John Anderson, (Chair) University of
Nevada, Reno
Rick
Aster,
New Mexico Tech
Glenn Biasi, University of Nevada, Reno
Geoff
Blewitt,
Nevada Bureau of Mines and Geology
Jim
Faulds,
Nevada Bureau of Mines and Geology
Lew Gustafson, Independent Consultant
Gene Humphreys, University of Oregon
John Louie, University of Nevada, Reno
Jon
Price,
Nevada Bureau of Mines and Geology
Phil Wannamaker, University of Utah
Steve Wesnousky, University of Nevada, Reno
EarthScope ushers in a new era in
exploring the Earth’s interior. The
history of Earth science ubiquitously reveals that major advances in
understanding have been driven by major advances in data collection. For example, the development of plate
tectonics relied fundamentally upon the intensive exploration of the ocean
floor following World War II. Viewing
the next decade and beyond in this light, we believe that primary new data sets
driving Earth Science on the continents and beyond will come from the
EarthScope Observatory.
Major components of the EarthScope
Observatory are scheduled to occupy the Great Basin in the time frame 2005-2007
and beyond. An enhanced scale of
community self-organization will be necessary for the first Major Research
Equipment and Facilities Construction initiative in our field to achieve its
full potential. We suggested for that reason that this was a propitious time
for a regional workshop exploring key science and outreach issues. Thus, we proposed a workshop on the Great
Basin and its Margins. The workshop was to be a multidisciplinary (geologic,
geodynamic, seismic, gravity, magnetotelluric, geochemical) workshop
structure.
The workshop organizers were informed about the
positive funding decision in October 2003.
The initial announcement was distributed in January 2004 (Appendix 1).
The workshop met from June 21-23, 2004 at
Granlibakken Resort and Conference Center, Tahoe City, California. The final agenda is shown in Appendix
2. Nearly 90 participants came to
Granlibakken for all or part of the workshop (Figure 1). A participant list is given in Appendix 3.
The workshop was timed to run back-to-back with the EarthScope Science and
Education Committee (ESEC). Thus it
became easier for members of ESEC to also attend GreatBREAK, and several took
advantage of the opportunity.
The
Basin and Range province (Figure 2) is a remarkably long and wide swath of
post-orogenically collapsed interior mountains. Before collapse, the mountains were presumably created by the
strongly contractional subduction-related Sevier-Laramide orogeny ~50-100 m.y.
ago. This orogeny was the most intense
of a series of tectonic and magmatic events that progressively grew and
reconstructed the western continent obliquely across earlier Archean and
Proterozoic structures. Mysteriously,
the onset of extension was associated with one of the largest continental
magmatic events known, and, also mysteriously, the region remains high standing
and intensely volcanic while being dismembered by transform entrainment with
the Pacific plate. The northern extent
of the Basin and Range coincides with the tracks of the Yellowstone and
Newberry hotspots, and the southern Basin and Range, which includes most of
Mexico, appears to have largely ceased tectonic activity coincident with the
transform opening of the Gulf of California 5 m.y. ago.
Although
extensional provinces are common in the geological record, few major
continental extensional domains are currently active and none are larger; the
northern Basin and Range therefore provides the best opportunity for studying
the processes underlying this phase of orogeny. This phase of activity is important to the Earth sciences because
1) during the course of extension the mass and area of continental lithosphere
have grown substantially, 2) the structural fabric of the continent,
consequences of both magmatic segregation and tectonic scrambling, have
overprinted prior structures, and 3) such extension may well be a primary aftermath
of orogeny.
The
initial part of the meeting focused on summaries of some of the characteristics
of the Basin and Range that are relevant for Earthscope.
Earthscope
can be expected to result in many surprising discoveries that will be placed in
the context of recent geophysical observations. From the geodetic viewpoint, the greatest activity is around the
margins of the Great Basin (Figure 3, 4, Blewitt; Figure 5, Bennett et al
2003). Figure 3 shows a
well-constrained zone with the highest shear strain rates along the San Andreas
system in California, and a second zone that is somewhat broader and less
intense along the Walker Lane, on the eastern side of the Sierra Nevada
mountains, branching northward from the San Andreas fault at the eastern end of
the San Andreas Big Bend. Figure 4
shows that major dilational zones are along the Wasatch Front in Utah, and
along the Walker Lane of Nevada and eastern California. While overall the Great Basin is extending,
those parts of eastern Nevada and western Utah best constrained by a high
density of observations are showing little extension or even low magnitude
conpression. Figure 5 generalizes these
zones and suggests a terminology convention that will be useful for subsequent
discussions.
