African Journal of Science and Technology (AJST) Science and Engineering Series Vol. 9, No. 1, pp. 50 - 71

PAN-AFRICAN DEFORMATION IN SE KENYA AND NE TANZANIA: GEOTECTONIC IMPLICATIONS FOR THE DEVELOPMENT OF THE NORTH-CENTRAL

aA.H. Bauernhofer, aC.A. Hauzenberger, aE. Wallbrecher, aG. Hoinkes, bS. Muhongo, cE.M. Mathu

aInstitute for Earth Sciences, University of Graz, Heinrichstraße 26, 8010 Graz bDepartment of Geology, University of Dar es Salaam, P.O.Box 35052, Tanzania cDepartment of Geology, University of Nairobi, P.O. Box 30197, Kenya email: [email protected]

ABSTRACT:- In the Taita Hills – Galana River area in SE Kenya, four tectonostratigraphic units can be distingui-shed. They differ in tectonic style, and in part in metamorphism, lithology and age of peak metamor-phism as recently documented by U-Pb zircon dating. An older Pan-African granulite facies metamor-phism (~644-629 Ma) is found in domains showing S-SW directed low angle thrusting (Taita Hills) and NNW-SSE strike slip deformation (western Galana River). In an eastward direction, a late Pan-African granulite facies metamorphism (~550 Ma) occurs in a domain where structures indicate ~NE-SW shor-tening (eastern Galana River). The nearby granulites of the Pare-Usambara mountains as well as rocks of the Umba Steppe in NE Tanzania show similarities in age of high-grade metamorphism and structural appearance. Two tectonothermal events which were noticed in north- central Kenya could have affected both regions. The Samburuan-Sabachian event (starting at ~820 Ma), possibly associ-ated with island arc accretion and structurally documented as early recumbent and overturned folds, may have resulted in granulite facies metamorphism at ≥ 644-629 Ma. The vestiges of a related suture are likely to be in SE Kenya and are assumed to continue southwards across the NE granulites of Tanzania. Geochemical and recent geochronological (U-Pb zircon) data suggest that arc-related magmatism started in the early Neoproterozoic (>900 Ma). The Baragoian- Barsaloian event ()∼ 620≤ 580Ma , as a tectonometamorphic overprint, presumably caused S-SW directed thrusting in the Taita Hills, cross folding and thrusting in NE Tanzania as well as a distinct scatter of lineations. This event could involve another collision and suturing of the Pan-African cycle and may be manifested in metamorphism and tectonic style of the Galana River area.

Keywords: Pan-African suture, polyphase deformation, transcurrent tectonics, north-central Mozambique Belt

INTRODUCTION and older crust (e.g. Denkler et al., 1994; Abdelsalam and Stern, 1996; Johnson and Woldehaimanot, 2003). By The Mozambique Belt of eastern Africa (e.g. Holmes, 1951) contrast, the high grade basement that outcrops in East is a part of the East African Orogen (e.g. Stern, 1994) and Africa (e.g., Shackleton, 1986; Key et al., 1989; Mosley, merges in a northward direction into the Arabian-Nubian 1993; Pinna, 1995) was associated with continent-continent (ANS). The orogen formed by the assembly of crustal collision (e.g. Burke et al., 1977). Compared to the ANS blocks belonging to East and West and shows ophiolitic rocks are more rare, but occur, e.g., in N to NW marked changes of lithology, metamorphism and tectonic Kenya (e.g. Vernacombe, 1983; Shackleton, 1986; Berhe, style. In the ANS, e.g., low grade island arc- and ophiolitic 1990; Fig. 1) where the occurrences may docu-ment two rocks are to be found (e.g. Ries et al., 1983; Berhe, 1990; collisional events (Shackleton, 1993b). Evidence for a suture Stern, 1994; Neumayr et al., 1996). The rocks, often in SE Kenya arose from a geo-chemical study (Frisch and emplaced as nappes (e.g., Shanti and Robol, 1979; Curch, Pohl, 1986). Below we present structural observations from 1988; Wallbrecher et al., 1993) relate to sutures between SE Kenya and NE Tanzania and, along with previous and Neoproterozoic arc terranes or Neoproterozoic arc terranes

AJST, Vol. 9, No. 1: June, 2008 50 Pan-African deformation in SE Kenya and NE Tanzania: geotectonic implications for the development of the north-central Mozambique Belt

Figure 1: Tectonic map of Precambrian structures in Kenya (simplified according to Mosley, 1993)

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Table 1: Simplified description of tectonometamorphic characteristics of the northern-central Mozambique Belt (according to Mosley, 1993; Key et al, 1989; Hepworth and Kennerly, 1970; Hepworth, 1972)

