Evidence for an Inactive Rift in the Precambrian from a Wide-Angle Reflection Survey Across the Ottawa-Bonnechere Graben

  1. Muawia Barazangi and
  2. Larry Brown
  1. Robert Mereu,
  2. Dapeng Wang and
  3. Oliver Kuhn

Published Online: 15 MAR 2013

DOI: 10.1029/GD014p0127

Reflection Seismology: The Continental Crust

Reflection Seismology: The Continental Crust

How to Cite

Mereu, R., Wang, D. and Kuhn, O. (2013) Evidence for an Inactive Rift in the Precambrian from a Wide-Angle Reflection Survey Across the Ottawa-Bonnechere Graben, in Reflection Seismology: The Continental Crust (eds M. Barazangi and L. Brown), American Geophysical Union, Washington, D. C.. doi: 10.1029/GD014p0127

Author Information

  1. Department of Geophysics, University of Western Ontario, London, Ontario, Canada N6A 5B7

Publication History

  1. Published Online: 15 MAR 2013
  2. Published Print: 1 JAN 1986

Book Series:

  1. Geodynamics Series

ISBN Information

Print ISBN: 9780875905143

Online ISBN: 9781118670118

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Keywords:

  • Earth—Crust—Congresses;
  • Continents—Congresses;
  • Seismic reflection method—Congresses

Summary

During the summer of 1982 the Canadian Consortium for Crustal Reconnaissance using Seismic Techniques (COCRUST) conducted a major long range seismic refraction and wide angle reflection experiment across the Grenville province of the Canadian Shield. One of the main aims of the experiment was to investigate the structure and origin of the Ottawa-Bonnechere graben from a set of in-line and fan-type profiles recorded both along the length of the Graben and in directions perpendicular to it. Wide-angle reflection observations from the Central Gneiss Belt to the north of the graben revealed a very simple one layered crust with a sharp Moho. Large amplitude wide-angle PmP reflected waves were clearly identified. This was in sharp contrast to the poor or non-existent PmP signals associated with the profiles obtained along the Graben. The data supports the theory that the Graben was part of the St. Lawrence rift system and that the Moho was disrupted with the entry of upper mantle material into the crust' The results of the first arrival direct wave observations showed that the near-surface velocities of rocks of the Grenville province varied from 5·8 to 6.4 km/s· There was no evidence for any intermediate discontinuity, however,regional differences in velocity gradients within the upper crust were pronounced and had a major influence on the appearance of the record sections. The complexity of the energy in the coda supported geological observations that the rocks of the Central Gneiss belt are more homogeneous than those associated with the Central Metasedimentary belt south of the Graben. Amplitude and direct wave arrival time fluctuations all indicated that the seismic velocity function is a fractal quantity. Seismic waves on travelling through the crust smooth out the small scale variations to create regions of high and low velocity each separated from the other by both lateral and vertical velocity gradients.