Strain analysis of the Ninemile fault zone, western Montana: insights into multiply deformed regions

Abstract

By means of detailed field mapping and a systematic strain analysis we investigated the development of the Ninemile fault in western Montana. The fault has been considered to be either a Mesozoic or mid-Tertiary strike-slip fault, or a Late Eocene-Early Oligocene normal fault. The rock of the study area experienced a Mesozoic to early Tertiary NE-SW shortening expressed by thrusts, folds, and pervasive cleavage. We estimated total strain by measuring preferred orientation of chlorite grains in samples from within and near the Ninemile fault zone and by calculating the strain according to the March theory. Our goal was to explain how strain accumulated by successive superposition of strains during the two deformation events onto an early compaction strain. In all samples the minimum-elongation direction (= maximum shortening) is nearly vertical. The maximum-elongation directions of all the samples outside the Ninemile shear zone are subperpendicular to the strike of the shear zone, whereas in the shear zone they are either parallel to the strike of the shear zone or form an angle of about 45°. The systematic variation in the principal strain directions is the result of two deformational events expressed by local structures created by early NE-SW shortening and later NE-SW extension. The observed strain requires that the Ninemile fault, a normal fault during the Tertiary, had earlier been either a thrust or a zone of contractional shear deformation, along which there was fault-perpendicular shortening and fault-parallel extension. Although the average value of maximum elongation is greater within the Ninemile shear zone than outside, it varies significantly inside the fault zone, suggesting a highly inhomogeneous strain distribution created by a complex deformation history.