ON THE GRAVITATIONAL POTENTIAL OF THE EARTHS LITHOSPHERE

The mean potential energy of the lithosphere Ul(g)BAR is useful for de fining the tectonic reference state (TRS) of the Earth and can be used to constrain the ambient state of stress in the plates. In the absenc e of external forces applied at the base or along plate boundaries a l ithospheric column with the potential energy of the TRS would remain u ndeformed. Thus the difference between the potential energy of a litho spheric column and the TRS determines whether the column is in an exte nsional, neutral, or compressional state of stress. We evaluate U(l)gB AR and intraplate variations about this mean, using a simple, first-or der lithospheric density model. This model assumed that the continenta l geotherm is linear, and density variations below a depth of 125 km h ave negligible influence on U(l)gBAR, and is consistent with observed geoid anomalies across continental margins. U(l)gBAR is estimated to b e 2.379 x 10(14) N m-1, which is equivalent to the potential energy of both near sea level continental lithosphere (-160 to +220 m for an as sumed crustal density, rho(c), in the range 2800 - 2700 kg m-3) and co oling oceanic lithosphere at a depth of 4.3 km. With the exception of Eurasia, which has anomalously high mean potential energy (U(l)pBAR = 2.383 x 10(14) N m-1), the mean potential energies of the continental plates are nearly identical to the global mean U(l)gBAR. The mean pote ntial of the oceanic plates was found to be a strong function of the m ean age of the oceanic lithosphere. Both the global and plate mean pot ential energies are relatively insensitive to a wide range in rho(c). The potential of the mid-ocean ridges (U(l)MOR), 2.391 x 10(14) N m-1, is greater than the global mean, which is consistent with the diverge nt nature of the ridges. Elevated continental lithosphere with a heigh t of about 70 m has an equivalent potential energy to U(l)gBAR, sugges ting that in the absence of external forces, continental regions will be in a slightly extensional state of stress. The importance of our po tential energy formulation is substantiated by the strong correlation between the torque poles associated with the potential energy distribu tions and the observed plate velocity poles for the South American, Na zca, Indo-Australian, and Pacific plates.