Precambrian Research 274 (2016) 82–109 Contents lists available at ScienceDirect Precambrian Research jo urnal homepage: www.elsevier.com/locate/precamres Lower crust exhumation during Paleoproterozoic (Eburnean) orogeny, NW Ghana, West African Craton: Interplay of coeval contractional deformation and extensional gravitational collapse a,∗ b c a,e d Sylvain Block , Mark Jessell , Laurent Aillères , Lenka Baratoux , Olivier Bruguier , f d d g Armin Zeh , Delphine Bosch , Renaud Caby , Emmanuel Mensah a Géosciences Environnement Toulouse, Observatoire Midi Pyrénées, 14 ave E. Belin, 31400 Toulouse, France b Center for Exploration Targeting, The University of Western Australia, 35 Stirling Highway, Crawley, Perth 6009, WA, Australia c Monash University, School of Geosciences, Wellington Road, Clayton 3800, VIC, Australia d Géosciences Montpellier, Université Montpellier 2-CNRS, cc 066, Place Eugène Bataillon, 34095 Montpellier Cedex 5, France e IFAN, Cheikh Anta Diop, Dakar, Senegal f Institut für Geowissenschaften, Altenhöfer Allee 1, D-60438 Frankfurt am Main, Germany g Geological Survey Department of Ghana, Accra, Ghana a r t i c l e i n f o a b s t r a c t Article history: We present new litho-structural and metamorphic maps of the Paleoproterozoic (2.25–2.00 Ga) West Received 1 December 2014 African Craton in northern Ghana, based on the interpretation of field observations and airborne geophys- Received in revised form 31 August 2015 ical datasets. The study reveals contrasting metamorphic domains consisting of high-grade gneisses and Accepted 17 October 2015 low-grade volcano–sedimentary belts, separated by shear zones and assembled during the Paleoprotero- Available online 29 October 2015 zoic Eburnean orogeny (2.15–2.07 Ga). Supracrustal rocks were buried and metamorphosed at conditions corresponding to the amphibolite–granulite facies transition, during a (D1) deformation event, consis- Keywords: tent with N–S horizontal shortening, which generated reverse shear zones. High and low metamorphic West African craton grade rocks were brought in contact along extensional shear zones formed during N–S extension (D2). Eburnean orogeny Paleoproterozoic These structures were overprinted by contractional deformation associated with E–W shortening and Anatectic migmatite dome N to NNE stretching (D3), coeval with amphibolite-facies metamorphism. The late-stage tectonic evolu- Collision tion (D4–D7) is characterised by strain localisation in successive generations of narrow shear zones, and Archean–Proterozoic transition the re-activation of inherited structures in a dominantly transcurrent regime. U–Pb dating of zircon and monazite from magmatic and metamorphic rocks reveals that D1-D3 deformations form a continuous and overlapping time sequence between ca. 2140 and 2110 Ma. The changes in deformation style from D1 to D3 are interpreted to reflect a shift from dominant horizontal tectonic forces to an interplay between tectonic and gravitational forces, which allowed for the exhumation of the lower crust in an anatectic migmatite dome. We suggest that doming is accommodated by lateral extensional sliding of the upper crust and amplified by coeval orthogonal shortening. The abrupt rotation of shortening directions points to a change in boundary conditions applied to the orogen. We hypothesise that it is due to the colli- sion of northern Ghana with the Paleoproterozoic province in modern-day south-western Burkina Faso, which shows a contrasting geological history. The results reveal that the Eburnean orogeny in NW Ghana shared some thermo-mechanical similarities with modern orogenic belts. The findings bring new insight in Paleoprotezozoic plate tectonics, at the transition between archaic and modern geodynamics. © 2015 Elsevier B.V. All rights reserved. 1. Introduction comprise domains affected by homogeneous LP-HT metamorphism and distributed strain, and are transect by sets of craton-scale Granite-greenstone terranes are archetypal of Archean and strike-slip shear zones (Cawood et al., 2009; Condie and Kröner, Paleoproterozoic cratons (e.g. De Wit and Ashwal, 1997). They 2008; Pease et al., 2008; Windley, 1992). Granite-greenstone ter- ranes are described by some authors as analogues of modern orogenic terranes formed in subduction-driven plate tectonic set- ∗ tings (e.g. Cawood et al., 2006; Condie and Benn, 2006; Condie Corresponding author. Tel.: +33 621333525. E-mail address: [email protected] (S. Block). and Kröner, 2008; De Wit, 1998; Kusky and Polat, 1999; De Wit, http://dx.doi.org/10.1016/j.precamres.2015.10.014 0301-9268/© 2015 Elsevier B.V. All rights reserved. S. Block et al. / Precambrian Research 274 (2016) 82–109 83 2004; Komiya et al., 1999; Kusky et al., 2001). However, evi- rhyolites (e.g. Baratoux et al., 2011; Kitson, 1918; Leube et al., 1990; dence for fold-and-thrust belts, blueschist and ultra-high pressure Pouclet et al., 2006). Birimian sedimentary basins are elongated to metamorphism, extensional gravitational collapse, all of which are broad units