Geothermal data has been indicating promising potentialities in the north-eastern Morocco. This p... more Geothermal data has been indicating promising potentialities in the north-eastern Morocco. This paper presents new temperature data, recently recorded in water boreholes located in the Berkane and Oujda areas. Generally, the observed temperature gradients are rather high. One hole near Berkane, revealed an average geothermal gradient of more than 110 °C/km at depths greater than 300 m. This result confirms the geothermal gradient estimated in a mining borehole located about 30 km west of the Berkane borehole, in which water temperature of 96 °C is reached at a depth of about 700 m. Such a high geothermal gradient, exceeding by far the ones already determined for northeastern Morocco, could act as a stimulus to programs aimed at the geothermal exploitation of high temperature aquifers.
The petrophysical properties of sediment drill core samples recovered from the Sardinian margin a... more The petrophysical properties of sediment drill core samples recovered from the Sardinian margin and the abyssal plain of the Southern Tyrrhenian Basin were used to estimate the downhole change in porosity and rates of deposition and mass accumulation. We calculated how the deposited material has changed its thickness as a function of depth, and corrected the thickness for the compaction. The corresponding porosity variation with depth for terrigenous and pelagic sediments and evaporites was modelled according to an exponential law. The mass accumulation rate for the Plio-Quaternary is on average 4.8×104 kg m−2 my−1 on the Sardinian margin and for the Pliocene in the abyssal plain. In the latter area, the Quaternary attains its greatest thickness and a mass accumulation rate of 11–40×104 kg m−2 my−1. The basement response to sediment loading was calculated with Airy-type backstripping. On the lower part of the Sardinian margin, the basement subsidence rate due to sediment loading has decreased from a value of 300 m my−1 in the Tortonian and during the Messinian salinity crisis (7.0–5.33 Ma) to about 5 m my−1 in the Plio-Quaternary. In contrast, on the abyssal plain this rate has changed from 8–50 m my−1 during the period 3.6–0.46 Ma, to 95–130 m my−1 since 0.46 Ma, with the largest values in the Marsili Basin. The correlation between age and the depth to the basement corrected for the loading of the sediment in the ocean domain of the Tyrrhenian Basin argues for a young age of basin formation.
We present techniques to infer thermo-hydraulic parameters from the analysis of underground tempe... more We present techniques to infer thermo-hydraulic parameters from the analysis of underground temperature data from boreholes. Two different cases are analyzed: thermal data from artesian wells and from boreholes drilled in unconfined aquifers. The former allow the assessment of the formation temperature gradient and the flow volume; the latter give estimates of the Darcy velocity and, consequently, allow the inference of the aquifer hydraulic conductivity. The analysis procedure consists in matching thermal logs with analytical models incorporating both heat and mass transfer. The thermo-hydraulic parameters are obtained from the coefficients of the advective models by means of the least-square fitting method. Examples of applications are given for a set of temperature logs from a geothermal area of northwestern Italy. Preliminarily, thermal gradients and the correction for the recent climate change were evaluated from thermal data of boreholes with no evidence of advection. The analysis of artesian boreholes gave high formation thermal gradients, in agreement with those obtained from boreholes with dominant conductive thermal regime. In boreholes drilled in permeable formations, the models predicting the temperature versus depth distribution resulted generally less sensitive to geological noise and fitted the field data better than the models based on the gradient analysis. If the shape of the thermal profile is taken into account as a qualitative descriptor of the type of flow, reliable estimates of the flow components can be obtained.
We propose a thermal model of the subducting Ionian microplate. The slab sinks in an isothermal m... more We propose a thermal model of the subducting Ionian microplate. The slab sinks in an isothermal mantle, and for the boundary conditions we take into account the relation between the maximum depth of seismicity and the “thermal parameter” L th of the slab, which is a product of the age of the subducted lithosphere and the vertical component of the convergence rate. The surface heat-flux dataset of the Ionian Sea is reviewed, and a convective geotherm is calculated in its undeformed part for a surface heat flux of 42 mW m−2, an adiabatic gradient of 0.6 mK m−1, a mantle kinematic viscosity of 1017 m2 s−1 and an asthenosphere potential temperature of 1300°C. The calculated temperature-depth distribution compared to the mantle melting temperature indicates the decoupling limit between lithosphere and asthenosphere occurs at a depth of 105 km and a temperature of 1260°C. A 70–km thick mechanical boundary layer is found. By considering that the maximum depth of the seismic events within the slab is 600 km, a L th of 4725 km is inferred. For a subduction rate equal to the spreading rate, the corresponding assimilation and cooling times of the microplate are about 7 and 90 Myr, respectively. The thermal model assumes that the mantle flow above the slab is parallel and equal to the subducting plate velocity of 6 cm yr−1, and ignores the heat conduction down the slab dip. The critical temperature, above which the subduced lithosphere cannot sustain the stress necessary to produce seismicity, is determined from the thermal conditions governing the rheology of the plate. The minimum potential temperature at the depth of the deepest earthquake in the slab is 730°C.
