Fold-thrust belt evolution expressed in an internal thrust sheet, Sevier orogen: The role of cataclastic flow

Abstract

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. Their emplacement and deformation account for a significant portion of the total mechanical energy budget in FTB evolution based on estimates using deformation features preserved within such sheets. Moreover, the deformation within dominant internal sheets can be used as a means to evaluate the tectonic history of the wedge as a whole. The 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 foreland-ward slope of the “brittl-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 toward 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 lat-stage cataclastic behavior exists in either ancient or modern (i.e., active) orogens, it is often not recognized or studied. Our work in the Canyon Range, central Utah segment of the Sevier FTB shows that unraveling the EF mechanisms, such as lat-stage cataclastic flow, is critical to understanding the total deformation history of an internal thrust sheet. After initial emplacement under plastic deformation conditions, the CR thrust sheet was continuously reactivated under EF conditions, thereby helping to maintain critical taper of the FTB wedge during its development. Structures preserved in the CR thrust sheet, therefore, record a significant portion of the deformation history of the FTB as a whole. The Canyon Range (CR) syncline, part of the CR sheet, underwent fold tightening within the EF regime and the cataclasized rocks are well preserved. The geometry and progressive deformation patterns of the CR syncline reveal four distinct to partly overlapping deformation stages. The history of the rocks in the CR thrust sheet has been tracked by incrementally retro-deforming the FTB and by unraveling details of the deformation within the CR syncline at various scales for each stage. We use the CR syncline as a type example to show that unraveling EF mechanisms is critical to (1) understanding the state of a FTB wedge during its evolution, (2) calculating the energy involved in FTB emplacement, and (3) developing accurate restorations. Placing constraints on the depth of the rocks for successive stages of deformation is essential for developing retrodeformable balanced cross sections, and in turn, geometric and kinematic models for wedge evolution.