dominated by greywacke and shales, and occasionally found in Phanerozoic orogenic belts (e.g. Brown, 2009; Chopin, contain lavas and chemical sedimentary rocks. Tarkwaian basins 2003; Dewey, 1988; Miyashiro, 1961), are lacking or ambiguous are fault-bounded tectonised sequences made of conglomerates, on the early Earth. The contrasts between the geological records quartzites and shales that are discordant on Birimian formations of Archean and Phanerozoic provinces have led a growing body of (Davis et al., 1994; Perrouty et al., 2012). research to point out the limits of a strictly uniformitarian approach Across the craton, the ages of inherited and detrital zircon to geodynamics and to propose alternative tectonic frameworks for grains reveal that magmatism started as early as ca. 2.30 Ga (e.g. the evolution of the continental crust (e.g. Albarède, 1998; Bédard Gasquet et al., 2003). Intense volcanic activity is recorded in green- et al., 2006, 2013; Hamilton, 1998, 2003; Stern, 2005; van Hunen stone belts between 2.25 and 2.19 Ga (Agyei Duodu et al., 2009; and van den Berg, 2008; Van Kranendonk et al., 2004, 2007). In par- Feybesse et al., 2006; Hirdes and Davis, 1998; Hirdes et al., 1996; in ticular, the rheological behaviour of the lithosphere, which is tightly Ghana; Schwartz and Melcher, 2003, in Burkina Faso; Delor et al., controlled by mantle temperature, must have changed markedly 1995, in Côte d’Ivoire, Lahondère et al., 2002, in Guinea); and is since the early Archean, due to the inferred secular cooling of the coeval with the formation of early granitoid suites (Dia et al., 1997; mantle (e.g. Korenaga, 2006; Rey and Houseman, 2006; Sizova et al., Gueye et al., 2008, in Senegal; De Kock et al., 2011; Feybesse et al., 2014). As a consequence, tectonic processes in orogens on a hotter 2006, in Ghana; Tshibubudze et al., 2013, in Burkina Faso). Mag- Earth must have been significantly different from those driving the matic activity is uninterrupted until ca. 2.10 Ga (e.g. Davis et al., evolution of modern orogenic belts. Strain and metamorphic pat- 1994; Doumbia et al., 1998; Hirdes et al., 1992, 1996; Kouamelan terns can be used to explore the rheological structure and thermal et al., 1997; Oberthür et al., 1998; Tapsoba et al., 2013). Younger regime of ancient orogens. They record secular changes and allow granitoids and lavas are emplaced between 2.10 and 2.07 Ga are speculating on the evolving geodynamic settings during the history mostly in the western portion of the Baoulé-Mossi domain and of the Earth (e.g. Brown, 2007, 2009). along the contact zone with the Archean craton (Egal et al., 2002; The debate on ancient geodynamic processes has mostly Hirdes and Davis, 2002; Liégeois et al., 1991). The magmatic activity focussed on the Archean, but the controversy lives on after the gradually declined after 2.10 Ga as the crust cooled down, as evi- Archean–Proterozoic transition. Indeed, “hot” orogen models have denced by K–Ar and Ar–Ar cooling ages between ca. 2100 and ca. been proposed for some Paleoproterozoic provinces (e.g. Cagnard 1900 Ma (Castaing et al., 2003; Chalokwu et al., 1997; Pigois et al., et al., 2006; Chardon et al., 2009; Vidal et al., 2009), despite 2003) a worldwide Paleoproterozoic metamorphic record that differs The structural trend of litho-tectonic units and the dominant from that of Archean terranes (Brown, 2009; Harley, 1992). In fabrics on the West African Craton formed during the Eburnean order to investigate the geodynamic changes in orogens around orogeny (Bonhomme, 1962; Eisenlohr and Hirdes, 1992; Feybesse the Archean–Proterozoic transition, we draw our attention to et al., 2006; Lemoine et al., 1990; Milési et al., 1989; Milesi the juvenile Paleoproterozoic (2.25–2.07 Ga) West African Craton et al., 1992; Perrouty et al., 2012). In most provinces of the cra- (Abouchami et al., 1990; Boher et al., 1992), which represents one ton, it consists of an early phase (2140–2100 Ma) characterised 6 2 ∼ of the youngest large ( 3.10 km ) cratonic domains on Earth. by contractional deformation, crustal thickening and medium- to This study focuses on north-western Ghana, where large sur- high-grade metamorphism (Block et al., 2015; Caby et al., 2000; faces of exhumed high-grade metamorphic rocks are juxtaposed to Debat et al., 2003; Galipp et al., 2003; Ganne et al., 2012; John et al., coeval low-grade metamorphic rocks. Detailed lithological, struc- 1999; Klemd et al., 2002; Liégeois et al., 1991; Opare-Addo et al., tural and metamorphic maps are produced from the interpretation 1993; Pitra et al., 2010; Triboulet and Feybesse, 1998), followed of field and geophysical data. A structural analysis is carried out by a 2100–2070 Ma phase of transcurrent tectonics and low-grade and coupled to geochronological constraints, in order to describe metamorphic overprint (e.g. Feybesse et al., 2006; Jessell et al., the deformation sequence during the Eburnean (2.14–2.07 Ga) oro- 2012; Ledru et al., 1991; Lompo, 2010; Nikiéma et al., 1993; White genic cycle.