Insights into the Paleostress Analysis of Heterogeneous Fault-Slip Data by Comparing Different Methodologies: The Case of the Voltri Massif in the Ligurian Alps (NW Italy)

Abstract

One of the most critical stages in fault-slip data stress analysis is separating the fault data into homogeneous subsets and selecting a suitable analysis method for each subset. A basic assumption in stress tensor computations is that fault activations occur simultaneously under a homogeneous stress regime. With that rationale, this work aims to attain improvements in the paleostress reconstruction from the polyphase deformed region of Voltri Massif in the Ligurian Alps by using already published heterogeneous fault-slip data inverted using best-fit stress inversion methods and in the absence of any tectonostratigraphic and overprinting criteria. The fault-slip data are re-examined and analyzed with a best-fit stress inversion method and the Tensor Ratio Method (TRM) in the absence of any tectonostratigraphic and overprinting criteria. This analysis defines crucial differences in the paleostress history of the Voltri Massif in the Ligurian Alps, and gives insight into the analysis and results of different stress inversion methodologies. Best-fit site stress tensors have substantial diversity in stress orientations and ratios, implying possible stress perturbations in the region. The reason for these diversities is that the Misfit Angle (MA) minimization criterion taken into account in the best-fit stress inversion methods allows for acceptable fault-slip data combinations, which under the additional geological compatibility criteria used by the TRM, are found to be incompatible. The TRM application on this already published and analyzed data defines similar site and bulk stress tensors with fewer diversities in stress orientations and ratios defined from fault-slip data whose orientations always satisfy the same additional geological compatibility criteria induced by the TRM, and not only from the MA minimization criterion. Thus, TRM seems to define stress tensors that are not as sensitive to the input of fault-slip data, compared to the best-fit stress tensors that appear to suffer from the ‘overfitting’ modeling error. Five distinct TRM bulk paleostress tensors provide a more constrained paleostress history for the Voltri Massif and the Ligurian Alps, which after the restoration of the ~50° CCW rotation, comprise: (a) a transpression–strike-slip stress regime (T1) with NNE-SSW contraction in Late Eocene, (b) an Oligocene NW-SE extensional regime (T2), which fits with the NW-SE extension documented for the broader area north of Corsica due to a significant change in subduction dynamics, (c) a transient, local, or ephemeral NE-SW transtension (T3) which might be considered a local mutual permutation of the T2 stresses, and (d) a Miocene transpression with a contraction that progressively shifted from ENE-WSW (T4) to NNE-SSW (T5), reflecting the stress reorganization in the Ligurian Alps due to a decrease in the retreating rate of the northern Apennines slab. Therefore, paleostress reconstruction can be fairly described by enhanced Andersonian bulk stress tensors, and requires additional geological compatibility criteria than the criteria and sophisticated tools used by the best-fit stress inversion methods for separating the fault-slip data to different faulting events.

Keywords

• fault-slip data
• stress inversion
• Tensor Ratio Method (TRM)
• ‘enhanced’ Andersonian tensors
• Ligurian Alps
• Northwest Italy