1. The LITHOPROBE Experience:
Active-source Seismology and Other Earth Science – An Essential Combination for Understanding Tectonic Evolution
Ron Clowes
University of British Columbia, Vancouver, BC
EarthScope Workshop, Bozeman, MT, Sept /05

2. My main points are …
For most geologists to participate meaningfully in the EarthScope program, multichannel reflection and refraction/wide-angle reflection seismology must be part of the scientific program
The active-source seismology program must be directed at geological targets that have fundamental significance for understanding tectonic evolution
The active-source seismology program must be coordinated with geological studies and Flexible Array

3. LITHOPROBE is … A national earth science research project
To investigate the three-dimensional structure and evolution of Canada’s landmass and continental margins
By probing the lithosphere, Earth’s relatively cold, strong, rigid outer shell which is typically 100 km or more thick

4. Why LITHOPROBE ?
To gain a basic understanding of the continent on which we live, from which we derive resources and which generates natural hazards
To obtain regional background information useful to mining and petroleum industries

5. How does LITHOPROBE work?
Multidisciplinary
Collaboration
Partnerships
Decentralized research

7. LITHOSPHERIC STRUCTURE
GEOPHYSICS
LITHOSPHERIC STRUCTURE
& TECTONIC PROCESSES
REGIONAL INFORMATION
FOR INDUSTRY
DETAILED STUDIES WITH
INDUSTRY

8. How did LITHOPROBE select fundamental geological features?
Call for integrated, multidisciplinary proposals from a New Transects Subcommittee
Peer-review nationally and internationally
Internal review by 3 disciplinary subcommittees [seismic, em, geology] and the senior Scientific Ctte
Review of evaluations by New Transects Subctte
Recommendation to Sci. Ctte. and LITHOPROBE Board of Directors

9. How does LITHOPROBE image internal structure?
Seismic reflection – best resolution; crust and upper mantle structure
Seismic refraction – medium resolution; P-wave velocity and composition of crust & upper mantle
Teleseismic studies – lower resolution; S-wave velocity and structure from crust to transition zone
Magnetotellurics – conductivity, fluids
Magnetics & gravity – tie with geology and general structure

10. Seismic Reflection and Refraction

11. Seismic reflection acquisition
Vibroseis trucks

12. Land seismic acquisition Vibroseis truck sources
Contracted crew. Research staff are only there for quality control

13. wide-angle reflection
Near-vertical
reflection
Refraction and
wide-angle reflection
Goals:
image differences between rock types and subsurface structures:
mapping the detailed
“structural fabric”
limited compositional information
Goals:
image the velocity structure
also image major
differences in rock types
provide compositional and
thermal constraints
6-12 km
>500 km
0 km
40 km

14. wide-angle reflection
Near-vertical
reflection
Refraction and
wide-angle reflection
impedance contrasts
structural fabric
velocity structure
& large impedance contrasts
compositional and thermal
constraints
Primary Goals
Models
imaging using deconvolution,
stacking, and migration
techniques
inversion for simplest
structure
Resolution
Upper crust
10 m < 0.5 km
Moho
200 m < 2.5 km
1 km km < 0.1 km/s
0.2 km/s
1.5-2 km km km/s
10-80 Hz
2–12 Hz
INTRO
TECHNIQUES
RESULTS
CONCLUSIONS

15. Tectonic Ages
Archean
Proterozoic
4Ga
Phanerozoic
3Ga
2Ga
1Ga
0Ga

16. Trans-Canada Crustal Cross-Section
80 km
1:1
6000 km
2:1

17. Tectonic Ages
Archean
Proterozoic
4Ga
Phanerozoic
3Ga
2Ga
1Ga
0Ga

18. Paleoproterozoic Trans-Hudson Orogen in Saskatchewan and Manitoba

20. Ashton et al., CJES, v. 42, 2005

21. Mylonites from Pelican Thrust Zone
Strain gradient around tectonic
inclusion of granodiorite-tonalite
Sheath fold defined by attenuated
mafic dyke in mylonite
Δ-winged porphyroclasts in
mylonitic granodiorite-tonalite –
Shows dextral shear component
Δ-winged porphyroclasts in
mylonitic pelitic migmatite –
Shows low-angle reverse shear
Ashton et al., CJES, v. 42, 2005

