Rifts and swells: Geophysical constraints on causality

Abstract

Analyses are developed to examine whether rifting can generate broad topographic swells and whether swells formed by lithospheric thinning can generate rifts. Although swells can be supported by deep lithospheric cracks filled with low-density intrusions, observed topography and gravity constrain how this might occur. Single deep dikes apparently cannot explain the width of swells, the low-gravity gradients on swell flanks, or the lack of a large negative free-air anomaly over swell crests. If swells are supported by deep cracks, there must be numerous narrow dikes distributed over the full breadth of each swell. Comparing the hydrostatic pressure of various columns of lithosphere to that of an ocean ridge crest suggests the conditions under which a swell might spread apart. Oceanic basins are under deviatoric horizontal compression due to ridge push. The uplift caused by lithospheric thinning will reduce this compression, but because swells never rise higher than ridge crests oceanic swells should not develop deviatoric tensile stresses. Continental platforms are under less compression, so formation of a continental swell can generate significant rifting forces.