Geometry and kinematics of the late Proterozoic Angavo Shear Zone, Central Madagascar: Implications for Gondwana Assembly

Tectonophysics 592 (2013) 113–129

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Geometry and kinematics of the late Proterozoic Angavo Shear Zone, Central Madagascar: Implications for Gondwana Assembly Tsilavo Raharimahefa a, Timothy M. Kusky b, c,⁎, Erkan Toraman d, Christine Rasoazanamparany e, Imboarina Rasaonina f a

Department of Geology, University of Regina, 3737 Wascana Parway, Regina, SK, Canada S4S 0A2 Three Gorges Research Center for Geohazards, China University of Geosciences, 388 Lumo Road, Hongshan District, Wuhan 430074, Hubei Province, China State Key Lab for Geological Processes and Mineral Resources, China University of Geosciences, 388 Lumo Road, Hongshan District, Wuhan 430074, Hubei Province, China d Department of Geology and Geophysics, University of Minnesota, Minneapolis, MN 55455-0219, USA e Department of Geology, Miami University, Oxford, OH 45065, USA f Département des Sciences de la Terre, Université d'Antananarivo, Madagascar b c

a r t i c l e

i n f o

Article history: Received 28 November 2012 Received in revised form 2 February 2013 Accepted 6 February 2013 Available online 18 February 2013 Keywords: Madagascar Angavo Shear Zone Tectonics Gondwana

a b s t r a c t This paper documents the 20 to 60 km wide N–S trending Angavo Shear Zone (ASZ) in central Madagascar and its tectonic implications by examining its structural styles, kinematics and geometry. Our study indicates that the ASZ is characterized by at least two ductile Late Proterozoic deformation events (D1 and D2) followed by a brittle neotectonic deformation (D3). The early D1 event produced a regionally extensive S1 foliation, stretching/flattening mineral lineation L1 and symmetrical structural fabrics such as recumbent and isoclinal intra-folial folds (F1), implying a flattening deformation. D1 deformational fabrics are locally overprinted by D2 structures. D2 is characterized by a penetrative S2 foliation, shallow south plunging L2 lineation, asymmetric and sheath folds (F2) consistent with a right lateral sense of movement exhibited by delta- and sigma-type porphyroclast systems and asymmetric boudinage fabrics. D2 represents a non-coaxial flow regime formed in a dextral west over east shear zone during a partitioned transpression in response to east–west-directed compression during the assembly of Gondwana. A close resemblance with the Achankovil shear zone in India is noticed; however the continuation of the ASZ in Africa is uncertain. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Madagascar's location prior to the amalgamation of eastern and western Gondwana has received great attention (Collins, 2006; Collins et al., 2000; Fritz et al., in press; Reeves and de Wit, 2001; Shackleton, 1996; Stern, 1994), and studies of major crustal scale shear zones within the island will help constrain reconstructions of the Gondwana supercontinent. The position of Madagascar in most reconstructed Gondwana supercontinent models was based on similar-aged deformation and intrusions, structural styles and metamorphism, lithologies, mineralization and paleomagnetism. However, the understanding of the tectonic evolution of the Precambrian basement of Madagascar has been limited by incomplete knowledge of the structural history and ages of deformations, especially those of major shear zones cutting the basement. The Angavo Shear Zone (ASZ) is an 800 km long zone of highly strained rocks that cuts across the Precambrian basement of Madagascar (Fig. 1), and was first named by Ralison and Nédélec (1997). The belt of deformed rocks is approximately 20 to 60 km wide and contained

⁎ Corresponding author. Tel.: +86 189 7157 9211. E-mail address: [email protected] (T.M. Kusky). 0040-1951/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tecto.2013.02.014

within the Antananarivo block (Collins, 2006; Collins et al., 2000; Kröner et al., 2000) which is a major Precambrian tectonic block that occupies much of central Madagascar (Fig. 1). The northern part of the ASZ is delimited by the Andriamena units (Goncalves et al., 2003, 2004) on the west, and the Beforona units (Fig. 1) on the east (Collins, 2006). However the southern part of the ASZ, known as Ifanadiana Shear Zone (Martelat et al., 1999, 2000) is delimited by the Betsimisaraka Suture (Collins, 2006; Collins and Windley, 2002; Collins et al., 2003c; Kröner et al., 2000; Raharimahefa and Kusky, 2006, 2009). The ASZ is well-exposed in the Moramanga–Anevoka area (Figs. 1–3) where it is characterized by north striking belts of heterogeneously strained plutonic and metamorphic rocks (Fig. 2). The Antananarivo block that hosts the Angavo shear zone in this area (Collins, 2006) consists mainly of Neoarchean gneisses and metasedimentary sequences intruded by Neoproterozoic and Cambrian plutons (Grégoire et al., 2009; Kröner et al., 2000; Meert et al., 2001; Nédélec et al., 1994; Paquette and Nédélec, 1998; Tucker et al., 1999) and its geological evolution has been the subject of different studies (Collins, 2006; Grégoire et al., 2009; Nédélec et al., 1995, 2000; Windley et al., 1994). Based on a transect from Antananarivo to Toamasina, Collins et al (2003b) divided the eastern Antananarivo block into four structural domains, and discussed some of the differences in structural

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Fig. 1. Geologic map of Madagascar simplified from Bésairie (1964), Kusky et al. (2003), Collins et al. (2000, 2001, 2003a,b,c), Collins and Windley (2002), and Collins (2006). Red box shows the location of Figs. 2 and 3.

fabrics in each domain. In contrast to the shear zones in southern Madagascar (Berger et al., 2006; de Wit et al., 2001; Martelat et al., 1999, 2000; Schreurs et al., 2010), little research has been done within the ASZ, which focused mainly on the magmatic and metamorphic history (Grégoire et al., 2009; Martelat et al., 1999; Nédélec et al., 2000). A large part of the ASZ has been left unknown and structural styles and kinematics are still in need of further documentation, after the pioneering work of Collins et al. (2003b). Hence the ASZ merits a detailed structural analysis despite the fact that major shear zones in

the southern Madagascar have been documented (Berger et al., 2006; de Wit et al., 2001; Martelat et al., 1999, 2000; Schreurs et al., 2010). This paper presents new structural data, describes the structural evolution along the ASZ and discusses its relationships to the tectonics of Madagascar and greater Gondwana. Our result suggests that two ductile Late Proterozoic (Raharimahefa and Kusky, 2010) deformational events have been involved in the structural history of the ASZ, a coaxial/ flattening D1 and a transpressional dextral west over east thrusting D2. These two ductile deformational events are later followed by a younger

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Fig. 2. Simplified geological map of the study area (shown in square box in Fig. 1). Modified from Bésairie (1964), Kusky and Pitfield (2006) and Rantoanina (1962).

