The Position of Madagascar Within Gondwana and Its Movements During Gondwana Dispersal ⇑ Colin Reeves

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Journal of African Earth Sciences xxx (2013) xxx–xxx

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The position of Madagascar within Gondwana and its movements during Gondwana dispersal ⇑ Colin Reeves

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Article history: A reassembly of the Precambrian fragments of central Gondwana is presented that is a refinement of a Available online xxxx tight reassembly published earlier. Fragments are matched with conjugate sides parallel as far as possible and at a distance of 60–120 km from each other. With this amount of Precambrian crust now stretched Keywords: into rifts and passive margins, a fit for all the pieces neighbouring Madagascar – East Africa, Somalia, the Madagascar Seychelles, India, Sri Lanka and Mozambique – may be made without inelegant overlap or underlap. This Gondwana works less well for wider de-stretched margins on such small fragments. A model of Gondwana dispersal Aeromagnetics is also developed, working backwards in time from the present day, confining the relative movements of Indian Ocean the major fragments – Africa, Antarctica and India – such that ocean fracture zones collapse back into Dykes themselves until each ridge-reorganisation is encountered. The movements of Antarctica with respect to Africa and of India with respect to Antarctica are defined in this way by a limited number of interval poles to achieve the Gondwana ‘fit’ situation described above. The ‘fit’ offers persuasive alignments of structural and lithologic features from Madagascar to its neighbours. The dispersal model helps describe the evolution of Madagascar’s passive margins and the role of the Madagascar Rise as a microplate in the India–Africa–Antarctica triple junction. Intrusions, extrusions and dykes observed in Madagascar and its neighbours, largely from aeromagnetic survey data, are related to the outbreak of the Karoo/Bouvet mantle plume at 182 Ma, the Marion mantle plume at 88 Ma and the Reunion mantle plume at 66 Ma. The dispersal model may be viewed and downloaded as an animation at: http://www.reeves.nl/ gondwana. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction 2. Gondwana reassembled

The purpose of this paper is to describe and define (a) a well- The continent of Gondwana became an entity as a result of substantiated central position for Madagascar within reassembled complex Neoproterozoic and early Cambrian tectonism – primarily Gondwana and (b) the disruption and dispersal process by which the collision of several pre-existing continents – that involved the disintegration of Gondwana led to the present situation of many parts of Africa, South America, India and Antarctica and led Madagascar as a small continent within the Indian Ocean. The to a continuous landmass that included more than half the land work is based on many years of building and refining plate tectonic area of the world. In the vicinity of Madagascar, the north–south- models of Gondwana using the ‘Atlas’ paleogeographic mapping striking East African orogeny was formed by the east–west colli- system (http://www.the-conference.com/cpsl/atlas.htm) and sion of India with the Tanzania craton following the closure of an experience in many hitherto-adjacent parts of Gondwana with ocean that previously separated them (Collins and Pisarevsky, the interpretation of regional geophysical data – particularly aero- 2005). This process was complete by early Cambrian times magnetic surveys – in support of geological mapping. The paper (530 Ma) and most of Gondwana, including certainly the central therefore attempts to set Madagascar into its global-tectonic con- parts around Madagascar, became stable for the next 250 myr. The text such that the geological results from the recent World Bank principal activities affecting Gondwana during this period of quies- project (BGS-USGS-GLW, 2008; GAF, 2009; BRGM-USGS, 2012) cence were the rafting of several fragments off the north coast of may be better seen in respect to other fragments of Gondwana the continent and the development of an orogeny that extended and their common tectonic history. along Gondwana’s Pacific coast (Trouw and de Wit, 1999), both processes remote from Madagascar itself. The central parts of the united continent were stable until Late

⇑ Tel.: +31 611356272. Carboniferous time when the so-called ‘Karoo’ rifting episodes E-mail address: [email protected] started and led to the preservation of continental sediments in

Please cite this article in press as: Reeves, C. The position of Madagascar within Gondwana and its movements during Gondwana dispersal. J. Afr. Earth Sci. (2013), http://dx.doi.org/10.1016/j.jafrearsci.2013.07.011 2 C. Reeves / Journal of African Earth Sciences xxx (2013) xxx–xxx an extensive rift system across southern and eastern Africa, India tinents of Gondwana and their outlines determined from and adjacent Antarctica (Catuneanu et al., 2005). The Karoo sedi- geological maps such as the Geological Map of the World (Bouysse, ments were later covered in southern Africa by extensive basalts 2010) and the digital map series of the USGS (2000). These out- that are attributed to the outbreak of the Karoo/Bouvet large igne- lines, where obscured by younger cover rocks, may often be deter- ous province at 182 Ma (Toarcian – Svensen et al. (2012)) which mined with assistance from airborne geophysical data, such as pre-dated the onset of Gondwana dispersal by about 15 myr. aeromagnetic surveys (e.g. Barritt, 1993), or gravity anomaly Starting with the pioneering insights of Du Toit (1937), the pre- images (e.g. Andersen and Knudsen, 2009). In the worst cases, cise geometry of the Gondwana reassembly has undergone re- boundaries have been estimated or interpolated conservatively, peated refinements (e.g. Smith and Hallam (1970)) and the minimising unlikely invention. These outlines have all been care- Gondwana geological map of De Wit et al. (1988) has been widely fully digitized and attached to the appropriate fragments. used. Lawver et al. (1997) introduced the idea of a tighter fit of the At the margins of these fragments, outcropping or near-out- continents off East Africa that has been pursued by the present cropping Precambrian basement rocks are down-faulted at the author, following the first publication of detailed ocean-floor (hidden) shoulders of rifts that, in many cases, developed into pas- topography from satellite altimetry (Smith and Sandwell, 1997) sive margins. Inboard of these shoulders it is assumed that each and its subsequently improved definition (Andersen and Knudsen, fragment has retained the same size and shape throughout Phan- 2009). The most recent reassembly (CR12AALE) for the region sur- erozoic times and so is made up of Precambrian continental crust rounding Madagascar is illustrated in Fig. 1 and the finite rotation of undiminished thickness. The area shown in Fig. 1 was, then, parameters that achieve this configuration of the fragments are gi- essentially continuous Precambrian crust through Paleozoic times. ven in Table 1. The fragments so defined, about fifty for all Gondwana, were This reassembly has undergone some refinement from that gi- assembled with, as far as possible, their rift shoulders parallel to ven by Reeves et al. (2002) but is not, in principle, greatly different and separated from the previously conjugate rift shoulder by a dis- from it. The rationale for the reassembly is that areas of rigid Pre- tance that is a reasonable estimate of the width of the strip of Pre- cambrian crust may be identified throughout the present-day con- cambrian crust that foundered within the rift zone and became stretched in the rifting processes that, in many cases, led to drifting and ocean formation. This value will, undoubtedly, vary from place to place, but a solution for all fragments with persuasive elegance is found with values in the range 60–120 km is shown in Fig. 1. The average of seven closest points of contact with its neighbours for Madagascar is a distance of 72 km. Two types of independent information are offered here to substantiate this reconstruction. First, data from global satellite gravity observations (Smith and Sandwell, 1997; Andersen and Knudsen, 2009) indicate clear linear anomalies may be found running parallel to many continental margins. These are in many cases attributable to the edges of the continental shelves, relics of a late stage of rifting turning into drifting with an area of stretched continental crust lying between them and the ‘inboard’ fragment margin described above. These (mostly offshore) anomalies have been traced and ascribed to the appropriate continental fragment so that they follow that fragment in any reconstruction. In Fig. 2 these features are shown reconstructed as per the model defined in Table 1. Madagascar dis- plays two distinctly different types of margin. In the east the margin is very narrow and the Madagascar anomaly coincides with that off India for a length of about 500 km; elsewhere there is underlap. The west coast of Madagascar has been extended by two episodes of rifting (Karoo and Cretaceous) leaving Madagascar with two anomalies, inboard and outboard, separated by about 175 km. The gravity margin of the East Africa and Somali coasts falls midway between these two anomalies in the reconstruction. A second demonstration comes from the USGS digital data set of (largely offshore) faults and basin depth contours (USGS, 2000). This data set has similarly been divided between the relevant Precam- brian fragments and reassembled in Fig. 3.InFig. 3(a) the data for all fragments except Madagascar is shown while in Fig. 3(b) only the data for Madagascar is displayed. The representations of faulting and basin depth contours fall mostly within the overlapping area of Fig. 1. Madagascar within Gondwana reassembled at 167.2 Ma according to model Madagascar with its putative neighbours where the original Pre- CR12AALE. Areas of Precambrian crust with same outlines as today shown in pink. cambrian crust will have been stretched to half or less of its original Archean terranes shown with diagonal cross-hachuring in white. Main terranes and thickness. Data from another source is shown for The Seychelles. structural features discussed in the text labelled. AT: Achankovil and Tenemalai shear zones; DSM: deep-seated magnetic feature; MC: Moyar-Cauvery shear zone; RA: Ranotsara ‘shear zone’; RV: coastal faults of the Ravuma basin in coastal Tanzania and Mozambique. Areas now covered by Deccan Trap cover indicated by 3. Madagascar’s neighbouring fragments horizontal hachuring. Dashed line between India and Madagascar shows the limit of near-surface Precambrian rocks off the western coast of India interpreted from Following clockwise around the coast of Madagascar from the aeromagnetic data, where available. Orthographic projection centred at 20°S, 46°E, present-day Madagascar coordinates. (For interpretation of the references to southwest corner, the conjugate fragments to the Madagascar Pre- colour in this figure legend, the reader is referred to the web version of this article.) cambrian fragment are as follows.

