MIDCRETACEOUS OROCLINAL BENDING OF NEW-ZEALAND TERRANES

Recently published results and new data suggest that the Jurassic-Cret aceous magmatic rocks of the Median Tectonic Zone of New Zealand, and the Cretaceous Separation Point Batholith that locally intrudes it, we re emplaced subparallel to the Mesozoic Gondwana margin. Together, the y provide a valuable piercing point on the Alpine Fault for 118 Ma. Th ey also lie almost exactly parallel to mean extension lineations in ex humed metamorphic core complexes and extension directions indicated by fault-bounded Cretaceous sedimentary basins and dike swarms in the ov erlying cover. Continental extension and subsequent breakup in the Tas man Sea and the eastern Bounty Trough was towards the northeast, almos t perpendicular to the overall trend of the Gondwana margin but parall el to the margin-related rocks in the central sector. Together, these relationships suggest almost 90 degrees of rotation and major dextral shear. New geochronology now constrains the rotation to the period bet ween the intrusion of the Separation Point Batholith at 118 Ma and the initiation of Cretaceous sedimentary basins at c. 101 Ma. Much of the rotation was probably completed before the intrusion of the Buckland Granite at 110 Ma. The relationships suggest that a prominent dextral ''Z'' bend developed in the basement terranes of New Zealand and in th e continental margin in the mid Cretaceous, and this may have influenc ed the site of the later Neogene plate boundary.