Joan Martí’s scientific contributions

The so-called Henkel's plot (1976) allows relating density and magnetic susceptibility of rocks. In this work we focused on three paramagnetic Pyrenean granites from the Mont Louis-Andorra, Maladeta and Marimanha granitic plutons. In total, 128 sites with 310 density measurements and > 2600 susceptibility readings cover the main range of variability of magnetic susceptibility and density of these bodies. Linear fittings were estimated for every granitic body as well as a global estimation for the entire dataset: r (kg/m 3) = 2566 (kg/m 3) + 0.541 k (10-6 S.I.) (R:0.97). This fitting is only valid in the paramagnetic domain, where iron is mostly fractioned in iron-bearing phyllosilicates and the occurrence of magnetite is negligible (or at least its contribution to the bulk susceptibility). This relationship, likely different in other bodies, allows transforming magnetic susceptibility data into density data (or the other way around) helping to constraint gravity and magnetic modelling when density data or magnetic susceptibility are scarce. Given the large amount of AMS studies worldwide, together with the quickness and cost-effectiveness of susceptibility measurements with portable devices, this methodology allows densifying and homogenizing the petrophysical data when modelling granite rock volumes based on both magnetic and gravimetric signal.

The acquisition and interpretation of gravity and magnetic data represents a cost-effective tool in geophysics since it allows to determine the geometry and distribution of the density and magnetic properties at depth of the subsurface rocks. The study area, where gravity and magnetic data have been interpreted, is the La Cerdanya basin (Eastern Pyrenees), a Neogene ENE-WSW oriented half graben located in the Axial Zone, the central part of the Pyrenees mainly formed by Paleozoic rocks. It is situated in the NW block of the La Tet fault and its Neogene sediments lie unconformably on top of the Paleozoic basement. Its dimensions are approximately 30 km long and 7 km wide. The tectonic evolution and geometry of the La Cerdanya basin is not well known and this work aims to add new constraints to help solving the Neogene tectonic evolution of the Eastern Pyrenees and to improve the knowledge of its 3D geometry. The magnetic anomaly map of the study area, based on airborne magnetic data, shows very little contrasts of the magnetic properties between the Neogene rocks of the La Cerdanya basin and the Paleozoic rocks surrounding it. Gravity data consist of previous and new acquired gravimetric stations and the residual Bouguer anomaly map shows density contrasts big enough to model the geometry of the basin and the neighbor intrusive bodies. They have been incorporated into a 3D geological model based on available geological and petrophysical data using the 3D GeoModeller software. The 3D potential fields model has been made taking into account the three most representative units outcropping in the study area: the Neogene rocks, the Late Carboniferous intrusive bodies and the Paleozoic basement. The resulting potential fields response of the model is consistent with the observed data. The 3D model shows a basin slightly deeper than shown in previous works and has helped to better define the 3D geometry of the basin and the along-strike geometry of the La Tet fault. Powered by TCPDF (www.tcpdf.org)

In this work we establish reliable correlations between density and magnetic susceptibility in three paramagnetic granites from the Pyrenees. In total, 128 sites (310 density measurements and >2600 susceptibility ones) were studied in the Mont Louis-Andorra, Maladeta and Marimanha granitic plutons covering the main range of variability of magnetic susceptibility. Regressions were calculated for every granitic body and an integrated linear function was obtained for the entire dataset: ρ (kg/m³) = 2566 (kg/m³) + 0.541κ (10⁻⁶ S.I.) (R:0.97). This relationship is only valid in the paramagnetic domain, where iron is mostly fractioned in iron-bearing phyllosilicates and the occurrence of magnetite is negligible (or at least its contribution to the bulk susceptibility). This relationship, likely different in other bodies, allows for transforming magnetic susceptibility data into density data, helping to constrain gravity modelling when density data from rock samples are scarce. Given the large amount of AMS studies worldwide, together with the quickness and cost-effectiveness of susceptibility measurements with portable devices, this methodology allows for densifying and homogenizing the petrophysical data when modelling granite rock volumes based on both magnetic and gravimetric signals.

In this work, we present new Bouguer and residual Bouguer anomaly maps of the Central Pyrenees calculated from 3590 stations, of which 1141 are new observations acquired from surveys performed between 2018 and 2019. The most prominent feature of the Bouguer anomaly is the long wavelength elongated minimum in its central part that continues to the W and ends towards the E with a positive gradient that seems to envelope the minimum. Other short and medium wavelength minima are superimposed, some placed over the batholithic outcrops. In the residual Bouguer, the main relative minima are related with outcrops of batholiths or interpreted buried granites except for the prominent minimum South of La Maladeta Granite, associated with Triassic evaporitic accumulations. These maps (shown on the Main Map) will help characterizing the Permo-Carboniferous batholiths of the Central Pyrenees, in particular La Maladeta and Andorra Mont-Louis granites in order to add constraints to the mechanism of their emplacement.

The GeoPiri3D project aims to characterize the geometry and kinematics of the Permo-Carboniferous batholiths of the Central Pyrenees through a combination of potential fields and structural data. During 2018 we have acquired 824 new gravity stations and 186 rock samples. The gravity data were processed and integrated into the IGME database and a new Bouguer anomaly was calculated. The two main granite bodies (La Maladeta and Andorra-Mount Louis) provide different gravimetric responses. The anomaly on the La Maladeta granite outcrop presents a zonation with small variations in its amplitude. The petrophysical data provide an average density of ca. 2.6 g/cm3, which is consistent with its granodioritic composition. The outcrop of the Andorra-Mount Louis pluton is characterized by a relative minimum, suggesting a dominant granitic composition. In addition, the minimum extends in an oblique NE direction indicating that part of the pluton is buried. The relative minimum to the SW of La Maladeta is probably linked to accumulations of Triassic evaporites.

Geophysical surveying (both gravity and magnetic) is of great help in 3D modeling of granitic bodies at depth. As in any potential-field geophysics study, petrophysical data (density [r], magnetic susceptibility [k] and remanence) are of key importance to reduce the uncertainty during the modeling of rock volumes. Several works have already demonstrated that ∂ 18 O or [SiO 2 ] display a negative correlation to density and to magnetic susceptibility. These relationships are particularly stable (and linear) in the so-called "non-magnetic" granites (susceptibilities falling within the paramagnetic range; between 0 and 500 10-6 S.I.) and usually coincident with calc-alcaline (CA) compositions (very common in Variscan domains). In this work we establish robust correlations between density and magnetic susceptibility at different scales in CA granites from the Pyrenees. Other plutons from Iberia were also considered (Veiga, Monesterio). The main goal is to use the available and densely sampled nets of anisotropy of magnetic susceptibility (AMS) data, performed during the 90's and early 2000's, together with new data acquired in the last few years, as an indirect measurement of density in order to carry out the 3D modelling of the gravimetric signal. We sampled some sections covering the main range of variability of magnetic susceptibility in the Mont Louis-Andorra, Maladeta and Marimanha granite bodies (Pyrenees), all three characterized by even and dense nets of AMS sites (more than 550 sites and 2500 AMS measurements). We performed new density and susceptibility measurements along two main cross-sections (Maladeta and Mont Louis-Andorra).