22. Concordia diagrams: the need for dating structures and rocks
Ashton et al., CJES, v. 42, 2005

23. Ashton et al., CJES, v. 42, 2005

24. L10 L9

25. Line 10
Line 9 east

27. Lines 9 and 10 Extensive Crustal Reflectivity
Limited mantle reflectivity

28. Lines 9 and 10 PT PT PT
Paleoprot
Sask
Paleoprot
Archean
Archean

29. LS2b L9

30. Line 9 PT PT Sask Paleoproterozoic Archean
Hajnal et al., CJES, v. 42, 2005

31. Line S2b Sask PT Paleoproterozoic Archean
Hajnal et al., CJES, v. 42, 2005

32. C C` C C`
Hajnal et al., CJES, v. 42, 2005

35. Archean Slave craton and Paleoproterozoic Wopmay Orogen in Northwest Territories

37. Corridor 1, west half: migrated reflection section and interpretation
Fort Simpson Terrane
upper crust (upthrust)
Fort Simpson
lower crust (delaminated)
Cook et al., Tectonics, v. 18, 1999

38. Corridor 1, east half: migrated reflection section and interpretation5 10
Time (s) 15 20 25 30 15
Hottah lower crust
Slave lower crust 30
Approx Depth (km) 50
Hottah mantle
(Archean &/or Proterozoic)
Archean Mantle 70
Deformed mantle
Proterozoic (?) 90
110
Cook et al., Tectonics, v. 18, 1999

39. Van der Velden & Cook, CJES, v. 38, 2002

40. Van der Velden & Cook, CJES, v. 38, 2002km 20 40

41. Van der Velden & Cook, CJES, v. 38, 2002

42. RESULTS Strong crustal reflectivity; related to surface geology
Strong mantle reflections . to depths of ~100 km . lateral continuity for 100s of km
Crustal delamination during continental collision
Deformation within the lithospheric mantle
Possibly some Proterozoic mantle beneath Archean Slave craton

43. 1100
1101
Yellowknife array
1112
1113

44. Offset distance (km)
Offset distance (km)

45. Offset distance (km)

46. H3 J H1 K

48. SNORCLE Line 11, Northwest Territories
1111
1101 H1 H2 J
1113
1100
P-Velocity (km/s)

49. RESULTS WA reflections correlate with NVI reflections
WA reflection at 180 km depth probably corresponds to base of lithosphere
Support for model of delamination, subduction and Proterozoic mantle below Archean Slave craton

50. Integration of seismic reflection and teleseismic results
Bostock, JGR, v. 103, 1998

51. Slave craton teleseismic study
Straub et al., CGU-AGU, Montreal, 2004

52. RESULTS Impulse responses show mantle stratigraphy
Teleseismic images correspond to interpretations of MCS and R/WAR data
Support for assembly of proto-Slave craton by processes of shallow subduction; and for subduction of Proterozoic lithosphere below Archean Slave

53. RESULTS Impulse responses show mantle stratigraphy
Teleseismic images correspond to interpretations of MCS and R/WAR data
Support for assembly of proto-Slave craton by processes of shallow subduction; and for subduction of Proterozoic lithosphere below Archean Slave
Low velocity anomaly at ~350 km depth centered to the south of the Lac de Gras kimberlite field

54. My primary points, now demonstrated, are …
For most geologists to participate meaningfully in the EarthScope program, multichannel reflection and refraction/wide-angle reflection seismology must be part of the scientific program
The active-source seismology program must be directed at geological targets that have fundamental significance for understanding tectonic evolution
The active-source seismology program must be coordinated with geological studies and Flexible Array

55. Summary
In order:
To maximize the scientific and infrastructure investment in EarthScope
To involve the geological and geochemical community in a meaningful way
To develop a true 4-D understanding of lithospheric development
Focused studies within EarthScope should include active-source seismology integrated with all other Solid Earth Science studies

56. Thank you!