To date, approximately 20% of the ocean floor has been surveyed by ships at a spatial resolution of 400 m or better. The remaining 80% has depth predicted from satellite altimeter-derived gravity measurements at a relatively low resolution. There are many remote ocean areas in the southern hemisphere that will not be completely mapped at 400 m resolution during this decade. This study is focused on the development of synthetic bathymetry to fill the gaps. There are two types of seafloor features that are not typically well resolved by satellite gravity: abyssal hills and small seamounts (< 2.5 km tall). We generate synthetic realizations of abyssal hills by combining the measured statistical properties of mapped abyssal hills with regional geology including fossil spreading rate/orientation, rms height from satellite gravity, and sediment thickness. With recent improvements in accuracy and resolution, It is now possible to detect all seamounts taller than about 800 m in satellite-derived gravity and their location can be determined to an accuracy of better than 1 km. However, the width of the gravity anomaly is much greater than the actual width of the seamount so the seamount predicted from gravity will underestimate the true seamount height and overestimate its base dimension. In this study we use the amplitude of the vertical gravity gradient (VGG) to estimate the mass of the seamount and then use their characteristic shape, based on well surveyed seamounts, to replace the smooth predicted seamount with a seamount having a more realistic shape.

The Australian-Antarctic Ridge (AAR) is the intermediate spreading system located between the Southeast Indian Ridge and Macquarie Triple Junction of the Australian-Antarctic-Pacific plates. KR1 is the easternmost and longest AAR segment and exhibits unique axial morphology and various volcanic structures. Within it, we identified three linearly aligned volcanic seamount chains positioned parallel to the seafloor spreading direction. We found that the seamount chains had formed asymmetrically and had developed through near-ridge volcanism at some distance away from the KR1 axis. Based on high-resolution bathymetric data, we identified the spatial distribution, morphology, and summit types of the isolated volcanic structures composing the seamount chains. The magnetic constraints on the age of the identified seamounts indicate that most had a formation time of less than ~600 kyrs, which primarily occurred during four distinct volcanic pulses from 0.3-0.8 Ma, 0.9-1.1 Ma, 1.6-2.1 Ma, and 2.2-2.7 Ma (or two major distinct pulses from 0.3-1.1 Ma and 1.6-2.7 Ma). When inconsistency existed between the observed and modeled ages of volcanic structures, volcanos were found to have a temporal gap of 200-650 kyrs between their formation and that of the underlying seafloor. Such volcanos are thought to have developed due to off-axis volcanism at a distance of 7-20 km. Considering the scale of off-axis volcanism and thickening lithosphere of such areas of ~20 km away from the axis of the intermediate spreading ridge, we propose that the seamounts originated from a deep plume source beneath the oceanic lithosphere.

Seamounts are isolated elevations in the seafloor with circular or elliptical plan, comparatively steep slopes, and relatively small summit area (Menard, 1964). The vertical gravity gradient (VGG), which is the curvature of the ocean surface topography derived from satellite altimeter measurements, has been used to map the global distribution of seamounts (Kim & Wessel, 2011). We used the latest grid of VGG to update and refine the global seamount catalog; we identified 10,796 new seamounts, expanding the catalog by 1/3. 739 well-surveyed seamounts, having heights ranging from 421 m to 2500 m, were then used to estimate the typical radially-symmetric seamount morphology. First, an Empirical Orthogonal Function (EOF) analysis was used to demonstrate that these small seamounts have a basal radius that is linearly related to their height – their shapes are scale invariant. Two methods were then used to compute this characteristic base to height ratio: an average Gaussian fit to the stack of all profiles and an individual Gaussian fit for each seamount in the sample. The first method combined the radial normalized height data from all 739 seamounts to form median and median-absolute deviation. These data were fit by a 3-parameter Gaussian model that explained 99.82% of the variance. The second method used the Gaussian function to individually model each seamount in the sample and further establish the Gaussian model. Using this characteristic Gaussian shape we show that VGG can be used to estimate the height of small seamounts to an accuracy of ~270 m.