Large hinterland thrust sheets are the dominant structures in retroarc fold-thrust belt (FTB) wedges. These internal sheets remain active throughout the history of an FTB and strongly influence the kinematics of the external portions of the FTB and the overall behavior of the entire tectonic wedge. Principal deformation mechanisms in different parts of an FTB wedge are strongly controlled by variations in the geothermal gradient. Retroarc FTBs typically show a steeper geothermal gradient in the hinterland than in the foreland, resulting in a forelandward slope of the “brittle-ductile transition”. Therefore, a change from plastic to elastico-frictional (EF) mechanisms is expected not only with decreasing depth, but also across strike as a dominant internal thrust sheet is transported towards the foreland and undergoes exhumation by synorogenic erosion. However, evidence for cataclasis overprinting early plastic deformation features is often not preserved in ancient FTBs because of extensive erosion of the wedge-top. Even where evidence for late-stage cataclastic behavior exists in either ancient or modern (i.e. active) orogens it is often not recognized or studied.

The Canyon Range (CR) thrust sheet, an internal sheet in the central Utah segment of the Sevier FTB was continuously reactivated at shallow crustal levels, so most of the deformation took place within the EF regime and the cataclasized rocks are well preserved. The deformation in the CR sheet was measured both by determining the total strain in the rocks and by estimating the energy involved in emplacing the CR thrust sheet. These values are compared to published results on strain distribution in the Sheeprock thrust sheet of the Provo salient (Mukul and Mitra, 1998) and to estimates of work in emplacement of the McConnell thrust sheet in the Canadian Rockies (Elliott, 1976). These sheets are both mechanically dominant internal thrust sheets in other parts of the Cordilleran FTB that have similar deformation histories to the CR sheet but exhibit mainly plastic deformation. The total work estimates in all three sheets are very similar, which suggests that EF mechanisms are a critical component to FTB wedge evolution and cannot be ignored.