Geological
studies summarized and extended by Wesnousky (Figure 6) are consistent with
this interpretation, although the boundaries of the provinces in Figure 5 would
need to be redrawn somewhat to be consistent with the geology, and the
northward extent of these geodetic provinces is poorly constrained by the
geodetic data and not clearly consistent with the available geological
mapping. Earthscope needs to support
sufficient geological research to get much more information on active faults,
in conjunction with the geodesy to constrain contemporary deformations. GPS results from several parts of the Great
Basin indicate that the uniform model for strain accumulation is probably not
consistent with observations (Figure 7), although the explanation for the
variability is not known. An exciting
opportunity for Earthscope is to understand the causes of this variability,
which may be related to processes in the lower crust or upper mantle.
From
the seismic viewpoint, historical activity is spatially consistent with geodetic
observations (Figure 8). Earthquake
occurrence rates (Figure 9) are consistent with the geodesy within
uncertainties. However, the sum of
seismic moment rates over all well-characterized faults in the region is a
factor of two to four smaller than the geodetic or historical rates. This points out the importance of a
significant amount of detailed geological mapping and fault characterization to
determine if that is the cause of the discrepancy.
Figure
10 shows locations of refraction profiles in the western US. Two features stand out. One is that there is little data since these
pre mid-1980’s studies, and the density of these studies in the Great Basin is
quite low. Larry Brown pointed out that
the capabilities for controlled-source seismic studies have improved enormously
so that for all of these profiles a much more detailed interpretation would be
possible if the survey were repeated using modern technology. Among the interesting targets for improved
controlled seismic source studies are seismic bright spots with possible
mid-crustal fluid origins (Figure 11), possible double-Moho reflections,
anisotropy, flatness of the Moho, low-angle normal faults, and opportunities to
trace deep structures back to the surface.
Given the sparseness of studies, it is possible that bright spots are
far more common that shown in Figure 11.
It is also possible that magma movement at bright spots could be
responsible for some of the fine structure on the GPS observations (Figure 4).
Magnetotellurics offers a means of constraining the
degree of hydration and tectonic activity of the lower crust (Figure 12). Both the eastern and central parts of the
Great Basin have conductive lower crust, but in the eastern Great Basin, lower
crust is generally more conductive. The
conductive zone is interpreted to contain up to a few tenths of a percent of
highly saline fluids interpreted to originate through exsolution from
underplated basaltic melts. These
fluids can be expected to have a first-order effect in reducing lower crustal
viscosity. Melt also causes conductive
zones, sometimes with correlated high heat flow. Better discrimination between melt and other conductive fluids
would result from joint geophysical interpretations of conductivity, seismic properties, and thermal regime.
Phil Wannamaker and students at the University of
Utah have been assembling a comprehensive magnetotelluric transect of the Great
Basin-Colorado Plateau transition zone in southern Utah. One of the goals has
been to examine the mode of ongoing consumption of the Colorado Plateau by
extensional processes of the Great Basin and to determine the geometry of the
deep transition. A preliminary two-dimensional inversion of the newly completed
results appears in Figure 13. Two conductive, diapiric structures in the upper
mantle project upward toward concentrations of low resistivity in the lowermost
crust and are interpreted to represent mantle melting, basaltic crustal
underplating, and fluid exsolution from melts. The western region lies approximately
below the Sevier Desert-Grand Canyon Quaternary basaltic eruption trend (Nelson
and Tingey, 1997). The eastern zone lies under only slightly extended crust of
the transition zone, suggesting that extension at depth significantly exceeds
that at the surface (vertically non-uniform). Also seen is an interesting set
of nested, whole-crustal detachment-like structures in the transition zone
soling into the lower crustal conductor, although the displacement is
indeterminate. The Colorado Plateau interior has only a slight, deeper lower
crustal conductor, perhaps in keeping with its nearly stable state. A recent
review of the structure and tectonics of this region appears in Wannamaker et
al. (2001).
The meeting attracted several scientists whose research
empasis is mapping and interpreting the geology of the region. It is recognized that the behavior of the
Great Basin is potentially influenced by its entire history, starting with the
Proterozoic geological backbone (Figure 14).
For instance, Humphreys and Karlstrom both suggested that the location
of the Wasatch Front could be controlled by the Proterozoic boundary between
the Mojave and Yavapai provinces. This
earliest imprinting, together with the subsequent Antler, Laramide and other
Phanerozoic orogenies, undoubtedly had considerable influence on the geology,
volcanism, and nature of extension that began when strike-slip motion began
along the Pacific coast about 40 Ma.
Geochemistry of the subsequent volcanism, associated with the extension,
carries the imprint of these earlier geological events, and also can be very
informative about the temperature, curstal and lithospheric thickness,
permeability of the lithosphere, and presence of water (Figure 15).
A significant event, occurring around the end of
subduction on the Pacific coast west of the Great Basin, was the wave of
volcanism (ignimbrite flare-up) that swept from north to south. The event is somewhat puzzling since at any
one time the main trend of the volcanism was east-west, normal to the trench.
