Breakup Of Antarctica Research Articles

Reconstruction of the dispersal history of Winteraceae R. Br. ex Lindl. according to phylogenetic analysis

Winteraceae family consists of 5 genera that are disjunctly distributed in Madagascar, Australasia, Central and South America. Phylogenetic relationships in Winteraceae were reconstructed using the molecular data (ITS-5.8S rDNA and trnL-trnF sequences). Paleobotanical data indicate the origin of Winteraceae in the north of Gondwana in the Lower Cretaceous. The calibration of the phylogenetic tree with 3 fossil records allowed to estimate the divergence time of Takhtajania at ca. 77 Ma, which correlates with the fragmentation of Eastern Gondwana (ca. 84-95 Ma). The origin of Drimys was caused by the breakup of Antarctica and Australia. Disjunctions in Zygogynum can be explained by several episodes of long-distance dispersal.

India‐Elan Bank‐East Antarctica Breakup, Crustal Architecture, and Margin Evolution: Results From Constrained Potential Field and Process‐Oriented Gravity Modeling of Conjugate Margin Segments

AbstractWe present here the results of a seismically well‐constrained potential field modeling along with process‐oriented gravity modeling across the Eastern Continental Margin of India (ECMI) and its conjugate East Antarctica Continental Margin (EACM). The study revealed both along‐strike variations in crustal architecture and effective elastic thickness (Te), showing a locally asymmetric pattern. Crustal stretching factors (β) estimated at these margins range between 1.5 and 4.0 with slightly lower values varying over the narrow zone in the Cauvery basin‐western Enderby basin and relatively higher values in other margin segments. The models reveal typical magma‐poor margin characteristics in the Krishna‐Godavari (K‐G) and Cauvery basin segments of ECMI. The ocean‐continent transition (OCT) in the K‐G basin and its conjugate (central Enderby basin) indicates asymmetric rifting marked by the development of a detachment fault separating the combined India‐Elan Bank margin and the EACM. The higher β values and magmatic underplating at OCT in the Mahanadi basin segment of ECMI may be related to the post‐breakup magmatic emplacement associated with the proximal activity of the Kerguelen plume after the separation of Elan Bank from India by a ridge jump during M2/M0 (124–120 Ma). The pre‐rift reconstruction model through the palinspastic restoration of margins suggests that the central part of ECMI (16–20°) is the ideal pre‐rift location for the Elan bank.

Transdimensional ambient noise tomography of Bass Strait, southeast Australia, reveals the sedimentary basin and deep crustal structure beneath a failed continental rift

Debate is ongoing as to which tectonic model is most consistent with the known geology of southeast Australia, formerly part of the eastern margin of Gondwana. In particular, numerous tectonic models have been proposed to explain the enigmatic geological relationship between Tasmania and the mainland, which is separated by Bass Strait. One of the primary reasons for the lack of certainty is the limited exposure of basement rocks, which are masked by the sea and thick Mesozoic–Cenozoic sedimentary and volcanic cover sequences. We use ambient noise tomography recorded across Bass Strait to generate a new shear wave velocity model in order to investigate crustal structure. Fundamental mode Rayleigh wave phase velocity dispersion data extracted from long-term cross-correlation of ambient noise data are inverted using a transdimensional, hierarchical, Bayesian inversion scheme to produce phase velocity maps in the period range 2–30 s. Subsequent inversion for depth-dependent shear wave velocity structure across a dense grid of points allows a composite 3-D shear wave velocity model to be produced. Benefits of the transdimensional scheme include a data-driven parametrization that allows the number and distribution of velocity unknowns to vary, and the data noise to also be treated as an unknown in the inversion. The new shear wave velocity model clearly reveals the primary sedimentary basins in Bass Strait as slow shear velocity zones which extend down to 14 km in depth. These failed rift basins, which formed during the early stages of Australia–Antarctica break-up, appear to be overlying thinned crust, where high velocities of 3.8–4.0 km s−1 occur at depths greater than 20 km. Along the northern margin of Bass Strait, our new model is consistent with major tectonic boundaries mapped at the surface. In particular, we identify an east dipping velocity transition zone in the vicinity of the Moyston Fault, a major tectonic boundary between the Lachlan and Delamerian orogens, which are part of the Phanerozoic accretionary terrane that makes up eastern Australia. A pronounced lineament of high shear wave velocities (∼3.7–3.8 km s−1) in the lower crust of our new model may represent the signature of relict intrusive magmatism from failed rifting in the early stages of Australia–Antarctica break-up along a crustal scale discontinuity in the Selwyn Block microcontinent which joins Tasmania and Victoria.

