Joint Audio-Magnetotelluric and Passive Seismic Imaging of the Cerdanya Basin

The structure of Cerdanya Basin (north-east of Iberian Peninsula) is partly known from geological cross sections, geological maps and vintage geophysical data. However, these data do not have the necessary resolution to characterize some parts of Cerdanya Basin such as the thickness of soft soil, geometry of bedrock or geometry of geological units and associated faults. For all these reasons, the main objective of this work is to improve this deficiency carrying out a detailed study in this Neogene basin applying jointly the combination of passive seismic methods (H/V spectral ratio and seismic array) and electromagnetic methods (audio-magnetotelluric and magnetotelluric method). The passive seismic techniques provide valuable information of geometry of basement along the profile. The maximum depth is located near Alp village with a bedrock depth of 500 m. The bedrock is located in surface at both sites of profile. The Neogene sediments present a shear-wave velocity between 400 and 1000 m/s, and the bedrock basement presents a shear-wave velocity values between 1700 and 2200 m/s. These results are used as a priori information to create a 2D resistivity initial model which constraints the inversion process of electromagnetic data. We have obtained a 2D resistivity model which is characterized by (1) a heterogeneous conductivity zone (<40 Ohm m) that corresponds to shallow part of the model up to 500 m depth in the centre of the profile. These values have been associated with Quaternary and Neogene sediments formed by silts, clays, conglomerates, sandstones and gravels, and (2) a deeper resistive zone (1000–3000 Ohm m) interpreted as Palaeozoic basement (sandstones, limestones and slates at NW and conglomerates and microconglomerates at SE). The resistive zone is truncated by a discontinuity at the south-east of the profile which is interpreted as the Alp-La Tet Fault. This discontinuity is represented by a more conductive zone (600 Ohm m approx.) and is explained as a combination of fractured rock and a fluid network. The result highlights that the support between different geophysical methods is essential in producing geophysical meaningful models.     

Mapping and compositional analysis of mare basalts in the Aristarchus region of the Moon using Clementine data

The process of accurately defining and outlining mare basalt units is necessary for constraining the stratigraphy and ages of basalt units,which are used to determine the duration and the flux of lunar volcanism.We use a combination of Clementine’s five-band ultraviolet/visible data and TiO2and FeO abundance distribution maps to define homogenous mare basalt units and characterize their compositional variations(with maturity)in the Aristarchus region.With 20 groups of distinct mare basaltic soils identified using the method in this paper,six additional spectrally defined areas and five basaltic units are constructed,and their mineralogic quantization values provide new constraints on their temporal and spatial evolution.Our results indicate that the Aristarchus region has diverse basalt units and a complex history of volcanic evolution.We also demonstrate that the techniques,from which spectrally distinct mare basalts can be mapped,performed well in this study and can be confidently expanded to other mare regions of the Moon.