Carlson showed a very effective movie illustrating how it occurred. Humphreys proposed the “Taco model” where
the underlying slab folds along a trench-normal axis as it sinks like a folded
taco, beneath southern Nevada.
There was a major pulse of extension about 14-19 Ma,
as shown by Elizabeth Miller (Figure 16).
Illustrating the hypothesis that the boundary of the Great Basin is
slowly eating its way into the Sierra on the west and the Wasatch on the
east, Miller showed a possible extension
and faulting cross section along a latitude just south of Carson City (Figure
17). A well constrained, detailed cross
section consistent with geodynamic modeling should be an Earthscope goal.
There was much discussion at the workshop about the
Walker Lane, and its similarities and differences to the San Andreas system
(Figure 18; e.g. Wesnousky, Faulds, King).
There seemed to be a consensus that the plate motion that takes place
currently on the San Andreas system is likely in the process of jumping inland
to the Walker Lane, and thus the Walker Lane is a possible example of the early
stage of the evolution of a major strike-slip fault plate boundary. Jim Faulds (Figure 19) suggested that the
jump of the North America – Pacific plate boundary will be complete when the
triple junction currently at Cape Mendocino has moved north to approximately
the California – Oregon border. In the
meantime, it is necessary to explain why the strike-slip offset across the Walker
Lane is greater in the south than in the north. One possibility is that slip is “bleeding off” to the northeast
along fault systems such as the Central Nevada Seismic Belt. Northeast-trending fault systems are also
preferentially the locations of geothermal systems. Questions for Earthscope to investigate include how strike-slip
systems develop, what controls strain localization, and the forces that are
driving the Sierra Nevada to move, including the relative role of forces
applied from the mantle, from the edges, or from the elevation of the Great
Basin.
World class Eocene mineral deposits, porphyry
Cu-Au-Mo and Carlin-type Au, offer well documented evidence of magmatic and
hydrothermal activity related to deep-seated and little understood structure,
and pose important questions for Earthscope (Figures 20).
Active geothermal systems are manifestations of
deep-seated structure and heat flow. As discussed below, there is much
potential for collaboration with the resource industries in the Great Basin, as
an understanding of these important resources is strongly linked with the
overall tectonic history. The mineral
industry companies have considerable geological and geophysical data,
particularly focused on evaluating structures in the upper two kilometers of
the crust.
Extensional tectonics on the
largest scale.
Moderator: Dennis Harry
Recorder: Gene Humphreys
Succinct
summaries were delivered by eight participants (see below) and discussed by the
group, followed by general discussion about the nature of deformation in the
northern Basin and Range and about how best to study it.
A
host of observations (some mentioned above) potentially bear on the
fundamentals of this activity. For
instance, while high gravitational potential energy (PE) and relatively low
strength are generally thought to be essential to active northern Basin and
Range tectonics, these same attributes appear to characterize the inactive
Mexican Basin and Range. Origins of the
requisite weakness and high potential energy, presumably inherited in some fashion
from the Laramide-Sevier orogeny, have not been explained, and the evolution of
the trans-tensional Mexican system to a narrow oceanic system in the Sea of
Cortez provides a point of comparison for the broadly distributed
trans-tensional northern Basin and Range (in which the transform component is
relatively young, suggesting to some an earlier stage of the same evolutionary
sequence).
Principal questions are:
Participant Summaries.
David
Blackwell. The broad deformation zone
of the Great Basin is a consequence of the broad zone of thermal weakness,
itself a consequence of its backarc setting.
Dennis
Harry. When considering the processes
of Great Basin extension, one must include the dynamic thermal evolution. For the Great Basin, important factors
include the initial hot and weak Sevier welt.
Crustal thinning leading to lithospheric strengthening can propagate
deformation to the rift margins and thus produce broad extension.
Craig
Jones. The transition between the
northern and southern Basin and Range is a profound boundary associated with a
step in topography, the narrowest width of the province, and the area upon
which mid-Tertiary magmatism converged (although it avoided magmatism itself);
this should be a focus of study.
Bill
Hammond. Dilation, shear and rotation
are all concentrated in the western Great Basin; the eastern margin is
moderately extensional. Three nearly
rigid domains can be defined: NE Nevada, southern Oregon, and NE California-NW
Nevada. Relative motion between blocks
results in central Nevada extension and northern California contraction.
Wanda
Taylor: If Sevier thrusting is important to current extension, we need to understand
the thrust history and kinematics.
Thrusting was unusually steep and deep on the west side of the Sevier
thrust belt.
