Gravity profiles across the Alpine Fault, New Zealand

Ten gravity traverses, between 7 and 18 km long, have been run across the Alpine fault, New Zealand. These traverses range from the Haast River in the south to the Wairau River 470 km to the north. The derived isostatic anomalies reveal only minor gravity variations in the vicinity of the fault. That these variations are caused mainly by random fluctuations in near-surface densities and in terrain corrections is revealed by an ‘average’ profile that was obtained by averaging the gravity values along the profiles projected parallel and onto a line normal to the fault and taking zero anomaly at the present trace of the fault. The only gravity anomaly in the average profile that could be due to the Alpine fault is less than 2 mgal in amplitude. If this anomaly is considered to be a ‘low’ superimposed on a slight regional gradient, it could represent a low density fault zone; if considered to be an inflection, it could be due to a small density contrast across the fault confined to the uppermost kilometer of the crust. The gravity characteristics of the Alpine fault are similar to those of the San Andreas fault.