Introduction: rocks as flight recorders
Metamorphic rocks can be thought of as flight recorders, black boxes recovered from the wreckage of an orogen that tell us something of that orogen's history – primarily the pressure or depth (P), temperatures (T), and times (τ) of the journey. P, T and τ are linked by the physics of heat transfer and the thermodynamics and kinetics of possible transformations in the rocks mass. For a simplified one-dimensional rock mass, this relation can be visualized as a surface in depth(= P)–T–τ space (Fig. 12.1 (A)). P–T–τ paths of rocks are lines on this surface. Tectonic histories (P–τ paths) studied by the structural geologist, P–T loops determined by the metamorphic petrologist, and the T–τ histories examined by the geochronologist are all projections of the P–T–τ path onto the appropriate surface (Fig. 12.1(B)).
Forward modelling of the thermal response to tectonism (for example, England & Thompson, 1984, Haugerud, 1986) illustrates the kinds of P–T–τ paths rocks must follow for certain tectonic scenarios and heat-transfer mechanisms. Much of metamorphic petrology addresses the inverse problem: determining P and T from rocks and from these data inferring tectonic history. When combined with radiometrically or kinetically determined τ, the resulting data set is directly comparable to a P–T–τ path obtained by forward modelling. Combining the forward and inverse approaches to metamorphic path studies promises to extend our understanding of orogenic history.