Fiona Sutherland. In the
Gulf of California, there was about 5 m.y. of distributed continental
Proto-Gulf transtension prior to the opening of the Gulf about 6 m.y. ago. The earlier phase was quite similar to that
currently active in the Great Basin.
Jim Trexler. Great Basin extension is clearly 10-12 Ma,
but the superposition of shear began only 2-2.5 m.y. ago in the Walker Lane.
Chris
Henry. Proto-Gulf extension occurred in
a thermally weak zone, and was accompanied by central Mexican extension. Extension occurred amagmatically in the area
that was later (?) magmatic in the mid
Tertiary; the intervening Sierra Madre Occidental was like the Sierra Nevada
(prior volcanic arc now acting rheologically strong).
Rheology
of the Mantle and its Relation to Current Tectonics; Why Are Some Parts of the
Basin and Range More Active than Others
Moderator:
Glenn Biasi
Recorder:
Rick Aster
Members of Breakout B shared a general recognition
that the rheology of the mantle has to be addressed in the context of the
rheology of the entire lithosphere.
Certain understandings set the backdrop for the
breakout discussion. Rheology is a
measure of the ratio of the shear strain rate to the stress. GPS shear strain maps have provided an
excellent and enlightening view of where, and to some extent how the crust is
deforming. Strain in the Great Basin is
concentrated on the east and west margins.
On the east side GPS suggests almost pure extension beginning somewhat
east of the Wasatch Front at about two millimeters per year. The central Great Basin comprises a more
rigid domain, spanning from central Utah to central Nevada. West of the Central Nevada Seismic Belt,
extensional and right-lateral shear strain combine to a strong NW shear that
matches the Sierran NW translation velocity.
A NE-trending re-entrant in the western strain field occurs at the
Central Nevada Seismic Belt that extends to around 40.5 degrees north. The strain field observed in years of GPS
monitoring is consistent with a paleoseismic transect. Rates inferred from paleoseismic slip
measurements over the last 20 ka match well both in the high and low strain
regions.
Sources of geological stress are much less well
understood. Main sources include
gravitational potential energy gradients arising from the topographically high
central Great Basin, and shear expressed in translation of the Sierra Nevada
block. The breakout discussion
recognized that data are presently insufficient to determine whether shear flow
east of the Sierran block is driven through edge stresses in the crust, or
viscous coupled flow from below, or some combination of the two. Likewise estimates of lithospheric stiffness
and the present expression of the gravitational collapse bear further
investigation.
The
discussion is here summarized into four themes.
Theme I:
Understanding the Style of GB Extension and Our Ability to Resolve Lithospheric
Rheology.
Theme II:
Fluids; Is magnetotellurics a perhaps underappreciated tool that can
significantly reduce non-uniqueness of mantle modeling (e.g., seismic)?
Theme III:
How can we reliably distinguish partial melt?
Theme IV:
What does the Moho tell us about the rheology of the mantle?
It was the consensus of the group that understanding
the rheology of the lithosphere is clearly an inherently interdisciplinary
topic because it incorporates geology, geophysics, mineral physics, and other
disciplines. Lithospheric rheology
strongly depends upon bulk composition, temperature, and hydration state. Geologic history can locally provide insight
into likely composition at depth, but reliable regional syntheses are needed to
decide how well locally understood geologic conditions can be extended for
purposes of regional tectonic modeling.
The group recognized a general need for better constrained geotherms and
heat flow data. While the Great Basin
is clearly elevated as a heat flow region, the use of the heat flow data to
infer Moho temperatures may be restricted to general ranges. Heat flow data have been compiled by Dave
Blackwell and made available in spreadsheet format, but a systematic effort to
uniformly correct the data for refraction and other systematic effects has not
been undertaken.
Phil Wannamaker discussed magnetotellurics as a
means of constraining the degree of hydration of the lower crust, as described
above. (Figures 12,13).
Tony Lowry
presented an interpretive means to infer regional equivalent elastic thickness
of the lithosphere. The method relies
on crustal thickness estimates compared to topography, and adjusts elastic
thickness to maximize their decorrelation.
Bigfoot stations will improve estimates and coverage of crustal
thicknesses, contributing both to understanding the elastic and crustal
thickness in general. Seismicity
patterns follow variations in elastic thickness, but concentrate in the regions
of thickness contrasts, and are generally not everywhere available.
Glenn Biasi presented a tomographic image from 50-70
km depth covering California and western Nevada with principle faults on it as
a possible direct means of mapping upper mantle relative viscosity. P-wave scaling that makes the mantle relatively
faster also can be expected strengthen it.
Correlation with mapped faulting where inferences of strength in the
mantle are generally encouraging, but the scaling of velocity to strength is
not unique. Also, the absolute
velocities are removed in imaging, and as a result, only relative strength can
be inferred. This effort may provide
the EarthScope community with a direct spatial means of inferring relative
strength of the upper mantle. Comments
from the breakout group indicated interest in the method if it can be better
quantified.
