Bathymetric gradients of lineated abyssal hills: Inferring seafloor spreading vectors and a new model for hills formed at ultra-fast rates Kelly A. Kriner, Robert A. Pockalny ⁎ , Roger L. Larson Graduate School of Oceanography, University of Rhode Island, Horn Laboratory, South Ferry Road; Narragansett, RI 02882, United States Received 28 July 2004; received in revised form 28 April 2005; accepted 2 May 2005 Available online 18 January 2006 Editor: R.D. van der Hilst Abstract 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. © 2005 Elsevier B.V. All rights reserved. Keywords: Abyssal hills; seafloor morphology; plate tectonics; mid-ocean ridges; slope analysis 1. Introduction Abyssal hills constitute ∼80% of the seafloor and thus are the most common landform on Earth [1]. The morphology of abyssal hills is best observed on relatively young ocean crust flanking mid-ocean ridges where sediment accumulation has not masked their appearance. In these settings, the hills are elongate parallel to the ridge axis, have relief of 50–500 m, and characteristic width of 1–10 km [2]. The proximity of abyssal hills to the ridge axis suggests these features are the result of the interplay between tectonic and volcanic processes occurring within the plate boundary zone. Once formed, the hills are transported onto the ridge flanks by plate motion and their basement morphology is preserved. Since the early days of seafloor mapping, it has been realized the general morphology of abyssal hills varies Earth and Planetary Science Letters 242 (2006) 98 – 110 www.elsevier.com/locate/epsl ⁎ Corresponding author. Tel.: +1 401 874 6926; fax: +1 401 874 6811. E-mail address: robp@gso.uri.edu (R.A. Pockalny). 0012-821X/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2005.05.046