The HSI bright reflector: further evidence for extensive magmatism in the Precambrian of western Canada

Abstract

Seismic reflection lines acquired in 1995 as part of LITHOPROBE, Canada's national geoscientific research program, revealed unusually bright reflectors of extensive lateral scale in the Archaean Medicine Hat block of southern Alberta. The herein-named Head-Smashed-In (HSI) reflector is imaged over an area of at least 6000 km2 in the upper half of the crust. The reflector appears as a mostly horizontal to subhorizontal feature. Although continuous, it has a complex and laterally variable internal structure. Comparison with a reflector of known impedance contrast in the overlying western Canadian sedimentary basin indicates a high-impedance structure to cause the reflections. Dense one-dimensional modelling is consistent with the reflector being a multilayered structure with layer thicknesses in the range of tens to hundreds of metres and lateral scales of discrete reflecting elements between tens of metres and hundreds of metres. Polarity constraints, overall geometry and reflection character strongly suggest that mafic/ultramafic horizontal sheets are the cause of HSI reflectivity. The HSI is the third major sequence of strong crustal reflectivity imaged by LITHOPROBE deep seismic data in the Precambrian crust of western Canada. All three features are very similar in overall geometry, reflector strength and character and have been interpreted as horizontal, sheetlike intrusions. Small-scale variations of the reflectivity signatures suggest a pronounced rheological heterogeneity in the upper crustal rocks within the host tectonic domains.