Erosion by tectonic carving in the Concordia Subglacial Fault Zone, East Antarctica

In this work we present the analysis of the footwall morphology of the Concordia subglacial extensional fault in the East Antarctic Craton. The Concordia Fault is a regional fault zone that extends for almost 200 km. The displacement, up to 1800 m, and the listric geometry were recognized by numerical modeling of the resulting hangingwall bedrock morphology and is responsible for the marked asymmetry that characterizes the corresponding scarp in the Concordia Subglacial Trench. The portion of the footwall in the proximity of the master fault exhibits an excavated morphology, about 500 m deep and up to 5 km wide, showing strong correlation with the master fault displacement. We excluded a predominant glacial and fluvial origin of this morphology considering: (i) the sharp topography of the Concordia Fault, suggesting that the fault activity started after the onset of the ice sheet; (ii) the ice-sheet/bedrock contact is characterized by a general negligible erosion/deposition rates still allowing clast removal; (iii) the lack of significant deposits in the Concordia Trench. We hence explored the possibility that this morphology may result from the combined action of fault-induced fracturing and passive clast removal and scattering by flow and plastic deformation within the ice sheet. We introduced the term tectonic carving for this process. Our modeling shows that tectonic carving relates to the relative fracture intensity in the Concordia fracture zone, that corresponds to the envelope of master and secondary fault damage zones. Fracture intensity depends on the frequency and the displacement of secondary faulting and can be approximated by a normal distribution. Using a Monte Carlo modeling approach we selected the set of parameters that best fits the data set with the carving theoretical curve. The final results of the Monte Carlo analysis show a root mean square of about 50 meters, comparable with the data resolution. This analysis demonstrates a method to unravel the presence of fracture zones in similar, weak erosional environments.