Alan S. Collins

The tectonic evolution of central and Northern Madagascar and its place in the final assembly of Gondwana

Recent work in central and northern Madagascar has identified five tectonic units of the East African Orogen (EAO), a large collisional zone fundamental to the amalgamation of Gondwana. These five units are the Antongil block, the Antananarivo block, the Tsaratanana sheet, the Itremo sheet, and the Bemarivo belt. Geochronological, lithological, metamorphic, and geochemical characteristics of these units and their relationships to each other are used as a type area to compare and contrast with surrounding regions of Gondwana. The Antananarivo block of central Madagascar, part of a broad band of pre‐1000‐Ma continental crust that stretches from Yemen through Somalia and eastern Ethiopia into Madagascar, is sandwiched between two suture zones we interpret as marking strands of the Neoproterozoic Mozambique Ocean. The eastern suture connects the Al‐Mukalla terrane (Yemen), the Maydh greenstone belt (northern Somalia), the Betsimisaraka suture (east Madagascar), and the Palghat‐Cauvery shear zone system (south India). The western suture projects the Al‐Bayda terrane (Yemen) through a change in crustal age in Ethiopia to the region west of Madagascar. Our new framework for the central EAO links the Mozambique belt with the Arabian/Nubian Shield and highlights the power of tectonic analysis in unraveling the complex tectonic collage of the EAO.

Alternative tectonic models for the Late Palaeozoic-Early Tertiary development of Tethys in the Eastern Mediterranean region

Abstract A summary and discussion is given of alternative models of the tectonic evolution of the Tethyan orogenic belt in the Eastern Mediterranean region, based on recent information. Model 1 (Robertson & Dixon 1984). A single Tethyan ocean continuously existed in the Eastern Mediterranean region, at least from Late Palaeozoic onwards. The dominant influences were episodic northward subduction of Tethyan oceanic crust beneath Eurasia, and the northward drift of continental fragments, from Gondwana towards Eurasia. During the Mesozoic, the south Tethyan area was interspersed with Gondwana-derived microcontinents and small ocean basins. Ophiolites formed mainly by spreading above subduction zones in both northerly (internal) and southerly (external) oceanic basins during times of regional plate convergence, and were mainly emplaced as a result of trench-passive margin collisions. In a related model, Stampfli et al. (1991) argued for spreading along the North African margin in the Late Permian. Model 2A (Dercourt et al. 1986). Only one evolving Tethys existed. Triassic-Jurassic oceanic crust (Neotethys) formed in a single Tethyan ocean basin located north of Gondwana-related units. Spreading later formed a small ocean basin in the present Eastern Mediterranean Sea area during the Cretaceous. Jurassic and Cretaceous ophiolites formed at spreading ridges and record times of regional plate divergence. In an update version, Model 2B (Dercourt et al. 1993), spreading extended along the northern margin of Gondwana, with an arm extending through the south Aegean, splitting off a large microcontinent. Further spreading in the Cretaceous then opened the Eastern Mediterranean basin and fragmented pre-existing carbonate platforms. The Mesozoic ophiolites were seen as being mainly far-travelled from northerly (i.e. internal) orogenic areas. Model 3 (Şengör et al. 1984). Subduction in the Late Palaeozoic was dominantly southwards, beneath the northern margin of Gondwana in the Eastern Mediterranean. This subduction led to opening of Triassic backarc basins; and a rifted Gondwana fragment (Cimmeria) drifted across a pre-existing Tethys (Palaeo-Tethys) to collide with a passive Eurasian margin. In their model, a backarc basin (Karakaya Basin) rifted and then closed prior to collision of a Cimmerian microcontinent in the Mid Jurassic, and this was followed by renewed rifting of a small ocean basin in the Early Jurassic. Mesozoic ophiolites mainly formed above subduction zones; they were variously seen as far-travelled (in the ‘Greek area’), or more locally rooted (in the ‘Turkish area’). Recent evidence shows that difficulties exist in detail with all three models. However, four key elements are met in Model 1: dominantly northward subduction in the north; multiple ocean basins from Triassic onwards in the south; supra-subduction spreading of the major ophiolites; and emplacement from both northerly and southerly Mesozoic oceanic basins. Palaeomagnetism has played an important role, in setting the large-scale Africa-Eurasia relative motion framework and in providing tests for the tectonic affinities of smaller units, but such smaller-scale studies have often been compromised by the geological complexity and by the remagnetisation of tectonically thickened units.

