Rosa Tejero

Thermal and mechanical structure of the central Iberian Peninsula lithosphere

Abstract 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 2.5×10 12 to 8×10 12 N m −1 in compression, and from 1.3×10 12 to 1.6×10 12 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.

Heat flows through the ice lithosphere of Europa

Some of the geological features on the surface of Europas ice shell suggest the existence of a brittle lithosphere that in some regions is no more than 2 km thick. The surface heat flow needed to put the brittle-ductile transition in ice at a depth of 2 km is at least ∼100 mW m−2, much higher than predicted by tidal heating models for a purely conductive ice shell. A possible explanation lies in the probable existence of a convective layer beneath the brittle lithosphere, which, owing to tidal dissipation in warm ice, could contribute to higher heat flows. If a convective subsurface layer and grain size dependent flow behavior of ice are admitted, the grain size at brittle-ductile transition depth cannot be >1 mm, which is consistent with the required value for the onset of convection in an ice shell thinner than 20 km.

Heat flow, lenticulae spacing, and possibility of convection in the ice shell of europa

Abstract Two opposing models to explain the geological features observed on Europa’s surface have been proposed. The thin-shell model states that the ice shell is only a few kilometers thick, transfers heat by conduction only, and can become locally thinner until it exposes an underlying ocean on the satellite’s surface. According to the thick-shell model, the ice shell may be several tens of kilometers thick and have a lower convective layer, above which there is a cold stagnant lid that dissipates heat by conduction. Whichever the case, from magnetic data there is strong support for the presence of a layer of salty liquid water under the ice. The present study was performed to examine whether the possibility of convection is theoretically consistent with surface heat flows of ∼100–200 mW m −2 , deduced from a thin brittle lithosphere, and with the typical spacing of 15–23 km proposed for the features usually known as lenticulae. It was obtained that under Europa’s ice shell conditions convection could occur and also account for high heat flows due to tidal heating of the convective (nearly isothermal) interior, but only if the dominant water ice rheology is superplastic flow (with activation energy of 49 kJ mol −1 ; this is the rheology thought dominant in the warm interior of the ice shell). In this case the ice shell would be ∼15–50 km thick. Furthermore, in this scenario explaining the origin of the lenticulae related to convective processes requires ice grain size close to 1 mm and ice thickness around 15–20 km.

Origin and emplacement of the Aguablanca magmatic Ni-Cu-(PGE) sulfide deposit, SW Iberia: A multidisciplinary approach

A model is proposed for the origin and emplacement of the ca. 341 Ma Aguablanca magmatic Ni-Cu-(platinum group element [PGE]) sulfi de deposit (SW Iberia) integrating petrological, geochemical, structural, and geophysical data. The Aguablanca deposit occurs in an unusual geodynamic context for this ore type (an active plate margin) as an exotic , magmatic subvertical breccia located at the northern part of the coeval gabbronorite Aguablanca stock (341 ± 1.5 Ma). Structural and gravity data show that mineralized breccia occurs inside the inferred feeder zone for the stock adjacent to the Cherneca ductile shear zone, a Variscan sinistral transpressional structure. The orientation of the feeder zone corresponds to that expected for tensional fractures formed within the strain fi eld of the adjacent Cherneca ductile shear. Two distinctive stages are established for the origin and emplacement of the deposit: (1) initially, the ore-forming processes are attributed to magma emplacement in the crust, assimilation of crustal S, and segregation and gravitational settling of sulfi de melt (a scenario similar to most plutonic Ni-Cu sulfi de ores), and (2) fi nal emplacement of the Ni-Cu sulfi de-bearing rocks by multiple melt injections controlled by successive opening events of tensional fractures related to the Cherneca ductile shear zone.

