1. Rheology of the Mantle and its Relation to Current Tectonics; Why Are Some Parts of the Basin and Range More Active than Others GreatBREAK Workshop

2. Glenn Biasi (UNR; Moderator) Rick Aster (NMT; Recorder) Charlie Wilson (Stanford) Jim Coleman (Stanford) Bob Phinney (Princeton) Wayne Thatcher (USGS) Eric Christianson (BYU) Jim Gaherty (Lamont) Tony Lowry (Colorado) Aasha Pancha (UNR) Geoff Blewitt (UNR) Bob Smith (Utah) Jim Ni (NMSU) Hersh Gilbert (Arizona) Greg Arehart (UNR) Phil Wannamaker (Utah) Lou Gustavson Jon Price (UNR) Larry Brown (Cornell)

3. General Interests of the Group: Mantle/Crustal stress-strain interactions; seismicity. Mantle flexural rigidity and lithospheric strength. Crustal and Upper Mantle Seismic Structures; what do they mean? Crustal structure and time-space history. Where and when are mafic intrusions happening? Properties of the lower crust. Structure and composition of upper mantle; effects on strain field. Basic controls on mineralization; Eocent magmatism Time-space patterns of magmatism in GB. Anisotropy in the mantle (surface wave and other techniques). Mantle rheology and dynamics and mantle flow; isotasy. MT/Fluids; proxy measurements for rheology, partial melt, mantle composition. Structural controls to mineralizaton/magmatism; economic geology. Initiation and propagation of large earthquakes. Moho character variations across the great basin can highlight mantle-crust interactions.

4. Theme I: Understanding the Style of GB Extension and General Rheology Resolution. Inherently interdisciplinary! Crust/Mantle Geochemistry rheological/hydration modeling, including laboratory studies. Heat flow needed. Seismology. Deformation field, including elastic rebound constraints. Role of rheology now vs. then vs. reactivated structures. T e (elastic thickness) across the Great Basin provides clues (gravity data/topography data; solve for load fields with elastic thickness assumption (statistical assumptions necessary). Where is the system weak/strong? Extent of crustal heating from stress work in the mantle. Traces of faults combined with lithosphere-scale (e.g., V p ) tomography show the locations and effects of strong/weak lithosphere (perhaps underappreciated). Velocity/strength correlation needs to be calibrated. Improved attenuation tomography should be pursued. How well welded are the mantle and crust? Anchoring, other strength/deformation effects of “mantle drips”/delaminating lithosphere (e.g., on Moho, basin formation).

5. Theme II: Can MT significantly reduce non-uniqueness of mantle modeling (e.g., seismic)? Conductive lower crust correlated with higher heat flow. Graphite, conductive fluids, melt. Lower crust appears to be widely hydrated; how do you trap fluids below the brittle-ductile transition? Improved geotherm/heat flow measurements will help. Conducting horizons may follow isotherms (e.g., 500 deg. C). Importance of the full geologic context. Complementary anisotropy measurements possible. Formal joint inversion schemes have been developed (e.g., “similarly shaped” constraints). Thin highly-conductive horizons may be resolvable in MT, yet invisible in seismic techniques.

6. Theme III: How can we reliably distinguish partial melt? Heat drives off water; could we have mid-crustal brines above mantle source zones. Can seismic attenuation/MT/other methods be used to resolve the question in some areas? Help welcom to constrain strong trade-off of scaling mechanisms. General correlation often noted between low velocity zones and recent eruptive centers.

7. Theme IV: What does the Moho tell us about the rheology of the mantle. Impedance contrast, roughness measurements (and thickness of the crust) are spatially variable. Crisp Moho noted beneath large extensions. What do these variations mean? Are mantle and lower-crust rheology coupled? Role of shear at/near the Moho? Can it be imaged? Don’t forget southern AZ as “Great Basin” style tectonics.

8. Theme V: History and Role of Water in Rheology Farallon/hydration/dehydration/heating/cooling history. Dehydration effectiveness in producing volcanism. Late Quaternary volcanism/monitoring of fluids/magma can provide insight. Melting depth variations (isotherm vs. water content). Analysis of deep fluids (“xenowhiffs”).