Session II
Examples of Integrated Geophysical Projects

LITHOPROBE is an Earth science megaproject that has been the focus of much solid earth science research in Canada for 20 years. The project is structured around collaborative, multidisciplinary research programs in carefully selected transects, or study areas. Based on the quality and quantity of its scientific results, its contributions to industry, its training of the next generation of Earth scientists and its strong public education and outreach programs, LITHOPROBE has been an outstanding success. One of these successes is the manner in which active-source seismology has been integrated with other geophysics, geology and geochemistry to further understanding of the tectonic evolution of the northern part of North America. In this presentation, I briefly outline a few basics related to the LITHOPROBE project, review the essentials about active-source seismology, and illustrate the integration of geology with active-source seismology to provide enhanced understanding of the tectonic evolution of the Canadian landmass.

The primary points that I want to make with respect to EarthScope in the Northern Rockies 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

One of the issues within EarthScope might be: "How do we determine the geological targets that have fundamental significance for understanding tectonic evolution?". While I don't believe there is necessarily only one approach to address this question, that taken by LITHOPROBE is the following.

• 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 Committee
• Review of evaluations by New Transects Subcommittee
• Recommendation to Scientific Committee and LITHOPROBE Board of Directors

Within LITHOPROBE there are many examples that demonstrate the primary points noted above. In this presentation, I will focus on results from LITHOPROBE's Trans-Hudson Orogen (THO) and Slave-Northern Cordillera Lithospheric Evolution (SNORCLE) transects. The THO is the world's largest and best exposed Paleoproterozoic orogen. It comprises suites of juvenile rocks and reworked Archean rocks that stitched together the Archean Hearne-Rae, Sask and likely Wyoming cratons to the west with the Archean Superior craton to the east. The SNORCLE Transect examines tectonic features from the small Archean Slave craton in the Northwest Territories westward through the Paleoproterozoic Wopmay Orogen and into the Cordillera of northern Canada. It represents the only region on Earth where 4 Ga of Earth history can be examined in one large area. In this presentation, I will exemplify results only from the Precambrian regions.

Figure 1. Map of Canada showing general tectonic age, i.e., the time since the last major deformation; and the location of Lithoprobe transects (polygons with lettering). Each transect involved a multidisciplinary Earth science research program that was spearheaded by active-source seismology. Transects are: AB, Alberta Basement; AG, Abitibi-Grenville; ECSOOT, Eastern Canadian Shield Onshore-Offshore Transect; GLIMPCE, Great Lakes International Multidisciplinary Program on Crustal Evolution; KSZ, Kapuskasing Structural Zone; LE, Lithoprobe East; SCORD, Southern Cordillera; SNORCLE, Slave-Northern Cordilleran Lithospheric Evolution; THOT, Trans-Hudson Orogen Transect and WS, Western Superior.

Results from the Colorado Plateau-Rio Grande Rift-Great Plains seismic transect (LA RISTRA) experiment show a pure shear extension mechanism for the Rio Grande rift (RGR) at the lithosphere scale. Receiver function results show crustal thickness ranging from 45 to 50 km beneath both the Colorado Plateau and the Great Plains, with crustal thinning to a minimum of approximately 37 km centered beneath the RGR axis. The centering of the thinnest crust on the rift axis indicates that the deep crust has undergone primarily pure shear extension. Inversion of LA RISTRA surface wave data and tomographic inversion of teleseismic body-wave delay times for upper-mantle structure show a broad low velocity region, also centered beneath the rift axis. This low-velocity region is interpreted as rift-centered lithospheric extension, indicating that lithospheric deformation, like that of the deep crust, is also primarily pure shear. A pure shear extensional mechanism for the RGR is consistent with geochemical evidence and regional gravity data which both suggest lithospheric thinning, with the greatest thinning centered beneath the rift axis. We find distributed lithospheric extension that is roughly twice the width of the surface expression of the rift. This geometry suggests less concentrated vertical mantle upwelling than would arise from more localized deformation, which may have led to decreased partial melting, magmatism, and less vigorous convection that might have otherwise occurred. While the upper crust along the RGR has undergone brittle deformation expressed as a series of asymmetric grabens, the lower crust and mantle lithosphere of the RGR have undergone distributed ductile deformation, symmetric about the rift axis, with a broad mantle signature that was likely controlled by pre-rift structure and thermal conditions. New data is currently being collected along an 18-station overlapping extension, RISTRA 1.5, that traverses the northwestern edge of the Colorado Plateau and extends into the easternmost Great Basin.

