Thermal and mechanical structure of the central Iberian Peninsula lithosphere

The central Iberian Peninsula (Spain) is made up of three main tectonic units: a mountain range, the Spanish Central System and two Tertiary basins (those of the rivers Duero and Tajo). These units are the result of widespread foreland deformation of the Iberian plate interior in response to Alpine convergence of European and African plates. The present study was designed to investigate thermal structure and rheological stratification in this region of central Spain. Surface heat flow has been described to range from 80 to 60 mW m⁻². Highest surface heat flow values correspond to the Central System and northern part of the Tajo Basin. The relationship between elevation and thermal state was used to construct a one-dimensional thermal model. Mantle heat flow drops from 34 mW m⁻² (Duero Basin) to 27 mW m⁻² (Tajo Basin), and increases with diminishing surface heat flow. Strength predictions made by extrapolating experimental data indicate varying rheological stratification throughout the area. In general, in compression, ductile fields predominate in the middle and lower crusts and lithospheric mantle. Brittle behaviour is restricted to the first 8 km of the upper crust and to a thin layer at the top of the middle crust. In tension, brittle layers are slightly more extended, while the lower crust and lithospheric mantle remain ductile in the case of a wet peridotite composition. Discontinuities in brittle and ductile layer thickness determine lateral rheological anisotropy. Tectonic units roughly correspond to rheological domains. Brittle layers reach their maximum thickness beneath the Duero Basin and are of least thickness under the Tajo Basin, especially its northern area. Estimated total lithospheric strength shows a range from 2.5×10¹² to 8×10¹² N m⁻¹ in compression, and from 1.3×10¹² to 1.6×10¹² N m⁻¹ in tension. Highest values were estimated for the Duero Basin.Depth versus frequency of earthquakes correlates well with strength predictions. Earthquake foci concentrate mainly in the upper crust, showing a peak close to maximum strength depth. Most earthquakes occur in the southern margin of the Central System and southeast Tajo Basin. Seismicity is related to major faults, some bounding rheological domains. The Duero Basin is a relative quiescence zone characterised by higher total lithospheric strength than the remaining units.     

Insights into chondrule formation process and shock-thermal history of the Dergaon chondrite (H4-5)

The Dergaon fall represents a shock-melted H4-5(S5) ordinary chondrite which includes at least ten textural varieties of chondrules and belongs to the high chondrule-matrix ratio type.Our study reveals that the chondrules are of diverse mineralogy with variable olivine-pyroxene ratios(Type Ⅱ),igneous melt textures developed under variable cooling rates and formed through melt fractionations from two different melt reservoirs.Based on the experimental analogues,mineralogical associations and phase compositions,it is suggested that the Dergaon chondrules reflect two contrasting environments:a hot,dust-enriched and highly oxidized nebular environment through melting,without significant evaporation,and an arrested reducing environment concomitant with major evaporation loss of alkali and highly volatile trace elements.Coexistence of chlorapatite and merrillite suggests formation of the Dergaon matrix in an acidic accretionary environment.Textural integration and chemical homogenization occurred at ~ 1 atmospheric pressure and a mean temperature of 765 C mark the radiogenic thermal event.Equilibrated shock features(olivine mosaicism,diaplectic plagioclase,polycrystalline troilite) due to an impact-induced thermal event reflect a shock pressure 45 GPa and temperature of 600 C.By contrast,the local disequilibrium shock features(silicate melt veins comprising of olivine crystallites,troilite melt veins and metal droplets) correspond to a shock pressure up to 75 GPa and temperature950 ℃.     

