New multibeam bathymetric data from the southeastern Weddell Sea show significant differences in surface morphology of the outer continental shelf and slope between two adjacent cross-shelf troughs. ...These are the Filchner Trough and a smaller trough to the east which we refer to as the ‘Halley Trough’. Multibeam bathymetric data, acoustic sub-bottom profiler and seismic data show major differences in the incidence and morphologies of submarine gullies, channel systems, submarine slides and iceberg scours, and in sediment deposition. These large-scale differences suggest significant variation in slope and sedimentary processes and in the environmental setting between the two troughs, leading to much greater deposition at the mouth of the Filchner Trough. Bedforms, including a terminal moraine and scalloped embayments on the outer shelf of the Halley Trough, provide insight into the relative timing and extent of past ice-sheet grounding and point to grounded ice near to the shelf edge during the Late Quaternary.
The new data reveal two large-scale submarine slides on the upper slope of the eastern Crary Fan, a trough mouth fan offshore from the Filchner Trough. Both slides head at the shelf edge (~500m water depth), with the largest slide measuring 20km wide and with an incision depth of 60m. Multibeam and seismic data show elongate slabs on the seafloor surface of the mid-slope. The lack of a discernible sedimentary cover suggests that they were generated after the Last Glacial Maximum (LGM). This is unusual because post-LGM submarine slides are very rare on the Antarctic continental margin, and to our knowledge, no other post-LGM slides have been documented on an Antarctic trough mouth fan. Because the slides occur on a part of the continental slope where the deposition of glacial debris was greatest, we speculate that weaker, unconsolidated sedimentary layers within the subsurface are important for slide initiation here.
•Geomorphology shows different slope processes at two cross-shelf trough mouths.•Two large and relatively young submarine slides occur on the Antarctic margin.•Outer shelf morphology suggests ice extent near shelf edge during Late Quaternary.
New
40Ar–
39Ar ages of 5.6 to 1.3
Ma for lavas from the fossil Phoenix Ridge in the Drake Passage show that magmatism continued for at least 2
Ma after the cessation of spreading at 3.3
±
0.2
Ma. The ...Phoenix Ridge lavas are incompatible element-enriched relative to average MORB and show an increasing enrichment with decreasing age, corresponding to progressively decreasing degrees of partial melting of spinel peridotite after spreading stopped. The low-degree partial melts increasingly tap a mantle source with radiogenic Sr and Pb but unradiogenic Nd isotope ratios implying an ancient enrichment. The post-spreading magmas apparently form by buoyant ascent of enriched and easily fusible portions of the upper mantle. Only segments of fossil spreading ridges underlain by such enriched and fertile mantle show post-spreading volcanism frequently forming bathymetric highs. The Phoenix Ridge lavas belong to the Pacific, rather than the Atlantic, mantle domain in regional Sr–Nd–Pb space. Our new data show that the southern Pacific Ocean mantle is heterogeneous containing significant enriched portions that are preferentially tapped at low melt fractions. Isotopic mapping reveals that Pacific-type upper mantle flows eastward through Drake Passage and surrounds the subducting Phoenix Plate beneath the Bransfield Basin.
► Magmatism along the fossil Phoenix Ridge continued for about 2 Ma after spreading ceased. ► Increasingly enriched basalts imply decreasing partial melting degrees as spreading slowed. ► Final magmas formed from an enriched mantle source similar EPR seamount basalts. ► Bransfield Basin and the adjacent Antarctic margin are underlain by Pacific-type asthenosphere. ► Enriched Phoenix Ridge basalts formed from enriched mantle unrelated to the Bouvet hotspot source.
