INVESTIGATION OF FOLD GROWTH IN NORTH CANTERBURY, SOUTH ISLAND, NEW ZEALAND

The presence of growing folds is as important as the recognition of emergent faults for assessing seismic hazard as many folds overlie blind thrust faults, which are capable of producing large magnitude earthquakes. The North Canterbury region on the South Island, New Zealand is an active example of a growing fold and thrust belt where blind thrusts underlie folds, creating a potential seismic hazard. The Montserrat Anticline is exposed along the North Canterbury coast where an extensive flight of marine terraces parallels the axis of the growing anticline and records late Pleistocene uplift. For our field study, we use AAR and OSL to date the terraces and assign the terraces to the associated highstand in the sea level curve. The uplift rates determined on the backlimb of the anticline from terraces are combined with a fault-related fold trishear kinematic model to determine the age of the fault, slip rates, and the potential recurrence interval on the blind thrust that drives folding. We focus here on the Glendhu Fault, a basement-rooted blind thrust fault that is deforming a ~1km thick Cenozoic-Quaternary sedimentary cover and associated structure, the Montserrat Anticline. Dating results combined with the sea level curve indicate that the terraces were formed at MIS 3 and MIS 5a, and surveys reveal that an additional highstand is preserved. Using uplift rates for constraint, we correlate this terrace to MIS 5e. The time-averaged regional uplift rate is 1.2 m/ka ± 0.2 m/ka. The folding began 1.2 Ma ± 0.2 Ma ago, which corresponds to a slip rate of 3.5 m/ka to 2.4 m/ka. The constraint on the uplift rate, fault slip, and fault age correspond to a recurrence interval for the Glendhu Fault for maximum displacement per Mw 6.2 earthquake of 260 to 600 years.