Theoretical aspects and interpretation of thermal measurements concerning the subsurface investigation of a cometary nucleus

Part of the lander payload for the comet rendezvous mission Rosetta is the thermal probe multi-purpose sensors for surface and subsurface science (MUPUS). In this paper, we discuss the relationship of the expected MUPUS data to structural and textural parameters of the near-surface layers of the cometary nucleus. Such properties could be crucial parameters concerning the formation and evolution of the nucleus. Thus, we calculate the thermal conductivity of a porous material in terms of microstructural parameters, using a geometrical model with a solid matrix, a surrounding pore space and a distinct contact area between different particles. We include the possibility that a significant amount of heat may be transported by pore filling vapour in addition to heat conducted via the matrix. Furthermore, we consider that the heat is transmitted through only a fraction of the grains and these are organized into a chain-like structure. These chains—and not the single grains—should be regarded as the basic unit of structure. Applying our model to measured thermal conductivities of porous water ice, we interpret the material in terms of microparameters and estimate the effective size of the contact area and the effective pore radius. The results are in good agreement with our knowledge of the prepared samples. Contrary, we can also show that popular models used in cometary research do not fit with laboratory data at all.     

Implications of large elastic thicknesses for the composition and current thermal state of Mars

The martian elastic lithosphere thickness Te has recently been constrained by modeling the geodynamical response to loading at the martian polar caps and Te was found to exceed 300 km at the north pole today. Geological evidence suggests that Mars has been volcanically active in the recent past and we have reinvestigated the martian thermal evolution, identifying models which are consistent with Te>300 km and the observed recent magmatic activity. We find that although models satisfying both constraints can be constructed, special assumptions regarding the concentration and distribution of radioactive elements, the style of mantle convection and/or the mantle's volatile content need to be made. If a dry mantle rheology is assumed, strong plumes caused by, e.g., a strongly pressure dependent mantle viscosity or endothermic phase transitions near the core-mantle boundary are required to allow for decompression melting in the heads of mantle plumes. For a wet mantle, large mantle water contents of the order of 1000 ppm are required to allow for partial mantle melting. Also, for a moderate crustal enrichment of heat producing, elements the planet's bulk composition needs to be 25 and 50% sub-chondritic for dry and wet mantle rheologies, respectively. Even then, models resulting in a globally averaged elastic thicknesses of Te>300 km are difficult to reconcile with most elastic thickness estimates available for the Hesperian and Amazonian periods. It therefore seems likely that large elastic thicknesses in excess of 300 km are not representative for the bulk of the planet and that Te possibly shows a large degree of spatial heterogeneity.     

Growth and evolution of small porous icy bodies with an adaptive-grid thermal evolution code: I. Application to Kuiper belt objects and Enceladus

We present a new 1-dimensional thermal evolution code suited for small icy bodies of the Solar System, based on modern adaptive grid numerical techniques, and suited for multiphase flow through a porous medium. The code is used for evolutionary calculations spanning 4.6×¹⁰⁹ yr of a growing body made of ice and rock, starting with a 10 km radius seed and ending with an object 250 km in radius. Initial conditions are chosen to match two different classes of objects: a Kuiper belt object, and Saturn's moon Enceladus. Heating by the decay of ²⁶Al, as well as long-lived radionuclides is taken into account. Several values of the thermal conductivity and accretion laws are tested. We find that in all cases the melting point of ice is reached in a central core. Evaporation and flow of water and vapor gradually remove the water from the core and the final (present) structure is differentiated, with a rocky, highly porous core of 80 km radius (and up to 160 km for very low conductivities). Outside the core, due to refreezing of water and vapor, a compact, ice-rich layer forms, a few tens of km thick (except in the case of very high conductivity). If the ice is initially amorphous, as expected in the Kuiper belt, the amorphous ice is preserved in an outer layer about 20 km thick. We conclude by suggesting various ways in which the code may be extended.     

