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... Migratory trend of emplacement of the Mesozoic alkaline ring (1976), Karche et al., (1976), and Rahaman et al., (1984) and is believe to be related to the general north-south trend of the complexes. (Black and Girod, 1970) stated that the most spect locations for the Mesozoic alkaline complexes was controlled by structural weakness in the lithosphere. Tectonic activity within and along plate margins may lead to fragmentation of the plates (Atherton and Ghani 2002). ...
... Migratory trend of emplacement of the Mesozoic alkaline ring-complexes of Nigeria has been reported by Bowden et al., (1976), Karche et al., (1976), and Rahaman et al., (1984) and is believe to be related to the south trend of the complexes. (Black and Girod, 1970) stated that the most spect locations for the Mesozoic alkaline complexes was controlled by structural weakness in the lithosphere. Tectonic activity within and along plate margins may lead to fragmentation of the plates (Atherton and Ghani 2002). ...
... Classical examples are the Tibchi ring structure, in Nigeria (Ike, 1983), Great Tonalite Sill plutons of Alaska USA (Ingram opolith granite Central Sweden (Cruden, 1998), the Coastal batholiths of Peru (Haerderlen geria has been reported by Bowden et al., (1976), Karche et al., (1976), and Rahaman et al., (1984) and is believe to be related to the south trend of the complexes. (Black and Girod, 1970) stated that the most spectacular arrangement of locations for the Mesozoic alkaline complexes was controlled by structural weakness in the lithosphere. Tectonic activity within and along plate margins may lead to fragmentation of the plates (Atherton and Ghani 2002). ...
... Dewey's (1982) reconstruction, one of the first attempts to close the Atlantic Ocean including intra-plate deformation in the African plate (Benue area); on the left, the Nürnberg and Müller's (1991) pre-rift reconstruction, one of the first models including intra-plate deformation in both African and South American plates; and, in the middle, our new "tightest" reconstruction (Moulin, 2003;Moulin et al., 2007). Geological constraints (blue lines: Kandi and Sobral conjugate lineaments, Sanaga/Ngoaoundere and Pernambuco/Patos lineaments, Rio Grande Fracture Zone conjugate segments: Black and Girod, 1970;De Almeida et al., 1970;Dumont, 1986;Guiraud and Alidou, 1981), hinge lines drawn according to an industrial compilation (Unternehr, pers. comm.), ...
... B) If the entire thinned crust is continental, a horizontal movement of 235 km is necessary to restore an un-thinned, 32 km-thick continental crust (orange area with red dotted box). Black and Girod, 1970;Dumont, 1986;De Almeida et al., 1970;Guiraud and Alidou, 1981). The hinge lines are drawn according to an industrial compilation (Unternehr, comm. ...
... West Africa (not seen on this picture) is fixed. In the Equatorial and Central Segment, blue lines represent Panafrican fault systems (Ngaoundéré-Sabaga, Patos-Pernambuco and Kandi-Sobral lineaments: Black and Girod, 1970;Dumont, 1986, De Almeida et al., 1970, Guiraud and Alidou, 1981. The hinge lines are drawn according to an industrial compilation (Unternehr, comm. ...
... Related to this rifting phase and prior to the beginning of seafloor spreading in the Early Middle Jurassic (Vogt & Einwich 1979), an extensive episode of basaltic magmatism produced sill and dike intrusions and volcanism. It affected a wide domain (Central Atlantic Magmatic Province) in West Africa (Black & Girod 1970;Cogney et al. 1971;Bardon et al. 1973;Logan & Duddy 1998;Wilson & Guiraud 1998) as well as in South and North America (Sutter 1988) and Iberia (Cebria et al. 2003). In Africa, this magmatism led to intrusion of dolerite, which extends from Morocco and Algeria to the Ivory Coast and is mainly localized to the west and north of the West African craton. ...
... In Africa, this magmatism led to intrusion of dolerite, which extends from Morocco and Algeria to the Ivory Coast and is mainly localized to the west and north of the West African craton. These dolerites form a very homogeneous compositional group and display identical mineralogical, textural and chemical features over distances exceeding 1500 km (Black & Girod 1970). According to Bertrand & Westphal (1977), these dolerites are quartzitic tholeiites and very similar to those outcropping in the Appalachians, particularly in the New Jersey. ...
... The centers normally overlap one another, and there is a general tendency for a southern shift in intrusion. However, NE trending alignments of complexes are noticeable, perhaps reflecting deep seated zones of weakness in the basement, but there are no obvious surface relationships between location and regional tectonic features (Black and Girod 1970). Mesozoic and younger (Roser and Korsch 1988) sediments cover the remaining parts of Nigeria and these are located in a number of sedimentary basins, comprising the Benue (central), Sokoto (north-west border), Chad (north-east), Bida (central, along the Niger valley), Dahomey (southwest) and Anambra (south-east) Basins, and the Niger delta (south coastal). ...
An evaluation was done to better understand the utilization of empirical formulas and discrimination diagrams in the determination of sediment origin, frequently in Central Africa. For this purpose, a database was established for geochemical data (major and trace elements) collected from present-day environments or Mesozoic deposits with variable environments and source geologies from two countries of Central Africa (Cameroon and Nigeria). The plotting of modern sediments and ancient rocks in the same diagrams has revealed the low impact of diagenetic processes on the chemical compositions. The results suggest that the proximal provenance proposed by several authors that used empirical formulas and discriminant diagrams to infer the origin of sediments are not always appropriate for all types of sediments. This study shows that chemical index of weathering (CIW) and plagioclase index of alteration (PIA) derive from chemical index of alteration (CIA). It is therefore recommended that they should be used with caution while taking into account the regional geological settings, the location of source area, the paleoweathering of source rocks, and hydraulic-sorting effects. In addition, it is also recommended to use other diagrams such mobile alkaline earth metals versus chemical index of alteration (CIA).
... The Nigerian Younger Granites are part only of a much larger igneous province extending far to the north in Niger Republic [8]. Here there are two main areas of ring complex occurrence, both on structural culminations where uplifted basement rocks emerge from the surrounding younger sedimentary formations. ...
A detailed study of the Landsat Thematic Mapper(Landsat-TM) image over Jos Plateau, Nigeria was carried out to identify linear geologic features that could be attributed to paleo-tectonic and/or neo-tectonic structures and to infer the influence of such structures on mineralization in the study area. The data was geo-referenced using the coordinates of the topographic map sheet of Naraguta. The application of directional edge enhancement techniques to band 5 of the Landsat-TM data using convolution models in ILWIS 3.2 academic software were to further enhance these linear features. Results of the structural analyses revealed several lineaments at the northwestern, central and southwestern parts of the study area. Trend analysis of the lineaments revealed structural trends in the NW-SE, NE-SW, N-S and E-W directions with the NW-SE and NE-SW been the dominant trends in the area. These trends correspond to the positions and directions of the paleo-tectonic fracture zones in the area. High lineament densities were also observed in areas where basement rocks outcrop or are close to the surface. The relationship between lineament densities and Younger granites occurrences in the study area is an indication of tectonic control probably associated with paleo-tectonic structures. This correlation is an indication that the emplacement of the Younger granite ring complexes may be associated with epeirogenic uplift. The epeirogenic uplift is believed to result from the intrusion of large masses of basic magmatic materials into the lower part of the continental crust in the area. The marked relationship between the Younger granites and the lineaments is believed to have controlled mineralization in the study area.
... The Nigeria-Cameroon Volcanic Provinces lie within the Pan-African collision belt of West Africa (Figure 1). In Nigeria, the volcanic provinces (Jos Plateau, Biu Plateau, Benue Valley, etc.) are confined to the northeastern and central regions [4]. The volcanic provinces are characterized by numerous volcanic cones and lava flows consisting of alkaline olivine basalts together with less important trachyte and phonolite intrusive rocks [5]. ...
