Tectonic pattern of the Azores spreading centre and triple junction     

Roger Searle 《Earth and Planetary Science Letters》1980,51(2):415-434

The major tectonic elements of the Azores triple junction have been mapped using long-range side-scan sonar. The data enable the Mid-Atlantic Ridge axis to be located with a precision of a few kilometres. Major faults and other tectonic and volcanic elements of the ridge maintain their regional trend of 010° to 020° past the triple junction area. There is no oblique spreading, and only minor transform offsets of the Mid-Atlantic Ridge occur here. The main effect of the triple junction or Azores hot spot is to diminish the amplitude of the median valley to 200 m or less. There is no axial high: a topographic high seen on several profiles is located to the east of the Mid-Atlantic Ridge spreading axis and does not appear to have any fundamental significance.The third arm of the triple junction includes the Azores srreading centre which appears to have developed as a series of en echelon rifted basins (the Terceira Rift) extending from Formigas Trough at 36.8°N, 24.5°W to a point near 39.3°N, 28.8°W. There are indications that recent activity in the spreading centre may be concentrated in a series of ridges which flank the older rifted basins. Until recently the northwest end of the Terceira Rift was connected to the Mid-Atlantic Ridge axis either directly at an RRR junction, or via a transform fault. The triple junction has probably moved south during the last 6 Ma to a positin on the Mid-Atlantic Ridge near 38.7°N.Initiation of the Azores spreading centre may have occurred during the 36 Ma B.P. rearrangement of poles, with an RFF triple junction north from the East Azores fracture zone to the North Azores fracture zone and transferring a wedge of European plate to the African plate.The tectonic elements revealed by this study are in good agreement with inferred earthquake mechanisms and with the RM2 plate tectonic model of Minster and Jordan, but east-west motion between North America and Africa does not seem to be compatible with the other motions at the triple junction unless it is of very recent (2>3 Ma) origin.  相似文献   

Uranium and copper exploration by nearshore lake sediment geochemistry, Kaipokok region of Labrador     

Dimitris G. Minatidis  Roger M. Slatt 《Journal of Geochemical Exploration》1976,5(1-2)

Uranium and copper concentrations were determined on 184 sediment samples collected systematically arount the periphery of 24 lakes in the Kaipokok region of Labrador. The results allowed us to define several zones across glacial drift and lake-covered terrain which are enriched in these elements and which warrant further exploration. Some zones are extensions of known U and Cu mineralization, but others had not previously been found.This survey demonstrates that systematic nearshore lake sediment sampling and geochemical analysis is an informative method of mineral exploration which can be used as a follow-up to reconnaissance level sampling.  相似文献   

Rare earth element geochemistry of potassic lavas from the Birunga and Toro-Ankole regions of Uganda,Africa     

Roger H. Mitchell  Keith Bell 《Contributions to Mineralogy and Petrology》1976,58(3):293-303

Neutron activation determination of La, Ce, Sm, Eu, Tb, Yb, Lu, Ta, Hf, Sc, Co and Th in potassic lavas from the Birunga and Toro-Ankole regions show that the rocks are characterized by high rare earth element (REE) contents (161–754 ppm) and form two groups based upon differing La/Yb ratios. One group is made up of katungite, ugandite and mafurite with La/Yb =146–312, and the other of rocks of the leucitite and phonolitic tephrite series, La/Yb =30–56. The trace element content of the ugandite group is similar to that of kimberlites. The data do not indicate any trends of differentiation or simple relationships between the two groups of rocks, although katungite is unlikely to be parental to rocks of lower La/Yb ratios. It is unlikely that in terms of La/Yb ratios that partial melting of mica-garnet-lherzolite mantle can form katungite because of the very small amounts of partial melting required (0.2%), although the La/Yb ratios of 150–200 (ugandites, mafurites) and 30–60 (leucitites, phonolitic tephrites) can be accounted for by 0.3–1.5% and 1–9% melting respectively, if the REE are then concentrated without further La and Yb fractionation. Partial melting of mantle which has been metasomatized by alkaline earths and REE bearing fluids or mixing of carbonatite and nephelenite are also compatable with the observed geochemistry of the lavas. It is considered that gas transfer processes which selectively enrich the light REE may have obscured REE evidence pertaining to early partial melting and/or differentiation processes and therefore that REE geochemistry is of little use in determining the petrogenetic processes involved in the formation of potassic lavas.  相似文献   

The evolution of the archaean crust of northeast Botswana     

Roger M. Key  Martin Litherland  John V. Hepworth 《Precambrian Research》1976,3(4):375-413