D3 brittle deformation (Kusky et al., 2007, 2010), which is not covered and discussed in this paper. 2. Tectonic framework Madagascar is an important place for studying the late Proterozoic supercontinents of Rodinia and Gondwana. The island is part of the Mozambique Belt, or the larger East African Orogen (EAO) (Fritz et al., in press; Holmes, 1954; Jacobs et al., 2003; Johnson et al., 2011; Kusky et al., 2003; Stern, 1994). The basement of Madagascar was classified into five main tectonic blocks (Collins, 2006; Collins and Windley, 2002; Collins et al., 2000;

Kröner et al., 2000): (1) the Bemarivo block (Fig. 1), forming the youngest Proterozoic orogenic belt of Madagascar, composed of granite batholiths, metasediments, metavolcanic rocks, gneisses and migmatites (Bésairie, 1964; Buchwaldt et al., 2002; Collins et al., 2001; Peters et al., 2003; Tucker et al., 2001); (2) the Antongil (Fig. 1) and Masora block (Collins et al., 2003b,c; Hottin, 1976; Paquette et al., 2003; Schofield et al., 2010) composed mainly of granodiorite, ortho- and paragneisses, migmatite and granite suites (Paquette et al., 2003). This block includes a 3187 Ma tonalitic gneiss and is intruded by ~ 2500 Ma granitic bodies (Tucker et al., 1999) as well as mafic suites; these Archean blocks of Madagascar experienced Late Archean greenschist–facies metamorphism; it is equivalent to

116 T. Raharimahefa et al. / Tectonophysics 592 (2013) 113–129 Fig. 3. Structural map of ASZ showing the foliation and lineation orientation as well as foliation trends derived from this work and compiled with previous maps. Group I (Gp I) named as Ambatoloana group, Group II: Moramanga, Group III: Anosibe An'Ala, Group IV: Anevoka. Stereonet diagram showing S1 (black line), S2 (blue line), □ L1, ■ L2, △ F1 hinge line, ▽ F2 hinge line, ✩ F1 axial plane, × F2 axial plane.

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the Indian Dharwar craton which is composed of mafic schist belts, ultramafic suites, mafic suites, anorthositic suites, the quartzofeldspathic Peninsular gneiss, and granite suites (Jayananda et al., 2006; Jayaram et al., 1984; Manikyamba et al., 2004; Reddy et al., 1998); (3) the Antananarivo block, pervasively affected by Neoproterozoic metamorphism and granite emplacement (Grégoire et al., 2009; Kröner et al., 2000; Nédélec et al., 2000; Raharimahefa and Kusky, 2010; Rakotondrazafy et al., 2001), delimited in the south by the Ranotsara Shear Zone. This block occupies almost 2/3 of the basement rocks of Madagascar and consists of 2520–2490 Ma orthogneisses (Tucker et al., 1999), paragneisses, migmatites and intrusions of Neoproterozoic and Cambrian granitoids including the ~630–560 Ma stratoid granites (Emberger, 1958; Nédélec et al., 1994, 1995; Paquette and Nédélec, 1998) which were emplaced in an extensional tectonic setting (Nédélec et al., 1995; Paquette and Nédélec, 1998) and a suite of 824–719 Ma extensive granitoids (Kröner et al., 2000; Raharimahefa and Kusky, 2010; Tucker et al., 1999) proposed to be emplaced above a subduction zone (Brewer et al., 2001), as well as a late Proterozoic event (~ 550–500 Ma) associated with the 537 Ma Carion granite (Meert et al., 2001). The “Schisto-Quartzo-Calcaire” known also as the Proterozoic Itremo group occupies the southwestern part of the Antananarivo block (Bésairie, 1964; Moine, 1968, 1974) and is interpreted as a shelf sequence that has been intruded by granitoids possibly associated with a supra subduction zone setting (Handke et al., 1999) around 800 Ma (Collins et al., 2003c; Handke et al., 1999; Tucker et al., 2007). The Itremo group metasediments were proposed to have an East Africa origin and were deposited at ~1700–1500 Ma (Cox et al., 1998, 2004; Fitzsimons and Hulscher, 2005) then deformed into large-scale recumbent isoclinal folds (Collins et al., 2003a; Tucker et al., 2007); (4) the Tsaratanana thrust sheet, composed of three allochthonous greenstone belts (de Wit, 2003), including the Maevatanana, Andriamena and Beforona–Alaotra belts; it is characterized by mafic gneiss, tonalities, metapelites, and ultramafic rocks partially affected by a 2.5 Ga ultra-high temperatures metamorphism (Goncalves et al., 2004; Nicollet, 1990; Paquette et al., 2004) and; (5) the Proterezoic Bekily block, which is dominated by the granulite, sillimanite–cordierite– garnet paragneisses, quartzite, charnockite and marble (Aurouze, 1953; de Wit et al., 2001; Kröner and Sassis, 1996; Kröner et al., 1999; Noizet, 1966; Windley et al., 1994) (Fig. 1), composed mainly of the granulite and amphibolite facies rocks of the Androyan system. In the simplified geological map of Madagascar (Fig. 1) the Tsaratanana thrust sheet is included in the Antananarivo block. The ASZ lies within the Antananarivo block, and is known as a major Neoproterozoic–Cambrian structure that runs N–S along the island (Grégoire et al., 2009; Raharimahefa and Kusky, 2010; Ralison and Nédélec, 1997; Windley et al., 1994). The ASZ is characterized by steeply-dipping N–S striking foliation and shallowly plunging lineation resulting from deformation during low-pressure granulitic conditions of 790 °C and 3.3 kbar (Nédélec et al., 2000; Ralison, 1998; Ralison and Nédélec, 1997) that overprints the Angavo–Nondiana belt of central Madagascar (Fig. 1). The southern part of the ASZ is known as the Ifanadiana Shear Zone (Martelat et al., 1999, 2000) and is characterized by two phases of deformation resulting from an east–west horizontal shortening. 3. Major lithological units The Precambrian basement lithologies of Madagascar were divided by previous workers into different systems, groups, or domains (Bésairie, 1964, 1968-71; BGS-USGS-GLW, 2008; Hottin, 1976; Jourde, 1971). From west to east, the basement units exposed in the study area consist mainly of: Ambatolaona migmatitic granitic gneisses (part of the Neoproterozoic Angavo–Ankazobe migmatitic granites and granitoid gneiss complex), Moramanga orthogneiss, migmatitic gneisses of Neoarchean Vondrozo group, Andasibe paragneiss,

