Western Gondwanaland Research Articles

Thirteen Dipterocarpoideae genomes provide insights into their evolution and borneol biosynthesis

Dipterocarpoideae, the largest subfamily of the Dipterocarpaceae, is a dominant component of Southeast Asian rainforests and is widely used as a source of wood, damar resin, medicine, and essential oil. However, many Dipterocarpoideae species are currently on the IUCN Red List owing to severe degradation of their habitats under global climate change and human disturbance. Genetic information regarding these taxa has only recently been reported with the sequencing of four Dipterocarp genomes, providing clues to the function and evolution of these species. Here, we report on 13 high-quality Dipterocarpoideae genome assemblies, ranging in size from 302.6 to 494.8 Mb and representing the five most species-rich genera in Dipterocarpoideae. Molecular dating analyses support the Western Gondwanaland origin of Dipterocarpaceae. Based on evolutionary analysis, we propose a three-step chromosome evolution scenario to describe the karyotypic evolution from an ancestor with six chromosomes to present-day species with 11 and 7 chromosomes. We discovered an expansion of genes encoding cellulose synthase (CesA), which is essential for cellulose biosynthesis and secondary cell-wall formation. We functionally identified five bornyl diphosphate synthase (BPPS) genes, which specifically catalyze the biosynthesis of borneol, a natural medicinal compound extracted from damar resin and oils, thus providing a basis for large-scale production of natural borneol in vitro.

Silurian and Devonian Nepiomorph Bivalves from Bolivia

Rare nepiomorph bivalves from the Silurian and the Devonian beds of the Central Andes Basin of Western Gondwanaland are herein presented. They were collected in four scattered outcrops in the Eastern Andes Cordillera and southern Interandean Bolivia. A total of 58 specimens of medium preservation were studied. Almost all of them are Ludlowian to Pridolian in age: Praecardium sp. A, Cardiola fluctuans Barrande, Dualina secunda Barrande, and one species is of Eifelian age: Praecardium sudamericanum (Kozlowski). This is both, the first report of the genus PraecardiumBarrande of Ludlowian to Pridolian age in this part of Gondwanaland and the first occurrence of the species Cardiola fluctuans Barrande in Bolivia. Through findings in the centre of the Silurian—Devonian Central Andes Basin (Pongo and Chaquimayu localities), this paper confirms the cosmopolitism of Dualina secunda Barrande. This research contributes to the as yet inaccurately known Silurian and Devonian paleogeography of the Central Andes Basin and to update old systematic descriptions of nepiomorph bivalves from late 19th and early 20th centuries.

A suture–related accretionary wedge formed in the Neoproterozoic Araçuaí orogen (SE Brazil) during Western Gondwanaland assembly

The Araçuaí orogen represents a branch of the Brasiliano orogenic system developed between the São Francisco and Congo cratons in Neoproterozoic time. We conducted detailed studies on a complex schist belt located to the west of the Rio Doce magmatic arc, along the assumed suture zone of the Araçuaí orogen. This 30km-wide and 100km-long, NS-trending belt includes pelitic schists with intercalations of quartzites, metaultramafic schists and diopsidites, intruded by collisional granites. U–Pb ages from detrital zircon grains point to distinct provenances for different portions of the schist belt. The lower succession shows an age spectra and maximum depositional age (819Ma) similar to passive margin deposits of the precursor basin. The upper succession yielded a maximum deposition age around 600Ma, pointing toward the Rio Doce arc as the main sediment source. Thrust to the west onto the Guanhães basement and separated on the east from the Ediacaran Rio Doce magmatic arc by reverse–dextral faults, the schist belt exhibits the architecture of an asymmetric flower structure developed in transpressional regime. The distribution of metamorphic assemblages across the schist belt characterizes a collisional, Barrovian-type metamorphic zoning, in which the sillimanite, kyanite, staurolite and garnet zones are locally duplicated by thrusts. P–T conditions range from 700°C at 7.5kbar, at the western base of the pile, to 550°C at 5.5kbar, at the eastern top of the package. Zircon U–Pb ages record metamorphic overprinting on the sheared top of the basement at 560±20Ma and crystallization of collisional granites at 544±10Ma. Our results allow us to interpret the schist belt as a suture-related accretionary wedge and suggest that basin closure during the assembly of West Gondwanaland lasted to the Ediacaran–Cambrian boundary in the Araçuaí orogen.

