Bathymetric gradients of lineated abyssal hills: Inferring seafloor spreading vectors and a new model for hills formed at ultra-fast rates

Abyssal hill morphology provides a preliminary measure of the direction and rate of seafloor spreading, however, additional information (e.g., magnetic anomaly data or a nearby mid-ocean ridge) is usually required to verify these estimates. Previous attempts to identify a unique spreading rate proxy from abyssal hill dimensions (e.g., height, length, width) have largely failed due to the relatively large scatter of data or the non-linear character of spreading rate trends. We present a new, stand-alone method of determining both spreading rate and spreading direction using the distribution of azimuths for slopes facing toward and away from the ridge axis. The spreading rate is determined with the Δ peak height parameter, defined as the difference in the height (maximum frequency) of the two dominant modes observed in the azimuthal histograms. This parameter exhibits a clear, nearly linear spreading rate trend and allows half spreading rates to be estimated to within 10–20 km/Myr. The spreading direction is determined with the Δ peak width parameter, which compares the average width of the two dominant modes in the azimuthal histograms. The wider distribution of slope azimuths is oriented away form the paleo-ridge axis for all spreading rates, and thus spreading direction can be determined. The trends in the peak height and width parameters are used to constrain a new model of abyssal hill formation at ultra-fast spreading rates, which require greater off-axis extensional faulting resulting in a few large-throw faults on the outward-facing hillsides, and many smaller throw faults on the inward-facing hillsides.