The Motunau Fault and other structures at the southern edge of the Australian-Pacific plate boundary, offshore Marlborough, New Zealand

Abstract

The boundary between the Indo-Australian and Pacific plates passes through New Zealand as the Alpine Fault, then links northeastwards with the Hikurangi subduction margin through the Marlborough shear zone, a group of five major, subparallel transcurrent faults. The southern limit of the Marlborough shear zone is situated at the Motunau Fault, a previously unrecognized offshore extension of the active Porters Pass Fault and lineation system. To the south of the Motunau Fault, the Kaikoura Sequence (Cretaceous-Cainozoic) sediments that underlie the Canterbury continental shelf are effectively undeformed and lie outside of the plate boundary zone. 3.5 kHz profiler records and other marine geologic data collected on G.R.V. “Tangaroa” Cruises 1097 and 1116 show that the shelf north of the Motunau fault is underlain by complexly deformed sediments. Faulting and folding continue in this area today, as evidenced by the presence of deformed late Pleistocene strata and by the distribution of earthquake epicentres. Within the Marlborough plate boundary zone the five major strike-slip faults delimit four substantial blocks, or “microplates”, 20–60 km wide and up to 220 km long. From north to south, these are the Spenser, Inland Kaikoura, Seaward Kaikoura and Conway blocks respectively. Kaikoura Sequence sediments are most strongly deformed near the margins of the blocks and their subsidiary fault systems; within each microplate, deformation is restricted to moderate folding and faulting, the overall structural pattern being consistent with dextral movement on the boundary-faults. The available marine geologic and geophysical data suggest that subduction of the Pacific plate may have propagated southwards during the late Neogene, causing successive activation of the Marlborough faults. Today, subduction is occurring right to the southern head of the Hikurangi Trough, where the motion is transferred to the Motunau fault system. Since earthquake epicentres only reach to intermediate depths beneath the southern part of the Conway microplate, and since the Motunau fault system is not such a continuous physiographic features as faults further north, it is likely that this southern margin to the plate boundary zone is still actively developing.