Appendix 4. Detailed summary reports from breakout sessions.
Breakout A
Breakout B
Lists of participants, disciplines represented, and questions raised are presented below.
Participants:
Charlie Wilson (Stanford); Jim Coleman (Stanford); Bob Phinney (Princeton); Wayne
Thatcher (USGS); Eric Christiansen (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); Lew
Gustafson (Independent consultant.); Jon Price (UNR); Larry Brown (Cornell)
By way of making introductions, individuals in the group summarized their general and
research interests. These included:
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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; Eocene magmatism
Time-space patterns of magmatism in the Great Basin.
Anisotropy in the mantle (surface wave and other techniques).
Mantle rheology and dynamics and mantle flow; isostasy.
Magnetotelluric implications for fluids; bearing on 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 as they bear on mantle-crust
interactions.
Theme I: Understanding the Style of GB Extension and Our Ability to Resolve
Lithospheric Rheology.
It was the consensus of the group that this topic, because it incorporates geology,
geophysics, mineral physics, and other disciplines, is inherently interdisciplinary!
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Crust/Mantle petrological/rheological/hydration modeling, including laboratory
studies on a range of temporal scales are clearly needed to approach this question!
What’s needed:
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Heat flow.
Seismological images of elasticity and anelasticity.
Deformation field (from GPS, including elastic rebound constraints).
Te (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 could be significant.
Traces of faults combined with lithosphere-scale (e.g., Vp) tomography show
the locations and effects of strong/weak lithosphere (perhaps under
appreciated). Velocity/strength correlation needs to be calibrated.
How well welded are the mantle and crust from a stress-strain transfer viewpoint?
Role of rheology now vs. then vs. reactivated structures is unclear (these are of
course general interpretation questions for geophysical images).
Anchoring effects in the lithosphere could be important for driving deformation,
other strength/deformation effects of “mantle drips”/delaminating lithosphere (e.g.,
on Moho, basin formation).
Theme II: Fluids; Is magnetotelluric an unappreciated tool that can significantly reduce
non-uniqueness of mantle modeling (e.g., seismic)?
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Conductive lower crust correlated with higher heat flow. Graphite, conductive
fluids, melt; discrimination requires consideration of many other geological context
factors. Thus, magnetotelluric is not a panacea!
Lower crust appears to be widely hydrated, but a general issue is how do you trap
such fluids below the brittle-ductile transition?
Improved geotherm/heat flow measurements will help. Conducting horizons may
follow isotherms (e.g., 500 deg. C).
Complementary anisotropy measurements between seismic and magnetotelluric
techniques are possible, which could help constrain mechanisms and constrain
depth of anisotropy.
Formal joint inversion schemes have been developed (e.g., “similarly shaped”
constraints) to co-invert for spatial anomalies.
Thin highly-conductive horizons may be resolvable with magnetotellurics, yet
invisible in seismic techniques in some cases.
Theme III: How can we reliably distinguish partial melt?
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Heat and differentiation can drive off water; could we have mid-crustal brines
above mantle source zones which could be confused with magma. Can seismic
attenuation/ magnetotelluric/other methods be used to resolve the question in some
areas?
Help welcome to constrain strong trade-off of scaling mechanisms (e.g., partial melt
vs. composition vs. heating).
General correlation often noted between low velocity zones and recent eruptive
centers (e.g., RISTRA) lends credence to the idea that we are indeed already
imaging partial melt in some cases.
Theme IV: What does the Moho tell us about the rheology of the mantle.
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Impedance contrast, roughness measurements (and thickness of the crust) are
spatially variable. Crisp Moho noted beneath large extensions in some cases. 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.
Breakout C
What is the mantle and lower crust in the Great Basin doing now?
Two major categories of process emerged in the discussion:
1. Mantle Processes: processes responsible for the persistence and evolution of
mantle features, mantle upwelling, magma bodies, partial melt, and
implication for future volcanism in the Great Basin.
2. Crust- Mantle Coupling: processes responsible for the transfer of force
between mantle and crust, and lower crustal flow.
During the discussion we identified some of the issues regarding lower crustal and mantle
rheology that EarthScope should focus on, considering physical processes that are
important, and on how interdisciplinary studies can contribute to understanding these
processes.