ABSTRACT We propose a thermo-mechanical model and a new interpretation of heat flux data for the ... more ABSTRACT We propose a thermo-mechanical model and a new interpretation of heat flux data for the Ligurian basin that may be a key to understanding the evolution of the NW Mediterranean. The model incorporates the removal of a portion of mantle lithosphere to explain the heat-flux and subsidence anomaly of the eastern (Corsican) margin of the basin. This process is envisaged as a result of eastward asthenosphere flow induced by the Apennines subduction system. After a heating phase, time-dependent conductive cooling and re-thickening of the lithosphere result in re-equilibrium of the thermal gradient to its initial value. Such a rifting mode can account for the asymmetric heat-flux and subsidence pattern observed across the basin and the present-day lithospheric thickness.
The change in the air temperature recorded at the Genoa University observatory over 155 years fol... more The change in the air temperature recorded at the Genoa University observatory over 155 years followed the change in the wind direction. A warming occurred both from 1838 to 1869 and from 1890 to 1950. The latter warm period was followed by a cooling in the 1950s, when the lowest mean annual temperature occurred. Generally, a warm period corresponds to a growth in the southern components of the ground wind direction; the increase in the southerly circulation in the Mediterranean yields increased stability in dry weather. Temperature-depth data measured in a geothermal borehole reflect this climate change. A comparison between the measurements and a synthetic temperature profile, based on the air temperature history recorded at the observatory, indicates that the average temperature prior to the meteorological time series was about 0.6 K higher than the average of the decade 1973-1982.
The ground surface temperature (GST) history in NW Italy was reconstructed for the last three cen... more The ground surface temperature (GST) history in NW Italy was reconstructed for the last three centuries by means of temperature–depth data recorded in a borehole in 1982. The results indicate relatively warmer periods in the 18th century and at the end the 1970s. A more recent set of underground temperature data was recorded in 1996. The comparison with the earlier thermal logging shows an evident temperature increase in the uppermost 80 m. Two different inversion techniques yield a subsurface temperature increase of 0.8–1.0 K since the 1980s. The inferred climatic model is consistent with the air temperature variations recorded at the Genoa University meteorological station.
We evaluate the different components contributing to the surface heat flux of the Sicilian–Calabr... more We evaluate the different components contributing to the surface heat flux of the Sicilian–Calabrian margin in the Tyrrhenian back-arc basin. The stretching factor, estimated from the change in crustal thickness, and the observed surface heat flux reveal a quite regular pattern, increasing from the coastline to the oceanic domain. The average value of the stretching factor is 2, while that of the surface heat flux, excluding data close to the Aeolian volcanic arc and those affected by hydrothermal circulation, is 125 mW m−2. A heat-flux model taking into account the stretching processes, the lithospheric radiogenic heat source and the heat flux from the asthenosphere cannot account for these average values. A likely source of the energy required to balance the 33 mW m−2 heat-flux deficit should be searched in the active subduction process occurring beneath the basin. The results show that when a moderately hot asthenosphere (in excess of 90 °C with respect to the normal) moves from high to low pressure with little change in entropy, it crosses its solidus and can generate partial melt. The produced magma is able to ascend and to pond at the base of the crust. By incorporating the heat produced by a 3.4 km thick magma layer and a crustal growth correction on the lithosphere extension, the thermal budget fits the observed surface heat flux. This indicates that extension, besides subduction, might have generated continental margin magmatism, and it requires a thermally anomalous mantle.
