Evolution of orthogonal sets of coeval extension joints   总被引：1，自引：0，他引：1  

Riccardo Caputo 《地学学报》1995,7(5):479-490

The formation of two orthogonal sets of extension joints either crossing or abutting each other is a typical product of brittle deformation. Such systems of joints, with the two joint sets being geologically coeval, have been called a fracture grid-lock. The two sets are of common genesis and thus a unique remote stress field can be inferred. This interpretation causes some perplexity if the two joint sets are purely extension fractures and formed perpendicular to the least principal stress. In the present paper a conceptual model to explain the origin and the evolution of such systems is proposed. In a volume of rock undergoing a tensional and uniform remote stress state, caused for example by a tectonic regime, two horizontal and negative (i.e. tensional) stress rates are assumed to exist. When the tensile strength of the rock is locally reached, failure occurs perpendicular to the least principal stress. Then, that direction locally experiences a positive stress drop due to the stress release. For this reason, the stress field, retaining the same principal directions, is locally distorted by a swap between the σ₃ and the σ₂ components in a volume of rock surrounding the fracture. As a consequence of the persisting remote stress rates, when elastic failure conditions are newly accumulated, a second fracture forms and propagates perpendicular to the previous one. Repeated failure events, stress-drops and stress swaps eventually generate a fracture grid-lock. The whole process is also described with a simplified analytical model by applying elasticity theory.  相似文献   

Constraints on the Genesis of Potassium-rich Italian Volcanic Rocks from U/Th Disequilibrium     

Avanzinelli  Riccardo; Elliott  Tim; Tommasini  Simone; Conticelli  Sandro 《Journal of Petrology》2008,49(2):195-223

We present new U-series isotope, ⁸⁷Sr/ ⁸⁶Sr, ¹⁴³Nd/ ¹⁴⁴Nd andtrace element data for a set of mafic, K-rich rocks from volcanoesin Central–Southern Italy. These shoshonitic to ultrapotassiclavas display strongly depleted high field strength element(HSFE) abundances with respect to other incompatible trace elementstogether with high but variable ⁸⁷Sr/ ⁸⁶Sr and low but variable¹⁴³Nd/ ¹⁴⁴Nd values. Such characteristics are thought to bedue to addition of subducted crust of variable amount and compositionto their mantle sources prior to magma genesis. Rocks from thenorthernmost region (i.e. Tuscan Magmatic Province and NorthernRoman Magmatic Province) display (²³⁰Th/ ²³⁸U) activity ratiosclose to radioactive equilibrium, suggesting that metasomatismof their sources occurred before 400 ka and recent melting tookplace at shallow depths, in the absence of garnet. A ²³⁸U excessof up to 27% has been measured in rocks from the NeapolitanDistrict. The occurrence of significant U excesses is a featureof arc magmas, but is typically seen in depleted lavas ratherthan in highly enriched rocks such as these (20 ppm Th). Thissignature requires a recent addition of a U-rich component tothe already strongly enriched mantle wedge beneath this regionof Italy. We suggest that a supercritical liquid, from deeplysubducted carbonate-rich sediments of the still-active Ionianslab, is responsible for generating a high-U, low-Th component,which produces the observed disequilibria. A 30% ²³⁰Th excessmeasured in a single unaltered sample from the Lucanian MagmaticProvince, along with a less marked negative HFSE anomaly, suggeststhe contribution of a deeper, garnet-bearing component in thegenesis of these magmas, plausibly related to the upwellingof asthenospheric mantle around the corner of the Ionian slab. KEY WORDS: U/Th disequilibria; potassic and ultrapotassic rocks; subduction: metasomatism; mantle melting; Central and Southern Italy  相似文献   

Wave systems in the chromosphere     

R. Giovanelli 《Solar physics》1975,44(2):299-314

Observations are reported of two, possibly three, distinct wave systems in the Hα chromosphere.