Mantle Processes
There is little doubt that the mantle is the dominant engine that drives deformation of the
Great Basin lithosphere. Imaging of most of the Great Basin is currently not very
realistic, so EarthScope provides an opportunity for the discovery of major unrecognized
features. For example, it will be important to investigate features that relate to possible
mantle flow on the trailing edge of a subducting slab. If we can find the remnant pieces
of the Farallon plate we may be able to make a reconstruction and address the issues
regarding upwelling counterflows. Farrallon plate remnants, the “Sierra Nevada drip,”
and other “high velocity blobs” need to be mapped out with high resolution seismic
imaging if we hope to understand the genesis of these features. This might require 35 km
instrument spacing for selected features, which would represent a factor of two higher
linear resolution than current EarthScope plans, and a factor of 4 increase in the density
of surface instrumentation. It will also be important to map out where partial melt exists
in the upper mantle. One technical problem that needs to be addressed is that density
cannot be uniquely derived from seismic velocity. This will require further development
of joint-inversion techniques that integrate data from topography, gravity, the geoid,
isostatic considerations, Vp/Vs ratio, Pn, Sn, body waves, and surface waves.
Crust-Mantle Coupling
The controlling variables for lower crustal flow include composition, pressure, deviatoric
stress and strain rate, temperature, and water. An end-member hypothesis that might be
tested is that, in the Great Basin, the lower crust and mantle are effectively fixed (as in
the “jelly sandwich model”). EarthScope should be used to seek any evidence of lowercrustal flow, through seismic anisotropy data and geodetic transient data. Seismic
anisotropy might work in the mantle, but its application to study flow in the lower crust is
a challenge. One problem is how to separate lower crustal flow from magmatism and
foliation of old features, which might produce seismic anisotropy.
We need good geodetic characterization of surface kinematics to provide clues to the
underlying dynamics.
Permanent PBO GPS stations will provide the required temporal resolution for transient
processes, but complementary GPS networks are also needed to provide sufficient spatial
resolution for dynamics operating at the scale of the crustal thickness. Possible source of
impulse to drive detectable lower-crustal flow include large earthquakes, magmatism, and
phenomena associated with (currently) enigmatic “silent slip” events.
There are currently many apparent “transients” in GPS station position time series that
cannot be explained. In order to take these transients at face value as reflecting real
rheological phenomena, the geodetic community will need to carefully assess GPS
accuracy and to integrate complementary data such as seismicity. There is little doubt
that EarthScope will provide GPS data with transients that need to be explained. Slow
earthquakes and lower crustal flow will be candidates to explain these transients. This is
a specific situation that requires a strong interdisciplinary approach within EarthScope. It
is therefore important that the community addresses how can we self-organize and be
prepared to model and explain such transients. What complementary measurements and
databases should be arranged?
Integrated geodetic-seismological analysis is essential, as demonstrated with the Slide
Mountain “silent event” that was presented at the Workshop. A multi-technique
approach in general provides stronger clues and conclusions. Bigfoot should provide
microseismicity, but we really need comprehensive microseismicity at least down to
magnitude 1 to provide confirming evidence of a lower crustal transient events.
Recommend looking at options to improve microseismicity coverage (permanent, and
regional arrays) in the region of PBO GPS stations.
Long-term (1-100 yr) GPS transients (changing station velocity) have been recently
inverted to provide information on lower crustal and upper mantle rheology. Inversions
can be ill-conditioned (with regard to layered rheology), but this might be improved with
dense GPS array deployments in area of transient deformation.
What are some of the important details that would need to be resolved? The role of water
in the lower crust is important to the rheology, but quantification of the water is a
challenge. Magnetotellurics should be considered to provide evidence of brine in the
vicinity of the crust-mantle interface. Another complicating issue is uncertainty on the
details of crustal structures that control the deformation, especially on the geometry of
normal faults. It seems that seismic reflection might be required, at least in some
intensive study areas, or perhaps on a Basin-spanning profile. Other factors related to
control of rheology might need further investigation, such as heat-flow, the issue of strain
strengthening versus strain weakening in the Walker Lane, and the need for constraints
on the boundary between dislocation creep and diffusion creep.
Breakout D
Appendix – Detailed notes from the breakout session
Important Questions:
 Inherited fabrics: NE and NW older trends visible in Pre-C basement, but N trends
established ~700 Ma (important for edge of CO plateau).
 What got reactivated? How important are inherited structures in future evolution?
Where do the Paleozoic-Mesozoic terranes of central GB really start (isotopic boundaries
quite variable)?