The evolution of rifting on the volcanic margin of the Pelotas Basin and the contextualization of the Paraná–Etendeka LIP in the separation of Gondwana in the South Atlantic

The Pelotas Basin is the classical example of a volcanic passive margin displaying large wedges of seaward-dipping reflectors (SDR). The SDR fill entirely its rifts throughout the basin, characterizing the abundant syn-rift magmatism (133–113 Ma). The Paraná–Etendeka Large Igneous Province (LIP), adjacent to west, constituted the pre-rift magmatism (134–132 Ma). The interpretation of ultra-deep seismic lines showed a very different geology from the adjacent Santos, Campos and Espírito Santo Basins, which constitute examples of magma-poor passive margins. Besides displaying rifts totally filled by volcanic rocks, diverse continental crustal domains were defined in the Pelotas Basin, such as an outer domain, probably constituted by highly stretched and permeated continental igneous crust, and a highly reflective lower crust probably reflecting underplating.The analysis of rifting in this portion of the South Atlantic is based on seismic interpretation and on the distribution of regional linear magnetic anomalies. The lateral accretion of SDR to the east towards the future site of the breakup and the temporal relationship between their rift and sag geometries allows the reconstitution of the evolution of rifting in the basin. Breakup propagated from south to north in three stages (130–127.5; 127.5–125; 125–113 Ma) physically separated by oceanic fracture zones (FZ). The width of the stretched, thinned and heavily intruded continental crust also showed a three-stage increase in the same direction and at the same FZ. Consequently, the Continental-Oceanic Boundary (COB) shows three marked shifts, from west to east, from south to north, resulting into rift to margin segmentation. Rifting also propagated from west to east, in the direction of the final breakup, in each of the three segments defined. The importance of the Paraná–Etendeka LIP upon the overall history of rupturing and breakup of Western Gondwanaland seems to have been restricted in time and in space only to the Pelotas Basin.

Permian bryophytes of Western Gondwanaland from the Paraná Basin in Brazil

Abstract: This study involves bryophyte fossils from the Teresina Formation (Permian–Guadalupian) collected in the Rio Preto Quarry in the state of Paraná, southern Brazil. Two new genera are proposed, with two new species: Capimirinus riopretensis sp. nov. and Yguajemanus yucapirus sp. nov. C. riopretensis sp. nov. included a lateral sporophyte that was organically attached to a gametophyte and is thus one of the oldest pleurocarpous fossils known today. This discovery provides clear evidence of the evolution of mosses during the Permian, because an acrocarpous moss from this period was also found in India. The preservation of such a fragile structure demonstrates that no prefossilization displacement took place and that the sediments accumulated in a low‐energy, fresh‐water environment, similar to a lake fed by rain, and isolated from the direct influence of the sea. Campimirinus riopretensis may be placed within the earlier pleurocarps, and Yguajemanus yucapirus sp. nov. may belong to the Bryalean families. However, we were not able to place neither of these species in any existing family today.

Eocene out-of-India dispersal of Asian dipterocarps

The Dipterocarpaceae, a well known and economically important family of trees of the tropical rain forests of Asia, comprise over 470 species. These angiosperm trees contribute to 30% of the total area in typical lowland evergreen forests in Southeast Asia. Despite their remarkable diversity and regional ecological dominance, the origins and phytogeographical evolution of the family are poorly understood. The earliest dipterocarp fossils recorded in SE Asia come from Oligocene (34–23 Ma) sediments of Borneo. Here, we report an occurrence of Asian dipterocarps from approximately 53 Ma old sediments from western India based on fossil resin chemistry and palynological data. An important implication of our finding is that Asian dipterocarps must have originated in Gondwana and dispersed from India into Asia once the land connection between the Indian and Asian plate was well established during the middle Eocene (49–41 Ma). Moreover, the present study supports the hypothesis which suggests that many angiosperms did not originate in the SE Asian region, but dispersed into the area from western Gondwanaland.

