229-3 In Defense of Petrological Depth Estimates

Session: Crustal Petrology (Posters)

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Abstract:

Petrological determinations of pressure-temperature conditions of metamorphism have long been used to estimate depths of burial, based on the assumption that metamorphic pressure corresponds approximately to the lithostatic load. This assumption comes under fire periodically, for one or more of the following reasons. 1) 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. Under metamorphic conditions, most crustal rocks cannot sustain differential stresses greater than ~200 MPa, which limits tectonic overpressure to ~100 MPa, within the uncertainties of thermobarometric measurements. Bodies of unusually strong rock within a deforming orogen may experience exceptional loads due to the particular geometry of the system, but this is likely to be limited both spatially and temporally. 2) 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 concept has been referred to as dynamic overpressure, and has been suggested to reach values of 2-3 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 is is unlikely to exceed ~120 MPa. At greater depths, and in oceanic subduction zones, higher values are possible, but will likely be limited by elastic flexure of the overlying lithosphere. 3) 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. Another suggestion is that metamorphic reactions proceeding by a dissolution/precipitation process during deformation may take place at pressures substantially different from equilibrium conditions. These suggestions are based on flawed arguments or unrealistic assumptions. Pressures inferred from regionally developed metamorphic mineral assemblages can therefore be used be used to infer depths of metamorphism with an uncertainty of approximately ±4 km.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-7364

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