Mozambique Orogenic Belt Research Articles

Geochemistry of Kolme granitoids: Implication for crustal growth in the Mozambique orogenic belt of southwestern Ethiopia

Petrological and geochemical (major and trace element) data are presented for Kolme granitoid from the Precambrian terrain of southwestern Ethiopia. Integrated field and petrochemical studies have been carried out to assess the source and discriminate tectonic setting of the granitoid as well as evaluate their implication for crustal growth. The petrological and the geochemical data indicate the presence of two groups of granitoid in the area: Borkara granitoid (BG) and Matera granitoid (MG). BG is granite in composition with higher SiO2 content (75.27–78.51 wt %), lower Al2O3 (13.05–13.45 wt %), and FeO*/(FeO*+MgO) ꞊ 0.92–0.94 wt %. BG is a high-K calc-alkaline, peraluminous, ferroan, and felsic I-type granitoid. Whereas MG is quartz-monzonite in composition with lower SiO2 (66.03–69.16wt %) and higher Al2O3 (14.85–16.25 wt %) and FeO*/(FeO*+MgO) (0.96–0.97wt %). MG is shoshonitic, metaluminous to peraluminous, alkalic, ferroan and mafic I-type granitoid. BG is derived from the partial melting of upper continental crust with the contribution of melts from subducted slab and asthenospheric mantle wedge, while MG is derived by fractional crystallization of mantle-derived mafic magma underplated in the lower crust with minor crustal involvement.The tectonic discrimination diagrams for Kolme granitoid infer that BG is a syn-orogenic granitoid, while the MG is a post-orogenic granitoid. Moreover, the transition from syn-orogenic calcic-alkalic BG magmatism to alkalic MG magmatism is marking the end of the syn-collisional stage and the beginning of the extension of the Neoproterozoic crust.

Lithospheric Controls on the Rifting of the Tanzanian Craton at the Eyasi Basin, Eastern Branch of the East African Rift System

AbstractContinental rifts most often nucleate within orogenic belts. However, some studies in the East African rift system have shown that continental rifts can also develop within cratons. This work investigated the ~1.5‐Ma Eyasi basin, which propagates in a WSW direction into the Tanzanian craton. The basin is located where the eastern branch of the East African rift system transitions from a narrow rift (~70 km wide) to the wider (~300 km wide) North Tanzanian Divergence. Unlike the rest of the eastern branch segments, the Eyasi basin does not follow the Mozambique orogenic belt located on the eastern margin of the Tanzanian craton. This work generated lithospheric‐scale sections across the basin using (1) digital elevation model to map surface rift‐related brittle structures; (2) aeromagnetic data to determine the depth to the Precambrian basement; and (3) World Gravity Model 2012 to estimate crustal and lithospheric thickness by applying the two‐dimensional radially averaged power spectral analysis and two‐dimensional forward gravity modeling. These cross sections show that the Eyasi basin nucleates within a previously unidentified suture zone within the Tanzanian craton and that this suture zone is characterized by thinner lithosphere that can be as thin as ~95 km. This zone of thinner lithosphere is offset southeastward from the surface expression of the Eyasi basin and might have facilitated the formation of other basins further south. Furthermore, the lithospheric thickness map indicates that the Tanzanian craton is heterogeneous and possibly composed of multiple smaller cratonic fragments.

Chapter 3 Regional geology

The present-day Gregory Rift Valley in northern Tanzania is an elongate north–south half graben flanked on its western side by a high, eastwards-facing escarpment; this differs from Kenya where there are also major faults on the eastern side of the rift valley. The words ‘present-day’ are used advisedly, as the present morphology is imposed upon an earlier, wider, volcano-infilled tectonic depression. As in Kenya, within and to either side of the rift valley there are extensive areas of Neogene and Quaternary volcanic rocks (Fig. 3.1). To the SW of the main volcanic area is the Eyasi half graben and, to the SE, the Pangani graben; these major features are infilled by volcanoes at their northeastern and northern ends, respectively. Fig. 3.1. Distribution of Neogene–Recent faults and volcanic rocks in northern Tanzania and adjoining parts of southern Kenya. The northern Tanzania volcanic province is younger overall than the Ethiopian, where magmatism was initiated at around 40 Ma, and Kenya provinces (Baker et al . 1972; George et al . 1998) suggesting that more recent mantle perturbations are responsible for the intra-plate magmatism and crustal fracturing in this part of Africa. The province stands astride the surface interface between the Archaean rocks of the Tanzania Craton and the north–south-trending Mozambique orogenic fold belt (Fig. 3.1). The rocks of the craton have been divided into three major formations, the Dodoman (oldest), the Nyanzian and the Kavirondian (Quennell et al . 1956; Schluter 1997) but, in brief, the craton is an amalgamation …

