Date of Award
Master of Science
Mervin J Bartholomew
Layer-parallel-shortening (LPS) strain records the first strain experienced by rocks during tectonic events. LPS indicators preserved in vertical to overturned fold limbs along the leading edge of the fold-thrust belt in southwestern Montana are used to determine which deformation came first there: Sevier or Laramide. LPS-strain was analyzed for 22 samples collected in the Dixon Mountain area from different beds containing ooids, Pentacrinus, or brachiopods within a 580-meter stratigraphic interval. Field orientations of strain ellipses were then adjusted for: 1) trend and plunge of fold axes; 2) rotation of thrust sheets relative to the trend of the Sevier-emplaced, overturned Deadwood Gulch syncline; and 3) bedding dip. Retrodeformed orientations of 15 samples show an average shortening direction of ~227o, consistent with Sevier shortening (~213o); 4 samples show a shortening direction of ~268o; and 3 samples show a shortening direction of ~171o, perhaps indicating Sevier shortening across a lateral ramp.
Loyacano, Christopher Dale, "Layer-Parallel-Shortening Strain: a Key to the Sevier-Laramide Deformational Sequence in the Tendoy Range, Southwestern Montana" (2017). Electronic Theses and Dissertations. 1752.