STRAIN VARIATION IN THRUST SHEETS ACROSS THE SEVIER FOLD-AND-THRUST BELT (IDAHO UTAH WYOMING) - IMPLICATIONS FOR SECTION RESTORATION AND WEDGE TAPER EVOLUTION

Although strain in fold-and-thrust belts is typically inhomogeneous, o n the largest scale strain within thrust sheets shows regular patterns . The strain, which is a quantitative measure of the deformation of an y thrust sheet is independent of the translation of the sheet. The int ernal deformation can be represented by a deformation profile (the cha nge in shape during thrusting of a fault perpendicular line) which can then be used to unstrain the sheet during restoration in cross-sectio n balancing. Deformation profiles in well-studied thrust sheets show r egular geometric patterns, and compatibility constraints can be used t o minimize the data needed to define the profiles in other sheets. Fro m the geometry of deformation profiles and direct field observations, the most important component of internal deformation of thrust sheets in many fold-and-thrust belts is early layer parallel shortening (LPS) . This can be removed in a straightforward manner during restoration, and significantly affects the geometry (i.e. taper) of the restored se ction. Original basin wedge taper may have important implications for the mechanical evolution of fold-and-thrust belts. Thus changes in ori ginal basin taper resulting from strain removal may affect significant ly the way in which we interpret the evolution of a fold-and-thrust be lt in an area.