Magma flow in dyke swarms of the Karoo LIP: Implications for the mantle plume hypothesis

The ~ 183 Ma old Karoo Large Igneous Province extends across southern Africa and is related to magmatism in Antarctica (west Dronning Maud Land and Transantarctic Mountains) and parts of Australasia. Intrusive events, including the emplacement of at least ten dyke swarms, occurred between ~ 183 Ma and ~ 174 Ma. We review here the field evidence, structure and geochronology of the dyke swarms and related magmatism as it relates to melt sources and the mantle plume hypothesis for the Karoo LIP. Specifically, the magma flow-related fabric(s) in 90 dykes from five of these swarms is reviewed, paying particular attention to those that converge on triple junctions in southern Africa and Antarctica. The northern Lebombo and Rooi Rand dyke swarms form an integral part of the Lebombo monocline, which converges upon the Karoo triple junction at Mwenezi, southern Zimbabwe. Dykes of the Northern Lebombo dyke swarm (182–178 Ma) appear to have initially intruded vertically, followed later by lateral flow in the youngest dykes. In dykes of the Okavango dyke swarm (178 Ma) there is evidence of steep magma flow proximal to the triple junction, and lateral flow from the southeast to the northwest in the distal regions. This is consistent with the Karoo triple junction and the shallow mantle being a viable magma source for both these dyke swarms. In the Rooi Rand dyke swarm (174 Ma) there is also evidence of vertical and inclined magma flow from north to south. This flow direction cannot be reconciled with the Karoo triple junction, as the northern termination of the Rooi Rand dyke swarm is in east-central Swaziland. The Jutulrøra and Straumsvola dyke swarms of Dronning Maud Land display evidence of sub-vertical magma flow in the north and lateral flow further south. The regional pattern of magma flow is therefore not compatible with direction expected from the Weddell Sea triple junction. The overall flow pattern in Karoo dykes is consistent with the triple junction being an important magma source. However, the Limpopo Belt and Kaapvaal Craton have significantly controlled the structure and distribution of the Lebombo and Save–Limpopo monoclines and the Okavango dyke swarm. The locus of magma flow in dykes of Dronning Maud Land is at least 500 km from the Karoo triple junction, as is the apparent locus for the Rooi Rand dyke swarm. In comparison with recent modelling of continental assembly, the structure and flow of the dyke swarms, linked with geochronology and geochemistry, suggests that thermal incubation during Gondwana assembly led to Karoo magmatism. A plate tectonic, rather than a fluid dynamic plume explanation, is most reasonably applicable to the development of the Karoo LIP which does not bear evidence of a deep-seated, plume source.     

Geochemistry of the Mesozoic regional basic dykes of western Dronning Maud Land,Antarctica

Regional dolerite dykes of Mesozoic age in western Dronning Maud Land are variable in both major and trace element composition and include picritic types (MgO>18 wt%). The range in incompatible element concentrations is considerable (e.g. Zr 40–478 ppm) and shows little correlation with MgO content. Both high-and low-Ti, Zr (HTZ and LTZ) magma types are present and there is a spread of compositions between these types. Major element oxide variations in dykes having MgO>10 wt% indicate that olivine and orthoyproxene fractionation occurred, presumably at an early high-pressure stage of magma evolution. Major element oxide variations in dykes having MgO<10 wt% indicate control by olivine and clinopyroxene. A minority of the more evolved dykes are compositionally similar to the nearby Kirwan basalts, but the majority cannot be related to the Kirwan basalts by any simple petrogenetic process as they contain higher concentrations of incompatible elements and have higher Mg-numbers. The HTZ Dronning Maud Land dolerites have incompatible trace element concentrations which are very similar to the HTZ basalt magma types of the Karoo of southern Africa with the exception of lower K and Rb in DML dolerites. The HTZ dolerites occur in the part of Dronning Maud Land which appears to have been tectonically stable since the Archaean and are not found to intrude the surrounding high-grade (about 1000 Ma) metamorphic rocks of the Sverdrup Group. These data provide qualified support for models which seek to relate spatially the HTZ Mesozoic basalt types of Gondwana to sources beneath stable Archaean cratons.     