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migmatitic granitoids of the Neoproterozoic Brickaville orthogneiss, and Neoarchean Beforona amphibolite bearing gneiss and migmatites. The units are intruded by various igneous intrusions and are metamorphosed under amphibolite to granulite facies conditions (Figs. 1, 2) (Bésairie, 1968, 1969; Grégoire et al., 2009; Kusky and Pitfield, 2006; Nédélec et al, 1995; Raharimahefa and Kusky, 2010; Rantoanina, 1962). 3.1. Ambatolaona migmatitic granitic gneisses Migmatite granitic gneiss occupies the western margin of the study area. The main granitoid-gneiss unit has a uniform texture of thick leucocratic bands (>5 cm) interlayered with very thin muscovite– biotite–hornblende layers (b2 mm). The felsic layers are composed mainly of quartz, K-feldspar, plagioclase and muscovite with rare sillimanite, in contrast the mafic layers which contain hornblende, muscovite, biotite, garnet and rarely pyroxene. Gneiss schlieren and K-feldspar porphyroclast are also often present within the migmatite granitic gneiss. 3.2. Orthogneiss The orthogneiss (Moramanga orthogneiss) is exposed in the central part of the study area and it consists mainly of foliated and augen medium- to coarse-grained biotite gneiss. The compositional layering of the orthogneiss is well-defined by the felsic and mafic layers, with leucosome composed of quartz, plagioclase, with minor amounts of K-feldspar, biotite, amphibole and locally pyroxene (both opx and cpx); the melanosome consists of amphibole, chlorite, biotite and plagioclase with lesser quartz, K-feldspar with rare hornblende, pyroxene, sphene and opaque minerals. The orthogneisses vary from granodioritic to tonalitic in composition and are locally charnockitized and grains of up to 0.5 cm long orthopyroxene are visible. 3.3. Migmatitic gneiss The N–S striking belt of migmatitic gneiss is exposed also in the central part of the study area and along the road from Moramanga to Anosibe An'Ala. The migmatitic gneiss (Vondrozo Group) mainly consists of migmatitic tonalitic gneiss, migmatitic quartzo-feldspathic gneiss and biotite–sillimanite bearing paragneiss intercalates with lenses of quartzite. It is composed of quartz, plagioclase, K-feldspar, biotite, amphibole (hornblende) and few garnets with pyrite and pyroxene as accessory minerals. Depending of the location, biotite– hornblende–clinopyroxene bearing migmatitic gneiss and quartzofeldspathic migmatitic gneiss can be distinguished within the migmatitic orthogneisses, which also contain some charnockites. The sillimanite bearing paragneiss is composed of quartz, plagioclase, biotite, sillimanite and locally garnet and graphite. The mineral assemblage of the coarsegrained quartzite is mainly quartz with lesser K-feldspar and biotite. 3.4. Migmatitic granitoids A partially melted weakly to intensely foliated coarse-grained granitoid (part of Brickaville orthogneiss) occupies the central-eastern and south-eastern part of the study area and it is composed of quartz, K-feldspar and plagioclase, with minor amounts of garnet, amphibole, biotite and opaque minerals with accessory titanite, apatite and magnetite. In some locations, microcline augens are common. 3.5. Paragneiss Fine to medium-grained foliated paragneiss (known also as Andasibe paragneiss) is exposed in the eastern part of the mapped area and comprises biotite±hornblende gneiss, migmatitic quartzofeldspathic gneiss, sillimanite-bearing gneiss, garnet-bearing gneiss,

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graphitic gneiss intercalates with schist, quartzite, muscovite-bearing gneiss and marble. It contains also deformed ultramafic pods and exhibits centimeter to a meter gneissic bandings composed of plagioclase–biotite–quartz gneiss layers and quartz–plagioclase amphibolite layers with garnet; the main accessory minerals are zircon, apatite and monazite. Graphitic gneiss is sporadically exposed within the paragneiss and is intercalated with graphite-bearing quartzite and biotite–sillimanite gneiss. 3.6. Amphibole-bearing gneiss Amphibole-bearing gneiss (part of the Beforona group) is exposed in the eastern margin of the study area and is associated with amphibolite and pyroxenite as well as magnetite-bearing quartzite and hornblende–biotite–garnet migmatite. The main gneiss unit is composed of plagioclase, hornblende, biotite, and quartz; accessory minerals are pyroxene and titanite. 3.7. Younger intrusions Beside the common quartz and pegmatite veins, the basement units of the ASZ and its adjacent region are intruded by various intrusions including massive pre-kinematic (relative to the ASZ deformation), syn-kinematic layered, and post-collisional granitic and gabbroic intrusions. Pre-tectonic granitoids are widespread within the area, those include the 825 M–720 Ma granitoids (Kröner et al., 1999, 2000; Raharimahefa and Kusky, 2010) that intrude the Ambatolaona migmatitic granitic gneisses; similar intrusions are located farther north in the northern part of the ASZ (outside the map area) and have been dated at 756.6 ± 4 Ma (Raharimahefa and Kusky, 2010). The most predominant pre-tectonic granites were identified: (1) a deformed granite intruded the Andasibe paragneiss and is composed of plagioclase, quartz, K-feldspar (perthitic) and minor phlogopite, biotite and amphibole. Accessory minerals include zircon, apatite, magnetite, and titanite. Numerous small pegmatite veins (b0.2 m) are common. In some localities the granite is characterized by reddishbrown biotite, muscovite, and hornblende. Sillimanite occurs as mats in muscovite; (2) a foliated medium-coarse-grained gneissic biotite rich granite with gneissic layering and alkali feldspar augen intruded the Moramanga orthogneiss. It is composed mainly of microcline, orthoclase, quartz, plagioclase, and biotite with secondary epidote. Plagioclase and quartz are locally in myrmekitic intergrowth; (3) intrusions of granitic gneiss ranging in composition from charnockitic to granodioritic within the Brickaville orthogneiss and the amphibolebearing gneiss of Beforona, and when present orthopyroxene is small and usually associated with amphibole and garnet. Along its margins, the ASZ is marked by potassic and ultra-potassic intrusions, the so-called stratoid granites (Emberger, 1958; Nédélec et al., 1994); the ~ 630 Ma granites (Paquette and Nédélec, 1998) with meter to kilometric-scale thicknesses are interlayered with migmatitic gneiss and amphibolite. Most of the granites dip gently west, and are petrographically divided into alkaline granite, and biotite–hornblende granodiorites associated with monzogranites and pyroxene-bearing granitoids (Nédélec et al., 1994). The syn-tectonic intrusions within the ASZ include the ~ 550 Ma granite (Raharimahefa and Kusky, 2010), which is intruded parallel to the main foliation of the Angavo migmatitic granitic gneiss host rock. The granite is mostly equigranular and medium-grained and contains zircon and monazite as accessory minerals. The most apparent post-tectonic intrusion within the mapped area is the mafic–ultramafic and felsic-alkaline suites of the ~ 90 Ma Ambatovy–Antapombato complex (Melluso et al., 2005). The dominant lithological units within the complex are gabbros, dunite, peridotite and syenite.