An integrated geophysical study of the northern Kenya rift

The Kenyan part of the East African rift is among the most studied rift zones in the world. It is characterized by: (1) a classic rift valley, (2) sheer escarpments along the faulted borders of the rift valley, (3) voluminous volcanics that flowed from faults and fissures along the rift, and (4) axial and flank volcanoes where magma flow was most intense. In northern Kenya, the rift faults formed in an area where the lithosphere was weakened and stretched by Cretaceous-Paleogene extension, and in central and southern Kenya, it formed along old zones of weakness at the contact between the Archean Tanzania craton and the Proterozoic Mozambique orogenic belt. Recent geophysical investigations focused on the tectonic evolution of the East African rift and on exploration for geothermal energy in the southern portion of the Kenyan rift provide considerable information and insight on the structure and evolution of the lithosphere. In the north, a variety of other data exist. However, the lack of an integrated regional analysis of these data was the motivation for this study. Our study began with the collection and compilation of gravity data, and then we used the seismic refraction results from the Kenya Rift International Seismic Project (KRISP), published seismic reflection data, aeromagnetic data, and geologic and drilling data as constraints in the construction of integrated gravity models. These models and gravity anomaly maps provide insight on spatial variations in crustal thickness and upper mantle structure. In addition, they show the distribution of basins and help characterize the distribution of magmatism along the axis of the northern sector of the rift. Our main observations are the following: (1) the region of thinning and anomalous mantle widens northward in agreement with previous studies showing that the crust thins from about 35 km in the south to 20 km in the north; (2) as observed in the south, gravity highs observed along the axis are due to mafic bodies associated with the main volcanic centers, densification of the upper crust due to diking, and horst blocks where Precambrian basement has been brought to or near the surface; (3) the axis of the rift is marked by a series of high amplitude magnetic anomalies whose wavelengths are less than 2.5 km, the positive anomalies coinciding closely with known Quaternary volcanoes.

Depositional environments and an apparent age for the Geci meta-limestones: Constraints on the geological history of northern Mozambique

Strongly contrasting rocks were juxtaposed during the long tectonometamorphic history of the Mozambique Orogenic Belt in northern Mozambique. The latest depositional event was expressed by accumulation of marine sediments, which were subsequently mildly metamorphosed and tectonically juxtaposed with granulite facies complexes. The low-grade metasedimentary rocks comprise the Geci group which was mapped, and studied petrographically, geochemically and isotopically in order to provide constraints on the depositional environments, age and latest history of the Mozambique Orogenic Belt. The group occurs as several large, tectonically dissected, intensively sheared, folded and mylonitised, SW–NE trending lenses within Unango Complex granulite rocks. In places, primary depositional features are well preserved. The dominant rocks are calcarenites, dolarenites, calcite matrix-supported and dolostone clast-supported carbonate breccias forming beds with erosional bases, normal and reverse graded bedding, and well-developed Bouma sequences. Dolomicritic, microbial and oolitic dolostone clasts were apparently derived from the margin of a shallow-water carbonate platform and redeposited by turbidity currents on a continental slope with calcareous sedimentation. The Geci meta-carbonate rocks have low SiO 2 and Al 2O 3 contents. MgO/CaO ratios fluctuate between 0.05 and 0.70 averaging 0.15 ± 0.18 (1 σ, n = 111). Acid-soluble constituents have moderate concentrations of Fe (777 ± 310 ppm), Mn (131 ± 85 ppm) and Sr (566 ± 145). Mn/Sr ratios are relatively low (0.26 ± 0.19), δ 13C and δ 18O values are invariably high, with only moderate scatter: +4.0 ± 0.6% V-PDB and 25.8 ± 0.5% V-SMOW, respectively. The least altered 87Sr/ 86Sr ratio of calcite is 0.70708, whereas dolomite is enriched in 87Sr with the lowest ratio of 0.70730. The least-altered 87Sr/ 86Sr and δ 13C ratios suggest an apparent depositional age of either 595–585 or 630–625 Ma. This provides a lower age limit for juxtaposition of the low-grade Geci group rocks and granulite facies rocks of the Unango Complex.