Regional and local magmatic anomalies and tectonics of rift zones between the Antarctic and South American plates

The study provides new understanding of magmatism at extinct and modern spreading zones around the western margin of East Antarctica from Bransfield Strait to the Bouvet Triple Junction (BTJ) in the Atlantic Ocean and reveals causes of geochemical heterogeneity of mantle magmatism during the early opening of the Southern Ocean. The results indicate the involvement of an enriched source component in the generation of parental melts, which was formed in several tectonic stages. The enriched (metasomatized) mantle generated at rift zones has geochemical characteristics typical of the western Gondwana lithosphere (with isotopic compositions similar to those inferred for the enriched HIMU and EM-2 sources). This mantle source may have been produced by the thermal erosion of the continental mantle during the early stages of the Karoo–Maud–Ferrar superplume activity. This enriched mantle generated in the apical parts of the plume (sub-oceanic) began to melt during tectonic displacement and fragmentation of Gondwana. The Bouvet Triple Junction, located along modern spreading zones between the Antarctic and South American plate, is characterized by a greater depth of melting and a higher degree of enrichment of primary tholeiitic magmas. The highest enrichment of magmas in this region is controlled by a contribution from a pyroxenite-rich component, which was also identified in the extinct spreading center in Powell Basin.     

New aeromagnetic view of the geological fabric of southern Dronning Maud Land and Coats Land,East Antarctica

The ice shield of Antarctica, which measures several kilometers in thickness, presents a challenge when attempting to unravel the subglacial geology. Here, we report about systematic airborne magnetic surveys conducted over the last decade, which investigated a significant part of Dronning Maud Land (DML), imaging for the first time the crustal architecture of the interior of this sector of East Antarctica. High-resolution data reveal parallel, elongated magnetic anomalies in southeastern DML. These NW–SE trending anomalies can be traced farther east into sparser Russian magnetic data sets. Several high amplitude magnetic anomalies with values above 400 nT have been observed in southwestern DML and Coats Land. They differ clearly in wavelength and amplitudes from the magnetic pattern found in the east and do not show any evidence of a Pan-African orogenic belt or suture zone connecting the Shackleton Range with eastern DML, as hypothesized in several studies. This leads to the assumption of the existence of a hitherto unrecognized large tectonic province in southeastern DML. Whereas an over 100 km long magnetic lineament in the interior of the Dronning Maud Land may reflect a major shear zone akin to the Pan-African age Heimefrontfjella shear zone. Both findings bring new evidences to the still open question about the amalgation of East and West Gondwana. In addition, the magnetic data allow mapping the eastern extent of the presumable cratonic province of Coats Land, a region considered as a key piercing point for reconstructions of Rodinia. Furthermore, the Beattie Magnetic Anomaly in southern Africa is assumed to continue into East Antarctica. Two magnetic highs in western DML are identified as possible eastward continuation of this prominent anomaly.     

阿尔泰大型-超大型矿床富集区地壳演化

通过构造、岩浆、变质、成矿等地质作用及其时空演化规律的对比分析,以及物探重力、航磁及遥感信息的综合研究,认为古生代阿尔泰富蕴地幔热柱成因的三联点裂谷形成与演化———“手风琴式”开合,是阿尔泰大型- 超大型矿床富集区形成与演化的根本原因。地幔热柱热地幔物质大规模上涌和横向扩张,产生三联点裂谷系统,导致古大陆解体,形成阿尔泰“洋岛型”蛇绿岩、阿尔泰型花岗岩、花岗岩化、高温低压变质带和主要大型- 超大型矿床;地幔热柱活动停止或间息,导致三联点裂谷系统发生A 型俯冲而封闭造山,形成三叉式陆间造山带和同造山花岗岩、     