Table 1 Finite rotation poles 0–167.2 Ma for the fragments of the model CR12AALE.

Fragment Latitude Longitude Rotation Remarks Africa 90.00 0.00 0.000 Africa ﬁxed Horn of Africa 13.55 26.75 4.650 o Seychelles 7.82 58.12 62.578 o Madagascar 11.42 111.81 19.343 o North Mozambique 36.00 165.00 0.500 o Peninsular India 31.36 38.59 63.245 o Sri Lanka 23.25 47.13 77.370 o East Antarctica 12.46 150.58 59.212 o Madagascar Rise 3.22 118.48 24.142 o Bombay High 31.09 39.31 62.777 o Saurashtra 31.34 39.32 63.005 o Tanzania (W) 4.00 30.00 2.000 o Tanzania (E) 37.67 30.00 0.654 o Beira High Fixed to Antarctica until 160 Ma, then ﬁxed to Africa

coverage of Tanzania (Batterham et al., 1983), is recognised across the NE terminus of the Selous Basin, interpreted as indicating the initial line of fracture of the continental crust already thinned by Karoo rifting. The ﬁt of Madagascar to Africa is similar too, but rather tighter than, that proposed by Reeves et al. (1987) and Yar- dimcilar and Reeves (1998). The westernmost angle of the Precambrian outcrop within Madagascar is aligned with the angle in the East Africa coast occupied by the Lamu delta. Within Africa, the Somalia Precambrian fragment is here separated from the Tanzanian fragment by the Anza graben which probably served as the third arm of a triple junction when Madagascar ﬁrst started separating from Africa (Re- eves et al., 1987). The Anza graben has been reactivated subsequently in more recent tectonic events such as the Cretaceous rifting of northern and central Africa (Guiraud et al., 2005). The northwestern coast of Madagascar, including the Majunga basin, is then situated conjugate to the Bur Acaba craton of Somalia. The position of Somalia and the entire Horn of Africa is about 180 km inland of its present position as a result of (a) undoing the opening of the Gulf of Aden during the East African rifting episode and (b) the clockwise rotation of North Africa during the Cre- taceous Rifting episode coincident with the early opening of the South Atlantic. Our reassembly then brings the prominent Marda fault in Somalia (Fig. 1) into alignment with the future axis of the Red Sea to the North and with the Narmada-Son lineament across India, together forming a shear zone of global dimension (Reeves et al., 2004). We place the small, semi-circular part of the Seychelles that is evidently Precambrian (from its expression in outcrop and in gravity anomaly images) against the northern tip of Madagascar. Fault patterns mapped around the archipelago then conform with fault directions in the adjacent fragments. Dextral strike-slip of the Sey- Fig. 2. Main world gravity anomaly features, particularly features related to chelles block along the NE coast of Madagascar is distinctly likely continental margins, digitsed from DNSC08 (Andersen and Knudsen, 2009) and at a later stage. The Kurduwadi or Trans-Peninsula lineament evi- attached to the relevant fragments in the reassembly shown in Fig. 1. Those related dent in the gravity (Geological Survey of India, 2010) and aeromag- to Madagascar shown in black, all other fragments shown in grey. netic images (Rajaram et al., 2006) of India shares its alignment with that of the straight NE coast of Madagascar and the straight The western margin of Madagascar’s exposed Precambrian margin of The Seychelles fragment (Fig. 1). rocks, where they disappear under the Karoo-age sediments of The Saurastra block of India has been re-positioned 75 km clo- the Morondava basin, is set parallel to the similarly arcuate eastern ser to NW India by closing east–west trending rifts and re-aligning margin of the Precambrian rocks in Tanzania and Kenya. The East its southern margin more closely with the Son-Narmada lineament and West branches of the East African Rift system have been closed (Fig. 1) that crosses India from west to east (Anand and Rajaram, in our model, bringing this coastal margin about 20 km inboard of 2004). The NW-striking margin of the Saurashtra block is then par- its present position with respect to central Africa as an estimate of allel with the NE coast of Madagascar. The N–S trending Cambay E–W stretching in the East African Rift System. The eastern margin rift that separates Saurashtra from the rest of northern India of Karoo rocks, as mapped in SW Madagascar, is aligned with the started accumulating sediment in Paleocene times. eastern margin of similar rocks in the Selous Basin in Tanzania. A The Bombay High, an offshore remnant of Precambrian crust line of deep-seated intrusions, seen clearly on the aeromagnetic that accommodates a major petroleum resource in the sediments

