LOCAL DISPLACEMENT RATE CYCLES IN THE LIFE OF A FOLD-THRUST BELT

The evolution of minor structures during the growth of major folds and thrusts, in the Chartreuse district of the French Subalpine thrust be lt, shows that each thrust evolved through a phase of distributed faul ting, major thrust propagation and displacement, followed by distribut ed shear modification of the hanging-wall fold. Microstructural studie s suggest that the distributed faulting phases, early and late in the history, were characterized by strain rates limited by diffusive mass transfer processes (c. 10(-15)-10(-16) s(-1)). Faulting whose rate is limited by DMT is too slow on its own to accommodate the regional time -averaged shortening rates for the thrust belt as a whole, implying th at the slow thrusts operated in tandem with those major, fast thrusts where deformation was primarily cataclastic. Consequently each thrust anticline experienced a displacement rate cycle and an array of thrust anticlines must amplify simultaneously. These interpretations raise i mportant issues for the dynamics of fault populations, the evolution o f thrust wedges and the history of fluid migration in thrust belts.