 Do we really know the profile of the base of crust despite one or two profiles from the
1980’s (or ’70’s)?
 What is the relation between mantle velocity profiles and magmatic provinces? How
has magmatism modified stress and structure?
 What is the cause of the thick band of continuous seismicity just west of the Wasatch
front?
 Does the eastern GB show non-uniform extensional geometries like the S Sierra, or is
extension generally uniform?
 Why does the western (Lahontan) depression have thin crust but little apparent upper
crustal extension, whereas the eastern GB is apparently well extended?
 Is the eastern GB undergoing magmatic underplating/inflation in places?
 What can vertical components of deformation measured (from GPS, in a global ref
frame) do to resolve differences?
 Fundamental at end of EarthScope to know geometry of Sierran and Wasatch fronts.
Was this the Andes before collapse?
 Can combo experiments verify contrast in extension/strike slip on the two sides of the
GB?
Solutions from EarthScope
 Fabric: Mid-scale lithospheric thickness variations may represent earlier strength
variations. Perhaps anisotropy can help answer it but, in the crust esp., it’s a geophysical
challenge?
 Flexible array follow-up needed to trace deep geophysical structures toward the
surface.
 Crustal Profiles: We really do not know profile of the base of crust except in limited
areas. Bigfoot receiver functions will be very valuable. Expensive to address with
reflection and areas need to be prioritized.
 Margin seismicity, rifting: Must establish mode of extension. Crustal thickness and
rheology can determine contribution of GPE variation. Need better earthquake locations
to correlate seismicity with known faults or hidden detachments.
 Magmatic provinces: Should expect some correspondence between interpreted
inherited fabric (velocity) and magmatic history and compositions. Need more work on
plutonic complexes to better define isotopic (Prot./Pal.) provinces for correlation to
subregional scale deformation.
 General margin contrasts: Despite some in the east, a big contrast in degree of shear in
the west. 5-6 myr old in the west, so maybe two sides more similar at earlier times.
Need to try to strip Late Cenozoic sedimentary fill to restore pre-extensional differences.
Contour map of depth to graben basement from gravity, perhaps jointly with seismic
methods. Interest in basin profiles in urban areas in east and west, and Yucca Mountain,
with respect to shaking and seismic hazard.
 Magmatic inflation, fluid movement: Vertical components of deformation (from GPS,
in a global ref frame) together with attenuation/bright spot/ conductivity structure.
Self-Organization
 Need agency partnerships with common purpose to fund expensive active source
surveys.
 Decide on areas to emphasize and determine which techniques will solve the questions.
Audience Remarks after Breakout Report
 Graben depths available at coarse resolution available performed (by R. Jachens,
USGS), a regional summary is available.
 Industry reflection transects already exist by Shell (Wasatch to central GB) and by
CGG (Mineral Mountains to CGB). SEG Research Committee expressed offer of help to
EarthScope through donated industry data.
 A SCEC-like fault geometry group for the Great Basin was advocated. SCEC (Tom
Jordan) would be happy to help.
 Re seismicity, is the Great Basin actually getting wider? Is its basic structure evolving?
How is both the stress and the strength evolving?
 Need continued discussion on the whole Great Basin versus focused areas regarding
research.
 We can define a Great Basin-wide transect as a complement to Bigfoot, but not
everything needs to be done at the same high resolution the entire way.
 In considering the Great Basin, don’t forget the northern boundary in addition to those
west and east.
 In transect design, take care not to just go the easy way (e.g., along the highway) since
this may not represent processes most fully expressed.
Breakout E
What do we know about how faults behave over time? Do they turn on and off,
speed up – slow down? If so, why?
It was emphasized by our group that understanding the process of how faults behave over
time is of fundamental importance to understanding the tectonic evolution of the Great
Basin. Geological study will be needed to provide context and extend the time-scale of
study beyond that represented in the seismological and geodetic measurements that will
be provided by the geodetic and seismological measurements of PBO and USarray,
respectively. Problems in the process of fault behavior through time ranged from the
discrepancy between geologic and geodetic rates of deformation the relationship of
geodetic strain measurements to patterns of earthquake recurrence and fault slip rate
during the late Quaternary
Important scientific problems:
 Discrepancy between Geologic and Geodetic Rates of Deformation
 The geometry of faults is critical to understanding the rates of deformation – Geometry
 The evolution of faulting
a.) Relationship of Walker Lane development to the Basin and Range
b.) The knowledge that ranges are characterized by pulses of rapid deformation-Why?