Fig. 3. Faults and sediment depth contours from USGS (2000) attached to the relevant Gondwana fragments and reassembled as in Fig. 1. (A): All fragments except Madagascar, (B): Madagascar alone. that overlie it, is replaced 100 km NNE of its present position off Precambrian rocks of Antarctica from the Gunnerus Ridge to Prydz the Indian west coast and has its western margin parallel to the Bay (Tingey, 1991), leaving a space immediately east of the Gunne- long, straight eastern margin of Precambrian rocks in Madagascar. rus Ridge that is exactly the right size and shape to accommodate The displacement could have occurred as a result of dextral strike the Precambrian rocks of Sri Lanka. This fragment is brought closer slip during movements between India and Madagascar that could to India by closing the Palk Strait and the Gulf of Mannar. Wider also have involved the syn- and post-Deccan Trap rifting of the spacing between fragments in the assembly would prohibit tight Saurashtra block from mainland India. reassembly such as this since the smaller fragments, such as Sri From aeromagnetic and gravity coverage of India (Rajaram et al., Lanka, become too large. While adjustments to each individual 2006), the western margin of the peninsula’s Precambrian rocks is matched pair of margins can be argued (e.g. Veevers, 2009), at an- within 20–30 km of the shoreline in the south (Kerala coast), but lies other level the overall aspect of a compact, holistic Gondwana further offshore towards the north where aeromagnetic coverage is reassembly is compelling and has already formed a useful basis lacking. The Bombay High is presumed to be a fragment of this (ex- for the further development of ideas. tended) crust that has been detached. The interpreted margin, The Gunnerus Ridge has been interpreted as a sliver of conti- shown partially from aeromagnetic data in Fig. 1, is placed parallel nental crust – whether containing Precambrian rocks or not is un- to the long, straight eastern margin of Precambrian rocks in Mada- known (Leitchenkov et al., 2008) – measuring about 400 km in gascar. This Madagascan continental margin has little or no ex- length that projects almost perpendicular to the coast of Antarc- tended crust still attached to it; the new aeromagnetic data (GAF, tica, separating ocean of Jurassic age to its west from Cretaceous 2009) shows shallow Precambrian rock extending to and beyond ocean to its east. It therefore marks the division between the ear- the shoreline while bathymetric data shows water depths of liest ocean created between Africa and Antarctica in Jurassic times 200 m within 20 km of the coastline and 2000 m within 40 km. and that created between India–Sri Lanka and Antarctica in the Between Madagascar, SE Tanzania and southernmost India Cretaceous. The western concave side of the Gunnerus Ridge has there is an equilateral triangular gap in the reassembly of side been fitted to the convex coast of northern Mozambique where about 300 km. A notional Precambrian core to the Madagascar Rise Precambrian rocks (and coast-parallel dykes) extend as far SE as has been placed here. This micro-continent has followed a complex the shoreline. evolution as a separate fragment, moving first slightly south with The straight line along the eastern margin of shallow Precam- India (125–88 Ma, Aptian to Cenomanian) then much more quickly brian rocks evident in the aeromagnetic coverage in North Mozam- on its own to a total distance of about 800 km from Madagascar, bique and into Tanzania as far north as the ‘entrance’ to the Selous partly after the outbreak of the Marion mantle plume at 88 Ma Basin marks the dextral strike-slip zone by which East Gondwana, and the departure of India from Madagascar but mostly as part including Madagascar, was to move south against Africa (the pro- of the ridge-reorganisation of the Africa–Antarctica–India plate to-Davie fracture zone) while East Gondwana remained intact. The system that accompanied the 66 Ma outbreak of the Reunion line is now covered by the Cretaceous and younger sediments of plume and the outpouring of the Deccan Traps. The Marion plume the Rovuma Basin in coastal Tanzania and Mozambique (Key (88 Ma) had already made a large contribution of magma to the et al., 2008). Madagascar Rise and to a number of other ocean-floor edifices dur- The line of the Davie fracture zone is still preserved in a sub- ing and after the separation of India from Madagascar and the sep- marine escarpment running N20°W immediately off the SW shore- aration of the Madagascar Rise from Madagascar itself. line of Madagascar. The alignment of the Achankovil and The Precambrian outline inboard of India’s east coast, from the Tenemalai shear zones at India’s southernmost tip (where they southern tip of India to the Mahanadi rift, fits tightly against the separate the Maiduri (or Southern) Granulite Terrain to the north

Please cite this article in press as: Reeves, C. The position of Madagascar within Gondwana and its movements during Gondwana dispersal. J. Afr. Earth Sci. (2013), http://dx.doi.org/10.1016/j.jafrearsci.2013.07.011 C. Reeves / Journal of African Earth Sciences xxx (2013) xxx–xxx 5 from the Khondalite belt in evidence in southernmost India (Geo- are evident in the regional aeromagnetic coverage there (Barritt, logical Survey of India, 1994)) also shares the strike direction and 1993), substantiating the relative positions of India, Madagascar alignment of the Davie fracture zone in the reconstruction (Fig. 1). and Mozambique in Fig. 1. The prominent Lurio Belt in Mozam- bique which is interpreted as a thrust zone between terranes of distinctly different origin (Grantham et al., 2007), may be contin- 4. Madagascar’s situation in the reassembly ued to the east along the line separating Sri Lanka and India in our reconstruction, so accounting for the distinct differences be- Some alignments of other features may be mentioned in sup- tween the Precambrian geology of India and its island neighbour. port of this reassembly and include the following: The angle between the Lurio Belt and the continuation of the In- The NE coast of Madagascar (here matched to the main faulting dia–Madagascar join into Mozambique follows the angle made be- direction off the Seychelles) aligns with the Kurduwadi or Trans- tween the east and west coasts of India at the continent’s southern Peninsula lineament of peninsular India (Krishna Brahmam and tip. A rather persuasive fit of aeromagnetic anomaly patterns from Negi, 1973). This runs across India approximately from Chennai India into Mozambique, across the future Davie fracture zone to Mumbai. It is a clear regional feature in both the national mag- (DFZ), emerges from our reconstruction. In Fig. 4 the arcuate anom- netic and gravity anomaly coverages (Geological Survey of India, alies, concave to the NE, in NE Mozambique are seen in reconstruc- 2010; Rajaram et al., 2006) and is manifested as parallel shears tion to sit opposite similarly shaped anomalies in southernmost over a width of about 100 km. Such a feature would be prone to India. re-activation in the dextral strike-slip regime that evolved as East While the new aeromagnetic coverage of Madagascar makes Gondwana moved south against West Gondwana in which the Da- important contributions, it is regrettably incomplete, as are the vie fracture zone was central. Significant displacements are not re- coverages of neighbouring fragments in the re-assembly. This lim- quired by the reconstruction model but limited movements on a its the ability to draw parallels between adjacent tectono-meta- smaller scale cannot be ruled out. morhpic terranes in our reconstruction from their geophysical The 2010 gravity image of India (Geological Survey of India, signatures. Even a gap of 60–100 km between fragments (or survey 2010) shows a distinct sinistral displacement of a whole section areas) introduces doubt into extrapolations, even without the pos- of southernmost India. On the aeromagnetic coverage (Rajaram sibility that the gap was the locus of a pre-existing structural et al., 2006), this appears as a number of parallel shears that lie just boundary of some sort within the entity of Gondwana that became south of the Archean Dharwar craton and mostly within the reactivated during dispersal. Erosion levels may also differ signifi- ‘Southern Granulite Terrane’ (Acharrya, 1999) that is dated as cantly from one fragment to another. Nevertheless, some observa- Pan-African in age. Different authors have mapped various com- tions are pertinent. plex fault patterns in India south of 12°N (e.g. Sambandam et al., Within India, the overall shape of the Archean Dharwar craton 1994; Collins et al., in press), but the simple alignment concerned is well-defined by its distinctive gravity signature (Geological Sur- here is well-substantiated by the regional geophysical data and so vey of India, 2010) with a clear eastern margin, resembling in size probably reflects the most recent movements. It follows the Moyar and geophysical expression the Yilgarn block in Western Australia shear of Sambandam et al. (1994) in the west and the Cauvery Riv- (Geoscience Australia, 2009a,b). No such detailed gravity anomaly er in the east where it separates Archean gneisses to the north from coverage exists for Madagascar and the aeromagnetic coverage Neoproterozoic migmatites to the south. In the present paper it is available for India offers far lower resolution than that now avail- referred to as the Moyar–Cauvery shear zone to distinguish it from able for the parts of Madagascar that have been covered by recent the Palghat–Cauvery shear zone that has often been referred to in surveys. A like-with-like comparison of the geophysical signatures the past. The Moyar–Cauvery shear zone is about 550 km in total of the Dharwar margins from India into Madagascar is therefore length and a central section of about 100 km is evidently only still for the future. recognised so far in its geophysical expression since the terrane Rocks surrounding cratons typically follow the direction of the of southern India (like much of Africa) evidently does not lend it- craton margins, in contrast to the structures within cratons that of- self easily to detailed geological mapping without the support of ten meet the craton margin at high angles. In many ways, the aero- high-resolution airborne geophysics. magnetic expression of rocks within Madagascar show (with the Of particular relevance to the study of Madagascan geology is exception of the Bemarivo domain in the north) a strike direction the alignment of the Moyar–Cauvery shear zone in our reassembly approximately parallel to the island’s east coast. But this direction with the Ranotsara ‘‘shear zone’’ that separates the geology of is also shared by many features (such as the Closepet granite) with- southern Madagascar from the remainder of the island. This fea- in the Dharwar craton itself. So a geophysical solution to the loca- ture is discussed elsewhere in this volume and is now revealed tion of the western margin of the Dharwar craton within by detailed geophysical survey (GAF, 2009) as a zone of ductile Madagascar and the role of the Betsimisaraka suture zone (Collins deformation within the Anosyen domain with a sinistral displace- and Windley, 2002; Key et al., 2011; Tucker et al., 2011a) cannot be ment of about 60 km. In southern India, sinistral displacement of offered at this stage. the rocks south of the Moyar–Cauvery shear zone, also by about The clearest aeromagnetic anomaly of regional extent within 60 km, is indicated by an offset in the gravity anomaly contours northern Madagascar is a deep-seated positive that tracks centrally across the Cauvery River near the town of Tiruchirappalli. The com- through the area of the Antananarivo domain from the western bined feature, Moyar–Cauvery and Ranotsara, is also closely paral- margin of the Bemarivo domain in the north to the SE end the Ran- lel to the Davie fracture zone (Fig. 1), though this may be purely tosara shear zone in the south (Fig. 1), a distance of about 1000 km. fortuitous. Earlier authors (e.g. Collins and Windley, 2002; BGS- This anomaly appears to arise from a deep-seated source and its USGS-GLW, 2008) have suggested a continuation of the Ranotsara location, about 35 km west of the Betsimisaraka shear zone, sug- ‘shear zone’ into Kenya that could even continue into the Aswa gests that this feature dips westwards to the base of the crust. shear zone in Uganda (Fig. 1) and from there into the Cretaceous The new geophysical data is regrettably discontinuous in its cover- rift systems of southern Sudan. age and little analysis has yet been carried out of deep-seated The long, straight eastern margin of Madagascar presumably sources (as opposed to outcropping geology) but, in a broader con- had a role as a major brittle shear zone at some stage, most prob- text, the geophysical data is not at variance with the idea that the ably in the closing stages of the East African orogeny. Shears western margin of the Dharwar craton falls largely within continuing this direction to the SW into Northern Mozambique Madagascar and that the common boundary of continental crust

