Po. Koons et Cm. Henderson, GEODETIC ANALYSIS OF MODEL OBLIQUE COLLISION AND COMPARISON TO THE SOUTHERN ALPS OF NEW-ZEALAND, New Zealand Journal of Geology and Geophysics, 38(4), 1995, pp. 545-552
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.