AJST, Vol. 9, No. 1: June, 2008 52 Pan-African deformation in SE Kenya and NE Tanzania: geotectonic implications for the development of the north-central Mozambique Belt recent work (structural-, geochronological- and in rocks of the Buna area (Williams and Matheson, 1991) geochemical data), outline a simple evolutionary tectonic sillimanite growth and the minor occurence of kyanite were model which may allow a connection to other parts of the associated with an anti-clockwise P-T-t path. orogen. In central-NE Tanzania (Fig. 2, Table 1), tectonic events Orogenic characteristics were subdivided into a Parangan-, Kondoan- and Bongan deformation, and a folding (eyed-folds) event (Hepworth Regional structural framework and tectono- and Kennerly, 1970; Hepworth 1972). The Parangan metamorphism (possibly pre-Pan-African) deformation can be found about 100 km from the Tanzania towards the east. In Three tectonic sectors were proposed for the Mozambique the north, Parangan foliations show a northeasterly, W- Belt in Kenya. These sectors, each probably having its dipping trend and probably involve two phases of nearly own tectonometamorphic history, are mainly separated by coaxial folding. In a southward direction, a NS foliation ductile shear zones (for a detailed discussion see Mosley, trend appears. The Kondoan (main Mozambiquian event; 1993; Fig. 1; Table 1) and may be evidenced also by Hepworth, 1972) reworked the Parangan domain and newly- geophysical data (e.g., Meju and Sakkas, 2007). The arranged foliations and fold axes in a sub-horizontal girdle western sector (Fig. 1) shows W to NW-verging thrust (NE trending cluster). The Kondoan deformation typically piles as well as tectonically repeated sequences. In western shows reclined (~SE dipping) folds, asymmetric folds and Kenya, the rocks of the Mozambique Belt tectonically distinct intersection lineations. Near by the craton a late interleave with older cratonic crust (e.g. Sanders, 1965). deformation event (Bongan) formed NE to NNE trending Duplex structures and fold interference patterns (recumbent flexural slip folds. In eastern Tanzania, the Pan-African folds with NE-trending axes being refolded on NW axes) event may also involve contrasting metamorphic histories. can be found, e.g., in the south of the sector (e.g., Loita Peak metamorphism in the eastern granulites (e.g. Pare Hills; Fig. 1). Thermobarometric studies (Key and Hill, 1989; Usambara-, Uluguru mountains; Fig 2) was related with Suwa et al., 1979) indicate temperatures and pressures of anticlockwise P-T-t paths (Appel et al., 1998), whereas, 500-700°C and 0.4-1.0 GPa, while the abundant occurrence e.g., Sommer et al. (2003) concluded a clockwise P-T-t of kyanite which is consistently overprinted by sillimanite history from rocks to the west of the Uluguru mountains. may be seen as evidence of a clockwise P-T-t history (Mosley, 1993). This is in agreement with the work Hetzel Crustal age domains and Strecker (1994), who found indications of both isothermal decompression in granulite facies gneisses and Based on Nd model ages, different crustal age domains mafic rocks and subsequent left-lateral shear under were discerned in eastern Tanzania (Fig. 2, Table 2; e.g. retograde amphibolite facies conditions. The latter event Möller et al. 1998; Maboko 2000 (1995); Maboko and was attributed by the authors to the Barsaloian deformation Nakamura 2002; Kröner et al. 2003; Vogt et al. 2006), (Key et al., 1989). including domains of Paleo- to Neoarchean (, Usagaran Belt, central-NE Tanzania)-, The central tectonic sector (Fig. 1) contains subhorizontal Neoarchean- to Paleoproterozoic (Uluguru mountains, thrust stacks (Sabachian event) that were formed and central-east Tanzania, SE Tanzania)-, and Meso- to largely reworked (Baragoian and Barsaloian deformation) Neoproterozoic model ages (Uluguru mountains, Pare- during Neoproterozoic orogenesis (Table 1; Key et al., 1989; Usambara mountains, Wami River area, SE Tanzania). Charsley et al., 1984). The rocks are often characterized by coarse grained fabrics, implying higher metamorphic In part, these model ages are supported by U/Pb zircon temperatures (Mosley, 1993). Nyamai et al. (1999) inferred (monazite) dating (see Table 2). For instance, Nd model PT conditions of 750°C and 0.66-0.7 GPa (amphibolitic ages from the Uluguru mountains in Central-E Tanzania rocks) and ~880-900°C and 0.6-0.63 kbar (gabbroic rocks), imply mixing between crustal material of different ages respectively. HT/HP conditions (~840-850°C; 1.5 GPa) were (Möller et al. 1998), which is consistent with U-Pb zircon deduced from dioritic rocks. In the eastern tectonic sector data that suggest that central-E Tanzania is underlain by (Fig. 1), east-verging thrusts are a characteristic structural -, Usagaran- as well as Neoproterozoic feature and there are indications of different metaigneous basement (Fig. 2; Table 2). Moreover, in tectonometamorphic histories. In the Adola-Moyale area addition to the widely documented Pan-African (Fig. 1), ultramafic, migmatitic- and low grade rocks may metamorphism, there are indications of Neoarchean- to evince a clockwise P-T-t history (see Mosley, 1993) while Paloproterozoic high-grade events (Table 2).

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Figure 2: Simplified tectonic sketch (based on Hauzenberger et al., 2004). Basement rocks in SE Kenya and the eastern granulites of Tanzania are highlighted. Span of peak metamorphism according to Möller et al. (2000); Muhongo et al. (2001); Hauzenberger et al. (2007). The age of ~580 Ma (syn-tectonic pegmatite) is assumed to relate to a high-grade event. Bold dotted line (Galana River area): eastern border of the shear zone possibly showing a terrane boundary. For a detailed outline of the Parangan- to Bongan tectonic domains in central-NE Tanzania see, e.g, Hepworth (1972)

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Table 2: Nd model age domains and U/Pb zircon (monazite) data for eastern Tanzania. Super- and subscripts: Cl (Collins et al. 2004), C (Coolen et al., 1982); Ct (Cutten et al., 2006); (Johnson et al., 2003); K (Kröner et al., 2003); Ma (Maboko 2000), Mb (Maboko 1995) MN (Maboko and Nakamura 2002); Ml (Möller et al., 1995), M (Möller et al., 1998), Mo (Möller et al., 2000); Mh (Muhongo et al., 2001); ML (Muhongo and Lenoir, 1994); R (Reddy et al. 2003), S (Sommer et al., 2003); Sa (Sommer et al., 2005a); Sb (Sommer et al., 2005b); T (Tenczer et al., 2006); V (Vogt et al., 2006).

Previous work in the study area Lugard’s Falls belt which, in a westward direction, passes into isoclinally folded graphitic-, garnet bearing- and ± SE Kenya hornblende to hornblende-garnet gneisses (Sanders, 1963). In the SE of the Taita Hills (Fig. 5), Saggerson (1962) To the south of the Taita Hills (Figs. 4, 5), gneisses and distinguished between a Kurase and a Kasigau series marbles indicate a west verging and ENE dipping fold (Kurase- and Kasigau group; Pohl and Niedermayr, 1979; pattern (Walsh, 1960), which was explained by a single or Table 3). Recumbent to overturned west-verging folds are different event(s) as a result of ~E-W shortening (F1-F2; typical for the Kurase group (F1; Fig. 3), while rocks of the Fig. 3). Fold axes and lineations gently (~10°) plunge to overlying Kasigau group generally show open folding (F2; the N and imply either tilting due to late Precambrian east- Fig. 3). The different fold patterns were interpreted as west faults (Walsh, 1960) or overthrusting from the north separate deformation events or, alternatively, as the result (Parkinson, 1947). Pohl and Niedermayr (1979) suggested of disharmonic folding (Saggerson, 1962). a deformation history similar to that in the Taita Hills (Fig. 3, see below) and documented the occurrence of granulite The Taita Hills were initially subdivided into an arenaceous facies rocks. and argillaceous group (Parkinson, 1947). Farquar (1960) described, e.g., asbestos mineralization and ultramafic rocks The area to the E and NE of the Taita Hills (Figs. 4, 5) of the northern area. Subsequent work (Pohl and comprises the Sobo formation and the Lugard’s Falls belt Niedermayr, 1979; Horkel et al., 1979; Pohl and Horkel, 1980) (Sanders, 1963; Table 3). In the east, the Sobo formation is showed a correlation to results of Saggerson (1962). The faulted against overlying arkoses, sandstones, shales and Kurase group was interpreted as shelf sequence that may limestones (Duruma Sandstones). The Sobo formation rest on a tectonically emplaced granulitic (charnockitic) indicates a tectonic contact to the steep westerly dipping basement (Pohl and Horkel, 1980), while the overlying

