Isostasy, magma chambers, and plate driving forces on the East Pacific Rise

TLDR: In this paper, the density distribution within a cooling plate is calculated for various degrees of isostatic compensation and the pressure field within the plate has a horizontal gradient at shallow depths away from zero age and a vertical gradient toward zero age at greater depths caused by the loading of the ocean.

Abstract: The density distribution within a cooling plate is calculated, which incorporates temperature and pressure effects. From this density distribution the pressure field within the plate and the gravity field at sea level are computed for various degrees of isostatic compensation. In this model the pressure field within the plate has a horizontal gradient at shallow depths away from zero age and a horizontal gradient toward zero age at greater depths caused by the loading of the ocean. Isostatic equilibrium is approached if one allows the loading due to the water to depress the seafloor and at the same time allows mass conservation by flow at depth toward zero age. A viscosity model based on a Newtonian rheology which included temperature and pressure effects has a high gradient close to the plane separating positive and negative pressure gradients which would facilitate the return flow and decouple the lithosphere from the asthenosphere. Addition of a crust to the homogeneous model does not substantially change these conclusions. Comparison of this model with examples of East Pacific Rise data suggests that some areas may not be in complete isostatic equilibrium, implying the existence of horizontal pressure gradients toward zero age in the asthenosphere. This model can be made to fit the general features of the East Pacific Rise but not the detailed gravity and topography near zero age. If one allows convective cooling of the crust by water, partial melting of the upper mantle, and intrusion of this partial melt into the crust, the water depths increase more rapidly near zero age, and an increased positive gravity anomaly is produced over the rise axis, both of which produce a better fit to East Pacific Rise data at 12°N. This study suggests that crustal magma chambers on the East Pacific Rise may be associated with anomalous positive gravity anomalies caused by a positive density contrast between magma and fractured porous crustal rocks.