Segment linkage process at the origin of slip surface roughness: Evidence from the Dixie Valley fault

The dynamics and geometry of slip along a fault govern the distribution of heterogeneities and, inturn, these heterogeneities organize slip. Some of these heterogeneities are morphological and are fossilized in the topography of the fault plane, i.e., its roughness. In the present study, our goal is to gain a better understanding of the processes involved during the creation and destruction of fault roughness. Our analysis is focused on the Dixie Valley (Nevada) normal fault, which has a particular outcrop that offers the opportunity to characterize the relationship between fault surface geometry and fault zone internal architecture. The fault morphology was measured in the field using a Light Detection And Ranging (LiDAR) apparatus. The data indicate that the fault surface topography is self-affine and characterized by two power law exponents, one parallel ( H / / = 0.6 ) and one perpendicular ( H ⊥ = 0.8 ) to the slip direction. Accordingly, self-affinity implies that the fault surface appears smoother as the spatial scale increases. More precisely, this self-affine property indicates that the standard deviation of the roughness amplitude scales as L 0.6 where L is the distance along the fault in the slip direction. We propose that fault roughness generation is dominated by damage processes that leave an imprint on fault geometry in the form of elongated lenses. Indeed, the fault zone displays a network of anastomosed discrete slip surfaces that bound bumpy lenses of damaged rock elongated in the direction of slip and that give the fault a rough topography. Such lenses are also observed in many other faults worldwide. Symmetry axis measurements of the lenses found in the Dixie Valley fault reveal that their geometry is scale-dependent. The maximum thickness T of the lenses scales with their length l, measured in the slip direction, as T ∝ l 0.6 . Based on previous experimental studies on the fault growth process, we suggest that the multi-scale aggregation of lenses explains why the standard deviation of the roughness amplitude of the fault surface scales as L 0.6 . We propose that elastic interactions related to the linkage of many discrete slip surfaces, controlling the generation of multi-scale bumpy lenses, are one of the physical processes at the origin of fault roughness.