Intra-arc transpression in the lower crust and its relationship to magmatism in a Mesozoic magmatic arc

Structural observations and U-Pb geochronology from Fiordland, New Zealand support a model of partitioned transpression within the lower crust of an early Mesozoic magmatic arc called the Median Batholith. We use this lower crustal section to test whether transpression was an efficient mechanism for transporting magma through the deep lithosphere. A continentward migration of magmatic activity occurred within the margin of Gondwana after ∼140 Ma followed by a period of concentrated magmatism in a vertical, 12-15 km wide lower crustal shear zone after ∼119 Ma. The shear zone, named the Indecision Creek Shear Zone, contains variably oriented dioritic intrusions and displays systematic variations in the three-dimensional orientation of ductile structures. From the margins to the center of the shear zone the pitch of stretching lineations on foliation surfaces changes from 10-35° to 55-82° with increasing finite strain. This increase in pitch is accompanied by a steepening and counter-clockwise rotation of foliation planes. These and other structural patterns indicate that arc-parallel sinistral oblique-slip and strike-slip displacements occurred at the shear zone margins and that deformation in its center was dominated by horizontal arc-normal shortening and near vertical extrusion. This style of partitioned transpression reflects the effects of rheological contrasts created by a heterogeneous pattern of magmatism within the arc. Field relationships and U-Pb dates on zircon suggest that the shear zone formed along the boundary between outboard (older) and inboard (younger) parts of the batholith and facilitated the transfer of small volumes of magma vertically through the lower crust until at least ∼111 Ma, when convergence and arc magmatism waned.