R.J. Thomas

Himalayan-type indenter-escape tectonics model for the southern part of the late Neoproterozoic–early Paleozoic East African– Antarctic orogen

The East African–Antarctic orogen is one of the largest orogenic belts on the planet. It resulted from the collision of various parts of proto–East and West Gondwana during late Neoproterozoic–early Paleozoic time (between 650 and 500 Ma). We propose that the southern part of this Himalayan-type orogen can be interpreted in terms of a lateral-escape tectonic model. Modern Gondwana reconstructions show that the southern part of the East African– Antarctic orogen can best be reassembled when a number of microplates (the Falkland, Ellsworth-Haag, and Filchner blocks) are positioned between southern Africa and East Antarctica. This microplate assemblage is unusual. The microplates probably represent shear-zone–bounded blocks, produced by tectonic translation during lateral escape, similar to those currently evolving in Southeast Asia. One of the escape-related shear zones is exposed as the 20-km-wide Heimefront transpression zone in western Dronning Maud Land. Coats Land, a crustal block within the orogen, probably represents a block of older crust that was not subjected to tectonometamorphic reworking ca. 500 Ma by lateral tectonic escape. The southern part of the orogen is also typified by very large volumes of late-tectonic A2-type granitoids, intruded ca. 530–490 Ma, probably as a consequence of delamination of the orogenic root and the subsequent influx of hot asthenospheric mantle during tectonic escape. Erosional unroofing of the orogen is documented by the remnants of originally massive areas covered by Cambrian– Ordovician molasse-type sedimentary rocks throughout Africa, Arabia, and Antarctica, testifying to the past extent and size of this largest of orogens.

Accretion and indentation tectonics at the southern edge of the Kaapvaal craton during the Kibaran (Grenville) orogeny

A comparison of the lithologic, geochronologic, and kinematic features of the ∼1Ga orogens in southern Africa (Namaqua-Natal belt) and the Heimefrontfjella (East Antarctica) shows that the three areas originally constituted a contiguous belt that evolved within a prolonged, consistently northeast-oriented stress regime (African azimuths). An early northeast- or southwest-directed thrusting event has been identified along the entire belt. A later episode of transcurrent shearing can account for both the emplacement of extensive late-tectonic granitoid plutons in Natal and for the development of the Koras and Sinclair basins in Namaqualand and Namibia. The geometry and sense of movement of the later shear zones are functions of the varying orientation of the adjacent Archean cratonic margin and a prolonged period of northeast-directed plate convergence. Consequently, the craton is interpreted as a southwest-directed indenter during the Grenville orogeny.