Flood Basalts of Vestfjella: Jurassic Magmatism Across an Archaean-Proterozoic Lithospheric Boundary in Dronning Maud Land, Antarctica

Continental flood basalts (CFBs) of Jurassic age make up theVestfjella mountains of western Dronning Maud Land and demonstratean Antarctic extension of the Karoo large igneous province.A detailed geochemical study of the 120-km-long Vestfjella rangeshows the CFB suite to consist mainly of three intercalatedbasaltic rock types designated CT1, CT2 and CT3 (chemical types1, 2 and 3) that exhibit different incompatible trace elementratios. CT1 and CT2 of north Vestfjella record wide ranges ofNd and Sr isotopic compositions with initial     

Neoproterozoic/Lower Palaeozoic Events in Central Dronning Maud Land (East Antarctica)

New geochronological data indicate that central Dronning Maud Land in East Antarctica underwent polyphase Neoproterozoic/Lower Palaeozoic metamorphism that can be correlated with the final amalgamation of E- and W-Gondwana. Central Dronning Maud Land most probably represents part of the southern continuation of the Mozambique Belt into E-Antarctica. The Neoproterozoic/Lower Palaeozoic metamorphism is preceded by a period of anorogenic anorthosite-charnockite magmatism at ca. 600 Ma. Polyphase metamorphism is recorded from ca. 580 to 515 Ma. Voluminous syntectonic magmatism has been documented at ca. 530 Ma, which is probably the most voluminous Neoproterozoic/Lower Palaeozoic syntectonic magmatism so far recorded in E-Antarctica. The Neoproterozoic/Lower Palaeozoic structural evolution evolved in an overall sinistral transpressional setting, and thus can be correlated with the broad tectonic setting of the Mozambique Belt in Africa.     

Tectonometamorphic Evolution of Jutulsessen,Gjelsvikfjella, cDML,East Antarctica

The Jutulsessen area, can provide a vital clue to the supercontinent assembly of Gondwana Land as it is situated within the Circum East Antarctic Mobile Belt just east of the Penksockett rift marking the divide between the central Dronning Maud Land from the Western Dronning Maud Land. This landmass is dominated by migmatitic quartzo-feldspathic rocks intruded by syn to post-tectonic granites. The work highlights the data from western part cDML area with a view to arrive at a more comprehensive model for the cDML and subsequently to the super continent assembly. Granitic and migmatitic gneisses comprising of amphibolitic and biotite rich enclaves. The gneisses show variations from quartzo-felspathic gneiss to amphibolitic gneiss. The area has witnessed complex geological history involving at different deformational episodes with concomitant metamorphism. The pervasive dominant foliation trends NW-SE with shallow to medium dips towards SW. In the Stabben area, a nonfoliated intrusive syenite-gabbro pluton limits the gneissic exposures. Compositionally, the orthogneisses plot in the monzogranitegranodiorite field where as the mafic dykes/enclaves plot in the basalt-andesite-rhyodacite field. The bulk geochemical characteristics suggest significant crustal contamination. Garnet-biotite Fe-Mg exchange thermometry gives peak metamorphic temperature of 483° C for the gneisses and 628° C for the dioritic enclave within gneisses. A peak metamorphic grade of upper amphibolite to granulite facies is deduced from the mineral assemblages. Widespread anatexis has led to extensive occurrence of migmatites in the area. Recent geochronological studies assign an age of 1170 Ma to 970 Ma for the migmatites/gneisses and an emplacement age of 501 Ma for the Stabben gabbro and syenite. The discriminant plots of the Jutulsessen rocks indicate diverse origin ranging from pre-plate collision to post-collision orogenic tectonic setting. The mafic enclaves/dykes show ocean island arc to MORB affinities. Voluminous addition of juvenile crust during the Pan-African orogeny strongly overprints earlier structures.     