Physics of The Earth and Planetary Interiors, 2005
The basal depth of the outer layer with internal magnetic sources was calculated from magnetic da... more The basal depth of the outer layer with internal magnetic sources was calculated from magnetic data available within a roughly 500 km wide and 1200 km long area, running from central Germany to southern Italy. The dataset, deriving from different aeromagnetic surveys, is reduced to the reference altitude of 3000 m a.s.l. and a reference year of 1980.0. The adopted method, which transforms the spatial data into the frequency domain, provides a relationship between the two-dimensional spectrum of the magnetic anomalies and the top and centroid depths of the magnetic sources. The magnetic layer bottom depth (MLBD) thus obtained is 29–33 km deep in the stable areas (central Europe Variscan units, Corsica–Sardinia Variscan block) and corresponds to the Moho, having an average temperature of 560 °C. From the Alps to the Apennines, MLBD ranges between 22 and 28 km and is clearly shallower than the Moho. In these units, the wide variation of MLBD appears to be compatible with the presence of shallow magnetised bodies, consisting of lower crustal rocks (Ivrea–Verbano zone), ophiolitic units (Penninic zone and Voltri Massif) and intrasedimentary basic volcanic bodies (Po Basin). An average value of 25 km can be attributed to MLBD, which corresponds to a temperature of 550 °C. In the peri-Tyrrhenian zone and the Ligurian Sea, MLBD is below the Moho, which ranges from 17 to 20 km depth, and it has a temperature matching approximately to the Curie temperature of magnetite (580 °C).
The surface heat-flow density of the Fennoscandian Shield, after removing the disturbances due to... more The surface heat-flow density of the Fennoscandian Shield, after removing the disturbances due to palaeoclimatic changes, shows a remarkable contrast from the Archaean terrains to the Late Proterozoic provinces which derive from tectonic reactivation and reheating of older materials. This involves lateral variations in the rheological behaviour of the lithosphere. On the basis of seismic structural data and assumptions about the petrological composition and flow parameters of steady-state dislocation creep, strength profiles and lateral viscosity variations have been deduced for several sites. In the northern and northeastern parts of the shield, where the Moho temperature and mantle heat-flow density are typically cratonic, the rheological thickness of the lithosphere, given by the depth at which the strength is reduced to 1 MPa, ranges from 120 to 140 km. In the southwestern shield, were enhanced Moho temperatures and mantle heat-flow density occur, the rheological thickness is reduced to 60-80 km. The depth of the brittle-ductile transition in the upper crust, above which most earthquakes occur, varies on average from 30 km in the northeast to 18 km in the southwest. The limiting temperature of the brittle uppermost layer on average is 365 ± 70°C. The essentially aseismic behaviour of the mantle agrees with the subcrustal predominating ductile deformations predicted by the models. The average lithospheric strength falls within the range 70-200 MPa, typical of the older stable areas. The viscosity of the upper mantle, at a reference depth level of 60 km, ranges from 10 21 to 10 24 Pa s and increases with the geological age, being maximum beneath the Archaean nucleus.
Physics of The Earth and Planetary Interiors, 2001
The regional thermal regime in the Fennoscandian Shield is outlined, and the consequent rheologic... more The regional thermal regime in the Fennoscandian Shield is outlined, and the consequent rheological structure is analysed from the Kola Orogen to the Sorgenfrei–Tornquist zone. Moho temperatures and the heat flux from the mantle are typical of cratonic areas. The deep thermal field shows a cold root in the north-eastern sector. Larger lateral variations of mantle heat flux and Moho temperatures occur in the southern area and at the edges of the shield. The thickness of the thermal lithosphere is maximum (200–220 km), where the mantle heat flux is minimum (about 15–20 mW m−2). Three lithospheric cross-sections illustrate the expected lateral variation in viscosity and failure mode throughout the lithosphere as a consequence of the different geothermal conditions and the rheological stratification. In the upper crust, the depth of the brittle–ductile transition varies on average from 30, in the north-east, to 15 km, in the south-west. In the subcrustal mantle, the models predict a mainly ductile behaviour. At a depth of 60 km, the upper mantle viscosity is maximum (2.5×1023 to 25.0×1023 Pa s) beneath the Archean province and minimum (2.5×1021 to 10.0×1021 Pa s) below the Svecofennian and Sveconorwegian units. The comparison of the rheological calculations with the seismic activity shows a general agreement with the brittle–ductile transition depth expected in the Archaean and Proterozoic provinces. There is a difference both in number of earthquakes and distribution versus depth between areas of different age. The occurrence of larger magnitude seismic events near the base of the seismogenic zone, in the region of inferred peak of shear resistance, suggests some causal relationship. The Proterozoic areas show a more regular decrease of shocks with depth and, compared to the Archean provinces, a tendency to shallowing.
The structural setting beneath the Ligurian Sea resuJts from several tectonic events reflected in... more The 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 is N120°E on average.
