GEOPHYSICS AND GEOLOGY: AN ESSENTIAL COMBINATION ILLUSTRATED BY LITHOPROBE INTERPRETATIONS

Lithoprobe (1984-2005) was Canada’s internationally acclaimed, collaborative, multidisciplinary, Earth science research project. Quality structural and tectonic interpretations derived from an effective combination of geophysics, mainly seismic reflection but including refraction, gravity, magnetic and magnetotelluric studies; and geology, including mapping, structural, geochemical, metamorphic and geochronological studies. Three examples are presented.

In the Cordillera of NE British Columbia, a reflection line extended westward from the W Canada Sedimentary Basin into the Foreland Belt to examine the craton-Cordillera transition. Coincident reflection and refraction data show differences in structure to 25 km depth. The seismic line lies north of outcrops of the Paleoproterozoic Muskwa Assemblage (MA). From surface geology and the reflection section, Mesoproterozoic strata were deposited on MA. The combined Proterozoic strata were thrust over older strata and tectonically wedged into Paleozoic and Mesozoic strata of the Foreland Belt. Lithological layering for MA was combined with estimated densities and velocities to generate synthetic seismograms that corresponded well with the recorded data.

In the central Grenville Province in eastern Québec, exposures of the Manicouagan Imbricate Zone enabled extensive geological data that can be related to a reflection profile recorded to the SE. The SE-dipping geology combined with the seismic data enabled derivation of 3-d crustal structure and generation of a cross-section across strike. A subsequent model for the tectonic evolution of the northern Grenville highlights two pulses of crustal thickening, the Ottawan (1050 Ma) and Rigolet (990 Ma) phases.

Within the Trans-Hudson Orogen in Saskatchewan and Manitoba, the Pelican Thrust Zone (PTZ) is the basal detachment between the Archean Sask craton and the Paleoproterozoic Flin Flon – Glennie volcanic-plutonic complex. Two reflection profiles cross the region and exemplify the crustal structure, showing the structural isolation of the Sask craton by the PTZ. Follow-up geological studies show structural details within the PTZ and include zircon dating. These enable generation of schematic cross sections illustrating development and subsequent deformation of the PTZ over a 100 My period.