Lycian melange, southwestern Turkey: An emplaced Late Cretaceous accretionary complex

The Lycian thrust belt is an important part of the Tethyan orogenic belt in the eastern Mediterranean region. It includes a unit of internally disrupted, thin-skinned thrust sheets (layered tectonic melange) and a melange rich in ophiolitic material (ophiolitic melange). The layered tectonic melange is dominated by Mesozoic distal deep-water sedimentary rocks, and the ophiolitic melange includes disrupted thrust sheets of early Mesozoic shallow-water limestone and blocks of basalt of both mid-ocean ridge and within-plate geochemical affinities. A complete record of closure of a Tethyan oceanic basin began with Late Cretaceous intraoceanic subduction, followed by latest Cretaceous trench-continental collision, and ended with mid-Tertiary continental collision, and orogenic collapse.

Structural and thermal history of poly-orogenic basement: U–Pb geochronology of granitoid rocks in the southern Menderes Massif, Western Turkey

Ion microprobe U–Pb dating of granitoid rocks from key structural outcrops of the Menderes Massif in western Turkey provides an important constraint to the thermal and deformational history of a structurally complex metamorphic belt within the Alpine chain. Crystallization ages of two granite protoliths, derived from the weighted means of rim ages and the ages of homogeneous prismatic zircon grains, are 541 ± 14 Ma and 566 ± 9 Ma, whereas the cores of zoned pyramidal and short-prismatic zircon grains range from Palaeoproterozoic to Neoproterozoic in age. These ages indicate that amphibolite- to granulite-facies metamorphic rocks in much of the Menderes Massif were deformed, metamorphosed and intruded during the Pan-African Orogeny, and neither crystallized nor remelted during an Alpine event. Structural and metamorphic evidence for Alpine convergence in Pan-African basement rocks is limited to greenschist-facies top-to-the-south shear zones, which occur on a regional scale across a number of tectonic units.

Late Neoproterozoic and Early Cambrian palaeogeography: models and problems

Abstract We present two alternative sets of global palaeogeographical reconstructions for the time interval 615–530 Ma using competing high and low-latitude palaeomagnetic data subsets for Laurentia in conjunction with geological data. Both models demonstrate a genetic relationship between the collisional events associated with the assembly of Gondwana and the extensional events related to the opening of the Tornquist Sea, the eastern Iapetus Ocean (600–550 Ma), and the western Iapetus Ocean (after 550 Ma), forming a three-arm rift between Laurentia, Baltica, and Gondwana. The extensional events are probably plume-related, which is indicated in the reconstructions by voluminous mafic magmatism along the margins of palaeo-continents. The low-latitude model requires a single plume event, whereas the high-latitude model needs at least three discrete plumes. Coeval collisions of large continental masses during the assembly of Gondwana, as well as slab pull from subduction zones associated with those collisions, could have caused upper plate extension resulting in the rifted arm that developed into the eastern Iapetus Ocean and Tornquist Sea but retarded development of the western Iapetus Ocean. As a result, the eastern Iapetus Ocean and the Tornquist Sea opened before the western Iapetus Ocean.

U–Pb SIMS dating of synkinematic granites: timing of core-complex formation in the northern Anatolide belt of western Turkey

