Arcuate accretionary wedge formation at convex plate margin corners: results of sandbox analogue experiments

Indentation of a rigid box into a sand layer leads to accretion of sand into a wedge which has an arcuate shape in map-view. Curvature of the wedge is most pronounced at the corners of the indentor, where deformation has been investigated in detail in this study. Several features of the arcuate wedge in the corner region depend on the indentation angle, i.e. the angle between the indentation direction and the normal to the frontal face of the indentor: (a) The ratio between the widths of the lateral and frontal parts of the wedge increases linearly with increasing indentation angle. (b) Orogen-parallel extension (OPE) in the corner area is initially high and leads thus to large finite OPE in the firstly formed nappes, but decreases and attains a stable level during progressive convergence. Finite OPE in the corner region is smaller at larger indentation angles. (c) Displacement vectors of accreted material exhibit fanning around the arc, but in oblique indentation the spread of their orientation is smaller than the change of structural strike (nappe traces and fold axes). Displacement vectors trend mainly in such positions normal to structural strike where displacement is parallel to the indentation direction. The results from these experiments can be applied to natural arcuate fold–thrust belts if natural conditions do not differ much from experimental boundary conditions. Such an application may give information about the former plate movement direction across an arcuate plate margin. In the example of the Eastern Carpathian arc, a Tertiary plate-movement direction of approx. 125–130° was determined. This direction is in accordance with current models of large-scale tectonics of the Carpathian region, which indicates the applicability of the experimental results to natural examples.