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.

Abstract Transform faults that offset mid-ocean ridge (MOR) segments accommodate plate motion through deformations that involve complex thermal and mechanical feedbacks involving both brittle and temperature-dependent ductile rheologies. Through the implementation of a 3D coupled thermal-mechanical modelling approach, we have developed a more detailed picture of the geometry of plate boundary deformation and its dependence on plate velocity and the age offset of MOR transforms. The modelling results show that cooling of near-ridge lithosphère (lateral heat transfer) has significant effects in the ductile mantle lithosphere for both the location and style of deformation. The region where străin is accommodated in the subjacent mantle lithosphere is systematically offset from the position of the overlying linear transform fault in the brittle crust. This offset causes the boundary to be oblique to plate motions along much of the transform’s length, producing extension in regions of significant obliquity modifying the location of the surface fault segments. An implication of this complex plate-boundary geometry is that in the near-ridge region, the older (cooler) lithosphère will extend beneath the ridge tip, restricting the upwelling of mantle to the MOR. The melt to generate the oceanic crust adjacent to the transform must migrate laterally from its offset source, resulting in a reduced volume and thinner crust. This near-ridge plate boundary structure also matches the pattern of core-complex extension observed at inside corners of many slow-spreading ridges. The oblique extensional structure may also explain mag-matism that is observed along ‘leaky’ transforms, which could ultimately result in the generation of new ridge segments that effectively ‘split’ large transforms.

Extension zones within continents have complex patterns of tectonic evolution. The Basin and Range Province of western North America provides an ideal location to study the mode of extension in continental regions. We have utilized numerical models to test two distinct geological models of extension that have been proposed for the Basin and Range: (1) a model in which extension takes place by uniform (or pure shear) stretching; and (2) a model in which extension occurs along discrete low-angle shear zones by a simple shear mechanism. These numerical models indicate that both styles of extension produce results generally consistent with observed heat flow, gravity, and elevation data. Distinctive patterns in these data are maintained primarily during the period of extension, implying that present day observations are dominantly a consequence of an ongoing process. The results further imply that the effects of present day extension will obscure the evidence of previous extensional episodes at least as far as the parameters of heat flow, elevation, and gravity are concerned.