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Transcript
Regional overview and workshop theme
David Foster - University of Florida
Paul Mueller - University of Florida
David Mogk - Montana State University
• Theme: >3.5 Ga of
lithospheric evolution:
segregation from the
mantle, stabilization and
modification.
• This area contains the
largest record of major
lithospheric events to be
analyzed by EarthScope.
• The Archean lithosphere of
Wyoming is still being
modified thermally and
chemically by the
Yellowstone system
Goals:
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Identify major geological
problems that EarthScope can
help address
Identify what additional
groundwork needs to be done
to maximize EarthScope
results
Formulate hypotheses and
pose tests of the major
questions
Prioritize major objectives
and possible additional
projects to maximize gains
while the instruments are
active in the area
Plan education and outreach
activities
Inform and engage the public
Integrate Northern Rockies
research in to broader
EarthScope science goals
SW Laurentian Basement
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Complex mosaic of Archean and
Proterozoic lithospheric blocks
Poorly exposed outside of Laramide
basement uplifts
What are the province boundaries?
Do the provinces extend into the
mantle lithosphere?
How was the basement and
lithosphere modified by younger
events?
What is the nature of the edge of
Laurentia
–
•
Has the ancient structure played a
roll in Phanerozoic
tectonomagmatic events?
–
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Idaho shear zone?
Does it continue to do so today?
What constitutes a craton in terms
of geochemical and geophysical
properties
Margins of Wyoming
craton
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Nature of the Great Falls
tectonic zone and
relationship to THO
Selway accreted terrane
Medicine Hat block - how far
does it extend?
Priest River block - is it a
separate late ArcheanPaleoproterozoic block?
How do terranes defined by
magnetic data equate to
crustal and lithospheric
structure?
Does the basement surface
map equate to what is at
depth in the lower crust and
mantle lithosphere?
– probably not
Sims et al. (2004)
Deep Crustal &
Lithospheric
Investigations
•
LITHOPROBE
DEEPPROBE seismic data
& U-Pb ages of xenoliths
suggest that a thick
Paleoproterozoic mafic
underplate underlies the
northern Wyoming
province
•
Underplate could be a
major source for tonalitegrandiorite magmas.
Many of these plutons age
give Nd TDM = 1.7.1.9 Ga
(e.g. Tobacco Root)
•
is some of it Archean?
•
Implications for PC
orogeny and cratonization
•
Less data than southern
Rocky Mountains
Clowes et al., 2002
Belt basin and Neoproterozoic rift
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What controlled rift
geometry?
What drove rifting?
How was the crust modified?
Metallogenesis and
magmatism
– Western edge of Belt basin?
Is most of it here or is most
of it missing?
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What is the Lewis and Clark
line & other
tectonostratigraphic
boundaries?
Implications of recently
recognized widespread
Neoproterozoic rocks west of
Belt basin in Idaho
Was the margin truncated
What age is the reentrant?
– Neoprotoeroic or
Mesozoic or both
Cordilleran Orogen
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Rich and complex history of
continental modification
Cretaceous-Eocene magmatic
provinces
Cordilleran fold-thrust belt
Laramide basement uplifts
Eocene metamorphic core
complexes
Cenozoic-Recent magmatism
Mineralized zones
Mesozoic accreted terranes
Idaho shear zone
Lewis and Clark fault zone
Hot Spot
Neotectonics
Seismic activity
Shallow subduction
Slab window
SW Montana
magmatic
province
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Orthogonal to
orogen axis
Intruded well into
foreland at ~75 Ma
Relationships
between thrusts and
plutons
Basement control,
shallow subduction,
or truncated
margin?
Lithospheric
response to
tectonics through
time
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In southern Rocky Mts major
variations in strength and fertility
occur between the Phanerozoic
and Proterozoic lithosphere
In northern Rocky Mts there are
major differences in strength and
partial melt productivity between
the Proterozoic and Archean as
well.
How far northeast did the effects
of Laramide shallow subduction
extend?
Asthospheric upwelling at 54-50
Ma (older than in the south)
Humphreys et al., 2003
Lewis and Clark fault zone
•
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Basin bounding fault along axis of
Belt basin
A lateral ramp during CretaceousPaleocene thrusting
A dextral accommodation zone
during Eocene extension
Basement control?
Eocene extension
and magmatism
•
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Reworking lower crust
Collapse of orogenic
wedge
Lithospheric response
to a slap window?
Relationship between
magmatism and
extension
– e.g., Bitterroot - older,
deep Eocene plutons
in footwall, younger
shallow plutons and
volcanic rocks in
hanging wall
Integration
Relationships
between
Precambrian
provinces and
Phanerozoic
tectonomagmatic
provinces
The Yellowstone system is
currently modifying the Archean
lithosphere of Wyoming. What is
the nature of the hot spot? Is
there lithospheric control on the
neotectonics and magmatism?
Humphreys et al., 2000
Summary
• Understanding the processes by which the Rocky
Mountains lithosphere formed and evolves requires
the integration of many disciplines (structural
geology, geochemistry, petrology, geophysics,
geochronology, geomorphology, etc.).
• EarthScope resources can play a pivotal role in
elucidating the complex history of the northern Rocky
Mountains, and thereby add significantly to our
understanding of both crustal genesis and continental
evolution, by providing images of crust and mantle
structures to combine with the age and composition
information of material added to the continental crust.
Example Questions for Northern Rockies Workshop Discussion
• How did the ancient lithosphere and structures control crustal
evolution?
• What is the structure of the oldest craton in southern Laurentia?
• How are cratons stabilized and once “cratonized” how are they
reactivated?
• Is there a typical crustal/lithospheric structure associated with major
mineralization?
• How strong is crust in orogenic zones and why does it extend?
• The nature of magmatic underplating and repeated mafic magma
intrusion in the craton?
• Is the Yellowstone Hot Spot a deep mantle plume?
• How diffuse is the plate boundary of western North American?
• What is driving modern extension and seismicity?
• Why are the Northern Rocky Mountains still at high elevation?
• How has the Cenozoic-Recent magmatism in the Snake River Plain
modified the lithosphere?
• What is the large-scale structure of the Mesoproterozoic Belt basin?
• Can we answer all important questions with passive seismic?
• What other methods are needed?
• What is the best way to involve educators and public in the science of
EarthScope?