The Antarctic margin and its possible hydrocarbon potential

Geological and geophysical data over the Antarctic margin are reviewed to define those areas where thick sedimentary sequences occur which may have potential to source hydrocarbons. Where possible, the Waples-Lopatin model has been used to calculate whether the degree of maturation of the sediments is sufficient for hydrocarbons to have been generated.Significant sedimentary sequences occur along the continental margins of Wilkes Land, western Dronning Maud Land, Antarctic Peninsula and Ellsworth Land and, as basins or troughs, on the continental shelf of Prydz Bay, Weddell Sea and Ross Sea. Maturation assessments are possible for western Dronning Maud Land, eastern Weddell Sea, western Antarctic Peninsula, Ellsworth Land and Ross Sea regions. Within the great limitations of the data, only western Weddell Sea and Ross Sea basins have maturation values sufficient for the generation of hydrocarbons.     

Mineral chemical evidence for extremely magnesian subalkaline melts from the Antarctic extension of the Karoo large igneous province

We present a comprehensive mineral chemical dataset (～400 analyses) on subalkaline meimechitic (Mg-number?=?74–80) and ferropicritic (Mg-number?=?67–69) dike samples from the Antarctic extension of the Karoo large igneous province (LIP) in Vestfjella, western Dronning Maud Land. Some of the meimechites, previously considered to be cumulates from ferropicritic magmas, are characterized by forsteritic olivine (with core composition up to Fo₉₂) that is in, or close to Fe-Mg equilibrium with the host rock. The olivines are subhedral to euhedral, contain Ti-rich (volcanic) spinel inclusions, have a high CaO content (≥0.19 wt. %), and are thus unlikely to represent xenocrysts from mantle peridotite. Igneous amphibole is found in olivine-hosted, crystallized melt inclusions, indicating that the parental magmas had a H₂O content of 1–2 wt. %. The olivine data suggests generation of extremely MgO-rich (up to 25 wt. %) melts during the Karoo magmatism. Based on our petrogenetic modeling, such melts are likely to have originated from the partial melting of garnet peridotite at high pressures (5–6 GPa) and mantle potential temperatures (>1,600°C) that are compatible with the involvement of a mantle plume in the generation of the Karoo LIP. A geochemical comparison of the Vestfjella meimechites with meimechites from the Siberian Traps LIP and the assumed komatiitic parental melts of the Horingbai picrites (Paraná-Etendeka LIP) reveals key similarities, suggesting that all these suites were generated from broadly similar sources and/or by similar melting processes in anomalously hot subcontinental mantle.     

Stenian A-type granitoids in the Namaqua-Natal Belt,southern Africa,Maud Belt,Antarctica and Nampula Terrane,Mozambique:Rodinia and Gondwana amalgamation implications

