Bathymetric Constraints on Dynamic Topography and Mantle Flow from Asymmetric Subsidence Across the Mid-Ocean Ridges

Abstract:

Although stresses from convective flow in the mantle should deflect Earth’s surface vertically, the amplitude of such deflections remains unclear despite their importance for historic shoreline and sea-level reconstruction. Indeed, obtaining topographic constraints on this dynamically-supported topography is challenging, especially on continents where isostatic topography is enlarged by the long and complicated history of tectonic deformation. In contrast, the well-understood half-space cooling of young seafloor offers a relatively weak filter for the signal of dynamic topography. In this study we attempt to constrain both the sources and scales of dynamic topography by comparing deviations in unperturbed seafloor bathymetry with predictions of dynamic topography from a numerical mantle flow model. Here we define unperturbed seafloor as regions free from the direct influence of volcanic hotspots, oceanic plateaus, tectonic plate boundaries, or sub-lithospheric thermal alteration. We identify dynamic deflections of the seafloor by focusing on regions where normal seafloor resides on both sides of a mid-ocean ridge, specifically across the East Pacific Rise and the Mid-Atlantic Ridge south of the Icelandic hotspot. Across both mid-ocean ridges the magnitude of the subsidence is greater in the direction of South America than away from it, which is consistent with long-wavelength dynamic subsidence of the seafloor above subduction-induced mantle downwelling. We compared models of dynamic topography driven by both negative and positive density heterogeneity (which induce upwelling and downwelling, respectively) in both the upper and lower mantle to the asymmetrical seafloor subsidence across these mid-ocean ridges. We used spherical harmonics up to degree eight for each model prediction of dynamic topography, and found that peak-to-peak amplitudes of 700-1200m sourced mainly from density contrasts in the lower mantle best explain the observed ridge asymmetries. Dynamic topography of this magnitude and pattern is best explained as being supported by active downwelling associated with subducted slabs in the lower mantle beneath eastern Asia and the Americas, without significant contribution from active upwelling driven by the seismically slow regions beneath Africa and the central Pacific.