Constraints on present-day plate motions south of 30°S from satellite altimetry

Abstract

Gridded free-air gravity anomalies derived from Geosat and Seasat altimetric observations are used to locate all mid-ocean ridge transform faults south of 30°S, and wherever possible, assign azimuths and uncertainties useful for plate kinematic analysis. A total of 131 ridge offsets longer than 30 km were located, of which 114 are clearly transform faults with well-defined valleys. Azimuthal uncertainties range from 0.6° for the longest transform faults to nearly 10° for the transform faults with lengths close to 30 km. To assess whether the altimetrically-derived azimuths are representative of present-day plate slip directions, azimuths of transform faults mapped with side-scan sonar and side-looking or conventional bathymetric systems were compared to their altimetrically-derived counterparts. Comparison of these two independently-estimated sets of azimuths using the x 2 test shows no significant difference, indicating that the altimetrically-derived azimuths can be used where bathymetrically-derived azimuths are not available. We also find no evidence that large age-offset transform faults are less representative of present-day slip directions than short age-offset transforms. Models constructed using the altimetrically-derived azimuths fit the NUVEL-1 spreading rates, transform azimuths, and earthquake slip vectors nearly as well as NUVEL-1, as well as decreasing the model uncertainties. Thus, the altimetrically-derived azimuths offer improved accuracy and decreased uncertainties in estimates of global plate velocities without increasing non-closures of plate circuits beyond the level already present in NUVEL-1. In practical terms, plate velocities predicted by models that substitute the altimetrically-derived azimuths for all NUVEL-1 transform azimuths from below 30°S never differ from velocities predicted by NUVEL-1 by more than 0.3 mm yr −1 and 4°; however, the several-degree azimuthal misfits to altimetrically-derived azimuths from the Africa-Antarctic and Pacific-Antarctic plate boundaries are statistically significant and suggest there are several-degree inaccuracies in the predictions of the NUVEL-1 angular velocities. The new catalog of southern hemisphere transform fault azimuths should prove useful for at least the following: (1) improvement of models of deformation of the Australian plate west of the Macquarie Ridge Complex; (2) improvement of estimates of motion between the Australian and Pacific plates; (3) revision of upward bounds on the amount of seafloor deformation south of Africa related to rifting of the African plate; and (4) studies of a possible deforming zone within the South American plate east of the South Sandwich trench.