EFFECTS OF FLAT LENGTH ON DEFORMATION AROUND A RAMP-FLAT-RAMP THRUST SYSTEM: A NUMERICAL APPROACH

Deformation associated with a single thrust fault ramp is characterized by an early shortening followed by rotation in both the hanging wall and footwall. When an intervening flat separates two distinct sequential ramps crossing different stratigraphic intervals, the thrust system will experience more complex deformations. The resultant deformations associated with sequential ramps would exhibit a spectrum of styles, of which two end members correspond to ‘overprinting’ and ‘interference’. Overprinting is bound to occur when an existing deformation field established at the lower ramp comes under the influence of the upper ramp. Interference refers to the situation where deformation fields at adjacent ramps overlap in space and time such that one hinders the other from fully developing. Previous field works on the northern part of the southern Appalachian fold and thrust belt suggest that the deformation style is particularly sensitive to the length of the intervening flat. With emphasis on the comparison with the northern part of the southern Appalachian fold and thrust belt, we construct numerical models of the ramp-flat-ramp geometry to quantitatively examine effects of flat length on the deformation style. We employ a novel finite element code, DynEarthSol2D, to have appropriate material models as well as non-trivial fault geometry. Our models confirm that the relative importance of overprinting versus interference is a function of the flat length as well as the length of the lower ramps and hanging wall displacement. When the flat is long compared to ramp lengths, and hanging wall displacement is sufficiently large (i.e., ~ flat length), structural overprinting is found in an element in the hangingwall that passes over the lower and upper ramps successively. Thus, recognition of overprinting is indicative of sequential ramps. In contrast, when the intervening flat is significantly shorter than the lower ramp (i.e., <= flat length), the flat is annihilated and a single long ramp of variable dip is produced. As a result, interference between the deformation fields of the two ramps occurs. We will further present stress and strain histories extracted from the models. If clearly distinguishable for different flat lengths, they will be referenced at the time of interpreting field observations.