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East African Rift System (EARS) Plume Structure: Insights from Quaternary Mafic Lavas of Turkana, Kenya 总被引：6，自引：2，他引：6
FURMAN TANYA; BRYCE JULIA G.; KARSON JEFFREY; IOTTI ANNAMARIA 《Journal of Petrology》2004,45(5):1069-1088
Quaternary mafic lavas from Lake Turkana (northern Kenya) provideinformation on processes operating beneath the East AfricanRift in an area of anomalous lithospheric and crustal thinning.Inferred depths of melting beneath Turkana (15–20 km)are shallower than those recorded elsewhere along the rift,consistent with the anomalously thin crustal section. The maficlavas have elevated incompatible trace element contents whencompared with mid-ocean ridge basalts, requiring an enrichmentevent in the source region. Basalts with low Sr isotopic ratios( 相似文献
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Rip currents as a geological tool 总被引：2，自引：0，他引：2
MICHA&#; GRUSZCZY&#;SKI§ STANIS&#;AW RUDOWSKI† JULIA SEMIL‡ JAN S&#;OMI&#;SKI† JERZY ZROBEK‡ 《Sedimentology》1993,40(2):217-236
This paper considers the nature and sedimentary significance of rip currents. Rip currents are the main factor responsible for the transport of coarse sediments from the littoral zone to greater depths. Such sediments, deposited outside the zone of wave deformation, may be identified as storm rip current increments within sediments deposited during fair weather. Composite beds deposited during a given wave cycle by storm rip currents are closely similar to ‘turbidites’ and many so called ‘fluxo-turbidites’ described from flysch deposits. Using data gathered in studies conducted on the modern Baltic coast, supplemented by experimental work and theoretical considerations, a sedimentary model is proposed. The model may be used to interpret possible rip current deposits among shallow water ‘turbidites’, and both modern storm sediments and ancient ‘tempestites’. 相似文献
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FURMAN TANYA; KALETA KELLY M.; BRYCE JULIA G.; HANAN BARRY B. 《Journal of Petrology》2006,47(6):1221-1244
The East African Rift System is important to understanding plume-initiatedrifting as manifest in the geochemistry of mafic lavas eruptedalong the rift throughout its evolution. We present new datafrom high-MgO Tertiary lavas from Turkana, northern Kenya, toinvestigate regional melt source components, to identify thedepths and degrees of melting, and to characterize spatiallyand temporally the chemical structure of the underlying mantle.The Turkana area is a region of high lithospheric extensionthat sits between two topographic uplifts thought to be surfaceexpressions of one or more upwelling mantle plumes. Thinningof local crust is believed to be accompanied by widespread removalof the mantle lithosphere, causing the asthenosphere to be inclose contact with the overlying crust. New geochemical dataon basanites, picrites and basalts (MgO >7 wt %) tightlyconstrain the primary melt source regions of Tertiary volcanism.Initial isotopic signatures (143Nd/144Nd = 0·51267–0·51283,87Sr/86Sr = 0·7031–0·7036) and trace elementabundances (Ce/Pb 30, La/Nb = 0·6–0·8 andBa/Nb = 3–10) in these lavas are consistent with derivationfrom sub-lithospheric sources. Basalts and picrites eruptedbetween 23 and 20 Ma have Sr–Nd–Pb–He isotopiccharacteristics indicative of high-µ influence, recordhigh depths and degrees of partial melting, and are associatedwith rift propagation to the north and south. Accordingly, theselavas sample a source region that is geochemically distinctfrom that reflected both in Oligocene Ethiopian flood basaltsand in the modern Afar region. The geochemical data supportnumerical and theoretical models as well as tomographic resultsproviding for a complex thermal structure in the mantle beneathEast Africa and are interpreted to reflect isotopically distinctplume heads beneath Tanzania and Afar that are derived fromthe chemically heterogeneous South African superplume. KEY WORDS: East African Rift System; mantle plumes; HIMU; geochemistry; Afar 相似文献
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A diverse range of crustal xenoliths is hosted in young alkalibasalt lavas and scoria deposits (erupted 3–5 ka) at thesummit of Huallai. Leucocratic xenoliths, including monzodiorites,diorites and syenogabbros, are distinctive among Hawaiian plutonicrocks in having alkali feldspar, apatite, zircon and biotite,and evolved mineral compositions (e.g. albitic feldspar, clinopyroxeneMg-number 67–78). Fine-grained diorites and monzodioritesare plutonic equivalents of mugearite lavas, which are unknownat Huallai. These xenoliths appear to represent melt compositionsfalling along a liquid line of descent leading to trachyte—amagma type which erupted from Huallai as a prodigious lava flowand scoria cone at 114 ka. Inferred fractionating assemblages,MELTS modeling, pyroxene geobarometry and whole-rock norms allpoint to formation of the parent rocks of the leucocratic xenolithsat 3–7 kbar pressure. This depth constraint on xenolithformation, coupled with a demonstrated affinity to hypersthene-normativebasalt and petrologic links between the xenoliths and the trachyte,suggests that the shift from shield to post-shield magmatismat Huallai was accompanied by significant deepening of the activemagma reservoir and a gradual transition from tholeiitic toalkalic magmas. Subsequent differentiation of transitional basaltsby fractional crystallization was apparently both extreme—culminatingin >5·5 km3 of trachyte—and rapid, at 2·75x 106 m3 magma crystallized/year. KEY WORDS: geothermobarometry; magma chamber; xenolith; cumulate; intensive parameters 相似文献
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JULIA CHRISTENSEN 《Geographical review》2013,103(2):219-229
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JULIA M. G. MILLER 《Sedimentology》1989,36(3):419-430
Episodes of glacial advance and retreat can be recognized through analysis of vertical facies sequences in the Permo-Carboniferous Pagoda Formation of the Beardmore Glacier area, Antarctica. The formation includes a remarkably complete record of continental sedimentation near the terminus of a temperate glacier. Facies sequence is pre-eminent for inferring glacial advance and retreat. Other important criteria are abundance and geometry of sandstone interbedded with diamictite, diamictite character and nature of bed contacts. Using these characteristics advance and retreat sequences 5–60 m thick are recognized. A sharp contact, with a striated surface and erosional relief, overlain by structureless diamictite (lodgement till) is typical of grounded ice advance. Grounded ice retreat is characterized by structureless diamictite (lodgement till), overlain by crudely stratified diamictite (melt-out till) and then by diamictite interbedded with sandstone and conglomerate (flow till and glacio-fluvial or glacio-lacustrine deposits). Gradational contacts between shale overlain by diamictite and diamictite overlain by shale characterize advance and retreat, respectively, in subaqueous settings. Pauses in sediment accumulation, minor(?) fluctuations of the ice margin, and/or changes in subglacial dynamics are indicated by specific features within diamictite units such as probable frost-wedge casts, single layer boulder beds, sharp sedimentary contacts and changes in diamictite character. These minor(?) events are superimposed upon the main advance-retreat cycles. Study of both the overall facies sequence and of individual diagnostic structures, albeit in an incomplete stratigraphic record, permits a distinction between major and minor advance-retreat events. As many as six major advance-retreat cycles exist in some Pagoda sections, but the number of cycles present varies in different sections. 相似文献