the pacific and african deep-mantle anomalies are not superplumes bruce r. julian dept. of earth...
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The Pacific and AfricanDeep-Mantle Anomalies
are not Superplumes
Bruce R. JulianDept. of Earth SciencesUniversity of Durham
Durham, U.K.
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Large Low Shear-Velocity Provinces(LLSVPs of Lay, 2005)
• First deep-mantle features reliably resolved by seismic tomography• “commonly referred to as superplumes” (Romanowicz & Gung, 2002)• Suggested source of plumes & flood basalts (e.g. Torsvik et al., 2006)
Della Mora et al., 2011
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Large Low Shear-Velocity Provinces(LLSVPs of Lay, 2005)
• Anomaly much weaker for VP than for VS.
Della Mora et al., 2011
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Seismic-Wave Speeds
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VS & Vφ anomalies have opposite signs!
Trampert et al., 2004
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Density anomaly is positive!
Trampert et al., 2004
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Anomalies are chemical, not thermal
Trampert et al., 2004
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Is there some kind of error?
• Anelasticity can make VS more sensitive to temperature, perhaps make chemical variation unnecessary.
• Effect is quantitatively inadequate (Brodholt et al., 2007).• Can’t explain VP , let alone Vφ .
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Is there some kind of error (cont.)?
• Is the resolution of seismic tomography adequate? Yes (Della Mora et al., 2011).
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Is there some kind of error (cont.)?
• Is the resolution of seismic tomography adequate? Yes (Della Mora et al., 2011).
Bottom line: No, there isn’t an error.
The “superplumes” are not hot.
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Large Igneous Provinces (LIPs)
Torsvik et al., 2006
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LIP/LLSVP Correlation?
Torsvik et al., 2006
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Hotspot/LLSVP Correlation?
Torsvik et al., 2006
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The “Hindsight Heresy”
• Constructing a hypothesis after looking at a data set, then using the same data set to test the hypothesis.
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An Example From U.S. History
• Thomas Jefferson and John Adams died on the same day: July 4, 1826. (P < 3 X 10-5)
• That was the 50th anniversary of the U.S. Declaration of Independence, which they co-wrote. (P < 10-4)
• Exactly 5 years later, on July 4, 1831, James Monroe (U.S. Founding Father) died. (P < 10-3)
• Joint Probability < 3 X 10-12
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Another Example
• Abraham Lincoln’s secretary was named Kennedy. (P = 0.00067)• John F. Kennedy’s secretary was named Lincoln. (P < 0.00012)• Joint Probability < 8 X 10-8
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Inadvertent Selection?
• Which LIPs/plumes to include? Many catalogs, of widely differing sizes, exist. A new one was generated.
• Which tomographic model? Three models were averaged, with differing weights.
• The definition of “margin” was a variable.• Geometry: within anomalies?, outside anomalies?, near
anomaly margins?, …• Excluding worst-fit LIPs (e.g. Siberian Traps) from statistics(!).
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Large Igneous Provinces (LIPs)
Torsvik et al., 2006
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Systematic Errors?• Age of crust:
Sandwell et al., 2005
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Conclusions
• The LLSVPs are dense, not buoyant.• They cannot be solely thermal effects.• They appear to be primarily chemical effects.• Correlation with LIPs or hot spots is probably
not real, but a case of the hindsight heresy and/or an artifact of systematic influences such as the age of the crust.
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THE END