1. Velocity films show waves propagating predominantly outwards along mottles and fibrils from as close as 2000 km to the network axis at velocities of the order of 70 km s^-1. The line-of-sight component of the velocity amplitude is estimated to be typically 5 km s^-1. The velocities are accompanied by propagating intensity fluctuations. The system is interpreted as one of basically Alfvén waves. Similar waves are observed propagating predominantly outwards along superpenumbral fibrils radiating from a small sunspot.
2. The velocities in the chromospheric granulation undergo fluctuations of an oscillatory character but without any observable horizontal propagation. The intensities show a close correlation with the velocities, maximum intensity occurring about T/4 after maximum downward velocity. The period is variable across the surface (2.5 min upwards). The intensity-velocity correlation is characteristic of a standing compressional wave.
3. Intensity cinefilms at Hα line centre show in places a horizontal drift of the chromospheric granulation pattern at about 12 km s^-1 without any accompanying vertical velocity fluctuations. It is not known whether this is due to a gas stream at sonic velocities, or to a horizontally propagating sound wave.

The Alfvén wave system is shown to make a significant contribution to coronal heating. Whether the velocity fluctuations in the chromospheric granulation also make an important contribution depends on whether there are upwardly propagating or standing waves; this is not yet established despite the intensity-velocity correlation.  相似文献   

An exploratory two-dimensional study of the coarse structure of network magnetic fields     

R. G. Giovanelli 《Solar physics》1980,68(1):49-69

Magnetograms in lines originating high in the solar atmosphere show, away from disk center, diffuse fringes of reverse polarity on the limbward side and diffuse centerwards extensions of normal polarity wherever the field is strong. Analysis of a Mg b₂ magnetogram reveals that, in active regions (and, hence, wherever the magnetic network is well developed) fields cover associated supergranules completely at heights mostly below 500–600 km (zero height is at ₅₀₀₀ = 1) but possibly up to 700–800 km at great distances (e.g. >10⁴km) from the network. These lie much lower than previously believed, mostly around the solar average temperature minimum. Near plagettes, the low-lying field has been measured out to 6000–7000 km. One consequence is that in active regions and plagettes, the chromosphere-corona transition region probably penetrates below 600 km; another is that potential theory is inapplicable at coronal heights below about 15 000 km.A more accurate analysis requires a specific atmospheric model for magnetic regions. Attention is drawn to the need for studying the consequences for acoustic wave propagation, reflection and dissipation in regions of strong network fields.Visiting Astronomer, Kitt Peak National Observatory.Operated by the Association of Universities for Research in Astronomy, Inc. under contract with the National Science Foundation.  相似文献   

Magnetograph response to canopy-type fields     

Harrison P. Jones  Ronald G. Giovanelli 《Solar physics》1982,79(2):247-266

The response of longitudinal-field magnetographs to magnetic fields which are semi-infinite or confined to a horizontal layer is discussed with respect to the interpretation of solar diffuse fields, observed towards the limb, in terms of magnetic canopy models. Numerical results are presented for several reference solar models and typical calibration curves are shown for the C I 9111 Å, Fe I 8688 Å, and Ca II 8542 Å lines in magnetostatic atmospheres derived from a mean model. A procedure is developed for determining the base heights of magnetic canopies from observations with an uncertainty not exceeding the order of a pressure scale height. Until definitive information regarding atmospheric structure inside flux tubes can be developed from theory or observation, reliable field strengths cannot be derived from the data.Operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.  相似文献   

The three-dimensional structure of atmospheric magnetic fields in two active regions     

Ronald G. Giovanelli  Harrison P. Jones 《Solar physics》1982,79(2):267-278

The magnetic field above two unrelated active regions on 11 and 12 September, 1974 has been studied using magnetograms obtained in C I 9111, Fe I 8688, Ca II 8542, and H. In C I 9111, originating low in the photosphere, the fields are strong and sharply defined. In Ca II 8542 and H they are very diffuse, with significant diffuseness also in Fe I 8688, due to the spreading of the field with height to form almost horizontal magnetic canopies over regions free of field at lower levels.Within a region between two small sunspots some 140 Mm apart, the canopy height found is typically 300–400 km. Within a small superpenumbra, the canopy height is 150–250 km. In extensive areas surrounding the active regions, over one-half the canopy bases are less than 400–500 km above the _c = 1 level, and over 80% less than 700 km.Arguments are given that the chromospheric fibrils (e.g., in H), taken to delineate the field configuration, are not due primarily to lateral variations in field but rather to differences in density or excitation of gas across the lines of force.Operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.  相似文献   