Fig. 4. Magnetic anomaly images of Northern Mozambique (courtesy of AMMP, Barritt, 1993) and southern India (Rajaram et al., 2006) re-projected and reassembled in their Gondwana orientation as per model CR12AALE (Fig. 1 and Table 1). Note that survey specifications differ between the two data sets and that no attempt has been made to reduce either data set to the pole or re-process the data in any other way. Even so, the continuation of sharply concave-to-the-NE geological features (present-day Africa coordinates) in southernmost India is repeated in adjacent Mozambique. Main fault zones described in the text are indicated. Orthographic projection centred at the point of contact of the two images. Both images have latitude and longitude lines at 1° intervals. between India and Madagascar is therefore a rare example of con- immediately north of the Ranotsara ‘shear zone’. The Archean rocks tinent–continent rupture crossing an Archean craton. Cratons typ- belong to pre-assembly East Gondwana and the younger rocks ically acquire younger material with geophysical anomalies were the products of reworking in the East African orogeny. The running parallel to their margins but in this case it is not easy to latter are framed to the west by the Tanzanian craton and West demarcate the western limit to the Archean Dharwar craton and Gondwana while a third part of the assembly including the Kalaha- elements accreted to it subsequently from geophysical evidence ri craton and much of Antarctica is postulated to have approached alone. from the south (Grantham et al., 2007; Collins et al., in press) with In Madagascar south of the Ranotsara ductile shear zone (Tuck- the suture now represented by the Lurio belt and the line of the er et al., 2011b), the major high-strain zones (Ampanihy and Berak- subsequent rift between India and Sri Lanka. eta) mapped in detail by GAF (2009) separating the Vohibory, Androyen and Anosyen domains strike strongly N–S (present-day 5. Building the Gondwana dispersal model Madagascar coordinates) over a distance of 350 km. An extension along strike of less than 200 km in our reconstruction would bring Reeves and de Wit (2000) described the process by which pres- these features into the SE corner of Tanzania, SE of the Selous Ba- ent-day continental fragments may be mapped back into their past sin. The aeromagnetic data available there (Batterham et al., positions by telescoping the conjugate halves of fracture zones into 1983) shows a predominantly E–W strike direction (present-day each other. The two halves, it is assumed, have throughout their Tanzania coordinates), though there is limited evidence of shearing growth been coincident and collinear with the transform section following the Madagascar direction further to the SW in northern (sensu stricto) offsetting the mid-ocean ridge. Remarkably little Mozambique (Reeves and de Wit, 2000). Any pre-existing shear simplification of mid-ocean ridge geometry during ocean develop- zone following the line of the Davie fracture zone (DFZ) might ren- ment can be found when the detailed images now available of der such correlations futile but the aeromagnetic correlations ocean-floor topography (e.g. Andersen and Knudsen, 2009) are shown in Fig. 4 suggest continuity of Precambrian fabric prior to studied, making this deduction possible on significant numbers the advent of the DFZ. Geophysically it seems clear, however, that of fracture zones. Not all transforms give exactly the same result correlations of the rocks and structures of southern Madagascar but, on average, there is good agreement so long as only one pair should be sought with those of Tanzania and northernmost of conjugate plates is studied. Mozambique, adjacent in our reconstruction. In the Indian Ocean, four distinct phases or regimes of ocean In summary, the reconstruction presented here places Mada- growth were described by Reeves and de Wit (2000). More detailed gascar in an important central position in the Early Phenerozoic studies using the improved ocean floor data (Andersen and Knud- assembly of Gondwana. The bulk of the island is seen to have Ar- sen, 2009), supplemented with important new elements of mag- chean geology, part of which may be considered a westward exten- netic anomaly data from key areas of the Indian Ocean (e.g. sion of the Dharwar craton in India (BRGM-USGS, 2012; GAF, König and Jokat, 2010), have refined this model in the present 2009). At the southern limit to the Dharwar craton in India, the study, but not changed it substantially. The history of relative Moyar–Cauvery shear zone, Archean rocks are in contact with movement of each pair of continents is then defined by a sequence granulites of Pan-African age. A similar transition from Archean of Euler interval poles. Within each interval, the fracture zones are to Pan-African rocks is observed in Madagascar south-to-north aligned with small circles about the active rotation pole. The pole