The Pacific Gondwana margin in the late Neoproterozoic–early Paleozoic: Detrital zircon U–Pb ages from metasediments in northwest Argentina reveal their maximum age, provenance and tectonic setting

U–Pb detrital zircon ages are reported from Puncoviscana Formation (late Neoproterozoic–Early Cambrian) and Mesón Group (Late Cambrian) greywackes of northwest Argentina, to constrain provenance and depositional environment. The new data are combined with previously-published detrital zircon ages, to show that Puncoviscana Formation age patterns contain two broad groups: late Mesoproterozoic–early Neoproterozoic (1150–850 Ma) and late Neoproterozoic–Early Cambrian (650–520 Ma); with their relative proportions varying inversely with youngest component age. The 1150–850 Ma age components are dominant in greywackes with oldest late Neoproterozoic components > 600 Ma. The former diminish considerably when late Neoproteozoic components become dominant and younger, to 520 Ma. A northernmost greywacke sample from Purmamarca, Jujuy, is distinctive: whilst its zircon age pattern partly resembles other Puncoviscana Formation samples, it contains no Cambrian–late Neoproterozoic ages, the youngest ages being early Neoproterozoic. This may reflect an early, Neoproterozoic, passive-margin depocentre for the Formation, or an older (early Neoproterozoic) succession within it, which may predate the Brasiliano orogeny in Brazil. The youngest age components, c. 520 Ma, in a greywacke from Rancagua (Cachi, Salta province), dominate an almost unimodal pattern suggestive of contemporary volcanic sources at a late Early Cambrian depocentre. Detrital zircon age patterns of the Mesón Group (Lizoite Formation) have major Cambrian–latest Neoproterozoic components resembling those of the Puncoviscana Formation, but its Mesoproterozoic component is diminished, and there are no significant age components of this age. Small youngest components at c. 500 Ma suggest a maximum Late Cambrian stratigraphic age. The Puncoviscana Formation detrital zircon patterns suggest a provenance in a continental hinterland having a stabilised, extensive late Mesoproterozoic orogen (with minor Paleoproterozoic and Archean precursors), and a more variable late Neoproterozoic orogen containing an evolving sequence of less extensive subcomponents. A direct relationship with the Brazilian Shield is suggested; with sediment supplies originating within active-margin orogens of the interior and collisional orogens at the suture between African and South American cratons, but ultimate deposition in passive-margin environments of western Gondwanaland.

Could ‘Snowball Earth’ have left thick glaciomarine deposits?

At least two of the Cryogenian (Neoproterozoic) glacials, viz. those of the Sturtian and the Marinoan, are said to have been so severe that the entire Earth was covered with ice (Snowball Earth). The most convincing evidence consists of diamicts with some glacial striae and of other glacial signatures (striated surfaces, polished rocks) that have been found in areas that are interpreted on the basis of paleomagnetic data as being positioned, at the time, at low latitudes. The extremely low temperatures must have contributed to entirely frozen oceans. Nevertheless, diamicts of exceptional thickness were formed in a marine environment. This cannot be explained satisfactorily, as icebergs cannot have floated in an entirely frozen ocean. It is suggested that at least a considerable part of the extremely thick Neoproterozoic ‘glaciomarine’ deposits represent syntectonic mass-flow deposits rather than glacial deposits. The existence of a huge mountain range between Eastern and Western Gondwanaland provided favourable conditions for such deposits.

