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 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 Canadas landmass and continental margins By probing the lithosphere, Earths 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 andgeneral 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 km < 0.1 km/s 0.2 km/s 1.5-2 km km km/s 10-80 Hz 212 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 -winged porphyroclasts in mylonitic granodiorite-tonalite Shows dextral shear component -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 andPaleoproterozoic 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 havefundamental 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!