Secondary ion mass spectrometry (SIMS) U–Th–Pb dating of magmatic zircon from the synkinematic Eğrigöz and Koyunoba granites and a leucogranite dyke dates core-complex formation in the northern Anatolide belt of western Turkey at 24–19 Ma. The granites intrude into the footwall of the Simav detachment and are strongly elongated in the NNE direction parallel to tectonic transport on the detachment. Although large parts of the granites are undeformed, localized mylonitic to ultramylonitic deformation occurs directly beneath the Simav detachment and preserves evidence of progressive deformation from ductile to brittle conditions. Oscillatory zoned rims of long-prismatic zircon from the Eğrigöz and Koyunoba granites yield identical and well-constrained intrusion ages of 20.7 ± 0.6 Ma and 21.0 ± 0.2 Ma, whereas inherited grains range from Palaeoproterozoic (2972 ± 13 Ma) to Neoproterozoic (653 ± 6 Ma to 500 ± 5 Ma) in age. A leucogranite dyke yields an intrusion age of 24.4 ± 0.3 Ma, with inherited Neoproterozoic (640 ± 7 Ma to 511 ± 6 Ma) grains. Our data, in conjunction with published 40Ar/39Ar biotite ages, indicate very rapid cooling (greater than c. 200 °C Ma−1) for the granites during and after synkinematic emplacement.

Detrital footprint of the Mozambique ocean: U-Pb SHRIMP and Pb evaporation zircon geochronology of metasedimentary gneisses in eastern Madagascar

Abstract The southern East African Orogen is a collisional belt where the identification of major suture zones has proved elusive. In this study, we apply U–Pb isotopic techniques to date detrital zircons from a key part of the East African Orogen, analyse their possible source region and discuss how this information can help in unravelling the orogen. U–Pb sensitive high-mass resolution ion microprobe (SHRIMP) and Pb evaporation analyses of detrital zircons from metasedimentary rocks in eastern Madagascar reveal that: (1) the protoliths of many of these rocks were deposited between ∼800 and 550 Ma; and (2) these rocks are sourced from regions with rocks that date back to over 3400 Ma, with dominant age populations of 3200–3000, ∼2650, ∼2500 and 800–700 Ma. The Dharwar Craton of southern India is a potential source region for these sediments, as here rocks date back to over 3400 Ma and include abundant gneissic rocks with protoliths older than 3000 Ma, sedimentary rocks deposited at 3000–2600 Ma and granitoids that crystallised at 2513–2552 Ma. The 800–700 Ma zircons could potentially be sourced from elsewhere in India or from the Antananarivo Block of central Madagascar in the latter stages of closure of the Mozambique Ocean. The region of East Africa adjacent to Madagascar in Gondwana reconstructions (the Tanzania craton) is rejected as a potential source as there are no known rocks here older than 3000 Ma, and no detrital grains in our samples sourced from Mesoproterozoic and early Neoproterozoic rocks that are common throughout central east Africa. In contrast, coeval sediments 200 km west, in the Itremo sheet of central Madagascar, have detrital zircon age profiles consistent with a central East African source, suggesting that two late Neoproterozoic provenance fronts pass through east Madagascar at approximately the position of the Betsimisaraka suture. These observations support an interpretation that the Betsimisaraka suture separates rocks that were derived from different locations within, or at the margins of, the Mozambique Ocean basin and therefore, that the suture is the site of subduction of a strand of Mozambique Ocean crust.

Evolution of the Lycian Allochthon, western Turkey, as a north-facing Late Palaeozoic to Mesozoic rift and passive continental margin

