GEODETIC ANALYSIS OF MODEL OBLIQUE COLLISION AND COMPARISON TO THE SOUTHERN ALPS OF NEW-ZEALAND

We present geodetic analysis of the relatively well characterised sand box analog of oblique collision. Video-digitising of surface displacem ent 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. Inter nal deformation accompanying basal sliding along low-angle decollement produces a surficial strain pattern dominated by a high-strain zone a t the toe where material moves into the orogen and at the indentor whe re it can exit. Deformation in the orogen centre is strongly partition ed in the vertical plane with the convergence component accommodated b y basal sliding and the lateral component accommodated by near-vertica l strike-slip faults. Strain along the base is invisible to surface ge odetic analysis whereas the lateral component is evident in significan t levels of angular strain within the orogen centre. Consequently, the characteristic signal of basal sliding of this three-dimensional crit ical wedge behaviour is rotation of the azimuth of principal horizonta l shortening (psi) to near parallelism with the indentor within this c entre 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 stra ins south of the step. A region of clockwise rotation lies along the s tep 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 va riations in boundary conditions can be identified through geodetic ana lysis and are important in determining the model and natural deformati on fields.