Astronomy and Astrophysics – Astronomy
Scientific paper
Apr 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003geoji.153....1f&link_type=abstract
Geophysical Journal International, Volume 153, Issue 1, pp. 1-19.
Astronomy and Astrophysics
Astronomy
21
Crustal Structure, Mantle Discontinuities, Plate Tectonics, Refraction Seismology, Slave Province, Wopmay Orogen
Scientific paper
The Slave geological province, a relatively small craton in NW Canada that includes the oldest rocks in the world, is one of five cratons that form the Archaean continental core of North America. The Palaeoproterozoic Wopmay orogen, lying between the craton and the Phanerozoic northern Canadian Cordillera, represents crustal growth through addition of a series of north-south oriented magmatic arcs and accreted terranes. As part of LITHOPROBE's multidisciplinary studies, Line 11 of the SNoRE 97 seismic refraction/wide-angle reflection survey investigated the structure of the lithosphere in this region. Interpretation of crustal and mantle arrivals from 13 record sections followed an iterative procedure of traveltime inversion and forward modelling. Results show that crustal thickness at 32-35 km remains relatively constant from the Archaean regions in the east to Proterozoic domains in the west, contrary to average values of crustal thicknesses from global compilations that suggest thicker Proterozoic crust. At the westernmost end of the line, where crustal extension and basin formation are inferred, the crust thins to 30 km. Upper crustal velocities to about 12 km depth lie in the range of 5.8-6.2 km s-1. The middle crust of thickness 8-10 km has velocities between 6.2 and 6.6 km s-1. Average velocities for the lower crust, differentiated as a 10 km thick layer, range between 6.6 and 6.9 km s-1 with a higher velocity region (7.1 km s-1) in the transition zone between the Fort Simpson magmatic arc and Hottah accreted terrane in the Wopmay orogen, where a Palaeoproterozoic subduction zone has been inferred from coincident multichannel reflection data. In the same area, anomalously low upper-mantle velocities (7.5 km s-1) have been found, indicating that the effects of collision, subduction and accompanying physical changes in the rocks comprising the mantle lithospheric wedge above the underthrusting structure, may have been retained for 1800 Ma. Data from most record sections with offset distances greater than 250 km show continuous wide-angle reflections from upper-mantle depths of about 75 km and almost 100 km. Although poorly constrained, velocities in this depth range vary from 8.4 to 8.8 km s-1. Interpretation of these phases and mantle reflections from equivalent depths in coincident, near-vertical incidence multichannel data relates them to subduction and underthrusting of oceanic crust and subsequent phase changes that occurred during the assembly of the craton and the evolution of the Palaeoproterozoic orogen.
Clowes Ron M.
Fernández Viejo G.
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