In Defence of Petrological Depth Estimates

ABSTRACT Petrological determinations of pressure–temperature conditions of metamorphism are commonly used to estimate depths of burial, based on the assumption that metamorphic pressure corresponds approximately to the lithostatic load. This assumption has been challenged for several reasons. Differential stress associated with rock deformation means that the mean stress (which may correspond to the pressure in the solid medium) differs from the vertical normal stress (which is likely to be the lithostatic load). This concept, known as tectonic overpressure , is mechanically sound and geologically reasonable but is limited by rock strength. During viscous flow of rock in a confined channel, such as a subduction channel, the pressure in the fluid may exceed the pressure outside the channel if flow is constricted, where the channel walls converge, for example. This dynamic overpressure has been suggested to reach values of > 1.5 GPa. It is limited, however, by the ability of the channel walls to contain it, which at depths of < 60 km in continental collision zones means that it is unlikely to exceed ~120 MPa. It has been suggested that at the grain‐scale, crystalline aggregates can exert very high pressures within their interiors by a mechanism analogous to a diamond‐anvil cell. The analogy is not valid. Elastic modelling of low viscosity inclusions in a higher viscosity matrix suggests that they may be overpressured, depending on the orientation of the inclusion with respect to the externally imposed stresses. This mechanism requires unrealistically high levels of differential stress in the matrix to produce significant overpressures in the inclusion. It has been proposed that ductile shear zones are likely to be overpressured because the differential stress within them is much lower than in their surroundings. This idea is based on the mistaken assumption that the strain rate in the shear zone is no higher than in the surroundings, which by definition is incorrect. The model also requires the shear zone to form at a high angle to the maximum compressive stress in the surroundings, which is mechanically unlikely. Another suggestion is that metamorphic reactions proceeding by a dissolution/precipitation process during deformation may take place at pressures substantially different from equilibrium conditions. This is based on a flawed argument. Pressures inferred from regionally developed metamorphic mineral assemblages can therefore be used to infer depths of metamorphism with an uncertainty of approximately ±4 km.