Deep seismic reflection profiling of Archean cratons

1. Deep seismic reflection profiling of Archean cratons • Outline: • Locations of available profiles • Causes of reflectivity: rules of thumb • “Shingle” reflections: indicators of horizontal tectonics • Mantle reflections: Archean subduction? • Vertical tectonics: an example • Conclusion Arie J. van der Velden Frederick A. Cook

3. Causes of reflectivity:rules of thumb • gneiss • fault zones • sills • Moho Generally reflective Generally poorly reflective • metavolcanic rocks • metasedimentary rocks • granite • mantle

4. Seismic reflection data sees mainly structure • Profiles from the Superior, Yilgarn and Slave provide a snapshot of the nature of tectonic activity at ~2.75-2.6 Ga

5. “shingle” reflections • characterized by dipping, listric reflections in the middle and lower crust • resemble imbricate slices within antiformal stacks, as seen in brittle-deformed foreland belts • however, shingle reflections occur within the ductile lower crust, and strain distribution within them is not well known

6. “shingle” reflections - Wabigoon S N 30 km

7. “shingle” reflections - Wabigoon S N 30 km

8. “shingle” reflections - Abitibi S N 30 km

9. “shingle” reflections - Abitibi S N 30 km

10. “shingle” reflections – eastern Yilgarn W E 10 km

11. “shingle” reflections – eastern Yilgarn W E 10 km

12. Analogy – thrust-and-fold structures E W ~3 m

13. Analogy – thrust-and-fold structures E W ~3 m

14. “shingle” reflections • generally considered to be indicators of horizontal foreshortening • often occur beneath greenstone belts • likely responsible for thickening crust beneath greenstone belts to present proportions • Hard to tell from reflection data whether the “imbricates” are parautochthonous slices, or if they are allochthonous rocks emplaced by subcretion.

15. Mantle reflections • gently to moderately dipping reflections extending from the Moho into the mantle • typically occur beneath greenstone belt to plutonic-gneiss domain transitions • typically dip beneath the protocraton (gneiss domain)

16. mantle reflections – Superior, Red Lake S N 30 km

17. mantle reflections – Superior, Red Lake S N

18. mantle reflections – Slave, Yellowknife W E 30 km

19. mantle reflections – Slave, Yellowknife W E

20. Mantle reflections • Mantle reflections likely accommodate compressional deformation • Difficult to tell from reflection data whether mantle reflections accommodate 10s of km, 100s km, or 1000s of km of displacement • Appear to be significant structures associated with accretion of “terranes” • By analogy with modern examples, mantle reflections are often interpreted as indicators of subduction

21. A Paleozoic example: Newfoundland Appalachians E W E W

22. North Caribou: an example of “vertical tectonics”

23. North Caribou: an example of “vertical tectonics”

24. Concluding thoughts • Seismic reflection data image structures within the crust associated with horizontal compression • Dome-and-keel structures are in some cases underlain by younger low-angle structures, while in other cases they may project to the base of the crust • Reflection patterns beneath Neoarchean cratons are remarkably similar to profiles across the hinterlands of younger orogens • There is no systematic change in reflection patterns with age

25. Acknowledgements • We would like to thank Tim Barton, Bruce Goleby, Barry Drummond and the ANSIR staff at Geoscience Australia for making available the Yilgarn data set • NSERC • GSA • GSA – graduate students association, U of C