FRACTURE PATTERNS OF SOUTH-CENTRAL KANSAS AND RELATIONS TO CONTRACTIONAL[?] STRUCTURES OF THE MID-CONTINENT REGION

Reconnaissance mapping in the western Flint Hills physiographic province of south-central Kansas reveals two prominent fracture sets: [1] subvertical, NE-striking systematic joints, and [2] subhorizontal to moderately NW-dipping listric fractures. Tension fractures are best developed in thin-bedded limestone and argillaceous strata [Lower Permian Chase Gp.; Fort Riley and Florence Fms.] along the western flanks of the Augusta North anticline. Fractographic markings supporting Mode-I opening include plumose structures and arrest/rib lines. While joint spacing differs, orientations are consistent with the Oklahoma Joint System [Melton, 1929] and Joint Set 1 of Ward [1968], both of which are linked to compressional deformation in the late Paleozoic Ouachita-Marathon-Sonora orogenic belt along the southern margin of Laurentia. Our structures are oblique to deep-seated Precambrian basement trends [Central Kansas and Nemaha Uplifts], buried extensional structures [Humboldt Fault System], and regional folds [Augusta North anticline-syncline pair]. Conversely, systematic joints are subparallel to the Fredonia and Silver City—Rose Dome tectonic zones of southeastern Kansas [Berendsen and Blair, 1991]. Listric fractures are typically observed in massive carbonate beds [Winfield Limestone Fm.] overlying calcareous shale. Structures are inconsistently spaced and show gently-curved surface traces that shallow with depth and merge with depositional contacts. Subplanar geometries are occasionally observed in the hinge zones of gentle-to-open, northerly-trending mesoscopic folds. Therein, low-angle fractures form part of a conjugate set. Northwest-dipping fractures are also subparallel to rare ESE-vergent thrust faults with minor throws [Aber, 1991]. Given their orientation, geometry, and association[?] with local contractional structures, we tentatively interpret these features as incipient thrusts. However, the lack of observable offset and kinematic information leaves open the possibility that structures are of extensional origin. In this context, listric fractures may involve the relaxation of contractional Precambrian basement faults [ancestral Rocky Mountains] or the reactivation of rift-related structural elements.