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Growth and collapse of a deeply eroded orogen : insights from structural, geophysical, and geochronological constraints on the Pan-African evolution of NE Mozambique

By G. Viola, I.H.C Henderson, B. Bingen, R.J. Thomas, M. Smethurst and S. De Azavedo

Abstract

This paper presents results of a large multidiciplinary\ud geological mapping project in NE Mozambique, with a\ud focus on the structural evolution of this part of the East\ud African Orogen (EAO). It integrates field structural\ud studies with geophysical interpretations and presents\ud new geochronological data. The tectonic architecture\ud of NE Mozambique can be subdivided into five\ud megatectonic units on the basis of lithology, structure\ud and geochronology: unit 1, Paleoproterozoic Ponta\ud Messuli Complex in the extreme NW corner of NE\ud Mozambique, which represents the local NW foreland\ud to the EAO; unit 2, a collage of Mesoproterozoic\ud metamorphic complexes, which forms the basement to\ud unit 3, a stack of Neoproterozoic, NW directed\ud imbricate thrust nappes named here the ‘‘Cabo\ud Delgado Nappe Complex’’ (CDNC); unit 4, restricted\ud Neoproterozoic metasedimentary basins; and unit 5,\ud two exotic Neoproterozoic granulite me´ lange\ud complexes. The units were assembled during a long\ud and complex history of NWdirected shortening, which\ud commenced with nappe stacking and emplacement of\ud the CDNC over the Mesoproterozoic basement\ud terranes toward the NW foreland. It is proposed that\ud the CDNC and the Eastern Granulites farther north in\ud Tanzania are remnants of Neoproterozoic volcanic arcs\ud and microcontinents formed ‘‘outboard’’ of the\ud Mesoproterozoic continent after 596 ± 11 Ma. Field\ud and potential field geophysical data show that the\ud nappes were folded by regional-scale NE–SW\ud trending folds that formed in response to a later stage\ud of the same shortening episode and this episode gave\ud rise to the Lurio Belt, a prominent structural feature of\ud northern Mozambique and a key element (often as\ud suture zone) in many Gondwana reconstructions. The\ud Lurio Belt is here interpreted as a structure generated\ud during folding of the CDNC during later stages of the\ud progressive shortening event. It is, however, a\ud repeatedly reactivated shear zone, probably at the\ud site of an older (Mesoproterozoic?) discontinuity, with\ud an intense pure shear deformation history. It is cored\ud by strongly attenuated lenses of a granulitic tectonic\ud me´lange, the Ocua Complex (megatectonic unit 5) and\ud is intruded by Late Pan-African granitoids of the\ud Malema Suite. The compressional phase of the orogen\ud was postdated by NW–SE directed extension. New\ud U-Pb zircon and monazite dates show that extension\ud was initiated at circa 540 Ma in the eastern Lurio\ud Belt. It is argued that extension was the result of a\ud major episode of orogenic collapse of the EAO,\ud initiated by gravitational instabilities resulting from\ud crustal thickening during the shortening phase

Topics: Earth Sciences
Publisher: American Geophysical Union.
Year: 2008
DOI identifier: 10.1029/2008TC002284
OAI identifier: oai:nora.nerc.ac.uk:5575
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