Precambrian evolution of the Sirwa Window, Anti-Atlas Orogen, Morocco

Abstract We present the results of a field, geochemical and geochronological study of a ∼5000 km2 area of the Sirwa Window of the Anti-Atlas Orogen of Morocco. The region includes the northern edge of the Palaeoproterozoic (Eburnean) West African Craton (Zenaga Complex) and the southern margin of the Neoproterozoic (Pan-African) Anti-Atlas Orogen. The Zenaga Complex comprises medium grade supracrustal schists and intrusive granitoid orthogneisses, three of which gave within-error U–Pb SHRIMP zircon dates of ∼2035 Ma. The Anti-Atlas Orogen contains a vast thickness of volcano-sedimentary rocks known collectively as the Anti-Atlas Supergroup. The oldest of these comprises three, probably coeval, sequences collectively known as the Bleida Group. The Bleida Group includes tectonic inliers of schists and orthogneisses which gave a SHRIMP date of 743±14 Ma; medium-grade ophiolitic rocks in the central part of the area (Khzama and Nqob fragments) and a low-grade clastic-chemical volcano-sedimentary sequence (Taghdout Subgroup) along the northern margin of the Zenaga Complex. These rocks are interpreted as representing island-arc, fore-arc basin ocean-floor, and rifted continental margin sequences, respectively. The rocks developed north of the West African Craton during Neoproterozoic subduction of oceanic crust and the development of an arc/fore-arc complex. The Bleida Group is overlain by the thick flysch-like volcano-sedimentary rocks of the Sarhro Group which were deposited before 615 Ma, according to the SHRIMP dates obtained from the oldest granitic bodies intruding them. The presence of glaciogenic diamictite units suggests a possible depositional age of ∼700 Ma. It is thought that the Sarhro Group was deposited in the fore-arc basin which developed between the island arc and the cratonic continental margin to the south. A reversal of plate movement vectors during Sarhro Group times led to a change from turbidite to coarse clastic deltaic deposition. This culminated in closure of the fore-arc basin and collision of the island arc with the Craton margin, ophiolite emplacement and deformation of the Sarhro Group and older rocks during the Pan-African Orogeny, which is probably dated at ∼660 Ma, the SHRIMP age given by metamorphic overgrowths on zircons from the arc rocks. The Pan-African collision event was associated with widespread late- to post-orogenic magmatism, including granitoids intruded at ∼614 Ma (Mzil Granite, Ida-ou-Illoun batholith) and a huge gabbro-diorite-granodiorite-granite I-type granitoid batholith at ∼580 Ma (Askaoun batholith). The granitoids are post-dated by the extensive post-orogenic volcano-sedimentary molasse sequences of the Ouarzazate Group and coeval polyphase granite plutons between 575 and 560 Ma. The Ouarzazate Group comprises immature, coarse clastic sedimentary rocks (conglomerates, arkoses, reworked volcanic rocks) acid to intermediate volcaniclastic rocks (lapilli and crystal tuffs, volcanic breccias, ignimbrites, etc.) and lavas (minor basalt, andesite and voluminous rhyolite). The volcanic component of this extensive succession was extruded from at least five separate interfingering volcanic centres, each with characteristic stratigraphies (designated subgroups), typically in fault-bound settings. The volcanic centres include calderas containing coeval high-level granites and quartz porphry bodies, along with rhyolitic plugs, domes and dykes. Post-orogenic magmatism terminated with the emplacement of a number of potassic leucogranite bodies, one of which has been dated at ∼560 Ma. This plutonic phase is recorded in the Zenaga Complex by Rb–Sr mica ages of ∼580–525 Ma. The Ouarzazate Group is conformably to disconformably overlain by a typical foreland basin succession (Tata Group). A sporadically-developed basal conglomerate unit is overlain by two marine carbonate-clastic/shale cycles. Stable isotope studies have indicated that the base of the Cambrian Era (∼544 Ma) lies near the top of the lowermost dolomitic unit. The evolution of the Sirwa Window may serve as a model for the entire Anti-Atlas Orogen, recording a cycle of cratonic rifting and ocean basin formation (∼800 Ma), subduction and island-arc formation (∼750 Ma), flysch deposition and volcanism in a fore-arc basin. Plate movement reversal from extension to convergence in Sarhro Group times led to eventual arc-continent collision and ophiolite obduction (∼660 Ma), late- to post-orogenic plutonism (∼615–580 Ma), post-orogenic extension, collapse, exhumation and molasse sedimentation, volcanism and plutonism (580–560 Ma) and marine foreland basin deposition (550 Ma to Palaeozoic).

Pb, Nd, and Sr isotope mapping of Grenville‐age crustal provinces in Rodinia

New Pb, Nd, and Sr isotope data are presented for geochemically similar, ∼1.1–1.2 Ga, granitoids and tonalitic‐granitic orthogneisses from Antarctica, southern Africa, and the Falkland Islands and adjoining plateau, areas originally within the supercontinents of Rodinia and Gondwana. These data support contentions for the presence of a Mesoproterozoic (∼1.2 Ga) destructive plate margin running from Namaqua‐Natal (southern Africa), through the displaced microplates of the Falkland Plateau and Falkland Islands, the Haag Nunatak crustal block (West Antarctica) and into western Dronning Maud Land (East Antarctica). The bulk of these granitoids represent juvenile Mesoproterozoic additions to the crust, except for in parts of East Antarctica (i.e., the Sverdrupfjella) where older (Paleoproterozoic or Archean) crust was involved in granitoid generation. Our isotope data permit plate reconstructions in which southern Africa, East Antarctica, and the Falkland Islands and plateau were adjacent within Rodinia.