... They tend to preweaken the lithosphere, especially in areas where the lithosphere is already thinned, as underneath most Pan-African shear zones crosscutting the African plate. These shear zones were affected by the Pan-African tectono-thermal event (650 -550 Ma), which caused a thin-ning of the lithosphere (Black and Girod, 1970;Thorpe and Smith, 1974;Bailey, 1992). Most of the younger volcanism appears to be restricted to these thinned areas (Bailey, 1992). ...
... A long waning felsic intrusive phase Wai complex, the present outcrop limits give the probable extent of the ed initial ring fracture (Fig. 3). Ring fracture lowed by cauldron subsidence is considered to be the major mode of intrusive emplacement for the younger granites and is thought to give rise to plutons roughly circular in plan with generally steeply dipping contacts (Turner, 1963(Turner, , 1972Black and Girod, 1973;Jacobson and MacLeod, 1977). ...
... Niger) and Younger Granites (Nigeria). Estimates from Black and Girod (1970), modified for AIr following Moreau (1987) and Moreau et al. (1994). ...
... African Archean cratons are surrounded by a number of younger Paleoproterozoic, Mesoproterozoic, and Neoproterozoic mobile belts ( Figure 1) formed dominantly by obduction. These tectonic sutures and polycyclic mobile zones created structural basement anisotropies, which often acted as weak zones that were later reactivated during the Phanerozoic and controlled the locus of extension, igneous activity, and initiation of rifting [e.g., Black and Girod, 1970;Thorpe and Smith, 1974;Roberts and Bally, 2012]. ...
We present new crust and lithosphere thickness maps of the African mainland based on
integrated modeling of elevation and geoid data and thermal analysis. The approach assumes local isostasy,
thermal steady state, and linear density increase with depth in the crust and temperature-dependent density in
the lithospheric mantle. Results are constrained by a new comprehensive compilation of seismic Moho depth
data consisting of 551 data points and by published tomography models relative to LAB depth. The crustal
thickness map shows a N-S bimodal distribution with higher thickness values in the cratonic domains of
southern Africa (38–44 km) relative to those beneath northern Africa (33–39 km). The most striking result is the
crustal thinning (28–30 km thickness) imaged along the Mesozoic West and Central African Rift Systems. Our
crustal model shows noticeable differences compared to previous models. After excluding the Afar plume
region, where the modeling assumptions are not fulfilled, our model better fits the available seismic data
(76.3% fitting; root mean square error = 4.3 km). The LAB depth map shows large spatial variability (90 to
230 km), with deeper LAB related to cratonic domains and shallower LAB related to Mesozoic and Cenozoic
rifting domains, in agreement with tomography models. Though crustal and lithosphere thickness maps show
similar regional patterns, major differences are found in the Atlas Mountains, the West African Rift System,
and the intracratonic basins. The effects of lateral variations in crustal density as well as the nonisostatic
contribution to elevation in the Afar plume region, which we estimate to be ~1.8 km, are also discussed.
... On the other hand, the negative second vertical derivatives ranging from -140,017.4 to -57.0 are believed to be as a result of felsic rock forming minerals. However, NE trending alignments of the ring complexes of the study area are noticeable, perhaps reflecting deep seated zones of weakness in the basement, but there are no obvious surface relationships between location and regional tectonic features [23]. ...
Structural interpretation over Naraguta area was carried out using aeromagnetic and Landsat ETM data with the objective of delineating the linear features of the study. Several image processing and analytical techniques were applied to the aeromagnetic and landsat data to improve the data quality and resolution. Linear features identified in the study area revealed principal trend directions in the NW-SE, NE-SW, N-S and E-W directions with the N-S trend been dominant. Results of the 2-D spectral analysis of the aeromagnetic data revealed a two depth source model. The depth to the deeper magnetic source bodies has an average depth of 2.03km. This layer may be attributed to magnetic rocks of the basement, lateral variations in basement susceptibilities and intra-basement features like faults and fractures. The shallower magnetic sources with an average depth of 0.265km could be attributed to near surface magnetic sources, which are magnetic rocks which intruded into the sedimentary overburden. Most of the interpreted lineations were observed to correspond to the trends and positions of the trans-oceanic fracture zones in the area. There is also a marked correlation of the location of the Younger Granite Ring complexes and the lineaments in the study area. This observed relationship may be attributed to tectonic control of secondary mineralization in the study area. Most of the lineaments also correlated with the orientation of the drainage lines indicating that the drainage system in the area may be structurally controlled. Finally, the drainage pattern in the study area was observed to be dendritic which is indicative of lithological heterogeneity.
... Secondly, local actors are good enough. A strong justification for the participatory approach is the argument that local people are capable thinkers who in many cases are able to think through problems and operationalise solutions (Birgegard 1993;Long and Van der Ploeg 1994;Fiege 1995;Tognetti 1995;Darkoh 1996;Heck 1996;Baidu Forson 1997;Bauer and Hoffman 1997;Fujisaka 1997;Monu 1997). Local people may therefore be competent socio-environmental managers, and in many cases they may be "reforming, adapting and managing modernisation" (Bebbington 1996). ...
This book looks at theories of nature-society relations and development, applied to soil erosion and environmental change in West Africa, and Ghana. Acknowledging the importance of power relations for environmental management, the book begins with an examination of the field of political ecology, alternately the "political economy of the environment." It assesses the role of this sub-discipline within geography and anthropology, and its role in practical conservation planning and the management of soil erosion. Political ecology tilts towards a structuralist approach, hence social theories (actor oriented studies and structuration theory) sensitive to actor analysis are examined as complementary frameworks. Structuration theory is discussed as attempting to tie the concepts of structure and actor together and provide a basis for the study of political, economic, socio-cultural and environmental contexts. These theories connect to the issues of livelihood and household analysis, gender relations and the economic, ecological and Green political variants of the paradigm of sustainable development.
... On the other hand, the negative second vertical derivatives ranging from -140,017.4 to -57.0 are believed to be as a result of felsic rock forming minerals. However, NE trending alignments of the ring complexes of the study area are noticeable, perhaps reflecting deep seated zones of weakness in the basement, but there are no obvious surface relationships between location and regional tectonic features [23]. ...
Structural interpretation over Naraguta area was carried out using aeromagnetic and Landsat ETM data with the objective of delineating the linear features of the study. Several image processing and analytical techniques were applied to the aero-magnetic and landsat data to improve the data quality and resolution. Linear features identified in the study area revealed principal trend directions in the NW-SE, NE-SW, N-S and E-W directions with the N-S trend been dominant. Results of the 2-D spectral analysis of the aeromagnetic data revealed a two depth source model. The depth to the deeper magnetic source bodies has an average depth of 2.03km. This layer may be attributed to magnetic rocks of the basement, lateral variations in basement susceptibilities and intra-basement features like faults and fractures. The shallower magnetic sources with an average depth of 0.265km could be attributed to near surface magnetic sources, which are magnetic rocks which intruded into the sedimentary overburden. Most of the interpreted lineations were observed to correspond to the trends and positions of the trans-oceanic fracture zones in the area. There is also a marked correlation of the location of the Younger Granite Ring complexes and the lineaments in the study area. This observed relationship may be attributed to tectonic control of secondary mineralization in the study area. Most of the lineaments also correlated with the orientation of the drainage lines indicating that the drainage system in the area may be structurally controlled. Finally, the drainage pattern in the study area was observed to be dendritic which is indicative of lithological heterogeneity.
... This is consistent with the thinking that the Alpine orogeny had nearly no effect on Africa to the south of the Atlas Mountains and with the fact that this mountain range underlines the northern boundary of the West African craton (Ennih and Liégeois, 2001). It is noteworthy that there is no Cenozoic volcanism in the cratonic part of Africa (Black and Girod, 1970). ...
