EPISODIC METAMORPHISM AND DEFORMATION IN LOW-PRESSURE, HIGH-TEMPERATURE TERRANES

In many cases, low-pressure, high-temperature metamorphism must be the consequence of advective heat transfer, e.g., the ascent of granitic magma. Whereas such heating mechanisms are necessarily short-lived, th e lifetime of the underlying orogenic processes is likely to be much l onger, and it is conceivable that, during the evolution of an orogen, repeated transient heating at shallow crustal levels is caused by epis odes of segregation and upward passage of melts generated at deep crus tal levels. The number and timing of these events would depend on such factors as critical melt segregation volume, Moho temperature, and st rain-rate evolution. We investigate some of the controls on this multi ple-event superposition using a simple thermomechanical model designed to predict, simultaneously, the strain-rate and the isostatic and the rmal evolution of convergent orogenesis, subject to a constant driving force. An appealing aspect of this formulation is that, in contrast t o models that have attempted to explain event cyclicity through episod ic processes acting on the orogenic boundaries, it accounts for the re peated occurrence of events through processes inherent to the orogenic system itself.