ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1994.tb00487.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1994.tb00488.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTCombined metamorphic, stable‐isotope and structural studies from the High Himalayan Crystalline sequence in the Langtang Valley of north‐central Nepal reveal a strong positive correlation between distance above the base of the section (the Main Central Thrust), the amount of melt material and evidence of prolonged fluid and deformation histories, thus suggesting that these processes are strongly interdependent. Kyanite‐grade rocks at the base of the section are unmelted and have undergone little syn‐ or post‐metamorphic internal deformation and little prograde or retrograde fluid‐rock interaction. By contrast sillimanite‐grade rocks higher in the section contain progressively larger volumes of melt, have suffered increasingly complex syn‐ and post‐metamorphic deformation and show increasing evidence for the presence of fluids. Although the factors that initiated these processes remain problematic, it is suggested that fluid distributions within the Langtang section have been passively controlled by the movement of melts. These melts may have provided a primary control on deformation during both the magmatic stage and, subsequently, through the exsolution of exsolv

ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1994.tb00489.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTTwo large cold masses in the deep mantle have been delineated by using long‐wavelength seismic tomographic models in conjunction with mineralogical experimental data at high pressure. These cold anomalies are found under the western Pacific and the Americas with temperatures more than 1000 degrees below the ambient mantle temperature. These strong cold anomalies existing in the lower mantle today would suggest that there might have existed not too long ago a substantial temperature jump across a thermal boundary layer between the upper and lower mantle. Numerical simulations in an axisymmetric spherical‐shell model incorporating the two major phase transitions have shown that very large pools of cold material with temperatures of around 1500 K can be flushed down to the core–mantle boundary during this tumultuous gravitational instability. A correlation is found between the current locations of these very cold masses and regions of past subduction since the Cretaceous. Correlation analysis shows that the slab mass‐flux into the lower mantle does not behave in a steady‐state fashion. These findings may support the idea of a strong gravitational instability with origins in the transition zone, as suggested by numerical models of mantle c

ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1994.tb00490.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTFractal geometry is an important concept for describing geological morphologies, providing not only a qualitative description, but also a basis for quantitative comparisons. The goal of the present study is to identify some of the geological factors which affect the fractal dimension of coastlines. In particular, a tentative classification is presented of the lithological factors influencing the coastlines of recently formed Italian volcanic islands ‐ Vulcano, Stromboli and Alicudi ‐ located in the Southern Tyrrhenian Ba

ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1994.tb00491.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTThe coastline of southern Fife on the east coast of the British Isles is presented here as an illustration of geological influences on spatial variability at different lengthscales. The coastline profile of Britain is widely referred to as an example of fractal geometry, as presented in Mandelbrot's (1967) original paper on statistical self‐similarity. I show here that at different lengthscales, different geological processes dominate the character of the coastline profile, and that estimates of the fractal dimension are strongly dependant on the scale and choice of sample. Significant periodic components in the tortuosity of the coastline are evident, and can be related to the causative, scale‐specific, geological proces

ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1994.tb00492.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTThe structural setting beneath the Ligurian Sea resuJts from several tectonic events reflected in the nature of the crust. The central‐western sector, called the Ligurian basin, is part of the northwestern Mediterranean. It is a marginal basin that was generated in Oligocene‐Miocene time by subduction of the Adriatic plate beneath the European plate and by the eastward drift of the Corsica‐Sardinia block. The eastern sector belongs to the Tyrrhenian basin system and is characterized by extensional activity which since Tortonian time superimposed an earlier compressional regime. Our effort has been addressed in particular towards simplifying the complex nature of the crust of the Ligurian basin by modelling its genesis using uniform extension and sea‐floor depth variation with age. In the rift stage of the basin's evolution, the initial subsidence reaches the isostatic equilibrium level of the asthenosphere by a thinning factor of 3.15. The additional passive process, corresponding to the cooling of the lithosphere since 21 Ma, leads to a total tectonic subsidence of 3.4 km, representing the boundary of the extended continental crust. For values up to 4.1 km a transitional‐type crust is expected, whereas for higher tectonic subsidence values a typical oceanic crust should exist. After setting these constraints, the boundaries of the different crust types have been drawn based on total tectonic subsidence observations deduced from bathymetry and post‐rift sediment thickness. Although there is a general agreement with the previous reconstructions deduced from other experimental data, the oceanic realm has wider extent and more complex shape. The northernmost part of this realm shows crust of sub‐oceanic type altemating basement highs with lower subsidence values. The observed surface heat flux is consistent with the predicted geothermal held in the Alpine‐Provençal continental margin and in the oceanic domain. However, a characteristic thermal asymmetry is clearly visible astride the basin, due to the enhanced heat flux of the Corsica margin. Even if the uniform extension model accounts well at a regional level for the present basement depth, a remarkable tectonic subsidence excess has been found in the Alpine‐Provençal continental margin. This evidence agrees with the reprise in compression of the margin; the direction of the greatest principal stress i

ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1994.tb00493.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTPrincipal component analysis (PCA) and correspondence analysis (CA) have been used to identify several critical mineral crystallization factors during metamorphism of Mn‐rich lithologies from the Iberian Massif, Spain. Three types of variables have been considered in the system: mineralogical variables, bulk composition variables and one physical variable (oxygen fugacity). PCA was applied to the compositional variables to obtain four components, which were used as new compositional variables. These, together with the mineralogical and physical variables, were treated in the CA. The plot of the variables on the factor 1‐factor 2 plane reveals that tephroite crystallization is controlled by a compositional variable representing low values of ratio Si/(Mn+Fe) in the rocks. Moreover, it is possible to deduce from this plot the importance of high oxygen fugacity conditions for the crystallization of piemontite. The third factor shows that the crystallization of spessartine does not require very restrictive physical‐chemical condi

ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1994.tb00494.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTIn order to identify the dominant non‐Newtonian effects which occur during the injection of a new Newtonian magma through a partially crystallized magma chamber, we have performed some preliminary analogue experiments which enable us to point out several features induced by the non‐Newtonian properties of the host fluid during injection processes. These experiments were performed in a three‐dimensional device and involve complex non‐Newtonian fluids—clay suspensions in which rheological properties such as bulk strength, yield strength and rheofluidification exponent may vary. Forced injection takes place through a slot which in the case of a Newtonian host fluid is the geometry that provides planar structures. Depending both on the density contrast and on the rheological contrast between the injected dyed water and the host fluid three kind of structures were observed: (1) permanent plumes when the injected water is lighter than the suspension exhibiting rheological properties close to Newtonian fluids; (2) pseudo‐fountains and spreading at the bottom of the tank with a destabilizing density contrast and in high yield strength/more viscous suspensions; (3) fountains with a slightly stablizing density contrast. The implications for magma chamber evolution are briefly discussed. In particular it seems that homogeneous non‐Newtonian media inhibit the formation of planar structures and partially crystallized magma may induce the spreading of the new magma at the bottom or at the top of the chamber regardless of the density contrast betwe

ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1994.tb00495.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTTheoretical studies have shown the possibility of high‐temperature (‘high enthalpy’) geothermal reservoirs in the pre‐Tertiary rocks at 4–5 km depth range within the Pannonian Basin. This expectation was proven by the hotwater/steam blowout of Fábiánsebestyén‐4 borehole (16.12.85–31.1.86). Exploration efforts carried out during 1987–88 in the broad vicinity of the borehole proved that reservoirs of this type can be found with the combination of seismic reflection, silica‐thermometry and magnetotelluric sounding methods. Deliberate prospection should be continued in all suitable areas within the basin, since high enthalpy reservoirs promise profitable operation of geoth

ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1994.tb00496.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY