Geoid anomalies over the north-east Pacific fracture zones from satellite altimeter data

Abstract

Summary. Seasat altimeter profiles have been used to recover geoid height anomalies over the north-east Pacific fracture zones. Geoid anomalies observed over fracture zones show a step-like signal, of 1–4m amplitude, downward from the younger, shallower side to the older, deeper side of the fracture zone. Assuming that fracture zones are in isostatic equilibrium at the base of the lithosphere, the geoid offset occurs because the portions of lithosphere on each side of the fracture have different thickness and density structures as predicted by thermal models of lithospheric cooling. Along the Mendocino Fracture Zone, whose age offset is of the order of 20–30Myr, the observed geoid anomalies present a clear change from east to west, both in shape and in amplitude: in the youngest part of Mendocino (around 130° W longitude), there is a marked step in the geoid (3–4m amplitude), while west of 140° W, the older part of Mendocino, the geoid offset does not exceed 1.5 m, the signal becoming more and more distorted. The (geoid offset/age offset) relation estimated at different locations along Mendocino presents a progressive decrease from east to west, e.g. from the young part to the old part of the fracture zone. For the Pioneer Fracture Zone, the geoid offset is barely visible, never exceeding 40 cm. Using the known bathymetry and crustal structure inferred from seismic refraction data at 128° 30°W longitude as well as ages inferred from magnetic lineation data, we have computed the theoretical geoid height anomaly at this location: assuming (local) isostatic compensation at the base of the lithosphere, we have deduced the subcrustal density structure from lithospheric cooling models, taking account of the age/thickness relationship. Comparison between computed and observed geoid anomaly at 128° 30°W longitude is very satisfactory, although a very slight overcompensation at depth needs to be assumed in order to give a better agreement. Over the western, older part of Mendocino, observed geoid anomalies approach isostatic anomalies. Since subcrustal lateral density contrasts are smaller than in the young part of the fracture zone, their isostatic shape and smaller amplitude imply smaller crustal discontinuities: this suggests that the crustal thickness discontinuity across a fracture zone may decrease with age.