FEEDBACK BETWEEN EARTH SURFACE PROCESSES AND DEFORMATION STYLES IN THIN-SKINNED THRUST-AND-FOLD BELTS: INSIGHTS FROM DYNAMICAL NUMERICAL MODELING

The “critical wedge” existence theories of Chapple (1978), Davis et al. (1983), and Dahlen (1984) adequately describe the first-order mechanics of thin-skinned thrust-and-fold belts (TFBs). However, because they assume the wedge is everywhere on the verge of failure, these existence theories are inappropriate for examining the mechanics within TFBs.

Dynamical numerical models are not similarly constrained, and allow examination of structures within the wedge, offering prospects of a deeper understanding of the Earth systems behavior of TFBs at the scale of individual thrust sheets. They provide insight into complex feedback relationships, especially those involving Earth surface processes (erosion and sedimentation) and regional isostasy, which are integral parts of the natural system. Neither geometric-kinematic analyses nor existence theories are capable of addressing these feedback relationships. Although the effects of erosion and sedimentation on orogenic-scale deformation are widely appreciated, a similar understanding within TFBs is lacking.

We describe numerical experiments of thin-skinned TFB deformation using a 2-D finite-element continuum mechanics code capable of accommodating very large strain. Each model is generally composed of initially layered materials (though arbitrary geometries are supported) of specified thickness, lateral extent, density, rheology (e.g., Coulomb plastic), and mechanical properties (which can vary, e.g., as a function of strain magnitude). During deformation, the upper surface of the thickening wedge is exposed to surface processes, allowing erosional removal and redistribution of material to sedimentary basins, and regional isostatic adjustment to thickening is accommodated.

Although faults are not modeled explicitly, narrow high-strain zones develop, forming structures very similar in style to those in TFBs. Model structures include far-traveled thrust sheets, irregular-roof and smooth-roof duplexes, antiformal stacks, backthrusts, pop-ups, nascent triangle zones, detachment folds, break-thrusts, and piggy-back basins. Variations in systems response (e.g., structural style) can result from subtle variations in input parameters. Model structures are compared to natural examples, especially from the Southern Canadian Rockies.