F. Mongelli

Geothermal ranking of Italian territory

Abstract The geothermal ranking of Italian territory to 3 km depth proposed in this paper is based on qualitative assessments of: 1. (1) the hydrogeological characteristics of the different rock complexes (including litho-stratigraphic sequence and structural setting), in order to identify major regional aquifers within a depth of 3 km. Clastic aquifers have not been considered in this study; 2. (2) the temperature range within the regional aquifers, and the impermeable rock complexes, based on available temperature, heat-flow data and geodynamic and structural conditions. Figures 1 and 2, and Plates 1–6, illustrate the different geothermal situations in Italy in terms of geodynamic, hydrogeological and thermal conditions to a maximum depth of 3 km. Italian territory has been divided into four main categories, A–D (in decreasing order of geothermal interest), each category being subdivided into classes, according to the estimated maximum temperature of the regional aquifer or of the impermeable formations within the reference depth, and according to the different potential use of the geothermal resource. The geothermal ranking of Italy shown in Plate 6 summarizes the objectives and limits of this work, and represents a reference base for identifying the priority areas for more detailed local studies and exploratory drilling.

Source parameters and three‐dimensional attenuation structure from the inversion of microearthquake pulse width data: Qp imaging and inferences on the thermal state of the Campi Flegrei caldera (southern Italy)

The three-dimensional P wave attenuation structure of the Campi Flegrei caldera and the estimate of source parameters for 87 local microearthquakes is obtained by the nonlinear inversion of pulse width and rise time measurements by using the method described by Zollo and de Lorenzo (this issue). Source radii represent the better resolved parameters with values ranging from 70 m to 230 m; the dip and strike angles defining fault orientations are usually affected by larger uncertainties and are well constrained only for 11 events. The dip fault is usually confined in the range 30°–60° (with an average uncertainty of 12°); the fault strikes mainly range between −60° and 60° and seem to define preferential directions oriented radially from the symmetry axis of the ground deformation. Stress drop estimates indicate rather low values (0.01–1 MPa) which suggest low strength properties of the incoherent and brittle materials filling the caldera (primarily yellow tuffs). The three-dimensional Qp images obtained from the inversion of P pulse durations show two significant low-Qp anomalies between 0 and 1 km of depth, in the north-eastern sector and at 2–3 km of depth in the central eastern sector of the caldera. The high degree of spatial correlation of the low-Qp zone and low-Vs (as inferred by Aster and Meyer (1988)) at 0–1 km in depth and other geophysical and geochemical observations suggest that this anomaly can be related to the presence of densely fractured, porous, and fluid-filled rocks in the NE sector of the caldera. The deeper low-Qp anomaly is interpreted as being related to a dominant thermal effect. We used the surface and deep borehole temperature measurements available in the area to obtain a local calibration curve to convert Qp in temperature at Campi Flegrei. The retrieved T(Qp) map shows a high thermal deep disturbance (450°–500°C) at depths between 2 and 3 km in the eastern sector of the caldera, where the most recent eruptive activity is concentrated. The present-day temperature field retrieved by Qp images has been interpreted by using a three-dimensional thermal conduction model assuming an extended heat source (initial temperature of 800°C) located underneath the attenuation anomalous region. The results indicate that the Qp-inferred temperature field can be related to the heat conduction effect of one or more molten bodies whose top should be at about 4-km depth, consistent with recent seismic estimates of the magma chamber top at Campi Flegrei (Ferrucci et al., 1992). This study suggests that the present thermal state and rock rheology of the inner caldera could be controlled by the cooling of molten bodies that originally intruded at depths of 1.4–1.6 km, during one or more recent (time of <10 kyr) eruptive events.

Interpretation of heat flow density of the Apennine chain, Italy

The Apennine chain is a consequence of the continental collision between Europe and Africa characterized by several compressive phases with events of intense shortening. During one of these phases, from Early Oligocene to Early Miocene, the continental crust broke into various segments which became packed one over the other, forming an Adriatic-verging structure of imbricated listric wedges. This event is thermally modeled in the Southern Apennines by the successive overthrust of two slabs of 7 km total thickness and explains the low heat flow density (HFD) (< 40 mW m−2) observed there. In the Northwestern Apennines, modeling is on the basis of the overthrusting of a slab 10 km thick, which reduced the HFD of the steady lithosphere (63 mW m−2) observed on the Iblean platform to 50.4 mW m−2. From the Late Miocene onwards, tensional tectonism developed in the internal areas of the chain. Application of McKenzies “simple stretching” model shows that the high regional heat flow (> 100 mW m−2), obtaining by filtering the observed data, is explained by a stretching of the lithosphere of a factor β = 3.1.

