Using GPR to Delineate the Porter's Pass Fault at the Acheron Rock Avalanche, New Zealand

The Porter's Pass Fault (PPF) is one of a number of en echelon range front faults along the coastal mountains to the east of, and parallel to, the Southern Alps and the Alpine Fault of the South Island of New Zealand. The PPF crosses the Acheron rock avalanche deposit in the Red Hill valley, just west of Lake Lyndon, near Porter's Pass. The relationship between the two is still unresolved, and ground penetrating radar (GPR) profiling was carried out in support of one study of the interrelationship of the two. Two parallel GPR profiles crossed a small ridge situated on top of a river terrace, and an additional two parallel profiles were acquired in the bed of the adjacent Bluff River. The first two profiles confirmed the presence of the PPF beneath the small ridge, and a trench yielded material for dating and for analysis of the detailed fault geometry. The second two profiles were acquired to attempt to trace the continuation of the PPF across the Bluff River, but no trace of the fault was observed. This indicates that the river has been active enough in erosion and deposition to obscure any trace of the PPF.