Periodic but inconsistent signals detected in abyssal hill profiles at the southern East Pacific Rise: Possible indicators of internal melt-supply fluctuations that overprint random fault-generated topography

Abstract:

The existence, or not, of periodicities in abyssal hill morphology has been vigorously debated in recent publications, and some have hypothesized that such periodicities are evidence of the impact of Milankovitch cycle-caused sea level fluctuations on the volcanic construction process at mid-ocean ridges. Periodicities are detected by the presence of spectral peaks that rise significantly above the random variations of sample power spectra associated with an aperiodic, continuous spectrum process, typically modeled as a band-limited fractal (von Kármán model). Here I formulate a new algorithm, based on the “P(λ) test” of M. B. Priestly, to “empirically pre-whiten” the sample power spectrum, which, without needing to model the continuous spectrum, flattens it to a zero-mean process. This greatly simplifies definition of the null hypothesis, and additional modeling approximates standard deviation levels that provide a conservative basis for detecting peaks that may be indicative of periodicity. I first test the algorithm on synthetic profiles generated by adding cosine functions of various amplitude and frequency to an aperiodic time series corresponding to the von Kármán statistical model. The algorithm is able to accurately detect the periodic components provided the amplitude is sufficient to rise significantly above the random fluctuations associated with the aperiodic component of the spectrum at that frequency. The algorithm is then applied to extensive bathymetric data flanking the southern East Pacific Rise referenced to a recently updated crustal age model. Significant periodicities are detected on many profiles analyzed, but the periods vary widely, and do not cluster at Milankovitch periods. The most substantial harmonic signals detected exhibit periods ~0.082-0.216 my, with root-mean square (RMS) heights approximately a quarter to a third of the RMS height for the aperiodic component of the profiles. It is hypothesized that the dominant aperiodic component of abyssal hills corresponds to morphology constructed by faults that follow a random distribution governed by scaling laws, whereas longer-scale periodic signals are associated with crustal thickness variations controlled internally by variations in melt supply.