FAULT-FRACTURE NETWORKS AT THE BASEMENT-SEDIMENTARY INTERFACE, OKLAHOMA: IMPLICATIONS FOR INDUCED SEISMICITY

Deformation in brittle, crystalline basement is manifested by multiscale fracturing and faulting that may control the fluid circulation and associated earthquake patterns at depth. We analyze fault-zones and fault-fracture networks in a 2 km² area of granite outcrops in southern Oklahoma. This granite is of similar age and tectonothermal history to the buried basement in Oklahoma hosting recent widespread induced seismicity. We combine multiscale (outcrop, drone, & satellite) characterization of the geometries, spacing, intensity, and cross-cutting relationships of the fracture/fault systems. Our analyses revealed the following features: (1) sub-vertical fracture sets with en-echelon segmentation and three general trends: a dominant one, NE-SW (245°±0.5), a secondary one, NW-SE (318°±1.7), and minor one, ~N-S (189°±1.5); (2) a ~260 m wide, NE-trending, strike-slip fault zone (Mill Creek Fault Zone, MCFZ), characterized by parallel fractures trending ~245° that commonly show iron-stains, epidote-fill, and rare gouge-filled shear fractures. These shear fractures occur within zones of tightly-clustered tensile fractures that are separated by damage zones; (3) prominent fracture spacing & intensity across the MCFZ, as determined by 1-D discrete Fourier transform, which show groupings of <0.1 m, 0.1 – 0.99 m, 1 – 9.99 m, and >10 m. These four groups of the fault-zone are interpreted as representing Near Slip Surface (NSS), Intra-Slip Zone (ISZ), Intra-Damage Zone (IDZ), and background damage compartments, respectively. We interpret that the damage within the MCFZ is the shallow extension of an immature, buried strike-slip fault. In fault-zones with similar fracture patterns, fault-parallel fluid migration pathways (primarily confined to the NSS and ISZ) dominate over fault-oblique flow pathways due a lack of widespread cross-fault fracturing. Comparisons between the MCFZ and recently injection-activated faults in Oklahoma suggest that the analyzed fracture patterns could serve as the fluid migration pathways at the basement-sedimentary interface associated with the triggered induced seismicity.