... The present-day tectonic settings include both continent-continent collisional and postcollisional regimes (Jolivet et al., 1999). Numerous magmatic provinces developed along seismically active tectonic lineaments within the Alps and Apennine fold belts (Lustrino, 2000;Savelli, 2002 and references therein), as well as further inland within the adjacent continents of Africa (Black and Girod, 1970) and Europe (Wilson and Downes, 1991). ...
... The Nigerian Younger Granites are part only of a much larger igneous province extending far to the north in Niger Republic [8]. Here there are two main areas of ring complex occurrence, both on structural culminations where uplifted basement rocks emerge from the surrounding younger sedimentary formations. ...
Abstract A detailed study of the Landsat Thematic Mapper(Landsat-TM) image over Jos Plateau, Nigeria was carried out to identify linear geologic features that could be attributed to paleo-tectonic and/or neo-tectonic structures and to infer the influence of such structures on mineralization in the study area. The data was geo-referenced using the coordinates of the topographic map sheet of Naraguta. The application of directional edge enhancement techniques to band 5 of the Landsat-TM data using convolution models in ILWIS 3.2 academic software were to further enhance these linear features. Results of the structural analyses revealed several lineaments at the northwestern, central and southwestern parts of the study area. Trend analysis of the lineaments revealed structural trends in the NW-SE, NE-SW, N-S and E-W directions with the NW-SE and NE-SW been the dominant trends in the area. These trends correspond to the positions and directions of the paleo-tectonic fracture zones in the area. High lineament densities were also observed in areas where basement rocks outcrop or are close to the surface. The relationship between lineament densities and Younger granites occurrences in the study area is an indication of tectonic control probably associated with paleo-tectonic structures. This correlation is an indication that the emplacement of the Younger granite ring complexes may be associated with epeirogenic uplift. The epeirogenic uplift is believed to result from the intrusion of large masses of basic magmatic materials into the lower part of the continental crust in the area. The marked relationship between the Younger granites and the lineaments is believed to have controlled mineralization in the study area. Keywords: Landsat-TM, lineaments, uplift, mineralization, younger granites, tectonic trend, Nigeria
... The Nigerian Younger Granites are part only of a much larger igneous province extending far to the north in Niger Republic [8]. Here there are two main areas of ring complex occurrence, both on structural culminations where uplifted basement rocks emerge from the surrounding younger sedimentary formations. ...
A detailed study of the Landsat Thematic Mapper(Landsat-TM) image over Jos Plateau, Nigeria was carried out to identify linear geologic features that could be attributed to paleo-tectonic and/or neo-tectonic structures and to infer the influence of such structures on mineralization in the study area. The data was geo-referenced using the coordinates of the topographic map sheet of Naraguta. The application of directional edge enhancement techniques to band 5 of the Landsat-TM data using convolution models in ILWIS 3.2 academic software were to further enhance these linear features. Results of the structural analyses revealed several lineaments at the northwestern, central and southwestern parts of the study area. Trend analysis of the lineaments revealed structural trends in the NW-SE, NE-SW, N-S and E-W directions with the NW-SE and NE-SW been the dominant trends in the area. These trends correspond to the positions and directions of the paleo-tectonic fracture zones in the area. High lineament densities were also observed in areas where basement rocks outcrop or are close to the surface. The relationship between lineament densities and Younger granites occurrences in the study area is an indication of tectonic control probably associated with paleo-tectonic structures. This correlation is an indication that the emplacement of the Younger granite ring complexes may be associated with epeirogenic uplift. The epeirogenic uplift is believed to result from the intrusion of large masses of basic magmatic materials into the lower part of the continental crust in the area. The marked relationship between the Younger granites and the lineaments is believed to have controlled mineralization in the study area.
... The Serra Geral-Kaoko volcanic province (Fig. 4B), the earliest manifestation of the present Tristan hot spot, apparently predates the start of South Atlantic opening (e.g., Campos et al., 1974). The present St. Helena hot spot (Black and Girod, 1970) is also probably at least as old. It is therefore pertinent to ask if, through a similar mechanism of lithospheric thinning and uplift-induced stress, the hot spots could have caused or at least influenced the rupture of West Gond-wanaland (Fig. 4B). ...
Drilling at Site 516 on the N shoulder of the main Rio Grande Rise has improved our understanding of the tectonic evolution and subsidence history of the Rise and of the entire Rio Grande-Walvis seamount and aseismic ridge system. The anomalous elevation of the Rise is attributed to an eastward ridge crest jump to the W end of the Rise, 91 Ma. A model for the Rio Grande-Walvis system involving a slow westward component of drift of the ridge crest off a hot- spot swell is suggested. Data from the site suggest that a single short off-axis event affected the entire crestal region of the Rise. Perhaps the same midplate hot spot that produced an 80-50Ma volcanic episode in the Serra Geral of Brazil was responsible, but that was not the present Tristan hot spot. The data from Site 516 have been incorporated into a detailed model for the subsidence history of the main body of the Rise. -from Author
... In the geological literature, this area has hitherto been described only in its regional context. Works concerning North Africa include Rogers et al. 1978, Black & Girod 1970, Ghuma & Rogers 1980. Works concerning the whole Tibesti include those of Wacrenier 1956 and Vincent 1963 while, at a local level, there are a few works concerning very small areas (e.g. ...
The northern part of the Tibesti Massif is built from Precambrian rocks which are metamorphosed in an amphibolitic facies. Due to varying resistance to weathering of the individual rock-types, the Lower Tibestian horizon forms a morphologically diversified outcrop which is 100 km long and 50 km wide. The identification of the various genetic types of intrusion has been done based on the cross-cutting relationship of the magmatic bodies within the complex intrusions. Type 1 -pretectonic or early-orogenic; type 2 -pre-shearing; type 3 -post-shearing; type 4 -post-tectonic. The outline of tectonic development of the Eastern Tibesti (base on a review of the existing publications on the geology of the region combined with results of a remote sensing analysis): The oldest, prekinematic intrusions were re-melted and deformed in later stages of tectonic-magmatic activity. Subduction of the oceanic crust took place towards the East, under the eastern part of Chadian-Awaynat craton. After orogenesis intensive and metamorphism granitoid intrusions penetrated the Lower Tibestian series. During the following subduction of the oceanic crust towards the West, this series was folded, metamorphosed and penetrated by calc-alkaline granitoids. The Lower Tibestian series was next subjected at that time to shearing and to, probable, clockwise, twist which resulted in deformation of the foliation and fold axes and also in rotation of the intrusive bodies. Several smaller phases of magmatic activity followed, with the youngest, the rhyolitic being of the lower Ordovician age. Introduction This work attempts to review what is known about the numerous granitic intrusions of eastern Tibesti (Fig. 1, 2). However, it should be emphasised, that owing to its inaccessibility and difficulties of climate and relief conditions (travelling is possible only along the valleys of intermittent rivers called wadis) the geological investigations in this area have not progressed much beyond the reconnaissance stage. In the geological literature, this area has hitherto been described only in its regional context. Works concerning North Africa include Rogers et al. 1978, Black & Girod 1970, Ghuma & Rogers 1980. Works concerning the whole Tibesti include those of Wacrenier 1956 and Vincent 1963 while, at a local level, there are a few works concerning very small areas (e.g. List & Stock 1969). Studies carried out in the 1970s on the wider scale (Hunting 1974) were directed towards the delimitation of metal resources in the area but this included only a part of the area under review here. The Authors present a working hypothesis concerning the genesis of the granite intrusions, emphasising their role in the structural evolution of this area. Where possible, remote sensing data were verified by data forthcoming from previous field studies in the area. They were also compared with previous models of the geology of Northern Africa in general and the Tibesti area specifically and also those of the magmatic and tectonic evolution of this area. No claim is made that our hypothesis concerning the different phases of intrusion applies to the whole of the Tibesti region but it is hoped that it will be accepted as a basis for wider scientific discussion and in the planning for further fieldwork.