Pan-Gondwanaland detrital zircons from Australia analysed for Hf-isotopes and trace elements reflect an ice-covered Antarctic provenance of 700–500 Ma age, TDM of 2.0–1.0 Ga, and alkaline affinity

Eastern Australian sediments of Cambrian, Ordovician, Silurian–Devonian, Triassic, and Neogene ages are known to be dominated by zircons dated 700–500 Ma (“Southwest Pacific–Gondwana igneous component”) by the U–Pb SHRIMP method, and thought to be derived from Antarctica, as suggested also by paleogeographical evidence. To extend the characteristics of the provenance we subjected SHRIMPed zircons from the Middle Triassic Hawkesbury Sandstone and four Neogene beach sands to LAM–ICPMS analysis for rock type and Hf-isotope T DM model ages. These data confirm the demonstration (from ages alone) that the beach sands were recycled from the Hawkesbury Sandstone. All five samples have a substantial fraction of 700–500 Ma zircons derived from alkaline rocks with T DM of 2.0–1.0 Ga. We analysed zircons from the nearest exposed alkaline rock of appropriate age in Antarctica: the 550–500 Ma Koettlitz Glacier Alkaline Province of the Ross orogen of the Transantarctic Mountains, emplaced during contemporary transtension. The rock types and T DM of the Koettlitz Glacier Alkaline Province zircons match those of the eastern Australian samples but only over the restricted range of 550–500 Ma. Rocks of 700–550 Ma age and alkaline type are unknown in Antarctic exposures. Rocks of this age and type, however, abound in the Pan-Gondwanaland orogens. The nearest of these is the Leeuwin Complex of the Pinjarra orogen of southwestern Australia, which connects through the Prydz–Leeuwin Belt with the Mozambique Orogenic Belt. In turn, the Leeuwin Complex is dominated by 700–500 Ma alkaline rocks generated by contemporary transtension. We find that zircons from a beach sand at Eneabba, 450 km north of the Leeuwin Complex, are dominated by alkaline types, so confirming demonstration by age alone that the sand came from the Leeuwin Complex. An important inference is that Pan-Gondwanaland terranes are potential provenances of sediment with zircons of ages 700–500 Ma, model ages 2.0–1.0 Ga, and alkaline affinity. We compiled the age spectra of our samples with others of the Southwest Pacific–Gondwana igneous component in southern Australia and reviewed their paleogeographical data. The Early Cambrian turbidites of the Kanmantoo Group were deposited from an axial northward paleocurrent that flowed across adjacent Antarctica in Wilkes Land. The Ordovician turbidite was deposited in fore-arc and abyssal fans likewise from a northward paleocurrent that flowed parallel to the uplifted Ross–Delamerian orogen. The Ordovician–Silurian–Devonian Mathinna Supergroup was deposited in a submarine fan system that prograded to the east-northeast from high ground in Antarctica. The Hawkesbury Sandstone was deposited in a braided-river system on a surface that sloped to the north-northeast from high ground in Antarctica. Together with the zircon data, the paleogeographical evidence in the Cambrian, Ordovician, Ordovician–Silurian–Devonian, and Triassic all point to a provenance in Wilkes Land, Antarctica of Pan-Gondwanaland (700–500 Ma) terranes with alkaline rocks.