The breakout group also discussed the degree to
which the crust and mantle are welded and what processes might contribute to
mechanical decoupling at the Moho.
Regions with large crustal extensions have been noted to commonly have
sharp Moho contrasts in receiver functions, reflection, and wide-angle
refraction. The Moho is also reported
to be relatively flat in some such regions, as though perhaps sheared and
re-equilibrated by lateral flow. In
these regions, some amount of new lower crust, likely by basaltic input,
appears to be required to support present crustal elevations. Together these features suggest a time of
low integrated strength and little or no welding of the crust and mantle. Elsewhere, such as in the Sierran block,
little or no relative motion of the crust relative to the mantle has
occurred. On mechanical grounds a
wholesale disconnection of the crust and mantle seems difficult, but
quantifying these relative mechanical strengths will be essential to
understanding the rheology of the Great Basin.
What
is the mantle and lower crust in the Great Basin doing now?
Moderator:
James Ni
Recorder:
Geoff Blewitt
Two major categories of process emerged in the
discussion: mantle processes, and crust-mantle coupling. EarthScope provides an opportunity to
discover major unrecognized mantle features, some of which may control
important dynamic processes. Examples
include the “Sierra Nevada drip,” and features relating to mantle flow associated
with the trailing edge of a subducting slab.
Joint inversion techniques require development to better infer the
density of mantle features.
Crust-mantle coupling and lower-crustal flow may be
responsible for transient phenomena in geodetic data. More accurate determination of GPS station position time series,
along with improved methods for imaging lower-crustal rheology through seismic
anisotropy are required. An
interdisciplinary approach is absolutely essential toward explaining geodetic
transients, as evidenced by recent successes in combined interpretation of
geodetic data with microseismicity.
Collocation of seismometers at PBO sites is strongly recommended to
improve the potential for discovery in this emerging area of interest, which
may have important implications for seismic and volcanic hazard assessment.
Contrasts
between the Eastern and Western Great Basin
Moderator:
Ron Bruhn
Recorder:
Phil Wannamaker
A fundamental difference between the eastern and
western margins of the Great Basin province is existence of Proterozoic
basement underlying the entirety of the former, in contrast to
Paleozoic/Mesozoic oceanic-type accreted terrains underlying the latter. We
don’t know with much certainty where that transition lies, other than somewhere
in central Nevada. In his keynote address, Gene Humphreys noted that the
Wasatch is near a boundary of late Proterozoic age in the Precambrian basement
of the eastern region. Karl Karlstrom showed that more generally, in the
Proterozoic, the tectonic provinces trend in an east to northeast direction
through North America. This raises the
basic question as to what existing structure, if any, was involved in defining
the current N-S oriented transition to the Colorado Plateau.
The 100 km-wide margin between the eastern Great
Basin and Colorado Plateau contains a thick band of seismicity (Figure D1,
Christine Pankow). This seismicity occurs east of the major normal fault
defining the Wasatch front, below mountains with high elevation and little
cumulative extension, but also high heat flow and high lower crustal electrical
conductivity. In contrast, the seismicity on the western margin is mostly east
of the major western-most normal faults, beneath the Basin and Range
topography. What is the cause of this
extended eastern seismicity – does it represent uniform or fundamentally
non-uniform extension versus depth? How might the mode of extension vary along
the transition, particularly in response to preconditioning by middle Cenozoic
plutonism? Extension appears to been essentially normal to the province
boundary, which is in contrast to the western Great Basin which exhibits a
large strike slip component in the Walker Lane fault zone. However, the latter
began only 5-6 Ma so the two province margins may have been more similar prior
to that.
Despite much symmetry across the Great Basin in
elevation, heat flow, crustal thickness, and gravity/magnetic patterns, there
is strong evidence that the degree of apparent upper crustal extension is quite
different. In particular, northwestern Nevada shows little upper crustal
extension but possibly substantial thinning and magmatic underplate. Leading
models for the eastern Great Basin, however, imply substantial upper crustal
extension to go along with the overall crustal thinning. An exciting new
technological advance is the improvement in estimates of vertical crustal
motion from GPS, which may address the possibility of magmatic inflation or
regional changes in hydrology. A fundamental goal to achieve by the end of
EarthScope is to know similarities and differences between the geometry,
kinematics, and dynamics of Great-Basin widening along the Sierran and Wasatch
fronts.
EarthScope data and modeling may address these
questions via a number of approaches. Mid-scale lithospheric thickness
variations may represent earlier strength variations, and terrain boundaries
may be mappable using anisotropy images. We know crustal thickness in only a
few places but this may be improved greatly through receiver functions at the
Bigfoot sites and in follow-up studies. Crustal thickness and rheology
estimates can help determine gravitational potential energy variations as a
driver of margin extension and seismicity. More work is needed on the copious
plutonic complexes to map out isotopic terrain boundaries on the basis of their
chemistry.