... Gorini and Bryan (1976) suggested that uplift of the ocean floor along the CL formed an effective barrier since the Cretaceous between the Niger Delta to the northwest and the Douala basin to the southeast. A general topography basement uplift with a N20E axis is related to the magmatic activity (Black and Girod, 1970). Both normal and reverse faulting is evident (Deruelle et al., 1991). ...
The near 1700 km long Cameroon Line (CL) is an African intraplate ‘fan-shaped’ alkaline volcano-plutonic rift zone of variable width (< 200 km), with a ca. 66 Ma history of magmatic activity without any systematic internal pattern of age variation. Although the CL features prominently in discussions about active rift tectonics and magmatism during the growth of the African Plate and the opening of the Central Atlantic Ocean, there is significant diversity of opinions about its local geometry, origin and evolution. Here, we review various geodynamic models for the CL based on large data-sets of field geology, petrology, geochemistry and geochronology. Results indicate that the Line traverses both oceanic and continental domains, extending NE-SW for about 700 km on oceanic lithosphere of the Gulf of Guinea (ca. 80-120 Ma), cutting obliquely across oceanic transform faults and across the ocean-continent boundary (OCB), and then continuous for ~ 1000 km (to Kapsiki), with a significant bifurcation at about 320 km, to the Adamawa and the Biu Plateaus along complex Neoproterozoic continental lithosphere of Central and West Africa (500-800 Ma, with embedded fragments of Paleoproterozoic-Archean crust). The continental lithosphere is transected by a dense network of major late Neoproterozoic shear zones (500-650 Ma) that were linked to conterminous extensions in South America before the opening of the Atlantic Ocean. The shear zones define a “Cameroon Block” to which the continental sector of the Cameroon Line is confined, but neither this continental tectonic anisotropy, nor its oceanic transform faults, appear to have influenced the geometry of the CL. Geochemical data confirm that the mostly alkaline CL magmas originate from deep mantle upwelling with variable input from at least four different mantle end-members (DMM, HIMU, FOZO and EM-I), and with different degrees of partial melts derived from above the 410-discontinuity. These melts then interacted diversely with lithospheric mantle, but without extensive crustal contamination. The oldest igneous activity is documented on the continent at the far end of the CL by the Late Cretaceous - Early Paleocene (on the Kapsiki Plateau at Golda Zuelda), and by the Miocene, contemporaneous volcanic activity spread along the oceanic and continental sectors. The intraplate magmatism along the CL is coincident with continent-wide extensional tectonics across central and northern Africa during the Cretaceous and throughout the Cenozoic, a time during which the African Plate experienced rotation of ~ 7o and slow but variable rates of convergence with Europe, involving several episodic changes in the pole of rotation between the African and South American plates that affected the opening of the South-Central Atlantic. The CL reflects a unique long-lived geodynamic setting and there appears to be no simple explanation for its origin.
... Analysis of satellite imagery to locate linear ground features (lineaments) is viewed as an integral part of strategic water resource management. It is noteworthy that wrench faults with east-northeasterly (dextral) and north-north-westerly (sinistral) orientations occur in West Africa (Black and Girod, 1970). Simpson and Sohani (2000) outlined the relationships between lineaments, drainage and waterconservation measures in a dryland part of Maharashtra State, India. ...
Political events intervened and the project participants next were reunited in 1999. The extended project term came to an end in 2000. The goal was to reduce gully erosion in southeastern Nigeria. The purpose was to discover reasons for the large numbers of gullies in the region and to design a strategy for the control and prevention of gully erosion. The funding agency was the International Development Research Centre, Ottawa. Hudec et al. (1998) described the geological engineering properties of those materials that are especially susceptible to gully erosion. The present account relates some aspects of project research to concepts of water resource management. Use of "strategic" in the title draws attention to the importance of this to the national security of Nigeria.
... This is consistent with the thinking that the Alpine orogeny had nearly no effect on Africa to the south of the Atlas Mountains and with the fact that this mountain range underlines the northern boundary of the West African craton (Ennih and Liégeois, 2001). It is noteworthy that there is no Cenozoic volcanism in the cratonic part of Africa (Black and Girod, 1970). ...
We review the northwest African Cenozoic volcanic fields, including their regional
geology. This provides a basis for understanding the relations between Hoggar volcanism
and the Africa-Europe collision. Volcanic alignments are related to structural
features, and no spatial age trend exists. In Hoggar, a close link is established between
the volcanism and Pan-African structure. During the Mesozoic rifting period, the
Hoggar area was already a topographic high well before any volcanism, which began
at ca. 35 Ma, just after the initiation of the Africa-Europe collision at ca. 38 Ma. Hoggar
volcanism continued episodically until now, as did the collision. We describe the
Hoggar volcanic province based on available field, petrological, geochemical isotopic,
and geophysical data, including data on gravimetry, heatflow, and seismic tomography.
The latter suggests that northwestern African volcanism is linked to mantle structure
down to 150 km but not deeper, implying a shallow mantle source. In Hoggar, lithospheric
structures deduced from the seismic tomographic model and from geology are
compatible when their respective resolutions are taken into account. The considerations just stated cannot be reconciled with a plume model. We propose
instead that intraplate stress induced by the Africa-Europe collision reactivated
the Pan-African mega–shear zones mainly in metacratonic terranes, inducing linear lithospheric delamination, rapid asthenosphere upwelling, and melting due to pressure
release. Edge-driven convection may contribute. The surface location of the volcanism
is influenced by Paleozoic and Mesozoic brittle faults.
... The present-day tectonic settings include both continent-continent collisional and postcollisional regimes (Jolivet et al., 1999 ). Numerous magmatic provinces developed along seismically active tectonic lineaments within the Alps and Apennine fold belts (Lustrino, 2000; Savelli, 2002 and references therein), as well as further inland within the adjacent continents of Africa (Black and Girod, 1970) and Europe (Wilson and Downes, 1991). The Cenozoic volcanic province of Hoggar (also named Ahaggar in the literature) is located within the African plate ~1200 km south of the Mediterranean coast (Fig. 1 ). ...
The Atakor massif is a part of the Hoggar volcanic province, which was emplaced on top of a basement swell initiated during the Cretaceous. There have been three main episodes of volcanic activity since the Miocene, separated by long periods of qui- escence. The lava fl ows and domes were emitted along lithosphere-scale fault zones. With its famous scenery, the Atakor massif is one of the largest (2150 km2) volcanic districts of the province. Mafi c volcanic rocks are abundant in the center of the mas- sif, but become scarce to the south, where only few scarps are observed. Phonolites occur only in the Assekrem area, whereas trachytes occur everywhere, with a marked enrichment in quartz to the south and the southeast (Tahifet area), where rhyolites are also exposed. Two magmatic groups have been identifi ed based on fi eld and petrologi- cal observations. The mafi c group has a basanite-phonotephrite association, forming uplifted plateaus, scoria cones, and valley-fi lling lava fl ows. The presence of mantle- derived amphibole ± biotite megacrysts and peridotite mantle xenoliths together with the nonprimary chemical compositions of the magmatic rocks suggest that magmatic differentiation may have occurred within the upper mantle. The felsic group is com- posed of two diverging trends, a silica-saturated benmoreite-trachyte-rhyolite trend and a silica-undersaturated trachyte-phonolite trend. The primary magmas are con- sidered to have been produced as a consequence of lithospheric mantle delamination along linear megashear zones inducing low degrees of decompression partial melting at variable depths (110-40 km) in the upwelling asthenosphere. The discrete volcanic episodes correspond to periods of reactivation of the major fault zones in response to discrete Neogene extensional tectonic events associated with Alpine orogenesis in the Western Mediterranean region induced by Africa-Eurasia collision.