On the upper mantle beneath the Kenya Rift

SUMMARY During the Kenya Rift International Seismic Project (KRISP 90) a 450 km long E-W seismic-refractionlwide-angle-reflection profile involving the deployment of 250 instruments was shot across the Kenya Rift. A reflected phase recorded between distances of 260 and 350 km from a 1000 kg shot at the western end of the line in Lake Victoria has been interpreted as originating from about 60 km beneath the western margin of the rift. Detailed processing of this phase has resulted in defining its polarity in relation to the first-arrival diving wave at the same range. Extensive kinematic and dynamic modelling shows there is a high-velocity zone at depths below 60 km under the western flank of the rift. We cannot exclude the presence of a layered alternating high-lowvelocity structure as found in the upper mantle beneath the northern part of the N-S seismic profile along the rift axis. Constraints from xenolith studies indicate that anisotropy may explain the high velocity found beneath the reflecting horizon (2 8.40 km s-’). Petrological modelling shows that if the anisotropy is due to the preferred orientation of olivine crystals, then either a transverse isotropic structure, in which the ‘a’ and ‘c’ axes are randomly orientated in the horizontal plane, or an orthorhombic structure, in which the fast ‘a’ axis is orientated along the direction of the E-W seismic line, is possible. The reflection could also be caused by a pre-rift structure associated with the Proterozoic collisional orogen involving the Mozambique Orogenic Belt and the Archaean Nyanza Craton, whose contact is subparallel to and lies about 70 km to the west of the Tertiary rift. The evidence presented here delimits the lateral extent of the upper-mantle region of anomalously low-velocity material that is confined to below the surface expression of the rift itself at depths below 60 km.

Neoproterozoic collision tectonics in the Mozambique Belt of East Africa: evidence from the Uluguru mountains, Tanzania

The fault-bounded Proterozoic metamorphic terranes lying to the E of the Tanzanian craton make up the Usagara tectonic domain and are a part of the transcontinental Mozambique Orogenic Belt (MB). The lithotectonic units in the MB of the East Africa consist of comparable rock assembles which underwent the same complex deformational history and are thought to represent large thrust sheets or nappes. Their shelf- and fore-deep terranes border the Tanzanian craton and make up the foreland terranes of the Pan-African Mozambique Belt. Granulite-gneiss nappes are ubiquitous in the orogen. Granulite-facies metamorphism, associated with recumbent folds, was due to crustal thickening, which took place during the collision between Gondwana fragments. Isotope data suggest a collision (and concomitant granulite-facies metamorphism) age of between 700 and 550 Ma. The orientations of planar and linear fabrics in the granulite-facies rocks of the Uluguru mountains are used to infer the relative crustal block motions during this collisional event. This Pan-African collisional event was characterized by NW-directed movements, oblique to the N-S trend of the orogen, and involved SE-directed backthrusting. The Ubendian Belt of Tanzania and the Aswa Shear Zone in Uganda and Kenya, which both bifurcate around the Tanzania craton, accommodated the tectonically thickened crust, created by the collisional event, through NW-SE sinistral strike-slip movements.

A crustal and uppermost mantle cross-sectional model of the Kenya Rift derived from seismic and gravity data

The most significant implication of a combined seismic and gravity model derived from the data along the KRISP 90 cross-rift line on the Equator concerns the compensation mechanism for the uplifted East African plateau. The seismic model shows that despite an elevation difference of 400 m the thickness of the crust at both ends of the profile is approximately the same at about 34 km b.s.l. A regional gradient needs to be added to the gravity field calculated from the seismic model to agree with the observed. This regional is consistent with Ebinger et al.'s (1989) proposal that the plateau is being dynamically supported by convective processes in the mantle. The principal features revealed by the seismic modelling are: 1. (1) the presence of the thick graben infill, and in particular a very deep (8 km b.s.l.) low-velocity body beneath the Kerio Valley adjoining the rift's western boundary, the Elgeyo fault; 2. (2) the correlation between seismic velocities in the upper crust and different Precambrian (Archaean and Pan-African Mozambique orogenic belt) crustal packages identified along the profile; 3. (3) a thickening of the crust beneath both margins of the rift; 4. (4) the apparent symmetry in crustal thinning mirroring the surface distribution of Tertiary volcanics and sediments, suggesting a pure shear mechanism of crustal extension; 5. (5) the presence of a wide, anomalously low-velocity (7.6–7.8 km s −1) body at the base of the crust identified along the axial line and which it has been argued must include partial melt. This may be continuous with a deep low-velocity zone beneath the rift previously identified from the study of teleseismic residuals; and 6. (6) the presence of an intra-mantle reflector at a depth of approximately 55 km beneath the western margin of the rift.