Of more direct societal interest perhaps, auxiliary
science under EarthScope may improve estimates of sedimentary thickness in
grabens especially toward the province margins where the population centers are
located. This has applications for ground shaking and seismic hazard, and for
reconstruction of the elevations of the eastern and western province areas for
the pre Late Cenozoic.
Self organization of the community is necessary to provide
optimal plans of attack and to maximize use of limited resources. In particular
regarding active source seismic experiments, agency partnerships toward a
common goal are clearly advantageous.
What
do we know about how faults behave over time? Do they turn on and off, speed up
– slow down? If so, why?
Moderator:
Anke Freiderich
Recorder:
Steve Wesnousky
General
Summary of Discussion
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
geodetic and seismological measurements of PBO and USArray, respectively. To ultimately reach a goal of understanding
the gross structural and physiographic characteristics of the Great Basin, we
will require an understanding of the behavior of faults since the inception of
extension in the Miocene. As well, this understanding will be required as a
framework to view and interpret analyses of seismological data that will be
collected with USArray, and geodetic observations of PBO.
Understanding the evolution of fault systems and
tectonic environments requires studies of fault behavior over all time scales
encompassing the evolution of the Basin and Range. Initial observations have shown discrepancies between geodetic
rates of strain accumulation and those measured by geologic studies. We
concurred that defining and understanding these discrepancies was important to
understanding ongoing deformation of the Basin and Range. . Thus far, studies
of active faults are spatially limited and generally consider only the last 20
– 40 ka of earth history. Extending the
spatial coverage as well as the time period over which fault slip rates are
averaged will be needed to ultimately understanding the relationship of strain
accumulation to strain release on faults in the Great Basin. It was agreed that the dating and mapping facilities outlined in
the GeoPBO white paper are needed to successfully achieve these objectives.
At yet longer time scales, their was unanimous
agreement among the group that effort toward documenting the inception,
development, and evolution of the Walker Lane strike-slip fault system is a
problem of fundamental import to understanding both the evolution of a fault
system as well as the western North America plate boundary. Similarly, a number of recent measurements
have suggested that individual mountain ranges within the Basin and Range are
characterized by rapid pulses of displacement and uplift in the Miocene with
intervening periods of relative quiescence.
The observations suggest that foci of fault deformation migrate over
time. Efforts to define these temporal
changes are critically important to understanding the pattern of faulting and
evolution of Basin and Range structure.
Understandng the pattern of faulting and evolution of the Basin and
Range structure will be a needed constraint in analyzing subsurface analyses of
USArray seismological data and the dynamic models of continent and mantle
evolution that arise. This points to the need to not localize geologic studies
in the Basin and Range to only those areas currently characterized by
relatively high rates of geodetic strain accumulation.
Other themes arising from our discussions was the
importance of understanding the control of rheology and fluids on fault
behavior, and the need to define fault dip geometries if we are to define calculate
rates of Basin and Range extension from geologic observations.
In sum, it was agreed that a conceptual goal of
GeoPBO (now has another name I think?)/Earthscope driven studies should be the
collection of observations that will allow development of a movie of how the
Great Basin has expanded through time.
Relations of Economic Resources to Tectonics
(Structure, Magmatism, Fluid and Heat Flow)
Moderator: David Blackwell
Recorder: Lew Gustafson
We organized discussion of major scientific problems and opportunities for EarthScope around the two major types of resources: mineral deposits of both Carlin Au and porphyry Cu-Au-Mo types, both formed in the Eocene, and presently active Geothermal systems.
Gold deposits of the Carlin Trend and similar deposits in Nevada produce nearly 8% of the world’s gold. These deposits, formed at 38±5MA, occur in general north-northwest striking belts through north-central Nevada (Figure 20). World-class deposits of the Park City-Bingham-Battle Mountain trend of porphyry are rich in copper, gold, and molybdumnum. These occur with an east-west strike crossing the Carlin trend and extending to the Wasatch Front (Figure 20), and were formed essentially contemporaneously with the Carlin deposits, although the deposits are widely separated spatially. The districts around the ore deposits have been thoroughly mapped, geologically, geochemically and geophysically. They present a number of problems about the deep structure, magmatism and fluid flow which are pertinent to deep processes important to Earthscope.