New isotope data from a neoproterozoic porphyritic garnitoid-charnockite suite from Natal, South Africa

Abstract The tectonic setting and age of emplacement of the Oribi Gorge Suite, voluminous rapakivi-textured granites and charnockites (“megacrystic granites”) from the ∼ 1 Ga (Kibaran) Natal Metamorphic Province has been controversial. Isotopic dates of ∼ 1.0 Ga have been obtained fromfive plutons (various isotopic systems), but two of these (Fafa and Oribi Gorge plutons) have also given ∼ 0.89 Ga RbSr whole-rock isochrons. The age problem has been exacerbated by apparently equivocal field and structural relationships, in which both syn- and post-tectonic settings have been argued. New UPb isotopic analyses for zircon fractions from a sample of the Fafa pluton give a concordia data of 1029±1010Ma. Single zircon evaporation analyses of selected grains from the Oribi Gorge pluton (Bomela locality) give a data of 1092 ± 2Ma RbSr model dates of biotite separates from three plutons range between 970 and 880 Ma, generally similar to the RbSr whole-rock dates. It is significant that no Pan-African (∼ 500 Ma) dates were obtained. The zircon data strongly support a > 1.0 Ga age for the emplacement of the Oribi Gorge Suite, an age similar to both the main regional tectogenesis and syntectonic granites of the Margate Suite. The Oribi Gorge Suite can thus be regarded as late (syn)tectonic and not partly post-tectonic as previously envisaged. Consequently, the need to invoke two periods of identical A-type magmatism is removed. The younger RbSr whole-rock and biotite dates may represent the time when the various plutons or the entire terrane cooled through the relevant blocking temperatures, though this was not a simple, homogeneous process.

Early Palaeozoic orogenic collapse and voluminous late-tectonic magmatism in Dronning Maud Land and Mozambique : insights into the partially delaminated orogenic root of the East African-Antarctic Orogen?

Abstract The late tectonic history of the southern part of the Late Neoproterozoic–Early Palaeozoic East African–Antarctic Orogen (EAAO) is characterized by lateral extrusion, extensional collapse and large volumes of high-temperature A2-type granitoids. This late-tectonic igneous province covers an area more than 15 000 km2 of the EAAO in Dronning Maud Land (East Antarctica) and its northerly continuation as the Nampula Complex of NE Mozambique. The magmatic province is bounded in the north by the Lurio Belt. New secondary ionization mass spectrometry (SIMS) U–Pb analyses of zircons from two major late-tectonic granitoid intrusions from Dronning Maud Land indicate crystallization ages of 501±7 and 499±4 Ma, whereas a major extensional shear zone was dated at 507±9 Ma. New SIMS zircon U–Pb analyses of late-tectonic granitoid sheets and plutons from the Nampula Province indicate ages of 512±4, 508±4, 508±2 and 507±3 Ma. Consequently, the late-tectonic magmatism can be bracketed between c. 530 and 485 Ma. It started with small gabbro bodies emplaced at c. 530–520 Ma, culminated with the intrusion of major granite–charnockite plutons at c. 510–500 Ma and terminated with the introduction of small volumes of sheet-like granite at c. 485 Ma. The new dates demonstrate that extensional shearing and granitoid intrusion are synchronous, and that orogenic collapse and the magmatism are related. We ascribe the distribution, structural style, geochemical composition and age of the late magmatic province to a process of partial delamination of the orogenic root in the southern third of the EAAO. It remains to be tested whether there is a relationship between orogenic collapse–granitoid magmatism and south-directed escape tectonics in the southernmost EAAO.

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.

Protolith interpretation in metamorphic terranes: a back-arc environment with Besshi-type base metal potential for the Quha Formation, Natal Province, South Africa

Abstract A scheme for protolith interpretation of high-grade gneisses is presented which integrates field, petrographic and geochemical criteria to interpret not only the nature of the original rock types, but also the geological setting in which they formed. The amphibolite to granulite-grade supracrustal gneisses of the ∼ 1.2 Ga Quha Formation from the Mzumbe Terrane, Natal Metamorphic Province, South Africa, represent a metamorphosed sequence of predominantly volcanic and volcaniclastic rocks of K-feldspar-absent basaltic to andesitic compositions, along with penecontemporaneous (greywacke) sedimentary rocks derived from their erosion. Interlayered, finely laminated quartz-garnet-pyrite rocks (coticules) point to localised volcanogenic exhalative activity, which may indicate the existence of Besshi-type base metal sulphide deposits. Two types of nearly concordant amphibolite layers represent metamorphosed dykes of pre- and late-tectonic age. Geochemical parameters including ACF, TAS and REE plots show strong calc-alkaline volcanic-arc signatures, while bulk compositional data can be successfully matched with proposed protolith mineral compositions. UPb dating of selected zircon separates from Quha metavolcanic volcanic rocks suggest an age of crystallisation of ∼ 1200 Ma, indistinguishable from that of the pre-tectonic, arc-related Mzumbe tonalite-trondhjemite with which the Quha Formation is intimately associated. However, none of the Quha rocks are cogenetic with the Mzumbe Suite.

