Gravity signature in the mid-ocean ridge-transform system: Insights from deep mantle rheology and shallow crustal structure

Gravity signals over the mid-ocean ridge-transform system reflect the distribution of underlying crustal and upper mantle mass anomalies. The gravity measurement, especially ‘residual’ gravity anomalies, relies on the gravitational corrections of both seafloor relief and lithospheric thermal structure. Lithospheric thermal correction typically uses a 1D plate cooling approximation or a 3D passive flow model that assumes isoviscous mantle rheology. As this rheological approximation is oversimplified and physically complex, how sensitive gravity anomalies are to an increasingly complex/accurate approximation for mantle rheology is still unresolved. Here we systematically examine the residual gravity anomaly discrepancies caused by assumptions of different mantle rheologies on 16 natural ridge-transform systems ranging from ultraslow- to fast-spreading. Our calculations show that estimated residual gravity anomalies are significantly lower (e.g., ~21 mGal lower at mid-ocean ridges) in the isoviscous flow models than in the static plate cooling models, primarily due to the effects of lateral heat advection and conduction. When the assumed mantle rheology is changed from uniform viscosity to a non-Newtonian viscosity with brittle weakening in cooler (faulting) regions, the mantle upwelling intensifies and local near-surface temperature generally increases, resulting in an increase in the residual anomaly. This increase is distributed uniformly along the ultraslow-and slow-spreading ridge axes, but is concentrated along transform faults at intermediate- and fast-spreading ridges. The amount of the rheology-induced gravity difference is most closely linked to transform age offset instead of spreading rate or transform offset length alone. Our analysis reveals that oceanic transform faults exhibit higher gravity anomalies than adjacent fracture zones, which may reflect thinner crust in the transform deformation zone.