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 gneiss fault zones sills Moho metavolcanic rocks metasedimentary rocks granite mantle Generally reflective Generally poorly reflective

Seismic reflection data sees mainly structure Profiles from the Superior, Yilgarn and Slave provide a snapshot of the nature of tectonic activity at ~ 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 SN 30 km

“shingle” reflections - Abitibi SN 30 km

“shingle” reflections – eastern Yilgarn WE 10 km

“shingle” reflections – eastern Yilgarn WE 10 km

Analogy – thrust-and-fold structures WE ~3 m

Analogy – thrust-and-fold structures WE ~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”

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