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101.
Coupling of oceanic and continental crust during Eocene eclogite-facies metamorphism: evidence from the Monte Rosa nappe,western Alps 总被引:1,自引:0,他引:1
Thomas J. Lapen Clark M. Johnson Lukas P. Baumgartner Giorgio V. Dal Piaz Susanne Skora Brian L. Beard 《Contributions to Mineralogy and Petrology》2007,153(2):139-157
High precision U–Pb geochronology of rutile from quartz–carbonate–white mica–rutile veins that are hosted within eclogite
and schist of the Monte Rosa nappe, western Alps, Italy, indicate that the Monte Rosa nappe was at eclogite-facies metamorphic
conditions at 42.6 ± 0.6 Ma. The sample area [Indren glacier, Furgg zone; Dal Piaz (2001) Geology of the Monte Rosa massif: historical review and personal comments. SMPM] consists of eclogite boudins that are exposed
inside a south-plunging overturned synform within micaceous schist. Associated with the eclogite and schist are quartz–carbonate–white
mica–rutile veins that formed in tension cracks in the eclogite and along the contact between eclogite and surrounding schist.
Intrusion of the veins at about 42.6 Ma occurred at eclogite-facies metamorphic conditions (480–570°C, >1.3–1.4 GPa) based
on textural relations, oxygen isotope thermometry, and geothermobarometry. The timing of eclogite-facies metamorphism in the
Monte Rosa nappe determined in this study is identical to that of the Gran Paradiso nappe [Meffan-Main et al. (2004) J Metamorphic Geol 22:261–281], confirming that these two units have shared the same Alpine metamorphic history. Furthermore,
the Gran Paradiso and Monte Rosa nappes underwent eclogite-facies metamorphism within the same time interval as the structurally
overlying Zermatt-Saas ophiolite [∼50–40 Ma; e.g., Amato et al. (1999) Earth Planet Sci Lett 171:425–438; Mayer et al. (1999) Eur Union Geosci 10:809 (abstract); Lapen et al. (2003) Earth Planet Sci Lett 215:57–72]. The nearly identical P–T–t histories of the Gran Paradiso, Monte Rosa, and Zermatt-Saas units suggest that these units shared a common Alpine tectonic
and metamorphic history. The close spatial and temporal associations between high pressure (HP) ophiolite and continental
crust during Alpine orogeny indicates that the HP internal basement nappes in the western Alps may have played a key role
in exhumation and preservation of the ophiolitic rocks through buoyancy-driven uplift. Coupling of oceanic and continental
crust may therefore be critical in preventing permanent loss of oceanic crust to the mantle. 相似文献
102.
Research shows that flood damage potential has increased significantly in the last 15 years. At the same time, flood policy has shifted away from simplistic flood defence towards 'living with floods' and 'making space for water'. This paper explores the mis-match between the aspiration in policy ideals, the reality of rising potential economic damages and the inability of the flood risk appraisal process to match the aspiration with the reality. Unless investment appraisal procedures are changed, the increase in damages will undermine policy changes that seek a different pattern of flood risk management, away from economically dominated decision-criteria towards more sustainable objectives. 相似文献
103.
Natural bedrock rivers flow in self‐formed channels and form diverse erosional morphologies. The parameters that collectively define channel morphology (e.g. width, slope, bed roughness, bedrock exposure, sediment size distribution) all influence river incision rates and dynamically adjust in poorly understood ways to imposed fluid and sediment fluxes. To explore the mechanics of river incision, we conducted laboratory experiments in which the complexities of natural bedrock channels were reduced to a homogenous brittle substrate (sand and cement), a single sediment size primarily transported as bedload, a single erosion mechanism (abrasion) and sediment‐starved transport conditions. We find that patterns of erosion both create and are sensitive functions of the evolving bed topography because of feedbacks between the turbulent flow field, sediment transport and bottom roughness. Abrasion only occurs where sediment impacts the bed, and so positive feedback occurs between the sediment preferentially drawn to topographic lows by gravity and the further erosion of these lows. However, the spatial focusing of erosion results in tortuous flow paths and erosional forms (inner channels, scoops, potholes), which dissipate flow energy. This energy dissipation is a negative feedback that reduces sediment transport capacity, inhibiting further incision and ultimately leading to channel morphologies adjusted to just transport the imposed sediment load. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
104.