Deposits on the Carlin trend appear to be controlled by deeply penetrating, pre-Antler normal faults with NNW trend, which are reflected in a major gravity discontinuity. They are related to deep, possibly magmatic sources of heat and fluid, driving potentially large-scale paleohydrothermal systems which formed these deposits. While most porphyry deposits are formed in trends parallel to subduction in zones of arc magmatism, the east-west trend of the Great Basin porphyry deposits is perpendicular to and far inland from the Laramide subduction zone, and not related to arc magmatism, indicating that a new model is required. These major ore deposits thus offer important boundary conditions at the surface for future experiments that might try to determine the tectonic controls on the east-west belt of intrusions which formed the porphyry deposits, and how these are related to both the southward-sweeping front of Eocene volcanism and to contemporaneous minor dikes but major hydrothermal systems of the Carlin trend, which formed nearly perpendicular to the porphyry trend.
The
active geothermal systems provide potential analogues with insights into the
paleogeothermal systems which formed the deposits, and could provide immediate
feedback into geophysical investigations related to active tectonics and
fluid/heat flow. Highest temperatures
indicate fluid circulation from at least 8 to 10 km, and He isotopes show a
mantle signature and indicate structures penetrating the lower crust and zones
of hydrous weakening. The Department of Energy is
providing significant funding for geothermal research, which could be available
to EarthScope projects.
Recent discovery of oil in an overthrust at the eastern edge of the Great Basin could forsage increased interest by the oil industry in geophysical studies in the region. Investigations related to potential volcanic or earthquake hazards near the Yucca Mt. nuclear waste disposal facility are of interest to both Earthscope and DOE.
General
Aspects of Industry Involvement with EarthScope. The resource industries are the repositories
of geologic understanding and large, near-surface data sets which could provide useful boundary
conditions on deep geophysical experiments.
While being potential sources of funding to a much less extent, the
industries could provide useful collaborators in EarthScope investigations if
mutual interests can be established by individual contacts. Such collaboration would 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.
Walker
Lane. When did it start? Why is the Sierra Nevada range moving north? How does the crust accommodate simultaneous
extension and strike-slip deformation in the Walker Lane? What does this mean for the strength of
faults? Is there anything special about
the lower crust and upper mantle?
Moderator: Jim Faulds
Recorder: Elizabeth Miller / Steve Wesnousky
(note
that not all of these topics were actually addressed in the flow of discussion)
The Walker Lane is a system of dextral faults that
strike northwesterly along the western margin of the Great Basin extensional
province. Geodetic measurements show
the fault system is currently accommodating about 20% of Pacific-North American
plate motion. Geologic estimates of
cumulative displacements taken up by the fault system range from about 40 km to
100 km. As such, the Walker Lane is a major tectonic element of the
Pacific-North American plate boundary.
An understanding of the evolution of the Pacific-North America plate
boundary cannot be complete without an understanding of the development of the
Walker Lane. The fundamental questions
that remain necessary to address are how, why, and when did this significant component of plate
motion step eastward to this particular location? Related to this overarching question, we discussed the potential relationship to the opening of the
Gulf of California to the inception of Walker Lane. All agreed that the Walker Lane also represents an excellent
opportunity to study the evolution, growth, and propagation of a strike-slip
fault system. Toward that end, effort
will have to be placed to obtain the slip histories of major faults comprising
the Walker Lane extending from the Holocene back to the initiation of the
system in the Miocene. Such geological studies in concert with EarthScope
driven geophysical and geological measurements holds the key to understanding
what has controlled the location and distribution of strain through time. The role of volcanism and heat may play a
significant role in localizing deformation in the Walker Lane. Only through
this integrated approach will we gain an understanding of the nature and role
of coupling between the upper mantle and crust in the Walker Lane. Similarly, both geological and geophysical
observables need to be collected if we are to understand the dynamic and
kinematic links of the Walker Lane to far-field plate boundary as well as local
stresses.
Today the Walker Lane is transtensional and we
discussed how the crust accommodates simultaneous extension and strike-slip
faulting. Similarly, the occurrence of
conjugate strike-slip faults and prior paleomagnetic measurements indicates the
presence of significance vertical axis crustal block rotations and the
possibility exists that these rotations might be observable geodetically. The
relationship of these facets of deformation and their relation in both space
and time will, when combined with the lower crustal and upper mantle
observations that will come forth with Earthscope, hold the key to unraveling issues bearing on the relative strength
of the crust and mantle, the strength of faults, and the dynamics driving the
strike-slip system.
Seismic
and Geophysical Methods, Crust and Mantle
Moderator:
Bob Phinney
Recorder:
John Louie
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 an 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, magnetotelluric, 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 New
Mexico Institute of Technoloyg, 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
magnetotellurics among others.
The panelists were Glenn Biasi, Geoff Blewitt, Ron
Bruhn, Anka Friedrich, Lew Gustafson, Gene Humphreys, Kaye Shedlock, Greg van
der Vink, and Steve Wesnousky. John
Anderson moderated the discussion.
