Finite strain and strain variation analysis in the Sheeprock Thrust Sheet: an internal thrust sheet in the Provo salient of the Sevier Fold-and-Thrust belt, Central Utah

The Sheeprock thrust sheet in west-central Utah is an internal thrust sheet in the Provo salient of the Sevier fold-and-thrust belt. We have measured finite strain in quartzites (the dominant lithology), sampled along a square grid within the thrust sheet, using the modified normalized Fry method (McNaught M.A. (1994) Modifying the normalized Fry method for aggregates of non-elliptical grains. Journal of Structural Geology16 493–503). The XY and XZ axial ratios from unsampled locations within the sample area were estimated using the spatial statistics approach. The strain ellipsoids exhibit a variable three-dimensional orientation pattern resulting from modification of the initial layer parallel shortening (LPS) strain ellipsoid by fault parallel shear in conjunction with vertical flattening and/or horizontal stretching indicating that the thrust sheet did not undergo plane strain deformation in the transport plane. This suggests that the plane strain assumption used in drawing restorable balanced cross-sections breaks down for internal thrust sheets with more than one penetrative-strain producing deformation event. The XZ strain axial ratios decrease away from the thrust towards the middle of the sheet. The XY strain axial ratios from interpolated image diagrams indicate transport-parallel stretching at the front end of the sheet and strike-parallel stretching at the back end of the sheet. The footwall and hanging wall finite strain patterns are similar indicating that most of the strain in the Sheeprock thrust sheet developed early in the deformation history of the thrust sheet before and perhaps during the growth of a large fault propagation fold pair.