INTERACTIONS BETWEEN SURFACE PROCESSES AND FOLD GROWTH IN FOLD-AND-THRUST BELTS: APPLICATION TO THE ZAGROS FOLD BELT

Thin-skinned fold-and-thrust belts result from the compressional deformation of a sediment pile over a basal decollement layer. The topographic evolution of fold-and-thrust belts results from the competition between climatic, tectonic and surface processes. Importance and complexity of interplays between these processes have been amply discussed and documented by numerous field, analogue and numerical studies. However some aspects of these interactions, such as the effects of erosion and sedimentation on fold growth rate are not well parametrized. The effects of erosion, sedimentation and an initial topography on the fold pattern, as well as the conditions of surface uplift and fluvial incision to form wind gaps, were systematically investigated with numerical modelling and applied to the Zagros Fold Belt. Constraints on denudation rates in the Fars Province were obtained by cosmogenic nuclides measured on river sand. A new finite-element based surface process model (SPM), which includes both sedimentation and erosion, was developed and coupled to the finite-difference 3D parallel mechanical code LaMEM (Lithospheric and Mantle Evolution Model). Both codes are written in C, using the PETSc library. This thesis describes the mechanical and surface process models, and how they are coupled. The 3D fully-coupled mechanical and surface process model was used to investigate the effects of erosion, sedimentation and initial topography on fold patterns. The mechanical model considers variations in density and viscosity of a sedimentary cover with internal weak layers detached over a much weaker basal layer representing evaporites. The model topography is modified through time by mass redistribution, which is achieved by a combination of fluvial and hillslope processes. At the scale of the fold-and-thrust belt, erosion and sedimentation do not strongly modify the general fold pattern or fold wavelength. However, erosion reduces the amount of shortening required to initiate folding and enhances fold growth. The main contribution of surface processes is their ability to modify the pre-folding topography and hence the initial random noise. If larger initial random noise is present, folds amplify faster, which is consistent with previous detachment folding theory. Variations in thickness of the sedimentary cover, introduced by the presence of an initial topography, also have a significant influence on the folding pattern, resulting in long, cylindrical folds that amplify faster. A regional slope reduces the critical distance required for the transition between linkage and no-linkage modes. Erosion and redeposition of sediments, on the contrary, increase this distance. The location of the saddle point, where fold segments link, is shifted and its vertical distances to fold crests are increased or decreased compared to the case without erosion or initial topography, which can potentially affect the characteristics of hydrocarbon trap. Both erosion and redeposition enhance fold elongation, once the erosion velocity overcomes the folding velocity. The quasi-cylindrical character of the long Fars anticlines and the persistence of small, high aspect ratio doubly-plunging structures suggest that an initial surface slope between 0 and 0.1% may have been present in this area before buckling. This initial slope could be associated with the formation of an accretionary prism during the collision. 3D highresolution numerical models reproduce well the variety of fold shapes (doubly-plunging folds, long straight or sinuous folds) and fold linkages (linear or oblique linkage), observed in the Fars Province, favouring the idea that large parts of the Zagros could have formed by buckling of the sedimentary cover. The new SPM was employed to quantify the interactions between surface uplift and river incision and predict the formation of wind gaps. Horizontal and vertical displacements were prescribed in a kinematic manner to better control the tectonic parameters. The model considers drainage network that initially developed on a tilted surface and subjected to vertical and horizontal movements that simulates fold growth and shortening, respectively. Three modes of morphological relationship between rivers and folds were identified: (1) transverse, (2) intermediate and (3) parallel. Besides, three modes of erosion pattern were distinguished: (1) maximum erosion rates at the fold, (2) maximum erosion rates in the upstream area of fold and (3) deposition of large sedimentary basins in the upstream area of fold. Transitions between these modes are not unidirectional and suggest that the investigated parameters are coupled and their effects on the drainage network are difficult to isolate. However, the parametric study allows determining a range of values for one parameter for which a simple relationship can be derived between drainage network and one or two other parameters. A range of values for the fluvial incision ratio, the maximum uplift rate, the time span of fold growth and the time at which the uplift rate is maximum, has been constrained for each incision and erosion mode, such that we are able to determine when transverse drainage and formation of large sedimentary basins occurs. Transverse drainage networks occur for uplift rates up to 0.8 mm.yr−1 and incision ratio between two successive layers between 0.5 and 5. Parallel drainage networks and formation of sedimentary basins occur for large values of incision ratio (> 20) and uplift rates between 1 and 2 mm.yr−1. In the Fars Province, the relation between rivers and folds and the presence of large sedimentary basins in synclines fit incision mode 2 and erosion mode 3. Comparing incision modes, uplift rates and stratigraphy from geological observation and measurements with results of the numerical models permitted to determine the incision ratio between two successive lithologies in some anticlines of the Central and Coastal Fars. Basin-averaged denudation rates for the Fars Province were constrained by cosmogenic nuclides (10Be) on river samples collected along the Mand river and two of its tributaries: The Shur and Qareh Aghaj rivers. Denudation rates range from 0.049 to 0.185 mm.yr−1 with a clear trend of ∼ 3 times faster rates in downstream areas, close to the Mountain Front Fault, than in upstream areas. This trend of erosion rates correlates with the present day GPS velocity field, which indicates faster shortening in the coastal area than in Central Fars. Results were not applied in numerical models, but should be used for future work.