Phylogenetic relationships among gekkotan lizards inferred from C-mos nuclear DNA sequences and a new classification of the Gekkota

A phylogeny of gekkotan lizards was derived from C-mos nuclear DNA sequence data. Forty-one currently recognized genera, representing all major gekkotan lineages, were included in the study. A total of 378 bp of partial C-mos gene sequences was obtained and aligned. Maximum parsimony (MP) and maximum likelihood (ML) trees were generated based on unweighted analysis using PAUP*; similar tree topologies were recovered by both methods. The Eublepharidae were monophyletic and its relationship to other major clades was poorly resolved. The Pygopodidae of Kluge (1987) was monophyletic, but relationships within this group differed from those retrieved by previous analyses. The Diplodactylini + padded carphodactylines were the sister group of pygopods + padless carphodactylines. The Gekkonidae were monophyletic, but we found no evidence in support of the Teratoscincinae, as Teratoscincus was embedded well within the gekkonids. Both MP and ML analyses supported the basal position of Sphaerodactylus within the gekkonids, in contrast to morphologically based hypotheses. We propose a new higher order classification of the Gekkota that reflect these results. Five gekkotan families: Eublepharidae, Gekkonidae, Pygopodidae, Diplodactylidae, and Carphodactylidae are recognized. The higher order status of the sphaerodactyls will require more intensive sampling of this group. Our results support the hypothesis that the early cladogenesis of the Gekkota was associated with the split of Eastern Gondwanaland from Western Gondwanaland. Divergences among living genera in the Eublepharidae and the Eastern Gondwanan lineages (Diplodactylidae, Pygopodidae and Carphodactylidae) may be older than those in the Gekkonidae. © 2004 The Linnean Society of London, Biological Journal of the Linnean Society, 2004, 83, 353–368.

Anorogenic Gross Spitzkoppe granite stock in central western Namibia: Part I. Petrology and geochemistry

The Gross Spitzkoppe is a small (~30 km 2 ) anorogenic granite stock that was emplaced into the Damara Orogenic Belt of central western Namibia in response to mantle plume activity and early Cretaceous rifting of western Gondwanaland. The epizonal Gross Spitzkoppe stock (GSS), penecontemporaneous mafic and felsic dikes in the country rocks, and synplutonic mafic dikes and mafic magmatic inclusions within the GSS indicate bimodal magmatism. The GSS consists of three zonally arranged granitic units: medium-grained biotite granite at the margins, coarse-grained biotite granite, and porphyritic granite at the center of the stock. Late-stage silicic dikes and pegmatites cut the granites of the stock. All the granites are mildly peraluminous, topaz-bearing, high-silica monzogranites that contain extremely Fe-rich biotite (siderophyllite-annite) as the only primary mafic silicate. Common accessory minerals include: topaz, fluorite, magnetite, zircon, monazite, thorite, ilmenite, columbite, and niobian rutile. Miarolitic cavities and pegmatite pockets within the stock suggest that fluid saturation was attained. Rectilinear structures of biotite-rich schlieren along the marginal parts of the stock suggest that magma flow was parallel to the walls of the chamber, whereas turbulent or plume-like flows are indicated by curved, circular, and ladder-dike schlieren. Geochemical variation among the granites can be attributed to fractional crystallization of major and accessory minerals and by accumulation of halogen complexes with rare-earth and high-field-strength elements. The negative, partially irregular, correlation between whole-rock contents of F and Cl suggests that Cl was partially lost to a vapor phase during magmatic degassing. The granites of the Gross Spitzkoppe show clear A-type and within-plate granite characteristics, corresponding to the actual tectonic setting. The granites are interpreted to have formed by partial crustal remelting related to mafic underplating in a continental rift environment

A new species of Lepidotes (Neopterygii: Semionotiformes: Semionotidae) from the Santana Formation, Lower Cretaceous of northeastern Brazil

ABSTRACT A new species of Lepidotes is described on the basis of two nearly complete specimens from the ?Albian Romualdo Member of the Santana Formation, in the Araripe Basin, northeast Brazil. The new species differs from the other species of Lepidotes previously recorded in western Gondwanaland by the apomorphic presence of an axe-shaped hyomandibula, and by a unique combination of other characters. This is the first record of an articulated Lepidotes from the Santana Formation of the Araripe Basin.

