Bounds on the width of mantle lithosphere flow derived from surface geodetic measurements: application to the central Southern Alps, New Zealand

Abstract

SUMMARY We show that the strain rate pattern measured at the surface of an oblique collision zone, the New Zealand Southern Alps, can be used to impose bounds on the width of present-day deformation in underlying mantle lithosphere. Geological and geophysical evidence supporting localized shearing along the downdip extension of the Alpine Fault, is used to constrain the upper slipping patch. Using finite-element models, we investigate the effect of different boundary conditions in the lower lithosphere on the predicted interseismic velocity profile at the surface. For a homogeneous elastic rheology, distributed lower lithosphere shear over widths greater than about 100 km causes the fit with interseismic velocities derived from Global Positioning System (GPS) measurements to significantly degrade. In comparison, either localized shear along a slipping patch or distributed shear over a width less than 100 km can fit the data. The strike-parallel velocity component is most diagnostic between the differing models. Finite-element models incorporating variations in elastic strength and non-linear viscous creep in the crust and mantle lithosphere, while indicating that some decoupling can occur between crust and mantle lithosphere, also support the conclusion that mantle lithosphere cannot be deforming over a width greater than about 100 km. In contrast, shear-wave splitting studies have been interpreted to show a wide, 200–400-km deforming zone beneath the Southern Alps. The discrepancy highlights the different timescales measured by GPS geodesy compared to estimates of finite strain that has accumulated over millions of years.