How is this important to geodetic measures…
(don’t ignore those regions which appear to now be characterized by low strain rate)
 History of earthquakes in time – hazard and evolution
 Explain the gross structure and physiography of the Basin and Range.
 Linkage of faults – How valid are block models?
 Connections of fluids to fault behavior …. Scales of faulting….. incorporation of time
into mechanical models of fault behavior.
Products:
 In a perfect world: A movie of the how the Great Basin has expanded through time.
What are scientific problems:
 The overriding emphasis of this group was that geology is needed to provide context to
all of the short term measurements of EarthScope
 The time scale of faulting…time sequence of events going back in time with additional
information of strain that accompanied these.
 A movie of how the Great Basin has expanded.
 A catalog of large earthquakes going back through time…
To explain the gross physiography of B&$ - the central basin high
To match geodetic budget to geologic budget
The evolution of the Walker Lane and other fault systems?
Extend the slip histories back to 10000’s of years
Fault physics/mechanics – incorporation of time into the models
Earthquake hazards…
Focal mechanisms of Nevada earthquakes ---but what does this have to do with
questions…
Relationship of B$R deformation to Walker Lane development
What is linkage of faults – how valid are block models
Don’t ignore slow strain areas…
Can we use geodesy to define tectonically ‘dead’ zones…
Whereas EarthScope is giving short term measurements – geology is needed to provide
time context – needed to understand evolution – also needed to need structural control on
current deformation observations…
Connection of fluids to fault behavior….
Geometry of fault systems remains a fundamental problem
Multiple scales of faulting
The evolution of faulting
Fundamental rheology and fluid characteristics of the crust at large spatial scales
3-d imagery facility –
Fault geometries – their movement through time
Maybe outline a well-thought out area…
What problems can EarthScope solve
(flex array)
 what does it mean to self-organize
 do we want to self-organize?
 what is interesting for outreach and how should we present it?
 Can we get imagery – it is not used enough – accessible enough?
 Geoff King: notes lack of use of satellite imagery – should this be an emphasis.
should put money into a visual data base….
 Elizabeth Miller: technique to measure mountain growth with time – faults moving 3
mm/yr? U/He technique
Faults move fast for short period of times
Area of faulting jumps around – takes place quickly – stops –then starts up again –
Thermal chronology are proxy for fault motion – what are the assumptions…assume
blocks are rotating through a flat set of isotherms..
Most cases – don’t measure fault surface…..
Need to know geothermal structure/gradient
 Karl Karlstrom (Soccorro): time scale of faulting and microseismicity
Fluid chemistry (water and gas) of travertine – depositing springs (etc) – a link between
tectonism in western U.S. and hydrology
Xenowhiffs
Hypothesis
Lower world waters: small volume warm, high CO2, high SR89/86, high HE3/he4,
saling, metals
Upper world – meteoric system
Travertine deposit gases to seismicity – a relation – water and gas chemistry as related to
seismicity – relationship of mantle to crustal…
 Leigh Preston (UNR): obtaining focal mechanisms for small earthquakes (M<3) from
1st motions and amplitudes  Ron Bruhn: looking at fault activity over hundreds of thousand of years – basic idea –
combo of seismic t tomography below faulted scarps to identify places to drill – this time
problem is enigmatic to the B&R – younger is pretty good… tomography down to 30 m
with resolution of 11-10m and 15 m cores.
Use tectonic geomorpology – previous trenching – seismic tomography – drilling –
dating
Slip rates on Wasatch front - ~ 0.7 mm/yr from geologist perspective – geodesy suggests
twice that size
Fault rupture mechanics – what to know
a fluids
b lithology
c thermal fields
d deformation rate/ direction
e dist of deformation
f geometry
Would like to know about anisotropy, fluid …. section boundaries – would like to have 3
resolution cells – down to 10 km say
 Rich Briggs: – brief little plots – raise question – with all of the geodetic data- will
demand geology to go with it… relationship of walker lane geodetic shear strain to where
it is accommodated. Primary problem is accounting for the slip budget.
 Anke Friedrich: what is today? that is focus of her talk… geodesy versus
paleoseismology versus geology
Growth and duration of faulting
One data set is topography
Thinks can see faults develop on 100m and 100k thousand years
 Geoff King: using ideas of fault propagation developed in Tibet and Aegean to explain
development of the Walker Lane – this addresses directly an idea on how particular faults
do turn on and off…
CFF used to look at activation and suppression of faulting – can play with this in the west
USA—most novel thought…in essence a kinematic model….