Some 1800 km² of Archaean terrain have been mapped (the Eastern Geotraverse of the Botswana Geological Survey, Geodynamics Project) including a part of the schist belt/granitic terrain of the Rhodesian Craton, and part of the Limpopo Mobile Belt in the south. The cratonic area includes the whole or part of four schist belts (greenstone belts) now referred to as ‘schist relics’ as they are shown to be mega-xenoliths surviving regional deformation and granitisation.The schist relics display a typical greenstone belt composition with basal ultra-mafic schist, extrusive meta-basalts, serpentinites, and meta-dacites, -andesites, -rhyolites and volcanoclastic rocks appearing up the succession. These are termed the Volcanic Group. They also contain marble, graphitic phyllite, meta-greywacke, banded ironstone and aluminous schist.The schist relics overlie thick sequences of granitised clastic sediments with intercalated volcanic and sedimentary layers which are regarded as an integral, lower part of the succession. The total sequence, including the Volcanic Group and underlying rocks is at least 30 km thick.The schist relics originally formed a more continuous and lithostratigraphically equivalent succession, which may have included some of the Rhodesian schist belts. Such a succession would be of the order of size of an island arc system in length and width.An early stage of regional folding (F1) is recognized, from preserved fabrics and structural analysis, which is tentatively proposed as nappe folding on a regional scale. This was accompanied by low-grade metamorphism. This first stage of deformation determined the regional pattern as it is still seen.Tonalite/monzonite plutons were emplaced within the Volcanic Group and probably protected the relics during regional granitisation.Regional granitisation, which isolated the schist relics, was accompanied by pyroxene hornfels metamorphism. This was followed by a regional, penetrative deformation (F2) with amphibolite facies (Barrovian type) metamorphism. This is the most prominent style of deformation which, although most intensely developed in the Limpopo Mobile Belt, is imposed throughout the whole area. The Limpopo Belt cuts across the area in the south as a large ductile shear zone and affects rocks largely similar to those of the craton, although anorthosites are not known outside it.The Limpopo Belt has its own style of deformation — interference folds, and several stages of folding related to transcurrent movement, and intense cataclasis.Metamorphism is similar throughout Limpopo Mobile Belt and Craton, each event being recognized in both domains and being of similar grades.In many respects the Botswana Eastern Geotraverse area resembles other Archaean terrains and analogies can be drawn. In attempting to find a geotectonic model into which it can be placed there is a considerable range of choices, and not enough constraints existing either within the imperfectly preserved geological record, nor within the expanding and elastic framework of Global tectonic theory.No direct evidence for subduction zones or plate boundaries has been found, nor is this surprising considering the subsequent history. On the other hand we see nothing in the rather detailed body of evidence that has been accumulated by mapping to preclude a place within Plate theory as outlined for the Phanerozoic. It allows a moderately uniformitarian interpretation rather than one supposing unique conditions preculiar to the Archaean, such as the concept of isolated volcanic depositaries, or of unique and peculiar crustal conditions.  相似文献   

Geochemistry of magnesian ilmenites from kimberlites in South Africa and Lesotho     

Roger H. Mitchell 《Lithos》1977,10(1):29-37

Magnesium ilmenite from discrete nodules and lamellar intergrowths with pyroxene from the Kao, Sekameng, Frank Smith, and Monastery kimberlites has been analysed for Ti, Fe, Mg, Nb, Zr, Cr, Cu, Zn, Mn, Co, and Ni. Each kimberlite contains discrete ilmenites which exhibit a wide compositional range within the ilmenite-geikeilite series. Lamellar ilmenites from Frank Smith and Monastery differ in composition but both show a limited range in composition which lies within the compositional range shown by discrete ilmenites from these pipes. The ilmenites are enriched in Nb, Zr, Cr, Co, and Ni and depleted in Mn, Cu, and Zn relative to Mg-poor ilmenites from basic intrusions. Nb, Zr, and Ni correlate with Fe, Ti and Mg variations but the abundances of the other trace elements are independent of major element variation. R-mode factor analysis is interpreted to imply that the geochemistry cannot be interpreted in terms of a differentiation hypothesis in which trace elements are removed from or concentrated in residua. Factor scores and major element abundances indicate that each pipe is characterized by a particular suite of discrete ilmenite nodules, which are considered to be phenocrysts in a proto-kimberlite magma. Lamellar ilmenite-pyroxene intergrowths are unlikely to have had a eutectic origin, and show no simple relationship to the discrete ilmenites.  相似文献   

Sea floor basalt alteration: Some chemical and Sr isotopic effects     

S. R. Hart  A. J. Erlank  E. J. D. Kable 《Contributions to Mineralogy and Petrology》1974,44(3):219-230

Major element, Sr isotope and trace element data for 16 elements are reported for various weathered zones in three submarine basalt pillows. During the initial stages of alteration, it appears that SiO₂, Al₂O₃, CaO, S, and Ga are lost from the basalt, whereas Fe₂O₃, total Fe, MnO, K₂O, H₂O, Cl, B, Rb, and Cs increase. Sr⁸⁷/Sr⁸⁶ ratios also increase during weathering. MgO, Na₂O, P₂O₅, Ba, Ni and Cu show significant (10–50%) but less consistent changes. TiO₂, Zr, Sr and V show only minor changes (<7%). Zn, Cr, Co, Y and Nb show no changes outside experimental error.  相似文献   

Surface history of some Apollo 17 lunar soils     

Robert L. Fleischer  Howard R. Hart 《Geochimica et cosmochimica acta》1974,38(10):1615-1623

