British Geological Survey

About: British Geological Survey is a government organization based out in Nottingham, United Kingdom. It is known for research contribution in the topics: Groundwater & Aquifer. The organization has 2561 authors who have published 7326 publications receiving 241944 citations.

Modelling frequency-dependent seismic anisotropy in fluid-saturated rock with aligned fractures: implication of fracture size estimation from anisotropic measurements

Abstract: Measurements of seismic anisotropy in fractured rock are used at present to deduce information about the fracture orientation and the spatial distribution of fracture intensity. Analysis of the data is based upon equivalent-medium theories that describe the elastic response of a rock containing cracks or fractures in the long-wavelength limit. Conventional models assume frequency independence and cannot distinguish between microcracks and macrofractures. The latter, however, control the fluid flow in many subsurface reservoirs. Therefore, the fracture size is essential information for reservoir engineers. In this study we apply a new equivalent-medium theory that models frequency-dependent anisotropy and is sensitive to the length scale of fractures. The model considers velocity dispersion and attenuation due to a squirt-flow mechanism at two different scales: the grain scale (microcracks and equant matrix porosity) and formation-scale fractures. The theory is first tested and calibrated against published laboratory data. Then we present the analysis and modelling of frequency-dependent shear-wave splitting in multicomponent VSP data from a tight gas reservoir. We invert for fracture density and fracture size from the frequency dependence of the time delay between split shear waves. The derived fracture length matches independent observations from borehole data.

The Ontong Java Plateau

Abstract: The Alaska-size Ontong Java Plateau (OJP) in the southwest Pacific is the largest of the world's large igneous provinces and formed entirely in an oceanic environment. Limited sampling of the upper levels of basaltic basement reveals strongly bimodal ages of122 Ma and 90 Ma. Geochemical signatures indicate two isotopically distinct, ocean-island-like mantle-source types for the 122 Ma basalts and that the 90 Ma source was almost identical to one of the 122 Ma sources, strongly suggesting that a single mantle plume caused both eruptive events. In the 125-90 Ma period, the OJP appears to have been located near the Pacific Plate Euler poles and thus moved little relative to a postulated hotspot at about 42°S, 159°W; the early phase of emplacement probably also occurred near a spreading center, but substantial volumes were emplaced off-axis. The eastern lobe of the plateau appears to have been rifted shortly after 90 Ma. Incompatible and major element data are consistent with 20-30% polybaric partial melting of a peridotite source, beginning in the garnet stability field and continuing in the spinel field. Most existing basaltic basement samples appear to have experienced 30-45% of crystal fractionation; the resulting cumulates would be wehrlitic to pyroxenitic in composition, with an average density of 3.25 g cm -3 . We conclude that these cumulates form much of the plateau's high velocity (7.6 km s -1 ) basal crustal layer. The relatively high density of this layer appears to have prevented emergence of much of the OJP above sea level despite a total crustal thickness exceeding 30 km. Although the deepest levels of the crust could be eclogitized (further increasing density), post-emplacement subsidence of the plateau has probably been tempered by the presence of a roughly 85-km-thick melt-depleted mantle root with a relatively low density (<3.25 g cm -3 ) for mantle material. Estimates of partial melting deduced from the apparent thickness of the mantle root imply that the OJP formed by 17-31% partial melting, in excellent agreement with geochemical modeling.

Petrogenesis of Cenozoic Basalts from Mongolia: Evidence for the Role of Asthenospheric versus Metasomatized Lithospheric Mantle Sources

Abstract: Diffuse Cenozoic volcanism in Mongolia forms part of a widespread tectono-magmatic province that extends from NE China to Lake Baikal, Siberia. Mafic lavas from the Gobi Altai, southern Mongolia (33 Ma) and Hangai, central Mongolia ( 70 km. Three isotopic end-members can explain the heterogeneity: (1) is similar to bulk silicate Earth with 206Pb/204Pb >17·8 and is asthenospheric; (2) is EM1-like, characterized by low 206Pb/204Pb (>17·062), and may represent mobilized ancient lithospheric mantle; (3) also lithospheric, is characterized by low 143Nd/144Nd (>0·512292) and shows similarities to EM2, although decoupling of isotopic systems suggests a complex enrichment process. The timing of lithospheric enrichment is unconstrained, but may be related to Mesozoic magmatic events and/or melts mobilized during the Cenozoic responding to higher than ambient potential temperature mantle. Published geophysical studies suggest anomalous material at the base of the lithospheric mantle; however, there is no evidence to suggest a high heat flux mantle plume. Volcanism is likely to occur where localized extensional conditions are favourable.

Late Caledonian (Acadian) transpression in north-west England: timing, geometry and geotectonic significance

Abstract: SUMMARY The late Caledonian structure of the Lower Palaeozoic slate belts which lie to the south of the Iapetus suture in Britain is not ‘Caledonoid’ (NE-SW) but characterised by arcuate trends. The significance of the major cleavage arc of northern England is the subject of this paper. Its exposed part, in the Lake District and adjacent inliers, is described and its regional extent inferred from the control exerted by Caledonian basement trends on early Carboniferous sedimentation patterns. The arc is shown to be a major feature of the orogen, marking a change from a NE-SW ‘Appalachian’ trend to the ESE-WNW ‘Tornquist’ trend of northern Germany and Poland. Evidence for the age of deformation in the British slate belts is reviewed. It is shown that the deformation was not ‘end-Silurian’ as previously supposed, but of early Devonian age, climaxing in the Emsian and approximately synchronous with the Acadian orogeny of Canada. The systematic variation in cleavage/fold transection angles around the arc is described and interpreted in terms of transpressive strains associated with the northward movement of a basement block, the Midlands Massif, which acted as a rigid indenter during accretion of the southern British terrane (Eastern Avalonia) onto the Laurentian margin. These new data on the timing and geometry of the Acadian accretion event in Britain go some way to resolving the current controversy concerning late Ordovician vs. Devonian closure of Iapetus.