Bivergent wedges result from the interaction between deformation, flexure and surface processes. Based on 2D sandbox simulations in conjunction with PIV, this study investigates the influence of these processes on the spatio-temporal evolution of strain-partitioning within, and the associated surface uplift of, bivergent wedges. To facilitate interpretation and to successfully communicate results, two new display types, i.e., the surface uplift and the evolution of deformation map are introduced.
Experimental results suggest a four-staged evolutionary pathway for bivergent wedges: an initial pop-up (stage I) or backfold is followed by a proto pro-wedge, in which frontal accretion dominates (stage II). Basal accretion commences if a mid-level detachment is present (stage III). Frontal accretion within the retro-wedge occurs during stage IV.
Furthermore, a conceptual model of cyclic accretion within bivergent wedges is proposed, whereby each cycle consists of a thrust initiation, an underthrusting and a re-activation phase. The latter determine the location and magnitude of deformation within, and surface uplift of, a bivergent wedge. Therefore, the accretion cycle is considered as an internal clock for wedge-scaled deformation and surface uplift. We also demonstrate that the geometry of the deformation front as well as the spatial distribution of surface uplift are indicative for the currently active phase within an accretion cycle. During the course of an accretion cycle, surface uplift of the axial-zone and the retro-wedge may reach up to 1/2 of the thickness of the incoming layer. Cycle duration is estimated to range between 104 to 105 years and is thus in a similar range as climatic cycles. This conceptual model provides also an explanation for the transient behaviour of deformation and surface uplift and for the discrepancy between geodetic and geologic estimates of fault slip.
We found that retro-wedge erosion amplifies the displacement of the basally accreted material, whereas pro-wedge erosion accelerates and additionally redirects the particle flow of the frontally accreted material. Pro- and retro-wedge erosion retard the propagation of deformation into the foreland. This effect is stronger for pro-wedge erosion. Retro-wedge erosion amplifies vertical growth and leads to increased strain accumulation along the retro shear-zone and the mid-level detachment. Thus, cause (retro-wedge erosion) and response (pro-wedge deformation) are significantly offset in space. Since pro-wedge erosion evokes a complete decoupling of the retro-wedge from the pro-wedge, cause and response are spatially more closely related. We also found that more focused erosion is associated with a more focused tectonic response.
Implications of these results are finally used to explore the evolution of foreland basins. Thereby, special emphasis is devoted to the Flysch to Molasse transition as well as to the occurrence of hydrocarbon and MVT deposits.
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