Chapter I distinct geotectonic models for the evolution of the Paleoproterozoic basement in West Africa. A traditional plate tectonic model of crustal buildup by thrust and fold belts has been proposed by Boher et al. (1992), Feybesse and Milési (1994), Ledru et al. (1994), Hirdes et al. (1996), Debat et al. (2003), Feybesse et al. (2006), Tshibubudze et al. (2009), Hein (2010). Pons et al. (1995) or Vidal et al. (2009) favor dome and basin structures caused by vertical tectonic gravitational processes.
Field mapping and structural studies in this region are seriously hindered by the presence of thick lateritic cover and scarcity of fresh outcrops. Rock exposure in the study area is about six percent. High-resolution airborne geophysical data have become an indispensable tool for geological mapping (Agyei Duodu et al., 2010) in West Africa. Interpretations of magnetic, gravimetric, gamma- ray spectrometry and satellite remote sensing data constrained by field observations proved to be efficient in deciphering the regional litho-structural hierarchies in the Precambrian terrains (e.g. Jaques et al., 1997; Schetselaar et al., 2000; Betts et al., 2003; Peschler et al., 2004; Direen et al., 2005;
Martelet et al., 2006; Peschler et al., 2006; de Souza Filho et al., 2007; Schetselaar et al., 2007;
Schetselaar et al., 2008; Teruiya et al., 2008; Aitken and Betts, 2009a; Stewart and Betts, 2010). The effective application of some methods alone without an integrated approach may be difficult in geological mapping especially in deeply weathered terrains due to the complex regolith-parent rock relationships (Wilford et al., 1997; Taylor and Eggleton, 2001). Additionally, the application of gamma-ray spectrometry data or satellite remote sensing data is more appropriate for regolith landform mapping rather than geological mapping (Craig, 2001; Wilford, 2002).
In this study, we present a new regional scale structural and lithological map of western Burkina Faso, based on an integrated analysis of field observations, geophysical, and remote sensing data. We will discuss in particular the significance of newly discovered structures and lithological relationships, in the light of the geodynamic scenario that we propose in chapter II. As a part of this study, we will offer an overview of the advantages and drawbacks of geophysical and remote sensing methods in the deeply weathered terrains of West Africa and compare our findings to similar works in other Precambrian terrains.
Chapter I Leube et al., 1990; Boher et al., 1992; Taylor et al., 1992; Pouclet et al., 1996; Béziat et al., 2000).
Radiometric dating of the volcanic units (Davis et al., 1994; Loh and Hirdes, 1996; Lüdtke et al., 1998; Lüdtke et al., 1999) places the main peak of the Birimian volcanism at around 2190–2160 Ma, while detrital zircons from the sedimentary basins yield ages as young as 2130 Ma (Lüdtke et al., 1999) or 2107 Ma (Doumbia et al., 1998). The Birimian volcanic and volcano-sedimentary units are unconformably overlain at several places across the craton by detrital shallow water sedimentary rocks, which are known as the Tarkwaian sediments (Whitelaw, 1929; Sestini, 1973; Leube et al., 1990; Oberthuer et al., 1998; Feybesse et al., 2006). The whole complex of volcanic, volcano- sedimentary and sedimentary units was intruded by several generations of granitoids, which were emplaced during several magmatic pulses from 2180 to 1980 Ma (Leube et al., 1990; Pons et al., 1995; Hirdes et al., 1996; Doumbia et al., 1998; Castaing et al., 2003; Gasquet et al., 2003; Naba et al., 2004; Siegfried et al., 2009; Thomas et al., 2009; Agyei Duodu et al., 2010). The general geochemistry of the granitoids evolves from Na-rich calc-alkaline to K-rich alkaline (Boher et al., 1992) while their shape depends on tectonic regime during their emplacement, ranging from undeformed circular plutons to elongate and complex interlocked bodies (Pons et al., 1991; Pons et al., 1992; Pons et al., 1995).
The polydeformed Baoulé-Mossi domain developed during the Eburnean orogeny (Bonhomme, 1962), which operated between ~2130 and 1980 Ma (Davis et al., 1994; Oberthuer et al., 1998; Feybesse et al., 2006). Most of the volcanic and sedimentary rocks underwent lower to upper greenschist facies metamorphism (Béziat et al., 2000; Feybesse et al., 2006; Kříbek et al., 2008).
