On the gravitational potential of the Earth's lithosphere

Abstract

The mean potential energy of the lithosphere is useful for defining the tectonic reference state (TRS) of the Earth and can be used to constrain the ambient state of stress in the plates. In the absence of external forces applied at the base or along plate boundaries a lithospheric column with the potential energy of the TRS would remain undeformed. Thus the difference between the potential energy of a lithospheric column and the TRS determines whether the column is in an extensional, joeutral, or compressional state of stress. We evaluate and intraplate variations about this mean, using a simple, first‐order lithospheric density model. This model assumed that the continental geotherm is linear, and density variations below a depth of 125 km have negligible influence on , and is consistent with observed geoid anomalies across continental margins. is estimated to be 2.379 × 1014 N m−1, which is equivalent to the potential energy of both near sea level continental lithosphere (−160 to +220 m for an assumed crustal density, ρc, in the range 2800–2700 kg m−3) and cooling oceanic lithosphere at a depth of 4.3 km. With the exception of Eurasia, which has anomalously high mean potential energy ( = 2.383 × 1014 N m−1), the mean potential energies of the continental plates are nearly identical to the global mean . The mean potential of the oceanic plates was found to be a strong function of the mean age of the oceanic lithosphere. Both the global and plate mean potential energies are relatively insensitive to a wide range in ρc. The potential of the mid‐ocean ridges ( ), 2.391 × 1014 N m−1, is greater than the global mean, which is consistent with the divergent nature of the ridges. Elevated continental lithosphere with a height of about 70 m has an equivalent potential energy to , suggesting that in the absence of external forces, continental regions will be in a slightly extensional state of stress. The importance of our potential energy formulation is substantiated by the strong correlation between the torque poles associated with the potential energy distributions and the observed plate velocity poles for the South American, Nazca, Indo‐Australian, and Pacific plates.