Pressure-to-depth conversion models for metamorphic rocks: derivation
and applications
Abstract
Pressure-to-depth conversion is a crucial step towards geodynamic
reconstruction but remains strongly debated. Here, we derive
pressure-to-depth conversion models using either one or two pressure
data points in conjunction. In the two-point method, we assume that both
peak and retrograde pressure are recorded at the same depth. This method
reduces the depth estimate uncertainty dramatically. We apply the
proposed pressure-to-depth conversions to a large set of $P$ data from
(ultra)high-pressure metamorphic rocks. We explore different cases to
explain the transition from peak to retrograde pressure by varying the
direction and magnitude of stress components. Our results show that (1)
even small deviatoric stresses have a significant impact on depth
estimates, (2) the second principal stress component
$\sigma_2$ plays an essential role, (3) several models
can explain the $P$ evolution of the data but lead to different depth
estimates, and (4) strain data offer a means to falsify two-point
models. The most commonly used pressure-to-depth conversion method uses
one pressure point and the assumption that pressure is lithostatic.
Then, the transition from peak to retrograde pressure is interpreted as
the result of deep subduction ($>100$ km), followed by
fast exhumation to mid-crustal depth. We show that alternative models
where a change in the stress state at a constant depth triggers the
pressure transition explain the data equally well. The predicted depth
is then shallower than the crustal root Moho ($<75$ km) for
all data points.