DYNAMICAL COUPLING BETWEEN FORELAND DEPOSITION AND THIN-SKINNED THRUST-AND-FOLD BELT STRUCTURAL STYLE: INSIGHTS FROM NUMERICAL MODELS

Foreland basin strata provide sensitive records of aspects of the tectonic history of adjacent orogenic belts. For example, the mechanical linkage provided by lithospheric flexural response implies that changes in foreland accommodation space or surface slope can be caused by changes in mass distribution due to thrust motion. In addition, these syndeformational sediments can strongly affect the structural style within the thrust-and-fold belt (TFB), as well as the timing and magnitude of motion on individual thrusts. The TFB / foreland basin system is dynamically coupled: structure influences surface processes (erosion and sedimentation), and surface processes influence structure.

We model aspects of the mechanics of thin-skinned TFBs, and dynamical feedback with surface processes, using an Arbitrary Lagrangian-Eulerian 2-D finite-element continuum mechanics code capable of accommodating very large strain. Narrow zones of high shear strain represent thrust faults, which yield structural styles very similar to natural TFBs. Model structures can include far-traveled thrust sheets, duplexes, antiformal stacks, backthrusts, pop-ups, break-thrusts, nascent triangle zones, and piggy-back basins.

In the absence of surface processes, thrusts tend to develop in simple in-sequence patterns, although individual faults may remain active for extended periods or be later reactivated. With syndeformational surface processes, and under appropriate mechanical conditions, the proximal portion of the flexurally subsiding foreland basin can achieve critical taper, prior to any internal deformation. Once at critical taper, the tip of the active TFB steps out into the foreland, incorporating the proximal foreland basin as a piggyback basin. Continued convergence results in out-of-sequence thrusts shortening the piggyback basin, as well as repetition of the process to accrete new piggyback basins at the TFB toe. This leads to a characteristic structural style of broad, little-deformed synforms, separated by more strongly deformed antiforms, similar to features in the Alberta Foothills and elsewhere. Although syndeformational sediments have been erosionally removed across the Foothills, the structural characteristics of dynamical feedback between deformational style and surface processes remain.