Oceanic lithosphere is understood to cool and subside away from mid-ocean ridges to a Pratt-like isostasy condition. However, the presence of ocean mass added on top of subsiding lithosphere necessitates an additional isostatic response that cannot be achieved through a Pratt model. In a manner similar to an ice cap on a continent, the addition of ocean mass on top of subsiding lithosphere drives a small degree of flow in the asthenosphere to accommodate the excess mass accumulated on top. The basic mathematics behind the isostasy-driven asthenospheric flow demonstrate that the flow occurs systematically from beneath younger seafloor toward older seafloor. The flow rate peaks beneath seafloor of about one-quarter the plate age. The maximum flow rate is a few tenths of a percent of the plate rate and is superimposed on the regional asthenospheric flow field. The modification to the regional flow induces a small, but systematically positive drag (mantle push) component on the overlying lithosphere. The form of the drag predisposes young lithosphere to extensional stress and older lithosphere to compressional stress.
- Received 29 May 2012.
- Revision received 24 June 2012.
- Accepted 6 July 2012.
- © 2012 Geological Society of America