Seismically Derived Gondwana and Proterozoic Sediments East of Cuddapah Basin, South Indian Shield and Its Possible Geotectonic Implications

The study area situated east of the intracratonic Proterozoic Cuddapah basin, that covers Nellore Schist belt, Ongole domain of the Eastern Ghats Belt and adjacent East Coast Terrain, forms a geodynamically important segment of the Dharwar craton. This region came into focus due to proposition that during Paleoproterozoic to Mesoproterozoic era, it underwent persistent oceanic subduction, continent–continent collision and multi-stage accretional growth. In view of this, we have reprocessed in detail the deep seismic sounding data over this region and integrated the obtained results with other geophysical studies to decipher the deep subsurface crustal structure underneath. The derived shallow seismic section across this region showed the presence of a thin layer (~250 m) of Gondwana sediments with a distinct velocity of 4.20 km/s and a lateral dimension of about 40 km around Vinjamuru region of the Nellore Schist Belt. Another important finding has been the depiction of 5.3 and 5.5 km/s velocity sediments (comparable to upper and lower Cuddapah sediments) between Nellore Schist Belt and the east coast, indicating the possible presence of an additional Proterozoic sedimentary terrain (named here as East Coast Sedimentary Basin), overlying the concealed Archean crystalline basement. The presence of this sedimentary terrain, which is separated from the Cuddapah basin by an exhumed horst-like feature, situated below Nellore Schist Belt, would indicate Proterozoic sedimentation in a much larger area during Columbia and Rodinia supercontinental assembly periods than hitherto known. Importantly, no seismic trace of the Eastern Ghats Belt has been found between Nellore Schist Belt and the east coast. It appears that during the Gondwana period, entire eastern segment of the east Dharwar craton has been persistently rifting, uplifting and eroding together with other Gondwana grabens in India, thereby leaving behind only a thin veneer of Gondwana sediments at many places. In view of the presence of number of Gondwana occurrences all along the east coast, it is felt that the rifting during the Gondwana period may have been a prelude to India–Antarctica breakup, with final separation between the two taking place much later during early Cretaceous.

First Cretaceous (Albian) invertebrate fossil assemblage from Batavia Knoll, Perth Abyssal Plain, eastern Indian Ocean: taxonomy and paleoecological significance

AbstractThe first sedimentary rocks from Batavia Knoll, on the western edge of the Perth Abyssal Plain, eastern Indian Ocean, have been recovered, yielding an assemblage of invertebrate fossils hitherto undocumented from this part of the world. The fauna consists of 22 species of Mollusca, including new gastropods, a calliotropidPlanolateralus acanthanodusn. sp.; a margaritidIgonoia levimargaritan. sp.; a procerithiidProcerithium arenacollicolan. sp.; and aporrhaidsDrepanocheilus bataviensisn. sp. andAnchura pelsaertin. sp. In addition, pleurotomariid, ringiculid, and architectonicid gastropod taxa were recovered. Bivalves are represented by members of the Nuculanidae, Inoceramidae, Pinnidae, Buchiidae, Lucinidae, Veneridae, and Hiatellidae. Scaphopods (Dentaliidae) and ammonites (two taxa, of Desmoceratidae and Hamitidae) are also present. Further recovered were one species of Serpulidae (Polychaeta), two of Trachyleberididae (Ostracoda), and a probable echinoid fragment. The fossil assemblage was dominated by shallow marine suspension-feeding taxa (39% of the suite). Detritivorous and herbivorous taxa comprised 22% and 9%, respectively, with nektic and epifaunal carnivores amounting to 30%. Taphonomic analyses of these fossils and their host sedimentary facies revealed the Batavia Knoll sandstone was deposited in a shallow marine environment during a mass-flow event. Biostratigraphic range data of the preserved macro- and microfossil assemblages imply an age of latest Albian, contemporaneous with the rifting of Batavia Knoll from Greater India during the broader India–Australia–Antarctica breakup in the mid-Cretaceous.