The <80 ka basalts–basanites of the Potrillo Volcanic Field (PVF) form scattered scoria cones, lava flows and maars adjacent to the New Mexico–Mexico border. MgO ranges up to 12·5%; lavas with MgO < ...10·7% have fractionated both olivine and clinopyroxene. Cumulate fragments are common in the lavas, as are subhedral megacrysts of aluminous clinopyroxene (with pleonaste inclusions) and kaersutitic amphibole. REE modelling indicates that these megacrysts could be in equilibrium with the PVF melts at ∼1·6–1·7 GPa pressure. The lavas fall into two geochemical groups: the Main Series (85% of lavas) have major- and trace-element abundances and ratios closely resembling those of worldwide ocean-island alkali basalts and basanites (OIB); the Low-K Series (15%) differ principally by having relatively low K2O and Rb contents. Otherwise, they are chemically indistinguishable from the Main Series lavas. Sr- and Nd-isotopic ratios in the two series are identical and vary by scarcely more than analytical error, averaging 87Sr/86Sr = 0·70308 (SD = 0·00004) and 143Nd/144Nd = 0·512952 (SD=0·000025). Such compositions would be expected if both series originated from the same mantle source, with Low-K melts generated when amphibole remained in the residuum. Three PVF lavas have very low Os contents (<14 ppt) and appear to have become contaminated by crustal Os. One Main Series picrite has 209 ppt Os and has a γOs value of +13·6, typical for OIB. This contrasts with published 187Os/188Os ratios for Kilbourne Hole peridotite mantle xenoliths, which give mostly negative γOs values and show that Proterozoic lithospheric mantle forms a thick Mechanical Boundary Layer (MBL) that extends to ∼70 km depth beneath the PVF area. The calculated mean primary magma, in equilibrium with Fo89, has Na2O and FeO contents that give a lherzolite decompression melting trajectory from 2·8 GPa (95 km depth) to 2·2 GPa (70 km depth). Inverse modelling of REE abundances in Main Series Mg-rich lavas is successful for a model invoking decompression melting of convecting sub-lithospheric lherzolite mantle (ϵNd = 6·4; Tp ∼ 1400°C) between 90 and 70 km. Nevertheless, such a one-stage model cannot account for the genesis of the Low-K Series because amphibole would not be stable within convecting mantle at Tf ∼ 1400°C. These magmas can only be accommodated by a three-stage model that envisages a Thermal Boundary Layer (TBL) freezing conductively onto the ∼70 km base of the Proterozoic MBL during the ∼20 Myr tectonomagmatic quiescence before PVF eruptions. As it grew, this was veined by hydrous small-fraction melts from below. The geologically recent arrival of hotter-than-ambient (Tp ∼ 1400°C) convecting mantle beneath the Potrillo area re-melted the TBL and caused the magmatism.
A suite of mafic dykes from the Underberg region of southern KwaZulu-Natal (South Africa) were intruded at ∼178 Ma, coincident in age with the major Okavango Dyke Swarm of Botswana, and also ...coincident with minor Karoo-related intrusions of the northern and central Lebombo. The dykes are all low-Ti–Zr tholeiites, they trend NW–SE and are presumed to continue into the Karoo central area of the Lesotho Highlands. In many respects, the Underberg dykes are similar to the majority of the low-Ti–Zr volcanic and subvolcanic intrusions of the Karoo; however, their 87Sr/86Sr and εNd isotope ratios are either ‘Ferrar-like’ (87Sr/86Sr ∼0·710; εNd < −3) or transitional between Karoo low-Ti–Zr and Ferrar low-Ti magmas. A potential Ferrar source for at least some of the Underberg dykes is supported by anisotropy of magnetic susceptibility analyses of the dyke suite, which demonstrate absolute flow direction from the SE to the NW, consistent with Gondwana reconstructions. The role of crustal contamination and combined fractional crystallization is also demonstrated to have played a key role in the petrogenesis of the Underberg dykes, involving a local upper crust contaminant. However, the composition of the ‘Ferrar-like’ dykes cannot be easily explained by AFC processes, but they do demonstrate that melting of a lithospheric mantle source enriched to a small degree by subduction-derived fluid was also important.
The isotopic difference between modern Indian Ocean and Pacific or North Atlantic Ocean ridge mantle (e.g. variably lower super(206)Pb/ super(204)Pb for a given epsilon sub(Nd) and super(208)Pb/ ...super(204)Pb) could reflect processes that occurred within a few tens of millions of years preceding the initial breakup of Gondwana. Alternatively, the Indian Ocean isotopic signature could be a much more ancient upper-mantle feature inherited from the asthenosphere of the eastern Tethyan Ocean, which formerly occupied much of the present Indian Ocean region. Age-corrected Nd, Pb, and Sr isotopic data for 46-150 Ma seafloor lavas from sites in the western Indian Ocean and ocean-ridge-type Tethyan ophiolites (Masirah, Yarlung-Zangpo) reveal the presence of both Indian-Ocean-type compositions and essentially Pacific-North Atlantic-type signatures. In comparison, Jurassic South Pacific ridge basalts from Alexander Island, Antarctica, possess normal Pacific-North Atlantic-type isotopic ratios. Despite the very sparse sampling of old seafloor, the age-corrected epsilon sub(Nd) (t) values of the old Indian Ocean basalts cover a greater range than seen for the much more thoroughly sampled present-day spreading axes and islands within the Indian Ocean (e.g. 18 epsilon sub(Nd) units for basalts in the 60-80 Ma range vs 15 epsilon sub(Nd) units for 0-10 Ma ones). The implications of these results are that the upper mantle in the Indian Ocean region is becoming increasingly well mixed through time, and that the Indian Ocean mantle domain may not greatly pre-date the age of earliest spreading in the Indian Ocean.
