BENEATH THE TOPOGRAPHY: COMPARATIVE U/PB AND 40AR/39AR THERMOCHRONOLOGY OF THE EXHUMED GRENVILLIAN CRUST OF NORTH AND SOUTH AMERICA AND IMPLICATIONS FOR OROGENIC STRUCTURE

The thermochronologic record from the ductile roots of mountain belts can be established for a wide range of temperatures. The combined use of U/Pb and 40Ar/39Ar dating of minerals with medium-to-high blocking temperatures can be used to establish a detailed cooling record for the middle and lower crust (10-40 km). Given the time required for these crustal levels to be exhumed to the surface, high and middle temperature geochronometers find widest application to older orogens. The exhumation rate (cooling rate divided by geothermal gradient) can be used to determine the paleodepth at a given time. Two assumptions are implicit to this exercise: first, a uniform geothermal gradient (verifiable using P-T conditions established from metamorphic assemblages); and second, short linear segments can be used to approximate the cooling curve in the immediate wake of crustal thickening and metamorphism. The advantage of the technique is that a large array of geochronological data with a wide range of blocking temperatures is reduced to a common dimension: depth. We apply this technique to new and compiled data from a restored transect across the Laurentian and Amazonian Grenville belts in a classic Rodinia reconstruction. Clear differences in the calculated paleodepths at 1.0 Ga across the restored orogen demonstrate that exhumation of the North American Grenville province level has proceeded to a greater degree than the South American counterpart. We suggest that the level of exhumation reflects a fundamental asymmetry in a collisional orogenic belt, with greater crustal thickening occurring in North America with strike-slip faulting largely accommodating strain in ancestral South America.