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Figure 3: Approximate axial directions of folds (shallow to moderate dip). Solid lines: data from the literature; dashed lines: this study. Plots are located according to the map (Fig. 3). Bold lines: overturned to recumbent folds. The ~NW-NNW lineation trend in unit I indicate a connection to folding (the NNW trend coincides with the strike directon of the p -pole of the foliations, see text) which could also be the case for the ~NNE-SSW lineation trend (Fig. 5). For the area to the south of the Taita Hills, two options are indicated (see text). The bold dashed line (F1(F2)) refers to the data of Walsh (1960)

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Figure 4: Study area, redrawn including some structural data (except stereoplots (see Fig. 5), structures at the sites m1- m3 and the Ikongwe anorthosite, and cross-striking zones) after Saggerson (1962), Bagnall (1960), Dundas (1965), Bagnall et al. (1963), Hartley and Moore (1965), and Möller et al. (2000). For details of the study area in SE Kenya, see Fig. 5 (and references in the figure caption). Not all isolated outcrops of the area are shown. Boundary of the Kurase- Kasigau group from Saggerson (1962) and Pohl and Horkel (1980)

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Figure 5: Geological map of the Taita Hills – Galana River area in SE Kenya (after Hauzenberger et al., 2004) modified from Sanders (1963); Pohl and Niedermayr (1979) and Horkel et al. (1979). Inserted diagrams: contoured distributions of lineation and foliation data (multiples of even distribution; equal area, lower hemisphere; see Wallbrecher 1986) from Bauernhofer (2003)

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Table 3: Simplified description of tectonometamorphic characteristics of the study area in SE Kenya and NE Tanzania (after Walsh 1960; Sanders 1963; Saggerson 1962; Horkel et al. 1979, Pohl and Horkel 1980; Frisch and Pohl 1986; Bagnall 1960; Dundas 1965; Bagnall et al. 1963; Hartley and Moore, 1965)

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Kasigau group could have formed in vicinity of a Shackleton (1993a) inferred a strain gradient for parts of continental margin or fast downwarping basin. The tectonic the area, insofar as intense planar shearing and linear contact of the groups (e.g. Wanjala thrust, Fig. 5), as well fabrics may merge into a more inclined fold and thrust as other thrust horizons, contains mafic to ultramafic rocks deformation. The latter deformation detected in structural of possible ophiolitic origin (Voi suture zone; Frisch and upper levels of the granulites and overlying metasediments Pohl, 1986). Three deformation-folding events (D1/F1-D3/ of the Umba Steppe was supposed to be younger. The F3, Fig. 3, Table 3) are documented in the area (Pohl and structural style and a pronounced ~ENE-WSW lineation Horkel 1980). Deformation D1/F1, which is largely trend in the area were associated with Samburuan- obliterated in the Taita Hills, appears, e.g, in the Mwatate Sabachian tectonics (Table 1). The change into a ~N-S area (Fig. 4) where also cross folding can be found. During lineation trend northward from the Pare-Usambara this event, ultramafic- and granulitic rocks were emplaced mountains was seen as late Mozambiquian incremental along thrusts. The second event, D2/F2, was accompanied strains that realigned earlier structures. Petrological studies by migmatism of the rocks and caused various deformation (Möller, 1995; Appel et al., 1998) indicate high pressure structures (shear-, flow-, ptygmatic folds, boudinage), fold granulite facies conditions as well as similar geothermal interference patterns and a reorientation of ultramafic rocks. gradients throughout the area. Open folds and gentle flexures (subparallel to F2 axes) represent late structures (D3/F3). Structural investigations

