Early evolution of faults in crystalline basement by organized fracturing and damage zonation

Basement faults evolve from multiscale distributed damage to localized shear, yet there remains a limited understanding of key controls on their internal structure leading to shear localization. We investigate the Off-Road Fault Zone (ORFZ), a ∼260-m wide immature fault exposed in the texturally homogenous granitic basement of southern Oklahoma. We utilize multiscale remote sensing and field mapping to characterize the multiple fault-fracture systems in the fault zone, and 2-D electrical resistivity imaging to investigate its down-dip structure. It was found that ORFZ hosts steep NW-dipping, NE-striking fracture clusters with predominantly tensile fractography, en-echelon segmentation, horizontally striated slickensided surfaces, hematite- and epidote veins, and distributed gouge-lenses. Scan-line fracture mapping revealed systematic fracture intensity zonation: a. >10 m⁻¹ intensities, interpreted as the core-cluster (CC); b. 1 – 9.99 m^-1, and 0.1 – 0.99 m^-1 intensities, regarded as the inner cluster and outer cluster of the damage zone; and c. <0.1 m^-1 intensities represent the background, country rocks. The CCs occur across the fault zone but are most predominant and densely packed in the southeastern margin. Also, the gouge lenses and slickensided fracture surfaces are localized in the CCs, indicating increasing shear deformation concurrently with increasing fracture saturation. The resistivity tomography image shows the principal slip zone as a narrow ∼70°NW-dipping conductor located at the southeastern margin hosting the widest CC and thick gouge lenses. The mapped deformation field presents a hanging wall-directed damage asymmetry and margin-confinement of highest strain zone, exemplifying dip-controlled early evolution of strike-slip faults in the crystalline crust.