Transcript Deep seismic reflection profiling of Archean cratons

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