Quasi-3-D model to describe topographic effects on non-spherical comet nucleus evolution

A new quasi three-dimensional code is presented and used to explore the thermal evolution of non-spherical-shape cometary nuclei. Calculations are done for spherical nuclei with different obliquities, spherical nuclei with crater-like depressions and spheroid-shape nuclei. The results obtained for both the gas and dust fluxes agree with previous simulations. Local dust crust formation can occur when the comet is located far from the Sun (around 3.5 AU), creating active and inactive areas on the surface. For a given set of parameters, the H₂O production rate is comparable to the one observed for comets. A pre-post-perihelion asymmetry exists for H₂O, CO₂ and CO production rates. It is shown that crater-like depressions can be erased within the lifetime of a comet and that spheroid-shape nuclei have a higher production rate than equal area spherical nuclei.     

10 and 20 Micron imaging with arrays

We discuss imaging with arrays in the thermal IR. Aspects of the design and performance of the Golden Gopher, an infrared array camera are presented. This instrument operates in a high-background environment, for ground-based astronomical imaging from 5 to 27 m. It is built around a 20×64 element Si:As Impurity Band Conduction (IBC) device manufactured by GenCorp Aerojet Electronic Systems Division, and has a noise-equivalent flux density (NEFD) of 23.5mJy min ^-1/2 arcsec^-2 at =10m, =1m, on the Mt. Lemmon 1.5m telescope. We present and discuss a sample of the data. In addition we discuss the design and expected performance of the Long Wavelength Spectrometer which is now under construction for the Keck telescope.     

塑料棚面日光温室设计与增温效果

通过对太阳、地球相对位置季节变化的研究,从天文学和气象学的角度,依据有关数学、物理学原理计算出塑料棚面日光温室的设计参数。结果表明:据此建造的塑料棚面日光温室在冬季农业生产中每日可延长日照时间约为15 min,较其他日光温室日平均温度提高2—3℃,并具有较好的保温效果。     

The heat flow of Europa

The heat flow from Europa has profound implications for ice shell thickness and structure, as well as for the existence of an internal ocean, which is strongly suggested by magnetic data. The brittle-ductile transition depth and the effective elastic thickness of the lithosphere are here used to perform heat flow estimations for Europa. Results give preferred heat flow values (for a typical geological strain rate of ¹⁰⁻¹⁵ s⁻¹) of 70-110 mW m⁻² for a brittle-ductile transition 2 km deep (the usually accepted upper limit for the brittle-ductile transition depth in the ice shell of Europa), 24-35 mW m⁻² for an effective elastic thickness of 2.9 km supporting a plateau near the Cilix impact crater, and >130 mW m⁻² for effective elastic thicknesses of ?0.4 km proposed for the lithosphere loaded by ridges and domes. These values are clearly higher than those produced by radiogenic heating, thus implying an important role for tidal heating. The ?19-25 km thick ice shell proposed from the analysis of size and depth of impact structures suggests a heat flow of ?30-45 mW m⁻² reaching the ice shell base, which in turn would imply an important contribution to the heat flow from tidal heating within the ice shell. Tidally heated convection in the ice shell could be capable to supply ∼100 mW m⁻² for superplastic flow, and, at the Cilix crater region, ∼35-50 mW m⁻² for dislocation creep, which suggests local variations in the dominant flow mechanism for convection. The very high heat flows maybe related to ridges and domes could be originated by preferential heating at special settings.     

Modelling of rock alteration due to coupled heat and mass transport

 A three-dimensional computer model is presented for studying the interaction of heat and mass transport regarding the temporal and spatial evolution of sandstones. The model simulates coupled heat and reactive mass transport in porous rocks. In general, mineral solubilities in water are low. Therefore, large fluid volumes are required to flow through the rock to explain observed mineral cements in sandstones. Besides mass transport, pore fluids transport heat which modifies rock temperatures. Very high flow rates result in strong temperature modifications and, therefore, enhance diagenesis. Low flow rates often cannot account for observed cementation. The model results show the effect of advective, convective and conductive heat transport on temperature and diagenetic evolution of sandstones for two different flow systems in a simple geological environment. Received: 6 August 1996/Accepted: 18 December 1996