Regional tectono-stratigraphic analysis allows widely distributed outcrops of mainly Mesozoic sedimentary rocks within the Lycian Allochthon, SW Turkey, to be correlated and placed within four regionally developed thrust sheets, the Karadag Thrust Sheet (lowest), the Teke Dere Thrust Sheet, the Koycegiz Thrust Sheet (highest), and the Yavuz Thrust Sheet. The Karadag Thrust Sheet records Late Carboniferous, Lower and Upper Permian continental shelf/lagoonal deposition. The overlying Teke Dere Thrust Sheet includes a rift succession of Late Permian age that was subaerially exposed during much of the Triassic; a marine transgression followed in the Early Jurassic succeeded by subsidence that formed a continental slope from Middle Jurassic to Palaeocene times. The overlying Koycegiz Thrust Sheet records Upper Triassic oceanic crust (along a rifted margin), overlain by a Lower Jurassic carbonate platform; this then subsided to form a continental slope that survived until Late Cretaceous times. The Lycian Allochthon is restored as a north-facing Mesozoic rift and passive margin taking account of structural evidence indicating southward thrust emplacement and comparisons of sedimentary successions. Mainly deep-water sediments of Triassic to Late Cretaceous age, preserved as blocks within melange units above the Lycian Thrust Sheets (Layered Tectonic Melange and Ophiolitic Melange), are interpreted as deep-water sediments deposited on Mesozoic (Neotethyan) oceanic crust. Subduction of the ocean basin proceeded from north to south, beginning with accretion of oceanic-derived melange and disrupted thrust sheets. Debris was shed into a continentward-migrating flexural foredeep, initially located along the distal edge of the continental margin in Campanian–Maastrichtian times; this foredeep then propagated southwards in stages over more proximal continental crust (including an intra-platform basin). The first main stage of southeastward propagation was in Palaeocene and Eocene times followed by a second stage in Oligocene–Miocene times. The Lycian Allochthon was finally emplaced over the most proximal (southeasterly) foredeep (i.e. the Kas basin) in Late Miocene time. Copyright

Testing models of Late Palaeozoic-Early Mesozoic orogeny in Western Turkey: support for an evolving open-Tethys model

Field evidence from north–south transects tests three tectonic models for Tethys in Western Turkey for when a Late Palaeozoic ocean was closing and an Early Mesozoic ocean opening. In Model 1, a Palaeozoic ocean subducted southwards, rifting continental fragments from Gondwana and opening a Triassic Neo-Tethys to the south. Closure and collision occurred by latest Triassic time. In Model 2, a wide Palaeozoic Tethys subducted northwards with an active Eurasian margin and a passive Gondwana margin. The northern Gondwana margin rifted in the Triassic; fragments either remained nearby (Taurides) or drifted northwards (e.g. Karakaya) attached to a north-subducting plate. New oceanic crust replaced Palaeo-Tethys with Neotethys and back-arc marginal basins opened along the south Eurasian margin (e.g. Küre). In Model 3, a Palaeozoic ocean also subducted northwards opening wide marginal basins. A wide Southern Neotethys opened along the Gondwana margin. Rifted Eurasian (Anatolides) and Gondwana (Taurides) fragments collided in mid-Tethys by latest Triassic time. Field evidence from the Pontides supports north-dipping subduction models (Model 2 or 3 above). Key features are a south-vergent, HP–LT accretionary prism, magmatic arc and back-arc basin system bordering the Eurasian margin. Also, evidence from the Tauride Mountains favours Model 2 over Model 3. Critically, the Anatolides and Taurides appear to have a common history and were unlikely to have been located on opposite sides of Tethys, as in Model 3.

Neoproterozoic extensional detachment in central Madagascar: implications for the collapse of the East African Orogen

A laterally extensive, Neoproterozoic extensional detachment (the Betsileo shear zone) is recognized in central Madagascar separating the Itremo sheet (consisting of Palaeoproterozoic to Mesoproterozoic sediments and underlying basement rocks) from the Antananarivo block (Archaean/Palaeoproterozoic crust re-metamorphosed in the Neoproterozoic). Non-coaxial deformation gradually increases to a maximum at a lithological contrast between the granitoids and gneisses of the footwall and the metasedimentary rocks of the hangingwall. Ultramylonites at this highest-strained zone show mineral-elongation lineations that plunge to the southwest. σ-, δ- and C/S-type fabrics imply top-to-the-southwest extensional shear sense. Contrasting metamorphic grades are found either side of the shear zone. In the north, where this contrast is greatest, amphibolite-grade footwall rocks are juxtaposed with lower-greenschist-grade hangingwall rocks. The metamorphic grade in the hangingwall increases to the south, suggesting that a crustal section is preserved. The Betsileo shear zone facilitated crustal-scale extensional collapse of the East African Orogeny, and thus represents a previously poorly recognized structural phase in the story of Gondwanan amalgamation. Granitic magmatism and granulite/amphibolite-grade metamorphism in the footwall are all associated with formation of the Betsileo shear zone, making recognition of this detachment important in any attempt to understand the tectonic evolution of central Gondwana.