Formation of seafloor swells by mantle plumes

Abstract

The prominent oceanic hot spots are located at the crest of broad topographic swells on the seafloor. Available evidence indicates that swells are supported by anomalously thin, or altered, lithosphere as might result from the intrusion of a mantle plume into the lithosphere. A set of experiments have been made to measure the free surface uplift and subsidence produced by intrusion of a low‐viscosity, buoyant plume into a rheological boundary layer. A deep layer of strongly thermoviscous sugar solution with a free surface was cooled from above to simulate the lithosphere‐asthenosphere system. Low‐viscosity, low‐density plumes were released at depth. The free surface displacement generated by the ascending plume was recorded as a function of time using a proximity measurement system. The plumes ascend through the asthenosphere as nearly spherical diapirs and collapse (flatten) upon intrusion into the base of the lithosphere. Evolution of the surface topography can be divided into three phases: (1) a low axisymmetric rise supported by the plume deep beneath the lithosphere, (2) rapid uplift and steepening as the plume reaches the lithosphere base, and (3) slow axial subsidence as the plume stagnates within the lithosphere. The measured uplifts are compared with the uplift calculated for buoyant plumes in a uniform viscous half‐space, using Morgan's image method. The agreement is good for thin lithosphere. For thick lithosphere the observed is less than the calculated uplift. Scaling laws have been derived which relate the measured maximum uplift and maximum uplift rate to diapir parameters. Extrapolation of these to the scale of ocean floor swells gives uplift rates of approximately 1 km in 5 m.y. by this mechanism. The most significant qualitative results of the experiments are that swells formed by interaction of low‐viscosity mantle plumes and oceanic lithosphere can be characterized by axial symmetry, rapid uplift, and shallow compensation.