Crustal structure from gravity signatures in the Iberian Peninsula

Through two-dimensional fi ltering and spectral analysis of gravity data, we infer the density structure of the Iberia Peninsula’s lithosphere (western Europe). The gravity anomaly map of the Iberian Peninsula displays long-wavelength anomaly minima related to Alpine ranges. These anomalies are primarily linked to a greater crustal thickness. Low-pass filtering of the anomaly map using a cutoff wavelength of 150 km was adequate for effective separation of shallow and deep sources used for computing the three-dimensional (3-D) Moho interface. Tsuboi’s technique (identical to the equivalent stratum theorem) is used here to map the 3-D Moho interface by selecting mean source depths from the results of the spectral analysis, and assuming a homogeneous density contrast of 350 kg/m3. The most characteristic feature of the 3-D Moho geometry was the presence of several lows associated with mountain ranges created by Alpine tectonics. In those areas, the gravity-derived Moho reaches a depth of up to 45 km in the Pyrenees and Cantabrian Mountains and close to 40 km under the betics, Central System, and Iberian Chain. Under the Iberian Massif, the western part of the Iberian Peninsula composed of a Variscan basement, the Moho was located at a depth of 30–36 km. These results are consistent with existing seismic data and indicate that gravity-based techniques can provide very good estimates of lithosphere structure.

Magma emplacement in transpression: The Santa Olalla Igneous Complex (Ossa-Morena Zone, SW Iberia)

The Santa Olalla Igneous Complex, a late-Variscan group of intrusions located in the Ossa-Morena Zone (SW Iberia), has been the focus of a gravity and structural study. The structure outlined by the foliation map is complex, showing two different structural domains: one characterized by vertical, and the other by horizontal, magmatic foliations. The vertical fabrics are restricted to the NE half of the complex, which is in direct contact with a Variscan sinistral strike-slip fault (Cherneca fault) whereas the horizontal fabrics are developed in the SW half of the complex, which is characterized by a horizontal tabular geometry. Gravity modeling indicates that the deeper floor of the plutons is closely related to the NE margin and the Cherneca fault. An emplacement and structural evolution model for this igneous complex is proposed following these structural and gravity results: (1) magma ascent was favored by releasing bends in the trace of the Cherneca fault; (2) when magma reached the present level it intruded to the SW with a horizontal sheet geometry generating the subhorizontal foliation domain; (3) after emplacement, the NE half of the complex suffered the external tectonic stress field provoked by sinistral motion along the Cherneca fault, subsequently generating the subvertical magmatic foliation domain.

3D gravity modelling of the Aguablanca Stock, tectonic control and emplacement of a Variscan gabbronorite bearing a Ni–Cu–PGE ore, SW Iberia

The Aguablanca stock is a Variscan mafic pluton located in the Ossa-Morena zone, southern Iberian Massif, hosting an unusual Ni–Cu–PGE mineralization associated with magmatic breccia pipes which intruded its northern part. The emplacement of the Aguablanca stock and the mineralized breccia pipes are related to the activity of the Cherneca ductile shear zone, a Variscan sinistral shear zone that favoured magma ascent through the upper crust. A detailed gravity study has been carried out in order to investigate the 3D geometry of the Aguablanca intrusion and to get insights about the emplacement mechanism and tectonic controls of the mineralization. The three-dimensional gravity modelling shows that the stock has an inverted drop geometry with a feeder zone in contact with the Cherneca ductile shear zone. The inferred orientation of the feeder zone suggests that the emplacement probably took place along an open tensional crack formed within the strain field of the adjacent Cherneca ductile shear zone. The modelling of the breccia pipes hosting the Ni–Cu–PGE ore shows that they are included inside the feeder zone, thus their emplacement is probably controlled by successive opening events of this tensional crack.

ESTIMATION OF HEAT SOURCES IN PLANETARY CRUSTS FROM ISOTHERM DEPTH, SURFACE HEAT FLOW, AND CRUSTAL THICKNESS

The depth to an isotherm provides clues on the intensity of heat sources within a planetary crust. In this work, we show that the depth to an isotherm and crustal thickness can be jointly used to give an approximate estimation of the fractionof the surface heat flow that is being originated from the crust of a terrestrial planet. Relationships between crustal heat generation rate and crustalthickness, and surface and mantle heat flow variations on a planet were also explored. The proposed methodology may serve to improve present descriptions of the crustaltemperature-depth profiles of terrestrial planets, and may also provide information on chemical and thermal evolution.