PASSIVE AND ACTIVE BEHAVIOR OF DOMINANT THRUST SHEETS DURING TAPER ENHANCEMENT IN THE HINTERLAND OF A FOLD-THRUST BELT WEDGE

The large scale evolution of fold-thrust belts (FTBs) can be described in terms of critical wedge theory. During progressive deformation in a FTB wedge, the taper tends to decrease due to a variety of causes. Critical wedge theory 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 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. 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 Lewis and central Utah segments of the Sevier FTB provide contrasting examples of dominant sheet behavior in the back of the FTB wedge.

In the Lewis segment the sedimentary basin had large initial taper (~9 degrees). Initial emplacement of the Lewis sheet required very little internal deformation of the sheet to achieve critical taper. Rocks close to the thrust were transported up from the quasi-plastic (QP) regime, and, after erosion, reached the elastico-frictional (EF) regime. Subsequent taper enhancement in the back of the wedge was accomplished by subthrust duplexing in weaker rocks that uplifted and/or transported the passive Lewis sheet. In the central Utah segment, by contrast, the initial taper of the sedimentary basin was significantly lower (~6 degrees). Initial emplacement of the dominant Canyon Range and Pavant sheets required significantly more internal deformation which was accomplished by quasi-plastic mechanisms in the lower part of the sheet. After erosion removed a significant portion of the overburden, subsequent taper enhancement of the wedge was accomplished by active deformation of the dominant internal sheets and their footwalls in the elastico-frictional (EF) regime. The contrasting styles of deformation of the dominant sheet(s) in the two examples result in two end-case scenarios of wedge evolution.