Linking mainland Australia and Tasmania using ambient seismic noise tomography: Implications for the tectonic evolution of the east Gondwana margin

For nearly half a century, a number of conflicting tectonic models have been postulated to explain the enigmatic geological relationship between Tasmania and Victoria, with a view to unifying our understanding of the evolution of the eastern margin of Gondwana in Australia. In this study, ambient noise data from an array of 24 broadband seismometers is used to produce a high-resolution 3-D crustal shear wave velocity model of Bass Strait, the key to understanding the missing link. We apply a novel transdimensional and hierarchical Bayesian inversion approach to construct group velocity maps in the period range of 2–30s, and subsequently invert group velocity dispersion for 3-D shear wave velocity structure. This allows us to image, for the first time, the entire crust beneath Bass Strait in high detail and elucidate the geometry and position of key crustal features with corroboration from complementary datasets. The three sedimentary basins related to the failed rifting event associated with the Australia–Antarctica breakup, in particular Bass Basin, clearly emerge from the tomographic solution model. A key feature of the 3-D shear wavespeed model is a distinct mid-lower crustal NW–SE high velocity zone which extends from northwestern Tasmania to south-central Victoria, confirming a Proterozoic geological connection. We also image three north–south high velocity belts that appear to span Bass Strait, with some interruption from velocity variations possibly related to more recent tectonic events. These belts are consistent with recent gravity and magnetic maps, and may indicate the presence of an exotic Precambrian terrane (the Selwyn Block). The model also images the crustal velocity structure of the southern Stawell and Bendigo Zones, and their internal large-scale multi-layer characters, a legacy of their Early Paleozoic intra-oceanic origins. Another high velocity anomaly imaged in the mid-lower crust is an east–west lineament beneath the northern part of Bass Strait, which may be an intrusive feature associated with the failed rift.

MINERAL COMPOSITIONS AND PETROGENETIC EVOLUTION OF THE ULTRAMAFIC-ALKALINE - CARBONATITIC COMPLEX OF SUNG VALLEY, NORTHEASTERN INDIA

The Sung Valley alkaline complex is a relatively small intrusion of Lower Cretaceous age emplaced slightly before or during the India–Antarctica break-up. It consists of ultramafic rocks (dunites, wehrlites, clinopyroxenites, uncompahgrites), mafic rocks (ijolites sensu lato ), felsic rocks (nepheline syenites) and carbonatites. The chemical composition of the mafic minerals indicates the expected enrichment in iron toward the felsic rocks. On the other hand, carbonatites feature very Mg-rich minerals, generally Cr-rich, indicating that their genesis is completely unrelated to that of mafic and felsic rocks (ijolites and nepheline syenites). The parageneses indicate that this complex was formed by batches of primitive magmas with a distinct magmatic affinity (olivine melilitites and olivine nephelinites, basanites, and possibly also carbonatites) which evolved independently, generating the observed spectrum of intrusive rocks. Clinopyroxenites have interstitial alkali feldspar and titanite, indicating that they formed from evolved feldspar-normative (phonotephritic, tephriphonolitic) magmas. The sequence perovskite–titanite and titanite–garnet noted in some ijolitic rocks indicates changes in the chemical composition of coexisting silicate melts and, most likely, an increasing f (O 2 ). The trace-element profiles of coexisting phases in interesting associations in a sample of ijolite were documented by means of LA–ICP–MS analyses.

Evidence from Antarctic mantle peridotite xenoliths for changes in mineralogy, geochemistry and geothermal gradients beneath a developing rift

Garnet and spinel peridotite xenoliths associated with the Phanerozoic Lambert–Amery Rift in eastern Antarctica contain evidence for several stages in the development of the mantle beneath the rift. Despite the fact that equilibria were only partly attained, a combination of petrography, whole-rock geochemistry, mineral chemistry and thermobarometry can be used to decipher four stages prior to entrainment of the xenoliths in the host magma during the initial stages of the breakup of Antarctica, India and Madagascar. The first chronological stage is represented by harzburgitic protoliths represented by rare occurrences of low-Ca olivines and orthopyroxenes in spinel lherzolites: these yield the lowest temperatures of 830–850 °C, and are also characterized by distinct trace element contents; lower Ti, Cr, V and Zn in olivine and orthopyroxene, and additionally lower Cu, Ni, Ga and Li in orthopyroxene. Some garnets are subcalcic, indicating that the spinel–garnet lherzolites also formed from harzburgitic protoliths. The second stage is the formation of garnet due to a pressure increase probably related to collision at 1.1 Ga. The third stage is marked by the growth of clinopyroxene, demonstrably in cpx-poor spinel lherzolites but probably in all xenolith groups: equilibrium of clinopyroxene with olivine and orthopyroxene was not attained in all samples, so that the non-judicious use of thermobarometers can produce bewildering results. The fourth stage is an enrichment episode that affected all spinel–garnet peridotites and about half of the spinel peridotites. During this stage, reaction rims were produced on the clinopyroxenes that formed during stage 3, the modal content of olivine and Mg/(Mg + Fe) in the rocks was reduced, CaO, Al 2O 3 and trace elements were enriched, and garnets were almost completely transformed to kelyphites. A later stage is documented by interstitial glasses and films around spinels related to infiltration of melt from the host magma. These post-date, and are more enriched in alkalies than, partially melted rims on clinopyroxenes, demonstrating that all the three earlier episodes were pre-entrainment events. Pressures indicated by the spinel + garnet lherzolites are restricted to 20–24 kbar at 1040–1180 °C. Early harzburgitic assemblages are interpreted to represent an earlier, cooler geotherm, whereas the kelyphite assemblages indicate temperatures 180–200 °C hotter than the main xenolith geotherm. This event also caused recrystallization of the clinopyroxene rims and is attributed to heating during rifting, but not due to the host magma itself. The preservation of evidence for three progressively hotter geotherms can be related to the upward movement of isotherms during the development of the sub-rift mantle.