The Fate of High-Angle Dipping Slabs in the Subduction Factory: an Integrated Trace Element and Radiogenic Isotope (U, Th, Sr, Nd, Pb) Study of Stromboli Volcano, Aeolian Arc, Italy   总被引：2，自引：0，他引：2  

Tommasini  Simone; Heumann  Arnd; Avanzinelli  Riccardo; Francalanci  Lorella 《Journal of Petrology》2007,48(12):2407-2430

The subaerial part of the Stromboli stratovolcano was builtup in the last 100 kyr through six periods of activity; theerupted magmas record the largest compositional variation ofall the Aeolian arc volcanoes (calc-alkaline, shoshonitic, andpotassic alkaline magma series). The trace element characteristicsof the less evolved magmas of each period of activity are coherentlycorrelated with their radiogenic isotope (Sr, Nd, Pb) composition,and are typical of volcanic arc rocks. In terms of U-seriesisotopes, samples from the different magma series have both²³⁸U and ²³⁰Th excesses, and this distinctive feature providesadditional constraints on source enrichment processes withinthe mantle wedge and on the mechanism of partial melting. Overallthe complete set of data demonstrates that the genesis of thedifferent magma series at Stromboli can be accommodated in amantle source that experienced two distinct enrichment processesby different parts of the subducting oceanic crust of the Ionianslab. The first was caused by supercritical liquids originatingfrom the basaltic and sedimentary parts of the subducting slabat >5 GPa and 900°C. The second was induced by aqueousfluids, again originating from the basaltic and sedimentaryparts of the slab, released from a shallower part of the subductedIonian slab (< 5 GPa and 800°C). U–Th disequilibriaconstrain the timing of the first metasomatic event (Stage I:supercritical liquids) at >435 ka, whereas the second event(Stage II: aqueous fluids) occurred at 100 ka. The high-angledip of the Ionian slab (70°) caused the superimpositionof the metasomatizing agents of the two enrichment processesin the same volume of the mantle wedge, explaining the occurrenceof such different magma series in a single volcanic edifice.The U–Th disequilibria provide evidence for dynamic meltingof the metasomatized mantle wedge combined with an ageing effectresulting from the restoration of secular equilibrium afterthe perturbation caused by the U-rich aqueous fluids of StageII. The trace element and radiogenic isotope (U, Th, Sr, Nd,Pb) signature of the mantle source of the magmas at Stromboliis thus dependent upon the amount of supercritical liquids andaqueous fluids released by the two components of the subductedslab, whereas the distinctive ²³⁸U and ²³⁰Th excesses of themagmas result from a combination of mantle ageing and time-dependentdynamic melting. The geochemical and radiogenic isotope signatureof the mantle source beneath Stromboli places important constraintson the isotopic polarity from Southern Latium to the Aeolianarc attributed to the effect of a HIMU mantle component followingeither lateral inflow of foreland mantle material or upwellingof a mantle plume in the centre of the Tyrrhenian basin. Ourgeochemical model demonstrates that the high ²⁰⁶Pb/²⁰⁴Pb ofthe putative ‘HIMU’ mantle component could be equallyformed during metasomatism of the pre-existing mantle wedgeby either the supercritical liquid (Stage I) or aqueous fluid(Stage II) released by the subducted altered basalt of the Ionianplate. KEY WORDS: radiogenic isotopes; U–Th disequilibria; mantle metasomatism; supercritical liquid; aqueous fluid; Stromboli  相似文献   

Improvements in the integration of remote sensing and rock slope modelling     

Francioni  Mirko  Salvini  Riccardo  Stead  Doug  Coggan  John 《Natural Hazards》2018,94(2):975-977

Natural Hazards - Forests are always dangerous to fire in China’s forested regions, and forest fire hazard has long been a serious issue in China. Zhejiang province is one of the key forest...  相似文献   

