East-west shortening during north-south convergence, example of the NW Himalayan syntaxis

The northwest terminus of the Himalayan range forms a syntaxis limited by two strike-slip faults, the Chaman Fault to the west, and the Karakorum Fault to the east. In between these faults, smaller-scale syntaxes are observed, as the Nanga Parbat and the Kashmir syntaxes. The fold axial traces of these syntaxes trend nearly north-south, revealing a component of east-west shortening in the geodynamic context of the north-south convergence between India and Asia. We constructed a sandbox with two edges oblique to the convergence to test the influence of the NW Himalayan syntaxis large-scale structural geometry on the construction of a wedge and on its evolution. We used sand and microbeads as standard analogues for brittle crustal layers approximately 30 km thick. We show that the obliquity of the edges can generate an east-west shortening in a north-south convergence. The amount of east-west shortening generated by the oblique edges of the box is up to 40% of the amount of the north-south convergence. This shortening is accommodated by oblique motion on transpressive faults parallel to the edges. The amount of strike-slip motion is up to 60% of the convergence vector. We conclude that the obliquity of the edges is sufficient to generate an east-west shortening in a north-south convergence, but not sufficient to generate a north-south axial-trending fold. One experiment, with a free southern boundary, allowed the development of pure strike-slip faults within the sand layer, conjugated to pure shortening on one north-south fold. The east-west component of the oblique motion generated by the obliquity of the faults bordering the NW Himalayan syntaxis could be absorbed by a north-south fold as the Nanga Parbat if the motion on the border faults is pure strike-slip.