Pan-African granulite-facies metamorphism in the Mozambique Belt of Tanzania: U-Pb zircon geochronology

Two new U-Pb concordia zircon ages are reported for granulite-facies rocks from the Mozambique Orogenic Belt in eastern Tanzania. These rocks are thought to have been generated during collision-related crustal thickening process and exhumed by deep erosion. A two-pyroxene granulite sample from the Pare Mountains has yielded a concordia upper intercept age of 645 ± 10 Ma, whereas a granulite-facies meta-anorthosite sample from the Uluguru Mountains has given an upper intercept age of 695 ± 4 Ma. Evidence for the involvement of older crustal material (pre-Late Proterozoic) was not found. These zircon data indicate crystallization during the peak of granulite-facies metamorphism during the Pan-African orogeny ( c . 950 to c . 550 Ma) consistent with a subduction-collisional environment.

Neoproterozoic structures in the Mozambique Orogenic belt of southern Ethiopia

This paper describes the complex arrangement of faults and dismembered ophiolites defined by geological mapping or interpreted from Landsat MSS imagery in the Mozambique Orogenic belt of southern Ethiopia. These rocks display the deep-rooted structures of collision zones. Metamorphic grade reached the amphibolite or locally the granulite facies; retrogression to greenschist facies occurs. In the description of the ophiolites, the problem of the location, orientation, and possible linkage of these masses is discussed; low- to high-angle fault (or shear-zone) orientations are presented. The faults were developed largely in a ductile regime and produced a wide range of mylonitic textures. The tectonic history started with a collision (Sabachian events > 826 Ma) followed by strike-slip faulting of basement rocks (a second orogeny <620,>530 Ma). These later events are expressed as: (i) early shears (Baragoian event, 620 Ma); (ii) late shears (Barsaloian event, 570 Ma) which trend north to northwest and have generally straight trends. The early and late shears formed upright folds, open and tight, respectively; (iii) in the southern region, northwest-trending reactivated shears produced gentle refolding and small-scale thrusting (Loldaikan-Kipsingian events 530 Ma). North- and northwest-trending shear zones (<620, > 530 Ma) are interpreted to be the reactivated surface expression of lateral and frontal ramps formed during the collision (> 826 Ma) between the Mozambique Orogenic belt/Arabian-Nubian shield and the old African continent.

Northward expulsion of the Pan-African of northeast Africa guided by a reentrant zone of the Tanzania craton

Research Article| November 01, 1992 Northward expulsion of the Pan-African of northeast Africa guided by a reentrant zone of the Tanzania craton F. F. Bonavia; F. F. Bonavia 1Université Pierre et Marie Curie, Département de Géotectonique (URA-CNRS 1315), Case 129, 4 Place Jussieu, 75252 Paris Cedex 05, France Search for other works by this author on: GSW Google Scholar J. Chorowicz J. Chorowicz 1Université Pierre et Marie Curie, Département de Géotectonique (URA-CNRS 1315), Case 129, 4 Place Jussieu, 75252 Paris Cedex 05, France Search for other works by this author on: GSW Google Scholar Author and Article Information F. F. Bonavia 1Université Pierre et Marie Curie, Département de Géotectonique (URA-CNRS 1315), Case 129, 4 Place Jussieu, 75252 Paris Cedex 05, France J. Chorowicz 1Université Pierre et Marie Curie, Département de Géotectonique (URA-CNRS 1315), Case 129, 4 Place Jussieu, 75252 Paris Cedex 05, France Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1992) 20 (11): 1023–1026. https://doi.org/10.1130/0091-7613(1992)020<1023:NEOTPA>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation F. F. Bonavia, J. Chorowicz; Northward expulsion of the Pan-African of northeast Africa guided by a reentrant zone of the Tanzania craton. Geology 1992;; 20 (11): 1023–1026. doi: https://doi.org/10.1130/0091-7613(1992)020<1023:NEOTPA>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract In southern Ethiopia and northern Kenya, higher grade Neoproterozoic rocks of the Mozambique orogenic belt and lower grade Arabian-Nubian shield are juxtaposed. Because both belts formed from a collision with African cratons (Tanzania, Eastern Sahara), the region of transition is interpreted to represent the root of northward expulsion of the Arabian-Nubian shield from the Mozambique orogenic belt. In this region, the northwest-trending late Mesozoic Kenyan rift, the Cenozoic Northern Kenyan rift, and the Main Ethiopian rift form a rift complex across Pan-African structures. In the model discussed here of successive thrusts throughout Pan-Africa, expulsion was guided either by subvertical or flat-lying faults along zones of decollement and decoupling. A northwest-trending reentrant zone in the Tanzania craton (one side of an indenter) guided the penetration of the Mozambique orogenic belt. The other side was the Schliesen-Mwembeshi-Chimaliro zone in central Africa. The interpreted structure has similarities with the gravity and magnetic pattern in the Kenyan rift. This suggests that Pan-African deformation in this region controlled late Mesozoic and Cenozoic rifting. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Structural pattern of Pan-African rocks around Moyale, southern Ethiopia

