Lateral Variations in Evolution of a Fold-thrust Belt Hinterland: Passive versus Active Behavior of Dominant Internal Thrust Sheets

Critical wedge theory (CWT) explains the large-scale architecture, kinematic evolution and strain distribution of fold-thrust belts (FTBs) for both plastic and Coulomb wedges. During progressive deformation in an FTB wedge, the taper tends to decrease due to a variety of causes. CWT requires continued deformation in the back of the wedge to maintain critical taper in the wedge as a whole. Since the overall taper of the FTB wedge is typically determined by the geometry of a dominant thrust sheet in the hinterland of the FTB, it is the deformation behavior of such a sheet that defines wedge behavior as a whole. After initial emplacement of a dominant hinterland sheet, deformation in the back of the wedge may result in passive uplift and/or translation of the dominant sheet, or it may result in active deformation of the entire back of the wedge. The Sevier fold–thrust belt (FTB) provides an excellent natural example for evaluating these CWT-predicted behaviors based on three decades of structural studies. In summary, the Lewis segment (initial taper ∼9◦), the Central Utah segment (initial taper ∼6◦) and the Provo segment (initial taper ∼3◦) of the Sevier FTB have different initial sedimentary prism geometries and lithotectonic configurations, providing contrasting examples that are end-case scenarios of dominant hinterland sheet behavior both during initial emplacement and during late stage deformation of the back of the wedge as an FTB evolves. Strain data from these three areas are compatible with the interpretive models.