NSF funded the Great BREAK workshop to assist interested geoscientists to prepare scientifically and organizationally for Earthscope in the Great Basin. Key goals included identification of key questions in Basin and Range origin, development and deformation, and the exploration of interdisciplinary solutions, including geologic, geodynamic, seismic, gravity, magnetotelluric, and geochemical contributions. The Great Basin is a type location for the breaking apart of a continent and the most accessible developing transform boundary in the world. The workshop was organized around plenary sessions that introduced leading geologic and geophysical problems and opportunities, and topical breakout sessions in which key questions and needs were discussed. These included:

Breakout A: Extensional tectonics on the largest scale. Why does the Sierra Nevada block move? What is the cause of Great Basin uplift? Present high elevation? What aspects of lithospheric rheology control Great Basin strength and deformation? What made the rheology as it is?

Breakout B: Rheology of the Mantle and its Relation to Current Tectonics; Why Are Some Parts of the Basin and Range More Active than Others? What does Great Basin deformation say about lithospheric rheology? The role of fluids: Can magnetotellurics reduce non-uniqueness of mantle seismic models?

Breakout C: What is the mantle and lower crust in the Great Basin doing now? What are the relevant mantle processes? How are the crust and mantle coupled?

Breakout D: Contrasts between the Eastern and Western Great Basin. What are the geophysical similarities and differences between east and west? Where is the boundary between the Proterozoic basement that underlies the E B&R and the Paleozoic/Mesozoic accreted terrain beneath the west? What drives seismicity east of the Wasatch Front?

Breakout E: How do faults behave over time? Do they turn on and off, speed up—slow down? If so, why? Where are the discrepancies between geodetic, geologic, and seismic rates of strain accumulation? Geological study and dating and mapping facilities were recognized as necessary.

Breakout F: Relations of Economic Resources to Tectonics (Structure, Magmatism, Fluid and Heat Flow) What are the large-scale controls on Eocene mineralization - Carlin Au (NNW trending) and porphyry Cu-Au-Mo (EW trending)? Industry collaborations could be mutually beneficial.

Breakout G: Walker Lane. When did it start? How does the crust accommodate simultaneous extension and strike-slip deformation in the Walker Lane?

Breakout H: Seismic and Geophysical Methods, Crust and Mantle. Three projects in sub-disciplinary self-organization were identified: (1) Promote the use of legacy data and models. (2) Develop a test site or proving ground. (3) Develop a community modeling environment (CME). Tom Jordan, drawing on successful experience with the Southern California Earthquake Center (SCEC), presented points recognized by the organizing committee as important for successful EarthScope efforts in the Great Basin: (1) Problem focus. (2) Common objectives. (3) Community data products and models. (3) Community identity and organization. (4) Collaboratory infrastructure to provide code validation, standardization of products. (5) Regular forums for assessing progress including workshops and annual meetings. (6) Funding to support collaboration/collaboratory activities.

GreatBREAK was held June 21-23, 2004 in Tahoe City, California. Powerpoint presentations and the complete workshop report are available on the web at www.seismo.unr.edu/greatbreak.

*John Anderson (Chair), Glenn Biasi, John Louie, Steve Wesnousky, University of Nevada, Reno
Rick Aster, New Mexico Tech
Geoff Blewitt, James Faulds, Jon Price, Nevada Bureau of Mines and Geology
Lew Gustafson, Independent Consultant
Gene Humphreys, University of Oregon
Phil Wannamaker, University of Utah

In April, 2003, the Department of Geology at Kansas State University hosted a workshop to bring together geoscientists with experience and/or interest in Great Plains geology and geophysics. About 40 people attended from over 25 colleges, universities, and state surveys. The meeting consisted of 3 days of invited talks, volunteered poster presentations, large-group brainstorming sessions and smaller working group sessions. The goal was to identify some of the major points of interest in the Great Plains cratonic region. The result was discussion of five major topic areas: rift features, terrane boundaries, basins and uplifts, Rocky Mountain-Great Plains transition, and occurrence of kimberlites and other anomalous intrusives. In the rift feature category there was interest in the Mid Continent rift, the New Madrid/ Reel Foot fault zone, and the Rio Grande rift and its far reaching effects. For the Mid Continent rift specific questions centered on the thick crust, mantle velocities, mantle fabrics, and possible lingering thermal effects. Basin discussion focused on the Michigan and Illinois basins with the uplift topic largely centered on the Black Hills uplift. Details of the transition from the Rocky Mountains to the Great Plains attracted attention from geoscientists from diverse disciplines such as mantle geophysics, fluvial sedimentation and erosion, and thermochronometry. Further geology studies coupled with geophysical investigations are needed to determine variation in timing, role of thermal processes, and nature of the mantle in this region. Mesozoic and Cenozoic intrusive activity in the Great Plains region remains a mystery. Petrologic studies coupled with geophysics could help address where the melts originate and the relation to other tectonic activity. This workshop resulted in several proposal submissions with a number of others still planned for the near future.