Late Ordovician sedimentary environments, glacial cycles, and post-glacial transgression in the Taoudeni Basin, West Africa

The sedimentary record and worldwide palaeontological and isotopic data support the existence of short-lived, latest Ordovician (Hirnantian) ice sheets over western Gondwanaland. Located in the cratonic Taoudeni Basin (West Africa), the study areas are distributed across a 500-km-long profile ranging from ice-proximal to ice-distal depositional conditions (Hodh and Adrar areas, respectively; central and northwestern Mauritania). Glaciation-related but mainly non-glacial deposits form the Tichitt Group, which rests upon Cambrian–Ordovician rocks on top of a basin-wide erosional surface. The glacial record consists of depositional successions bound by unconformities of glacial origin in ice-proximal areas, or of sub-aerial origin in ice-distal areas. Facies associations reflect a variety of environments (braided streams, flood-dominated alluvial plains, delta plain to delta slope, tidal or storm-wave influenced shallow-marine settings). The upper bounding surface of the Tichitt Group generally corresponds to a wave–ravinement surface, overlain by uppermost Ordovician to lower Silurian shales. Four, inter-regionally distributed units are vertically superimposed and laterally juxtaposed. A unit is a hundreds of kilometres long sedimentary body, up to 100 m thick, which is laterally discontinuous as it typically infills palaeodepressions or palaeovalleys. In ice-proximal areas, aggrading fluvial deposits are identified. Coarse-grained braided stream deposits, including glacial surfaces related to minor glacial advances, predominate upstream. Downstream, finer-grained flood-dominated fluvial deposits are identified. In ice-distal areas, thick fluvial-dominated delta sediments are deposited. Each of the four units, of climatic significance, and built under high accommodation conditions, records a recession stage of the northern Gondwana ice sheet following a major glacial advance. The overall backstepping of the glacial units characterises the large-scale depositional architecture. The fourth unit, characterised by glaciomarine deposits in ice-proximal areas, and non-glacial, bioturbated storm-dominated deposits in ice-distal areas, records the final retreat of the northern Gondwana ice sheet. The lower bounding surface of this later glacial unit marks a Late Hirnantian major transgressive surface of a Late Ordovician–Silurian relative sea-level rise starting before the glaciation.

Petrology, geochemistry and Sr–Nd isotopes of the Trindade and Martin Vaz volcanic rocks (Southern Atlantic Ocean)

Geochemical, mineralogical and Sr–Nd isotopic analyses were performed on the strongly undersaturated volcanic rocks of the Trindade and Martin Vaz islands, located in the South Atlantic Ocean (Brazil). The magmatic activity at Trindade was essentially bimodal, generating various ultrabasic rocks and mesocratic to leucocratic phonolites. The youngest volcanic events are made up exclusively of ultrabasic rocks, whereas the older episodes are characterized by abundant phonolites. Basic rocks (45<SiO 2<52 wt.%) are rare, and commonly carry cumulate xenoliths of phaneritic alkaline rocks (e.g., clinopyroxenites, ijolites, malignites). The ultrabasic rocks belong to two distinct groups requiring the participation of different mantle sources. The more primitive group of olivine and pyroxene melanephelinites (SiO 2=41–42 wt.%, MgO=10–12 wt.%) is distinguished by significantly higher contents of K (K 2O=2–3 wt.%) than the more evolved one (basanites and tephrites; SiO 2=43–44 wt.%, MgO=8–9 wt.%, K 2O=0.4–1.0 wt.%). Fractional crystallization processes may explain the differentiation from melanephelinites or basanites–tephrites to phonolites. The Trindade stratigraphy and clinopyroxene mineral chemistry data strengthen the hypothesis that the extrusive materials probably came from stratified magma chambers, replenished in the late stages of magmatic activity with ultrabasic magmas. The phonolites display a wide range of compositions, also caused primarily by fractional crystallization, as indicated by mass-balance modelling, evolving in the extreme to peralkaline phonolites. According to Sr–Nd isotopic systematics, mixing between predominant (70%) depleted mantle and subordinate EMI components can explain the isotopic variations. The decoupling between Sr–Nd isotopic compositions (depleted relative to the Bulk Earth) and the Rb/Sr–Sm/Nd elemental ratios indicate that metasomatic events occurred in the mantle source(s) prior to melting processes. The enrichment episode took place either recently (in the last tens of millions of years) or at about 450 Ma as indicated by Nd model ages. This age agrees with those of the Brasiliano Cycle, suggesting the involvement of detached continental lithosphere, related to the breakup of Western Gondwanaland.