Questions before the panel are given on the last page of the
agenda. Questions were asked of the
entire audience, with the panel stimulating the discussion as needed. There was wide participation from the
audience. The questions were discussed
in the order given below.
The first question is essentially about community
self-organization. Essential elements
for maintaining an effective community of Earth scientists focused on the Great
Basin are:
All of these points deserve some further
elaboration.
The idea of an annual science meeting is relatively
self-explanatory. A model for this
might be the SCEC community, where the annual workshops mainly concentrate on
science, with a limited amount of time (~10%) devoted to discussion of
important future directions. Following
the meeting we sent an email message to the entire set of participants, with a
questionnaire including an inquiry about interest in continuing meetings.
The concept of the Earth model was the focus of
considerable discussion. The vision is
that the model would be spatially referenced and contain visualization tools
for all of the information that would be entered. The scale would begin with a whole-earth scale, and include
embedded data on successively smaller scales down to the scale of local shallow
studies. Data to be included would
include bulk properties including mean P- and S- wave velocities and Q,
density, viscosity, electrical conductivity, and anisotropy. On the crustal
scale, it might follow the SCEC example of defining faults and material
properties for the crustal blocks between each of the faults. On this scale, it would also include
earthquake hypocenters. In a map mode it would give surface geology, elevation,
gravity, heat flow, location and age of volcanoes, geothermal sources, and so on.
Such a model is a considerable undertaking, and the expectation is that
this would build incrementally starting with frameworks developed by the Southern California
Earthquake Center, and others. There
would not be a single preferred model, but rather this would be able to include
and trace various, conflicting interpretations of the data.
The vision for a modeling environment envisions
sharing of documents and well-tested codes for various types of modeling,
ranging from synthetic seismograms to data analysis to prediction of the
dynamics of continental deformation.
The Earth model should be capable of providing input for these
models.
The test bed would be an EarthScope-wide facility,
as described in the write-up for Group H.
Ongoing working groups are necessary to maintain
momentum for some of the objectives in between annual meetings also if the
community is to become a cohesive group.
There was a sense in the room that this is important. One group did organize during the meeting to
work on development of the Earth model.
This was discussed relatively briefly, as it was
recognized by the audience that the Earth model and modeling environment, and
the community development are an important part of the desired EarthScope
products.
The desired legacy of EarthScope fall into four
categories:
The Earth model is likely to be the first legacy of
EarthScope in the Great Basin, or across the entire country for that
matter. Building such a model (or
models) is a complex community effort that has been pioneered by the global
seismic communities and, probably more applicably, in southern California under
SCEC and other efforts. A distinct and
sustained funded effort is probably necessary for this to occur.
The invigorated Earth science community is perhaps
already happening of its own accord, as evidenced by the enthusiastic and
wide-spread participation at this workshop.
There was a sense that the participants enjoyed the opportunity to take
part in GreatBREAK, and the community building that started here is already an
early benefit of EarthScope. Clearly,
additional community efforts, such as national EarthScope workshops and
EarthScope events at other meetings (e.g., IRIS, AGU, GSA, etc.), as well as
special issues in leading journals will be necessary to continue to build the
EarthScope community.
The broadest statement that we desire a scientific
understanding of the plate boundary can be subdivided into an understanding of
several key processes.
Some additional considerations would be that the
multidisciplinary character of EarthScope should help to tackle the problem of
non-uniqueness in inversion of data to Earth properties, or to determine Earth
history.
For EarthScope to live up to its revolutionary
potential in the Earth sciences, it is essential that connections continue to
be nurtured between the research and the education and outreach
communities. Some possibilities in the
Great Basin region summarized by Rick Aster include
Perhaps
one of the more important talks of the meeting regarding the evolving community
was given by Tom Jordan, who discussed the SCEC model for community
organization. The following is Jordan’s
list of ingredients for successful science integration, annotated in italics with the organizing
committee’s assessment of the status for the Great Basin.
•
Problem focus
–
Regional tectonics &
hazards
–
In the Great Basin, this is the same.
•
Common objectives
–
Community data products
& models
–
The workshop concluded that a community model of the
Great Basin, giving fault locations and crustal structure, is an important goal
for Earthscope in this region.
•
Community identity &
organization
–
History of
collaboration, interactive working groups
–
The GreatBREAK workshop is the starting point for this
community identity development.
•
Collaboratory
infrastructure
–
Code validation,
standardization of products
–
For this, we can collaborate closely with SCEC, as the
codes we need are generally similar if not idetnical.
•
Regular forums for
assessing progress
–
Workshops, annual
meeting
–
GreatBREAK may be the first of these events. The organizers believe that annual meetings
focused on Earthscope in the Great Basin are appropriate. The organizers are aware of at least a few
collaborations that developed as a result of the workshop.
•