Fine Structure of the Crust near Yellowknife

Abstract

A detailed reflection–refraction experiment was conducted near Yellowknife, Northwest Territories, in 1969. Over 3000 seismograms were recorded along two profiles with a geophone separation of 250 m. The high quality, quantity and unexpectedly high frequency content of the data has revealed a crustal structure which correlates well with the local geology and Bouguer gravity. Extensive use has been made of a ray-tracing program which includes an estimate of the amplitudes based on first-order seismic theory.The model includes a belt of greenstones to the east of the Yellowknife Fault, surrounded by granitic rocks. The velocities are, respectively, 6.17 and 6.0 km/s at depths of 1 km and greater, but the granites are overlain by a transition zone with the velocity at the surface being 5.5 km/s. In the granites at a depth of 8–10 km lies a low velocity layer (Vp = 5.8 km/s). The bottom of the greenstones corresponds roughly to the bottom of the LVL and both are underlain by a zone in which Vp = 6.2–6.4 km/s.The highly complex nature of the Mohorovičić reflection coda suggests that the M transition is characterized by strong lateral variations of velocity over 20 km or less which give rise to sideways reflections and interference patterns at the surface, t2, x2 calculations using some of the M reflections suggest a crustal thickness of about 30 km and an average crustal velocity of 6.22 km/s. Both values are considered low for a stable shield region.