Systematics of Amaryllidaceae based on cladistic analysis of plastid sequence data

Cladistic analyses of plastid DNA sequences rbcL and trnL-F are presented separately and combined for 48 genera of Amaryllidaceae and 29 genera of related asparagalean families. The combined analysis is the most highly resolved of the three and provides good support for the monophyly of Amaryllidaceae and indicates Agapanthaceae as its sister family. Alliaceae are in turn sister to the Amaryllidaceae/Agapanthaceae clade. The origins of the family appear to be western Gondwanaland (Africa), and infrafamilial relationships are resolved along biogeographic lines. Tribe Amaryllideae, primarily South African, is sister to the rest of Amaryllidaceae; this tribe is supported by numerous morphological synapomorphies as well. The remaining two African tribes of the family, Haemantheae and Cyrtantheae, are well supported, but their position relative to the Australasian Calostemmateae and a large clade comprising the Eurasian and American genera, is not yet clear. The Eurasian and American elements of the family are each monophyletic sister clades. Internal resolution of the Eurasian clade only partially supports currently accepted tribal concepts, and few conclusions can be drawn on the relationships of the genera based on these data. A monophyletic Lycorideae (Central and East Asian) is weakly supported. Galanthus and Leucojum (Galantheae pro parte) are supported as sister genera by the bootstrap. The American clade shows a higher degree of internal resolution. Hippeastreae (minus Griffinia and Worsleya) are well supported, and Zephyranthinae are resolved as a distinct subtribe. An Andean clade marked by a chromosome number of 2n = 46 (and derivatives thereof) is resolved with weak support. The plastid DNA phylogenies are discussed in the context of biogeography and character evolution in the family.

O complexo Atuba: um cinturão paleoproterozóico intensamente retrabalhado no Neoproterozóico

Studies of terranes between the northern Ribeira and southern Dom Feliciano Belts allow the characterization of three geotectonic domains with different evolutions: the Luís Alves, Curitiba and Paranaguá terranes. The Atuba Complex occurs in the Curitiba Domain, which has a northwestern limit with metassediments of the Açungui and Setuva Groups and a southwestern limit with the granulitic gneisses of the Luis Alves domains. The contacts are expressive shear zones. The predominant rocks of the Curitiba Domain are banded, migmatitic gneisses in amphibolite grade with biotite-amphibole gneissic mesosomes and tonalitic/granodioritic to granitic leucosomes, here called the Atuba Complex. The migmatites are Paleoproterozoic (2.000 ± 200 Ma) and remigmatized in Neoproterozoic (600 ± 20 Ma). During the latter period temperatures reached more than 500º C. The structural pattem indicated shear-controlled tectonics with an important lateral component, and low-angle, south-southeastwards transport direction. The terranes of the Atuba Complex appear to represent deep-level rocks which were migmatized, granitized and then added to the border of the Luis Alves Microplate during the Neoproterozoic. This late Neoproterozoic tectonic scheme which continued to the Cambro-Ordoviciano seems to be the result of larger-scale processes of continental agglutination which ended with the formation of western Gondwanaland.

The Neotropical-African Moss Disjunction

The neotropical flora shares with Africa about 334 specific and infraspecific moss taxa. Their disjunct range may be explained by an ancient land connection or by long-distance dispersal. Certain species may represent lineages whose taxonomic identity has been preserved for millions of years, but in species with rapid evolutionary rates the long-distance dispersal hypothesis is favored. An alternate explanation for the neotropics proposes secondary centers of dispersalfrom an ancestral continuous distribution in western Gondwanaland. Other hypotheses are inadequate because they concern themselves with few species or with portions of the general range.

