Modelling interactions between fold–thrust belt deformation, foreland flexure and surface mass transport

Authors


Guy Simpson, Department of Earth Science, Geological Institute, Haldenbachstr. 44, HAD F1, ETH Zentrum, CH-8092, Zurich, Switzerland. E-mail: simpson@erdw.ethz.ch

ABSTRACT

Interactions between fold and thrust belt deformation, foreland flexure and surface mass transport are investigated using a newly developed mathematical model incorporating fully dynamic coupling between mechanics and surface processes. The mechanical model is two dimensional (plane strain) and includes an elasto-visco-plastic rheology. The evolving model is flexurally compensated using an elastic beam formulation. Erosion and deposition at the surface are treated in a simple manner using a linear diffusion equation. The model is solved with the finite element method using a Lagrangian scheme with marker particles. Because the model is particle based, it enables straightforward tracking of stratigraphy and exhumation paths and it can sustain very large strain. It is thus ideally suited to study deformation, erosion and sedimentation in fold–thrust belts and foreland basins.

The model is used to investigate how fold–thrust deformation and foreland basin development is influenced by the non-dimensional parameter inline image, which can be interpreted as the ratio of the deformation time scale to the time scale for surface processes. Large values of inline image imply that the rate of surface mass transport is significantly greater than the rate of deformation. When inline image, the rates of surface processes are so slow that one observes a classic propagating fold–thrust belt with well-developed wedge top basins and a largely underfilled foreland flexural depression. Increasing inline image causes (1) deposition to shift progressively from the wedge top into the foredeep, which deepens and may eventually become filled, (2) widespread exhumation of the fold–thrust belt, (3) reduced rates of frontal thrust propagation and possible attainment of a steady-state orogen width and (4) change in the style and dynamics of deformation. Together, these effects indicate that erosion and sedimentation, rather than passively responding to tectonics, play an active and dynamic role in the development of fold–thrust belts and foreland basins. Results demonstrate that regional differences in the relative rates of surface processes (e.g. because of different climatic settings) may lead to fold–thrust belts and foreland basins with markedly different characteristics. Results also imply that variations in the efficiency of surface processes through time (e.g., because of climate change or the emergence of orogens above sea level) may cause major temporal changes in orogen and basin dynamics.

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