Multiscale spectral analysis of bathymetry on the flank of the Mid‐Atlantic Ridge: Modification of the seafloor by mass wasting and sedimentation

Abstract

Abstract. The results of a multiscale spectral analysis of bathymetric data on the flank of the Mid-Atlantic Ridge are described. Data were collected during two cruises using Hydrosweep multibeam (tens of kilometers to -0.2 km scale range) and Mesotech scanning pencil-beam sonar attached to remotely operated vehicle Jason (-1 km to -0.5 m scale range). These data are augmented by visual data which enabled us to identify bathymetric profiles which are over unsedimented or thinly sedimented crust. Our analysis, therefore, is focused primarily on statistical characterization of basement morphology. Work is concentrated at two sites: site B on -24 Ma crust in an outside-corner setting, and site D on -3 Ma crust in an inside-comer setting. At site B we find that an anisotropic, band-limited fractal model (i.e., the "von Kfirmfin" model proposed for abyssal hill morphology by Goffand Jordan [1988]) is not sufficient to describe the full range of scales observed in this study. Our observations differ from this model in two ways: (1) strike and cross-strike (dip) spectral properties converge for wavelengths smaller than -300 m, and (2) in both strike and dip directions the fractal dimension changes at -10 m wavelength, from -1.27 at larger scales to -1.0 at smaller scales. The convergence of strike and dip spectral properties appears to be associated with destruction of ridge-parallel fault scarps by mass wasting, which develops canyon-like incisions that cross scarps at high angles. The change in fractal dimension at -10 m scale appears to be related to a minimum spacing of significant slope breaks associated with scarps which are created by faulting and mass wasting. At site D, although there is no significant abyssal hill anisotropy, the spectral properties at all scales are consistent with the von Kfirmfin model. The fractal dimension at this site (-1.15) is less than at site B. This difference may be reflect different morphology related to crustal formation at inside-corner versus outside-comer position or, more likely, differences in the degree of mass wasting. The smoothing of seafloor morphology by sediments is evident in Hydrosweep periodograms where, relative to basement roughness, spectral power decreases progressively with decreasing wavelength.

Topographic height dependence of internal wave generation by tidal flow over random topography

“…The spectra in Figure 1e for the topography in Figures 1b-1d exhibit power law decay given asymptotically by k −2.5 (dashed line), corresponding to = 2.5, which is consistent with = 2.46 given in Goff and Tucholke [1997]. The spectra in Figure 1e for the topography in Figures 1b-1d exhibit power law decay given asymptotically by k −2.5 (dashed line), corresponding to = 2.5, which is consistent with = 2.46 given in Goff and Tucholke [1997].…”

Monostatic and bistatic reverberation statistics west of the Mid-Atlantic Ridge

“…2͒, at the -scale appropriate to backscatter the roughness is taken as horizontally isotropic ͑see Refs. 15,18,19,and Fig. 3͒.…”

Love-wave excitation due to the interaction between a propagating ocean wave and the sea-bottom topography

“…A small κ increases the PSDF at larger wavenumbers, which makes the fluctuation of the sea bottom rougher. The studies using the von Karman‐type PSDF reported that the value of κ ranges between 0.5 and 1.0 for the sea‐bottom fluctuation (Goff & Jordan 1988, Goff & Tucholke 1997). Substituting into we obtain where we use the scaling h 1 =ɛ 1 a for the sea‐bottom fluctuation.…”