The global three-dimensional Lagrangian chemistry-transport model STOCHEM has been used to follow the changes in the tropospheric distributions of the two major radiatively-active trace gases, methane and tropospheric ozone, following the emission of pulses of the short-lived tropospheric ozone precursor species, methane, carbon monoxide, NOx and hydrogen. The radiative impacts of NOx emissionswere dependent on the location chosen for the emission pulse, whether at the surface or in the upper troposphere or whether in the northern or southern hemispheres. Global warming potentials were derived for each of the short-lived tropospheric ozone precursor species by integrating the methane and tropospheric ozone responses over a 100 year time horizon. Indirect radiative forcing due to methane and tropospheric ozone changes appear to be significant for all of the tropospheric ozone precursor species studied. Whereas the radiative forcing from methane changes is likely to be dominated by methane emissions, that from tropospheric ozone changes is controlled by all the tropospheric ozone precursor gases, particularly NOxemissions. The indirect radiative forcing impacts of tropospheric ozone changes may be large enough such that ozone precursors should be considered in the basket of trace gases through which policy-makers aim to combat global climate change. 相似文献
105.
E. L. Gégo G. S. Johnson M. Hankins 《Stochastic Environmental Research and Risk Assessment (SERRA)》2001,15(1):47-64
Stochastic techniques, such as Monte Carlo experiments, are more and more frequently used for the study of flow and transport
in heterogeneous aquifers. When the aquifer is composed of distinct hydrofacies, a common way to model heterogeneity is to
first generate equally-possible hydrofacies fields, and then convert these hydrofacies fields into hydraulic conductivity
(K) fields by assigning a single K value to each facies. This technique assumes relative homogeneity of K within each facies
but may not be appropriate for the most conductive facies that often exhibits substantial variability. In this paper, we assessed
the impacts of assigning multiple random K, rather than a uniform K value, to the highly conductive facies on the results
of a flow and transport model. A set of fifty stochastic hydrofacies maps depicting an environment similar to the Snake River
Plain aquifer (SRPA) in south-east Idaho were generated. Simulations demonstrated that a uniform K value, if carefully chosen,
can reasonably reproduce the specific discharges and early particle arrival times produced by multiple K values. Yet, the
results obtained with a uniform K value are dramatically less variable than those obtained with multiple K values. It is therefore
concluded that stochastic simulations with uniform K assigned to the most conductive and variable facies do not necessarily
portray the entire uncertainty in the analyses. 相似文献
106.
Dolomitization of the Waulsortian Limestone (Lower Carboniferous) in the Irish Midlands 总被引:1,自引:0,他引:1
Jay M. Gregg Kevin L. Shelton Aaron W. Johnson Ian D. Somerville & Wayne R. Wright 《Sedimentology》2001,48(4):745-766
The Waulsortian Limestone (Lower Carboniferous) of the southern Irish Midlands is dolomitized pervasively over a much larger region than previous studies have documented. This study indicates a complex, multistage, multiple fluid history for regional dolomitization. Partially and completely dolomitized sections of Waulsortian Limestones are characterized by finely crystalline (0·01–0·3 mm) planar dolomite. Planar replacive dolomite is commonly followed by coarse (≥0·5 mm) nonplanar replacive dolomite, and pervasive void‐filling saddle dolomite cement is frequently associated with Zn–Pb mineralization. Planar dolomite has average δ18O and δ13C values (‰ PDB) of –4·8 and 3·9 respectively. These are lower oxygen and slightly higher carbon isotope values than averages for marine limestones in the Waulsortian (δ18O=–2·2, δ13C=3·7). Mean C and O isotope values of planar replacive dolomite are also distinct from those of nonplanar and saddle dolomite cement (–7·0 and 3·3; –7·4 and 2·4 respectively). Fluid inclusions indicate a complex history involving at least three chemically and thermally distinct fluids during dolomite cementation. The petrography and geochemistry of planar dolomites are consistent with an early diagenetic origin, possibly in equilibrium with modified Carboniferous sea water. Where the Waulsortian was exposed to hydrothermal fluids (70–280 °C), planar dolomite underwent a neomorphic recrystallization to a coarser crystalline, planar and nonplanar dolomite characterized by lower δ18O values. Void‐filling dolomite cement is isotopically similar to nonplanar, replacive dolomite and reflects a similar origin from hydrothermal fluids. This history of multiple stages of dolomitization is significantly more complex than earlier models proposed for the Irish Midlands and provides a framework upon which to test competing models of regional vs. localized fluid flow. 相似文献
107.
