Causes of Oceanic Crustal Thickness Oscillations Along a 74‐M Mid‐Atlantic Ridge Flow Line

Abstract

Gravity, magnetic, and bathymetry data collected along a continuous 1,400-km-long spreading-parallel flow line across the Mid-Atlantic Ridge indicate significant tectonic and magmatic fluctuations in the formation of oceanic crust over a range of time scales. The transect spans from 28 Ma on the African Plate to 74 Ma on the North American plate, crossing the Mid-Atlantic Ridge at 35.8°N. Gravity-derived crustal thicknesses vary from 3-9 km with a standard deviation of 1.0 km. Spectral analysis of bathymetry and residual mantle Bouguer anomaly show a diffuse power at >1 Myr and concurrent peaks at 390, 550, and 950 kyr. Large-scale (>10 km) mantle thermal and compositional heterogeneities, variations in upper mantle flow, and detachment faulting likely generate the >1 Myr diffuse power. The 550-and 950-kyr peaks may reflect the presence of magma solitons and/or regularly spaced~7.7 and 13.3 km short-wavelength mantle compositional heterogeneities. The 390-kyr spectral peak corresponds to the characteristic spacing of faults along the flow line. Fault spacing also varies over longer periods (>10 Myr ), which we interpret as reflecting long-lived changes in the fraction of tectonically versus magmatically accommodated extensional strain. A newly discovered off-axis oceanic core complex (Kafka Dome) found at 8 Ma on the African plate further suggests extended time periods of tectonically-dominated plate separation. Fault spacing negatively correlates with gravity-derived crustal thickness, supporting a strong link between magma input and fault style at mid-ocean ridges.

Time‐Dependent Crustal Accretion on the Southeast Indian Ridge Revealed by Malaysia Airlines Flight MH370 Search

“…The observed characteristic timescale of 300–400 kyr is consistent with the hundreds of kyr‐long crustal accretion cycles observed along single crustal profiles at the MAR (Canales et al, 2000; Shinevar et al, 2019) but shorter than the ~3 Myr cycles reported by Bonatti et al (2003). Below this corner period, the shorter‐period component of the bathymetric spectra (i.e., shorter wavelength and higher frequency, to the right in Figures 2d and 2e) deviates considerably from the MBA spectra, providing evidence that this short‐period signal represents the faulted component of bathymetry.…”

“…This spacing is consistent with the characteristic timescale of 300–400 kyr revealed by our bathymetric spectra, implying accretion cycles on the same timescale. Relative crustal thickness, derived from satellite free‐air gravity anomaly, does not show any resolvable dependence upon M (Figure S5), as similar to findings on the Mid‐Atlantic Ridge (Shinevar et al, 2019). The limited range in M values here (0.85 to 1.0) may preclude this relationship from being evident.…”

“…This result supports the notion that there is a delicate balance between magma supply and crustal accretion style; however, since this single profile is restricted to within 20 km (or ~1.6 Myr) of the ridge axis, it is difficult to apply more generally. Crustal thickness variability on timescales between 100 kyr and 1 Myr were also inferred from a 74 Myr‐long gravity and bathymetry survey across the Mid‐Atlantic Ridge (Shinevar et al, 2019). These authors noted, however, that the thick sediment cover of older abyssal hills could obscure the relationship between seafloor characteristics and crustal geometry.…”

“Empirical Prewhitening” Spectral Analysis Detects Periodic but Inconsistent Signals in Abyssal Hill Morphology at the Southern East Pacific Rise

“…The most significant harmonic contributions to abyssal hills, typically with RMS heights around a quarter to a third of the aperiodic RMS, exhibit periods that are larger than the characteristic time. Such variations in bathymetry could plausible be associated with variations in crustal thickness associated with temporal variations in magma supply (Canales et al., 2000; Parnell‐Turner et al., 2020; Shinevar et al., 2019). Parnell‐Turner et al.…”

“…Vaughan (2005), for example, formulated a methodology for detecting periodicities in the presence of a red (negative trend with frequency) background noise, which can be fit with simple power law, and where the null hypothesis variability about this trend is characterized by a χ 2 distribution. Vaughan (2005) noted, however, that the modeling fit itself adds additional uncertainty, leading him to strongly suggest that confidence levels 3–4 times the standard deviation (σ) should be applied when determining the significance of a spectral peak in indicating a periodic component of a mixed spectrum, rather than the oft‐assumed 2σ confidence level (Boulahanis et al., 2020; Crowley et al., 2015; Shinevar et al., 2019).…”