Effects of topography on surface fault geometry and kinematics: examples from the Alps, Italy and Tien Shan, Kazakstan

The kinematics and extent of surface faulting are usually determined using observations on morphostructures, such as escarpments, and measurements of the net displacement of geologic deposits and topographic features, such as gullies and ridges. The present paper shows that these measurements can be misleading in rugged terrains if one does not take into account the effect of topography. To illustrate this effect, field data on surface deformations collected along Holocene faults of the Italian Alps and Kazakstan Tien Shan are compared with fault plane geometry and local topography. The reverse Foscagno Fault, Italy, which is also linked to deep-seated gravitational slope deformation, splays upward at a 50-m-high ridge inducing scarps with apparent normal offset. The transpressional Satè Fault, Tien Shan, changes its dip near the surface where it intersects ridges at least 50–60 m high. Here, fault inclination changes up to 60–80° with a complete inversion of fault dip, a normal fault appearing at the surface. The transpressional Beskaragai Fault, Tien Shan, manifests an opposite sense of slip at a 80-m-high ridge. A detailed measurement of surface displacement along these Holocene structures shows that the offset apparently increases at ridges and tends to zero at inter-ridge depressions. These data are here explained with a new model which considers the greater mobility of the rock mass of the ridge which has unbuttressed sides. This produces upward or horizontal downhill expulsion of part of the ridge in a direction perpendicular to the fault strike if faulting has vertical motions, while the expulsion direction is parallel to the fault strike when strike–slip motions occur. This expulsion can be accompanied by tilting which produces oversteepening of the downhill slope and landsliding.