Age and thermal evolution of the Mesoproterozoic Cape Meredith Complex, West Falkland

U-Pb SHRIMP zircon and 40Ar/39Ar mineral ages are reported for rocks of the Cape Meredith Complex, West Falkland. Felsic gneisses of the oldest Big Cape Formation give a zircon date of 1118 ± 8 Ma, interpreted as the time of extrusion of the rhyolitic protoliths. The three phases of granitoid which intrude the Big Cape Formation, namely granodiorite orthogneiss (Gl), syntectonic granite gneiss (G2) and post-tectonic granite (G3), gave U–Pb dates of c. 1090, 1067 ± 9 and 1003 ± 16 Ma respectively. Metamorphic overgrowths on zircons from the felsic gneisses were dated at c. 1000 Ma, coeval with the G3 granite, whereas a date of 1135 ± 11 Ma from inherited cores in G2 zircons were probably derived from a slightly older component of the Big Cape Formation. 40Ar/39Ar hornblende ages from amphibo-lites of the Big Cape Formation (1009 ± 14 and 1015 ± 6 Ma), along with muscovite (989 ± 3 Ma) and biotite (989 ± 7 Ma) from G3 pegmatites show that the complex cooled relatively rapidly to below 350°C, with no evidence of any Pan-African (c. 500 Ma) thermal overprinting. These data show that the older rocks of the Cape Meredith Complex are significantly younger than the equivalent rocks of the Natal Metamorphic Province, SE Africa (c. 1200 Ma), though they are comparable in age to those of Western Dronning Maud Land (East Antarctica), areas which Gondwana reconstructions place west and east of the Falkland Microplate respectively. The older rocks of the Cape Meredith Complex may therefore represent younger outboard arc terranes relative to those exposed in Natal. However, the syn- to post-tectonic granites of the Cape Meredith Complex are comparable in age in all three areas (1070–1000 Ma), suggesting similar collisional and post-collisional histories throughout the entire region. The lower temperature history of the Cape Meredith Complex is comparable to data from Natal, but does not show the Pan-African (c. 500 Ma) overprint which is characteristic of much of western Dronning Maud Land. This situation is consistent with the proposed position of the Falkland Microplate between SE Africa and East Antarctica in Gondwana, where a general eastward increase in the intensity of the Pan-African thermal effects has been recorded.

Proterozoic-Cambrian history of Dronning Maud Land in the context of Gondwana assembly

Abstract Dronning Maud Land contains a fragment of an Archaean craton covered by sedimentary and magmatic rocks of Mesoproterozoic age, surrounded by a Late Mesoproterozoic metamorphic belt. Tectonothermal events at the end of the Mesoproterozoic and in Late Neoproterozoic-Cambrian times (Pan-African) have been proved within the metamorphic belt. In western Dronning Maud Land a juvenile Mesoproterozoic basement was accreted to the craton at c. 1.1 Ga. Mesoproterozoic rocks were also detected by zircon SHRIMP dating of gneisses in central Dronning Maud Land, followed by a long hiatus for which geochronological data are lacking, an amphibolite to granulite facies metamorphism and syntectonic granitoid emplacement of Pan-African age have been dated. During this orogeny older structures were completely overprinted in a sinistral tranpressive deformation regime, leading to the mainly coast-parallel tectonic structures of the East Antarctic Orogen. Putting Antarctica back in its Gondwana position, the East Antarctic Orogen continues northward in East Africa as the East African Orogen, whereas a connection to the marginal Ross Orogen at the Pacific margin of East Antarctica is suggested along the Shackleton Range. The East Antarctic-East African Orogen resulted from closure of the Mozambique Ocean and collision of West and East Gondwana, i.e. western Dronning Maud Land was part of West Gondwana. During this collision the lithospheric mantle probably delaminated, allowing the asthenosphere to underplate the continental crust and producing heat for the voluminous, typically anhydrous, Pan-African granitoids of central Dronning Maud Land.