New constraints on the age and evolution of the Wishbone Ridge, southwest Pacific Cretaceous microplates, and Zealandia–West Antarctica breakup

We present analytical results from four dredge locations across the eastern Zealandia continental margin and adjacent ocean crust. The 115 Ma dacites dredged from the West Wishbone Ridge (WWR) are isotopically primitive, weakly adakitic, slab-derived lavas. The 97 Ma A-type granites and a basalt from the easternmost Chatham Rise enlarge the known area of postsubduction Gondwana magmatism. Amphibolite-grade schists from a fault block south of the Chatham Rise provide a critical bridge between the Zealandia and West Antarctica belts of Jurassic–Cretaceous accretionary prism rocks. The new recognition of the WWR as a remnant of a 115 Ma intraoceanic subduction system means that previous hypotheses of the WWR as a fracture zone or spreading ridge require modification. The dacite ages constrain the start of Osbourn Trough spreading, which caused breakup of the Hikurangi-Manihiki igneous plateau, to before 115 Ma. We speculate that, after 115 Ma, the WWR was rifted by an intraoceanic spreading center that developed along its southeast side. Impingement of this spreading center against the Gondwana margin led to widespread 95–100 Ma postsubduction magmatism, variable lithospheric stretching, and ultimately continental splitting of Zealandia and West Antarctica across basement trends.

Teleseismic delay-time tomography of the upper mantle beneath southeastern India: imprint of Indo-Antarctica rifting

A 3-D P-velocity map of the crust and upper mantle beneath the southeastern part of India has been reconstructed through the inversion of teleseismic traveltimes. Salient geological features in the study region include the Archean Dharwar Craton and Eastern Ghat metamorphic belt (EGMB), and the Proterozoic Cuddapah and Godavari basins. The Krishna—Godavari basin, on the eastern coastal margin, evolved in response to the Indo—Antarctica breakup. A 24-station temporary network provided 1161 traveltimes, which were used to model 3-D P-velocity variation. The velocity model accounts of 80 per cent of the observed data variance. The velocity picture to a depth of 120 km shows two patterns: a high velocity beneath the interior domain (Dharwar craton and Cuddapah basin), and a lower velocity beneath the eastern margin region (EGMB and coastal basin). Across the array velocity variations of 7–10 per cent in the crust (0–40 km) and 3–5 per cent in the uppermost mantle (40–120 km) are observed. At deeper levels (120–210 km) the upper-mantle velocity differences are insignificant among different geological units. The presence of such a low velocity along the eastern margin suggests significantly thin lithosphere (<100 km) beneath it compared to a thick lithosphere (>200 km) beneath the eastern Dharwar craton. Such lithospheric thinning could be a consequence of Indo—Antarctica break-up.

Early Cretaceous volcano-sedimentary successions along the eastern Australian continental margin: Implications for the break-up of eastern Gondwana

