Continuous deformation of lithospheric mantle beneath New Zealand

Transform faults within continental areas - such as the San Andreas Fault in California and the Alpine Fault in New Zealand - are sites of relatively frequent earthquakes and visible offsets. From this it is clear that the crust is breaking in a brittle manner. What is not so clear is how the underlying lithospheric part of the mantle is behaving. Recent work from New Zealand suggests that the lithospheric mantle beneath New Zealand is deforming continuously - and not breaking.

New Zealand forms the boundary between the Pacific and Indo-Australian Plate, and interestingly the Pacific Plate is being subducted beneath the Indo-Australian Plate to the northeast of New Zealand, while the Indo-Australian Plate is being subducted beneath the Pacific Plate to the south of New Zealand (see map). The subduction zone boundaries are joined by the 800 km long Alpine Fault, which extends along the length of the South Island and part way across the North Island. Over the past 45 m.y. there has been 460 km of offset on the Alpine Fault. (note offset of Cambro-Devonian metamorphic rocks)

Researchers from the US and New Zealand have carried out a detailed study of seismic data from earthquakes in the region. They have used the data to argue that the lithospheric mantle at depths of 100 to 120 km has deformed continuously across a width of 300 to 400 km - and has not slipped along one, or a small number of faults. (see figure below)The implication - in this type of situation - is that the lithospheric mantle flows, and that it is not as LITHOspheric as we might have thought.

While it is still clear that the crustal plates include both crustal and lithospheric mantle material, and that for the most part they move together as a unit, if these authors are correct there may be a partial de-coupling of the crust and the lithospheric mantle at plate boundaries.

Reference: Molnar, P. and others, Continuous deformation versus faulting through the continental lithosphere of New Zealand, Science, Vol. 286, p. 516-519, October 15^th 1999.