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 ContentPrepared 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

0 100 200 300 400 500 600

0

20

40

60

80

100

120

140

160

mantlelithosphere

asthenosphere0-10Ma

crust

Reveille &L. MeadMojave

DeathValley

TransitionZone , Colorado PlateauE. Sierra

Sp

Gar

Z

1.0

2.0

3.0

4.0

5.0

0

1700°C

1600°C

1450°C1435°C

East Pac ificRise

Snake RiverBasalts

Wang, Planck, Walker and Smith, JGR, 2002

Oregon

Idaho

Wyoming

Nevada

California

Utah

Arizona

Basalt

Andesite

Rhyolite

< 5 5-10 10-17 Ma

87Sr/86Sr > 0.706

Post 17 Ma Volcanism in the Western US (after Luedke and Smith, 1984)

Great Basin

Observations

B&R does not notice PC continental boundary

Volcanism concentrated at B&R margins, particularly Northern boundary

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

OR

ID

MT

WY

NV

CA

UT

WA

OWL

VFZ

BFZEDFZ

MFZ

ESRP

SAFZ

14

1210

86

42

0

0

2

46

810

NNR

North American Plate14-26 km/Ma

JdFPlate

NB

Post 10 Ma Volcanism in the Western US (After Smith and Luedke, 1984)

87Sr/86Sr > 0.706

Observations

Two migrating volcanic traces, one with plate motion, the other against it

Migrating volcanism defined by rhyolites, basaltic activity continuous along both traces

Volcanism in High-Lava-Plains associated with right-lateral strike-slip faulting as extension dies out to the North

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?

Gravity Topography

Strong spatial changes in volcanic rock composition

Sr isotopic composition of post-10.5 Ma tholeiites from across central Oregon

0.704

0.705

0.706

0.707

115116117118119120121

Longitude

IdahoOregon

4

5

6

7

8

9

10

5 10 15 20

Age (Ma)

Fractionation

Mantle Melts

Black - CRB, Red - Steens, Open-Brown - Post 10.5 Ma tholeiites

Strong temporal changes in volcanic rock composition

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?