We report on three large volume Early Cretaceous volcanic and sedimentary provinces: the Whitsunday Volcanic Province and Great Artesian Basin system, both of northeastern Australia, and the Otway/Gippsland basin system along the southeastern margin of Australia. The Whitsunday Volcanic Province is part of a mafic to silicic, high-K calc-alkaline pyroclastic volcanic belt that extends for more than 900 km along the central and southern Queensland coast. Estimated extrusive volumes are >10 5 km 3. Volcanic and intrusive activity shows a broad range of ages from 132 to 95 Ma, but ages are dominated by an event between ∼120 and 105 Ma. Contemporaneous with volcanism in the Whitsunday Volcanic Province, sedimentary basins in interior and eastern Queensland were receiving large volumes (>10 6 km 3) of volcanogenic sediment. The Otway and Gippsland basins 1500 km to the south, were initiated by the break-up of Antarctica and Australia. These basins contain >4×10 5 km 3 of Aptian–Albian extrabasinal volcanogenic sediment supplied from the east. This volcanogenic sedimentation post-dates rift-related volcanism within the basin system. These three provinces are each significant for: (1) the accumulation of large volumes of volcanic and/or coeval volcanic-derived material; (2) the compositional similarity between phenocryst and detrital plagioclase, augite and hornblende; and (3) age data recording a major volcanic episode between 125 and 105 Ma. A causal relationship between volcanism in the Whitsunday Volcanic Province and volcaniclastic sedimentation in the Otway/Gippsland and Great Artesian basin systems is therefore suggested. We propose these provinces record volcanism related to the break-up of eastern continental Gondwana and the formation of the modern eastern Australian passive margin. The scale and volume of volcanic products, coupled temporally with emplacements of oceanic plateaux in the Southwest Pacific, demonstrate that this volcanic event along the present eastern Australian plate margin should be considered as another Early Cretaceous large igneous province.

Offshore Investigations on Wilkes Land-Victoria Land Margin, Antarctica: ABSTRACT

In January 1984, the U. S. Geological Survey research vessel S. P. Lee carried out investigations of the Antarctic continental margin in the Wilkes Land and Victoria Land areas, using 24-channel and high-resolution seismic, sonobuoy refraction, gravity, magnetic, and bottom-sampling methods. This investigation augmented previous surveys of the Dumont d'Urville area by the French Petroleum Institute and explored new areas west and east to the boundary between the onshore Wilkes basin and the Victoria Land highlands. These surveys defined sediment thickness distribution and seismic stratigraphy in this frontier area. The tectonic style of the boundary between the East Antarctic craton and the younger crust of West Antarctica in the Ross Sea is revealed by one multichannel s ismic line across this important boundary. The initial breakup of Antarctica from Australia occurred as a slowly spreading phase during the middle Cretaceous. According to Deep Sea Drilling Project results on the Tasman Rise, conditions of restricted circulation existed in the growing basin between the continents before the late Eocene. After the late Eocene, the major oceanic circulation pattern was established. Before that time, conditions were favorable for preservation of organic-carbon deposits on the sea floor. Among the questions to be addressed with this data are the following. (1) How do apparent subsidence rates of this passive margin compare with others around the world? (2) Does the onshore subglacial Wilkes basin continue onto the continental shelf? (3) Do Antarctic counterpart basins to the Otway and Ceduna basin of Australia exist? (4) What is the effect of the icecap on the stratigraphy of this margin? (5) Do the two major Tertiary ice advances have conspicuous seismic-stratigraphic signatures? End_of_Article - Last_Page 474------------

Structural Style and Hydrocarbons of Bass, Gippsland, and Otway Basins: ABSTRACT

The Bass, Gippsland, and Otway basins lie mainly offshore southeast of Australia. Offshore fields in the Gippsland basin contain recoverable reserves of approximately 8.0 Tcf of gas and 1.7 billion bbl of oil and currently supply two thirds of Australia's oil requirements. No commercial hydrocarbons have been discovered in the adjacent Bass and Otway basins. The basins were created during the Jurassic to mid-Cretaceous continental breakup of Antarctica, Australia, and New Zealand. Their initial tectonic framework was primarily tensional with basining achieved by normal faulting. End_Page 1440------------------------------ The Otway basin has a classic structural style with a series of normal faults parallel with the spreading axis. The Gippsland and Bass basins are similar but their complete pull-apart was prevented by the influence of the Tasmanian continental block (subplate). The separation of New Zealand from Australia had only a minor effect on the structural framework of the Gippsland basin in the shallow-slope areas. Following the continental breakup and margin collapse, the Otway basin remained structurally quiescent but the Bass and Gippsland basins continued to subside and accumulate thick Tertiary sections. However, during the late Eocene and Miocene all basins were subjected to an east-west right-lateral-shear deformation. In Gippsland, the shear generated large en echelon anticlines and rejuvenated some of the old tensional faults. Hydrocarbons accumulated in these anticlines or their eroded remnants. In Bass and Otway the shear was less severe and confined to the northern margins. The deformation in these basins was correspondingly less intense. However, structural styles in each basin are similar enough that their varying intensity does not explain fully the lack of success in the areas explored in Bass and Otway. Other factors such as the presence or absence of organic matter, maturation, and migration must have influenced hydrocarbon distribution. End_of_Article - Last_Page 1441------------