RATES AND MECHANISMS OF ACTIVE FAULTING AND FOLDING IN THE TRANSITION FROM HIKURANGI SUBDUCTION TO NORTH CANTERBURY TRANSPRESSION, SOUTH ISLAND, NEW ZEALAND (Invited Presentation)

The North Canterbury fold and thrust belt (NCFTB) on the South Island of New Zealand accommodates a component of the Pacific-Australia plate boundary strain in the transitional region between the Hikurangi subduction zone and the transpressive Alpine Fault. This region is seismically active, (e.g. the 2016 M7.8 Kaikoura earthquake), and active faults pose a significant seismic hazard. We investigate the rates and kinematics of fault-related folding in the NCFTB using forward and inverse kinematic modeling of fold growth, constrained by rates of uplift and folding derived from Quaternary marine terraces and Neogene growth strata. Fault slip rates inferred from our models vary among structures, with average rates of about 1-2 mm/yr onshore (with a significant exception of 4 mm/yr in one case) and <0.5mm/yr offshore. Slip rates on some faults also vary over time, including a decrease from 4 mm/yr to 1.5 mm/yr over about 30 ka. Normal faults inherited from earlier stages of Late Cretaceous rifting and reactivated as reverse faults play a major role in localizing deformation, while additional newly-formed faults may exist as well. A listric fault geometry with trishear folding ahead of the upward propagating fault tip (with inclined shear in the hanging wall) achieves successful models of the geometry of the reactivated normal faults and related folds. The depth to a proposed regional detachment is not well constrained by kinematic models and has a significant effect on shortening rates calculated from model results. Models with depth of detachment at 10-15 km, (as suggested by some previous studies in the region) result in shortening rates (~0.5-1.5 mm/yr on individual structures onshore) that are comparable to geodetic shortening rates from previous studies of 3-4 mm/yr across the fold belt. Restoration of a regional cross section across the onland part of the fold belt provides an estimate of total shortening of c. 2-3 km that is consistent with both the geodetic shortening rates and the lower end of estimates of the age of the fold belt (~0.8-1.2 Ma). The models tested in our reconstructions investigate the feasibility of fault-related folding in accommodating strain at the southern edge of the Hikurangi subduction zone and provide a new evaluation of rates of uplift, fault slip, and shortening in the seismically-active NCFTB.