Walker Lane Breakout

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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 48-75
km in central part to 0 in NE California.
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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?
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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
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Faulting along the western edge of the Basin
and Range- the process of stretching begins!
(Lahren and Schweikert, 1995 from Surpless,
1999).
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• 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?

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• Discussion of major questions and more
• 1.When did strike slip faulting start?
• Depends where you look, 15-19 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.
• 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.

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• Nature of block rotations in developing shear
  zones?
• Seems that this is happening, how are deep and
  shallow structures coupled?
• Where does the Walker Lane go to the north?
• Splits up into normal faults? Causes
  compression (N-S shortening) ?, gaining momentum
  and hasnt propagated yet?
• What is the coupling between upper mantle and
  crustal strain? We dont know, but Earthscope
  might help resolve.
• 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

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• 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?
• 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 everyones 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

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gt150 Extension
0
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
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).