This book examines the agricultural history of Ghana from the perspective of political ecology, a paradigm particularly suited for the
analysis and understanding of complex African environments. The issues of power, need, marginalization, management, structures, and actors are strong parameters that emerge in such a study. Covering as it does a long period of time, and an extremely complex, multi-ethnic
and multi-cultural country, the book summarises many points and priorities, some over others. The overriding topic is the historical period covered (pre-colonial to the early 1990s), which is the fundamental formative period for Ghanaian agricultural and environmental development.
The present thesis is inserted in the SALSA project (Seryipe Alagoas Seismic Acquisition), which was conducted by a collaboration between the Department of Marine Geosciences : IFREMER, the Laboratory of Oceanic Domain IUEM, the Faculdade de Ciências da Universidade de Lisboa (IDL, Portugal), the Universidade de Brasilia (Brazil) and PETROBRAS (Brazil).Seismic shot, Multi-Channel Seismic acquisition (MCS) and Ocean Bottom Seismometers (OBS) deployments were performed by the R/V L'Atalante (IFREMER) along 1-2 profiles.Among them, five were extended onshore by Land Seismic Stations (LSS). P-wave velocity models were constructed based on the joint interpretation of multichannel and wide-angle seismic data using the RAYINVR software.We present models derived from wide-angle refraction and coincident reflection data along two parallel profiles located on the Sergipe-Alagoas basin (SL01 and SL02), extending approximately 220 km and 200 km from the continental shelf to the distal Sergipe basin north to the Vaza-Barris Transfer zone, and one of them, the SL02 prolongates for 150 km through the continent, on Sergipe -Alagoas continental basin and its basement, the Sergipana FoId Belt.The foward modelling of the passive margin in the Sergipe Alagoas basin precisely constrains crustal geometries and segmentation. The crustal geometry puts in question the origin of the Sergipana Fold Belt, when compared with the geometries of the adjacent geological domains, the Sao Francisco Craton and the Borborema Province.The differences between the profiles illuminate the influence of the tectonic inheritance resulted by the Vaza-Barris Transfer Zone. These integrated information allowed some new conjectures around the geodynamic history of the region.
The structure and nature of the crust underlying the Camamu-Almada-Jequitinhonha-Sergipe-Alagoas basins System, in the NE Brazilian margin, were investigated based on the interpretation of 12 wide-angle seismic profiles acquired during the SALSA (Sergipe ALagoas Seismic Acquisition) experiment in 2014. In this work, we present two 220-km-long NW-SE combined wide-angle and reflection seismic profiles, SL 01 and SL 02, that have been acquired using 15 ocean-bottom-seismometers along each profile, offshore the southern part of the Sergipe Alagoas Basin (SAB), north of the Vaza-Barris Transfer zone. The SL 02 has a 150-km long inland continuation with 20 land-seismic-stations until the Sergipano Fold Belt (SFB). Wide-angle seismic forward modeling allows us to precisely divide the crust in three domains: beneath the continental shelf, a ∼100 km wide necking zone is imaged where the continental crust thins from ∼35 km on the Unthinned Continental Domain, which displays a three-layered crust structure, to less than 8 km on the Oceanic Crust Domain. In the necking zone, the upper and the middle layers thin dramatically and almost disappear, while the Moho discontinuity shows clear PmPs. The Continental-Oceanic Crust Boundary (COB) is located at ∼80 km from the coastline and is marked by intracrustal seismic reflectors and changes in the seismic velocity, showing a sharp transition. On profile SL02, the oceanic crust is perturbed by a volcanic edifice together with an anomalous velocity zone underneath the area.
Kimberlite dykes and pipes found in many parts of the West African craton are probably mainly of Cretaceous age, but some may be much older. Several are diamondiferous. A suspected kimberlite in Nigeria is probably a basaltic explosion vent.
In the Benue Trough there is a pre-Cretaceous suite of mainly acid igneous rocks. During the Upper Cretaceous mainly basic rocks were erupted and intruded among the sediments of the trough.
Cenozoic volcanism is of alkaline affinities and is virtually confined to Pan African areas east of the craton. On the Jos Plateau early Cenozoic(?) volcanism produced the Fluvio-Volcanic Series of intercalated sediments and basaltic lavas. Scattered Plio-Pleistocene volcanism dominated by basaltic cones and plateaux, with minor phonolite and trachyte plugs, characterised the Hoggar and Air regions and also the Jos Plateau and Benue Trough. Many basalts contain megacrysts and upper mantle inclusions. The Cameroun line forms the major part of the Gulf of Guinea province, with large shield volcanoes and basalt plateaux, also plugs of phonolite, trachyte and rhyolite. There was minor Cenozoic volcanism in the Dakar region of Senegal.
Most of the Cenozoic magmatism occurs in long-established areas of crustal doming or uplift and cannot realistically be related to fixed hot spots in the mantle.
There are several rich alluvial diamond fields on the craton, some of which can be traced to kimberlites. The economic potential of the Cenozoic volcanic rocks is confined to their use as roadstone or other aggregate, although alluvial tin is found among some of the Fluvio-Volcanics on the Jos Plateau, and the clays are used for ceramics. Some of the basalt plateaux have aquifers of limited size. Small amounts of sapphire occur in alluvial deposits at one locality.
This book reconstructs climatic changes in deserts and their margins at a variety of scales in space and time. It draws upon evidence from land and sea, including desert dunes, wind-blown dust, river and lake sediments, glacial moraines, plant and animal fossils, isotope geochemistry, speleothems, soils, and prehistoric archaeology. The book summarises the Cenozoic evolution of the major deserts of the Americas, Eurasia, Africa and Australia and the causes of historic floods and droughts. The book then considers the causes and consequences of desertification and proposes four key conditions for achieving ecologically sustainable use of natural resources in arid and semi-arid areas. Climate Change in Deserts is an invaluable reference for researchers and advanced students interested in the climate and geomorphology of deserts: geographers, geologists, ecologists, archaeologists, soil scientists, hydrologists, climatologists and natural resource managers.
The Younger Granite ring complexes of West Africa extend from Air to Cameroun and range in age from Palaeozoic to Tertiary. They are dominated by granites and are emplaced in Pan African basement rocks, on ‘swells’ bordering sedimentary basins.
The complexes probably represent the root zones of ancient volcanoes of the type characterised by caldera collapse. Early stages of magmatic activity involved the eruption of large volumes of rhyolitic ignimbrites. Further eruptions occurred along more or less circular ring faults. Magma subsequently solidified in these fractures and formed marginal ring dykes of granite porphyry that define the outer limits of some complexes. Inside the peripheral ring fracture, a variety of mainly granitic rocks was emplaced, both as massive ring dykes and as more or less cylindrical stocks and bosses. The ideal pattern is one of concentric intrusions, becoming progressively younger towards the centre, but many complexes depart from this ideal.
The dominant granites of the complexes range from peraluminous to peralkaline in composition and they are associated with smaller amounts of syenite, gabbro and anorthosite. Some of these magmas originated in the upper mantle, but the overwhelming preponderance of granite suggests that there was a contribution from crustal melting also.
Regional tectonic controls on emplacement of the complexes remain a matter of speculation. On the local scale, emplacement of individual complexes must have been controlled by basement fracture systems, but these are not easily identified.
