Testing the hypogenic hypothesis for the origin of granite multi-concave topography using electrical resistivity tomography: A case study from the Southern Albera Massif (Pyrenees, Spain)

The pediment developed on the unweathered granite rocks of the South Albera Massif exhibits the characteristic morphology of an exhumed weathering front, where prominent residual landforms (tors and granite boulders) coexist with >240 weathering basins that shape a typical multi-concave topography. Active hydrothermalism is observed in the area, supporting the hypothesis that such weathering depressions may also be hypogenic in origin. Here we use Electrical Resistivity Tomography (ERT) to investigate the geo-electrical structure of the subsurface beneath four temporary ponds and associated depressions. The resistivity images reveal sub-vertical zones with resistivity low grades (150 and 450 Ohm·m) consistent with weathered granite, which spatially correlate with the shear zones identified at the surface. These weathered zones reach widths of approximately 100 m and extend vertically to depths beyond the effective ERT investigation depth (>170 m). The potential hypogenic origin of weathering depressions is supported by the following observations: (1) the occurrence of upwelling hydrothermal springs within one of the areas; (2) the widening of some weathered zones with depth; and (3) the preservation of unaltered rock masses overlaying altered zones. Spatial relationships between the studied depressions and the sub-vertical weathering structures are intricate. While some depressions entirely or partially overlap hypogenic alteration zones, in other cases, they fall outside the limits of these structures. This feature highlights the complexity of the weathering front evolution, where epigenic and hypogenic alteration processes coexist, driven by both structural factors and convergence of fluid flows of different nature.