Unbalanced topography. Localised denudation/sedimentation and dynamic strain partitioning in 3D experimental Coulomb wedges: a synopsis.

In this paper, sandbox modelling was used to investigate the dynamic effect of growing topography on the deformation mechanics of accretionary systems. High obliquity was chosen as the main boundary condition to analyse 3D strain partitioning of a doubly-vergent Coulomb wedge, for the particular case in which a subduction polarity reversal event affected wedge development. In the experiments, the first phase of shortening produces a pre-existing topography affecting wedge kinematics during the second phase. Different degrees of obliquity set for the first phase produced different amounts of inherited topography, thus giving way to variable effects on wedge response to shortening during the second phase. In addition, the elevation potential during second phase could be varied in the models via sequential events of syn-tectonic denudation and/or sedimentation, performed on distinct sectors of the deforming wedge. Experimental results suggest that both timing and mode of retrothrust activation are crucial factors in influencing critical wedge mechanics as a whole. These may control localisation of compressive stress surpluses in the axial region, thus temporarily suppressing margin-parallel total strike-slip partitioning. Differential disequilibrium characterizing the topographic profile with respect to a given basal/internal coefficients of friction ratio will have a strong impact on the wedge growth history. Moreover, the parallelism between the imbricates at the thrust front and the strike-slip fault at the rear of the prism predicted by theoretical models is valid only at steady-state, when failure conditions exist everywhere at the basal décollement. Before this stage, uneven mechanical conditions at the base of the prowedge region produced an irregular distribution of the velocity fields within the deforming wedge, which caused, in turn, non-collinear mass transfer vectors between the prowedge and retrowedge regions. This led to unexpected wedge behaviour during which superficial extension or compression, both located in the axial zone, predated the full-development of a strike-slip fault.