On the shallow origin of hotspots and the westward drift of the lithosphere

Intraplate migrating hotspots, which are unrelated to rifts or plate margins in general, regardless of their origin in the mantle column, indicate relative motion between the lithosphere and the underlying mantle in which the hotspot source is located. Pacific plate hotspots are sufficiently fixed relative to one another to represent an independent reference frame to compute plate motions. However, the interpretation of the middle asthenosphere rather than the deep lower mantle as the source for intraplate Pacific hotspots has several implications. First, decoupling between the lithosphere and subasthenospheric mantle is greater than recorded by hotspot volcanic tracks (>100 mm/yr) due to undetectable shear in the lower asthenosphere below the magmatic source. The shallower the source, the larger the décollement. Second, computation of the westward drift is linked to the Pacific plate and assumes that the deep lower mantle, below the decoupling zone, sources the hotspots above. The Pacific plate is the fastest plate in the hotspot reference frame and dominates the net rotation of the lithosphere. Therefore, if decoupling with the subasthenospheric mantle is larger, the global westward drift of the lithosphere must be faster than present estimates, and may possibly vary between 50 and 90 mm/yr. In this case, all plates, albeit moving at different velocities, move westward relative to the subasthenospheric mantle. Finally, faster decoupling can generate more shear heating in the asthenosphere (even >100 °C). This amount of heating, in an undepleted mantle, could trigger scattered intraplate Pacific volcanism itself if the viscosity of the asthenosphere is locally higher than normal. The Emperor-Hawaiian bend can be reproduced when bent viscosity anisotropy in the asthenosphere is included. Variations in depth and geometry in the asthenosphere of these regions of higher viscosity could account for the irregular migration and velocities of surface volcanic tracks. This type of volcanic chain has different kinematic and magmatic origins from the Atlantic hotspots or wetspots, 735 *E-mail: carlo.doglioni@uniroma1.it. Doglioni, C., Green, D., and Mongelli, F., 2005, On the shallow origin of hotspots and the westward drift of the lithosphere, in Foulger, G.R., Natland, J.H., Presnall, D.C., and Anderson, D.L., eds., Plates, plumes, and paradigms: Geological Society of America Special Paper 388, p. 735–749, doi: 10.1130/2005.2388(42). For permission to copy, contact editing@geosociety.org. ©2005 Geological Society of America. which migrate with or close to the oceanic spreading center and are therefore platemargin related.

Thermal state of the Campi Flegrei caldera inferred from seismic attenuation tomography

Athree-dimensional Qp image of the Campi Flegrei caldera between 0 and 3 km of depth has been inferred by the inversion of P rise time and pulse width data of 87 local earthquakes recorded during the last bradiseismic crisis by a local array deployed in the area by the University of Wisconsin. The availability of both thermal measurements in 5 deep boreholes and of a heat flow surface map of the area allowed us to calibrate the local temperature F vs. Qp relationship. The comparison of Qp, Vp and Vp/Vs images, combined with hydrogeological and geochemical data from deep boreholes, allowed us to distinguish some low-Qp anomalies related to the presence of fluids in the rocks from a deep low-Qp anomaly related to the conductive cooling of a magma reservoir. The deep anomaly is located in the same zone where several authors believe that the volcanic and magmatic activity migrated after the Neapolitan Yellow Tuff eruption. Moreover this anomaly includes the area where the existence of a magma chamber at depth between 4 and 5 km was inferred by an active seismic experiment. # 2001 Published by Elsevier Science Ltd. All rights reserved.

Surface heat flow density at the phlegrean fields caldera (SOUTHERN ITALY)

The Phlegrean Fields area is a Holocene caldera located west of Naples, southern Italy. The recent post-caldera activity is characterized by several eruptive centers inside the collapsed area. In order to investigate the still active volcanic processes, surface heat flow measurements were carried out in 1995 in 30 sites of the Phlegrean Fields, and a heat flow map compiled. Filtering of the map reveals some well-defined anomalies superimposed on a general southward-increasing trend. Local anomalies are related to small magma bodies, whereas the observed general trend has been attributed to the effect of groundwater flow. This effect was calculated and removed. The undisturbed mean value of the surface heat flow density in the eastern sector is 149 mW\m2, which is above the regional value of 85 mW\m2 assigned to the eastern part of the Tyrrhenian Sea, and which is probably influenced by a very large, deep magmatic body.

Thermal conductivity, diffusivity and specific heat variation of some Travale field (Tuscany) rocks versus temperature

Abstract Thermal parameters of eight rock samples (limestone, calcareous dolomite, quartzite, micaceous quartzite, phyllite, micaceous schistose quartzite, plagioclasic micaschists, gneiss) have been measured by the transient state cut-core method at temperatures ranging from 20° to 250°C. An average 22% decrease for the conductivity, 34% decrease for the diffusivity, and 19% increase for the specific heat have been observed. From a geothermal point of view (heat extraction from hot dry rocks) the most promising rocks are micaschists whose specific heat varies between 1.48 · 103 J kg−1 K−1 at 20°C and 1.62·103J kg−1 K−1 at 220° C.

Influence of water recharge on heat transfer in a semi-infinite aquifer

Abstract A simple model is proposed for the temperature field within an unconfined semi-infinite thick aquifer, with groundwater flowing parallel to the terrestrial heat flow in the recharge zone, and perpendicular to heat flow in the other parts of the aquifer. The results enable evaluation of the extent of the influence of the recharge zone, once the thermal and hydrological parameters are known. Beyond this zone, where thermal equilibrium between water and rock is reached, water temperature reaches the constant highest value, and water movement cannot be revealed by temperature measurements. Moreover, over,on the basis of the range of influence of the recharge zone, the regional water pore velocity can be inferred.

Thermal effects of long intrusions

The cooling of igneous intrusions into the lithosphere is treated by a time-dependent analytical model with a varying initial temperature distribution throughout the country rock. One important result is that, for long times after the intrusion, the effect of the intrusion on the surface gradient is approximately the same as for a host rock with a uniform temperature equal to the mean of the initial temperature distribution through that interval. The method is of practical importance because it allows us to study the thermal anomaly produced within the lithosphere by a hot batholith or by an astenolith of long vertical extension just after its emplacement.

Terrestrial heat flow measurements near rosignano solvay (Tuscany), Italy

Abstract Heat flow measurements were made in three wells close together on the Tyrrhenian coast of Tuscany. The average value of the temperature gradient was 62.8 °C/km, that of thermal conductivity of the rocks was 3.79 mcal/(cm s °C). The heat flow obtained was 2.55 μcal/(cm 2 3) (107 mW/m 2 ). These are values corrected for local effects (palaeoclimatic variations, the influence of the sea). In particular, a correction was made for the climatic variations of the last 10,000 years based on climatological evidence.