The Younger Granites of Nigeria in particular are famous for their tin (cassiterite) mineralisation, which is mainly associated with the biotite granites. These rocks also contain significant quantities of the niobium-rich mineral col- j umbite as an accessory. Most of the workable deposits of cassiterite and columbite are in alluvial concentrations. The peralkaline granites also contain accessory uranium-bearing minerals, which probably provided the primary source for the sedimentary uranium deposits of Niger.
This book takes an incisive, political ecology perspective of the development of agriculture in the West African nation of Ghana (formerly the British colony of the Gold Coast), focusing on agricultural, socio-cultural, economic, political and environmental issues during the pre-colonial (pre-16th century), colonial and post colonial periods (after 1957, up until 1992, the start of the last Republic). This integrated approach for the study of agricultural development and environmental change is very relevant to the Ghanaian context. Here, complex historical, political and socio-cultural factors have been combined, within a fairly short period, to contribute to an environmental metamorphosis. A complicated mosaic of indeterminate vegetation climaxes, such as depleted and secondary forests, derived savanna and savanna woodland, has gradually replaced the primeval rain forest, deciduous forest and savanna. Problems include soil erosion and desertification risk. This, largely due to human agency, is explainable by examining the political and economic relations behind the resource exploitation methods devised by the different actors in the political, economic and social system.
The aeromagnetic intensity field anomaly of the Middle Benue Trough, Nigeria has been continued upward to an elevation of 100 km above flight height. The results of the calculations are usually free from the effects of inclination of the earth's main field and polarization vector. This was followed by progressive application of low-pass filters and their performance was compared. The upward continued field, which usually reveals the effects of deeper sources, is interpreted to be due to-two juxtaposed magnetic dipoles of dipolar length approximately 200 km. The configuration of the dipoles suggests an asymmetric geometry and has implications for the tensional regimes that prevailed in the area. The general indication is that the sedimentary trough might have resulted from simultaneous strike-slip motion and fault-normal transtension, similar to the way asymmetry is produced in larger transform basins. Depth estimates from five Werner profiles across the Cretaceous cover of the study area reveal variable sedimentary thickness (basement configuration) with the thickness nowhere exceedhig 4.80 km. The basement topography has a NE-SW oscillatory pattern revealing a maximum wavelength of 50 km with a tilted block-faulting pattern in which the anti-forms correspond to the trend of the inter-tonguing basement in some portions of the study area.
Deserts are excellent geological, geomorphic, and archaeological museums. The very aridity to which they owe their existence has made them remarkable repositories of past depositional and erosional events (Frostick and Reid 1987). The fossil river valleys of the Sahara, the great salt lakes of Australia, China, and Patagonia, the dissected volcanic mountains of the Arabian peninsula and the Afar Desert — all are legacies of former tectonic, volcanic, and climatic episodes which ultimately gave rise to the deserts we see today. Each desert reflects its own individual geological inheritance and geomorphic history; each is unique in its assemblage of landforms; each ideally deserves detailed and separate study in its own right.
Using both field relationships and some absolute ages, the sequence of volcanic units in the Cenozoic Tibesti Volcanic Province (TVP) (Chad) is established as follows: (1) plateau volcanism, between at least 17 and 8 Ma, consisting of flood basalts and silicic lava plugs, with intercalated ignimbritic sheets in the upper basalt succession increasing in amount upwards. Ages decrease from NE to SW, following the migration of the small NW-SE flexures concentrating the feeding dike swarms; (2) Late Miocene large central composite volcanoes exhibiting diverse and original structures. Some of them (Tarso Toon, Ehi Oyé, and Tarso Yéga) are located along a major NNE fault, representing the main tectonic direction in Tibesti since Precambrian times; (3) construction of three large ignimbritic volcanoes, associated with significant updoming of the basement, ending with the collapse of large calderas: Voon (about 5–7 Ma), Emi Koussi (2.4–1.33 Ma), and Yirrigué (0.43 Ma); (4) basaltic activity, starting at about 5–7 Ma, and essentially consisting of cinder cones and associated lava flows (Tarso Tôh, Tarso Ahon, and Tarso Emi Chi); and (5) final volcanic activity represented by post-Yirrigué caldera activity in the Tarso Toussidé Volcanic Complex, and especially Ehi Toussidé (the only active volcano in Tibesti), plus Ehi Timi and Ehi Mousgou volcanoes, similar to Ehi Toussidé. The two tectonic directions controlling some volcanic features of the province correspond to the major old lithospheric structures delimiting the volcanic province, namely, the great NW-SE Tassilian flexure to the SW and a major NE-NNE fault zone to the E. Unusual conditions of uplift and erosion in the TVP enable exceptional exposure of the internal structure of its volcanoes.
West Africa consists essentially of a Precambrian granitized craton (radiometric ages of 2700 to 1600 m.y.) covered by a thin sedimentary blanket, the oldest sedimentary rocks being as much as 1000 m.y. old. The Precambrian basement has been warped into three major uplifts trending approximately east-northeast (Fig. 1): the Léo uplift (Dorsale de Léo, also known as the Liberia-Upper Volta uplift, and Bouclier éburnéen); the Reguibat uplift; and the Anti-Atlas uplift. Deposition during most of Paleozoic time apparently occurred in epicontinental seas. The principal preserved depositional areas are the Taoudeni basin in Mauritania and Mali (Fig. 2); the Tindouf basin in Morocco, Algeria, Mauritania, and Spanish Sahara; the Volta basin in Ghana and Upper Volta; and the Bové basin in Guinea and Portuguese Guinea.
Anorogenic magmatism in West Africa began in the Palaeozoic, soon after the Pan African event, but most of it occurred from Mesozoic times onwards. A number of petrographic provinces can be identified. Permo-Triassic dolerites are widespread in the southwestern part of the region, emplaced in fracture systems that developed during the separation of Africa from North and South America. The Younger Granites of Nigeria and Niger form a discontinuous north-south belt nearly 1500 km long, with ages ranging from Ordovician in the north (Aïr) to Jurassic in the south (Jos Plateau). Another line of mainly Tertiary granites extends NE for about 1000 km along the Cameroun volcanic line. Carbonatites and related rocks of probable Permian age have recently been discovered in NE Mali. There is good spatial separation between these two provinces, the Permo-Triassic intrusives being confined to the West African craton, the Younger Granites to Pan African terranes. Post-Jurassic magmatism occurred mainly in the Pan African terrane east of the craton. Kimberlites and peralkaline intrusions were emplaced at several localities on the West African craton, some probably of Cretaceous age, others perhaps older. Otherwise, igneous activity in the Cretaceous was confined to the Benue Trough. Cenozoic alkali basaltic volcanism was most voluminous along the Cameroun line, but occurred in many other places.
The Permo-Triassic dolerites intrude rocks of the West African craton and the overlying Palaeozoic sediments. They are dominated by dykes and sills. The Freetown igneous complex is a layered gabbro-anorthosite intrusion, in which the layering dips concentrically inwards to define a cone-shaped structure. All the rocks in this province fall in the age range 275–175 Ma, and they are of tholeiitic affinities. The distribution of the West African dolerite intrusions, when considered with those of similar type and age in other countries bordering the Atlantic, suggests that they were emplaced into a strongly tensional environment, during the early stages of formation of the Atlantic Ocean. A tensional environment also controlled emplacement of the newly discovered nepheline syenite-carbonatite complexes on the eastern margin of the West African craton, which are dated as Lower Permian (c. 265 Ma) and were emplaced in a well defined continental rift setting.
