Two- and three-dimensional resistivity models of the RMT data of the DeepEarthShape project (Chile) imaging the Critical Zone

In 2019, as part of the interdisciplinary DFG priority program SPP1803 „EarthShape - Earth Surface Shaping by Biota“, the DeepEarthShape project was launched. The main goal of this German-Chilean research initiative was to gain a broader understanding of the interaction between geological, geochemical and biological processes controlling the weathering in the first tens to hundred metres of the subsurface. The elongated Chilean Coastal Range was selected as the ideal study area to investigate the effects of vegetation, precipitation and erosion on the transformation of intact bedrock into regolith within the so-called critical zone (CZ). This area encompasses several climate zones, from dry to humid, within a similar geological complex. We have carried out a radio-magnetotelluric (RMT) survey using a horizontal magnetic dipole (HMD) transmitter to image the electrical resistivity distribution, the lateral extent of the near-surface layers and the CZ at two sites of the DeepEarthShape project - Santa Gracia and Nahuelbuta. We inverted the CSRMT data for selected profiles using the finite element code GoFEM and the finite difference ModEM algorithm to obtain the first resistivity models of both areas. The models show boundaries between the conductive weathering front of the soil layer at the surface, the regolith layer and the intact bedrock down to a depth of 100 m. The depth of the CZ is consistent with borehole logs and drill cores from shallow boreholes, as well as the results of a seismic study. We interpret these near-surface conductivity structures, in particular the conductive channels as pathways for fluids, which might exhibit some correlation of topography and meteoric waters. In particular, the results from Nahuelbuta illustrate that geophysical imaging prior to planning and implementation is essential and protects against misinterpretation. A comparison of the two locations with different climate and precipitation rates based on our conductivity models supports the assumption that the extent of the CZ is related to both parameters.