4. Structural features of the Angavo Shear Zone Three generations of deformational fabrics have been recognized in the ASZ. These represent the products of discrete deformational events (D1, D2 and D3). We focus only on the ductile D1 and D2 deformation because D3 structures result from neotectonic faulting which characterizes the Angavo rift (or Ankay); a younger event that overprinted structures of the ASZ, resulting a steep brittle to brittle– ductile deformation (Kusky et al., 2007, 2010; Laville et al., 1998). Our analysis is based on detailed structural analysis including field observations of cross-cutting relations, measurements of structural fabrics and microstructural analysis of thin-sections that has been carried out within the ASZ and surrounding region. Thirty three (33) well-exposed locations were chosen in an effort to characterize the structural style of the ASZ (Fig. 3). The ASZ is a zone of anastomosing highly strained north-striking gneissic to mylonitic rocks (Figs. 2, 3, 5A, B, D) with intense foliation development. These high strain zones locally wrap around granitic and gabbroic intrusions and form the anastomosing Angavo Shear Zone, with individual mylonite zones ranging from a meter to several kilometers in width. In contrast, the low-strain zones are typically developed in the pre-tectonic plutons with few developed in syntectonic granites; these zones are typically weakly to moderately foliated and lineated. Shear zone fabrics and kinematics recognized during the detailed structural analysis are used to identify the limit of the ASZ. The attitude of foliations, lineations and other fabrics were analyzed using lower hemisphere equal area projections (Lisle and Leyshon, 2004; Marshak and Mitra, 1998; Ramsay and Huber, 1987). 4.1. Deformational phase D1 D1 fabrics are widely-developed in the study area and can be seen in both low and high strain zones. D1 is responsible for the development of an S1 foliation, L1 lineation, symmetric boudins and F1 isoclinal and recumbent folds. 4.1.1. Meso and macroscopic structure 4.1.1.1. Foliation S1. The most widely developed planar fabric is a penetrative foliation S1 within layering of mafic and felsic layers. Foliations are very closely spaced (Figs. 4A, B, C, D, G, J, 7A) and generally strike NNW–SSE to NE–SW with moderate to steep dip angles (Fig. 3) with some outcrops exhibiting shallow dips. The S1 foliation is defined by aligned and flattened minerals including amphibole, biotite, K-feldspar, hornblende, and quartz that are parallel to the compositional layering or gneissosity (G1) in the gneisses (Fig. 4D). Generally, the primary planar surfaces S0 cannot be identified unequivocally, consequently compositional layering of leucosome and melanosome banding are treated as S1; however, a few locations still exhibit S0 (Fig. 4F). S1 is axial planar in relation to F1 (Fig. 4C). Sillimanite bearing rocks are present and the sillimanite grains are aligned parallel to the S1 N–S regional foliation. 4.1.1.2. Lineation L1. L1 flattening/stretching and mineral lineations (including in L tectonite and SL tectonite) are common along the ASZ. L1 plunges mainly to the SSW with a subhorizontal angle (Fig. 3) and is generally formed by elongated feldspar and platy quartz. However, depending on the lithology these mineral lineations are also defined by alignment of hornblende, amphibole, and biotite. 4.1.1.3. Fold F1. The earliest recognized F1 folds occur as tight symmetric (Fig. 4C) to isoclinal folds (Fig. 4G) and possess an axial planar S1 foliation. Folding of bedding is only recognized in weathered paragneiss where it forms recumbent folds (Fig. 4F).

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Fig. 4. D1 related fabrics. A: Symmetric boudins formed by stretched pegmatite veins within a weathered foliated gneiss, the long axes of the boudins are parallel to the S1 foliation (XZ plane view). B: Symmetric boudins with well-defined pinch and swell in amphibolite, boudins long axes are parallel to the S1 foliation. C: Symmetric tight fold in migmatitic granitoid (XZ plane view). D: Symmetric mantle quartz porphyroclast in gneissic mylonite (XZ plane view). E: Symmetric boudin-like quartz–feldspar porphyroclast aggregates in granitic gneiss (XZ plane view). F: Recumbent fold in highly weathered paragneiss seen in a subvertical outcrop (XZ plane view). G: Intra-folial isoclinal folds in granitic mylonite, with shallow plunging folds axes (XZ plane view). H: A nearly symmetric boudins within a strongly foliated muscovite–biotite rich shear zone. I: Symmetric and elongated porphyroclast of quartz and feldspar in mica rich quartz mylonite, suture grain boundary is seen on some of the grains (cut parallel to lineation and perpendicular to foliation). J: Symmetric and elongated quartz, plagioclase and feldspar in micaschist, some of the quartz grain exhibit subgrain (cut parallel to lineation and perpendicular to foliation). K: Symmetric porphyroclast of feldspar in quartz mylonite (cut parallel to lineation and perpendicular to foliation).

Based on the geometry, orientation of hinge lines and axial planes, the relationships between planar surfaces and the nature of overprinting, the folds are classified into many styles and attitude groups including recumbent folds, open to tight symmetric folds (Fig. 4C), and isoclinal and rootless intrafolial folds (Fig. 4G). D1 also produced outcrop-scale symmetric porphyroclasts (Fig. 4D, E) and symmetric boudins (Fig. 4A, B, H) that are flattened parallel to S1.

4.1.2. Microscopic structure In thin-section, D1 fabrics are developed and preserved, these fabrics include symmetrical porphyroclasts and weak pressure shadows (Fig. 4I, J, K). Quartz grains are flattened parallel to S1 (Fig. 4I, J) and feldspar porphyroclasts are nearly symmetric. Subgrains are locally developed within strained quartz grains, and feldspar grains exhibit sutured grain boundaries. Within quartz rich micaschist some of the

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quartz grains are stretched into cigar shapes (Fig. 4I, J). S1 is generally defined as a continuous fine foliation and is characterized by a lepidoblastic structure with flattened domains of feldspar, quartz, and plagioclase separated by micaceous bands (Fig. 4I, J, K). Strong grain shape-preferred orientation (Fig. 4I, J) of flattened porphyroclasts of feldspar and quartz are also present in thin-section (XZ plane). 4.2. Deformational phase D2 D2 fabrics are locally developed and preserved in the anastomosing high-strain zones that form the strained core of the ASZ. The D2 shear zones are characterized by decreasing strain from the center towards the periphery and by the development of asymmetric shear sense indicators. 4.2.1. Meso and macroscopic structure 4.2.1.1. Foliation S2. In eastern and southern parts of the study area (Fig. 3), a penetrative foliation S2 is preserved in many outcrops. In several locations S2 is identified as axial planar (Fig. 5E) to the folding of S1 (i.e. F2), and in some outcrops both foliations become parallel. The tectonic layering developed in granitic, gneissic and metasedimentary rocks comprises alternating fine-scale leucosome and melanosome layering (Fig. 5A, C, E, H, L, M). Some layers are remylonitized through the successive development of stronglyattenuated isoclinal folds (Fig. 5B, J). Accordingly, isoclinal, sheath and rootless F2 folds are preserved in various stages of development (Figs. 5B, J, 6A, B). The S2 foliation strikes generally between NS to NW–SE with moderate to steep dips (Fig. 3) and is defined by alignment of biotite, quartz and sillimanite, with mineral elongations (L2) measurable in outcrop. 4.2.1.2. Lineation L2. L2 stretching mineral lineations mainly trend to the south (mean average N185) with gentle plunges (Figs. 3, 8). Depending on the locality and the lithology, the L2 lineation is formed by rotated and elongated quartz, biotite fish, amphibole fish, alignment of rotated winged feldspar and quartz/feldspar rods indicating a dextral thrusting movement. In sillimanite bearing rocks, where S1 is parallel to S2, the L2 is formed by aligned sillimanite. 4.2.1.3. Fold F2. The D2 event is responsible for the formation of the second generation of folding (F2) and is best visible in locations 17, 18, 21, 29, and 32 (Fig. 3). During D2, the S1 foliation is folded into F2 folds (Fig. 5B, E, K). In some places, the determination of the ASZ as a D2 structure is based on overprinting relationships observed along the margin of the shear zone and the evolution of these fabrics with increased shear strain. In gneisses, the G1 gneissosity is re-oriented by a second and late warping of G2 (Fig. 6C). F1 is overprinted by F2 (Fig. 6B) and both F1 and F2 are sub-parallel to S2 and granitic veins. In strongly foliated gneiss, felsic quartz–plagioclase layers alternate with biotite–hornblende layers. In the felsic layers, large quartz–plagioclase porphyroclasts define augen (Fig. 6A). The discordant layer in Fig. 6A suggests that G2 has an axial planar relationship to F2. 4.2.1.4. Kinematic indicators. Shear sense indicators associated with the D2 event within the ASZ are present at both regional (Fig. 3) and outcrop scales (Fig. 5). These include asymmetric boudins (Fig. 5B, C, N), sheath folds preserved in YZ planes (Fig. 5F), asymmetric and sigmoidal quartz aggregates (Fig. 5G), asymmetric and winged porphyroclasts