Although John et al. (1999) and Galipp et al. (2003) described regional MP/MT conditions (500–
600°C, 5–6 kbar) in Ghana, amphibolite facies metamorphism is mostly restricted to the contact aureoles of granitic plutons (Debat et al., 2003). The Eburnean orogeny is generally divided into two major deformation phases. The first phase, which caused major crustal thickening (Boher et al., 1992;
Milési et al., 1992; Allibone et al., 2002; Feybesse et al., 2006; Vidal et al., 2009), operated approximately during 2130–2100 Ma. The second phase that lasted up to 1980 Ma was responsible for the formation of regional scale transcurrent shear zones and faults, which transect all lithologies. Gold mineralization in West Africa is generally related to these shear zones (e.g. Milési et al., 1989; Milési et al., 1992; Blenkinsop et al., 1994; Bourges et al., 1998; Allibone et al., 2002; Feybesse et al., 2006).
The consolidated Eburnean basement was then locally affected by a N-S oriented compressional event (Nikiéma et al., 1993; Debat et al., 2003; Hein, 2010) and unconformably overlain by the Neoproterozoic sediments of the Taoudeni, Lullemeden, and Volta basins. Dyke swarms cross the entire Proterozoic domain in several directions and are formed by at least six different generations (Jessell et al., 2010).
Chapter I
2.2. Western Burkina Faso
The Paleoproterozoic basement of western Burkina Faso consists of three N-S trending greenstone belts, from east to west known as Boromo, Houndé, and Banfora (Castaing et al., 2003).
The greenstone belts are separated by composite granitoid domains, which we call the Koudougou- Tumu, Diébougou, Sidéradougou, and Niangoloko domains for the purpose of this work (figure I-1).
The detailed stratigraphic relationships and litho-tectonic evolution are described in detail in chapter II. Shallow water detrital sediments, considered genetically similar to the Tarkwaian meta-sediments in Ghana, occur as a ~400 km long and ~1–2 km wide unit within the Houndé belt (Bossière et al., 1996). In the west, sub-horizontal units of the Neoproterozoic Taoudeni basin overlay the Paleoproterozoic basement (Bronner et al., 1980). Doleritic dykes transect all of the present lithologies.
The greenstone belts of western Burkina Faso and neighboring northern Ivory Coast and Ghana are composed of mafic tholeiitic lithologies, followed by intermediate and acid calc-alkaline effusive suites, typical for volcanic arc environments (Lüdtke et al., 1998; Béziat et al., 2000). U-Pb zircon ages in rhyolites associated to the mafic rocks yield ~2176–2160 Ma (Lüdtke et al., 1998; Le Métour et al., 2003). The volcanic assemblages are often intercalated with volcano-sedimentary units, considered as their lateral stratigraphic equivalents (Hirdes et al., 1996; Lüdtke et al., 1998; Lüdtke et al., 1999; Castaing et al., 2003). The Birimian sediments and volcano-sediments form belt-parallel units, such as the Bambéla volcano-sedimentary unit, which extends to the Ivory Coast where the unit is known as the Bambéla basin (Hirdes et al., 1996; Lüdtke et al., 1998; Lüdtke et al. 1999), or the Batié volcano-sedimentary unit situated within the Boromo belt. These sedimentary units might postdate the main volcanic activity in the region, as suggested by radiometric ages obtained on detrital zircons – 2126 Ma (Lüdtke et al., 1999).
The granitic rocks form a complex assemblage resulting from several magmatic episodes (Castaing et al., 2003), for which several types of classifications have been proposed cratonwide (Leube et al., 1990; Davis et al., 1994; Hirdes et al., 1996; Doumbia et al., 1998; Egal et al., 2002;
Castaing et al., 2003; Gasquet et al., 2003; Naba et al., 2004; Lompo, 2009). The petrographic composition of the different suites is variable. Based on field reconnaissance and the character of the intrusive bodies observed in the geophysical data, we will distinguish four groups of granitoid
Chapter I best visible in magnetic data due to their high susceptibility. Coarse-grained undeformed gabbros of unknown age (ME4) are at places associated with the granitoid intrusions. More radiometric, petrographic, and geochemical data are needed for a more consistent classification of the plutons into the particular magmatic episodes.
The structural patterns of the granite-greenstone belt contacts vary between bifurcating and linear patterns, as described by Kusky and Vearncombe (1997), which suggest both thrust-fold and strike-slip related deformations. As for the whole Paleoproterozoic domain, the deformation history of western Burkina Faso is polyphase, showing evidence of at least two distinct Eburnean orogenic phases (Ouedraogo and Prost, 1986; Castaing et al., 2003) discussed in detail in chapter II.