Please cite this article in press as: Reeves, C. The position of Madagascar within Gondwana and its movements during Gondwana dispersal. J. Afr. Earth Sci. (2013), http://dx.doi.org/10.1016/j.jafrearsci.2013.07.011 C. Reeves / Journal of African Earth Sciences xxx (2013) xxx–xxx 7 to define such movement is determined by constructions joining clarity that has emerged between the first and second regimes of conjugate termini in the ‘Atlas’ plate reconstruction software. Re- ocean-floor spreading (Reeves and de Wit, 2000). peated iteration of approximate solutions making small adjust- The earliest relative motion of East and West Gondwana is now ments within the limits of error leads to an increasingly robust modeled as strike-slip along (a) the northern section of the Lebom- solution by way of a great deal of careful work. bo ‘monocline’ (north of 24°300S) that forms the eastern margin of In the most recent iteration of the reconstruction, the fracture the Kaapvaal craton in South Africa and (b) the line of intrusions zones shown on the Geological Map of the World (Bouysse, revealed by aeromagnetic survey at depth below where the Kar- 2010) have been used. There is little evidence for ocean growth oo-age Selous Basin intersects the newly-formed continental mar- being other than symmetrical about mid-ocean ridges, except in gin in Tanzania. This travel direction changes by about 23° exceptional circumstances. The ridge ‘jumps’ associated with the clockwise, going into the Cretaceous, becoming parallel to (a) the regime changes are limited in number and the velocity of plate southern section of the Lebombo ‘monocline’ and (b) the coastal separation is restricted to being fairly constant, if only to conserve faults of the Rovuma Basin in Tanzania–Mozambique (Fig. 1). momentum. It follows that observing animations of any dispersion While the first motion would be slightly transtensional along the model (conveniently in both forward and reverse time) is impor- proto-Davie fracture zone, the second is envisaged as pure strike- tant in assessing the credibility of the model. Jerky or irregular slip or slightly transpressional along this major shear direction. movement of any fragment – particularly the large fragments – The change in spreading direction occurs as continent–conti- at any time is physically unlikely. nent contact across the Davie fracture zone is lost between East Once continental dispersion began, there is no evidence of any Gondwana (Madagascar) and West Gondwana (Mozambique newly-created ocean crust being consumed or greatly shortened mainland) at about the start of the Cretaceous (145 Ma). By this within central Gondwana. It follows that all plate margins em- time continental Antarctica has cleared South Africa going south ployed in a model must either be concerned with growth at ridges and starts to move increasingly westward and to rotate slightly or conservative strike-slip motion at transforms. This is a further clockwise. The westward departure of the ‘toe’ of South America severe constraint on possible dispersal models. (Falklands Plateau etc) makes room for this to happen, following The progress of ocean growth may be time-calibrated by way of establishment of the Tristan mantle plume, the Bouvet triple junc- ocean floor magnetic anomalies, though regrettably few of these tion and the initiation of spreading in the South Atlantic Ocean. have been identified in the older parts of the Indian Ocean and Presumably at least in part under the influence of the outbreak an important period (including two regime changes) is devoid of of the Kerguelen mantle plume (Coffin et al., 2002), Greater India anomalies on account of the Cretaceous Normal Superchron or and Madagascar then fail to keep pace with Antarctica–Australia Quiet Zone,126–84 Ma approximately (Barremian to Santonian). and the growth of a new ocean between Greater India and Antarc- Plate motions at global scales are pre-disposed to having Euler tica–Australia accelerates. Gaina et al. (2007) and Borisova and Sy- poles that are as distant as possible from the active mid-ocean monds, 1997 offer magnetic anomaly identifications off Antarctica ridges. In this way the plate margins separate at the mid-ocean and Australia respectively that substantiate this timing. This new ridges while remaining approximately parallel to each other. The ocean growth is at the expense of that between Madagascar–India relative motion of the plates is then largely ‘translational’ with and Africa (Somalia)-Arabia. The latter ocean eventually becomes the active Euler pole about 90° distant on a great circle. This allows extinct by about 120 Ma when Madagascar became part of the Afri- ridge-push to operate efficiently with a constant value per unit ca plate, along with the Mozambique Rise. The Africa–Antarctica length of ridge. Efficiency is lost where separating margins are in mid-ocean ridge inboard of this Rise had jumped about 1000 km the vicinity of an Euler pole as the rate of separation declines to- to the south by this time, establishing a long transform offset that wards zero as the pole is approached. While it is true that any mo- has persisted until the present day. tion on a sphere has a pole about which that motion is a rotation, From about 117 Ma the movement of Antarctica relative to Afri- whether the pole is proximal or distant, any fragment moving with ca became stable and primarily translational until disrupted by the respect to another about an Euler pole that is proximal undergoes a outbreak of the distant Reunion hotspot 68–60 Ma (Maastrichtian motion that is locally more ‘rotational’ than ‘translational’. It is to to Selandian). While this sequence of events is rather difficult to be noted that the larger sections of mid-ocean ridge active today describe, it is easily visualised by viewing the animation globally (e.g. the South, Equatorial and Central Atlantic, the Ocean (www.reeves.nl/gondwana). between Australia and Antarctica) are all operating in their ‘Euler The ‘wedging apart’ of the South Atlantic Ocean in the Early Cre- tropics’ with translation predominating. In general it is left to the taceous (principally 140–125 Ma approximately) also caused the smaller crustal fragments (e.g. Sri Lanka, the Seychelles) to under- Cretaceous rifting in North Africa (Guiraud et al., 2005) that moved go movements that are closer to rotational since they contribute NE Africa and Arabia to the east by perhaps as much as 200 km, relatively little inefficiency to the use of the global energy budget. bringing the Somali ocean under compression. So both the Kergue- The model presented here has been built as far as possible from len plume south of India and the Tristan plume in the South Atlan- first principles to avoid the influence of, or making choices be- tic Ocean contributed – over a transition period from about 145 Ma tween, interpretations made by previous authors. Several itera- to about 118 Ma (Tithonian to Aptian) – to the locus of ocean tions of the dispersal model have been completed since Reeves growth east of Africa being transferred from north of Madagas- and de Wit (2000), that in place late in 2012 (CR12AALE) has been car–India (Regime 1) to south of it (Regime 2). In effect, Gondwana used to create figures in this paper. A more recent model with disruption morphed from being two plates (East and West Gondw- some dynamically significant fine-tuning (CR13AAGQ) has been ana) before this period to two plates (Africa–Madagascar–Greater used in making the animation available for viewing and download- India–Sri Lanka as ‘North Gondwana’ – and South America–Antarc- ing via the internet at http://www.reeves.nl/gondwana. tica–Australia as ‘South Gondwana’) after it. Madagascar, Africa The current model iteration uses critical new information on and India underwent very little relative movement between about conjugate magnetic anomalies in the ocean between Africa and 120 Ma and 88 Ma and the outbreak of the Marion plume. Antarctica (König and Jokat, 2010) to eliminate many uncertainties Relative motion between India and Madagascar is now thought in the relative motion of these two major fragments that existed to be limited to less than 150 km with both dextral and sinsitral previously, particularly in the Early Cretaceous, and so the scope episodes during Regime 1 and the transition to Regime 2. Reeves for error in other parts of the model is much reduced. Of particular and de Wit (2000) advocated considerable (1000 km) dextral relevance to the situation of Madagascar in this context is the new strike-slip in their model in order to bring India back against the

Madagascar Rise by telescoping the Vishnu fracture zone to its con- rocks from that time (BGS-USGS-GLW, 2008) suggests this date jugate ends. This is now seen as an over-simplification since (a) the (87.2 ± 1.0 Ma) for magmatic activity in Madagascar. Pande Gondwana ‘fit’ position of India has been revised to be somewhat et al. (2001) obtained an age of 85.6 ± 0.9 Ma for their supposed In- further south with respect to Madagascar and (b) it has been rea- dian equivalents in St Mary’s Islands. This age allows less than lised that the Madagascar Rise continued to move south, away 4 myr for ocean-floor spreading between India and Madagascar be- from Madagascar, for some time after the outbreak of the Marion fore Anomaly 34 time (84 Ma) when marine magnetic anomaly mantle plume and India’s departure from Madagascar. control returns. The precise evolution of the rift between Madagascar and India is still a matter of considerable economic interest but the stretched 6. The separation of Madagascar from its Gondwana neighbours continental crust and its sediment load is now to be found almost entirely on the continental shelf of India’s west coast and perhaps Fig. 5 illustrates in six frames the sequence of events by which partly in the Mascarene fragments (Salha da Maya, Nazareth Bank, Madagascar separated from its neighbours in Gondwana as quan- The Seychelles, etc). The eventual rupture line fell within a few tified in the model CR12AALE. A full animation of a slightly up- tens of kilometers of Madagascar’s present-day east coast, leaving dated model may be freely downloaded from the following URL: the island with very little stretched crust on its eastern seaboard. http://www.reeves.nl/gondwana. The present model initiates the third regime of Indian Ocean The long-lived stability of Gondwana was first disturbed by the growth at 88 Ma (Coniacian), though this is poorly constrained propagation of a system of rifts across central Gondwana from the by ocean data, being within the Cretaceous Quiet Zone that is de- Tethys Ocean to the Gondwanide orogeny during Karoo times void of marine magnetic anomalies. Recent dating of magmatic (Permian to Triassic, 300–200 Ma approximately). The Southern

Fig. 5. Key stages in the separation of Madagascar from its neighbours in Gondwana. (A) The ‘ﬁt’ position shown in Fig. 1. (B) 145 Ma (Berriasian). East Gondwana ceases to be a single plate at about this time as Antarctica starts to rotate clockwise. (C) 120 Ma (early Aptian). Somali Ocean between Madagascar and Somalia fails, ocean between Antarctica–Australia and Greater India fully active. (D) 88 Ma (Coniacian). Immediately before the outbreak of the Marion mantle plume and the start of India’s rapid NE-ward movement. (E) 66 Ma (start Paleocene). Deccan Traps being erupted; major plate reorganisation between India and Madagascar; Mascarene Basin headed for extinction. (F) Present situation with Comores volcanicity.