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(Fig. 5H, L), asymmetric Z folds (Fig. 5J), and changes of foliation orientations in shear zones (Fig. 5I). Within the ASZ, D2 related asymmetric boudins exhibit a dextral sense of movement and are characterized by foliation-oblique boudin trains (Fig. 5C, N) in which the S1 foliation is preserved in the sigmoidal lenses of amphibolite and reoriented and deformed by S2 in the wall rocks of granitic gneiss. Sigma and delta type porphyroclast systems are the most common shear sense indicators in the ASZ, and exhibit right lateral movement (Fig. 5G, H, L, M). In many cases it is difficult to distinguish sigma from delta indicators showing opposite senses of shearing, due to the recrystallized nature of minerals and high intensity of metamorphism in the area. Only the most-clear examples were used. Delta-type mantled porphyroclasts have been rotated and provide evidence of a dextral sense of shear (Fig. 5L). Hornblende clasts are rotated clockwise rotational movement within a slightly recrystallized matrix (Fig. 5L). Equally, sigma type mantled objects (Fig. 5M) are common, with non-rotated amphibole in the amphibolite gneiss. Asymmetries in feldspar porphyroclast tails (Fig. 5H) are also good kinematic indicators. These winged porphyroclast indicate west over east thrusting, with a dextral oblique slip component. Asymmetric folds (Fig. 5B, E, J, K) observed on the XZ plane of the strain ellipsoid, form Z shaped folds indicating a dextral sense of movement. Fig. 5J indicates an F1 fold of an offset pegmatite vein redeformed by a dextral shearing. Sheath folds classified to be a cat's eye-fold or type C fold (Alsop and Holdsworth, 2006; Cobbold and Quinquis, 1980; Minigh, 1979) are observed in the ASZ (Fig. 5F), when observed in the YZ plane, perpendicular to the sense of shear, and the folds plunge sub-horizontally. The right lateral movement indicators are the most dominant at the outcrop scale (Fig. 5). However, sinistral movement and dextral movement indicators are observed together in a few localities and treated as conjugate sets, perhaps indicating dominantly pure shear. 4.2.2. Microscopic structure In thin sections and chips cut parallel to the lineation and perpendicular to the foliation, quartz and biotite pressure shadows around feldspar and shear bands indicate dextral shear senses (Fig. 7A). Many of the porphyroclasts show asymmetric delta shapes and exhibit up to 1 cm elongated tails at both ends. Quartz–biotite pressure shadows are typically adjacent to feldspar and quartz porphyroclast, and they are relatively small (~0.15 mm). Elongated grains of quartz and feldspar (Fig. 7A, B, H) are typical for mylonites in the ASZ. Quartz grains deformed during the deformation exhibit a right lateral movement, the same as determined from the K-feldspar. Mylonites of the ASZ contain asymmetric lens-shaped grains of mica and mica fish also indicate a dextral sense of shear (Fig. 7C). S–C fabrics are present both at the field and thin section scales in deformed granite and gneiss units. In thin section the S direction is represented by the mineral foliation, and the C fabric appears as spaced foliation planes or shear bands associated with grain size reduction and exhibit a dextral sense of movement along the shear plane C (Fig. 7B). The mineral assemblage of quartz, hypersthene, plagioclase, biotite, K-feldspar and sillimanite (±garnet) are typical in addition to the presence of amphibole (hornblende), K-feldspar, quartz and sillimanite visible at the outcrop scale. The structure of minerals shows lobate grain boundaries (Fig. 7D) in the mylonitic gneisses. Different kinds of myrmekites are present in the granitic gneiss samples from the ASZ, including the regular vermicular form resulting

Fig. 5. Field photographs of D2 fabrics, A: closely spaced foliation in granitic gneiss, B: Z-shaped folds and deformed pegmatite veins showing a dextral sense of movement in migmatitic granitoid, C: Shear band oblique boudins in gneiss showing internal fabric S1 and external fabric S2; D: Amphibolite boudins in gneiss, E: Folding of S1 with S2 in gneiss, F: Sheath fold seen in its YZ plane in gneiss, G: Elongated and sigmoidal quartz granitic gneiss (XZ plane view), H: feldspar fish derived from a dextral movement deformed granite (XZ plane view), I: small scale dextral shear zone showing a changing orientation of foliation S1 in migmatitic granitoid (XZ plane view), J: Rootless fold (felsic) offset by right lateral movement in amphibolitic gneiss (XZ plane view), K: Asymmetric F2 fold in coarse-grained migmatitic granite, L: Delta type kinematic indicator in migmatitic granitic gneiss (XZ plane view), M: sigma-type fabric of dextral sense of shear in paragneiss (XZ plane view), N: Foliation parallel boudins in orthogneiss (XZ plane view).

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from changing plagioclase to K-feldspar, and those resulting from dynamic recrystallization during deformation (Fig. 7E). The presence of subgrains or chessboard texture in quartz (Fig. 7F) is also common as well as grain boundaries and mosaic texture in mylonitic rocks. In some samples parallelism of boundaries between elongated quartz grains and feldspar (Fig. 7B) helps define the foliation. Grain boundary migration (Fig. 7G) is present in thin-section and suggests hightemperature deformation with dynamic recrystallization (dragging microstructure). Within quartz–mylonite, quartz ribbons are wrapped around feldspar porphyroclasts with a strog shape-preferred orientation (Fig. 7H). This is a much lower-temperature fabric than that in Fig. 7G and other sections, and may be related to progressively cooler conditions during deformation, perhaps related to exhumation.

5. Regional deformation Dividing the study area into large structural domains of internally similar structure helps to understand more about the regional strain pattern. Four structural domains (Fig. 3), named the Ambatoloana shear zone (Group I), Moramanga (Group II), Anosibe An'Ala (Group III) and Anevoka (Group IV) are defined based on their structural patterns, deformation history, and location (see also Collins et al., 2003b for a division of this region into four domains). The Ambatoloana domain marks the western edge of the ASZ, while the easternmost Anevoka domain marks its eastern edge where D2 fabrics are still locally spotted. However, Moramanga and Anosibe An'Ala domains contain the most prominent D2 fabrics of the ASZ; the two domains are

differentiated by the intrusion of the Cretaceous Ambatovy complex into the Moramanga domain. 5.1. Ambatoloana (Group I) In this domain both S1 and S2 foliations are preserved. S1 foliation is the most dominant and its strike varies from SW–NE to NNW–SSE with the majority of strikes being N–S with moderate dips to the west, associated with low angle lineations plunging to the SSW (Fig. 8). The S2 foliation strikes N–S and dips steeply with a subhorizontal stretching lineation (Fig. 3). Fold axes in the Ambatoloana zone (Fig. 8) plunge shallowly to the west (beta axis of 32-285). L, S > L, and S tectonites are common in this section. The majority of observed rotated quartz and feldspar porphyroclasts and extensional shear bands indicate a west over east thrusting, with a dextral oblique slip component. 5.2. Moramanga (Group II) Subhorizontal L1 lineations which are parallel to the beta axis of the girdle of the S1 foliation characterize the shear zone segment located north of Moramanga. A large recumbent fold is interpreted to exist in this location with a fold axis plunging horizontally and trending to the south. The plunge of the fold axis is consistent with the subhorizontal quartz and feldspar stretching L1 lineation that trends to ~ N190 (Fig. 8). Asymmetric folds with axial planes parallel to the S2 foliation are produced by oblique shortening followed by a dextral subhorizontal

Fig. 6. Drawing of photographs of gneissosity and fold (F1, F2) relationships. A: A vertical outcrop showing the intra foliation F2. B: Refolding of F1 by F2. C: First gneissosity G1 oriented by the second gneissosity G2. Blue dot is contour of Malagasy coin of 10 Ariary (~2.5 cm in diameter) used as scale.

