MAKING MOUNTAINS ALONG THE NEW ZEALAND PLATE BOUNDARY: IMPORTANCE OF TRANSITIONS AND TRANSIENTS

Sitting astride the Pacific-Australia plate boundary, New Zealand exhibits the consequences of transpressional plate motions. Of particular importance are the distinctively different responses to similar plate kinematics along the plate boundary. In the far south of the South Island, transpression is accommodated by localized subduction of Australia beneath Fiordland, which undergoes an abrupt transition into the more translational Alpine Fault system. To the north, transpression is accommodated by the subduction of the Pacific plate along the Hikurangi margin, which is migrating to the south encroaching on the Australian lithosphere. At the transitions between these plate regimes there is substantial tectonism that records the consequences of the changes in plate boundary structure. The development of the New Zealand mountains is not a simple transpressional orogenic event, but rather reflects the superposition and juxtaposition of variable styles of tectonism affecting specific plate boundary components. Key features are (1) a tear within the Australia plate at the transition from Fiordland subduction to Alpine Fault transpression; (2) offsets between the near-surface Alpine Fault and lithospheric mantle Pacific-Australia shear zone through the South Island; and (3) delamination of the sub-crustal Australia lithosphere to make room for the Pacific slab, subducting beneath the North Island, as it migrates southward. The consequences of these changes in plate interactions include (1) uplift of the Fiordland mountains driven by the flexure of the underlying, strongly deformed subducting Australia slab; (2) variations in uplift and deformation along the Southern Alps as the coupling between and location of the crustal and upper mantle plate boundary changes; (3) a pattern of uplift-downwarp-uplift as the southern edge of the Hikurangi slab drives delamination of a sliver of Australia lithosphere; and (4) mountain building in the north of the South Island that reflects the superposition of Southern Alps orogenesis with crustal deformation associated with the delamination. As a result of these transitions and their migrations, orogenesis along the ~ 1000 km of the New Zealand plate boundary varies substantially over short distances and short time intervals.