Late Precambrian rocks exposed around Moyale are part of the Mozambique Orogenic Belt. Field mapping indicates that post-Baragoian (650 Ma) tectonic events have overprinted earlier tectonic fabrics of metasediment-metavolcanic rocks and intruded gabbroic-granitic plutons outcropping in the region. The main regional structure is defined by generally north-trending, upright, doubly plunging, tight to isoclinal folds (Barsaloian event: 580 Ma). Later events produced a large scale fold with a northeast-southwest axial trace in the central-western region of the map area, and north-south to northwest-southeast shear orientations. It is interpreted that northwest-southeast shears activated thrusting and that the region can be interpreted as a “flower structure” in section.

The Kabus ophiolitic melange, Sudan, and its bearing on the western boundary of the Nubian Shield

The Precambrian high-grade gneisses of a continental terrane and the late Proterozoic (Pan-African) low-grade volcanogenic oceanic assemblage of northeastern Nuba Mountains are separated by the Kabus ophiolitic melange. This zone reflects a structural style of imbricate thrusting and strata1 disruption that is attributed to the collision of the Pan-African oceanic assemblage of the Nubian Shield against the pre-Pan-African continental crust to the west. The geological, structural, geochemical and geophysical data indicate that the melange zone marks a fossil plate boundary. The tectonic units of the region demonstrate in-situ deformational patterns that argue against the detached nature that had been previously suggested for the region. Hence, the low-grade assemblage of northeastern Nuba Mountains represent the southwestern extension of the ensimatic Nubian Shield. It is suggested here that the western margin of the Shield is marked by a N-S trending suture extending from the Bayuda Desert in northern Sudan up to the western margin of the Mozambique Orogenic Belt in northwestern Kenya. This suture passes through the Kabus ophiolitic melange.

