Effects of Boundary Conditions and Convergence Geometry on Deformation Style at Oblique Convergent Margins

The accommodation at convergent margins of the partitioned components of oblique plate motion can result in spatially complex patterns of deformation. In thin-skinned fold-belts this can result in deformation that has distinct zones of nearly pure dip-slip thrusting at the deformation front and strike-slip within the fold-belt or in the accretionary wedge. Most convergent margins have along-strike variations in obliquity and corresponding variations in deformation style. We measure the horizontal strain field in a variety of frictional analogue models of oblique convergence in which we examine the kinematic and structural effect on deformation of buttressing due to changes in along-strike margin geometry. We find that even at oblique convergent margins that have a constant along-strike obliquity to plate motion considerable along-strike variations in deformation can result from boundary conditions associated with the finite length of the margin. Such boundary conditions affect the manner in which strain is accommodated over the entire length of the margin, not just locally. We find that significant along-strike variations in strain accommodation can also result from curved margin geometries commonly observed in convergent settings. At margins with such a geometry the gradual along-strike variations in plate motion obliquity will yield accommodation structures in the analogue models that transition from one structural style to another (e.g., margin normal compression to along-strike extension). While the local style of deformation will reflect the local margin obliquity the along-strike variations in deformation style may not be obvious in the gross morphology of the model fold-belts. In many cases the change in deformation style is only apparent in the strain rate or velocity fields.