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

Frédéric‐Victor Donzé, Yann Klinger+4 more|arXiv (Cornell University)|Oct 14, 2020

earthquake and tectonic studies64 references17 citations

TL;DR

This study develops a mechanical model using discrete element simulations to link the spacing and twist angle of en-echelon Riedel shear structures to the thickness of the brittle crust. It derives a predictive relationship (Equation 8) showing that Riedel spacing is approximately three times the brittle layer thickness under pure strike-slip conditions, enabling estimation of crustal thickness on Earth, Mars, and icy moons from observed fault patterns.

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.

Motivation & Objective

• To establish a quantitative relationship between en-echelon Riedel structure geometry and brittle crust thickness.
• To validate this relationship using discrete element modeling and analog experiments with sand, clay, and gypsum.
• To apply the derived relationship to estimate brittle layer thickness on tectonically active bodies including Earth, Mars, and icy moons.
• To assess the consistency of predicted brittle thickness with seismogenic depth on terrestrial strike-slip faults.
• To explore the applicability of the model to extraterrestrial bodies with limited geophysical data.

Proposed method

• Used a bonded particle discrete element model (YADE DEM) to simulate brittle faulting in cohesive-frictional materials.
• Simulated progressive fault development under varying deformation modes (pure strike-slip to transtensional).
• Tracked the evolution of Riedel shear structures, measuring their spacing (S) and twist angle (ω) relative to the brittle layer thickness (h).
• Performed nonlinear least-squares fitting to derive Equation 8: S/h = f(ω), relating spacing ratio to twist angle.
• Validated model results against published analog experiments (sand, clay, walnut, gypsum) and field observations.
• Applied Equation 8 to estimate brittle crust thickness from observed Riedel structures on Earth, Mars, and icy moons.

Experimental results

Research questions

• RQ1 How does the spacing and twist angle of en-echelon Riedel structures depend on the thickness of the brittle crust?
• RQ2 Can a consistent mechanical relationship be derived between Riedel geometry and brittle layer thickness across different materials and deformation modes?
• RQ3 Does the predicted brittle thickness from Riedel geometry align with seismogenic depth on major terrestrial strike-slip faults?
• RQ4 Can this relationship be reliably extended to estimate brittle layer thickness on Mars and icy moons where direct measurements are unavailable?
• RQ5 What are the implications of the derived thickness estimates for the mechanical behavior and evolution of planetary crusts?

Key findings

• For pure strike-slip conditions, the maximum spacing between Riedel structures is approximately three times the brittle layer thickness.
• In transtensional regimes, the surface spacing between Riedels becomes significantly smaller than the brittle layer thickness due to increased extensional strain.
• The model-derived Equation 8 successfully predicts brittle crust thickness on Earth, with estimates for the Greendale Fault aligning with seismogenic depth.
• Application to Mars’ Memnonia region yields a predicted brittle layer thickness of 2.1–2.8 km for en-echelon structures with 22° twist and 3–4 km spacing.
• For Ganymede, thickness estimates range from 1.8–2.2 km in Anshar Sulcus and 2.1–2.8 km in Transitional Terrain, consistent with tidal stress models.
• On Europa, predicted ice shell thicknesses range from 2.2–3.6 km, overlapping with values from mechanical models but challenging paleo-structure or thickness variability interpretations.