Massimo Verdoya

Heat flux and timing of the drifting stage in the Ligurian-Provençal basin (northwestern Mediterranean)

Abstract The surface thermal flux in the Ligurian-Provencal basin is accounted for by a three-component model for the continental margins and a cooling model for the oceanic domain. In the continental margins the surface heat-flux data, corrected for the main perturbations, range from 65–70 to 85–90 mW m − ; the background heat flowing out from the asthenosphere is 38 mW m − . Heat-flux data from the oceanic domain indicate that the drifting stage should have begun 4–5 Ma earlier and ended 2–3 Ma later than previously inferred, being consistent with recent paleomagnetic results. A decoupling of Corsica from Sardinia during drifting is also validated by the different periods of oceanic accretion which, in the northern sector, seems to be shorter. The rift stage started in the Oligocene and ended about 27 Ma ago between Sardinia and the Gulf of Lion. The rotation of Corsica and Sardinia, which was accompanied by westward crustal accretion, ended about 17 Ma ago. Such a wider time span for drifting involves a more reliable average spreading rate of 2.5 cm a − . An outline of the development of the basin during each evolutionary stage is drawn.

Heat from radioactive elements in young volcanics by γ-ray spectrometry

Abstract Radioactive heat production data for volcanic rocks ranging in age from 430 ka to the historical time and cropping out on the three main islands of the Aeolian arc (Southern Italy) are presented. They were derived from uranium, thorium and potassium concentration measured from γ-radiations originating from the decay of 214Bi (238U series), 208Tl (232Th series) and the primary decay of 40K. Concentration results from a NaI(Tl) detector, compared with independent determinations by means of mass spectrometry, reveal significant secular radioactive disequilibrium between 214Bi and 238U, in the rhyolitic rocks, which is ascribable to their young age. The use of the low-energy portion of the spectrum, where there are a number of γ-rays produced by 234Th, minimises the disequilibrium problem in the uranium determination. The Th/U ratio remains almost constant at 3.3 in all the rock types. Due to their low isotopic content, basalts show a low radioactive heat production rate (0.6 μW m−3), whereas the highest values (6.6–7.1 μW m−3) are found in trachytic and rhyolitic lavas.

Editorial to “Heat Flow: Recent Advances”

Geothermics, one of the principal geophysical disciplines, is a science pertaining to the earth’s interior heat. It focuses primarily on the investigations of the thermal structure of the earth, distribution of internal heat sources and heat transfer mechanisms through experimental and theoretical studies. Geothermics is relatively young and much of its advances have been achieved only over the last 80 years. Despite its youth, during this time, geothermal studies have broadened and expanded into further scientific domains, sometimes into obvious sub-disciplines such as radiometry and hydrology, and sometimes into surprisingly less apparent fields of interest such as paleoclimatology and global warming. Of special importance is the sub-discipline of applied geothermics addressing geothermal energy and its use in electric power generation andspace heating. During the 26th General Assembly of the International Union of Geodesy and Geophysics held in Prague, from 22 June to 2 July, 2015, Symposium S13 Terrestrial Heat Flow was organized. This Symposium addressed various aspects of geothermics, namely heat flow data and their interpretation, heat flow and tectonics, subsurface temperature field, borehole temperature inversion, borehole climatology and applied geothermics. This session was the largest of its kind realized in the last several years and in 24 oral and 28 poster presentations well reflected the contemporary level of knowledge. Here we present twelve selected contributions providing a representative illustration. Several contributions of that session were already submitted to different journals, and therefore, are not included here. Four papers in this volume address the interpretation of observed heat flow data in terms of crustal/lithosphere structure in a specific region or area. Jacek Majorovicz describes the geothermal activity of the Western Canadian foreland basin and addresses the problem of why its northwestern part was characterized by relatively high heat flow of about 80 mWm−2 while the southeastern part evidenced only 50 mWm−2, when the whole underlying Precambrian basement and 200 km thick lithosphere do not practically differ and no significant heat flow vs. radiogenic heat production statistical relationship was found. To explain the existing large heat flow contrast the author had to conclude that the high heat production layer of the upper crust must vary in thickness by as much as factor of 2 or that heat production measured at top of the Precambrian basement is not representative for deep rocks. The author speculates that the previous explanation that the heat in the basin is redistributed by the regional fluid flow systems driven from high hydraulic head bound from higher to lower elevations is inconsistent with the observed Darcy fluid velocities and/ or basin geometry. Given the available facts, the author concludes that at the moment no definite explanation for the observed high heat flow is at hand, and high heat generation of the thicker than normal, 20 km, upper crust, remains the preferred explanation. Tectonically, the continental margin of Brazil (CMB) is a passive continental margin where heat flow is not expected to be high. Surprisingly, Valiya Hamza et al. discover within the CMB a narrow belt where heat flow is, in general, higher than 70 mWm−2, in contrast to lower than 60 mWm−2 * Vladimir Cermak cermak@ig.cas.cz

Heat Conduction and Thermophysical Parameters

This chapter presents the basic equations for conductive heat transfer and the main thermal parameters of the rocks, in particular the thermal conductivity and radiogenic heat. Also, it outlines the most commonly used techniques for measuring these parameters. Models involving the application of mixing laws for a mineral aggregate are discussed together with techniques for estimating the in situ thermal conductivity and volumetric heat capacity . Finally, methods for determining the radiogenic heat in the crust are introduced.

Subsurface Temperature and Climate Change Reconstruction

Subsurface temperature field forms an independent archive of past climate changes that is complementary to both the surface air temperature observations and the traditional climate proxy data, each with particular strengths and limitations. In this chapter, we focus on the contribution of temperature measurements in deep boreholes to the inference of climate history. The ground surface temperature variations can be reconstructed by inverting the borehole temperature perturbations. Moreover, the joint analysis of temperature recorded in borehole and surface air temperature time series yields estimates of the pre-observational mean of surface air temperature. We also show how the subsurface temperature variations by a short wavelength climate change can be modelled with a known time-dependent temperature boundary condition.

Heat Conduction and Thermal Parameters

This chapter presents the basic equations for conductive heat transfer and the main thermal parameters of the rocks, in particular the thermal conductivity and radiogenic heat. Also, it outlines the most commonly used techniques for estimating these parameters. Models involving the application of mixing laws for a mineral aggregate are discussed together with techniques for estimating the in situ thermal conductivity and volumetric heat capacity. Finally, methods for determining the radiogenic heat in the crust are introduced.

Heat in the Groundwater Flow

The presence of groundwater flow implies other heat transfer mechanisms rather than pure conduction. Several strategies have been developed to explore the heat transport associated with water flow. This chapter presents some analytical methods and shows how subsurface temperatures can provide a quantitative tool for inferring water flow in permeable layers and heat advection in hydrothermal systems. Thermal convection in deep aquifers and its potential are then analyzed by means of the dimensionless Rayleigh number.