 Gordon Seitz – SDSU: developing paleoseismology offshore – using Tahoe – or for
Tahoe
Look at Tahoe as a ‘model’ basin of basin and range – perhaps can get longer
paleoseismic history. Age of slide is 50k – need core to get down to that level.
 Graham Kent: vertical offset west Tahoe – 0.5 mm/yr
Less on state line
Doesn’t really address the above questions
 Ron Bruhn comment:why can’t we explain gross physiography of B&R to general
public – it is so evident from even space – spectacular – should aim to understand it…
 Elizabeth Miller comment: high level of elevation in B&R – anomalous mantle=
Cenozoic lithospheric problem – hot spot – would be interesting to compare crustal
structure on both side of and across B&R – many hypotheses – what is the answer –
should be simple…symmetry is striking.
Breakout F
Relations of Economic Resources to Tectonics
(Structure, Magmatism, Fluid and Heat Flow)
We organized discussion of major scientific problems and opportunities for
EarthScope around the two major types of resources: Eocene mineral deposits of both
Carlin Au and porphyry Cu-Au-Mo types, and presently active Geothermal systems.
Issues related to oil and gas resources in a speculative overthrust domain, and to nuclear
waste disposal at Yucca Mt. were briefly considered.
Carlin-type Au deposits
The Carlin Trend was formed at 38±5MA, with similar deposits in Nevada
produces nearly 8% of the world’s gold, and has been documented in great detail by the
mining industry. It presents important geologic problems and potential data resources for
EarthScope. Geologic features requiring deeper resolution below the near surface zone
are:
Major controls are deeply penetrating, pre-Antler normal faults with NNW trend,
which are reflected in a major gravity discontinuity. How are these related to deep,
possibly magmatic sources of heat and fluid; fluid inclusions have a 3He signature
indicating a mantle derived component? The Northern Nevada Rift does not guide ore
deposits, but another near-parallel crustal discontinuity, the Battle Mountain–Eureka
linear does control ore deposits of multiple ages; why?
How are the deposits and minor Eocene dikes parallel to the deposits related to
deep magma sources. How big are the paleohydrothermal systems which formed these
deposits?
Are the major guiding faults truncated at the lower crust, indicating shear along
the boundary?
Porphyry Cu-Au-Mo Deposits
World-class deposits of the Park City-Bingham-Battle Mountain trend were
formed essentially contemporaneously with the Carlin deposits, although they are widely
separated spatially. They too are sources of important problems and potential data
sources.
While most porphyry deposits are formed in trends parallel to subduction in zones
of arc magmatism, the east-west trend of these deposits is perpendicular to and far inland
from the Laramide subduction zone, and not related to arc magmatism – a new model is
required.
Can any deep structural control be seen at depth to control this E-W magmatism,
and how is it related to driving forces of the Carlin trend. . Gene Humphreys’ “taco”
model offers a potential explanation for the magmatism, along with the southward sweep
of volcanism, but this needs testing by EarthScope imaging, and the discrete time period
of porphyry formation needs explanation.
Are the same magma sources responsible for the porphyries also responsible for
the diking in the Carlin trend, possibly at a higher elevation in the crust?
Present Geothermal Systems
These systems offer potential analogues for the paleogeothermal systems at Carlin
and other epithermal ore deposits, as well as offering both insights into active tectonics,
shear partitioning, high strain and fluid/heat flow. Being presently active, they do not
involve nearly as much interpretation as 40my old systems, and can provide immediate
feedback into geophysical interpretations.
Highest temperatures are normally on NE-trending faults, formed by active
extension (?), and highest temperatures indicate fluid circulation from 8 to 10 km.
He isotopes indicate a strong mantle magmatic signature in major fields along the
SW margin of the Great Basin. Strongly diluted deep input is indicated in the NW part of
the Great Basin, while most of the basin is devoid of 3He except in isolated occurrences.
The 3He signature offers an indication of structures penetrating the lower crust, and zones
of hydrous weakening.
Geothermal offers possible links to indications of fluids in midcrustal brightspots,
possible links to high conductivity magnetotelluric anomalies deeper in the crust, and
possible correlations with mantle tomography.
Department of Energy is providing significant funding for geothermal research,
which could be available to EarthScope projects.