Sixty crude oils from the Termit Basin (Eastern Niger) were analysed using biomarker distributions and bulk stable carbon isotopic compositions. Comprehensive oil-to-oil correlation indicates that there are two distinct families in the Termit Basin. The majority of the oils are geochemically similar and characterized by low Pr/Ph (pristane to phytane ratios) and high gammacerane/C-30 hopane ratios, small amounts of C-24 tetracyclic terpanes but abundant C-23 tricyclic terpane, and lower delta C-13 values for saturated and aromatic hydrocarbon fractions. All of these geochemical characteristics indicate possible marine sources with saline and reducing depositional environments. In contrast, oils from well DD-1 have different geochemical features. They are characterized by relatively higher Pr/Ph and lower gammacerane/C30 hopane ratios, higher amounts of C-24 tetracyclic terpane but a low content of C-23 tricyclic terpane, and relatively higher delta C-13 values for saturated and aromatic hydrocarbon fractions. These geochemical signatures indicate possible lacustrine sources deposited under freshwater, suboxic-oxic conditions. This oil family also has a unique biomarker signature in that there are large amounts of C-30 4 alpha-methylsteranes indicating a freshwater lacustrine depositional environment. The maturity of the Termit oils is assessed using a number of maturity indicators based on biomarkers, alkyl naphthalenes, alkyl phenanthrenes and alkyl dibenzothiophenes. All parameters indicate that all of the oils are generated by source rocks within the main phase of the oil generation stage with equivalent vitrinite reflectance of 0.58%-0.87%.
Correlation and R-mode factor analysis were used in the interpretation of multielement geochemical data of the peralkaline arfvedsonite granites from the northern groups of ring-complexes, Nigerian Younger Granites province. These ring-complexes are namely Dutsenwai, Kudaru, Ririwai Kila-Warji and Fagam. The specific objective is to distinguish subtle features related to mineralization from the more obvious geochemical expression of lithology and environmental factor. The results of correlation analysis showed that Nb is significantly positively correlated with Na2O, Zr, U and heavy REE and explained 48.7 % of the total data variance in the albitized variety of the peralkaline granites. The R-mode factor analysis revealed a five-factor model, which accounted for 63.5 % of the total data variance. The metal associations Zn - Pb - Rb - Nb - Th - Zr - U and Zr - Nb - Hf- Rb - Pb - Zn - U - Ta - Na are interpreted as indicative of lithology and Nb - mineralization respectively in the peralkaline granites.
The Quixeramobim granitic batholith, located in the State of Ceará, northeastern Brazil, distant 160 Km to the SW of Fortaleza, is made up by six main petrographics facies, informally called Muxure Velho, Muxuré Novo, Agua Doce, Serra Branca, Uruque and Late Mobilizates and the minor units Boa Fé and Uruque transitional Muxuré Novo. The Muxuré Novo, Serra Branca facies and the Boa Fé subunits constitute a medium-K calc-alkaline series, made up by quartz-diorites, tonalites, predominant granodiorites and monzogranites, all with biotite and hornblende. Sizes and forms of feldspars vary in these facies from lathlike (20 to 6 cm, Serra Branca; 6 to 1,5 cm, Muxuré Novo) to almost equidimensional (ca. 2,5 cm; Boa Fé). The Água Doce rocks are mainly aphyric, greyish-bluish, medium-grained, low-K calc-alkaline types showing predominant quartz-diorites and tonalites, with biotites and sometimes amphiboles. The Uruquê facies is constituted by leucocratic, light grey to yellowish grey, aphyric, medium-grained biotite granodiorites and monzogranites; they form together with their enclaves, a medium-K calc-alkaline series. The Muxuré Velho facies, a tonalite series with variable K contents is found as enclaves and synplutonic dikes within the other granitoids facies; they are dark grey, medium-fine grained types, usually containing K-feldspar xenocrysts. As a whole, the Quixeramobim rocks with SiO 2 contents between 51% and 73 %, are meta-aluminous, enriched in alkalies, Sr, Ba and LREE and poor in HREE, MgO and CaO. LREE abundances are 2 to 3 times those usually found in normal calc-alkaline suites. The ubiquitous presence of enclaves and synplutonic dikes within batholith suggests that magma mixing played a significant part in rock genesis. In Quixeramobim, the mathematical modeling and inclination of mixing curves, as observed in variation diagrams, points to a contribution up to 65% of crustal magmas to the final composition, the remainder being attributed to mantle-derived magmas. The high contents of Sr, Ba, and LREE, with no anomaly of Eu and low abundances of HREE, indicate that the mantle derived magmas that occur in Quixeramobim batholith, were formed by melting controlled by phlogopite and hornblende of a metasomatized lithospheric mantle. Mixing of magmas occurred at a stage when they were mainly liquid. The mixed magmas evolved later by crystal fractionation, with the mechanism "side-wall crystallization" well adapted to Quixeramobim batholith. Fractionation modeling using REE, K, Rb, Ba, and Sr suggests that a source similar to Agua Doce facies can generate Muxuré Novo magmas, in turn producing, by a 35-45% fractionation, a less dense and alkali-rich liquid that form the outer shell of Serra Branca types. Regular injections of the more basic magmas Muxuré Velho liquids add heat to the crystallizing magma chambers, allowing for growth of megacrysts. Extract of residual magmas formed the Uruquê rocks.
In the Northeastern part of Nigeria (Upper Benue region) uranium mineralizations occur widespread along major fracture zones within the Precambrian crystalline basement bounding the Cretaceous deposits of the Benue trough. In two mineralized areas (Mika and Ghumchi) structural analysis and UPb dating of these mineralizations have been performed.Isotopic data indicate an age of 148 ± 12 M.a. for the crystallization of Mika primary pitchblende, followed by a simple UPb evolution (without leakage of intermediate decay products) and a strong recent mobilization of the uranium (autunite and coffinite formation). The pitchblende crystallized in “en echelon” array megatension gashes due to regional dextral wrench mechanism along a N140E trend. On a regional scale, this fracturing episode and the uranium concentration phase are contemporaneous with the emplacement of a bimodal volcanism dated at 147 ± 7 M.a. which is related to the early stages of opening of the Benue trough (Popoff et al., 1982).Isotopic data of Ghumchi mineralization provide an age of 14 ± 3 m.y. for the crystallization of cryptocrystalline coffinite and like in Mika, actual- and strong mobilization of uranium affect the mineralizations (autunite formation). The mineralizations crystallized along passive preexisting structures (mylonites, faults and lamprophyric dykes) which acted as favorable traps for uranium concentration. Pb/Pb data on galena microcubes, associated with the coffinite, provide an isotopic composition comparable to those of recent alkali basalt series which intrude the African plate (e.g. Mandara-Cameroon, Kenya, Ahaggar, Canary Islands). Then, such a noticeable Pb isotopic composition together with the post-tectonic character and the Neogene age of the Ghumchi mineralization suggest that their crystallization is linked with the emplacement of Neogene alkali basalt lava flows and trachytic plugs which occur in the vicinity of Ghumchi area.
Three-dimensional modelling of gravity data shows that the Dutsen-Wai ring complex of the Nigerian Younger Granite Province can be a high-level, thin, tabular body with subsurface extension of felsic material, dominantly biotite granite, well beyond the outcrop limits of the complex in all directions. This geometry is consistent with emplacement of the body by near-surface lateral spreading of an intrusive diapir. Emplacement by this mechanism creates horizontally directed tensile stresses in the overburden which leads to the formation of ring fractures above the spreading intrusive.
