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Deep seismic reflection profiling of Archean cratons Arie J. van der Velden Frederick A. Cook 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 Causes of reflectivity: rules of thumb Generally reflective • • • • gneiss fault zones sills Moho Generally poorly reflective • • • • metavolcanic rocks metasedimentary rocks granite mantle • 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 “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 “shingle” reflections - Wabigoon S N 30 km “shingle” reflections - Wabigoon S N 30 km “shingle” reflections - Abitibi S N 30 km “shingle” reflections - Abitibi S N 30 km “shingle” reflections – eastern Yilgarn W E 10 km “shingle” reflections – eastern Yilgarn W E 10 km Analogy – thrust-and-fold structures E W ~3 m Analogy – thrust-and-fold structures E W ~3 m “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. 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) mantle reflections – Superior, Red Lake S N 30 km mantle reflections – Superior, Red Lake S N mantle reflections – Slave, Yellowknife W E 30 km mantle reflections – Slave, Yellowknife W E 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 A Paleozoic example: Newfoundland Appalachians E W E W North Caribou: an example of “vertical tectonics” North Caribou: an example of “vertical tectonics” 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 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