Trans-Africa Shear Zone (STASS) (Reeves et al., 2004) was an The Somali mid-ocean ridge became extinct by Aptian times important part of this system and other, parallel, trans-Gondwana (about 120 Ma) when Madagascar itself became part of the African alignments served to ‘shake ancient cratons loose’ from the broad- plate (Fig. 5(c)). It has remained in a fixed position with respect to er matrix of Precambrian terranes, particularly the mobile belts Africa ever since, except for slight movements since the advent of that were apparently less resistant to rupture than the cratonic nu- the East African Rift System when the Davie fracture zone could clei. This Karoo rift system, wrapping around cratons such as Zim- have been reactivated. A fairly robust position for India with re- babwe and Tanzania, included the western part of Madagascar and spect to Madagascar (with the reservations mentioned above) is allowed extensive Karoo sedimentation there, as is now in evi- obtained by retracing the fracture zones created once India started dence in the Morandava and Majunga basins (Fig. 3b). its rapid northward movement from this fixed Madagascar posi- Early in the Jurassic, a major mantle plume struck Gondwana in tion (Fig. 5(d)). This is the situation that was disrupted by the im- the vicinity of the present-day Limpopo estuary in southern Africa. pact of the Marion mantle plume that we place at 88 Ma. Two principal surface manifestations of this were (a) the eruption Surprisingly little of the output of this plume is to be found on of extensive ‘Karoo’ or Stormberg basalt sheets across all of south- land in Madagascar, much having been removed by erosion. Relics ern Africa at 182 ± 1.0 Ma (Riley et al., 2004) and (b) an extremely of volcanic rocks and dykes along the eastern coastal belt dated at large swarm of dykes that cuts across northern Botswana to the about 87 Ma are shown in the recent geological mapping (BGS- Okavango Delta and beyond (Reeves, 1978) in a well-defined linear USGS-GLW, 2008). In India, relics are confined to the columnar bas- trend about 1300 km in length. These dykes cut the STASS but alts of St Mary’s Islands (Pande et al., 2001) and a number of coast- show no sign of displacement by subsequent movement of it, indi- parallel dykes that are evident primarily in aeromagnetic surveys cating that strike-slip on the STASS has been negligible since the (Devaraju, 1995). Unfortunately, only incomplete, low resolution dykes were emplaced at between 179.0 ± 1.2 and 178.4 ± 1.1 Ma aeromagnetic coverage is as yet available for the west coastal belt (Le Gall et al., 2005 and references therein). of India. Limited dating gives ages of 90–85 Ma and 70–65 Ma Instead of following pre-existing weaknesses across southern (Radhakrishna, 2007) for some of these dykes while others give Africa such as the STASS, the line of separation of Gondwana into Paleoproterozoic ages. Coast-orthogonal dykes 200 km inland in East and West followed the Karoo rift system only as far SW as the Karnataka state have yielded ages of 90.0 ± 1.0 Ma and present coastline of Tanzania. The Selous Basin of Tanzania was 87.5 ± 0.9 Ma. It seems that relics of both the Marion plume out- abandoned and a new line of fracture followed an approximately break and that of the Reunion plume that produced the extensive north–south pathway (present-day Africa coordinates) between Deccan Trap volcanism around the end of the Cretaceous period what is now northern Mozambique on the one side and Madagascar, are in evidence. India and Sri Lanka on the other, along the line of the Davie fracture The model discussed here has India starting its rapid northward zone (Fig. 5(a)). Backward extrapolation of M-series magnetic movement at 88 Ma from the fairly well-substantiated situation in anomalies in the subsequently-formed oceans off South Africa Fig. 5(d). Copious supplies of magma from the Marion plume could places an estimated start for this movement at 167.2 Ma (Bathonian) be extruded to fill the early opening between the two continents (König, 2005), about 10 myr after the major dyke intrusion. and form the main part of the Mascarene fragments Saya de Malha, There is no evidence for East Gondwana being anything other Nazareth Bank, etc to maintain a bridge up to 180 km wide be- than a single intact fragment in the earliest phases of its relative tween the two continents for 8 myr after this separation started. movement against West Gondwana (Fig. 5(b)). However, if India It is envisaged that almost all of this material moved off with India and Madagascar follow the well-substantiated path of Antarctica to the NE, leaving Madagascar virtually devoid of continental shelf against Mozambique in this time, Madagascar will separate from on its eastern coast. South of Madagascar, meanwhile, the Marion the Davie fracture zone. This may be avoided by allowing Madagas- plume fed the growth of the Madagascar Rise and sent it moving car to move relative to India along their common boundary and this south, away from Madagascar, even after the departure of India, can be imagined as an effect of ridge-push in the early Somali ocean. until it reached its present position, probably by as late as 60 Ma. This conceptual movement is at first sinistral (167–147 Ma) then Its notional Precambrian core would by that time have separated dextral (147–110 Ma) and finally sinistral again but need be no more from Madagascar by a distance of about 720 km. For some small 150 km in total to keep Madagascar in contact with the Davie frac- but unknown part of the distance, the Madagascar Rise and India ture zone. Some movement could also have occurred on the exten- may have travelled together, prior to the 88 Ma event proposed sion of India’s Kurduwadi lineament along the NE coast of here. Madagascar, making Madagascar more distant from India at the It is worth noting that the new aeromagnetic coverage of south- end of this period than in its original ‘fit’ position through transten- ern Madagascar does not extend far enough south to indicate any sional rifting. faulting or southerly termination of the island’s Precambrian rocks. The paucity of geological knowledge of the conjugate margins of They are still in evidence from their magnetic anomalies below India and Madagascar extends to the extensive continental shelf off only shallow cover at the southern limit of the survey area at 25° India’s west coast where little is known of the stratigraphy before 11 min S. A continental shelf with (thinned?) Precambrian base- the deposition of the Deccan Trap basalts at about 66 Ma. Investi- ment may extend up to 100 km south of the mapped Precambrian gation of the Mascarene fragments might reveal any sedimentary outcrops. A clear, linear escarpment off the SW coast of Madagas- succession that exists there and their possible role as part of the car remains aligned with the Davie fracture zone during our recon- extended continental crust between Madagascar and India. struction and is also approximately collinear at this time with the Propensity for transtensional rifting between India and Mada- Achankovil and Tenmalai shear zones mapped clearly at the south- gascar would probably have been greatest early in the Cretaceous ern tip of India (Fig. 1). as Antarctica started its clockwise rotation with respect to Africa Anomaly 34, the oldest of the magnetic anomalies produced and an ocean between Antarctica–Australia and Greater India since the Cretaceous Quiet Zone, allows accurate reconstruction started to form. India would be situated between two opposing of continental positions at about 84 Ma at the end of the Creta- mid-ocean ridges at this time, that to the north being in decline ceous Quiet Zone. Initially, spreading between Madagascar and In- and that to the south being in the ascendancy. There is therefore dia was shared between (a) the Mascarene Basin between potential for prolonged rifting and sediment accumulation along Madagascar and the Seychelles and the Mascarene fragments a 1600 km length of conjugate passive margins through the Early (opening propagating south to north, the larger part) and (b) India Cretaceous (Bastia et al., 2010). and The Seychelles–Laxmi Ridge (propagating north to south)

