Assessing the brittle crust thickness from strike-slip fault segments on Earth, Mars and Icy Moons

TL;DR

This study links the geometry of Riedel shear structures to brittle crust thickness across planetary bodies, using models and experiments to estimate crustal properties on Earth, Mars, and icy moons.

Contribution

It introduces a new relationship between Riedel shear spacing and brittle layer thickness, applicable across different planetary environments.

Findings

Maximum Riedel spacing is about three times the brittle layer thickness.

The relationship aligns with seismogenic depths on Earth.

Estimated brittle thicknesses for icy moons suggest brittle behavior in their ice shells.

Abstract

Segment lengths along major strike-slip faults exhibit a size dependency related to the brittle crust thickness. These segments result in the formation of the localized 'P-shear' deformation crossing and connecting the initial Riedels structures (i.e. en-echelon fault structures) which formed during the genesis stage of the fault zone. Mechanical models show that at all scales, the geometrical characteristics of the Riedels exhibit dependency on the thickness of the brittle layer. Combining the results of our mechanical discrete element model with several analogue experiments using sand, clay and gypsum, we have formulated a relationship between the orientation and spacing of Riedels and the thickness of the brittle layer. From this relationship, we derive that for a pure strike-slip mode, the maximum spacing between the Riedels are close to three times the thickness. For a transtensional mode, as the extensive component becomes predominant, the spacing distance at the surface become much smaller than the thickness. Applying this relationship to several well-characterized strike-slip faults on Earth, we show that the predicted brittle thickness is consistent with the seismogenic depth. Supposing the ubiquity of this phenomenon, we extent this relationship to characterize en-echelon structures observed on Mars, in the Memnonia region located West of Tharsis. Assuming that the outer ice shells of Ganymede, Enceladus and Europa, exhibit a brittle behavior, we suggest values of the corresponding apparent brittle thicknesses.