INTERACTIONS BETWEEN GROWING EXTENSIONAL FAULTS AND PRE-EXISTING DISCONTINUITIES: IMPLICATIONS ON SURFACE EFFECTS AND SEISMIC HAZARD STUDIES

Fault propagation depends on the applied stress and on the mechanical characteristic of the host rocks. In an isotropic system, a fault develops with substantial constant rate and dip. Natural systems, differently, are far from being uniform and anisotropies affects the development of faults. Rocks variability, fluids, and pre-existing faults are examples of anisotropies that may accelerate, stop or decelerate the propagation of the faults from depth up to the surface.

Active tectonic studies on extensional systems relay on mapping of their surface traces. In many active areas, however, the extension is a young process and the related faults may be still confined at depth or poorly expressed at the surface.

A commonly used tool in earthquake geology is to infer the seismic potential of a fault from its surface displacement and from empirical relationships relating magnitude with fault length. This approach cannot be applied when active faults are blind. During the blind phase, the fault propagation is hard to be detected and also the role of pre-existing faults on the upward propagation of extensional faults cannot be observed. To understand these topics, we simulated a blind extensional fault interacting with differently oriented mechanical discontinuities in analogue models. This tool can effectively describe blind faults evolution, their surface effects, and their interactions with pre-existing discontinuities.

We simulated seven different settings where pre-existing discontinuities are placed above the upper tip of a 45° dipping master fault. We quantified the strain distribution within the models and we reconstructed the evolution of the fault-related folds at significant steps.

The results indicate that during the evolution of a growing extensional fault, pre-existing discontinuities affect the ability of new faults to reach the surface, the development of secondary brittle structures related to fault-propagation folding, and the shape of the related basins together with the migration through time of the folds axes. All these elements are differently affected depending on the geometrical relationships between the main fault and the inherited discontinuities. These results can provide clues for interpreting surface evidence while analyzing areas where active faults are blind or hidden.