TRANSIENT AND LONG-TERM CORRESPONDENCE OF EROSION LEVEL AND THE TOPS OF GRANITE PLUTONS

Granites, used in the sense of granitoids, are characteristically conf ined to continental crust and the bottoms of granite plutons are rarel y exposed. Indeed the roots of a major granite batholith have yet to b e unequivocally described as far as we know. In contrast the tops and upper parts of numerous granite plutons have been described as they co mmonly lie near to the present level of erosion of the continental cru st. Some regions have so many exposed tops that it is unlikely to be a chance coincidence of erosion level and granite exposure. Such situat ions are of two general types (1) Long Term. Stabilized crust in which the tops of plutons oscillate about +/-3 km of sea-level for hundreds of millions of years e.g. the 400-470 Ma Caledonide plutons of N. Bri tain and Ireland (2) Transient. Unstabilized crust in which relatively young granites form topographically high plutons in which erosion mig ht cut down into some of the plutons within tens of millions of years e.g. the Cretaceous and Tertiary plutons of Peru and the Sierra Nevada , Western USA. At least some of the latter group must eventually pass into the former group possibly by structural lowering e.g. due to crus tal extension. The main control appears to be isostatic stabilization accompanying and following plutonism, combined with the final consolid ation of granites in a narrow layer below the prevailing topography be cause of termination of crystallization by a topographically related w ater table.