Locating the deep extent of the plate boundary along the Alpine Fault zone. New Zealand: Implications for patterns of exhumation in the Southern Alps

The tectonic evolution of the South Island of New Zealand records the consequences of a transition from nearly translational to transpressional plate motions since the Late Miocene. Although it is clear how that transition was accommodated in the upper crust-primarily through the development of the Southern Alps orogen-how the lithospheric system responds to this change in plate kinematics is unclear. Coupling kinematic and deformational modeling with an analysis of the existing seismic data that images the deformational fabric in the lithospheric mantle leads me to propose that a substantial amount of the plate boundary strain is accommodated by a reorientation of the plate boundary structure and maintenance of simple shear deformation as plate motions change. This leads to a developing geographical mismatch between the location of upper crustal strain and that within the lower crust and lithospheric mantle. One possible result of this offset in the locus of plate boundary strain can be the development of a detachment surface within the lower crust that effectively decouples the upper crust from its underlying foundation. Consequences of this include different styles of deformation of the lower crust with position along the orogen and significantly different strain histories for crustal rocks involved in the Southern Alps orogen as a function of whether they are part of the thin-skinned or thick-skinned regime.