Grabens d'âge paléozoïque inférieuer et volcanisme tholéiitique associé dans la région de Garoua au Nord-Cameroun

Numerous hills of flat-lying sedimentary rocks rest uncomformably upon the metamorphic-granitic basement around Garoua (Northern Cameroon). Two types are distinguished among them: (i) Sedimentary hills of Cretaceous age, coeval with the infilling of the Yola- Garoua graben (the eastern branch of the Bénoué Trough), commonly cut across by Tertiary basaltic dykes, and (ii) volcano-sedimentary or sedimentary hills of early to middle Paleozoic age. The Paleozoic hills, the subject of this paper, appear as outcrops of a few square kilometers and represent the remnants of small ESE-WNW aulacogens infilled by sandstones and volcanic material of of late Ordovician to early Devonian age. Hosséré Balché (Hosséré=hill in Peulh language) is bounded along its whole perimeter by two types of faults: sub-vertical syn-sedimentary faults marked by the presence of coarse breccia and faults which postdate the sedimentary and volcanic infilling and are roughly parallel to the previous ones. Hosséré Mangbai probably corresponds to a stratovolcano developed in a small sedimentary basin; here all the boundary faults clearly postdate volcanism and sedimentation. In general, the sedimentary infilling consists of coarse-grained continental material derived from the erosion of associated volcanic rocks and/or the Precambrian basement. Volcanic rocks comprise early basalts of continental tholeiitic affinity and trachyandesites, rhyolites and rhyolitic ignimbrites. All of them have been affected by hydrothermal metamorphism which has partially obliterated their primary mineralogy and chemical composition. As a consequence, it is suggested, although not demonstrated, that these basic and acidic volcanic lavas might derive from a simple fractional crystallization process. The small aulacogens from northern Cameroon present lithological similarities with the older, late Pan-African, molassic basins of the Transaharian Fold Belt which run with an approximate NS direction from southwestern Algeria to Ghana and Togo. However, the aulacogens are younger and in fact coeval with the lower section of the sedimentary cover of the Tassilis surrounding the Hoggar massif. The moderate and localized extensional tectonics dated Ordovician to Devonian and responsible for the opening of the small aulacogens of Northern Cameroon could be also at the origin of the development of the wide, low subsiding, platformal, cratonic basins in Western Gondwanaland.

Paleomagnetism of the Hervey Group, Central New South Wales and its tectonic implications

Paleomagnetic work on the Hervey Group in southeast Australia reveals a predeformational magnetization as suggested by the retention of a predeformational compaction produced magnetic fabric and the effect of deformation on both the magnetic fabric and the direction of the magnetic remanence in cleaved rocks. The formation mean direction D=29.2°, I=−19.3° with α95=15.5° gives a latest Devonian‐Early Carboniferous palaeomagnetic pole at 54.4°S, 24.1°E with DP=8.4°, DM=16.2°. Comparing this pole with existing poles from Australia and North America, it is suggested that: (1) The Lachlan Fold Belt of southeast Australia has been part of cratonic Australia since at least the late Devonian, and consequently, the Late Palaeozoic palaeomagnetic data from the LFB may be applied to the whole of Gondwanaland. (2) Cratonic Australia and the New England Fold Belt were all in an equatorial position during the Early Carboniferous. (3) There was probably a rapid anticlockwise rotation of Gondwanaland during the Late Devonian around an axis close to Australia. (4) A “V”‐shaped ocean existed between Laurussia and the northern margin of western Gondwanaland during the late Devonian‐Early Carboniferous. The Appalachian‐Hercynian Orogenic Belt is probably the result of the closure of this ocean.

