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. 16-18/05

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

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

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

How does LITHOPROBE work? Multidisciplinary Collaboration Partnerships Decentralized research

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

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

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

Seismic Reflection and Refraction

Seismic reflection acquisition Vibroseis trucks

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

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

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 3-20 km < 0.1 km/s 0.2 km/s 1.5-2 km 20-50 km 0.1 km/s 10-80 Hz 2–12 Hz INTRO TECHNIQUES RESULTS CONCLUSIONS

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

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

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

Paleoproterozoic Trans-Hudson Orogen in Saskatchewan and Manitoba

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

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

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

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

L10 L9

Line 10 Line 9 east

Lines 9 and 10 Extensive Crustal Reflectivity Limited mantle reflectivity

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

LS2b L9

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

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

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

Archean Slave craton and Paleoproterozoic Wopmay Orogen in Northwest Territories

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

Corridor 1, east half: migrated reflection section and interpretation 5 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

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

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

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

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

1100 1101 Yellowknife array 1112 1113

Offset distance (km) Offset distance (km)

Offset distance (km)

H3 J H1 K

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

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

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

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

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

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

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

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

Thank you!