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Grand Challenge: Why Does Volcanism
Occur Where and When it Occurs in the
Basin and Range?
Richard W. Carlson
Carnegie Institution of Washington
Department of Terrestrial Magn(m)atism
EarthScope Great Breaks Workshop
June 22, 2004
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Animation of Volcanism vs. Time produced by Allen Glazner
(UNC) from the NAVDAT database (navdat.geongrid.org)
Western US Mafic Volcanism Contoured According to Potassium Content
Prepared by Allen Glazner, UNC from the NAVDAT dataset (navdat.geongrid.org)
Low
High
What Information Can be Extracted from the Magmatism?
• Temperature
– Mafic magmas
• Tholeiite - hot (wet), shallow
• Alkalic basalt - cooler, deeper
• Lots of crustal contamination --> warm crust
• Lithosphere Thickness
– Mafic rock composition relates to last depth of
equilibration, in order of increasing depth
• Tholeiitic basalt < alkalic basalt < kimberlite
• Crustal Thickness/Thermal Profile
– Depth of crystallization of mafic magmas
– Source depth for silicic magmas
What Information Can be Extracted from the Magmatism?
• “Permiability” of Lithosphere
–
–
–
–
–
More alkaline --> deeper --> lithospheric barrier?
Existence - or not - of a mafic underplate
Amount of crystallization reflects ease of crustal transit
Presence or absence of bimodal basalt-silicic volcanism
Relation of volcanicity to faulting and extension
• Presence of Water - Subduction
– Location of past subduction (shallow - steep)
– Slab windows/tacos
– Effect of non-volcanic shallow-dip subduction in “priming” the
lithosphere for large volume volcanism
• Flood basalts and ignimbrite sweeps
• Age of Magma Source
– Identify buried terrane boundaries
Mafic magmas as a lithosphere thickness monitor
East Pac ific
Rise
E. Sierra
Death
Valley
Mojave
Reveille &
L. Mead
Transition
Zone , Colorado Plateau
0
0
crust
20
1.0
40
mantle
lithosphere
60
2.0
Z
1435°C
80
1450°C
Sp
100
3.0
Gar
1600°C
120
4.0
1700°C
140
Snake River
Basalts
asthenosphere
0-10Ma
160
0
100
200
300
400
500
5.0
600
Wang, Planck, Walker and Smith, JGR, 2002
Post 17 Ma Volcanism in the Western US (after Luedke and Smith, 1984)
87Sr/86Sr
Observations
> 0.706
< 5 5-10 10-17 Ma
Basalt
Andesite
Rhyolite
B&R does not notice PC
continental boundary
Idaho
Volcanism concentrated at
B&R margins, particularly
Northern boundary
Oregon
Wyoming
Utah
California
Great Basin
Nevada
Arizona
Volcanism does not
obviously correlate with
amount of extension
Largest volume basalt
province (CRB) has no
associated silicic volcanism.
Bimodal volcanism
characteristic of B&R
Post 10 Ma Volcanism in the Western US (After Smith and Luedke, 1984)
Observations
WA
MT
87Sr/86Sr > 0.706
North American Plate
14-26 km/Ma
JdF
Plate
OWL
NB
0
ID
2
4
6
OR
4
6
8
8
10
EDFZ
Migrating volcanism defined
by rhyolites, basaltic activity
continuous along both traces
0
2
VFZ
ESRP
BFZ
12
MFZ
10
WY
14
NNR
CA
NV
SAFZ
Two migrating volcanic traces,
one with plate motion, the other
against it
UT
Volcanism in High-Lava-Plains
associated with right-lateral
strike-slip faulting as extension
dies out to the North
Gravity
Topography
Gravity and seismology show thick mafic underplating under SRP, but not HLP. Is
it there and compensated? How do you compensate with minimal extension?
Both migrating traces start around the Owyhee Plateau, which is surrounded by silicic
centers, and B&R faults, but contains neither. Is this another “Colorado Plateau” and
if so, why?
Strong spatial changes in volcanic rock composition
Oregon
Idaho
0.707
0.706
0.705
0.704
121
120
119
118
117
116
115
Longitude
Sr isotopic composition of post-10.5 Ma tholeiites
from across central Oregon
Strong temporal changes in volcanic rock composition
Black - CRB, Red - Steens, Open-Brown - Post 10.5 Ma tholeiites
10
Mantle Melts
9
8
7
Fractionation
6
5
4
5
10
15
20
Age (Ma)
Magnesium concentration as a monitor of changing extent of
basalt differentiation through time, northern B&R and CRB
Conclusions (Questions) Provided by B&R Volcanism
• Extension and volcanism: some local correlations, but
generally not. What is causing melting if not adiabatic
ascent due to stretching? Lithosphere filter?
• Ignimbrite sweep related to steepening subduction, but
how? Directly to retreating slab or caused by inflow of hot
asthenosphere under hydrous lithospheric mantle?
• Plumes and migrating volcanic traces - is this the answer?
Why is basaltic volcanism continuous along traces? Why
is there an “anti-plate motion” trend?
• Spatially correlated compositional variation: Due to
variations in lithosphere thickness, sublithospheric
temperature, crustal impedance --> extension/faulting?
• Temporally correlated compositional variation: How much
have lithospheric characteristics changed in 15 Myr?