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Fig. 7. Microphotographs of strained rocks from the ASZ. A: Polished chip of granitic gneiss mylonite with sigma and delta porphyroclasts showing dextral movement. B: S–C structure showing dextral shear sense in C plane (in deformed granite), note the elongated quartz and feldspar (cut parallel to lineation and perpendicular to foliation). C: Mica-fish and porphyroclasts, showing delta types shear sense indicators in gneiss (cut parallel to lineation and perpendicular to foliation). D: Lobate grain boundaries in quartz characteristic of high temperature deformation in deformed granitoid. E: Myrmekite replacing microcline in ASZ pre-tectonic granite. F: Chessboard structure in quartz in deformed granitoid. G: Grain boundary migration between quartz and pyroxene during dynamic recrystallization in granulite. H: Quartz ribbons showing bulging deformation mechanism indicative of lower temperature deformation (~400 C). Quartz ribbons surround feldspar porphyroclast in quartzo-feldspathic mylonite (cut parallel to lineation and perpendicular to foliation).

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Fig. 8. Stereonet plots of foliations and lineations of the four main domains. Stretching lineation: □ L1, ■ L2; black bold cross: beta axis or fold axis, black dash: calculated girdle of foliation planes.

oblique sense of movement. The N–S foliation (S2) is interpreted to be reactivated by current active tectonics in the area (Kusky et al., 2007). The D2 event is strongly preserved in this area. F2 axial planes are parallel to the foliation (Figs. 5G, 6C) and F2 fold axes are in many places parallel to the L2 stretching lineation, they are probably a-type folds (Kelly et al., 2000; Malavieille, 1987; Mattauer, 1975; Mattauer et al., 1983; Wang et al., 2005) formed during west over east dextral oblique D2 shearing. 5.3. Anosibe An'Ala (Group III) In this part of the ASZ, the D2 west over east dextral oblique shearing is prominent at the outcrop scale and the S2 foliation strikes N–S almost

parallel to S1 (Fig. 3). The L2 stretching lineation is also subparallel to the L1 lineation (Figs. 3, 8) with some exceptions of north plunging subhorizontal L2. The prominent L2 (~33-N003) mineral elongation and stretching lineations are defined by biotite, amphibole and hornblende flakes arranged within the F2 fold axial surfaces. F2 deformed the earlier S1 foliation which steepened the dip of S1 (Fig. 3; locations 20 and 21). In this domain, the same as other strongly strained zones along the ASZ, sheath folds (Fig. 5F) with NS trending axes are preserved. When rocks are subjected to shear, layers in the rock commonly form asymmetric folds whose sense of asymmetry reflects the sense of shear. These folds result from velocity gradients in the shear

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zone, and in the Anosibe An'Ala domain they have a mean axis plunging to the south with a subhorizontal angle. These folds are non-cylindrical and asymmetric. Their axial planar surface tends to be parallel to the shearing plane and characterized by tube-shaped folds with an elliptical or even a circular section (Alsop and Holdsworth, 2006; Carreras et al., 1977; Cobbold and Quinquis, 1980). They developed with their a-axis parallel to the direction of shearing. 5.4. Anevoka (Group IV) In the Anevoka area (eastern), the D2 west over east dextral oblique shearing is responsible for the folding of the S1 fabrics (Fig. 5E). The S2 foliation strikes NNE–SSW with L2 stretching lineations trending N to NE. The prominent L2 NE–SW mineral elongation and stretching lineations are defined by amphibole and biotite flakes. Inclined and isoclinal F2 folds deformed the earlier S1 foliation under an east–west contractional regime. Folding of S1 foliations also define large recumbent folds characterized by subhorizontal beta axes (Fig. 8), which are parallel with the south trending L1 lineation. Since F2 axial planes are not parallel to the foliation and F2 fold axes are parallel to the L1 stretching lineation, they probably produced isoclinal and reclined folds during D2 shearing associated with a down to the east displacement. Locally, sheath folds have formed with NE– SW trending axes, parallel to L2 and consistent with the S–C structures (Fig. 7B) and other shear sense indicators in the area. The eastern boundary of the ASZ is located in this area, however, the ASZ boundary in Fig. 2 shows only the general boundary of the ASZ since mylonitic rocks affected by the same deformation of the ASZ are still located to the east of the general boundary (location 32, Fig. 3), confirming the anastomosing structure of the ASZ. The boundary is drawn where the deformation related to the ASZ appears to be a small component of the outcrop. 6. Discussion 6.1. Structural styles and deformations Structural analysis reveals that the development of the anastomosing Angavo Shear Zone is characterized by at least two ductile deformational episodes D1 and D2. D1 fabrics are widely distributed across the ASZ and are locally overprinted by D2 deformation, which resulted in transposition of the S1 fabric into the prominent S2 within the anastomosing high-strain zones. The attitude of foliations varies and folds are well-developed (Figs. 3–5, 8). Lineations developed on the foliation surface dominantly plunge shallowly to the south–south-west (SSW). Characteristics typical of ductile shearing are indicated by the intensity of deformation, decreasing from the center towards the periphery and the location and transition of augen gneiss into hornblende–biotite gneiss (Fig. 5A–B–D). The early penetrative D1 deformation involved a prominent component of east–west shortening that resulted in symmetric structural fabrics such as symmetric boudin trains, symmetric porphyroclasts, and symmetric and isoclinal folds (F1). These symmetrical fabrics suggest a coaxial deformation with a constrictional component. The rootless intrafolial folds within the S1 foliation are also interpreted as a result of the D1 pure shear developed during a predominantly flattening deformation. In the western part of the study area, the D1 event is well preserved in the Antananarivo block, where east-vergent kilometer-scale recumbent folds are recognized. In the eastern blocks, the initial geometry of the D1 event is little disturbed by the later D2 tectonic event. The early D1 flat-lying foliation strikes N–S to NNE–SSW, whereas the L1 stretching and mineral lineations, for instance in domains 1, 2 and 3 (Fig. 3) trend SSW with subhorizontal plunges. L1 lineations and associated D1 kinematic indicators are well preserved along the central part of

