Processes controlling vertical coupling and decoupling between the upper and lower crust of orogens: Results from Fiordland, New Zealand

The pre-Cenozoic configuration of western New Zealand allows determination of the effects of magmatism and a changing lower crustal rheology on the evolution of a Cretaceous orogen from upper to lower crustal levels (10-50 km). Beginning at ∼126 Ma, a composite batholith dominated by diorite was emplaced into the lower crust. During emplacement, deformation was partitioned into zones weakened by magma and heat, leading to the development of two layer-parallel shear zones at the upper and lower contacts of the batholith. Transient vertical decoupling of the crust above and below the batholith occurred from ∼126 Ma until ∼120 Ma as magma was emplaced into and moved through a weak, thick lower crust. By ∼116 Ma, however, much of the batholith had crystallized and the lowermost crust had cooled from 750 °C< T <850 °C to T=650-700 °C. Cooling was aided by the juxtaposition of pre-existing crust against hot new crust and by the efficient extraction of partial melts out of the lower crust. Cooling together with dehydration of the lower crust and mafic compositions led to the development of a strong, dry, lower crustal root by ∼116 Ma. A strong lower crust resulted in high degrees of vertical coupling between the upper and lower crust during contraction from ∼116 to ∼105 Ma even as magma continued to be emplaced into the mid-upper crust. A narrow, focused orogenic style in the upper crust at this time reflected a highly viscous lower crust through which compressional stresses were transferred vertically. The results imply that changes in plate boundary dynamics rather than the thermal weakening of thick lower crust during convergence controlled the onset of regional extension at ∼108-105 Ma.