Fault-fracture strain in Wingate Sandstone

Abstract

The Laramide deformation of the Triassic Wingate Sandstone along the northeast flank of the Uncompahgre uplift has occurred by faulting at various scales. Macroscopically smooth flexures of beds within the Wingate occur by small displacements across a myriad of intraformational, mesoscale faults. The deformation resultant from these small faults may be approximated by a strain tensor, provided the measurement domain satisfies certain size criteria. Equivalent strain (ε) measurements, obtained from 22 locations in the East Kodel's Canyon, range from 1% to 15.5% (the maximum contractional strains range from −0.9% to −13.4%). The faults producing this strain have displacements ranging from a fraction of a millimeter to 18.5 cm. The fault intensity increases with increasing ε, although in a distinctly non-linear fashion. At low strains, incremental increases in the deformation produce additional, small displacement faults. At larger strains, incremental increases in the deformation occur via progressive displacement along existing faults. The principal strain axes are consistently non-coaxial with the inferred principal stresses (average σ 1 Λε 1 is 18.5°). This non-coaxiality results from the non-uniform development of the conjugate fault systems. This same inequality of the conjugate systems produces a non-zero rotation tensor, ω, but ω is not related to σ 1 Λε 1. The non-uniform development of conjugate shears (and the associated non-coaxiality of σ 1 and ε 1) may be an intrinsic characteristic of a Coulomb material.