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the study area (Lat~ 48 10 E) despite the steepening of the foliation and rotation due to the D2 event. L1 and L2 stretching lineations with moderate to steep plunges are restricted to a narrow zone not mappable at our map scale, but can be located in locations 11, 15 18, 29, and 32 (Fig. 3). In thin sections cut perpendicular to the foliation plane and parallel to lineation flattened porphyroclast and grain aggregates with symmetric tails also indicate pure shear (flattening). This D1 flattening is interpreted to be the result of the initial assemblage of the Gondwana supercontinent where considerable thickening took place. The Masora/Antongil cratons may have acted as stiff block causing rock particles to undergo orogen parallel flow (Fritz et al., in press). D2 fabrics are the most prominent indicator of strong shearing within the ASZ. This event is characterized by structural fabrics such as: strong S2 foliation; mineral and stretching L2 lineations and F2 folds. The kinematic indicators associated with D2 exhibit a dextral sense of movement and subhorizontal to moderate oblique dip slip motion; these features consist mainly of sigma and delta porphyroclast systems (Figs. 5G, H, L, M, 7A), sheath fold (Fig. 5F), asymmetric boudins (Fig. 5C, N), and S–C fabrics. The consistent orientation of the fabric in the western part of the study area suggests that the orientation of shear did not change significantly (mean of foliation N003 33W) during shearing. The augen in the mylonites, asymmetric pressure shadows and muscovite rich recrystallization tails indicate that the sense of shear is west over east dextral thrusting, and to the north a dextral oblique normal slip system, which is consistent with the west side up sense of movement derived from S–C fabrics in gneiss. In the western part of the ASZ, poles to foliation from the strained gneisses and granitic rocks in the Ambatoloana shear zone (Angavo escarpment) define a weak girdle pattern, consistent with folding about a shallowly west and southwest plunging fold axis (Figs. 3, 8). The lineation is almost parallel to the fold axis, defining a transpressive shortening. These asymmetries of D2 fabrics point to a non-coaxial flow regime for the D2 deformation, which might be associated with the late phase of the amalgamation between Antananarivo block and Masora/ Antongil cratons. Such simple shear dominated transpression is known to cover the field of small and cold orogens due to dominant lateral displacement (Fritz et al, in press). The metamorphic grade of the ASZ is only constrained by the mineral composition and deformation textures. The presence of amphibole, biotite, hornblende, garnet, sillimanite and pyroxene in the assemblage indicates amphibolite to granulite facies conditions. The ductile nature of quartz deformation combined with recrystallization especially of the feldspar into microcline and the occurrence of myrmekite in feldspars suggest that shearing may have taken place at low to medium grade conditions of 400–500 °C (Passchier and Trouw, 2005). However, high temperature conditions of the deformation in the ASZ could be constrained by the presence of subgrains or chessboard texture in quartz (Fig. 7G), a texture very common in granulite (Martelat et al., 1999; Masberg et al., 1992) and in deformed granites (Blumenfeld et al., 1986; Vernon, 2000), by lobate grain boundaries (Krabbendam et al., 2003; Tullis et al., 1973; Urai et al., 1986; White et al., 1980), and intergrowths of quartz and feldspar. The straight grain boundaries and mosaic texture confirm the high intensity of deformation in mylonitic rocks. These structures formed during plastic deformation (Hippertt et al., 2001) through the coalescence of scattered quartz grains associated with enhanced grain boundary mobility in high-grade metamorphic rocks. Diffusion creep features at temperatures of up to 650 °C (Gower and Simpson, 1992) are represented by parallelism of boundaries between elongated quartz grains and feldspar (Fig. 7A) within the foliation. This structure may enhance the partial melting process, which is ubiquitous in orthogneisses as interstitial intergrowths of quartz and feldspar (Dell'Angelo et al., 1987). Evidence of high grade conditions of deformation is also preserved in quartz (Fig. 7F). Elongated quartz forming ribbons and dextral

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sigmoidal minerals were probably formed by dislocation and diffusion creep (Lafrance and Scoates, 1996; Olesen, 1998). This deformation probably continued to lower temperature conditions, as indicated by the dislocation glide mechanisms and bulging visible in the quartz in Fig. 7H. 6.2. Implications for Gondwana Assembly The amalgamation of eastern and western Gondwana formed orogenic belts in Africa, Madagascar, Antarctica and India, and the ASZ is located within one of these Precambrian belts. There is considerable controversy concerning reconstructions of Gondwana and linkages of specific shear zones that are now on different continents (Collins, 2006; Collins et al., in press; Fritz et al., in press; Ito et al., 1997; Kusky et al., 2003; Lawver et al., 1998; Manish et al., 2005; Martelat et al., 1999; Plavsa et al., 2012; Sacks et al., 1997; Windley et al., 1994). Madagascar is located in the heart of the Gondwana supercontinent in the East African Orogen (EAO, Stern, 1994), a large orogenic belt continuing from East Africa to southern Africa and extending to East Antarctica. Reconstruction of the Precambrian crust of Madagascar, East Africa (Somalia?) and India relies on similar-aged deformation and intrusions, structural styles and metamorphism, lithologies, mineralization and paleomagnetism of suture zones. That assembly of different continents would be improved via the ASZ if its continuation is found in India and Africa. In contrast, debates concerning the India–Madagascar shear zone connection are controversial. Some researchers proposed that the Ranotsara Shear Zone in southern Madagascar is the continuation of the Achankovil Shear Zone (AKSZ) of southern India (Paquette et al., 1994; Ramakrishnan, 1991; Windley et al., 1994). Others disagreed

and proposed that the AKSZ may continue to the Central Granites– Gneiss–Migmatites Belt (CGGMB) or probably other shear zones (Rajesh and Chetty, 2006; Rambeloson et al., 2003; Sacks et al., 1997, 1998). Our structural data and previous isotopic age data from the ASZ (Raharimahefa and Kusky, 2010) suggest that the most likely candidate for the AKSZ is the Angavo Shear Zone (Fig. 9). Both shear zones have many similarities including similar sense of movement, sequence and timing of deformation events, and rock units. The AKSZ is characterized by sub-horizontal lineations, porphyroclasts and asymmetric structures that are remarkably similar to those seen within the ASZ. Lithologies within the AKSZ are mainly gneisses, migmatites, granulites and charnockites as well as ultramafic rocks relatively similar to those exposed within the Angavo–Nondiana belt of the Antananarivo block. Rajesh and Chetty (2006) reported an initial ductile dextral deformation (D2) of the AKSZ which may fit with the dextral sense determined for the ASZ. The AKSZ yields a post-tectonic granite age of ~540–560 Ma (Santosh et al., 2005). However, the timing of deformation within the ASZ was reported by Raharimahefa and Kusky (2010) in which a LA–MC–ICP–MS zircon age of 549.8 ±8 Ma (weighted mean 206Pb/238U age of 549.8 ± 4 Ma) for a syntectonic granite from a location south west of the Ambatoloana village (Fig. 3) was interpreted as the age of the D1 event. Kröner et al. (1999) also dated a postkinematic (?) alkali granite from near Antananarivo as 556 ± 1.7 Ma (zircon 207Pb/ 206Pb evaporation age), and interpreted the age as the probable younger age limit for granulite metamorphism in the Antananarivo block. In addition Paquette and Nédélec (1998) reported a 561± 4 Ma (U–Pb zircon) of a granitic dyke south of Antananarivo. Martelat et al. (2000) argued that two events can be identified in southern Madagascar (Ifanadiana Shear zone) including a D1 age of ca ~590–530 Ma, and D2 ca ~530-500 Ma. Hence, shearing along the ASZ took place during the East African Orogeny and prior to 540 Ma, which is coeval with the activation of AKSZ in India. The continuation of the ASZ in Africa is still unclear due to the lack of information about the shear zones in Somalia. The ASZ cannot be connected to the transpressional Nabitah shear in the Arabian–Nubian shield (Abdelsalam et al., 2003; Johnson and Woldehaimanot, 2003; Kusky et al., 2003) because no suture evidence was seen in the ASZ and it is not a terrane boundary. The best candidate in Madagascar for linking with the Nabitah shear zone in the Arabian–Nubian shield would be the Betsimisaraka suture (Collins, 2006; Collins and Windley, 2002; Collins et al., 2003c; Kröner et al., 2000; Raharimahefa and Kusky, 2006, 2009) a remnant of the Mozambique Ocean located further east of the ASZ, although this is debated by Tucker et al. (2011).

