Collaborative Research: Neoproterozoic Vergence in the East African Orogen: A Structural and Geochronologic Analysis of the Polydeformed Itremo Group, Central Madagascar

Saint Louis University, Saint Louis MO

Investigators

Abstract

The creation of the supercontinent of Gondwana is one of the most significant geologic events in the history of our planet yet the processes that led to its formation are poorly understood. In its predrift position near the dawn of the Cambrian, Madagascar was situated in the interior of Gondwana near the eastern edge of the East African Orogen (EAO). This orogen marks the join between East and West Gondwana, and it contains within it pieces of recycled continental crust, ophiolite, and island-arc and continental-arc terranes that formed within and marginal to the former Mozambique Ocean. Madagascar is thus critically positioned to reveal three key and currently enigmatic features of the EAO essential for understanding the transformation from Rodinia to Gondwana: (1) the nature of the volcanic terranes in the central part of the EAO and their possible extensions in Somalia, Ethiopia, and Rajasthan (India); (2) the chronology of terrane-suturing and continental collision events in the critical join between East and West Gondwana, and; (3) the structure of the eastern part of the EAO in the key transitional area from dominantly terrane-accreted geology (in the north) to continental collision and crustal overthickening (in the south). Pl's Tucker and Kusky propose to undertake a two-year structural and geochronologic investigation of the polydeformed and metamorphosed Precambrian rocks of central Madagascar (Itremo region). Work in this area holds great promise for resolving two first-order geologic questions in the eastern part of the East African Orogen: (1) What is the direction of early structural vergence (nappe emplacement) in the eastern part of the East African Orogen and what is the geometry of the superposed folds of central Madagascar? Is the structural vergence westerly or southwesterly, as has been recently proposed (Racolison 1997, Fernandez et al. in revision), or is it easterly or southeasterly as advocated by others (Tucker et al. 1997 and in revision; Collins et al. 2000 a,b)? Are other solutions possible? (2) When did Pan-African defon-nation and metamorphism begin in the southern part of the EAO. Were the nappes of central Madagascar initially transported and metamorphosed prior to -800 Ma (Cox et al. 1998; Collins et al. 2000 a,b; Hulscher et al 2001)? Alternatively, were they emplaced and metamorphosed after -800 Ma (Tucker et al. 1997, and in revison; Fernandez et al. in revision; Cox et al. 2001). These questions are central to understanding the timing and geometry of plate convergence within the EAO. The Itremo region of central Madagascar is the best place to study and address these issues: it is situated within the region of Neoproterozoic overprinting, only - 1 50 km from the proposed suture with East Gondwana; bedrock exposure is superb and the contrasting rock types permit accurate mapping of folds and faults; stratigraphic facing directions are abundant and hence the inverted limbs of recumbent folds and thrust nappes can be readily identified, and; reliable cross-sections may be constructed owing to the consistent regional plunge of the youngest-generation folds. Research will proceed through an integrated program of geologic mapping, structural analysis, and U-Pb geochronology. Structural investigations and sample collections will be made in three key areas within the Itremo region, tentatively identified as comprising the inverted limbs of large-scale east-directed told- or thrust-nappes (Tucker et al. in revision). Stratigraphic analysis, structural investigations and detailed mapping projects are designed to test our structural hypothesis and increase our knowledge of the regional geometry of the folds and faults. U-Pb geochronology of key samples will constrain the age of nappe emplacement and regional metamorphism, and provide a critical test of our collision model. This work will lead to a new understanding of the kinematics, age and overall structural vergence in the critical, eastern part of the EAO where strongly divergent views of its evolution are emerging. Research will involve collaboration between professional colleagues in Madagascar and the U.S., and allow for the participation and training of American and Malagasy students in field- and laboratory-based research.