The development of the East African Rift system in north-central Kenya

Between 1980 and 1986 geological surveying to produce maps on a scale of 1:250,000 was completed over an area of over 100,000 km 2 in north-central Kenya, bounded by the Equator, the Ethiopian border and longitudes 36° and 38 °E. The Gregory Rift, much of which has the structure of an asymmetric half-graben, is the most prominent component of the Cenozoic multiple rift system which extends up to 200 km to the east and for about 100 km to the west, forming the Kenya dome. On the eastern shoulder and fringes two en echelon arrays of late Tertiary to Quaternary multicentre shields can be recognized: to the south is the Aberdares-Mount Kenya-Nyambeni Range chain and, to the north the clusters of Mount Kulal, Asie, Huri Hills and Marsabit, with plateau lavas and fissure vents south of Marsabit in the Laisamis area. The Gregory Rift terminates at the southern end of Lake Turkana. Further north the rift system splays: the arcuate Kinu Sogo fault zone forms an offset link with the central Ethiopian Rift system. In the rifts of north-central Kenya volcanism, sedimentation and extensional tectonics commenced and have been continuous since the late Oligocene. Throughout this period the Elgeyo Fault acted as a major bounding fault. A comparative study of the northern and eastern fringes of the Kenya dome with the axial graben reinforces the impression of regional E-W asymmetry. Deviations from the essential N-trend of the Gregory Rift reflect structural weaknesses in the underlying Proterozoic basement, the Mozambique Orogenic Belt: thus south of Lake Baringo the swing to the southeast parallels the axes of the ca. 620 Ma phase folds. Secondary faults associated with this flexure have created a “shark tooth” array, an expression of en echelon offsets of the eastern margin of the Gregory Rift in a transtensional stress regime: hinge zones where major faults intersect on the eastern shoulder feature intense box faulting and ramp structures which have counterparts in the rift system in southern Ethiopia. The NE- and ENE-trending fissures of the eastern fringes of the Kenya dome, notably in the Meru-Nyambeni areaand in the Huri and Marsabit shields, parallel late orogenic structures dated at around 580-480 Ma. Alkaline trends characterize the petrochemistry of the Cenozoic volcanics: In the Gregory Rift, voluminous Miocene alkali basalts, associated with hawaiite/mugearite lavas, define a trend culminating in the Miocene flood phonolites of the eastern shoulderand in the trachyphonolites, trachytes and peralkaline rhyolites, with associated pyroclastics, in central volcanoes such as Korosi, Paka and Silali. Such trends may manifest in the products of a single volcanic centre, also regionally on a broadly cyclic basis. On the eastern flanks of the Kenya dome the flood phonolites are less evident, but the same alkaline trends dominate the lava sequences, supplemented by nephelinitic extrusives in parts of the Nyambeni Range and in the Laisamis area. Results from recent seismicity surveys in the Laisamis area indicate that crustal extension may be currently active on the eastern fringes of the Kenya dome, but manifest at greater depths than in the axial Gregory Rift-Lake Turkana zone: a correlation is suggested with the ultra-alkaline petrochemistry of some of the eastern multicentre shields.

Superimposed upper proterozoic collision-controlled orogenies in the Mozambique Orogenic Belt of Kenya

The geological history of the northernmost part of the Mozambique Orogenic Belt, exposed in north-central Kenya, is described. A basement of ∼ 1200-Ma (Kibaran) migmatites is overlain by altered sediments with lateral and vertical facies changes defining eight lithostratigraphic groups. There is no evidence for any Archaean-early Proterozoic crust within the mapped part of the orogenic belt. A tectonothermal (Samburuan-Sabachian) event in the amphibolitegranulite facies at ∼ 820 Ma produced major recumbent folds with ductile thrusting to interleave the basement, metasedimentary cover and slices of oceanic metavolcanic complexes. There was syntectonic emplacement of crustal melt granites and metabasic dykes. We interpret this tectonothermal event to be related to plate collision, oblique to the north-south orogenic strike, between the Archaean Tanzanian craton in the west and an eastern Kibaran craton. A one-dimensional manifestation of the suture between these cratonic areas is possibly exposed at West Pokot in western Kenya. Post-collisional greenschist-amphibolite facies (Baragoian-Barsaloian) deformation at between 620 and 570 Ma produced regional upright folds and vertical ductile strike-slip shear zones striking subparallel to the orogenic strike. The metamorphism culminated during the intrusion of syntectonic granites. High-level open folding and brittle shears record the final events of the orogeny. Uplift and cooling, which extended beyond the confines of the orogen, are dated by ubiquitous mineral ages of ∼ 500-480 Ma. Whereas the roughly contemporaneous (Pan-African) Nubian Shield to the north preserves large segments of island arc/ophiolitic material, in the Mozambique Orogenic Belt this volcanic material was mostly consumed during the Upper Proterozoic oblique continent-continent collision. The roots of the collision zone are now exposed in the highgrade Mozambique Orogenic Belt and contrast with the higher exposure levels seen in the Nubian Shield.

