Transcript Crustal Thickness in the Northern Sierra and Northern Nevada John Louie and Michelle Heimgartner With: James Scott, Weston Thelen, Christopher Lopez, Mark Coolbaugh, and.

Crustal Thickness in the Northern Sierra
and Northern Nevada
John Louie and Michelle Heimgartner
With: James Scott, Weston Thelen, Christopher Lopez,
Mark Coolbaugh, and Satish Pullammanappallil
University of Nevada, Reno
Nevada Seismological Laboratory
Great Basin Center for Geothermal Energy
www.seismo.unr.edu/geothermal
Optim, Inc.
Overview
• Goals of research
• Overview of seismic
refraction experiments
• Discuss new crustal
thickness/velocity results
• Discuss correlation of
crustal thickness with
geology & geothermal
occurrences
Goals of refraction experiments
1. Compile existing crustal information
2.
Establish a facility for long-range crustal surveys
3.
Collect three new crustal refraction profiles
4.
Integrate new and prior results: create a regional crustal model
that is available to others
5.
Relate crustal model to geology
Refraction profiles
Results in Heimgartner’s M.S. Thesis:
• Areas of extremely thin
crust (approx. 20 km thick,
northern Nevada)
• Crustal root beneath the
northern and central Sierra
Nevada
• Crustal thickness
correlates with heat flow in
the Great Basin
• Not all geophysical data
sets agree (Teleseismic vs.
refraction/reflection)
Seismic refraction
Source
Receivers
Cross-over distance: from the source, the distance at which
refracted rays arrive before direct rays
Northern Walker Lane (NWL) transect
NWL- Barrick GoldStrike blast
• Deployed 199 Texans over
450 km distance
• Seismic source: 38,000 kg
Barrick GoldStrike mine blasts
• Blast arrivals are visible over
300 km from source
Barrick GoldStrike mine blast, 8-30 Hz
filtering
NWL- Data & Synthetics Reduced by 7.4 km/s
NWL- Optimized Time Fits, Reduced at 7.8 km/s
West
East
NWL velocity-depth model
Louie et al., 2004
1. Crustal root beneath Sierra
Nevada Mtns. (>50 km)
2. Thin crust 19-23 km thick near
Battle Mountain, NV
3. Published in Tectonophysics in
2004
Idaho-Nevada-California (INC) transect
INC continuous crossing over the Sierra
INC-Barrick GoldStrike blast
SW
•Deployed 411 Texan instruments
along a 600 km transect (spaced
approx. 1.5 km apart)
•Recorded several 77,000 kg blasts at
Barrick GoldStrike
•Blast arrivals are visible 400 km from
the source
NE
INC-Tom’s Place earthquake
• Earthquake magnitude 1.6
• Shallow epicenter located
directly beneath the transect
line
• Provides crustal velocities
along the interior of the transect
Revised INC velocity-depth model
Moho
• Sierra crustal root, approx. 50 km depth
• 30+ km crust southeast of Battle Mtn.,
agrees with PASSCAL 1986
• Lose resolution north of GoldStrike, but
cross-over distances of less than 90 km
suggest thinner crust
Compiled Sierran Root Data
Molnar & Jones, GJI 2004
Northern Nevada Utah transect (NNUT)
• Several large mine
blasts & an
earthquake on the
Wasatch Front
• Historically large
blast for Simplot
• Provide
information for the
Great BasinWasatch transition
• Provide refraction
control through
northern Utah
• Work in progress
2005- All Active & RecFn GB Crustal Thicknesses
Selected Crustal Thicknesses
2007 Revised
crustal thickness
map of the
western Great
Basin:
100-km long area
of 20-km crust SW
of Battle Mtn.isolated but
corroborated
Crustal root under
northern and
central Sierra
Nevada
Gravity map with
crustal thickness
data
Gravity map from Oppliger, 2003,
University of Nevada, Reno
Temperature
Gradient map
with crustal
thickness data
After David Blackwell,
Southern Methodist
University (Coolbaugh
et al., 2005)
Geothermal favorability vs. Crustal thickness
NBMG
Map 151
Conclusions
• Showed that large mine blasts are effective
– Can collect data in regions not previously surveyed
• Thin crust southwest of Battle Mountain, NV
– Within a limited region 100 km long, 19-23 km-thick crust
– Thin crust limited by the INC transect (30 km crust) - Moho dips at least 15°
– Thin crust near Battle Mountain supported by crossover distances from the
INC and NWL experiments
– Gravity data supports thin crust
• Deep root under the Sierra Nevada
– Evidence for deep root in northern Sierra and no root in southern Sierra
• Integrate old and new crustal data
– Select survey techniques for consistency
Acknowledgements
This material is based upon work supported by the
U.S. Department of Energy under instruments
numbered DE-FG07-02ID14211 and DE-FG3602ID14311, managed through the DOE Golden Field
Office.
The instruments used in the field program were
provided by the PASSCAL facility of the Incorporated
Research Institutions for Seismology (IRIS) through
the PASSCAL Instrument Center at New Mexico Tech.
Data collected during this experiment will be available
through the IRIS Data Management Center. The
facilities under the IRIS Consortium are supported by
the NSF under Cooperative Agreement EAR-0004370
and the DOE National Nuclear Security
Administration.
The California Integrated Seismic Network (USGS
Cooperative Agreement 04HQAG0004) provided
earthquake locations used in the experiment.
We would like to thank Barrick GoldStrike, Round
Mountain, Kennecott Bingham Canyon, Simplot and
Cortez mines for their cooperation and willingness to
help.
Back-up slides
INC refraction transect
Barrick GoldStrike mine,
Battle Mountain, NV
(above)
Texan Instruments (right)
• Seismic source: mine blasts
(200,000 lb) and small local
earthquakes (magnitude 1.53.8)
• 24-bit single channel, portable
seismograms (Texans)
connected to 4.5 Hz
geophones
• Deployed 411 Texan
instruments along a 600 km
transect (spaced approx. 1.5
km apart)
• Instruments recorded for 96
hours (four 24-hour periods)
INC-Barrick GoldStrike blast, reduced time
SW
NE
Crustal thickness
map
Temperature
Gradient map
After David Blackwell Southern
Methodist University (Coolbaugh
et al., 2005)