We carried out SHRIMP zircon U-Pb dating on A-type granitic intrusions from the Namaqua-Natal Province,South Africa,Sverdrupfjella,western Dronning Maud Land,Antarctica and the Nampula Province of northern Mozambique.Zircon grains in these granitic rocks are typically elongated and oscillatory zoned,suggesting magmatic origins.Zircons from the granitoid intrusions analyzed in this study suggest~1025-1100 Ma ages,which confirm widespread Mesoproterozoic A-type granitic magmatism in the Namaqua-Natal(South Africa),Maud(Antarctica) and Mozambique metamorphic terrains.No older inherited(e.g.,~2500 Ma Achean basement or~1200 Ma island are magmatism in northern Natal)zircon grains were seen.Four plutons from the Natal Belt(Mvoti Pluton,Glendale Pluton,Kwalembe Pluton,Ntimbankulu Pluton) display 1050-1040 Ma ages,whereas the Nthlimbitwa Pluton in northern Natal indicates older 1090-1080 Ma ages.A sample from Sverdrupfjella,Antarctica has~1091 Ma old zircons along with~530 Ma metamorphic rims.Similarly,four samples analysed from the Nampula Province of Mozambique suggest crystallization ages of~1060-1090 Ma but also show significant discordance with two samples showing younger~550 Ma overgrowths.None of the Natal samples show any younger overgrowths.A single sample from southwestern Namaqualand yielded an age of~1033 Ma.Currently available chronological data suggest magmatism took place in the Namaqua-Natal-MaudMozambique(NNMM) belt between~1025 Ma and~1100 Ma with two broad phases between~1060-1020 Ma and 1100-1070 Ma respectively,with peaks at between~1030-1040 Ma and~1070-1090 Ma.The age data from the granitic intrusions from Namaqualand.combined with those from Natal,Antarctica and Mozambique suggest a crude spatial-age relationship with the older1070 Ma ages being largely restricted close to the eastern and western margins of the Kalahari Craton in northern Natal,Mozambique.Namaqualand and WDML Antarctica whereas the younger 1060 Ma ages dominate in southern Natal and western Namaqualand and are largely restricted to the southern and possibly the western margins of the Kalahari Craton.The older ages of magmatism partially overlap with or are marginally younger than the intracratonic Mkondo Large lgneous Provinee intruded into or extruded onto the Kalahari Craton,suggesting a tectonic relationship with the Maud Belt.Similar ages from granitic augen gneisses in Sri Lanka suggest a continuous belt stretching from Namaqualand to Sri Lanka in a reconstituted Gondwana,formed during the terminal stages of amalgamation of Rodinia and predating the East African Orogen.This contiguity contributes to defining the extent of Rodinia-age crustal blocks,subsequently fragmented by the dispersal of Rodinia and Gondwana.     

East Antarctic sources of extensive Lower–Middle Ordovician turbidites in the Lachlan Orogen,southern Tasmanides,eastern Australia

Lower to upper Middle Ordovician quartz-rich turbidites form the bedrock of the Lachlan Orogen in the southern Tasmanides of eastern Australia and occupy a present-day deformed volume of ～2–3 million km³. We have used U–Pb and Hf-isotope analyses of detrital zircons in biostratigraphically constrained turbiditic sandstones from three separate terranes of the Lachlan Orogen to investigate possible source regions and to compare similarities and differences in zircon populations. Comparison with shallow-water Lower Ordovician sandstones deposited on the subsiding margin of the Gondwana craton suggests different source regions, with Grenvillian zircons in shelf sandstones derived from the Musgrave Province in central Australia, and Panafrican sources in shelf sandstones possibly locally derived. All Ordovician turbiditic sandstone samples in the Lachlan Orogen are dominated by ca 490–620 Ma (late Panafrican) and ca 950–1120 Ma (late Grenvillian) zircons that are sourced mainly from East Antarctica. Subtle differences between samples point to different sources. In particular, the age consistency of late Panafrican zircon data from the most inboard of our terranes (Castlemaine Group, Bendigo Terrane) suggests they may have emanated directly from late Grenvillian East Antarctic belts, such as in Dronning Maud Land and subglacial extensions that were reworked in the late Panafrican. Changes in zircon data in the more outboard Hermidale and Albury-Bega terranes are more consistent with derivation from the youngest of four sedimentary sequences of the Ross Orogen of Antarctica (Cambrian–Ordovician upper Byrd Group, Liv Group and correlatives referred to here as sequence 4) and/or from the same mixture of sources that supplied that sequence. These sources include uncommon ca 650 Ma rift volcanics, late Panafrican Ross arc volcanics, now largely eroded, and some <545 Ma Granite Harbour Intrusives, representing the roots of the Ross Orogen continental-margin arc. Unlike farther north, Granite Harbour Intrusives between the Queen Maud and Pensacola mountains of the southern Ross Orogen contain late Grenvillian zircon xenocrysts (derived from underlying relatively juvenile basement), as well as late Panafrican magmatic zircons, and are thus able to supply sequence 4 and the Lachlan Ordovician turbidites with both these populations. Other zircons and detrital muscovites in the Lachlan Ordovician turbidites were derived from relatively juvenile inland Antarctic sources external to the orogen (e.g. Dronning Maud Land, Sør Rondane and a possible extension of the Pinjarra Orogen) either directly or recycled through older sedimentary sequences 2 (Beardmore and Skelton groups) and 3 (e.g. Hannah Ridge Formation) in the Ross Orogen. Shallow-water, forearc basin sequence 4 sediments (or their sources) fed turbidity currents into outboard, deeper-water parts of the forearc basin and led to deposition of the Ordovician turbidites ～2500–3400 km to the north in backarc-basin settings of the Lachlan Orogen.     

