The geological setting of the Darfield and Christchurch earthquakes

The 2010-2011 Canterbury earthquake sequence occurred near the southeastern margin of Neogene deformation associated with the Australia-Pacific plate boundary. Basement comprises indurated rocks of the Torlesse Composite Terrane, of Permian to Early Cretaceous age, overlain by 1-2 km of less-indurated Cretaceous-Neogene rocks and unconsolidated Quaternary sediments. Proximity to the subduction interface between Gondwana and the paleo-Pacific Ocean produced a Mesozoic-age structural grain in the basement rocks, aligned broadly east-west in the Canterbury to Chatham Rise areas. These structures provided an inherited weakness that was likely reactivated by present-day stress. Mid- to Late Cretaceous extension, marked by localised fault-bounded grabens, was followed by deposition of a Late Cretaceous to Paleogene passive-margin transgressive sedimentary sheet and minor intraplate basaltic volcanics. Mid-Cenozoic inception of the modern Australia-Pacific plate boundary heralded deposition of a regressive succession of Neogene sediments and further episodes of volcanism, most notably constructing the Late Miocene Banks Peninsula intraplate volcanoes. The east- to northeast-striking faults associated with the Darfield and Christchurch earthquakes are probably aligned with the Mesozoic structural grain within the Torlesse basement rocks.