Implications of hydrostatic pore pressures and high crustal strength for the deformation of intraplate lithosphere

Observations from deep boreholes at several locations worldwide indicate th at (i) hydrostatic pore pressures persist to depths of as much as 12 km in the upper crust, (ii) the brittle crust is in a state of failure equilibriu m according to Coulomb frictional-failure theory, and (iii) bulk permeabili ty is high - 10(-17)-10(-16) m(2) - apparently due to fluid flow along crit ically stressed faults. As a result of these factors, the brittle crust is stronger than it would be under near-litho static pore pressure conditions. This result provides a constraint on models of lithospheric deformation. P ostulating that the upper and lower crust and lithospheric mantle are total ly coupled and that the total strength of the lithosphere is equal to the m agnitude of tectonic driving forces (similar to3 X 10(12) N m(-1)), we have calculated lithospheric strain rates under representative thermal and rheo logical conditions such that the integrated differential stress over the en tire thickness of the lithosphere equals the plate driving force. For a str ike-slip stress state and surface heat flow of 60 +/- 6 mW m(-2), average s train rates are approximately 10(-18) s(-1) under hydrostatic upper crustal pore pressure conditions, and approximately 10(-15) s(-1) under near-litho static pore pressures. The latter strain rates are higher than either obser ved geodetically using very long baseline interferometry (VLBI), or estimat ed on the basis of plate tectonic reconstructions. Hence we argue that hydr ostatic upper crustal pore pressures enable lithospheric plates to behave r igidly over time scales of tens to hundreds of millions of years. (C) 2001 Elsevier Science B.V. All rights reserved.