NE Tanzania Tectonostratigraphy

The Pare-Usambara mountains (Bagnall, 1960; Dundas, Considering previous work, four tectonostratigraphic units 1965; Bagnall et al., 1963; Fig. 5; Table 3) indicate a thick (I-IV) were distinguished in SE Kenya nearby Voi (Figs. 5, sequence of enderbitic (charnockitic) gneisses (Appel et 6; see also Hauzenberger et al. 2004; Bauernhofer, 2003). al., 1998) that contain meta-carbonaceous, metapelitic and These are Galana East (unit I)- and Galana West (unit II), metacalcareous layers. In the North Pare mountains within the Tsavo East National Park and the Taita Hills (Bagnall, 1960), the rocks underwent a phase of recumbent (unit IV), located some km west of Voi. A Transition zone folding (F1, ~NE trending axes; Fig. 3), while localized (unit III) marks the tectonic change of the units II-IV. This cross-folding (F2, ~NW-trending axes) could relate to later subdivision, based on different tectonic styles (Figs. 5, 6), thrust tectonics. Pegmatitic rocks (Bagnall, 1960; corresponds to the Sobo formation (unit I), the Lugard’s Loizenbauer et al., 2000) occur, e.g., as foliated and Falls belt (unit II or Galana River shear zone) and parts of concordant layers that were cut by discor dant and the Kurase-Kasigau group in the Taita Hills (unit IV; Horkel unfoliated types, late pegmatites form (± vertical) NW- et al., 1979). Unit III includes gneissic lithologies to the trending dyke-like structures. In the South Pare mountains, west of unit II. A possible correlation of the Sobo formation the rocks contain moderately ~ENE-dipping foliations and and the Kurase group was supposed by Saggerson (1962). were probably affected by recumbent folding (Dundas, In addition, structural data from NE Tanzania (i.e. Pare- 1965). For instance, a large and gently NE plunging Usambara mountains; Umba Steppe) are presented but not recumbent structure has been documented about 15 km in terms of a tectonostratigraphy. The work depdended east of Same (Fig. 5). The plunge of minor fold axes and much more on specific sections and the structural lineations vary in orientation from NE to ESE. The rocks of correlation of the different domains was not studied in the Usambara mountains (Bagnall et al., 1963) contain tight detail. recumbent folds (~NE dipping axes), which were subjected to axial plane shear, as well as to a later stage of minor The results of petrological investigations and details of warping (~NNW trending axes). The pattern of pegmatitic the metamorphic history of the Taita Hills-Galana River rocks is similar to that described from the North Pare area are presented in Hauzenberger et al. (2004). In brief, mountains. The Umba Steppe (Fig. 4; Hartley and Moore, migmatitic gneisses and intercalated amphibolites/mafic 1965) predominantly comprise rocks of metasedimentary granulites can be found throughout the area, origin with foliations that generally strike N-S, while metasedimentary rocks frequently occur in unit I and parts lineations show a gradual change in trend from NE in the of unit IV. Normally the rocks are well foliated and show south to ~15° in the north. The common deformation coarse grained high-grade metamorphic fabrics which is structures are recumbent isoclinal folds and the area may often the case also for distinct shear horizons. Peak have been affected by two periods of folding of similar metamorphic conditions deduced from metapelites, style (Hartley and Moore, 1965). granulitic gneisses and amphibolites/mafic granulites (Hauzenberger et al., 2004) are estimated at 760-820°C (Grt-

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Figure 6: Simplified tectonic sketch (units I-IV). Common deformation structures include deformed pegmatitic (granitic) to migmatitic veins and mafic layers, shear (flow) folds and shear bands. Faults (thrusts) and folded strata in unit I (marbles and metasediments) are based on the map of Sanders (1963). Black layer (unit IV) shows mafic to ultramafic bodies at the boundary of the Kurase- and Kasigau group. The rocks possibly have been emplaced due to an older ~E - W shortening event (see text)

Bt-,Grt-Opx-, Grt-Cpx thermometer) and 0.75-0.95 GPa the rocks that outcrop in the Taita Hills-Galana River area. (GASP-, GRAIL barometer and Grt, Cpx, Opx, Pl, Qtz mineral Gneisses sampled from unit III indicate M-type granitoid equilibria) for unit I and parts of unit II. For unit III and unit genesis and could be clear evidence of a subduction IV, thermobarometric results indicate peak PT condtions process. Recent U-Pb zircon dating (Hauzenberger et al., of 760-840°C (Grt-Cpx thermometer) and 1.0-1.2 GPa (Grt- 2007) documents that such rocks were emplaced in the Cpx-Pl-Qtz-, GADS barometer). A follwing amphibolite early Neoproterozoic, between ~955 Ma and ~845 Ma. facies event in unit III and unit IV may involve temperatures The oldest dated gneiss (~970 Ma) suggesting a and pressures of ~590-700°C and ~0.6-1.0 GPa. SHRIMP subduction related-signature was sampled from unit II. The U-Pb zircon dating (Hauzenberger et. al., 2007) suggests geochemical data of mafic rocks from the Taita Hills (unit two periods of granulite facies metamorphism in the area. IV) support previous results (Frisch and Pohl, 1986) that An older event (644 ± 15-629 ± 6.8 Ma) may be evidenced indicate a subduction origin for amphibolitic rocks of the in the units II, III and unit IV, while a younger event (550 ± Kasigau group and a -related (within-plate) source for amphibolitic rocks of the Kurase group. Furthermore, the 14 Ma) was ascertained in unit I. In addition, and based on data from the Kurase group may allow to conclude that the petrographic and thermobarometric evidence, clockwise rocks relate to a back-arc setting. Similarly, mafic- to P-T-t histories were deduced for the units. ultramafic rocks from the Pare mountains (sites m1, m2, m3; Fig. 4) could document a former rift- and subduction A few notes are given on geochemical data (Bauernhofer, setting. 2003) that forms an essential part of the below model (Fig.

