CRUSTAL SHORTENING AND EOCENE EXTENSION IN THE SOUTHEASTERN CANADIAN CORDILLERA - SOME THERMAL AND RHEOLOGICAL CONSIDERATIONS

Metamorphic core complexes in the southeastern Canadian Cordillera wer e formed during Eocene crustal extension, shortly (within a few millio ns of years) after Late Jurassic-Paleocene crustal shortening. Thermal -rheological modeling, constrained by geological and geochronological studies of the Valhalla core complex and other core complexes in this region, is used to investigate two major problems concerning the forma tion of these core complexes: (1) the dynamic links between crustal sh ortening and extension and (2) the cooling history and unroofing rates during extension. Thermal-rheological effects associated with crustal shortening are integrated through the history of crustal compression, since crustal shortening in this region was a long and slow process a nd cannot be treated as an instantaneous event. Our results suggest th at crustal shortening may have played an important role in Eocene exte nsion in the southeastern Canadian Cordillera by (1) producing a thick ened and therefore unstable crust and (2) thermally weakening the lith osphere. However, heat generated by crustal shortening is not enough t o account for the thermal state of the Valhalla complex, and additiona l heat sources at depth may be necessary. We then investigate thermal evolution during extension in both a simple shear model and a progress ive pure stretching model. We show that the geotherm in an extensional region is time- and space-dependent and is affected by many variables including the preextensional history and the mode of extension. Thus caution needs to be exercised when inferring unroofing rates from ther mochronologic data. The cooling history of the Valhalla core complex m ay be explained by unroofing at rates of 1-2 mm/yr.