Geodetic analysis of model oblique collision and comparison to the Southern Alps of new Zealand

We present geodetic analysis of the relatively well characterised sandbox analog of oblique collision. Video-digitising of surface displacement patterns permits the description of the mechanical model in terms of the same strain components (gamma 1, gamma 2), dilatation (sigma), and rotation (omega) often used to describe natural deformation. Internal deformation accompanying basal sliding along low-angle decollement produces a surficial strain pattern dominated by a high-strain zone at the toe where material moves into the orogen and at the indentor where it can exit. Deformation in the orogen centre is strongly partitioned in the vertical plane with the convergence component accommodated by basal sliding and the lateral component accommodated by near-vertical strike-slip faults. Strain along the base is invisible to surface geodetic analysis whereas the lateral component is evident in significant levels of angular strain within the orogen centre. Consequently, the characteristic signal of basal sliding of this three-dimensional critical wedge behaviour is rotation of the azimuth of principal horizontal shortening (psi) to near parallelism with the indentor within this centre corridor. The natural geodetic signal in central Otago displays this characteristic pattern. Variation in boundary conditions parallel to the indentor yields spatial divergence in gamma 1, gamma 2, sigma, and omega. A step in sand thickness, simulating Marlborough, produces a zone of irrotational deformation associated with large angular strains south of the step. A region of clockwise rotation lies along the step where deformation is dominated by simple shear. Velocity gradients parallel to the plate boundary arising from the thickness step reduce the rotation of psi in the centre of the orogen. Boundary parallel variations in boundary conditions can be identified through geodetic analysis and are important in determining the model and natural deformation fields.