Please cite this article in press as: Reeves, C. The position of Madagascar within Gondwana and its movements during Gondwana dispersal. J. Afr. Earth Sci. (2013), http://dx.doi.org/10.1016/j.jafrearsci.2013.07.011 10 C. Reeves / Journal of African Earth Sciences xxx (2013) xxx–xxx where magnetic anomalies off the coast of NW India are mapped at about 66 Ma. Relative movement between the Madagascar Rise and identified as ages 82–65 Ma (Acharyya, 1999) across a width and Madagascar through ocean growth on the micro-ridge separat- of about 400 km off the Bombay High. ing them was therefore long-lived; strike-slip between India and Shortly after the Cretaceous–Paleogene boundary (66 Ma, Madagascar could have started as early as Bathonian times Fig. 5(e)), the Mascarene basin became an extinct ocean. At this (167 Ma) when any Precambrian core of the Rise would probably time the ridge between India and the Mascarene fragments re- have followed India. A total separation of more than 700 km will organised during a period of rapid ocean growth and became the have occurred between then and the end of the Mesozoic. Potential only locus of relative movement between India and the Africa plate for significant sedimentary accumulations on the Rise seems lim- that by then included the Mascarene basin. The situation estab- ited, however. A great deal of volcanicity occurred there from lished at that time in the ocean around Madagascar persists to about 88 Ma and continued while the feature lay close to the triple the present day, further growth of the ocean between India and junction, building the present edifice on the ocean floor. Madagascar being confined to ridges beyond the Mascarene fragments. All the coastlines of Madagascar have, therefore, been 7.3. Madagascar’s long, straight east coast essentially passive margins since the departure of India at about 88 Ma. The model developed here predicts transtensional rifting and The value of viewing this series of events as an animation is sedimentation off eastern Madagascar that may have started as stressed again (http://www.reeves.nl/gondwana). early as when East Gondwana first started moving south with respect to Africa (Bastia et al., 2010). Keeping SW Madagascar in con- 7. The evolution of the passive margins of Madagascar tact with the Davie fracture zone requires that Madagascar had a slightly different movement from the rest of East Gondwana in The history of the evolution of Madagascar’s passive margins Jurassic times, the difference being taken up with movements, both can therefore be divided into four coastal sectors, each with a dis- dextral and sinistral, along their line of contact. The development tinct tectonic history. of a large rift, about 1600 km in length, with provenance from both Madagascar and Greater India (and even Antarctica until about the end of the Jurassic) is possible, terminating with the volcanic epi- 7.1. The Karoo and younger rocks of the Madagascar NW and west sodes recorded in the basalts of eastern Madagascar and in St coast Mary’s Islands of India related to the outbreak of the Marion hotspot at about 88 Ma. The present-day continental shelf off eastern This is a sedimentary area with a long history going back to Madagascar is remarkably narrow, and probably preserves little of Permian times or earlier (Catuneanu et al., 2005). New evidence this sedimentary section. from the recent aeromagnetic surveys indicates that even the old- When ocean ridges re-organised and simplified at about 66 Ma, est exposed sediment overlapped the rift margin onto the Precam- by far the larger part of the extended crust in the Madagascar–In- brian rocks since the contact between them is clearly in most dia rift (along with much of the material erupted by the Marion locations a gradual deepening of the magnetic anomalies of the plume) remained attached to India as a broad continental shelf. Precambrian basement from east to west rather than a faulted con- This is, as yet, only poorly explored, largely on account of the tact. The opposing margin in Tanzania is clearly faulted, implying extensive Deccan Trap volcanic rocks (66 Ma) that may post-date that this rift was, like many rifts, a half-graben. the start of any sedimentation by as much as 80–90 myr if the early After a period of acquiescence in early Jurassic times, this rift transtensional rifting suggested here proves correct. was reactivated in part. First an embayment of the Tethys ocean was created with limited marine access along the Somali coast. Then strike-slip along the coastal fault direction now evident in 7.4. The NE coast of Madagascar Tanzania–Mozambique led to sediment accumulating in the Rov- uma Basin, a transform margin, along that coast (Key et al., 2008) This relatively short stretch of coast has its own distinct history. and to sediment of similar age overlying the Karoo rocks of Mada- The alignment of the coast is probably determined by an older frac- gascar in the Morondava basin above the larger part of the earlier ture line, recognised in India as the Kurduwadi lineament (Krishna Karoo sediment. On the northwest coast of Madagascar, the coast Brahmam and Negi, 1973). In the present Gondwana reassembly, conjugate with Somalia developed as a passive margin with the this line separates what is now Madagascar from Saurashtra, the underlying, extended continental crust again lying mainly on the Seychelles and other small fragments lying on the north side of Madagascar coast below the Majunga basin. Both the Morondava the Narmada–Red Sea megashear (Reeves et al., 2004). Its history and Majunga basins are currently the subject of exploration for since the start of Gondwana disruption is one of dextral strike-slip hydrocarbons in Madagascar. A fuller discussion of the sedimen- movement that could have been pure strike-slip, while deviating tary history of these basins is beyond the scope of this paper. from time to time into transtension and transpression at the mercy of the movements of larger fragments in the vicinity. The presence of the parallel graben in Madagascar in which Nosy Mangaby is sit- 7.2. Passive margins between southern Madagascar and the uated and dykes radiating inland from it indicates the occurrence Madagascar Rise of at least one episode of extension in this region and the possibil- ity of sediment accumulation in a rifted setting offshore. This conjugate pair of margins initially acquired complexity as a result of relative movements between India and Madagascar in Ap- tian times as Madagascar came to rest as part of the Africa plate 8. Igneous activity in and around Madagascar while India – probably with the early Madagascar Rise attached – was still incompletely separated from Antarctica. The margins Four episodes of igneous activity in and around Madagascar continued to evolve until as recently as about 60 Ma (Paleocene) may be recognised (BGS-USGS-GLW, 2008). The oldest is not yet as the Madagascar Rise became involved as a microplate in the directly recognised in outcrop but would relate to the Jurassic sep- evolution of the triple junction between the India, Africa and Ant- aration of Madagascar from Africa. The remaining three are rela- arctica plates, particularly from 88 Ma until after the ridge reorga- tively well-known: (a) the outbreak of the Marion mantle plume nization that followed the outbreak of the Reunion mantle plume at about 88 Ma; (b) the outbreak of the Reunion mantle plume at