Mesozoic and Cenozoic reconstructions of the South Atlantic

Abstract The movement of Antarctica with respect to South America has a number of implications for paleocirculation as well as for the reconstructions of Gondwanaland. Recent papers on the Southwest Indian Ridge have published new or revised poles of opening for Africa and Antarctica which can be combined with the poles of opening between South America and Africa to give resultant motions between South America and Antarctica. The first indication of a complete closure between South America and the Antarctic Peninsula is at anomaly 28 time (64 Ma) as the two continents are now configured. Between anomaly 28 time (64 Ma) and anomaly M0 time (119 Ma) the amount of closure does not change greatly, and the small computed overlap can be explained by minor uncertainties in the rotation poles used for the reconstructions or some slight extension between East and West Antarctica. By 135 Ma some rotation or translation of the Antarctic Peninsula with respect to East Antarctica must be postulated in addition to any presumed extension between East and West Antarctica in order to avoid an overlap of South America with the Antarctic Peninsula. Having determined what we feel to be a viable reconstruction of Western Gondwanaland and holding South America fixed, we rotated Africa and Antarctica, with respect to South America, for eight different times during the past. Africa moved away from South America in a more or less consistent manner throughout the time period, closure to present, while Antarctica moved away from Africa in a consistent manner only between 160 Ma and 64 Ma. At 64 Ma its motion changed abruptly: it slowed its north-south motion with respect to Africa and began slow east-west extension with respect to South America. This change supports the hypothesis that a major reorganization of the triple junction between Africa, Antarctica and South America occurred between 60 and 65 Ma. The triple junction changed from ridge-ridge-ridge to ridge-fault-fault at the time of the major westward jump of the Mid-Atlantic Ridge just south of the Falkland-Agulhas Fracture Zone. The Mesozoic opening of the Somali Basin moved Madagascar from its presumed original position with Africa in Gondwanaland. The closure of Sri Lanka with India produces a unique fit for India and Sri Lanka with respect to Africa, Madagascar and Antarctica. This fit juxtaposes geological localities in Southeast India against similar localities in Enderhy Land. East Antarctica. The late Jurassic opening in the Somali Basin is tied to opening of the same age in the Mozambique Basin. Since this late Jurassic movement represents the initial break-up of Gondwanaland, it is assumed that similar movement must have occurred in what is now the western Weddell Sea and may also explain the opening evidenced by the Rocas Verdes region of southern South America.

Palaeomagnetism and the reconstruction of gondwanaland

The past existence of the supercontinent Gondwanaland has been regarded as the basis of a now respectable hypothesis which explains the present distribution of rock types peculiar to the southern hemisphere continents and India. Gondwana geologists were clearly attracted to the hypothesis even though specific details of the reconstruction remained unresolved. Much credence has been given to reconstructions based on computer matching of continental-shelf morphologies: the palaeomagnetic data provided insufficient resolution to be used exclusively for detailed intercontinental reconstructions. The geometrical fit of South America to Africa and Australia to Antarctica left little to question whereas the relationship of western Gondwanaland (Africa and South America) to eastern Gondwanaland (Australia, Antarctica and India) has long been a topic for discussion. The detail of that fit depended primarily on the pre-drift position of Madagascar. Recent palaeomagnetic data from Madagascar have provided the unequivocal result that the island was situated off East-Africa near Kenya and Tanzania. The predrift relationship of eastern and western Gondwanaland established previously on morphological grounds has now been verified from the palaeomagnetic data. Palaeomagnetic data for the Mesozoic of Australia have also long been considered anomalous within the framework of Gondwanaland. The data were inconsistent with any form of reconstruction from the geological and morphological viewpoints. However, a new investigation of Mesozoic rocks in Australia has resolved that anomaly and the existence of a single group of palaeomagnetic poles for the Triassic—Jurassic of all continents comprising Gondwanaland confirms that the Smith-Hallam reconstruction is as realistic a model as currently available. The Smith-Hallam model has been used to combine palaeomagnetic data from the Gondwana continents for the Late Palaeozoic and Mesozoic. The large amount of data for the Late Palaeozoic and Early Mesozoic of South America clearly implies that the supercontinent was drifting during that period and the data are best considered in ‘time groups’ that cross Period boundaries. The unity of Gondwanaland has been confirmed until mid-Jurassic time.