Geothermal data has been indicating promising potentialities in the north-eastern Morocco. This p... more Geothermal data has been indicating promising potentialities in the north-eastern Morocco. This paper presents new temperature data, recently recorded in water boreholes located in the Berkane and Oujda areas. Generally, the observed temperature gradients are rather high. One hole near Berkane, revealed an average geothermal gradient of more than 110 °C/km at depths greater than 300 m. This result confirms the geothermal gradient estimated in a mining borehole located about 30 km west of the Berkane borehole, in which water temperature of 96 °C is reached at a depth of about 700 m. Such a high geothermal gradient, exceeding by far the ones already determined for northeastern Morocco, could act as a stimulus to programs aimed at the geothermal exploitation of high temperature aquifers.
The petrophysical properties of sediment drill core samples recovered from the Sardinian margin a... more The petrophysical properties of sediment drill core samples recovered from the Sardinian margin and the abyssal plain of the Southern Tyrrhenian Basin were used to estimate the downhole change in porosity and rates of deposition and mass accumulation. We calculated how the deposited material has changed its thickness as a function of depth, and corrected the thickness for the compaction. The corresponding porosity variation with depth for terrigenous and pelagic sediments and evaporites was modelled according to an exponential law. The mass accumulation rate for the Plio-Quaternary is on average 4.8×104 kg m−2 my−1 on the Sardinian margin and for the Pliocene in the abyssal plain. In the latter area, the Quaternary attains its greatest thickness and a mass accumulation rate of 11–40×104 kg m−2 my−1. The basement response to sediment loading was calculated with Airy-type backstripping. On the lower part of the Sardinian margin, the basement subsidence rate due to sediment loading has decreased from a value of 300 m my−1 in the Tortonian and during the Messinian salinity crisis (7.0–5.33 Ma) to about 5 m my−1 in the Plio-Quaternary. In contrast, on the abyssal plain this rate has changed from 8–50 m my−1 during the period 3.6–0.46 Ma, to 95–130 m my−1 since 0.46 Ma, with the largest values in the Marsili Basin. The correlation between age and the depth to the basement corrected for the loading of the sediment in the ocean domain of the Tyrrhenian Basin argues for a young age of basin formation.
We present techniques to infer thermo-hydraulic parameters from the analysis of underground tempe... more We present techniques to infer thermo-hydraulic parameters from the analysis of underground temperature data from boreholes. Two different cases are analyzed: thermal data from artesian wells and from boreholes drilled in unconfined aquifers. The former allow the assessment of the formation temperature gradient and the flow volume; the latter give estimates of the Darcy velocity and, consequently, allow the inference of the aquifer hydraulic conductivity. The analysis procedure consists in matching thermal logs with analytical models incorporating both heat and mass transfer. The thermo-hydraulic parameters are obtained from the coefficients of the advective models by means of the least-square fitting method. Examples of applications are given for a set of temperature logs from a geothermal area of northwestern Italy. Preliminarily, thermal gradients and the correction for the recent climate change were evaluated from thermal data of boreholes with no evidence of advection. The analysis of artesian boreholes gave high formation thermal gradients, in agreement with those obtained from boreholes with dominant conductive thermal regime. In boreholes drilled in permeable formations, the models predicting the temperature versus depth distribution resulted generally less sensitive to geological noise and fitted the field data better than the models based on the gradient analysis. If the shape of the thermal profile is taken into account as a qualitative descriptor of the type of flow, reliable estimates of the flow components can be obtained.
We propose a thermal model of the subducting Ionian microplate. The slab sinks in an isothermal m... more We propose a thermal model of the subducting Ionian microplate. The slab sinks in an isothermal mantle, and for the boundary conditions we take into account the relation between the maximum depth of seismicity and the “thermal parameter” L th of the slab, which is a product of the age of the subducted lithosphere and the vertical component of the convergence rate. The surface heat-flux dataset of the Ionian Sea is reviewed, and a convective geotherm is calculated in its undeformed part for a surface heat flux of 42 mW m−2, an adiabatic gradient of 0.6 mK m−1, a mantle kinematic viscosity of 1017 m2 s−1 and an asthenosphere potential temperature of 1300°C. The calculated temperature-depth distribution compared to the mantle melting temperature indicates the decoupling limit between lithosphere and asthenosphere occurs at a depth of 105 km and a temperature of 1260°C. A 70–km thick mechanical boundary layer is found. By considering that the maximum depth of the seismic events within the slab is 600 km, a L th of 4725 km is inferred. For a subduction rate equal to the spreading rate, the corresponding assimilation and cooling times of the microplate are about 7 and 90 Myr, respectively. The thermal model assumes that the mantle flow above the slab is parallel and equal to the subducting plate velocity of 6 cm yr−1, and ignores the heat conduction down the slab dip. The critical temperature, above which the subduced lithosphere cannot sustain the stress necessary to produce seismicity, is determined from the thermal conditions governing the rheology of the plate. The minimum potential temperature at the depth of the deepest earthquake in the slab is 730°C.
