Quantifying the efficiency of river erosion in up-warped fault blocks and its tectonic implications

Abstract:

Models that couple tectonics and surface processes commonly predict that efficient erosion and sedimentation help focus crustal deformation onto fewer, longer-lived faults. However, because their surface evolution parameters are difficult to calibrate against real landscapes, the sensitivity of tectonic deformation to a realistic range of surface process efficiency remains poorly known. The challenge lies in both defining and measuring erosional efficiency in a manner that applies to a wide range of settings and allows a straightforward parameterization in numerical models.

Here we focus on quantifying river incision acting on structurally simple half-graben and horst structures in continental rift zones. Numerical simulations predict that infinitely-efficient erosion and deposition can more than double the maximum offset that can be accommodated on a normal fault before crustal strain localizes elsewhere. Specifically, leveling footwall relief promotes the migration of strain towards the hanging wall to form new grabens instead of horsts. To test whether the efficiency of river incision can vary sufficiently across real rifts to exert a control on tectonic styles, we analyze the profiles of rivers draining half-graben footwalls and horst blocks in the Basin & Range, Taupo, Rio Grande, and East African rifts. We adapt the standard methodology of χ-analysis to account for spatial variations in uplift expected from crustal flexure in a fault-bounded block. Erosional efficiency (EE) is defined as the inverse of the (dimensionless) slope of uplift-corrected χ-plots for a reference drainage area of 10⁶ m², and stream power incision exponents m and n equal to 0.5 and 1, respectively.

Measured EEs range between ~0.5 and ~10, reflecting natural variability in lithology, climate, and uplift rates across sites. At certain locations, differences in EE recorded by rivers flowing towards and away from the master fault can be clearly attributed to lithological contrasts. We also find that horsts tend to be associated with lower EEs compared to half-grabens. The range of EE that we document in real landscapes is sufficient to induce measurable changes in the tectonic makeup of simulated rifts. The most notable effect is the transition from half-graben to horst-like structures in systems subjected to a low EE (~1 and less).