Role of melting process and melt–rock reaction in the formation of Jurassic MORB-type basalts (Alpine ophiolites)     

Maria?Rosaria?RennaEmail authorView author&#;s OrcID profile  Riccardo?Tribuzio  Alessio?Sanfilippo  Matthew?Thirlwall 《Contributions to Mineralogy and Petrology》2018,173(4):31

This study reports a geochemical investigation of two thick basalt sequences, exposed in the Bracco–Levanto ophiolite (northern Apennine, Italy) and in the Balagne ophiolite (central-northern Corsica, France). These ophiolites are considered to represent an oceanward and a continent-near paleogeographic domain of the Jurassic Liguria–Piedmont basin. Trace elements and Nd isotopic compositions were examined to obtain information about: (1) mantle source and melting process and (2) melt–rock reactions during basalt ascent. Whole-rock analyses revealed that the Balagne basalts are slightly enriched in LREE, Nb, and Ta with respect to the Bracco–Levanto counterparts. These variations are paralleled by clinopyroxene chemistry. In particular, clinopyroxene from the Balagne basalts has higher Ce_N/Sm_N (0.4–0.3 vs. 0.2) and Zr_N/Y_N (0.9–0.6 vs. 0.4–0.3) than that from the Bracco–Levanto basalts. The basalts from the two ophiolites have homogeneous initial Nd isotopic compositions (initial ε_Nd from +?8.8 to +?8.6), within typical depleted mantle values, thereby excluding an origin from a lithospheric mantle source. These data also reject the involvement of contaminant crustal material, as associated continent-derived clastic sediments and radiolarian cherts have a highly radiogenic Nd isotopic fingerprint (ε_Nd at the time of basalt formation?=???5.5 and ??5.2, respectively). We propose that the Bracco–Levanto and the Balagne basalts formed by partial melts of a depleted mantle source, most likely containing a garnet-bearing enriched component. The decoupling between incompatible elements and Nd isotopic signature can be explained either by different degrees of partial melting of a similar asthenospheric source or by reaction of the ascending melts with a lower crustal crystal mush. Both hypotheses are reconcilable with the formation of these two basalt sequences in different domains of a nascent oceanic basin.  相似文献   

V404 CYG/GS 2023+338: Monitoring in the Optical with Robotic Telescopes of the MASTER Global Network during the 2015 Superburst     

Lipunov  V. M.  Kuznetsov  A. S.  Gorbovskoy  E. S.  Kornilov  V. G.  Tyurina  N. V.  Vladimirov  V. V.  Krushinskiy  V. V.  Zalozhnykh  I. S.  Vlasenko  D. M.  Zimnukhov  D. S.  Balanutsa  P. V.  Gabovich  A.  Shakura  N. I.  Giovanelli  F.  Titarchuk  L. G.  Buckley  D. A. H.  Gress  O.  Budnev  N.  Rebolo  R.  Serra-Ricart  M.  Tlatov  A. G.  Senik  V. A.  Yurkov  V. V.  Ershova  O.  Grinshpun  V. G.  Ishmukhametova  Yu. V. 《Astronomy Reports》2019,63(7):534-549

2070 unique, homogeneous photometric and polarization observations of the microquasar in a binary system with a black hole V404 Cyg/GS2023+338 obtained in 2015 with the MASTER global network of robotic telescopes (16 robotic telescopes located at eight points on the Earth in Russia, Spain, South Africa, and Argentina) are presented. MASTER was the first telescope network to obtain optical observations of the microquasar after its gamma-ray outburst in 2015. Observations were carried out from 18:34:09 UT on June 15, 2015 until December 2015 in four polarizations and in the four standard BV RI filters. The paper presents the results of these observations and a comparative analysis of optical and X-ray data. The observations confirm the previously discovered super-long delays of the optical radiation relative to the X-ray outbursts. Possible mechanisms causing the delay in the optical variations relative to the X-ray variations are discussed. Variability of the optical polarization discovered earlier is confirmed another similar episode reported.

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