Non-Pratt component of oceanic isostasy

Abstract

Oceanic lithosphere is understood to cool and subside away from mid-ocean ridges to a Pratt-like isostasy condition. However, the pres- ence 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 fl ow in the asthenosphere to accommodate the excess mass accumulated on top. The basic mathematics behind the isostasy- driven asthenospheric fl ow demonstrate that the fl ow occurs systematically from beneath younger seafl oor toward older seafl oor. The fl ow rate peaks beneath seafl oor of about one-quarter the plate age. The maximum fl ow rate is a few tenths of a percent of the plate rate and is superimposed on the regional asthenospheric fl ow fi eld. The modifi cation to the regional fl ow 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.