Geosat‐derived geoid anomalies at medium wavelength

Abstract

Geosat profiles_ of the Exact Repeat Mission have been averaged over a 1-year period and high-pass-filtered using inverse method techniques. The geoid surface constructed with both ascending and descending profiles shows at medium wavelengths band-shaped anomalies preferentially elongated in the east-west direction. These anomalies have an average amplitude of ~30 cm and dominant wavelengths of 750 krn and 1100 km. We have performed numerous tests to show that the lineations are not artefacts created by the filtering process. Moreover, two-dimensional (2-D) filtering with the inverse method applied on a regional basis over the Pacific gives essentially similar results, indicating that the filtered geoid is not affected by directional bias. Seafloor topography in the Pacific filtered by 2-D inverse method also shows east-west trending depth anomalies positively correlated to medium-wavelength geoid lineations. Along the East Pacific Rise, there is a clear correlation between geoid lineations and regional variations in axial depth. Cross-spectral analysis carried_out on geoid and topography data over the Pacific area gives coherence maxima at 750-km and 1100-km wavelengths and admittance values of 2-3 m/km. Observed admittance matches the magnitude and shape of admittance predicted by lithospheric cooling models, suggesting that the lineations are related to regional variations in the plate cooling process. Convection models produce much higher admittances than observed unless a low-viscosity layer is assumed so that dynamical support cannot be completely discarded. In most instances, however, the position and direction of the lineations seem to be controlled by major fractures zones which is in favor of a lithospheric origin. In the south central Pacific where the lineations appear parallel to absolute plate motion, there may be a combination of both lithospheric and sublithospheric processes.

“Plate‐like” subsidence of the East Pacific Rise–South Pacific superswell system

“…Previous studies, for example, have revealed short-to-intermediate wavelength gravity [Haxby and Weissel, 1986] and geoid anomalies [Cazenave et al, 1992] in the EPR-superswell region which have been interpreted, at least in part, in terms of small-scale convection in the sublithospheric mantle that is aligned in the direction of absolute Pacific plate motion. The problem at large scales is that there is disagreement on the sign of the long-wavelength gravitational field over the superswell.…”

“…Since the general shape of these larger-scale features is difficult to collectively parameterize, U, f, and z were not applied on this pass, but S r was set to 750 km in order to avoid the possibility of MiMIC linking up fracture zones troughs (e.g., Marquesas and Austral). Topography previously associated with intermediate-wavelength ($1000 km) geoid anomalies [Cazenave et al, 1992] corresponds spatially to identifiable features (i.e., swells and plateaus) and so, if present, is difficult to distinguish.…”

“…Most of these values become positive after a correction [Turcotte and Schubert, 2002, equation 5 -158] of +16 m for a ''standard'' [Parsons and Sclater, 1977] plate is applied. To prevent the lowest order dominating and generating artefacts the coefficients should therefore be tapered [Sandwell and Renkin, 1988;Cazenave et al, 1992]. Given the probable spectral overlap however, the taper would have to be chosen with care.…”

Resolution of Seamount Geoid Anomalies Achieved by the SARAL/AltiKa and Envisat RA2 Satellite Radar Altimeters

“…Because the ridge is narrow and trends nearly east-west, the geoid anomaly is rather short-wavelength in the alongtrack direction of this pass (Watts et al 2006). The peculiar geographic pattern in linear, small amplitude geoid anomalies in this region has been the subject of many studies (Haxby and Weissel 1986;Cazenave et al 1992;Wessel et al 1994;Sandwell et al 1995;Watts et al 2006).…”

Geoid lineations of 1000 km wavelength over the central Pacific

“…Maia and Diament (1991) rand Baudry and Kroenke (1991) found evidence of 400-600 km wavelength geoid lineations in the south central Pacific. Longer wavelength (800-1200 km) lineations have been reported by Cazenave et al (1992), an observation recently confirmed by Wessel et al (1994). Although these medium-wavelength lineations have been obtained by different investigators using different satellite data and different filtering methods, their existence is not yet considered as firmly established and is often considered as an artifact of the filtering process.…”