7). The findings suggest a volcanic arc related origin for

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Galana East (unit I) NNW. Apart from specific (high-strain) domains a non- coaxial component of deformation (often sinistral) is Typical for the easternmost basement rocks are moderate indicated by the asymmetry of pinch and swell veins or southwesterly (up to ~45°) to gently or even rotated boudins, strain shadows (s- to d-shaped felsic subhorizontally to the NE(E) plunging foliations (Fig. 5, segregations) around mafic enclaves and shear (flow) folds. Fig. 6). The foliation pattern indicates close folding of the Distinct shear bands, often having dextral offset and a largely metasedimentary sequence (Sanders, 1963). northerly trend, are usually infilled with migmatitic Measured fold axes overlap a NNW (NW)-SSE (SE) (pegmatitic) melts. They sporadically intersect the gneissic lineation trend (Fig. 5) which also contains the p-pole (153°/ fabric or structures (e.g. pinch and swell veins and folds) 4°) of the partial girdle formed by foliation poles (Fig. 2). and occasionally indicate considerable displacement. At the outcrop scale folds often appear as tight structures, isoclinal folds (e.g. migmatitic layers, variably oriented) Transition zone (unit III) and isoclinal recumbent structures (e.g. marbles) additionally occur. Asymmetric folds with layer-parallel Rigde-shaped gneiss bodies, often tonalitic to dioritic in limbs, partly flattened, are also to be found. Boudins composition, appear in a domain stretched out to several occasionally display an internal fabric and substantiate tens of km northwards of Voi (Fig. 5). The size of the the influence of non-coaxial deformation. The observations structures which trend subparallel to unit II varies from indicate ~NE-SW shortening, however, there possibly exist more than 10 km in length and several km in width (Sagala different shear directions. In the eastern area, outcrops Hills; Fig. 5) to km scale and smaller. In part, these rock sections at high angles (>80-60°) to the ~NW-SE lineation types probably also occur in the Taita Hills, e.g, in the northern area. The gneisses frequently show prolate fabrics trend display asymmetric folds which could be evidence that occasionally appear also in distinct mylonitic horizons. of ~WSW-SSW directed shear. By contrast, in the more Normally, the gneisses enclose more metabasic rocks (thin western domain (near to unit II) the opposite follows from strips, lenses and nodules, less frequent massive garnet- the interpretation of Sanders (1963) (Fig. 6). The lineation amphibolites) than gneisses in unit II. The biggest pattern (Fig. 5) reveals an ambiguity too. In addition to a structure, the Sagala Hills (Fig. 5), was interpreted (Sanders, ~NW-SE lineation trend there appear subordinate ~NE- 1963) as an open anticlinal fold pervaded by a series of SW and ~E-W trends. high-angle faults down-throwing to the east. A fold Galana West (unit II) structure is supported also by our data. Foliation poles scatter upon a great circle and show a steep (>50°) ENE To the west of Ithangethi (Fig. 5), a more than ~25 km plunging cluster (Fig. 5). The lineation data indicate a NNW- westward extending domain of often steep (~70°) WSW- SSE trend which overlaps the p --pole (346o/14o) of the dipping foliations marks the Galana River shear zone, which foliation distribution. A spatial relation-ship of is probably a part of major NNW to N-S trending shear deformation structures can be observed from smaller zones in Kenya (Fig. 1). The lineation data evince a strong gneiss bodies. Two pronounced sets of mylonitic shear preferred orientation (NNW-SSE trend; Fig. 5) with a gentle zones (cm - dm thickness; in part former acidic or (~15°), more regular dip to the NNW. Steeper lineations intermediate veins) are discernable at some exposures (Fig. (Fig. 5) scatter along a great circle that contains the NNW- 6). They show either steep (>70oC to ± vertical) easterly SSE trend lineation trend. A SSW dipping lineation dip or plunge shallowly (<20°) to the NNW or SE. submaximum can be found at the change to unit I (Fig. 5). Precipitous sections (± parallel to the strike of the bodies; Intense deformation is obvious from most outcrops. Typical Fig. 6) frequently indicate northward displacement of structures comprise folded and boudinaged often coarse folded or boudinaged mafic vein segments and layers, grained pegmatitic- to granitic veins that usually are although the implied displacement could also be the effect imbedded in granodioritic to tonalitic gneisses. The veins of the intersection of outcrop surface and mylonitic vary in thickness from centimetre- to ~half-metre scale and horizons. On subhorizontal top surfaces, mafic layers are frequently point to flattening strain (horizontal and vertical sometimes boudinaged and in places indicate dextral strike extension; Fig. 6). Often, boudinaged veins are slip movement (Fig. 6), although opposed shear senses concordantly aligned to the strike and dip of the foliation can also be found (e.g. shear bands). On sections ± trend. Pinch and swell structures are more specific to larger perpendicular to the longitudinal extension of the gneiss felsic veins while competent mafic layers often form barrel bodies (Fig. 6) one can note the folding of a gneissic (irregular) shaped- to tapering boudins normally containing foliation and, occasionally, the varying orientation of migmatitic segregations in the interboudin domains. Fold ptygmatic veins or folded layers that contain blocky axes of observed minor isoclinal folds plunge gently to the boudins.

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hypothetical p-pole of the foliations (104o/17o). A few Taita Hills (unit IV) steeper lineations (Fig. 4) may be related to later dip- or strike slip tectonics on steep (N) NNE-SSW dipping foliation In the Taita Hills (Fig. 5), foliations of the Kurase- and planes. At some outcrops the latter deformation may o Kasigau group usually dip gently (<20 ) to the NNE. balance the shortening imposed by folding. In addition, Lineations scatter within NNW to NE directions but also SSW plunging lineations can be found. o o show a clear northerly oriented cluster (12 /14 , Fig. 4; see also Pohl and Horkel, 1980). Thus it seems likely that the At the northern foothills of the N-Pare mountains, a Taita Hills are imbricated by S-SW verging low angle lenticular body of mafic to ultramafic rocks (m3, Fig. 4) thrusts that probably define also the Voi suture zone. appears to be subjected to S to SSW thrusting. Numerous Although the outcrops frequently contain shear fabrics, ductily deformed phacoid-shaped fragments, partly movement directions are not always unambiguous. A embedded in pegmatoidic segregations, are discernible. thrust horizon, however, exposed near Dembwa (opposite The foliation and lineation data of a fragment show 35°/35o to Murry girls high school) about 6 km north of Mwatate and 25o/20o, respectively. The influence of later, localized (Fig. 4) could corroborate the above assumption. ductile strike slip shearing can be inferred from steep to Asymmetric fabrics (similar to S-C fabrics) and small scale subvertical northerly trending zones which often represent duplex structures indicate thrusting towards the S to SSW. thin (cm – dm scale) acidic veins. A well exposed vein The mylonitic banding of the rocks, which is more (foliation 260o/80o) indicates dextral shear but probably pronounced compared to the surrounding gneissic involves a significant oblique slip component. The internal lithology, as well as sharp boundaries (probably synthetic fabric of other mafic to ultramafic pods (m1, m2; Fig. 4) is shear domains) on the top of phacoid-shaped mafic more concealed. Some tens of km ESE of Same (m2, Fig. 4) enclaves are conspicuous. The measured lineations a foliation is hardly recognizeable in the rocks. The dip of frequently show N or NNE dip and most likely indicate the a few readings vary from moderate (50-60o) SSE-dip to sub- direction of the tectonic transport. The deformation could vertical E or W dip, while the surrounding gneisses dip have commenced under high grade metamorphic gently (<10o) to the east. To the SE of Same (m1, Fig. 4) on conditions, as indicated, e.g., by alumosilicate bearing can find steep WNW-ESE striking faults and strongly gneisses. One of several generations of sillimanite weathered domains (~N-S trend) within a pod. (Hauzenberger et al., 2004) is aligned with the lineation and foliation of the rocks and define in part microscopic A short visit of the Ikongwe meta-anorthosite body (Fig. shear bands. In addition, recumbent folds can be found 4) suggests that many structures relate to syn to post- with axes parallel to the ~NNE lineation trend. At the SE emplacement deformation (e.g. sinistal to dextral strike slip foothills of the Taita Hills, foliations define an northerly shears). Conspicuous structures are localized fine-grained dipping open anticline while towards unit II, the folding of mylonites (cm - dm scale), which indicate NNW-SSE strike the rocks becomes tighter. and a steep westerly dip (~24o/80o). The mylonites intersect the older fabric and could relate to the pattern observed Pare-Usambara mountains – Umba Steppe (V) from site m1. The rocks of the Umba Steppe often dip to the east at moderate angels (<40o) at which our data show The foliations observed in rocks of the Usambara a cluster at 81o/27o. Steeply dipping foliations (55o-80o dip) o mountains (Fig. 4) show a gently E-dip (maximum at ~99 / were found in marbles and calcsilicate rocks to the east of o o o 21 ), while lineations display a maximum at 63 /15 . A Mwakijembe (Fig. 4) as well as metasedimentary rocks comparable pattern exists in the Pare mountains (Fig. 4), occuring between Daluni and Umba (Fig. 4). Pronounced where the orientation of foliations and lineations evince a lineations, although seldomly noticed, indicate strong o o similar cluster (~81 /24 ). A feature of the two lineation preferred orientation (cluster at 15o/8o). patterns is a distinct scatter along great circles which, to a lesser extent, can be detected also in the Taita Hills (Fig. Regional implications 5). Two cross-striking zones (probably associated with localized folding) were observed, respectively, one in Pare The presented and previous observations indicate that mountains and one in the western peneplain (Fig. 4). The the study area is affected by a variety of structural styles. foliation data of these domains outline a fold structure There can be found thrust-related tectonics, e.g., in o o with a moderate northerly dipping maximum (~12 /42 ). The mountainous regions (e.g., Taita Hills, Pare- Usambara orientation of lineations mostly varies between ENE to SE mountains; Pohl and Horkel, 1980; Bagnall 1960; o o plunge and shows a cluster (~96 /15 ) near to the Shackleton, 1993a) or strike slip tectonics, such as the

