A BRIEF SUMMARY OF THE SEISMIC CHARACTERISTICS OF CRATONIC MANTLE LITHOSPHERE

Tomographic images of the Earth’s upper mantle demonstrate a correspondence between cratonic crust at the surface and fast seismic wave speeds that extend to depths of ~150-200 km. These high velocities are thought to indicate the presence of cold, thick mantle lithosphere. This relationship presents a paradox as cold temperatures should increase density, reduce buoyancy and increase instability within the lithosphere. However, the cratonic lithosphere appears to be convectively stable, indicative of a rheology capable of withstanding the tectonic demolition that more commonly occurs along continental margins. Why cratonic lithosphere appears to be seismically and rheologically distinct from lithosphere elsewhere may be related to processes that originally formed it. In the past decade, mounting evidence has pointed to complex, layered structure within cratons and shields at depths of ~80-150 km. Sometimes referred to as mid-lithospheric discontinuities (MLDs), the origin of intra-lithospheric layering has prompted considerable discussion, particularly as to how they may result from craton formation and/or evolution.

While a number of mechanisms, such as the presence of melt or anisotropy, have been proposed to explain the origin of MLDs, the presence of hydrous minerals, such as amphibole or phlogopite, is perhaps the currently favored explanation. While cratons are nominally dehydrated, which is required for them to maintain their convective stability, the presence of hydrous minerals at mid-lithospheric depths corresponds well to seismic observations and it is hypothesized that these hydrous minerals may be the result of slab-derived volatile-bearing phases which interacted with the overlying lithosphere during ancient subduction. Evidence from xenoliths in Australia, one region where MLDs are prevalent, indicates that in some instances metasomatism of the cratonic lithosphere may have occurred prior to 2 Ga. Another intriguing hypothesis is that MDs formed as a result of cratonic thickening during the closure of ocean basins in the Archean and Proterozoic. While a third hypothesis, supported by observations of dipping MLDs, is that the discontinuities are themselves remnant subducted slabs and that growth of the cratonic lithosphere happened in part through subduction-related thickening.