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

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A. M. HOPGOOD Department of Geology, University of St. Andrews, Fife, Scotland 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 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/81/11/3473/3443248/i0016-7606-81-11-3473.pdf by guest on 27 September 2021 CAINOZOIC GRANITES and GNEISSES Sands,cloys and grits PUD-PLEISTOCENE I 1 Pegmat/tic granites . Fault Toro and Karagwe-Anlcolean PRECAM BRIAN I acid gneisses ' Geological boundary Shales and phyllites OTHER ROCKS Locality examined Mica schist KARAGWE- Amphibolites and hornblende schists ANKOLEAN Quartzites with schists and sandstones SYSTEM 30KllOMfTRES Calcsilicate and hornblende gneisses 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 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/81/11/3473/3443248/i0016-7606-81-11-3473.pdf by guest on 27 September 2021 TECTONIC SEQUENCE 3475 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). Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/81/11/3473/3443248/i0016-7606-81-11-3473.pdf by guest on 27 September 2021 3476 A. M. HOPGOOD—STRUCTURAL REORIENTATION, UGANDA 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<i) predominant in the banded quartzo- geometry, it is difficult to determine the atti- feldspathic gneiss and "granite gneiss" (Fig. 7). tude of their fold axial planes which dip gently Subsequently, strong lineation and folding to the west. The structures are well developed (Fj) with a regular southeast trend and plunges in gneisses and phyllites that crop out in fault either to the northwest or to the southeast affected most structures (Fig. 8). These later 2 The 160° to 340° direction of folding, as well as folds exhibit west-dipping axial planes which that of p4, might also be the local equivalent of the northwesterly trend recognized in the Karagwe- 1 Although F& folds have a similar alignment.
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