Partial melting in the Inzie Head gneisses: the role of water and a petrogenetic grid in KFMASH applicable to anatectic pelitic migmatites 总被引:5,自引:0,他引:5
A sequence of prograde isograds is recognized within the Dalradian Inzie Head gneisses where pelitic compositions have undergone variable degrees of partial melting via incongruent melting reactions consuming biotite. Three leucosome types are identified. At the lowest grades, granitic leucosomes containing porphyroblasts of cordierite (CRD‐melt) are abundant. At intermediate grades, CRD‐melt mingles with garnetiferous leucosomes (GT‐melt). At the highest grades, CRD‐melt coexists with orthopyroxene‐bearing leucosomes (OPX‐melt), while garnet is conspicuously absent. The prograde metamorphic field gradient is constrained to pressures of 2–3 kbar below the CRD‐melt isograd, and no greater than 4.5 kbar at the highest grade around Inzie Head. A petrogenetic grid, calculated using thermocalc , is presented for the K2O–FeO–MgO–Al2O3–SiO2–H2O (KFMASH) system for the phases orthopyroxene, garnet, cordierite, biotite, sillimanite, H2O and melt with quartz and K‐feldspar in excess. For the implied field gradient, the reaction sequence predicted by the grid is consistent with the successive prograde development of each leucosome type. Compatibility diagrams suggest that, as anatexis proceeded, bulk compositions may have been displaced towards higher MgO content by the removal of (relatively) ferroan granitic leucosome. An isobaric (P = 4 kbar) T–aH2O diagram shows that premigmatization fluids must have been water‐rich (aH2O > 0.85) and suggests that, following the formation of small volumes of CRD‐melt, the system became fluid‐absent and melting reactions buffered aH2O to lower values as temperatures rose. GT‐ and OPX‐melt formed by fluid‐absent melting reactions, but a maximum of 7–11% CRD‐melt fraction can be generated under fluid‐absent conditions, much less than the large volumes observed in the field. There is strong evidence that the CRD‐melt leucosomes could not have been derived by buoyantly aided upwards migration from levels beneath the migmatites. Their formation therefore required a significant influx of H2O‐rich fluid, but in a quantity insufficient to have exhausted the buffering capacity of the solid assemblage plus melt. Fluid : rock ratios cannot have exceeded 1 : 30. The fluid was channelled through a regionally extensive shear zone network following melt‐induced failure. Such an influx of fluid at such depths has obvious consequences for localized crustal magma production and possibly for cordierite‐bearing granitoids in general. 相似文献
108.
109.
J. A. Walker M. J. Carr L. C. Patino C. M. Johnson M. D. Feigenson R. L. Ward 《Contributions to Mineralogy and Petrology》1995,120(3-4):378-390
Lavas erupted behind the volcanic front in southeastern Guatemala have many important distinctions from lavas erupted on the volcanic front. These include: generally higher MgO, Nb, Sr, TiO2, and rare earth element concentrations; higher La/Yb and Nb/Y ratios; and lower Ba/La, La/Nb, Ba/Zr and Zr/Nb ratios. These major and trace element distinctions are caused by reduced fractionation during ascent and storage in the crust, lower degrees of melting in the source, and greatly reduced contributions from the subducted Cocos plate in the source. In addition, because all of these important distinctions are even borne in lavas erupted within 20 km of the front, there is little apparent petrogenetic continuity between front and behind-the-front magmas. What little geochemical continuity exists is in radiogenic isotopes: 143Nd/144Nd falls across the arc, Pb isotopic ratios (except 206Pb/204Pb) rise across the arc, and 87Sr/86Sr rise across the arc after an initial discontinuity within 20 km of the front. These continuous across-arc changes in radiogenic isotopes are caused by increased contamination with older, more isotopically disparate rocks, away from the front. Once the effects of crustal contamination are removed, the remaining isotopic variability behind the front is non-systematic and reflects the inherent isotopic heterogeneity of the source, the mantle wedge. Geochemical disconnection in southeastern Guatemala suggests that behind-the-front magmas are produced by decompression melting near the top of the wedge, not by flux-dominated melting near the base of the wedge. 相似文献
110.
A detailed (5 km track separation) seismic reflection survey of a portion of the upper flank of Reykjanes Ridge supports the existence of an oblique aseismic ridge, previously postulated from other data. The oblique basement ridge may have been formed by a magma center moving southwest under this portion of the Reykjanes Ridge at about 6 cm/yr between 7 and 5 mbyp. The oblique ridge is complex, being interrupted by saddles about every 30 km length. This spacing could reflect incipient, very weakly developed transverse fractures, or more probably the concentration of volcanic activity at particularly active vents, which shift southwestward every million years or so in response to the south-westward moving magma chambers entrained in the asthenosphere. Minor irregularities in the oblique ridge parallel crustal isochrons; such small features are probably elongate fissure eruptions restricted to a narrow spreading axis. 相似文献