COUPLING OF THE CRUST AND MANTLE IN TRANSPRESSIONAL SYSTEMS

South Island, New Zealand, is an ideal place to study the relation of finite stain, geodetic motion, and the mechanical behavior of different parts of the lithosphere in a transpressional setting. Upper crustal deformation is determined by either integrating geodetic displacement over geologic time or utilizing finite strain markers. While the former is more tractable, it assumes steady-state deformation. This assumption can be tested with finite strain markers, and shown to be incorrect. Using two finite markers, the Junction magnetic anomaly and the Moonlight/Alpine fold belt in the Haast Schist, one can infer that the plate motions must have changed with time. This latter result is consistent with plate motions studies.

Our knowledge of mantle deformation in New Zealand is derived primarily from shear wave splitting, a result of anisotropic seismic wave propagation caused by the LPO of minerals (mainly olivine). Field studies, numerical models, and well-constrained experiments all indicate that LPO, and therefore shear wave splitting, tracks finite strain. Comparison of the finite strain analysis on crustal rocks and the mantle fabric shows a strikingly similar pattern. This indicates that deformation of these lithospheric layers are mechanically coupled and that the mantle does not partition deformation, for example into discrete high-strain zones. We can test the assumption of mantle flow driving crustal deformation in New Zealand by applying models of rigid-body rotation, which distinguish between bottom-driven and side-driven deformation. The results of this analysis are consistent with bottom-driven deformation.