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Publication Date


Document Type

Honors Project




Faults (Geology)-Wyoming, Geology, Structural-Wyoming, Calcite, South Fork fault, Strain, Calcite twinning, South Fork Fault (Wyoming), Twinning (Crystallography)


The origin of the South Fork fault, a 10km by 40 km rootless detachment fault, has been in question for over 90 years. In order to better understand its emplacement history, this project used calcite twinning analysis to calculate stress and strain associated with the South Fork detachment. Oriented samples of the Jurassic Sundance Formation were collected within both the upper and lower plate of the South Fork detachment. Calcite twinning analysis, using a universal stage, was conducted to calculate the strain ellipsoid, the principal stress axes and the differential stress preserved within the South Fork detachment. The twinning present within the upper and lower plates show compression and shortening axes oriented to the NW-SE with shallow plunges. Because transport of the allochthon was to the southeast, the stress and strain calculated from the calcite twins are likely due to emplacement of the allochthon itself. Samples within the lower plate also have steeply dipping compression and shortening axes representative of loading by the upper plate, providing further evidence for South Fork detachment-related twinning strain. The average differential stress calculated is ~20 MPa, which agrees with this interpretation of the calculated stress and strain ellipsoid orientations. The calcite twins within the Jurassic Sundance Formation of the South Fork fault indicate a localized, detachment related deformation. There is no sign of regional stress or strain induced by the proximal and temporal orogenic events, which is seen in many carbonates throughout the area. !




iv, 58 p. : ill. some col), col maps. Honors project-Smith College, Northampton, Mass., 2011. Includes bibliographical references (p. 53-55)