An Integrated geophysical study of Valhalla gneiss complex, southeastern British Columbia
Creators: name: Eaton, David W. S.
Publisher: name: University of Calgary
Year: 1988
Subjects: subject: QE 511 E27 1988a
subject: Geology, Stratigraphic
subject: Earth - Crust
subject: Geology - British Columbia

description: Bibliography: p. 109-115.
description: LITHOPROBE seismic reflection data in combination with petrophysical, aeromagnetic and gravity data have been used to study crustal structure beneath Valhalla gneiss complex, a Cordilleran metamorphic core complex in southeastern British Columbia. The region is characterized by a thin crust relative to most parts of North America, high surface heat flux and elevated lower crustal and/or upper mantle electrical conductivity, and is tectonically interposed between a convergent plate margin along Canada' s west coast and the stable North American craton to the east. Laboratory measurements of rock properties coupled with observed seismic reflections from mylonite zones suggest that mylonite reflectivity is influenced by interdependent strain related changes in density, velocity and p-wave anisotropy and elucidates some aspects of the more general problem of the nature of reflections in crystalline terranes. Ma.gnetic susceptibility measurements have been used to correlate aeromagnetic anomalies with exposed rock units. Bandpass filtering has been applied to the aeromagnetic data in order to further classify anomalies on the basis of wavenumber spectrum, and upward continuation of the data has been used to investigate the depth extent of Eocene Coryell intrusions. Regional/residual separation of Bouguer gravity data based on a best fit quadratic surface indicates that the Valhalla gneiss complex is coincident with a 15 mGal gravity high. Two dimensional numerical modelling of this gravity feature suggests that the area is underlain by rocks with a bulk density of 2710 kg/m3 . This inferred density value along with the absence of a pronounced regional aeromagnetic signature implies that "typical" oceanic basement is not present in the upper crust beneath Valhalla gneiss complex. Reprocessing of LITHOPROBE seismic line SCC-5 traversing the Valhalla gneiss complex has been undertaken to clarify a number of features important for the interpretation. A very high amplitude west dipping reflection beneath the Valhalla gneiss complex is imaged on line SCC-5. The Valhalla reflection may originate from the buried Gwillim Creek shear zone, a significant thrust-related mylonite zone that is exposed 30 km north of the seismic profile. A minimum of 20 km of overlap between two pairs of reflection zones separated by the Valhalla reflection supports the interpretation that the Valhalla reflection images a significant thrust fault. A convex upward narrow bandwidth reflection at 12 s two way time on the west side of line SCC-5 is interpreted to originate from the Moho, suggesting that the base of the crust undulates in depth, perhaps partially accounting for the virtual absence of Moho reflections elsewhere on this survey. The lower crust is both non-reflective and electrically conductive beneath the Valhalla gneiss complex, in contrast to other regions of high lower crustal conductivity where a reflective lower crust is often predominant. The combination of geophysical techniques used in this investigation illustrates the usefulness of adopting an interdisciplinary approach for studies of the earth' s crust.