ABSTRACT We propose a thermo-mechanical model and a new interpretation of heat flux data for the ... more ABSTRACT We propose a thermo-mechanical model and a new interpretation of heat flux data for the Ligurian basin that may be a key to understanding the evolution of the NW Mediterranean. The model incorporates the removal of a portion of mantle lithosphere to explain the heat-flux and subsidence anomaly of the eastern (Corsican) margin of the basin. This process is envisaged as a result of eastward asthenosphere flow induced by the Apennines subduction system. After a heating phase, time-dependent conductive cooling and re-thickening of the lithosphere result in re-equilibrium of the thermal gradient to its initial value. Such a rifting mode can account for the asymmetric heat-flux and subsidence pattern observed across the basin and the present-day lithospheric thickness.
The change in the air temperature recorded at the Genoa University observatory over 155 years fol... more The change in the air temperature recorded at the Genoa University observatory over 155 years followed the change in the wind direction. A warming occurred both from 1838 to 1869 and from 1890 to 1950. The latter warm period was followed by a cooling in the 1950s, when the lowest mean annual temperature occurred. Generally, a warm period corresponds to a growth in the southern components of the ground wind direction; the increase in the southerly circulation in the Mediterranean yields increased stability in dry weather. Temperature-depth data measured in a geothermal borehole reflect this climate change. A comparison between the measurements and a synthetic temperature profile, based on the air temperature history recorded at the observatory, indicates that the average temperature prior to the meteorological time series was about 0.6 K higher than the average of the decade 1973-1982.
The ground surface temperature (GST) history in NW Italy was reconstructed for the last three cen... more The ground surface temperature (GST) history in NW Italy was reconstructed for the last three centuries by means of temperature–depth data recorded in a borehole in 1982. The results indicate relatively warmer periods in the 18th century and at the end the 1970s. A more recent set of underground temperature data was recorded in 1996. The comparison with the earlier thermal logging shows an evident temperature increase in the uppermost 80 m. Two different inversion techniques yield a subsurface temperature increase of 0.8–1.0 K since the 1980s. The inferred climatic model is consistent with the air temperature variations recorded at the Genoa University meteorological station.
We evaluate the different components contributing to the surface heat flux of the Sicilian–Calabr... more We evaluate the different components contributing to the surface heat flux of the Sicilian–Calabrian margin in the Tyrrhenian back-arc basin. The stretching factor, estimated from the change in crustal thickness, and the observed surface heat flux reveal a quite regular pattern, increasing from the coastline to the oceanic domain. The average value of the stretching factor is 2, while that of the surface heat flux, excluding data close to the Aeolian volcanic arc and those affected by hydrothermal circulation, is 125 mW m−2. A heat-flux model taking into account the stretching processes, the lithospheric radiogenic heat source and the heat flux from the asthenosphere cannot account for these average values. A likely source of the energy required to balance the 33 mW m−2 heat-flux deficit should be searched in the active subduction process occurring beneath the basin. The results show that when a moderately hot asthenosphere (in excess of 90 °C with respect to the normal) moves from high to low pressure with little change in entropy, it crosses its solidus and can generate partial melt. The produced magma is able to ascend and to pond at the base of the crust. By incorporating the heat produced by a 3.4 km thick magma layer and a crustal growth correction on the lithosphere extension, the thermal budget fits the observed surface heat flux. This indicates that extension, besides subduction, might have generated continental margin magmatism, and it requires a thermally anomalous mantle.
