REFLECTION SEISMIC EVIDENCE FOR EXTENSIVE SYN- TO LATE-OROGENIC MELTS IN GRENVILLE OROGEN OF EASTERN CANADA: READING BETWEEN THE LINES IN SEISMIC DATA

The Grenville Orogen of Eastern Canada represents the mid- to lower-crustal remnants of a Himalayan-scale, hot, collisional orogen. New statistical approaches to interpretation of seismic reflection data from the Grenville orogen provide images that contain strong evidence of extensive syn- to late orogenic migmatization and/or melt emplacement in the middle to lower crust. The interpretation technique is based on classification of the geometric and scaling properties of the reflection wave field which are arguably related to meso-scale petrofabrics. Zones in the seismic images in which fabrics associated with terrane accretion and continent-continent collision are disrupted or destroyed are interpreted as resulting from either re-scaling of the fabric due to migmatization or wholesale destruction of the fabric by intrusion. On the basis of this interpretation, the seismic data map intrusive bodies or migmatite terranes ranging in lateral scale from 10 to 60 km with relatively complex geometries and evidence for the incorporation of large blocks of the country rock. The timing of the interpreted melting event is not well constrained by surface dates but, cross-cutting relations in the seismic data suggest either the latest stage (Rigolet, 1000-980 Ma) of Grenville continent-continent collision or alternatively an association with a post-collision extension evident in the seismic data.

Although the seismic data are sparse with respect to the area of the Grenville Orogen, the new images suggest that the process that produced melt operated for at least 500 km along the strike of the orogen and must have exerted a major influence on mechanics and architecture of the late stages of Grenville orogenic development. The lack of surface dates consistent with the timing inferred from the seismic data enforces the notion that some parts of the continental crust young downward and significant episodes in the evolutionary history of an orogenic belt may be under-represented at the surface.