Petrology of the talc-kyanite-yoderite-quartz schist and associated rocks of Mautia Hill, Mpwapwa District, Tanzania

Talc-kyanite-yoderite-quartz schist and associated rocks belonging to the Proterozoic Usagaran System occurring along the western edge of the Mozambique Orogenic Belt (450–600 Ma) were studied using petrographic, X-ray diffraction, electron-microprobe and fluid inclusion methods. The main rock types studied in the area include talc-kyanite-yoderite-quartz schist, piemontite quartzite, epidote-phogopite quartzite, kyanite-quartz-biotite schist and biotite gneiss. Fluid inclusion studies on the selected rock types indicate the presence of usually two-phased H 2O-rich and CO 2-rich fluids with a range of filling from 0.6 to 0.95. Some CO 2-rich fluids may be one-phased (liquid) at room temperature with their degree of filling ranging from 0.4 to 1.0. Most of the CO 2-rich inclusions show negative crystal shapes. Fluid inclusions trapped in kyanite in the talc-kyanite-yoderite-quartz schist with isolated negative crystal shapes are considered primary. The presence of CO 2-rich fluids indicates low water fugacity during the formation of the talc-kyanite assemblage, and so pressure was probably lower. Primary fluid inclusions could be trapped at pressures between 5.2 and 5.6 kb and temperatures ranging from 540 to 570°C; this gives the possible P-T range of the peak of the first phase of progressive metamorphism.

A provisional metamorphic map of Zambia—explanatory notes

Examination of the provisional metamorphic map of Zambia shows the existence of two major metamorphic belts. One, the ‘Lufilian Belt’, parallels the Lufilian structural arc, while the other, the ‘LuangwaKariba Belt’, includes parts of the ‘Zambezi’, ‘Irumide’ and ‘Mozambique’ tectonic belts. Structural, metamorphic and geochronological considerations suggest that the whole of the Luangwa-Kariba Belt forms part of the Mozambique orogenic belt.

Interpretation of Project MAGNET Aerornagnetic Profiles Across Africa

Summary Long aeromagnetic profiles recorded across central Africa are analysed in terms of the tectonics and geology. The short wavelength magnetic anomalies (6 km ≤ widths of anomalies ≤ 60 km) are correlated with the local geology while the long wavelength anomalies (widths of anomalies > 60 km) are compared with the regional tectonics and geology. Important features observed in the magnetic data are (a) the‘magnetic quiet zone’across the Mozambique Orogenic Belt, (b) the low magnetic signature of the East African Rift, (c) the double-peaked anomalies across the Lake Malawi-Shire Rifts, (d) the high amplitude long and short wavelength magnetic anomalies recorded across a vast region of western central Africa which includes the Kasai Shield, the Angola Basement and the Kalahari and Congo Sedimentary Basins to the south of latitude 4° S, and (e) the extraordinary Bangui and Kibalian Magnetic Anomalies. The qualitative interpretation is supplemented with an average depth to magnetic body analysis using the Statistical Models Method (Spector & Grant; Green). This analysis reveals two ensembles of magnetic bodies for most of the lengths of profile which recorded significant magnetic anomalies (>15 gammas). The shallow ensemble of magnetic bodies are at an average depth of 0.2 km beneath the exposed Precambrian Shield, while across the large sedimentary basins they are significantly deeper at 0.8 km. The depths to the deep ensembles of magnetic bodies covers a wide range from 4 to 30 km.

The Mozambique orogenic belt and its foreland in northeast Tanzania: a photogeologically-based study

The tectonic relationships within a rather large part (130,000 sq. km) of the Mozambique orogenic belt in N.E. Tanzania were investigated by photogeological interpretation and ground control Within the granitoid foreland (part of the Tanganyika Shield) the Dodoman and Nyanzian Greenstone belts, and charnockitie granulites are recognized. These were intruded and replaced by the chelogenie Katarchaean granite. Within the orogenic belt pre-Mozambiquian gneisses (termed Parangan) are exposed at the western margin, and in the floor of the orogen. These were tectonically reworked and transformed into L-tectonites and flaggy gneisses in the main Mozambiquian tectonism (termed Kondoan): this took place by strong folding about south-easterly-plunging axes, the folds being characteristically reclined. It is observed that structures transverse to the regional trend are a feature of other orogenic fronts such as the Grenville and Caledonian. In Tanzania they are succeeded by flexure folds on nne axes (termed Bongan), which locally become the predominant structures, parallel to the regional trend. In the north they are overturned towards the foreland and over-ride it. The Mozambiquian tectonism deforms the margin of the Shield and may penetrate it for considerable distances, perhaps contributing to the deformation in the Greenstone belts.