Physics of The Earth and Planetary Interiors, 2005
The basal depth of the outer layer with internal magnetic sources was calculated from magnetic da... more The basal depth of the outer layer with internal magnetic sources was calculated from magnetic data available within a roughly 500 km wide and 1200 km long area, running from central Germany to southern Italy. The dataset, deriving from different aeromagnetic surveys, is reduced to the reference altitude of 3000 m a.s.l. and a reference year of 1980.0. The adopted method, which transforms the spatial data into the frequency domain, provides a relationship between the two-dimensional spectrum of the magnetic anomalies and the top and centroid depths of the magnetic sources. The magnetic layer bottom depth (MLBD) thus obtained is 29–33 km deep in the stable areas (central Europe Variscan units, Corsica–Sardinia Variscan block) and corresponds to the Moho, having an average temperature of 560 °C. From the Alps to the Apennines, MLBD ranges between 22 and 28 km and is clearly shallower than the Moho. In these units, the wide variation of MLBD appears to be compatible with the presence of shallow magnetised bodies, consisting of lower crustal rocks (Ivrea–Verbano zone), ophiolitic units (Penninic zone and Voltri Massif) and intrasedimentary basic volcanic bodies (Po Basin). An average value of 25 km can be attributed to MLBD, which corresponds to a temperature of 550 °C. In the peri-Tyrrhenian zone and the Ligurian Sea, MLBD is below the Moho, which ranges from 17 to 20 km depth, and it has a temperature matching approximately to the Curie temperature of magnetite (580 °C).
The surface heat-flow density of the Fennoscandian Shield, after removing the disturbances due to... more The surface heat-flow density of the Fennoscandian Shield, after removing the disturbances due to palaeoclimatic changes, shows a remarkable contrast from the Archaean terrains to the Late Proterozoic provinces which derive from tectonic reactivation and reheating of older materials. This involves lateral variations in the rheological behaviour of the lithosphere. On the basis of seismic structural data and assumptions about the petrological composition and flow parameters of steady-state dislocation creep, strength profiles and lateral viscosity variations have been deduced for several sites. In the northern and northeastern parts of the shield, where the Moho temperature and mantle heat-flow density are typically cratonic, the rheological thickness of the lithosphere, given by the depth at which the strength is reduced to 1 MPa, ranges from 120 to 140 km. In the southwestern shield, were enhanced Moho temperatures and mantle heat-flow density occur, the rheological thickness is reduced to 60-80 km. The depth of the brittle-ductile transition in the upper crust, above which most earthquakes occur, varies on average from 30 km in the northeast to 18 km in the southwest. The limiting temperature of the brittle uppermost layer on average is 365 ± 70°C. The essentially aseismic behaviour of the mantle agrees with the subcrustal predominating ductile deformations predicted by the models. The average lithospheric strength falls within the range 70-200 MPa, typical of the older stable areas. The viscosity of the upper mantle, at a reference depth level of 60 km, ranges from 10 21 to 10 24 Pa s and increases with the geological age, being maximum beneath the Archaean nucleus.
Physics of The Earth and Planetary Interiors, 2001
The regional thermal regime in the Fennoscandian Shield is outlined, and the consequent rheologic... more The regional thermal regime in the Fennoscandian Shield is outlined, and the consequent rheological structure is analysed from the Kola Orogen to the Sorgenfrei–Tornquist zone. Moho temperatures and the heat flux from the mantle are typical of cratonic areas. The deep thermal field shows a cold root in the north-eastern sector. Larger lateral variations of mantle heat flux and Moho temperatures occur in the southern area and at the edges of the shield. The thickness of the thermal lithosphere is maximum (200–220 km), where the mantle heat flux is minimum (about 15–20 mW m−2). Three lithospheric cross-sections illustrate the expected lateral variation in viscosity and failure mode throughout the lithosphere as a consequence of the different geothermal conditions and the rheological stratification. In the upper crust, the depth of the brittle–ductile transition varies on average from 30, in the north-east, to 15 km, in the south-west. In the subcrustal mantle, the models predict a mainly ductile behaviour. At a depth of 60 km, the upper mantle viscosity is maximum (2.5×1023 to 25.0×1023 Pa s) beneath the Archean province and minimum (2.5×1021 to 10.0×1021 Pa s) below the Svecofennian and Sveconorwegian units. The comparison of the rheological calculations with the seismic activity shows a general agreement with the brittle–ductile transition depth expected in the Archaean and Proterozoic provinces. There is a difference both in number of earthquakes and distribution versus depth between areas of different age. The occurrence of larger magnitude seismic events near the base of the seismogenic zone, in the region of inferred peak of shear resistance, suggests some causal relationship. The Proterozoic areas show a more regular decrease of shocks with depth and, compared to the Archean provinces, a tendency to shallowing.
The structural setting beneath the Ligurian Sea resuJts from several tectonic events reflected in... more The 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 is N120°E on average.
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