GPR profiles for characterizing subsidence deformation in lake sediments within a maar crater

Large scale sinking (ca. 15 m) of lake deposits within the Parangueo maar crater (México) has been observed after drying of its perennial lake in the mid 1980's. Deformation is characterized by a main ring-shaped normal fault accompanied by gravitational sliding and gliding of mud blocks, and folding at the foot of the slides. Domes, caused by mud injections triggered by overpressure, are associated with the folds. A GPR survey of the crater included two profiles ~1000 m each that were continuously recorded across the crater. A SIR-20 equipment with 200 MHz antenna was employed for the surveys and processing of GPR data included a detailed topographic correction. Reflectors in the radargrams were correlated with layers observed in small excavations made in the lacustrine sequence and allowed the estimation of a propagation velocity of 0.075 m/ns. Shallow reflectors were recorded interpreted as layers, fractures, faults, or mud domes as observed in unstacked profiles. Likewise, the radar signature of continuous reflectors can be related with the spatial distribution of evaporite concentrations within the lake basin where water content increases. The complete GPR profiles allowed the identification of major deformation structures and give insights on the differences in the structural styles along the ring fault. The GPR results were of great importance for a better understanding of the geometry and distribution of structures near the surface related to land subsidence in fine grained materials.