Several late-collision and intraplate features are not entirely integrated in the classical plate tectonic model. The Pan-African orogeny (730-550 Ma) in Saharan Africa provides some insight into the contrasting behaviour of cratons and mobile belts. Simple geophysical considerations and geological observations indicate that rigidity and persistence of cratons are linked to the presence of a thick mechanical boundary layer, the upper brittle part of the continental lithospheric mantle, well attached to an ancient weakly radioactive crust. It is suggested that the 5000 km wide Pan-African domain of Saharan Africa suffered regional continental lithospheric mantle delamination during the early stages of this orogeny. Delamination and juxtaposition of crust against hot asthenosphere can explain many features of the late Pan-African (around 600 Ma): reactivation of old terrains, abundant late-tectonic high-K calc-alkaline granitoids, high temperature-low pressure metamorphism, important displacements along mega-shear zones and mantle-derived post-tectonic granitoids linked to a rapid change in mantle source. -from Authors
Attention is drawn to the existence of a negative gravity lineament linking the domally uplifted Cainozoic volcanic centres of North and West Africa to the negative Bouguer anomaly associated with the East African Rift System. The gravity lineament is shown to have similar dimensions to the Rift System anomaly and is interpreted as resulting from attenuation of the continental lithosphere. As such the lineament may represent an earlier stage than the East African Rift System in the processes that could eventually lead to continental disruption.
According to present concepts, a caldera is a more or less circular volcanic depression larger than a crater which is caused by subsidence. It is commonly considered that the subsided mass consists of a block or blocks encircled by a ring fracture. Caldera collapse is generally correlated with a major explosive eruption. The present investigation is concerned with six features which do not conform well with the favored caldera model. Attention is given to downsagged calderas, the distribution of postcaldera vents in calderas, vent rings, the size of calderas and cauldrons, incremental caldera growth, and caldera-forming events. It is found that no single structural or genetic model applies to all calderas. Thus, the fact of subsidence may be the only common feature. It is pointed out that most known ring dikes occur in Precambrian crust. This may mean that the subsiding piston mechanism operates best where the crust is sufficiently rigid and strong.
Except for the northwestern and southern extremities, the African plate has been anorogenic throughout the Phanerozoic, yet the interior has been subject to spectacular episodes of uplift, rifting and magmatism from Mesozoic to present. From Cretaceous to Recent times, alkaline magmatism (including carbonatites and kimberlites) has been the norm. Contemporaneous plate-wide activity, repetition at individual sites, and localization by existing anisotropies in the plate, are all indicative of responses to external events capable of affecting the whole plate. Spatial and temporal relations thus show that rifting and alkaline magmatism are permissive, and may themselves be symptoms of more fundamental processes that cause the break-up of continents. -from Author
Sediments deposited in a variety of environments including fluviatile sands and gravels, aeolian sands, lacustrine clays and diatomites, and swamp clays crop out in and around Adrar Bous, an isolated ring-complex in the northern Tenere Desert of Niger. During the early Holocene a small lake, stable at 710 m a.s.l. supported a community of Epi-Palaeolithic hunter-gatherers who also ate aquatic foods until the lake dried out at around 7500-7000 BP. Occupation of the piedmont plains resumed during 6000-4000 BP when Neolithic pastoralists grazed their cattle around a smaller lake stable at 700 m a.s.l. -from Authors
Major ring-intrusions in the Gardar Province are frequently elliptical in cross-section and are located at the intersections of regional dyke swarms with sinistral wrench-fault systems. Fault movement was contemporaneous with igneous intrusion and the eUipticity may be explained by ductile deformation of hot intrusions undergoing large-scale simple-shear. Geometrical measurements on four of the intrusions give shear-strain values (γ) ranging from 0.6-1.2. Away from the intrusions, deformation is entirely by brittle fracture and the amount of fault movement correlates well with the sum of brittle and ductile movement within the intrusions.
The ring complexes of the Nigeria–Niger Younger Granite province occur in three separate districts: Air, the Zinder–e area of S Niger Republic and the Jos Plateau area of Nigeria. Together these form a north-south zone 1300 km long, characterized petrographically by alkaline granites and associated volcanic rocks, and structurally by ring dykes, volcanic cauldrons and high level granite plutons. The three districts are separated by areas of younger sedimentary rocks. Between Air and the S Niger district are Cretaceous rocks, filling the saddle between the sedimentary basins of Iullemmeden and Chad. Between the S Niger and Nigerian districts is an embayment of the Chad basin containing deposits of Pliocene to Pleistocene age (Fig. 1).
The Nigerian district is the best known (Jacobson et al. 1958; Buchanan et al., 1971). Many of the complexes contain rhyolites within areas of cauldron subsidence. Intrusions into and beneath these rhyolites are mainly granitic but include minor syenites and gabbros. The granites contain such mafic minerals as fayalite, sodic amphibole and pyroxene and biotite. By far the most abundant type is biotite granite, often with tin and niobium mineralization. A Jurassic age of about 160 m.y. has been established for the Nigerian Younger Granites (Jacobson et al., 1963).
In the S Niger district (Karpoff 1948; Black 1963) most of the Younger Granite rocks are concentrated in the Mounio massif, which extends for over 100 km south from Goure Rhyolites occur extensively, and the main intrusive rocks are peralkaline granites and syenites. Traces of cassiterite have
Granitoids as categorized by tectonic environment are (1) island arc granitoids (IAG), (2) continental arc granitoids (CAG), (3) continental collision granitoids (CCG), (4) postorogenic granitoids (POG), (5) rift-related granitoids (RRG), (6) continental epeirogenic uplift granitoids (CEUG), and (7) oceanic plagiogranites (OP). Of these, the IAG, CAG, CCG, and POG are considered orogenic granitoids, and the RRG, CEUG, and OP are considered anorogenic granitoids.
The discrimination of granitoids is based on the major-element chemistry. Various discrimination plots are presented which sequentially discriminate the different tectonic environments. OP are separated from all other granitoids on the K2O versus SiO2 plot. Discrimination between group I (IAG + CAG + CCG), group II (RRG + CEUG), and group III (POG) granitoids can be achieved by using plots of Al2O3 versus SiO2, FeO(T)/ [FeO(T) + MgO] versus SiO2, and AFM and ACF ternary diagrams. In the figures, group I and group II plot in individual fields. Identification of group III is different, in that group III does not have a unique field in which it plots. Group III is identified because it consistently displays characteristics of both group I and group II. Further discrimination within group I can be accomplished on the basis of Shand's index. Only CCG have A/CNK [AL2O3/(CaO + Na2O + K2O)] values greater than 1.15. It is not possible to discriminate between IAG and CAG. Further discrimination within group II is done using the TiO2 versus SiO2 plot.
The proposed discrimination scheme is applied to the Proterozoic granitoids of the midcontinent of the United States. It is shown that the Arbuckle granitoids are not anorogenic as previously thought.
The Hoggar Region of Algeria, situated in the central part of the Sahara, is geologically complex and logistically difficult. Programmes of geological mapping and mineral exploration are being conducted by Algerian Government Agencies in which geophysical studies are playing useful roles. The region is covered by aerogeophysical surveys at regional scale. The interpretation of this data, integrated with the results of ground magnetic, radiometric and gravity traverses and of measurements of geophysical properties of outcrop samples is the first stage in the mapping programmes. Examples of results from the Timissao-Timgaouine region are presented. The several episodes of granite were identified and mapped by their radiometric expressions. The sequence of granites is interpreted, from older (early Pan African) to younger (Eocambrian), as Timgaouine granite, migmatitic granite, Abankor and Timissao magmatic granites and finally the intrusive granites of Taourirt and Bahouinet. The various magnetic bodies (conformable metavoicanics, diorite and gabbro intrusives, effusive complexes and mafic dikes) show characteristic aeromagnetic signatures by which they can be mapped. Source body parameters are calculated from magnetic and gravity data. The evoiution of the Pharusian Chain west of the great 4°50' submeridional fault has been re-interpreted in the light of modern theories of global tectonics. In the Timgaouine region, the computed structures, the distribution of magnetic units and the systems of faults all support the hypothesis that the Pharusian Chain evolved as a back-arc basin during a period of separation of the West African Craton from the Touareg Shield as a result of the Pan African Orogeny.
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