Unexplored Overthrust Belt with Petroleum Potential
There are speculations on an unknown overthrust belt in central Nevada, and the
question has been asked “are there any autochonous rocks in Nevada”.
This speculation received recent impetus with the discovery of oil in a thrust
regime at the eastern edge of the Great Basin in southern Utah. If westward extension of
this thrusting is confirmed, this could provide a major incentive for major oil companies
to support very significant detailed seismic investigations and increase the oil industry’s
interest in EarthScope.
Yucca Mountain
Investigations of nuclear waste disposal involve significant issues of potential
volcanism, related to basalt conduits and older calderas. These are of scientific interest to
EarthScope and could attract DOE funding for projects.
General Aspects of Industry Involvement with EarthScope
The resource industries are the repositories of large, near-surface data sets and
geologic understanding which could provide useful boundary conditions on deep
geophysical experiments. While to a much less extent potential sources of funding, the
industries could provide useful collaborators in EarthScope investigations if mutual
interests can be established by individual contacts. Such collaboration would in be
required to obtain significant input from most companies. There is a very diverse set of
capabilities and interests within companies involved in the different industries, and each
must be approached on a case by case basis.
Breakout G
Walker Lane Breakout Group
Big Questions:
 How does a strike-slip fault system develop
 What controlled distribution of strain?
 Coupling between upper mantle and crust?
 Dynamic and kinematic links with plate boundary far field and local stresses?
 What is Walker Lane? A complex system of dextral faults that interact with Basin and
Range Extension.
 When did it start? How long where and why and in what pulses did it move?
 How does the crust accommodate simultaneous extension and strike slip?
 What controls the location?
 Anything at depth-lower crust and mantle that specify this feature?
 How can EarthScope help us?
 Rich Schweickert: U. Nevada Reno
Believe they have figured out how extension and strike slip is accommodated in the Lake
Tahoe Region. Two figures, the bathymetry and fault map and a fault domain map.
3 major zones of currently active faults N. Tahoe incline village fault down with slight
oblique, and Tahoe Sierra Fault is the westernmost one and there is one in the middle I
forgot the name to. Kinematics and Rate of Activity along these now known in fair
detail.
Lake Tahoe basin and Carson valley are bound by large normal faults that are seismically
not so active, but faults are active. There are other regions with lots of little earthquakes
at N end of Lake Tahoe Basin and near Gardnerville. These have strike slip first motions,
both left and right lateral. So normal fault segments are still, the “transition zones are
moving.
I wonder what comes first? They cant tell if opening angles are small to the n-s or to the
east west Rich says they are flipping, at least that what they look like they are doing.
EarthScope: Need detailed experiment, not Bigfoot. Need to understand the temporal
and spatial partitioning of the two types of faulting. I think it would be easier if they
were just normal fault terminations. Or that they are left-stepping strike slip faults. Craig
Jones suggested keep looking at earthquakes.
It’s a great example of something important…Can you map an area that has gone on for a
longer amount of time and figure out how this strain has organized with greater strain?
 Pat Cashman U. Nevada Reno
Looking at sedimentary successions in the Walker Lane Zone.
Most of the record is in the Basins, and many of these basins are not exposed, but we
wish they were. Near Reno are exhumed basins, so you can look a them.
Gardnerville Basin, Verdi Basin Boca Reservoir, Long Valley and Hone Lake, Virginia
highlands Sunrise Pass
Gardnerville. 1/2 graben bound by normal fault, fanning upwards sequence, sit on
crystalline rocks in Pine Nut, where they are 7 Ma, there are older sequences beneath the
Basin, deposition continuous upward. So fault began its slip prior to 7 Ma
Virginia highlands: 12-7, tilted, overlain by 7 my basalts
Verdi basalts interceded low in basin, just over 10 Ma, continuous sedimentary basin, no
evidence for active faulting in the basin, topped by 3-2 Ma, then cut by faults
Boca Basin 9 Ma volcanic rocks at base, on up to 2.5 then faulting.
Long Valley 10 Ma to 4, then faulted. Honey Lake as old as 10.5 and as young as 2.5.
The exhumation is much younger than they thought.
Altogether, all basins began to form (elm by inference faulting began) not much older
than 10 Ma- probably not before 11 Ma. This is pretty close to Joe’s faulting ages as
recorded by apatite fission track dating.
Doesn’t think the basins are necessarily connected.
