LITHOPROBE REFLECTION STUDIES OF ARCHEAN AND PROTEROZOIC CRUST IN CANADA

LITHOPROBE, Canada's national collaborative earth science research pro ject established to develop a comprehensive understanding of the evolu tion of the North American continent, is a multidisciplinary program s pearheaded by seismic reflection studies. Five recently recorded seism ic lines, discussed in this paper, are located in Precambrian regions: the Mesoproterozoic Grenville Province, the Paleoproterozoic Trans-Hu dson orogen (THO), the Pareoproterozoic/Archean basement of Alberta, a nd the Archean Superior Province. Data acquired across the Grenville o rogen in eastern Quebec show strong reflectivity throughout the crust; upper crustal reflections can be correlated with exposed structural e lements, including extensional shear zones and packages of deformed hi gh-pressure rocks (eclogites). In a marine survey across the Grenville orogen off southeastern Labrador, seismic images show variably dippin g reflections and a structural high associated with a major gravity an omaly. Data acquired across central Alberta show crustal-scale thrust stacking and imbrication of the Archean Hearne craton. To the east acr oss the Trans-Hudson orogen, images of similar collisional features ar e observed. Geochronologic constraints indicate contemporaneity of tec tonic activity between the two regions at 1.8 Ga, suggesting that coll isional tectonic activity was coeval over a broad crustal region, ca. 1000 km across strike. In the Superior Province, seismic data across a collision zone involving the northern Abitibi greenstone belt and the are-related Opatica plutonic belt show spectacular crustal reflectivi ty and dipping reflections that extend 8 s (similar to 30 km) into the mantle, The latter feature is interpreted as representing a relict 2. 69-Ga-old suture associated with subduction, providing the first direc t evidence that plate tectonics was active in the late Archean. These five examples, supported by other LITHOPROBE results, refute a number of generalizations about crustal reflectivity that have been made in t he past and illustrate how reflection studies, combined with other geo science studies, can lead to a better understanding of Precambrian tec tonics. Reflectivity persists throughout the crust; there is no genera l separation into a poorly reflective upper crust and a reflective low er crust. Crustal reflectivity in Archean and Proterozoic regions is a s pervasive as that in areas of more recent tectonism. The Precambrian reflection Moho is generally well defined but shows a range of charac teristics. Relative ages of reflectors can be discerned and tectonic s ignificance can be attached to characteristic features of the crustal reflectivity.