MODELING THE EVOLUTION OF THE MONTSERRAT ANTICLINE AND HAPPY VALLEY FAULT IN THE NORTH CANTERBURY REGION ON THE SOUTH ISLAND OF NEW ZEALAND

Uplift rates from marine terraces constrain the structural evolution of an anticline and associated thrust fault in the seismically active North Canterbury thrust belt on the South Island of New Zealand. The thrust belt is in a region where the Pacific plate transitions from subduction to transpression along the Alpine Fault leading to lateral migration of the Pacific plate’s edge. This lateral migration causes shortening beneath the South Island because basement normal faults related to Cretaceous-Paleogene rifting are reactivated as thrusts. These basement thrust faults generate folds within the North Canterbury thrust belt. In this region, the Montserrat Anticline is exposed along the coast where a spectacular flight of marine terraces records late Pleistocene tectonic uplift. The landward vergent anticline folds a Cretaceous to Pleistocene cover sequence. Paralleling the anticline’s northwest limb, a fault thrusts Mesozoic basement rock against a Pliocene formation at the anticline’s north end and thrusts a Miocene formation against an early Oligocene formation at the south end. This contact reflects motion on a seaward dipping reverse fault rooted within the early Mesozoic basement. To constrain uplift and fault slip rates, fault-related fold kinematics were modeled using the trishear fold model appropriate for basement faults that drive cover sequence folding. Batch inverse models were run to generate the bedrock geology and bedding attitudes and to develop probability density distributions for important variables such as fault dip and slip. Marine terraces were dated using amino acid racemization and optically stimulated luminescence. Preliminary results date the upper terrace at 114.3 +/-5.8ka, MIS 5 indicating a regional uplift rate of 1.1 +/-0.1m/ka. On the anticline’s southeast limb, a fault scarp offsets the MIS 5 terrace by 15m and has an uplift rate of 0.13 +/-0.07m/ka. Since 1840, 16 plus earthquakes of Mw6.0 or greater have been recorded within this region. Mw6.0 earthquakes average 0.6m of vertical offset, which means a recurrence interval of 4,600 years for this fault. A recurrence interval of 7,600 years would be expected for Mw7.0 event.