63 AJST, Vol. 9, No. 1: June, 2008 A. H. BAUERNHOFER

Galana River shear zone, in the adjacent lowlands which, Interpretations of Bagnall (1960) and Parkinson (1947), who in part, are characerized by more metasedimentary suggested superimposed thrust tectonics to explain either lithologies (e.g., eastern Galana River area, Umba Steppe, cross folding (NW trending axes; Fig. 3) or a northward Kurase group). In this setting, the Galana River area forming dip of lineations in SE Kenya. part of the southeastern-most extension of the central tectonic sector (Fig. 1) could be of particular interest. A feature of the study area could be different metamorphic histories though similar peak metamorphic conditions The Galana River shear zone probably represents a high- were noticed (760-840oC, 10-12 kbar, Hauzenberger et al. grade (granulite- to amphibolite-facies) transpressional 2004; ~ 810oC, 9.5-11 kbar Appel et al., 1998; e.g., Grt-Cpx structure. The steeply dipping zone, which indicates phases thermometer, GADS barometer). Based on a magmatic of early sinistral and later dextral shearing, could continue underplating model Appel et al. (1998) concluded an anti- northwards as Athi-Ikutha shear zone (Fig. 1) and may clockwise P-T-t path for NE Tanzania. This would be also correspond to the main Barsaloian shear zone (~580 accordable with crustal growth as supposed, e.g., for Ma, Key et al. 1989; Fig. 1) showing dextral (Key et al. Phanerozoic volcanic arc settings (e.g. Hamilton, 1995). If 1989) or rather sinstral sense of shear (Mosely, 1993). A so, the present erosion level does not uncover the lower relationship between these shear zones would be relevant mafic parts of the crust, though some of the basic to assuming that the Galana River (and Barsaloian?) shear ultrabasic rocks of the area may show up-thrust sections. zone could form a terrane boundary. A geochronological In contrast, Hauzenberger et al. (2004) found a near isobaric correlation of both structures follows from two pre- to syn- cooling path following the peak of metamorphism. This is deformational pegmatites sampled from the shear zone. in accordance with the findings of Appel et al. (1998). The rocks have intrusion ages of ~580 and ~564 Ma However, Hauzenberger et al. (2004) concluded that tectonic (Hauzenberger et al., 2007). If the Galana River shear zone setting is rather related to collisional thickening and thus continues in NE Tanzania, it could run east of Mwakijembe the prograde PT-path may be different. In the Pare- (Fig. 4) now masked by upper Paleozoic sediments. In the Usambara mountains a more sporadic (late stage) sillimanite Umba Steppe there may exist localized strike-slip shears, growth was associated with fast uplift (Appel et al., 1998). however our observations do not support a tectonic style As in the Taita Hills, this may involve thrusting and could compareable to the Galana River shear zone. The main reconcile the paths at an advanced stage of the tectonic ~NW-SE lineation trend in unit I (Figs. 5) could be evolution. The different P-T-t situation, if it exists, could associated with folding caused by the shear zone result from the complex tectonometamorphic history of development. Thus, concerning the latest deformation, the volcanic arc systems. In particular, conditions can vary trend rather displays fold axes than stretching or shear along the strike (e.g. Hamilton, 1995) and could mark directions as assumed for lineations of the shear zone. A specific areas of an arc system now veiled or juxtaposed tectonic relation between unit I and unit II is supported by due to intense deformation following a collisional event. similar 40Ar-39Ar hornblende cooling ages (~525-519 Ma; Bauernhofer 2003). The subordinate lineation trends (Fig. Similar conclusions were also reached by Cutten et al. 5) could show earlier increments of this deformation or (2006), who proposed, based on U-Pb zircon dating in may relate to an older event. For instance, an older combination with P-T-t constraints from the eastern deformation event is indicated by a complex, shallowly granulites (Appel et al., 1998) and Mautia Hill (Fig. 2; Jöns ~SW dipping fold structure observed in unit I. The fold, and Schenk, 2004), a single-stage collision model for the presumably a former recumbent or sheath fold-like Mozambique Belt in Tanzania, culminating in continent- structure, encloses an obtuse angle (~115o) to the p -pole continent collision and peak metamorphism at ~550 Ma (153o/4o) of the foliation distribution. (see also Möller et al. 2000; Appel et al., 1998).