Change in regime at 3 Ma???
EarthScope: How can geophysics help? How about studies of analogous basins that
aren’t uplifted and deformed? Joe Colgan’s NW Nevada region has 10 Ma faults, the
basins are all subsurface, but they should have a similar stratigraphy.
Why do basins like this get inverted? Is there a general reason, does it have something to
do with the transition zone?
 Chris Henry: Why is the Walker Lane there?
Strike slip faulting has jumped eastward across the cold strong batholith. On the north
end, though, it goes right across the Sierra Nevada….as the sierra turns east,
magmatically speaking.
Suggests that 15 Ma magmatism warmed the area up….Ancestral Cascade magmatism it
is called. Best evidence for Cascade arc, 27 Ma andesites, but at 17, very well defined,
clear cut arc going straight up north to the Cascades.
Great Movie. Shows the ashflow tuffs in central Nevada. All magmatism 15 Ma and
younger—the westernmost edge of magmatism perfectly coincides with western edge of
project? Yeah!! There is a good point here.
How about looking at seismicity? Is seismicity associated with structures that are
conduits for young lavas? Are there any xenoliths? No, they are andesites to basalts.
Thinks they are arc. Could be all subduction related?
The British Wonder who lives in Paris speaks. What is his name? Slip partitioning. I
don’t think his talk has much relevance to the projects at hand, so I am not going to take
many notes.
Let’s get back to the Walker Lane.
Basin sequences and their inversion as indicators of important changes in fault orientation
and other regional tectonic events. The fact that there are many big basins in the Basin
and Range that have deposits dating back to the Oligocene or Miocene, are still bound by
the main range bounding faults and are not cut up or eroded or anything, mean that
history of faulting, crustal structure, strain history has remained pretty much the same for
a long time. When these basins are cut up and/or exposed, and tilted, it is generally
because of a singular event or change. Should be able to figure this out because you have
the deposits that give you the original orientation of the valley and then the superimposed
faults that cut the basins. How about areas that are just elevated and the basins are only
dissected? That is something else instead. I think a careful cataloguing of what is known
about basins and fault slip history combined is something we should strive for. Also, it is
important to tell whether basins are the result of fault bound motion, and are distinct,
versus large regions of regional deposition. It sounds like the fanning upward sequences
of Cashman's basins suggest they were individual fault bound basins. Should have at
least a couple of km of section, if they are significant fault bound basins.
Back to the big questions:
1. If we are going to find out if this is a crustal or mantle penetrating structure, we will
need to image it by controlled source experiments.
2. Key: to get together with Margins Initiative results—this is the beginning of the Gulf
of California Story. Very important.
What’s his name- what is holding you guys back re seismic reflection, I mean we just got
a 20 M 3-D seismic vessel. .
Thumper Trucks. They will do it.
But… maybe not such a good idea to try to image the walker lane (elm) too complicated,
what will you see, etc. etc.
3. When did strike slip faulting start? Why should strike slip move inland?
Test this: Did locus of strike-slip faulting move west with westward encroachment of the
Basin and Range? How about older stuff?
4. Geodesy? Does it tell you what the upper crust is doing? Or what the mantle beneath
the upper crust is doing? Block rotations in shear zone?
5. Where does the dextral shear go further to the North? Gaining momentum (what does
that mean?) goes to N-S shortening, do people have evidence for this? Jim says it goes
into NW great Basin extension.
Can you create the mess that is Walker Lane by just moving the locus of deformation
around within the general zone? Maybe it is simpler than it appears….
I like the idea of it being the proto-Gulf of California spreading center.
6. Gene: What are the energetics of initiating and driving such a system? Balance of
forces?
7. PBO role: Constraining width of shear zone
8. Mantle anisotropy and crustal anisotropy
Ray points out that if Gulf is an example, we should have already made oceanic crust
here, and we haven’t. Seisoman says in Baja, some people think it is 12.
How do we self-organize?
Penrose Conference coming up.
9. Wed margins with walker land work. Steve Harlan says that it won’t work because
Marine G and G will not go for it.
10. Pick an area to focus on. Tahoe to the east? Northern Nevada?
Breakout H
Seismic and Geophysical Methods, Crust and Mantle
Phinney, moderator; Louie, recorder
This group considered questions of outreach and self-organization first, before discussing
scientific grand challenges and the Earthscope legacy in the Great Basin. Since
Earthscope was proposed and designed with geophysical objectives in the forefront, the
group wished to simply emphasize a few key issues.