7. Conclusions

Fig. 9. Map of possible reconstruction of the relative paleoposition of East Africa, Madagascar and India. ASZ: Angavo Shear Zone, AKSZ: Achankovil Shear Zone, BS: Betsimisraka suture, MSZ: Moyar Shear Zone, PCSZ: Palghat–Cauvery Shear Zone, RSZ: Ranotsara Shear Zone.

From the research that has been carried out, we can conclude that the crustal scale anastomosing Angavo Shear Zone records evidence of a two phase deformational history (D1 and D2). D1 deformation resulted in the formation of symmetric structural fabrics such as symmetric boudins, symmetric porphyroclasts, and symmetric and isoclinal folds. These symmetrical fabrics indicate a coaxial deformation. In the ASZ, the overall D2 fabrics demonstrate that the movements across mylonite zones were predominantly subhorizontal dextral oblique slip. All convincing shear sense indicators observed in the field indicate dextral motion along the almost N–S striking shear zone. Based on fold axes, hinge lines and overprinting relationships, folding was classified into F1 and F2, respectively, associated with D1 and D2. D2 fabrics documented in both macro and micro scales point toward a non-coaxial flow regime for the D2 deformation, a result from a transpressive compression during the amalgamation of western and eastern Gondwana. Considering the similarity in structural styles and the age of deformation, the Achankovil shear zone in Indian would be the best possible extension of the ASZ; however more evidences are still needed for the reconstruction of the Gondwana supercontinent

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because of the existence of other similar shear zones in India. Further isotopic dating on deformations along the ASZ is still needed. Acknowledgments This work was supported by the NSF grant EAR-0221567 awarded to T. Kusky, and some of the field work was carried out under the auspices of the British Geological Survey and the U.S. Geological Survey, under the auspices of the World Bank Development of Madagascar Program and the Mineral Resource Assessment of Madagascar Project. The study was jointly funded by the National Natural Science Foundation of China (Grants 40821061 and 91014002) and the Ministry of Education of China (B07039). We greatly appreciate Kathryn M. Goodenough for her constructive suggestions. We thank Alan S. Collins and an anonymous reviewer for comments that substantially improved the manuscript. References Abdelsalam, M.G., Abdel-Rahman, E.-S.M., El-Faki, E.-F.M., Al-Hur, B., El-Bashier, F.-R.M., Stern, R.J., Thurmond, A.K., 2003. Neoproterozoic deformation in the northeastern part of the Saharan Metacraton, northern Sudan. Precambrian Research 123, 203–221. Alsop, G.I., Holdsworth, R.E., 2006. Sheath folds as discriminations of bulk strain type. Journal of Structural Geology 28, 1588–1606. Aurouze, J., 1953. Etude géologique des feuilles Fotadrevo-Bekily campagne. Tray Bureau Géologique Madagascar 42, 1–44. Berger, A., Gnos, E., Schreurs, G., Fernandez, A., Rakotondrazafy, M., 2006. Late Neoproterozoic, Ordovician and Carboniferous events recorded in monazites from southern-central Madagascar. Precambrian Research 144, 278–296. Bésairie, H. 1964. Madagascar, Carte Géologique. 1/1,000,000. Service Géologique de Madagascar. Bésairie, H., 1968. Description du massif ancien de Madagascar. Vol. 1: Centre Nord et centre Nord-Est. Document Bureau Géologique Madagascar. (177 pp.). Bésairie, H., 1968. Description géologique du massif ancien de Madagascar. Document Bureau Geologique Madagascar. no. 177. no. 177 a: centre nord et centre nord-est; 177b: région côtière orientale; 177c: région centrale- système de graphite;177 d: région centrale - système du Vohibory; 1 77 e: le sud; 1 77f le nord. Bureau Géologique Madagascar, Antananarivo. Bésairie, H., 1969. Description geologique du massif ancien de Madagascar. Troisieme volume: la region centrale. 1. Le Systeme du Graphite, Groupe d'Ambatolampy. Documentation du Bureau Geologique. Service Géologique de Madagascar 177c, Tananarive (73 pp.). BGS-USGS-GLW, 2008. Révision de la cartographie géologique et minière des zones Nord et Centre de Madagascar. République de Madagascar Ministère de L'Energie et des Mines, Antananarivo (1020 pp.). Blumenfeld, P., Mainprice, D., Bouchez, J.L., 1986. C-slip in quartz from subsolidus deformed granite. Tectonophysics 127, 97–115. Brewer, T.S., Collins, A.S., Kroner, A., Windley, B.F., Razakamanana, T., 2001. Multiphase granitoid magmatism in central Madagascar: evidence for subduction of the Mozambique Ocean. European Union of Geoscience 11, 362. Buchwaldt, R., Tucker, R.D., Dymek, R.F., 2002. Petrogenetic implication of three contrasting terranes in northern Madagascar. Abstracts with programs. Geological Society of America 34, 272. Carreras, J., Estrada, A., White, S., 1977. The effects of folding on the C-axis fabrics of a quartz mylonite. Tectonophysics 39, 3–24. Cobbold, P.R., Quinquis, H., 1980. Development of sheath folds in shear regimes. Journal of Structural Geology 2, 119–126. Collins, A.S., 2006. Madagascar and the amalgamation of central Gondwana. Gondwana Research 9, 3–16. Collins, A.S., Windley, B.F., 2002. The tectonic evolution of central and northern Madagascar and its place in the final assembly of Gondwana. Journal of Geology 110, 325–340. Collins, A.S., Razakamanana, T., Windley, B.F., 2000. Neoproterozoic crustal-scale extensional detachment in Central Madagascar: implications for extensional collapse of the East African Orogen. Geological Magazine 137, 39–51. Collins, A.S., Fitzsimons, I.C.W., Kinny, P.D., Brewer, T.S., Windley, B.F., Kröner, A., Razakamanana, T., 2001. The Archaean rocks of Central Madagascar: their place in Gondwana. In: Cassidy, K.F., Dunphy, J.M., Van Kranendonk, M.J. (Eds.), 4th International Archaean Symposium 2001, Extended Abstracts: AGSO-Geosc. Australia, 37, pp. 294–296. Collins, A.S., Johnson, S., Fitzsimons, I.C.W., Powell, C., Hulscher, B., Abello, J., Razakamanana, T., 2003a. Neoproterozoic deformation in Central Madagascar: a structural section through part of the East African Orogen. In: Yoshida, M., Windley, B.F., Dasgupta, S. (Eds.), Proterozoic East Gondwana: supercontinent assembly and breakup: Geological Society of London Special Publications, 206, pp. 363–379. Collins, A.S., Ian Fitzsimons, C.W., Hulscher, B., Razakamanana, T., 2003b. Structure of the eastern margin of the East African Orogen in central Madagascar. Precambrian Research 123, 111–133.

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