Walker Lane Breakout Presiding: Jim Faulds Recorder: E. Miller What is Walker Lane? - A complex system of dextral faults that interact with Basin and Range extension. Currently accommodates about 20% of P-NA plate motion. Displacement ranges from km in central part to 0 in NE California.

Big Questions: ·How does a strike-slip fault system like the Walker Lane develop? ·What controlled its location and distribution of strain through time? ·What is the nature of coupling between upper mantle and crust in the Walker Lane? ·What are its dynamic and kinematic links with plate boundary (far-field) and local stresses? ·How does the crust accommodate simultaneous extension and strike-slip faulting? Is there strain partitioning?

Topics Presented: Rich Schweickert (U.Nevada Reno) proposed model for active transtensional deformation in Lake Tahoe Region. Pat Cashman (U. Nevada, Reno) discussed dissected Neogene basins in the SN-GB transition zone. Chris Henry (U. Nevada, Reno) Showed magmatic history movie Margin of Basin and Range extension migrated west with magmatism. Similarly, the thermal regime of the Basin and Range may dictate the position of the Walker Lane Jeff King talked about slip partitioning in other places and its relevance to the B and R

(Lahren and Schweikert, 1995; from Surpless, 1999). Faulting along the western edge of the Basin and Range- the process of stretching begins!

General discussion brought up the following important points: Graham Kent pointed out the tie to Gulf of California history. BR/WL may represent a snapshot of the early (pre-opening) phase of the Gulf of California 6-10 Ma ago. Locus of strike slip with respect to western margin of B and R extension needs to be better documented to see what this relationship is. Need to image mid to lower crust and mantle to characterize the WL at depth. Is it a big shear zone? Or something more complex due to coeval extension? Geodesy: How are the parts of the WL moving with respect to one another? Blocks rotations have been documented by paleomag. Can block rotations be measured geodetically?

Discussion of major questions and more: 1.When did strike slip faulting start? Depends where you look, in central part, 3-6 elsewhere. 2.Why should strike slip faulting associated with plate boundary move inland? Related to foundering of batholithic roots? Related to the jump of San Andreas into Gulf of California? Related to heating and thermal weakening of the crust? Timing, T structure of crust. 3.How does/did the system propagate? From south to north, but mechanics are unknown. 4. How does locus of faulting change through time? Not clear if it follows the western boundary of extension, it appears to do that in Owens Valley but is further east to the north and again further west at its northernmost extent.

5.Nature of block rotations in developing shear zones? Seems that this is happening, how are deep and shallow structures coupled? 6.Where does the Walker Lane go to the north? Splits up into normal faults? Causes compression (N-S shortening) ?, gaining momentum and hasn’t propagated yet? 7.What is the coupling between upper mantle and crustal strain? We don’t know, but Earthscope might help resolve. 8.Balance of Forces, local extension, strike-slip, plate boundary If we could model the WL, we could see what the models would predict and then compare it with what is there. 9. PBO Plate Boundary Observatory: could play a role in terms of constraining the width of the shear zone

10. Mantle and crustal anisotropy: Earthscope could resolve upper mantle and lower crustal anisotropy, but to do that need flexible array. Is there flow, can you see the driver for surface deformation? 11.Discussion of transect selection. If you could pick the best site for detailed studies that would accompany deep crustal seismic reflection profile, a modern but shorter COCORP like transect, designed to answer everyone’s questions and problems, where would it be? (Answer heavily weighted by Reno/Stanford groups) ·Across boundary of Sierra Nevada and Basin and Range World class example of unextended to extended crust ·Across Walker Lane Zone World class example of plate boundary motions moving into a region of diffuse continental deformation ·Characterize one of the most seismically active regions of deformation in the Basin and Range; link time-scales of observation neotectonic to geologic past

In the Basin and Range, multiple sets of normal faults have operated over time to thin the upper, brittle crust. The youngest normal faults cut older, rotated normal faults (red) and interact with a modern ductile-brittle transition zone that lies between 6 and 10 km depths (Surpless, 1999). 0% >150% Extension Sources of data: Klemperer et al. (1986), McCarthy and Thompson (1988), Vetter et al. (1983), Hill et al. (1991), NCEDC,CNSS, Blackwell et al. (1991), Blakely (1995), Jarchow and Thompson(93) Surpless et al. TECTONICS 2002