Tectonic and climatic events controlling deposition in Tanzanian Karoo basins

The depositional megasequence of the Tanzanian Karoo resulted from an intracratonic phase of sedimentation prevailing during the maximum extension of Pangea in Late Palaeozoic and Triassic times. Karoo rocks are contained in a number of basins, extending from northeastern-most Tanzania to Lake Nyasa and beyond into neighbouring countries. The type section of the Tanzanian Karoo is the Songea Group of the Ruhuhu Basin, situated at the NE-shoulder of the Nyasa Rift. The succession, which reaches a thickness of more than 3000 m, is of Late Carboniferous to Mid-Triassic age. It exhibits five distinctive sequences, each commencing with rudaceous sediments and fining up towards the top. A sixth sequence of Middle to Late Triassic age is recognized in the Selous Basin, NE of the Ruhuhu Basin. The climate ranged from cold, semi-arid conditions in the Stephanian and Asselian to generally warm to hot climates, with fluctuating precipitation in the remaining Permian and Triassic. A marked peak in precipitation is evidenced in the Early Triassic. Each of the sedimentary sequences reflects tectonic movements related to the formation of non-volcanic rift systems during the Permian, and to detachment faults and crustal foundering during the Triassic. The intracratonic Karoo rifts were part of the Malagassy Trough, a large chasm emanating from the Tethyan margin of Gondwana in early Permian times. The Karoo rifts were terminated by their transformation to a pericratonic, passive margin in the Early Jurassic.     

New age constraints for Grenville-age metamorphism in western central Dronning Maud Land (East Antarctica), and implications for the palaeogeography of Kalahari in Rodinia

New SHRIMP zircon data from Gjelsvikfjella and Mühlig–Hofmann–Gebirge (East Antarctica) indicate that the metamorphic basement is composed of Grenville-age rocks that are most likely part of the north-eastern continuation of the Namaqua–Natal–Maud Belt. Crystallisation ages of meta-igneous rocks range between ca. 1,150 to 1,100 Ma, with little inheritance recorded. Metamorphic zircon overgrowth during high-grade metamorphism is dated between ca. 1,090 to 1,050 Ma. Both, the crystallisation ages and the metamorphic overprint are similar to U–Pb data from a number of areas along a ca. 2,000-km stretch from Natal in South Africa to central Dronning Maud Land. The basement underwent in part strong high-grade reworking during the collision of East and West Gondwana at ca. 550 Ma. The timing of Grenville-age metamorphism has important implications for the position of Kalahari in Rodinia. It also questions that Coats Land is part of the Maud Belt because the undeformed volcanic rocks of Coats Land are older than the main metamorphism within the Maud Belt and, therefore, must rest on older basement. This interpretation explains why the pole of Coats Land at ca. 1,110 Ma differs from the Kalahari poles by 30°, i.e. Coats Land had not yet amalgamated to Kalahari. On the other hand, the palaeopoles from Coats Land and Laurentia at 1,110 Ma are identical within error. Thus, Coats Land could have been part of Laurentia prior to the final amalgamation of Rodinia, the Namaqua–Natal–Maud Belt could have been a part of the Grenville Belt and the entire Kalahari Craton could indeed have opposed Laurentia on its eastern side.     