A classification of the lineation patterns in the Pare- Orogen-wide implications Usambara mountains appears to be difficult. The data (Fig. 4) show several submaxima which, due to polyphase Pan-African tectonics (peak metamorphism ³ 644 - deformation, could relate to a mixture of stretching- and b- £ 620 Ma) lineations. Strain geometry itself (e.g. flattening) may imply a spread of lineations and the indicated cross folding In combination with major tectonometamorphic events probably contribute to this scatter (Fig. 4). The S to SW- proposed for north-central Kenya (Table 1), a single and directed thrusting as detected in the Taita Hills could exist multistage collision are considered below (Fig. 7cI, cII). in the Pare-Usambara mountains as well. This is consistent Key et al. (1989) argued for a SE movement of the upper with the conclusion of Shackleton (1993a) or the

AJST, Vol. 9, No. 1: June, 2008 64 Pan-African deformation in SE Kenya and NE Tanzania: geotectonic implications for the development of the north-central Mozambique Belt

Figure 7: Simplified evlolutionary tectonic model for the study area (modified from Bauernhofer, 2003 (2006, un-published data)

(a) Premises that the Conogo craton was a part of (see text). (b) Formation of arc crust. (cI) East African after

Stern (1994) (Key et al., 1989). (cII) Polyphase orogeny (this study; Shackleton, 1993b; Meert (1995; 2003); Boger et al., 2002)

with preceding cI as a result of arc terrane assembly. Shaded triangles: island (continental) arc terranes (single arcs or continous system). Subduction directions are speculative. In the case of a back-arc setting (see text) a westward subduction (not shown) may be assumed. The situation to the west of the study area allows for sutures indicated by Berhe (1990) (see also Cutten et al. 2006)

65 AJST, Vol. 9, No. 1: June, 2008 A. H. BAUERNHOFER plate during plate collision ( £ 820 Ma) and a ENE-WSW by Nd model ages or Neoproterozoic metagranitoid rocks motion of nappes and thrusts. Referring to the authors, (Table 2). In all areas, the rocks are enmeshed in late Archean Shackleton (1993b) proposed a collision to take place to early basement which can be found towards between ~800-750 Ma including obduction of the Baragoi the Tanzania craton (Table 2, Fig. 2; see, e.g., Muhongo, and associated ophiolites and, based on geochemical 1994; Fritz et al., 2005, Cutten et al., 2006). Thus, in contrast evidences (dated metagranitoid samples of the former to a continent-continent collision, early Pan-African authors), suggested an island (continental) arc setting at granulite facies metamorphism ( ³ 644 - £ 620 Ma) could ~800 Ma. As mentioned at the beginning, the Samburuan- originate from arc-arc collisions of Neoproterozoic volcanic Sabachian event (~830-800 Ma) is rather interpreted as arcs or such arcs with older cratonic crust. For instance, linked to a major island arc accretion-collision event than Fitzsimons and Hulscher (2005) suggested that, after to continent-continent collision for the study area. closure of a Paleomozambique ocean, the Antongil block Furthermore, the time frame is modified to that effect that (Dhawar craton) and Malagasy arc (Antananarivo block) the event invol ves high grade metamorphism in the range amalgamated at ~720 Ma, while the subsequent closure of of ³ 644 - £ 620 Ma, and that the beginning overlaps with a Neomozambique ocean, between Malagasy arc and the age of volcanic arc magmatism as inferred for the study , was proposed to be followed by collision area (~970-850 Ma; Fig. 7). Similarly, the time frame of the and high-grade tectonism (~640-540 Ma), including a Baragoian-Barsaloian event (~620-580 Ma) is extended to cratonassociated Tanzanian arc. Another Madagascar- lower ages (£ 580 Ma) considering that tectonic events based model, recently proposed by Emmel et al. (2008) occur with a time lag in different parts of the orogen. (Jöns and Schenk, 2008), is also not inconsistent with our model, particularly with regard to a possible connection to Following Shackleton (1993a,b), the tectonic style of the the study area (Bauernhofer, 2003; see below). The authors study area could stem from a superposition of the inferred a ~650-600 Ma collisonal event associated with Samburuan-Sabachian and Baragoian-Barsaloian (~620-580 small-scale arc terrane accretion (Vohibory terrane) in SE Ma) tectonic events. However, here is supposed that the Madagascar, which resulted in high-grade metamorphism Baragoian-Basaloian event, as a transpressional system between ~640-610 Ma. In southern Tanzania and northern and more distinct in SE Kenya, and not the Samburuan- Malawi, NE directed thrusting and dextral transcurrent Sabachian event caused similar thrusting in the Taita Hills motion mark early Pan-African deformation (~740 - ~600 and PareUsambara mountains. The Samburuan-Sabachian Ma; see Ring, 1993 and discussion therein). Referring to deformation is possibly preserved as early recumbent Hoffman (1992) who suggested an andean-type setting (overturned) folds (Fig. 3) changing from a NE axial trend for the granulite areas of eastern Tanzania, Ring (1993) (Pare-Usambara mountains) to a northerly (Kurase-Kasigau related the thrusting to the deformational style of the area) or rather NNW axial trend (Taita Hills). Although the hinterland. structures may involve sheath folds (Shackleton 1993a), this interpretation was not favoured by Shackleton (1993b) No matter what the type of collision(s), a substantial for all areas. As an alternative, we assume that the argument for a collisional event associated with the early recumbent folds imply an older E (SE) - W (NW) shortening phase of Pan-African high grade metamorphism comes from and shear event at a high angle to the axial trend. In the the age data of SE Kenya (Hauzenberger et al, 2007). The Taita Hills, this possibly corresponds to the emplacement data suggest that, provided there were no of mafic to ultramafic rocks (F1; Horkel et al. 1979, Fig. 6). tectonometamorphic events in between, around 300 – 200 The variation of the axial trend could be influenced by the Ma elapsed from island arc plutonism to granulite facies shape of the Tanzania craton and the Usagaran Belt (Fig. metamorphism, a fact which cannot solely be explained by 2) as well as by the Baragoian-Barsaloian overprint. This a magmatic underplating scenario (see also discussion in overprint probably has realigned lineations and minor fold Appel et al., 1998). An early Neoproterozoic arc setting axes (e.g Pare-Usambara mountains) to spread along would have also implications for the Rodinia concept. If shallowly ~E dipping great cirlces (Fig. 4). the rift rocks in SE Kenya do not relate to a back-arc setting they may indicate that continental break-up started during Aside from kinematic aspects it is likely that a volcanic arc the late Mesoproterozoic, or even earlier (see also setting exists also in NE Tanzania. Both areas contain Fitzsimons and Hulscher, 2005). This would fit to the fact juvenile crust (e.g. Möller et al., 1998, Maboko, 2000; that the position of the Congo craton is uncertain in the Muhongo et al., 2001; Hauzenberger et al., 2007). The early Neoproterozoic (e.g. Weil et al., 1998; Dalziel et al., setting could continue in eastern Tanzania (e.g. Wami River, 2000) albeit a connection to Australia was envisaged Uluguru mountains, SE Tanzania, Fig. 2) as indicated, e.g, (Powell and Pisarevsky, 2002; Fig. 7a).

