IASPEI - Picture Gallery

IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS014 Oral Presentation 2262
Distributed vs. narrow deformation in the mantle lithosphere beneath the
central South Island, New Zealand: Insights from GPS and geology
Dr. Laura Wallace
Natural Hazards Group GNS Science
Susan Ellis
In the South Island, New Zealand, most of the crustal deformation between the Pacific and Australian
plates occurs on known, active fault zones. GPS and geological data there can be explained (to first
order) by the existence of a few, distinct tectonic (elastic) blocks with reasonably defined boundaries.
However, there is ongoing debate regarding the nature of interplate deformation at depth, within the
mantle lithosphere beneath the South Island. Some workers suggest that plate boundary deformation is
broadly distributed within the mantle lithosphere (over a zone up to 400 km wide), while others contend
that the overall relative plate motions are accommodated across comparatively narrow shear zones at
depth. One of the assumptions of the elastic block model previously used in the interpretation of GPS
and geological data in the South Island is that the relative plate motions occur at depth over a narrow
zone. To assess the possibility of distributed deformation within the mantle lithosphere, we integrate the
elastic block approach with analytical equations describing surface displacements due to a wide
deformation zone at depth (below the elastic part of the crust). This is the first time that a fully threedimensional
inversion accounting for both localised and distributed shear within the lithosphere has
been carried out. The GPS data (on its own) can be fit by either a wide or narrow shear zone at depth.
However, the long-term surface deformation patterns we obtain for the two end-member models are
quite distinct, suggesting that geological and geodetic studies together may help to distinguish between
these two conceptual models for plate boundary deformation. In particular, to fit the GPS data with a
wide shear zone in the mantle, we find that a quite unrealistic permanent deformation pattern in the
upper crust (in comparison to the known active faulting) is needed.
Keywords: crustal deformation, tectonics, gps