Magmatism-dominated intracontinental rifting in the Mesoproterozoic: The Ngaanyatjarra Rift,central Australia

The Late Mesoproterozoic (1085–1040 Ma) Ngaanyatjarra Rift, previously referred to as the Giles Event, is the dominant component of the Warakurna Large Igneous Province (LIP) that affected much of central and western Australia. This rift is well preserved and provides excellent examples of rift structure at a variety of crustal levels and times in the rift's evolution. Geological knowledge is integrated with geophysical interpretations and models to understand the crustal structure and evolution of this rift. Two phases are identified: an early rift stage (1085–1074 Ma) that is characterised by voluminous magmatism within the upper crust and relatively little tectonic deformation; and a late rift stage that is characterised by tectonic deformation, synchronous with the deposition of a thick pile of volcanic and sedimentary rocks (1074–1040 Ma). Compared to modern rift examples, this rift is unusual in that the crust was thickened by ~ 15 km and overall extension was very limited. However, its structure and evolution are very similar to the near-contemporaneous Midcontinent Rift, which shows the addition of a similar quantity of magmatic material as well as crustal thickening and limited extension. For these Mesoproterozoic rifts, we suggest that magmatism was the dominant process, and that the extension observed was a response to magmatism-induced crustal thickening and the gravitational collapse of the crustal column. Other Proterozoic rifts show similar characteristics (e.g. Transvaal Rift), whereas most Phanerozoic rifts are dissimilar, showing instead a dominance of extension, with magmatism largely a result of this extension. This change in the style of rifting from the Precambrian to the Phanerozoic may relate to the influence of a typically cooler and stronger lithosphere, which has caused stronger strain localisation and a greater role for extension as the controlling factor in rift evolution.     

冰芯和台站记录的近50 a来东南极冰盖边缘地区气候变化格局

对取自东南极冰盖Lambert冰流东、西两侧共支雪芯,恢复了过去50 a来稳定同位素温度序列和积累率序列.对比发现,位于Lambert冰流东侧,即位于Wilks地和Princess Elizabeth 地的5支雪芯(GC₃0, GD03, GD15, DT001和DT085),过去50 a来积累率总体为上升趋势,δ¹⁸O上升速率介于0.34～2.6 kg·m^-2·a^-1; 稳定同位素显示其气温亦呈整体上升趋势, 上升速率介于0‰～0.02‰·a^-1. 但对位于Lamb ert冰流西侧, 即位于Dronning Maud地、Mizuho高原和Kamp地的5支雪芯(Core E,DML 05,W2 00, LGB16和MGA),过去50 a来积累率总体为下降趋势,下降速率介于-0.01～-2.3 6 kg·m^-2·a^-1; 稳定同位素温度变化则十分复杂:Dronning Maud 地西侧为上升, Mizuho高原和Kamp地为下降或变化不明显. 分布于LGB两侧沿岸气象站记录也印证了上述格局. 这种格局可能是南大洋独特的环流形式-环南极波(ACW)-在特殊地形( 如大的冰盆)影响下, 在南极冰盖边缘的表现形式.     

