studies were concentrated on the variation in cent to the margin of the Western Rift Valley in attitude of one set of these structures along Ankole District, southwestern Uganda. Plunges lines perpendicular to the rift. Results were of one set of the structures in the sequence, expected to be free of complications such as tight folds (F^, and associated lineations, are those caused by contemporaneous and post- dispersed about a 35°-trending subhorizontal rifting vulcanicity which affect comparable axis which is subparallel to the rift trend of measurements involving topography (Bishop N. 30° E. in this area. This direction is not and Trendall, 1967; Saggerson and Baker, parallel to a local fold axial trend, however, and 1965, p. 60). Nevertheless, although while the the dispersion of p4 is attributed, therefore, to trend of the local axis of basement warping was basement warping related to Tertiary rifting. determined with a reasonable degree of preci- sion, I found that the effects of splinter fault- INTRODUCTION ing, associated with the rift fault, precluded In the autumn of 1967 approximately 250 sq the determination of the sense and degree of mi of Ankole District in southwest Uganda warping immediately adjacent to the rift. were investigated. The area lies south and west of Bushenyi and Ishaka and about 40 mi STRUCTURE west of Mbarrara and is included in the The rocks examined include gray and buff- 1:50,000, Series Y732 topographic sheets 85/1 weathering schist, biotite muscovite schist, and 80/2. It is underlain by rocks grouped quartzo-feldspathic gneiss, phyllite, slate, with the Karagwe-Ankolean System by Barnes quartzite, and porphyroblastic granitic gneiss, (1956) and with the Toro or Karagwe-Ankolean and some intrusive granite. Nearly all the Systems by Seal and Williams (1960; Fig. 1). rocks were deformed during the Kibaran The Buganda-Toro System is older than 1800 orogeny (1100 + 200 m.y.) and even the m.y., and includes argillites, amphibolites, and "granites" tend to exhibit alignment of the phyllites, some of which are granitized, and feldspar megacrysts. Mesoscopic structures in- which, in the east, are folded on "predominant- clude two generations of isoclinal folds: the ly steep east-north-easterly axes" and variably first with closely appressed limbs and flattened, in the west (Macdonald, 1966b). The Karagwe- sometimes detached hinges lying in the folia- Ankolean System is younger than 1800 m.y. tion, and the second with rounder hinges and and older than 1300 m.y., and includes argil- often with a characteristic pod-shape in longi- lites, quartzites, sandstones, conglomerates, and tudinal section. These have been affected by metacalcareous rocks which rest unconformably folding of more open style as well as bands of on the Buganda-Toro System and are folded on kink folds and crenulations, and finally by open steep, northwesterly and northeasterly axial warps with pinch and swell structures. Associ- planes. ated with some of these folds is a strong linea- In this paper the orientations and mutual tion, and the over-all structural sequence is relationships of small scale (mesoscopic) struc- complicated by phases of both acid and basic tural elements were determined in order to intrusion, migmatization, metamorphism, and, establish a tectonic sequence for the metamor- in the south of the area, pegmatite emplace-

Geological Society of America Bulletin, v. 81, p. 3473-3480, 12 figs., November 1970 3473

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Mica schists (Igora Series) TORO SYSTEM GEOLOGY OF ANKOLE DISTRICT Figure 1. Geological map of southwest Ankole District (after Mbarrata recorded and locations with respect to Western Rift Valley (inset). Sheet SA 36-1, 1961), showing localities from which structural data were

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ment. The degree of regularity of the attitude of fold axes and axial surfaces has been deter- mined statistically in most cases, using a Schmidt net, and for this purpose just over one hundred plots were prepared. The tectonic sequence was determined from the interrela- tionships of structures affecting the basement. TECTONIC SEQUENCE From their banded nature it appears that the original rocks were in most cases (apart from basement granite gneiss) both sedimen- tary (quartzite and slates) and igneous (interca- lated basic lava flows or later sills, or both). Intrusion of basic dykes, prior to, or early Figure 3. Random orientation of feldspar crystals in granite. in the deformational history, coupled with the emplacement of basic sills (Fig. 2) was followed rectilinear quartz veins close to granite mar- by the intrusion of largely undeformed granitic gins. Most FI isoclinal folds are represented rocks containing large euhedral feldspar crystals only as rootless intrafolial fold hinges; some- in random orientation (Fig. 3) and xenoliths times, too, they are represented mimetically of the earlier basic rocks. The schists close to by quartz and quartzo-feldspathic lenses and some of the granites are spotted owing to ther- "folded" veins. Related to the latter are non- mal metamorphism. The first phase of folding rectilinear quartz veins which cut the early (Fi) resulted in the early development of folia- isoclinal folds (Fi). These quartz veins are tion in the country rocks surrounding the affected by large isoclinal folds (F^) that have granites; this foliation was parallel to the axial distinctive pod-shaped longitudinal sections surfaces of what are now tight isoclinal, broadly and plunge generally in an easterly direction. similar folds shown refolded in Figure 4. Small A set of distinctive, approximately north- feldspar porphyroblasts are aligned parallel to trending kink folds (Fs) affects the orientation the margins of the granite bodies and to the of folds of preceding generations as well as the foliation (Fig. 5). Augen gneiss and biotite quartz and feldspar veins. These folds show schist at the margins of the granites appear to some variation in attitude either due to later result from a combination of marginal flow deformation or to the fact that they comprise before consolidation that was followed by parts of conjugate sets intersecting at an acute shearing associated with the FI similar folding. angle. Rodding and fine ribbing generally with This was intensified by later differential move- a northerly trend are sometimes associated ment between the granitic rocks and the with this structure. country rocks which has offset cross-cutting The most conspicuous folding in the area