New whole rock Rb-Sr and zircon U-Pb geochronological data and Sm-Nd isotopic data are presented from the central magmatic arc domain of the Antarctic Peninsula in the area of north-west Palmer Land ...and south-west Graham Land, Rb-Sr isochrons indicate an age of 169 ± 6 Ma for basement orthogneisses and 132 ± 9 to 71 ± 9 Ma for plutons. A U-Pb age of 183 ± 2.1 Ma, with no detectable inheritance, on zircons from an orthogneiss from Cape Berteaux provides the first reliable age for the orthogneisses, which are interpreted as metamorphosed silicic volcanic rocks, and Sm-Nd data indicate derivation in a mature volcanic arc. The age indicates they may be correlatives of the Jurassic ‘Chon Aike’ volcanism of the eastern Antarctic Peninsula. A U-Pb zircon age of 107 ± 1.7 Ma on a terrestrial volcanic sequence overlying an uncomformity strongly suggests a mid-Cretaceous age for the extensive volcanic cover of north-west Palmer Land that was previously thought to be Jurassic. The unconformity is interpreted to have been a result of compressional uplift related to the Palmer Land event. This is the first date for the event in the western part of the central magmatic arc terrane of the Antarctic Peninsula.
Protector Shoal, the northernmost and most silicic volcano of the South Sandwich arc, erupted dacite–rhyolite pumice in 1962. We report geochemical data for a new suite of samples dredged from the ...volcano. Geochemically, the dredge and 1962 samples form four distinct magma groups that cannot have been related to each other, and are unlikely to have been related to a single basaltic parent, by fractional crystallization. Instead, the silicic rocks are more likely to have been generated by partial melting of basaltic lower crust within the arc. Trace element and Sr–Nd isotope data indicate that the silicic volcanics have compositions that are more similar to the volcanic arc than the oceanic basement formed at a back-arc spreading centre, and volcanic arc basalts are considered to be the likely source for the silicic magmas. The South Sandwich Islands are one of several intra-oceanic arcs (Tonga–Kermadec, Izu–Bonin) that have: (1) significant amounts of compositionally bimodal mafic–silicic volcanic products and (2) 6.0–6.5 km s−1 P-wave velocity layers in their mid-crusts that have been imaged by wide-angle seismic surveys and interpreted as intermediate-silicic plutons. Geochemical and volume considerations indicate that both the silicic volcanics and plutonic layers were generated by partial melting of basaltic arc crust, representing an early stage in the fractionation of oceanic basalt to form continental crust.
The Earth's convecting upper mantle can be viewed as comprising three main reservoirs, beneath the Pacific, Atlantic and Indian oceans. Because of the uneven global distribution and migration of ...ridges and subduction zones, the surface area of the Pacific reservoir is at present contracting at about 0.6 km2 yr-1, while the Atlantic and Indian reservoirs are growing at about 0.45 km2 yr-1 and 0.15 km2 yr-1, respectively. Garfunkel and others have argued that there must accordingly be net mantle flow from the Pacific to the Atlantic and Indian reservoirs (in order to maintain mass balance), and Alvarez further predicted that this flow should be restricted to the few parts of the Pacific rim (here termed 'gateways') where there are no continental roots or subduction zones that might act as barriers to shallow mantle flow. The main Pacific gateways are, according to Alvarez, the southeast Indian Ocean, the Caribbean Sea and the Drake passage. Here we report geochemical data which confirm that there has been some outflow of Pacific mantle into the Drake passage-but probably in response to regional tectonic constraints, rather than global mass-balance requirements. We also show that a mantle domain boundary, equivalent to the Australian-Antarctic discordance, must lie between the Drake passage and the east Scotia Sea.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
U-Pb SHRIMP ages are reported for three rhyolite flows from the Lebombo rift region of the Karoo volcanic province. Two flows are interbedded with the Sabie River Basalt Formation and a third sample ...is from the overlying rhyolitic Jozini Formation. The interbedded rhyolites yield ages of 182.0±2.1 and 179.9±1.8 Ma, whilst the overlying Jozini Formation rhyolite yields an age of 182.1±2.9 Ma. Combined with existing 40Ar/39Ar geochronology, the new SHRIMP data fine-tunes the chronology of the Karoo volcanic province and indicates the 12 km succession of volcanic rocks in the Lebombo rift were erupted in 1-2 million years and lends considerable support to the links between the Pleinsbachian-Toarcian extinction event and the global environmental impact of Karoo volcanism.
Determining the location and geometry of possible subglacial rifts in western Dronning Maud Land is a key element to address processes leading to early Gondwana break‐up. However, previous ...geophysical investigations did not lead to unambiguous delineation of rift structures over this region. We interpret high‐resolution airborne radar and aerogravity data to image subglacial rift structures. Subglacial topography, free‐air and Bouguer gravity maps, coupled with 3D inverse gravity models, image a rift–rift–rift triple junction at the intersection of the Jutulstraumen ice stream and the Pencksökket glacier. These continental rifts were associated with alkaline and tholeiitic intrusions, minor dyke swarms and flood basalts of Jurassic age, but not with huge volumes of Karoo magmatism, such as that which characterizes the southern Africa conjugate margin. The western Dronning Maud Land triple junction may be linked to the Karoo mantle plume and represents an early stage of magmatism and rifting during Gondwana break‐up.
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