Mantle processes during Gondwana break-up and dispersal

This paper reviews the Mesozoic continental flood basalts (CFBs) associated with the break-up and dispersal of Gondwana from 185-60 Ma, the conditions for melt generation in mantle plumes and within the continental mantle lithosphere, and possible causes for lithospheric extension. The number of CFB provinces within Gondwana is much less than the number of mantle plumes that are likely to have been emplaced beneath it in the 300 Ma prior to its initial break-up. Also, the difference between the age of the peak of CFB volcanism and that of the oldest adjacent ocean crust decreases with the age of volcanism during the break-up and dispersal of Gondwana. The older CFBs of Karoo and Ferrar appear to have been derived largely from source regions within the mantle lithosphere. It is only in the younger Paranâ-Etendeka and Deccan CFBs that there are igneous rocks with major, trace element and radiogenic isotope ratios indicative of melting within a mantle plume. These younger CFBs are also clearly associated with hot spot traces on the adjacent ocean floor. The widespread 180 Ma magmatic event is attributed to partial melting within the lithosphere in response to thermal incubation over 300 Ma. In the case of the Ferrar (Antarctica) this was focussed by regional plate margin forces. The implication is that supercontinents effectively self-destruct in response to the build up of heat and resultant magmatism, since these effects significantly weaken the lithosphere and make it more susceptible to break-up in response to regional tectonics. The younger CFB of Paranâ-Etendeka was generated, at least in part, because the continental lithosphere had been thinned in response to regional tectonics. While magmatism in the Deccan was triggered by the emplacement of the plume, that too may have been beneath slightly thinned lithosphere.     

Strike-slip faults parallel to crustal spreading axes: data from Iceland and the Afar Depression

Slickenside studies in regions of crustal spreading such as Iceland and the Afar Depression, East Africa, reveal that a significant number of faults parallel and close to rift axes are strike-slip rather than normal. Therefore, the pattern of brittle deformation in these regions does not conform to the classic two-dimensional schemes of oceanic tectonics and pre-oceanic rifting. Dip-slip and strike-slip faulting presumably alternated along or in the vicinity of spreading axes, indicate a varying stress field and a combination of transverse and longitudinal movements. In Iceland, strike-slip faults parallel to rifts are observed both west and east of the rift system as well as in a median area between overlapping rifts; the mechanisms proposed for their origin include accommodation of oblique convergence or divergence of crustal sections due to variations of spreading directions along axis and the interaction of overlapping rifts. In the Afar Depression this kind of fault is recorded west of the rift of Asal and can be imputed to reflect an interaction among rifts in the vicinity of the Afar triple junction. Rift-parallel strike-slip faults cannot however be assumed to be a feature of all crustal spreading axes due to the peculiarity of the examined regions: both of them are hot-spot areas and the Afar Depression lies at a triple junction.     

Continental rift systems and anorogenic magmatism

Precambrian Laurentia and Mesozoic Gondwana both rifted along geometric patterns that closely approximate truncated-icosahedral tessellations of the lithosphere. These large-scale, quasi-hexagonal rift patterns manifest a least-work configuration. For both Laurentia and Gondwana, continental rifting coincided with drift stagnation, and may have been driven by lithospheric extension above an insulated and thermally expanded mantle. Anorogenic magmatism, including flood basalts, dike swarms, anorthosite massifs and granite-rhyolite provinces, originated along the Laurentian and Gondwanan rift tessellations. Long-lived volcanic regions of the Atlantic and Indian Oceans, sometimes called hotspots, originated near triple junctions of the Gondwanan tessellation as the supercontinent broke apart. We suggest that some anorogenic magmatism results from decompression melting of asthenosphere beneath opening fractures, rather than from random impingement of hypothetical deep-mantle plumes.     

Propagators and ridge jumps in a back‐arc basin,the West Philippine Basin

We explore the tectono‐magmatic processes in the western West Philippine Basin, Philippine Sea Plate, using bathymetric data acquired in 2003 and 2004. The northwestern part of the basin formed through a series of northwestward propagating rifts. We identify at least five sequences of propagating rifts, probably triggered by mantle flow away from the mantle thermal anomaly that is responsible for the origin of the Benham and Urdenata plateaus. Gravitational forces caused by along‐axis topographic gradient and a ～30° ridge reorientation appear to also be driving the rift propagations. The along‐axis mantle flow appears to be reduced and deflected along the Luzon‐Okinawa fracture zone, because the spreading system remained stable west of this major fault zone. North‐east of the Benham plateau, a left‐lateral fracture zone has turned into a NE–SW‐trending spreading axis. As a result, a microplate developed at the triple junction.