Figure 2. Pinch and swell structure developed in Figure 4. Isoclinal folds (Fi) curved around the amphibolite. hinge of later (F4) folds (above and to left of lens cap).

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Figure 5. Feldspars in granite aligned close to the Figure 7. Lineation and rodding (L4) associated contact with spotted schists. with p4 in quartzo-feldspathic gneiss.

(F4) is characterized by axial planes dipping distinguish them from the F4 fold. This sense toward the north-northeast. Folds of this of dip, coupled with the fact of their less com- generation trend 120° and plunge either east- mon occurrence shows that they are genetically southeast or west-northwest; their axial direc- distinct from p4 structures and do not con- tion constitutes the most regular structural stitute part of a complimentary set of conju- feature in the whole area. They may be the gate p4 folds.2 Their north-northwest trend in local equivalent of the two trends (northwest this particular area may be due to reorientation and northeast) described in the Karagwe- by later (Fg) folds. Mineral alignment and Ankolean System by Macdonald (1966b)1 The rectilinear quartzo-feldspathic pegmatite veins folds are generally chevron in profile, although parallel to the axial planes of Fe folds are some are so tightly closed that they are almost common. isoclinal while others are comparatively open Folds of the latest set (Fe) appear to be con- in style (Fig. 6). This period of deformation centric, and are very open structures that are appears to have been responsible for the in- associated in some banded gneisses with pinch tense regular rodding and fine mullion struc- and swell features (Fig. 2). Because of their tures (L

Figure 6. A typical profile of an F4 fold in quartzo- Figure 8. Folds (F5) and lineation (at lens cap) feldspathic gneiss. plunging at 40° toward 340°. Locality 41.

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Figure 9. Gently west-dipping curved axial pknes of F6 affecting phyllites.

splinters in the rift wall on the western border of the area (Fig. 9). Figure 10. Equal-area, lower hemisphere plot of 200 DISCUSSION attitudes of F4 lineations (Li) and fold axes that show a spread on a small circle whose center plunges gently Although the rocks have been mapped toward 215° (subparallel to the 30° trend of rift faulting previously as part of two major systems, (dashed in this area), thus indicating rotation on an axis Karagwe-Ankolean and Buganda-Toro, (Mac- parallel to the rift wall. Contours are at 1,2.5, 5, 10, and donald, 1966a, b), it was not possible to distin- 15 percent per one-percent area. guish two separate sequences in this work. Also, the relationship of the granitoid rocks in the area to the structural sequence implies to this trend (Fig. 10) and there are no fold fundamental differences from previous pro- directions in the district investigated which could posals (Barnes, 1956; Nicholson, 1965). Barnes be responsible for this dispersion. Moreover all considered that the structures resulted from a the linear elements (Fi to Fe) form maxima single deformational episode during which the similarly distributed on small circles whose granite domes were developed along with the center is parallel to the rift trend (Fig. 11). schistosity, and Nicholson considered that the Although at first the pattern appears to indicate Karagwe-Ankole metasediments resulted from that warping took place on a steep axis, the deformation in two stages, with the basement effect is simply due to interference as shown rocks, during dome formation. It is clear from by the form of the two principal maxima (in this new work that the deformational history the northern half of the plot) which represent after the formation of the granite domes and the latest structures (p4 and Fs). These struc- their associated structures is considerably more tures, which are late in the Precambrian se- complex than was suggested previously. quence, are affected least by later deformation Of particular importance in this paper is the and show the same post-Fs dispersion along significance of the varying orientation of the small circles with an essentially horizontal axis conspicuous p4 folding because it is the most of rotation as the linear attitudes (F4) of Figure consistently recognizable structure in the se- 10. quence and has a particular bearing on the The reorientation of the Precambrian tec- much younger rift fault movements. The aver- tonite fabric clearly demonstrates that base- age orientation of the strong rodding, related ment warping associated with rift faulting has fine mullion structures, and fold axes that were taken place and also defines the axis of this recorded and plotted from each of the 80 warping. However, the sense of warping could localities in the district (Fig. 1) shows, with not be detected by structural means in this minor exceptions, generally no consistent instance. Furthermore, rotation due to minor change in plunge of the lineation in a direction faulting parallel to the rift must have absorbed relative to the rift trend of N. 30° E. in this some of the warping and, thereby, reduced the area. However, when the attitudes are plotted, total effective dispersion. Displacement on they are distributed on a broad girdle normal these faults probably has been irregular and

