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

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The Position of Madagascar Within Gondwana and Its Movements During Gondwana Dispersal ⇑ Colin Reeves Journal of African Earth Sciences xxx (2013) xxx–xxx Contents lists available at ScienceDirect Journal of African Earth Sciences journal homepage: www.elsevier.com/locate/jafrearsci The position of Madagascar within Gondwana and its movements during Gondwana dispersal ⇑ Colin Reeves Earthworks BV, Achterom 41A, 2611 PL Delft, The Netherlands article info abstract 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 man- tle 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 1464-343X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jafrearsci.2013.07.011 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 sub- stantiate 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 rif- ting (Karoo and Cretaceous) leaving Madagascar with two anoma- lies, 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.
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