Pressure-Temperature-Time Paths of Regional Metamorphism I. Heat Transfer during the Evolution of Regions of Thickened Continental Crust

The development of regional metamorphism in areas of thickened continental crust is investigated in terms of the major controls on regional-scale thermal regimes. These are: the total radiogenic heat supply within the thickened crust, the supply of heat from the mantle, the thermal conductivity of the medium and the length and time scales of erosion of the continental crust. The orogenic episode is regarded as consisting of a relatively rapid phase of crustal thickening, during which little temperature change occurs in individual rocks, followed by a lengthier phase of erosion, at the end of which the crust is at its original thickness. The principal features of pressure-temperature-time {PTt) paths followed by rocks in this environment are a period of thermal relaxation, during which the temperature rises towards the higher geotherm that would be supported by the thickened crust, followed by a period of cooling as the rock approaches the cold land surface. The temperature increase that occurs is governed by the degree of thickening of the crust, its conductivity and the time that elapses before the rock is exhumed sufficiently to be affected by the proximity of the cold upper boundary. For much of the parameter range considered, the heating phase encompasses a considerable portion of the exhumation (decompression) part of the PTt path. In addition to the detailed calculation of PTt paths we present an idealized model of the thickening and exhumation process, which may be used to make simple calculations of the amount of heating to be expected during a given thickening and exhumation episode and of the depth at which a rock will start to cool on its ascent path. An important feature of these PTt paths is that most of them lie within 50 °C of the maximum temperature attained for one third or more of the total duration of their burial and uplift, and for a geologically plausible range of erosion rates the rocks do not begin to cool until they have completed 20 to 40 per cent of the total uplift they experience. Considerable melting of the continental crust is a likely consequence of thickening of crust with an average continental geotherm. A companion paper discusses these results in the context of attempts to use metamorphic petrology data to give information on tectonic processes. 1. I N T R O D U C T I O N Observations of metamorphic rocks exposed over regional extents form one of the important classes of data on which our present understanding of the evolution of orogenic belts is based. This understanding is at best patchy, owing to the many combinations of horizontal and vertical motions that may occur during the lifetime of an orogenic belt and to the complex, or sometimes merely insensitive, response to these processes of the geological indicators from which histories of strain, deviatoric stress, pressure or temperature may be inferred. In this paper we discuss the problems of inferring one class of these histories, the pressure-temperature-time (PTt) path followed by a rock during regional metamorphism; more specifically, we concentrate on the PTt paths resulting from the burial and exhumation of rocks during episodes of continental thickening. UournaJ of Petrotojy. VoL 25, P in 4, pp. 894-928, 1984] at U niersite C lude B enard (L on 1) on M arch 2, 2015 http://petroxfordjournals.org/ D ow nladed from PRESSURE-TEMPERATURE-TIME PATHS. I 895 Heat and mass transfer during the development of an orogenic belt may involve the thickening of continental crust by thrusting or magma addition, the thinning of crust by intra-plate rifting or the sliding of nappes under gravity; heat may be supplied to the crust by magmas generated from partial melting of the mantle, may be transferred within the crust by conduction or by the motion of fluids, and is generated to a significant extent by the decay of radioactive isotopes within the continental crust. In understanding the development of an individual orogenic belt, and the evolution of the continental crust as a whole, it is desirable to know the extent to which these, and other, processes have influenced the thermal history of the rocks that are now exposed in regional metamorphic terrains. It has long been recognized that PT data derived from mineral assemblages are, on their own, of limited use in this endeavour, and certainly cannot be used to infer directly, say, the ambient geothermal gradient during metamorphism. Thus the problem divides into two portions, of which the first is to determine the likely PTt paths that rocks may experience in given tectonic environments, and the second is to find, if possible, features of these paths that are diagnostic of particular processes and are likely to be recorded in the mineral assemblages when the rock finally reaches the surface. Rocks that record mineral geobarometric pressures of several kilobars are widespread on the surface of crust of normal continental thickness; this, with geophysical evidence for greatly thickened crust in regions of present day orogenic activity implies that crustal thickening is of major importance in orogenic evolution. Several models, of varying complexity, have been constructed to describe the thermal regimes of regions of thickened continental crust (e.g. Oxburgh & Turcotte, 1974; Bickle et al., 1975; Bird et al, 1975; England & Richardson, 1977; Toksoz & Bird, 1977; England, 1978; Richardson & England, 1979) but most of these have been tied to specific geological areas and often the full range of variability of the parameters to the models has not been considered. In the following sections we construct a system that we believe may contain the principal elements dictating the thermal development of thickened continental crust and investigate the PTt paths that may be followed by rocks undergoing burial and exhumation during an orogenic episode. A companion paper (Thompson & England, 1984, hereafter referred to as Part II) deals with the petrological problems of inferring such paths from the rocks preserved at the land surface.