Fracture clustering in the brittle crystalline crust: Manifestations of incipient faulting

Folarin Kolawole, Columbia University
Ze'ev Reches, University of Oklahoma
Brett Carpenter, University of Oklahoma
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Abstract

Basement faults evolve from distributed damage to localized shear, yet there remains a limited understanding of key controls on their internal structure leading to shear localization. Here, we analyze the internal structure of the Off-Road Fault Zone (ORFZ), a strike-slip fault in the exposed granitic basement of southern Oklahoma. The fault displays a ~260 m-wide zone that is dominated by multiple fracture systems which were analyzed by utilizing satellite, high-resolution drone images, field observations, and a 2-D shallow electrical resistivity imaging. The ORFZ hosts sub-vertical NE-striking fracture clusters with predominantly tensile fractography, en-echelon segmentation, few horizontally striated slickensided surfaces, hematite-veins, epidote veins, and distributed gouge-lenses. Scan-line fracture mapping revealed three fracture intensity zones: Core-clusters with intensity of >10 fractures/m, inner and outer clusters with intensities of 0.1– 10 fractures/m, and, country rocks with background lower intensity. The gouge lenses and slickensided fractures are restricted to the core zone indicating increasing shear deformation concurrent with increasing fracture saturation. The mapped deformation, which is a wide zone dominated by tensile fractures and minor shear, is interpreted as the early evolution of a major strike-slip fault in homogenous crystalline basement. Subsurface electrical resistivity tomography across the fault reveals its NW-dip and principal focusing of damage coalescence at the fault margin. Altogether, the observations suggest 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.

Session

Deformation in the upper crust