Publishing House SB RAS:

Publishing House SB RAS:

Address of the Publishing House SB RAS:
Morskoy pr. 2, 630090 Novosibirsk, Russia



Advanced Search

Russian Geology and Geophysics

2001 year, number 10

Thickening of mantle lithosphere beneath New Zealand and the role of buoyant crust in this process

P. Molnar, T. Stern, and G. A. Houseman
Keywords: Subduction, mantle lithosphere, Earth's crust, New Zealand
Pages: 1497-1503

Abstract

P-wave delays recorded by two linear arrays of seismographs across the Southern Alps of New Zealand require a high-speed zone in the upper mantle beneath the Alps. The pattern of residuals does not match that predicted for subduction of one plate of mantle lithosphere beneath the other. Instead, the high-speed zone seems to mark a blob of cold lithospheric mantle sinking beneath the eastern part of the Southern Alps, directly beneath the thickest crust, and hence where mantle lithosphere has thickened by pure shear. Calculations of Rayleigh-Taylor instability for a thickening heavy layer buoyed up by a lighter layer show that the form of the sinking blobs of dense material depends strongly on the ratio of effective viscosities of crust and mantle. Where the effective viscosity of the buoyant upper layer (crust) is large, a single blob sinks beneath the area of maximum thickening of the upper layer. In New Zealand, where a single sinking blob is inferred, large-strike-slip shear of the region may have lowered the viscosity of the mantle. Elsewhere, if the viscosity of the upper layer is small, however, two sinking blobs can form adjacent to the area where the more buoyant layer (crust) is shortened horizontally. Between the blobs, the unstable layer (lithospheric mantle) thins. Such a process may occur within the Tien Shan.