WHAT SHAPES A RESTRAINING BEND? AN ANALYSIS FROM THE EASTERN CALIFORNIA SHEAR ZONE

Restraining bends have important controls on topography, strike-slip evolution, and earthquake rupture dynamics, however the specific factors governing their geometry, uplift, and development are not well established in real-world systems. These relationships are a challenge to investigate in field examples due to cannibalization and erosion of earlier structures with cumulative strain. To address this, we analyzed 22 basement-cored km-scale restraining bends along seven low net-slip faults (<10 km) within the southern Eastern California shear zone (SECSZ). The low net-slip limits their size and erosion resulting in preservation of form and early transpressional structures. We mapped restraining bend faults and geomorphic features (e.g., exhumed sediments, erosional surfaces) to constrain geometries and uplift, and conducted morphometric analyses to derive empirical relationships. The SECSZ restraining bends are variable in size and fault configuration, but most have focused relief (<100-600 m) between two high-angle bounding faults that merge at shallow depths (<5 km) as positive flower structures. They exhibit self-similar form with a characteristic arrowhead shape in map view and a ‘whaleback’ longitudinal profile (avg. aspect ratio of 2.9:1) with good correlations between basic parameters such as width vs relief. Bend form and fault geometry are size-sensitive, with larger structures having wider bifurcation angles between bounding faults and more symmetric transverse profiles, suggesting predictable growth and self-similarity that reflect driving forces. We also compare observed fault geometries and uplift rate patterns to predictions from numerical 3D deformational modeling using Poly3D. Modeled uplift rates align well with topographic observations, giving confidence in interpreted fault geometries and subsequent strain analyses. We propose a kinematic model for the evolution of these bends where high shear strain at bend corners lead to the initial formation of secondary faults, which then enables more efficient accommodation of uplift of the inner wedge between the bounding faults. Continued topographic buildup of the wedge may work to warp the bounding faults and lead to the growth and migration of uplift with increasing local convergence.