Please cite this article in press as: Reeves, C. The position of Madagascar within Gondwana and its movements during Gondwana dispersal. J. Afr. Earth Sci. (2013), http://dx.doi.org/10.1016/j.jafrearsci.2013.07.011 C. Reeves / Journal of African Earth Sciences xxx (2013) xxx–xxx 11 about 66 Ma and (c) the more recent events principally manifested in the Comores Islands. Yardimcilar and Reeves (1998) noted, from the interpretation of aeromagnetic data for Tanzania, Kenya and western Madagascar, the existence of a number of mostly buried intrusions below their conjugate coastlines that probably had a common origin in the earliest separation of Madagascar from Africa. Estimates on the depth of the largest of these bodies in the entrance to the Selous Basin – 3–10 km – suggests that their emplacement pre-dated the deposition of the post-Karroo sedimentary section, as determined by seis- mic surveys. We propose that these intrusions would date from about 170 Ma, when relative movement of East Gondwana against Africa began. Their location is about 2000 km distant from the main centre of the Bouvet mantle plume that broke out at 182 Ma. Radhakrishna et al. (1999) recognise some dykes as being of Upper Jurassic age (144 ± 6 Ma) in India within the Granulite terrane south of the Moyar–Cauvery fault zone. The geometry of Androy, the large caldera in southern Madagas- car, is revealed clearly by the recent World Bank airborne geophysical surveys. In particular, the magnetic anomaly patterns reveal the geometry of several swarms of dykes in the vicinity of this feature (Fig. 6). One swarm lies south of the caldera and strikes N120°E, present-day Madagascar coordinates. Together with a small number of arcuate dykes trending NNW in SE Madagascar, the presence of another igneous centre off the SE corner of Mada- gascar is suggested, from which these dykes radiate. This interpretation is reinforced by dykes in India that share the strike direction of those in Madagascar in our reconstruction (B, Fig. 6). The other dykes and the Androy caldera itself are among the volcanic rocks in Madagascar dated so far which are either 87 Ma or of East Afri- can Rift age. The basalts lying close to the eastern shoreline of Madagascar are also dated at about 87 Ma and have their counterparts in the St Mary’s islands of India (Pande et al., 2001). Fig. 6. Igneous rocks of Madagascar and southern India in a reassembly at 88 Ma. The Marion mantle plume created voluminous outpouring of Precambrian rocks shaded pink with white diagonal hachuring on areas of Archean igneous material but, as in examples elsewhere, very little evi- age. Orange diagonal hachuring on areas of Karoo sediments. Dykes, igneous dence of plume activity has been left onshore. Offshore, mean- centres and basalt flows indicated in purple. (A) Androy caldera; (B) postulated while, large magma volumes are preserved (a) in and around the igneous centre between Madagascar and India with dykes also evident in India. (For interpretation of the references to colour in this figure legend, the reader is referred Madagascar Rise (with its notional Precambrian core), (b) probably to the web version of this article.) at least part of the Mascarene fragments that initially followed In- dia away from Madagascar and (c) in the Comorin Ridge off the southern tip of India which filled the earliest space between the sent later events that fortuitously coincided geographically with older ocean surrounding Sri Lanka and the Madagascar Rise when the pre-existing continental/oceanic fragments. India first started its northeastward journey. It may be noted from Dykes observed and dated along the western coast of India the animation that India and Madagascar remain virtually station- (Devaraju, 1995; Fig. 6) have given ages that can be associated ary above the future location of the Marion Plume from about either with the 87 Ma event or with the 68–66 Ma activity of Dec- 120 Ma (compare Fig. 5(c and d); Meert and Tamrat, 2006). can Trap volcanicity. Shallow samples from the Mascarene fragments have all The volcanic features in the north and central parts of Madagas- yielded dates that suggest they resulted from output of the Re- car represent a separate phase of igneous activity related to the union mantle plume as it tracked south to its present position be- East African Rift valley and the Comores volcanicity (Fig. 6(f)) neath Reunion Island following the creation of the Deccan Traps and are not discussed here (BGS-USGS-GLW, 2008). from about 68 Ma (Bouysse, 2010). This is much younger than the Marion-plume age origin preferred by the present author. The angular outline of at least the larger of these fragments (Saya de Malha, Nazareth Bank) suggests that the bulk of them broke 9. Summary and conclusions up from an earlier entity which is here (as in Reeves and de Wit, 2000) postulated to have occupied the initial graben or proto- A simple reconstruction of Gondwana, consisting of about 50 ocean between Madagascar and India. They were first detached Precambrian fragments identifiable today, allows repositioning of from Madagascar and later left remote from it, stranded on the Madagascar’s neighbouring Precambrian terranes with parallel far side of the Mascarene Basin. With the outbreak of the Reunion margins, provided a gap of only 60–120 km is allowed for Precam- plume and the eruption of the Deccan basalts, an acceleration of brian material, now lost by crustal extension into rifts and passive the rate of spreading between Madagascar and India occurred margins created during the Phanerozoic. Veevers (2009), for exam- but outside of the Mascarene Basin. Its influence extended far en- ple, has argued for a larger separation between India and Antarc- ough SW to cause a temporary re-orientation of the spreading tica. This is not necessarily at variance with the model presented direction between Africa and Antarctica that persisted for about here, but if all the smaller fragments such as Madagascar and Sri 8 myr (see animation). It is suggested that the samples that have Lanka are enlarged by a similar amount of extra material offshore, yielded younger dates for some of the Mascarene fragments repre- they become too large to fill the spaces available for them in our

Please cite this article in press as: Reeves, C. The position of Madagascar within Gondwana and its movements during Gondwana dispersal. J. Afr. Earth Sci. (2013), http://dx.doi.org/10.1016/j.jafrearsci.2013.07.011 12 C. Reeves / Journal of African Earth Sciences xxx (2013) xxx–xxx reconstruction and all the advantages of the Reeves et al. (2004) (BGS-USGS-RLW, 2008) as well as for the St Mary’s Islands off India model, refined in the present paper, are lost. (Pande et al., 2001) all cluster around 87 Ma. At date of 88 Ma has In the reconstruction presented here, the main divisions of the been adopted for the outbreak of the Marion plume and the NE Precambrian of Madagascar fit well with those proposed for the drift of India in our latest model (CR13AAGQ) used to build the ani- Precambrian of India, positioned closely to the east of Madagascar mation (www.reeves.nl/gondwana). (Fig. 1). The eastern terranes of Madagascar containing material It is hoped that a number of interesting ideas will be stimulated with Archean ages probably formed the western extremities and by the model presented here. The model is derived, as far as possi- margins of India’s Dharwar craton. The younger terranes south ble, from first principles and with a minimum of invention where and west of the Archean in Madagascar were subjected to tecto- data is sparse or missing. The model is subject to on-going revision nism during the Neoproterozoic-to-Cambrian East African orogeny and input from those with access to new data is invited. when East Gondwana (and, in particular, the Dharwar craton) col- lided with West Gondwana (and in particular the Tanzania craton). The Southern Granulite Terrane of India falls against the Androyen Acknowledgements domain of southern Madagascar with the alignment of the Ranot- sara ‘shear zone’ with the Moyar–Cauvery shear zone identified Alan Smith and Lawrence Rush of Cambridge Paleomap Services here from geophysical data over southern India. Recent work by Limited are thanked for many years of stimulating discussions and Grantham et al. (2007) postulates a separate collision history for unfailing support in the operation of the ‘Atlas’ paleogeographic the Kalahari craton and Australia–Antarctica (‘Southern Gondw- modeling software. ana’), south of the terranes of Madagascar. The continuity of fea- Suzanna Reeves devoted much time to carefully and selectively tures of southern Madagascar into SE Tanzania and NW digitizing elements of the Geological Map of the World at 1:25 M, Mozambique invites further investigation. 3rd edition, copyright CCGM-CGMW 2010 (www.ccgm.org) with A working model (www.reeves.nl/gondwana) of the dispersion permission from the copyright owners. of these Precambrian fragments consistent with fragment geome- ITC Students whose M.Sc. and Ph.D. projects (1993–2004) with try and evidence from the ocean floor created in the process does aeromagnetic data from many different countries, wittingly or not necessarily (unlike the Reeves and de Wit (2000) model) re- unwittingly, contributed to ideas discussed here include B.K. Sahu, quire large relative motion between India and Madagascar in a C. Yardimcilar, A.G.S.R. Perera, M.S. Mubu and S. Chavez Gomez. dextral transtensional sense during Jurassic times. However, the I thank Bernard Moine, Alan Collins and Roger Key for construc- dextral strike-slip regime which brought East Gondwana as a sin- tive input during the review process. gle entity south from Africa must have resulted in reactivation of I extend a final word of appreciation to all the unsung heroes the pre-existing shear zone separating India and Madagascar if who have collected airborne geophysical data over the decades in Madagascar itself were to remain in contact with the Davie frac- the hope – sadly yet to be fulfilled in many cases – that this data ture zone. Such a long, straight shear zone is likely to have had will become readily and generally accessible for future generations an earlier, large lateral offset across it, probably during the East of earth scientists to interpret as geology. Africa orogeny, so the search for detailed correlations of Precam- A contribution to IGCP 628, The Gondwana Map Project – the brian geology across it may prove fruitless. 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