THE HSI BRIGHT REFLECTOR - FURTHER EVIDENCE FOR EXTENSIVE MAGMATISM IN THE PRECAMBRIAN OF WESTERN CANADA

Seismic reflection lines acquired in 1995 as part of LITHOPROBE, Canad a's national geoscientific research program, revealed unusually bright reflectors of extensive lateral scale in the Archaean Medicine Hat bl ock of southern Alberta. The herein-named Head-Smashed-In (HSI) reflec tor is imaged over an area of at least 6000 km(2) in the upper half of the crust. The reflector appears as a mostly horizontal to subhorizon tal feature. Although continuous, it has a complex and laterally varia ble internal structure. Comparison with a reflector of known impedance contrast in the overlying western Canadian sedimentary basin indicate s a high-impedance structure to cause the reflections. Dense one-dimen sional modelling is consistent with the reflector being a multilayered structure with layer thicknesses in the range of tens to hundreds of metres acid lateral scales of discrete reflecting elements between ten s of metres and hundreds of metres. Polarity constraints, overall geom etry and reflection character strongly suggest that mafic/ultramafic h orizontal sheets are the cause of HSI reflectivity. The HSI is the thi rd major sequence of strong crustal reflectivity imaged by LITHOPROBE deep seismic data in the Precambrian crust of western Canada. All thre e features are very similar in overall geometry, reflector strength an d character and have been interpreted as horizontal sheetlike intrusio ns. Small-scale variations of the reflectivity signatures suggest a pr onounced rheological heterogeneity in the upper crustal rocks within t he host tectonic domains. (C) 1998 Elsevier Science B.V.