AJST, Vol. 9, No. 1: June, 2008 66 Pan-African deformation in SE Kenya and NE Tanzania: geotectonic implications for the development of the north-central Mozambique Belt

Late Pan-African tectonics (peak metamorphism £ 580 The geotectonic setting and deformational style of the Ma) Galana River area could be found, in addition to eastern Tanzania, in south to western Madagascar, the latter A pivotal question in this setting is whether the following from plate tectonic reconstructions and tectonometamorphic evolution in the Galana River area structural considerations (e.g., Reeves et al., 2002; Kriegsman, 1995). Several N (NNE) to NW trending (units I-II) results from a single collision event (Fig. 7cI) and eastward propagation (i.e. progressive reworking) of metamorphic belts limited by shear zones exist in southern Madagascar (e.g. Windley et al., 1994), under the Vohibory the orogen or not (Fig. 7cII). Hetzel and Strecker (1994) suggested that the Barsaloian shear zones (Fig. 1) are likely Belt, a shelf sequence of interthrusted orthogneisses and related to tectonic escape of thickened Mozambique Belt ophiolitic- to island arc rocks, probably containing vestiges crust along the Tanzania craton towards the ANS (see of the Neoproterozoic Mozambique ocean (e.g., Jöns and also, e.g., Bonavia and Chorowitz, 1992; Stern, 1994). Schenk, 2008). In SE Madagascar, metamorphic and However, the younger granulite facies event (~550 Ma; sychronous magmatic events were dated between ~588 Hauzenberger et al. 2007) along with a transpressional style Ma and ~523 Ma (e.g. Paquette et al., 1994; of unit II could also allude to a separate suturing event, Andriamarofahatra et al., 1990). For this time frame, Martelat which, in turn, probably gave rise to escape tectonics. In et al. (2000) showed in southern Madagascar a strain the Uluguru mountains (Fig. 2), Rossetti et al. (2008) interference pattern of two presumably interrelated high- described an episode of intracontinental compression grade events. The first event (D1; 590-530 Ma) caused a deformation (<580 Ma) that resulted in NW directed nappe flatlying granulitic foliation (E-W stretching lineations) and stacking (~550 Ma) under semi-brittle to brittle conditions. isoclinal folds which were re-folded by vertical This nappe-forming event wa s associated to a Kuunga- transpressional shear zones (D2; 530-500 Ma). In western Malagasy orogeny (570-550 Ma; see, e.g., Collins and and central Madagascar, a comparable period of Pisarevsky, 2005). magmatism and high-grade metamorphism (~580-520 Ma) was related to a collision event (Tucker et al., 1999). By A second orogeny that affected the East African contrast, de Wit et al. (2001) suggested that the high-grade Mozambique Belt would agree with Shackleton (1993b) deformation in southwestern Madagascar should result who proposed that the West Pokot ophiolite (Fig. 1, around from the East African orogen (~650-630 Ma and ~628-627 Sekerr) involves oceanic closure (see also Vernacombe, Ma for D1 and D2, respectively). If this is the case, 1983) and collision (~580 Ma) during the Baragoian- recumbent folds (D1, NE trending axes) and a re-folding Barsaloian event. Assuming that there was another major (D2, NW trending axes) noticed in the Vohibory Belt (e.g. orogeny, the recent data could allow to establish a de Wit et al., 2001) may correlate to the deformation in the connection to other Gondwana regions. A granulite facies southern study area (e.g. Pare-Usambara mountains, Umba event at ~550 Ma in Sri Lanka, Madagascar or Antarctica Steppe). (e.g., Andriamarofahatra et al., 1990; Kröner, 1993; Shiraishi et al., 1994) was related by Meert et al. (1995) (Meert, 2003) ACKNOWLEDGEMENTS to a Gondwana-wide event, namley (~570- 530 Ma) which supposably is composed of two sutures We would like to thank our colleagues from the geological (Boger et al., 2002). A Mozambiquian suture (~570-520 Ma), Departments of the University of Nairobi and Dar es Salaam parallel aligned to the East African Orogen, should result who have supported our work. N. Opiyo-Akech is thanked from the collision of West Gondwana and Indo-Antarctica for discussion and editorial work. We are indebted to G. (Madagascar) and would be a possibility for the study Niedermayr, who readily provided maps and reports from SE Kenya. A.B. and C.H. are grateful to T. Schlüter and D. area (Fig. 7cII). The more southern Kuunga suture (~550- 490 Ma) roughly E-W trending on a global scale may Hollnack for their help. Many thanks also to H. Fritz for assemble the former blocks and East Antarctica-Australia. providing some lineation and foliation data from the Pare- However, and supporting the classical interpetation (Fig. and Usambara mountains. A.B. is indebted to several reviewers for their efforts and detailed criticism of earlier 7cI), Squire et al. (2006) argued for a Kuunga intracratonic zone rather than for a Kuungan event in terms of a major versions of this manuscript, including further reading. This continent-continent collision, which was based, inter alia, is gratefully acknowledged. Special thanks go also to the on a Gondwana-wide accumulation of quartz-rich sediments rangers of the Tsavo East National Park. This study was and detrital zircon age spectra (Gondwana superfan- made possible by funding from the Austrian Science system). Foundation (FWF grants P12375-GEO and P15599).

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