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

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

Geological evolution of Northeastern Mozambique, in the context of the Pan-African Gondwana assembly

Results of an integrated geological mapping, geochronological, geochemical and airborne geophysics programme in NE Mozambique bring improved constraints on the sequence of events leading to the assembly of Gondwana along the East African Orogen (see Reference List). From the NW (foreland) to the SE (hinterland), a crustal transect across NE Mozambique shows division into 4 major lithotectonic units

Geochronology of the Precambrian crust in the Mozambique Belt of NE Mozambique, and implications for Gondwana assembly

Zircon and monazite U–Pb data document the geochronology of the felsic crust in the Mozambique Belt in NE Mozambique. Immediately E of Lake Niassa and NW of the Karoo-aged Maniamba Graben, the Ponta Messuli Complex preserves Paleoproterozoic gneisses with granulite-facies metamorphism dated at 1950±15 Ma, and intruded by granite at 1056±11 Ma. This complex has onlyweak evidence for a Pan- African metamorphism. Between the Maniamba Graben and theWSW–ENE-trending Lurio (shear) Belt, the Unango and Marrupa Complexes consist mainly of felsic orthogneisses dated between 1062±13 and 946±11 Ma, and interlayered with minor paragneisses. In these complexes, an amphibolite- to granulitefacies metamorphism is dated at 953±8Ma and a nepheline syenite pluton is dated at 799±8 Ma. Pan-African deformation and high-grade metamorphism are more intense and penetrative southwards, towards the Lurio Belt. Amphibolite-facies metamorphism is dated at 555±11Ma in the Marrupa Complex and amphibolite- to granulite-facies metamorphism between 569±9 and 527±8Ma in the Unango Complex. Post-collisional felsic plutonism, dated between 549±13 and 486±27 Ma, is uncommon in the Marrupa Complex but common in the Unango Complex. To the south of the Lurio Belt, the Nampula Complex consists of felsic orthogneisses which gave ages ranging from 1123±9 to 1042±9 Ma, interlayered with paragneisses. The Nampula Complex underwent amphibolite-facies metamorphism in the period between 543±23 to 493±8 Ma, and was intruded by voluminous post-collisional granitoid plutons between 511±12 and 508±3 Ma. In a larger context, the Ponta Messuli Complex is regarded as part of the Palaeoproterozoic, Usagaran, Congo-Tanzania Craton foreland of the Pan-African orogen. The Unango, Marrupa and Nampula Complexes were probably formed in an active margin setting during the Mesoproterozoic. The Unango and Marrupa Complexes were assembled on the margin of the Congo-Tanzania Craton during the Irumidian orogeny (ca. 1020–950 Ma), together with terranes in the Southern Irumide Belt. The distinctly older Nampula Complex was more probably linked to the Maud Belt of Antarctica, and peripheral to the Kalahari Craton during the Neoproterozoic. During the Pan-African orogeny, the Marrupa Complex was overlain by NW-directed nappes of the Cabo Delgado Nappe Complex before peak metamorphism at ca. 555 Ma. The nappes include evidence for early Pan-African orogenic events older than 610 Ma, typical for the Eastern Granulites in Tanzania. Crustal thickening at 555±11Ma is coeval with high-pressure granulite-facies metamorphism along the Lurio Belt at 557±16 Ma. Crustal thickening in NE Mozambique is part of the main Pan-African, Kuunga, orogeny peaking between 570 and 530 Ma, during which the Congo-Tanzania, Kalahari, East Antarctica and India Cratons welded to form Gondwana. Voluminous post-collisional magmatism and metamorphism younger than 530Ma in the Lurio Belt and the Nampula Complex are taken as evidence of gravitational collapse of the extensive orogenic domain south of the Lurio Belt after ca. 530 Ma. The Lurio Belt may represent a Pan-African suture zone between the Kalahari and Congo-Tanzania Craton. © 2009 Elsevier B.V. All rights reserved