Cosmic ray track densities in Apollo 17 soil samples are used to infer surface exposure times of soils from a trench at Van Serg Crater, from on and near a boulder at Camelot Crater, and from the position of the heat flow and neutron flux experiments (the ALSEP site). The topmost 2 cm of soil at Van Serg was exposed for 11 m.y., the top cm at Camelot for 36 m.y. A layering chronology and average deposition rate are proposed for the trench. For all soils the median track densities imply predispositional irradiation in the top 15 cm of the lunar surface for times that were long compared with the actual residence in the stratigraphic positions from which the soils were collected. Van Serg crater is inferred to have been formed approximately 24 m.y. ago.  相似文献   

Application of the UKCIP02 climate change scenarios: Reflections and lessons learnt     

Megan Gawith  Roger Street  Richard Westaway  Anna Steynor 《Global Environmental Change》2009,19(1):113-121

The UKCIP02 climate change scenarios have become the standard reference for climate change in the UK since their release in 2002. This paper describes and reflects on the ways in which they have been applied. It then identifies some strengths, weaknesses and barriers to their application, and extracts key lessons that may inform the development and provision of future climate change scenarios.Analysis of the application of UKCIP02 shows that the scenarios have been used primarily as a communication device, as well as for scientific research and to inform policy and decision-making on climate change. They have played a critical role in raising awareness on climate change and in engaging organisations in the need to adapt. Their presentation in an accessible style, and their availability in a variety of formats, greatly facilitated their uptake. However, analysis has also revealed weaknesses which served as barriers to their uptake. Some of these, such as file format and accessibility issues, were readily overcome through technical solutions. Others, such as the issue of how to use uncertain information in decision-making, have only been partially addressed and remain an outstanding challenge for future scenario packages. Two key lessons have emerged which may benefit the provision of future climate scenarios in the UK and elsewhere. First, it is not enough to simply make climate change scenarios available. Their provision must be accompanied by ongoing guidance and support to ensure widespread and appropriate uptake. Second, on-going dialogue between those providing scenarios and the communities using them is fundamental to constructively meet the challenges associated with delivering credible scenarios that balance user requirements and expectations with what the science can deliver.  相似文献   

Late Pleistocene to Holocene sedimentation and hydrocarbon seeps on the continental shelf of a steep,tectonically active margin,southern California,USA   总被引：1，自引：0，他引：1  

Amy E. Draut  Patrick E. Hart  Thomas D. Lorenson  Holly F. Ryan  Florence L. Wong  Ray W. Sliter  James E. Conrad 《Marine Geophysical Researches》2009,30(3):193-206

Small, steep, uplifting coastal watersheds are prolific sediment producers that contribute significantly to the global marine sediment budget. This study illustrates how sedimentation evolves in one such system where the continental shelf is largely sediment-starved, with most terrestrial sediment bypassing the shelf in favor of deposition in deeper basins. The Santa Barbara–Ventura coast of southern California, USA, is considered a classic area for the study of active tectonics and of Tertiary and Quaternary climatic evolution, interpretations of which depend upon an understanding of sedimentation patterns. High-resolution seismic-reflection data over >570 km² of this shelf show that sediment production is concentrated in a few drainage basins, with the Ventura and Santa Clara River deltas containing most of the upper Pleistocene to Holocene sediment on the shelf. Away from those deltas, the major factor controlling shelf sedimentation is the interaction of wave energy with coastline geometry. Depocenters containing sediment 5–20 m thick exist opposite broad coastal embayments, whereas relict material (bedrock below a regional unconformity) is exposed at the sea floor in areas of the shelf opposite coastal headlands. Locally, natural hydrocarbon seeps interact with sediment deposition either to produce elevated tar-and-sediment mounds or as gas plumes that hinder sediment settling. As much as 80% of fluvial sediment delivered by the Ventura and Santa Clara Rivers is transported off the shelf (some into the Santa Barbara Basin and some into the Santa Monica Basin via Hueneme Canyon), leaving a shelf with relatively little recent sediment accumulation. Understanding factors that control large-scale sediment dispersal along a rapidly uplifting coast that produces substantial quantities of sediment has implications for interpreting the ancient stratigraphic record of active and transform continental margins, and for inferring the distribution of hydrocarbon resources in relict shelf deposits.  相似文献   

Human and climate forcing of zooplankton populations: Introduction     

Dagg  Michael; Uye  Shin-ichi; Valdes  Luis; Harris  Roger 《ICES Journal of Marine Science》2008,65(3):277-278

Zooplankton play a key role in the pelagic foodweb by controllingphytoplankton production and shaping pelagic ecosystems. Inaddition, because of their critical role as a food source forlarval and juvenile fish, the dynamics of zooplankton populationshave a significant influence on recruitment to fish stocks.In 1961, ICES convened the First Zooplankton Production Symposiumin Charlottenlund, Denmark. ICES also played a leading rolein the Second Zooplankton Production Symposium on "ZooplanktonProduction: measurement and role in global  相似文献