LITHOPROBE: a scientific update-new images of the continental crust

Abstract

LITHOPROBE, Canada's national earth sciences research program which integrates geophysical, geological and geochemical investigations in a collaborative effort among scientists from universities, government and industry to extend and relate surface geology to structures at depth, is up and running. The Lithoprobe Secretariat is established at the University of British Columbia. The Lithoprobe Seismic Processing Facility (LSPF) for research reprocessing of Lithoprobe and related data is installed at the University of Calgary. Access by the Canadian research community will be available through remote site connections. All scientific aspects of the program are in operation and plans for the future are in hand. Some of the spectacular new images of continental crustal structures that have been obtained and their tectonic significance are highlighted in this summary. Individual contributions of specific studies by Lithoprobe scientists are included in this volume.Along the eastern portion of the Southern Cordillera Transect, 270 km of crustal reflection data acquired in 1985 and subsequent magnetotelluric results define the westward extent of autochthonous North American basement (truncated by the crustal scale east-dipping Slocan Lake fault zone), the thickening of Purcell Proterozoic supracrustal rocks by folding and thrusting and a major compressional shear zone underlying the Valhalla gneiss complex. In 1988, Seismic profiling was extended westward across the allochthonous terranes and plutonic complexes to the west coast. About 900 km of 60-fold, 18 s data were recorded by an industry contract crew; initial processing by contract is in progress.In our Appalachian transect, Lithoprobe East, more than 3000 km of marine crustal reflection data have been acquired around Newfoundland and in the Gulf of St. Lawrence. Three deep crustal blocks, each of which exhibits a characteristic Seismic signature, can be traced through the region. Intracrustal reflectors on the Seismic line run across the continental margin northeast of Newfoundland help determine the nature of extensional tectonics and associated sedimentary basin evolution on the rifted margin.The GLIMPCE Transect has enabled a glimpse of the mid-continent crust by recording marine reflection data in the Great Lakes. The sections from Lake Superior reveal — 10 km of sediments overlying a 20 km thick sequence of strongly reflective volcanics and inter-flow sediments, perhaps representing the greatest vertical extent of intra-continental rift deposits on Earth.The original Archean/Early Proterozoic crust is greatly thinned beneath the rift basin. In the Lake Huron/Georgian Bay profile, the Grenville front is imaged as the westernmost reflection of a spectacular 70 km wide band of southeast-dipping reflections that clearly truncates prominent sub-horizontal crustal reflections of the Superior granite-rhyolite terrane.The Kapuskasing Structure Zone is a sequence of lower crustal rocks that have been thrust upward along the Ivanhoe Lake cataclastic zone and are now exposed at the surface. As part of the Kapuskasing Transect, 340 km of regional deep reflection data and 20 km of “ high resolution” data (20 m station spacing, 20 m source interval and a higher sweep range) over the cataclastic zone were recorded in 1987–1988. The Abitibi sub-province of Ontario and Quebec contains mineralized belts for which it is famous. As a preliminary program for the Abitibi-Grenville Transect, 130 km of regional and 65 km of high-resolution Seismic profiles also were recorded in 1987–1988. These were centred over major fault zones with which mineralization is related. Data quality is excellent and the data are in the process of being interpreted.