In considering interesting examples of outreach activities, the group noted the high value
of small, local educational projects. An example is the Nevada K-12 Seismic Network of
K. Smith and C. Snelson. This effort is distinguished by the fact that educators are
leading the educational tasks, with seismologists providing technical support. Schools are
competing for stations, and the effort will directly involve students in Earthscope, as it
happens. Smith has also developed and effective and inexpensive network
communications architecture that Earthscope may benefit from.
This group of mostly crustal geophysicists, with many controlled-source seismologists,
came to a consensus to promote three projects in subdisciplinary self-organization. The
first is to promote the use of legacy data and models. New data and models benefit from
comparison to previous results, and existing data will fill in some geographical gaps in
Earthscope flex-array efforts, as well as the significant gap of crustal and uppermost
mantle depths that USArray is not addressing. A significant example of the conservation
of legacy data is the NSF sponsorship of Cornell to put the COCORP shot records on
line, and the three years of cold storage for their original field tapes. Snelson pointed out
that current earth science efforts to build data archives and libraries do not cover
digitizing of analog (or paper) data; nor are these libraries yet offering effective database
environments or any data analysis tools.
The second consensus on self-organization was that Earthscope geophysicists need to
develop a test site or proving ground. A convenient location having previously-identified,
well-characterized features could be occupied to prototype flex array deployments,
calibrate techniques, and determine the most efficient survey techniques that can find a
structure or constrain a phenomenon. A test site would also promote the interdisciplinary
collaborations that are crucial to expanding the funding base of this group’s work: among
the controlled-source, natural-source, MT, geodetic, and potential-fields geophysics
communities. There was some enthusiasm for putting Socorro forward as a test site (with
its magma bodies of interdisciplinary interest, proximity to the USArray operations center
at NMT, and prospect of catching mass movements in 4-D).
The final consensus on self-organization was the immediate need for a community
modeling environment (CME). The time pressure of having the transportable array in the
Great Basin, in only two years, requires that some elements of a community model be
distributed as soon as possible. Then a properly designed modeling environment can be
developed incrementally in collaboration with efforts like SCEC’s. J. Louie presented an
example tool that builds on the ideas of the SCEC CME, in that it integrates wide-scale
with local data. But the tool’s functionality is limited to creating velocity grids for
synthetic seismogram modeling, from legacy data and results. The tool’s simplicity is
such that it is available now to all investigators. This breakout group wishes to promote
such efforts, so long as they are coordinated with those of other disciplines and centers
such that a common software framework and comprehensive CME will result. Along
with the need for this incrementally developed CME, there are needs for computational
resources, and for the calculation of a synthetic test data set to challenge both active- and
passive-source imaging routines (like the Marmousi synthetic among petroleum
seismologists).
The breakout group did have three topics to emphasize among the ideas that belong to the
category of grand challenges to Earthscope, or of Earthscope’s legacy. The first is a call
to find new methods for more efficient, more affordable controlled-source experiments.
These methods may be developed in collaboration with other groups and disciplines, such
as with the Network for Earthquake Engineering Simulation (NEES). NEES vibrators are
already scheduled for testing under Klemperer’s Earthscope-funded experiment in the
NW Great Basin this September. The USGS is another group where collaboration
between controlled-source seismologists has been and will continue to be very fruitful.
Collaboration with various industries will help to accomplish controlled-source surveys,
through recording of mine blasts, the use of legacy industry seismic lines, or even “group
shoots” with petroleum exploration interests. Collaboration with regional seismic
networks may promote development of natural-source imaging.
The second grand challenge is to map how anisotropy varies with depth. This information
keys into mantle motions and ancient continental fabrics. However, depth analysis of
anisotropy requires excellent constraints on crustal variations. There is much legacy data
to yet to mine for such analyses, as well as much effort that will be needed in this
direction on USArray data.
Finally, a key legacy of Earthscope will be the ability to resolve structure at 1-2 km
resolution (principally in the crust and upper mantle). This group anticipates that such a
goal will guide the use of USArray’s flex array. Resolution capabilities can be proved at a
community test site, and recording methods developed to sample the teleseismic
wavefield at <10 km spacing. To achieve such fine resolution, the inversion and
interpretation of all such data will be an interdisciplinary activity, including specialists in
controlled-source seismology, hydrogeology, and TM among others.