Item

Abstract

Strain partitioning is a common observation in obliquely convergent subduction zones. Partitioning results in trench parallel material transport that, in the case of localisation of deformation, may be accommodated along large-scale margin parallel strike-slip faults. Such strike slip faults are located rather far inland from the trench (100-300km; e.g. Peru, North-Chile, Sumatra). Field studies as well as numerical and analogue models have shown the influence of convergence obliquity as a first order condition for the onset and degree of strain partitioning in the convergent wedge. The effect of friction at the wedge base is under discussion. In this context, the existence and position of the singularity (S-point, i.e. S-line for the 3D case) on the plate interface, defining the location from which basal shear forces are not further transmitted into the overlying wedge, has to be accounted for. This singularity is an inescapable necessity of numerical and analogue models. It may have its natural analogy in the line of intersection of the subducting slab with some brittle-ductile boundary in the convergent wedge. Up to date analogue experiments show strain partitioning in the deforming accretionary prism above the S-line, while the hinterlandward wedge remains undeformed. This, however, is not in accordance with natural examples: the formation of strike-slips faults is triggered further inland and is separated from the position of the assumed S-line. We therefore designed a series of scaled 3D analogue sandbox experiments, for which obliquity of convergence, basal friction and the position of the S-line can be varied. Analysis of the 3D surface displacement with CAD and PIV (particle imaging velocimetry) is used to evaluate material transport and to differentiate kinematic and dynamic domains of the convergent wedge. We test the influence of the relative position of the S-line with respect to the deformation front and the effect of the existence or non-existence of a basal shear force in the hinterlandward wedge. This shows whether shear along the entire base of the brittle wedge is required to facilitate lateral material transport at greater distance from the trench.