PASSIVE AND ACTIVE GEOMORPHIC MARKERS OF RECENT DEFORMATION

The studies of landscape forms to unravel tectonic deformation of continental Earth surfaces mostly relies on two distinct approaches: one commonly used approach is based on deformed passive geomorphic markers, that record in a passive manner the surface deformation after their abandonment; the other approach, still under development and used to quantify the vertical displacement, is based on active or dynamic marker and relies on the hypothesis that a geomorphic element of the landscape can reach an equilibrium topography for which erosion and uplift are equal and opposite. In this talk, I will present general views on these two approaches emphasising on the present state of knowledge and on the requirements, as well as the limitations, in using such markers.

The use of passive markers requires both precise dating of the time of abandonment, i.e. the age, of the marker and the reconstruction of the initial marker geometry. Through several examples, it will be shown how it is difficult to estimate such initial geometry and why the accuracy of such method degrades with the wavelength of the studied deformation field.

The use of active marker has mostly developed around the study of river geometry and can potentially provide information on vertical deformation at a wider scale and for longer time-scale than passive markers. Such method requires first to demonstrate that geometric equilibrium of the active marker has been reached, and second to link this present geometry with incision rate, and then uplift rate, through a fluvial incision model. Until recently, only empirical models have been used and provided some relative index of the tectonic activity, and were requiring independent uplift rate estimates (through passive markers study for example) to end up with absolute estimates of surface uplift rates. Consequently, their applicability is generally only local and these models need to be adapted from one region to another one in particular because lithology and climate are distinct. Recent theoretical and experimental developments have brought however new constrain on incision models and will help in the future to precise the relation between river geometry and incision rate, and also to decipher in river geometry the respective roles of the tectonic uplift and incised lithology.