THREE-DIMENSIONAL FINITE ELEMENT MODELING OF A THIN-SKINNED FOLD-THRUST BELT WEDGE: INSIGHTS FROM THE EXAMPLE OF THE PROVO SALIENT, SEVIER FTB, UTAH

Fold-thrust belts (FTBs) typically show arcuate patterns with thrust traces convex toward the foreland in map view. This arcuate geometry clearly reflects three-dimensional (3-D) deformation patterns caused primarily by non-planar and non-cylindrical geometry of the underlying thrust faults and variations in material behavior over space and time. This type of complex 3-D deformation, predicted by simple geometric and material considerations, is inadequately addressed by current theoretical and numerical plane strain models. We use a commercial finite element (FE) program (MARC-MENTAT) to model a 3-D FTB wedge using nonlinear analysis involving large displacement and large strains. This program allows us to generate input files for complex geometries and boundary conditions from its user friendly graphic interface, as well as representing the model results graphically and quantitatively. We have chosen the Provo salient as our prototype to take advantage of its well known geological setting, tectonic history and available kinematic information within a confined region; in addition, its variable geometry (lateral ramp at the northern end and oblique ramp at the southern end) allows us to study the effects of different types of lateral boundaries on the 3-D evolution of a wedge. The 3-D numerical model provides useful information (e.g. maximum principal strain, maximum shear strain, and maximum principal compressive stress orientations, and material displacement direction) in interpreting the kinematics and mechanics for the development of an FTB wedge. It demonstrates that the 3-D FTB wedge has different deformation characteristics in different parts, such as the middle of the salient versus oblique and lateral ramps, and its behavior is closely related to pre-existing templates such as predeformational basin shape. From the model results we interpret that conventional plane strain assumptions in FTBs are reasonable in the middle of a salient, where material displacements are confined to a vertical plane parallel to the regional transport direction. However, the results clearly indicate non-plane strain and non-coaxial deformation at the boundaries of the salient, and material paths show motion out of the regional transport plane.