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Figure 11. Equal-area, lower hemisphere plot of the attitudes of all (900) fold axes and lineations recorded in Figure 12. Sense of rotation in regional crustal up- doming compared with that due to normal faulting in the district. The two dominant maxima (from F4 and F5 folds) are dispersed on small circles whose center lies rifting resulting from the updoming. Modified from parallel to the rift trend (dashed), and the whole plot Cloos, in Holmes (1965, Fig. 758, p. 1049). appears to form a small circle girdle whose center "o" lies parallel to the rift trend. Contours are at 1, 2,3, 4, 5, 6 and 7 percent per one-percent intervals. of the crust (Holmes, 1965, p. 1049) could be expected to produce, along a very narrow zone would disrupt any uniform variation in struc- close to the rift, a sense of rotation opposite to tural reorientation perpendicular to the axis that due to the updoming (Fig. 12), depending of warping, that is, parallel to the rift. This on whether or not the faulting was synthetic or differential displacement on minor faults is antithetic. The axis of rotation attributed to probably the reason why plots of successive this faulting will be closely parallel to the rift attitudes measured away from the rift have not trend, whereas that due to regional crustal up- shown a continuous variation in orientation. doming may not (Fig. 1, inset). Information from a systematic structural survey over an extended area immediately adjacent to the CONCLUSIONS ritt could provide a useful addition to the data Where variation in orientation of a single accumulated on rift structures and their structural element can be determined with associated warping and vulcanicity. certainty, the technique used provides a more precise means of determining the trend of the local axis of basement warping than methods ACKNOWLEDGMENTS based on topographic measurements, which I am indebted to the Commissioner of the are more suitable for determinations of a more Geological Survey and Mines Department, general nature involving crustal warping over Entebbe, Uganda, for assistance during the a wider area. Using the approach outlined in course of the investigation and for permission this work, it will be possible to decide con- to publish the results of this work. Also, I am clusively on the nature of the basement warp- grateful to Professor W. Q. Kennedy for help ing adjacent to the rift, to follow closely the and encouragement during the planning stages swings in trend of the axis of this warping, and, of the work, to Dr. Macdonald, Makerere, and more importantly, to determine the sense of Dr. A. Reedman, Mbarrara, and to Dr. T. N. rotation of the basement along the edge of the Clifford for helpful criticism of the manuscript. rift and thus provide structural evidence of its Grants toward the cost of the fare to Uganda normal or reverse nature (Brock, 1965, p. 100). from the Royal Society of London and the Drag due to normal faulting that is associated University of St. Andrews are gratefully with rifting of the center of a broad updoming acknowledged.

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REFERENCES CITED Uganda Geol. Survey Rept., no. RM/34, lip. Barnes, J., 1956, Pre-Cambrian structure and the Nicholson, R., 1965, The structure and metamor- origin of granitoid rocks in south-west Uganda: phism of the mantling Karagwe-Ankolean sedi- Ph.D. thesis, London Univ., London, En- ments of the Ntungamo gneiss dome and their gland.* time-relation to the development of the dome: Bishop, W. W., and Trendall, A. F., 1967, Ero- Geol. Soc. London Quart. Jour., v. 121, p. sion-surfaces, tectonics and volcanic activity 143-62. in Uganda: Geol. Soc. London Quart. Jour., Saggerson, E. P., and Baker, B. H., 1965, Post- v. 122, p. 385-420. Jurassic erosion-surfaces in eastern Kenya and Brock, B. B., 1965, The Rift Valley Craton, in their deformation in relation to rift structure: The World Rift System: Canada Geol. Survey- Geol. Soc. London Quart. Jour., v. 121, p. 51- Paper 66-14. p. 99-123. 72. Haughton, S. H., 1963, Stratigraphic History of Seal, R. G., and Williams, E. W., 1960, Rukungiri Africa South of the Sahara: Edinburgh, Oliver sheet 84 (S.A. 35 F.N.), 1:100,000: Uganda and Boyd, 378 p. Holmes, A., 1965, Principles of Physical Geology: Geol. Survey. London and Edinburgh, Nelson, 1288 p. * Available on microfilm or by interlibrary loan. Macdonald, R., 1966a, Notes on parts of south- west Uganda: Uganda Geol. Survey Rept., no. RM/29, lip. MANUSCRIPT RECEIVED BY THE SOCIETY JUNE 1966b, Summary of the geology of Uganda: 26, 1970

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