Transcript The “Superplumes” are not Plumes

The Pacific and African
Deep-Mantle Anomalies
are not Superplumes
Bruce R. Julian
Dept. of Earth Sciences
University of Durham
Durham, U.K.
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
Large Low Shear-Velocity Provinces
(LLSVPs of Lay, 2005)
• Anomaly much weaker for VP than for VS.
Della Mora et al., 2011
Seismic-Wave Speeds
VS & Vφ anomalies have opposite signs!
Trampert et al., 2004
Density anomaly is positive!
Trampert et al., 2004
Anomalies are chemical, not thermal
Trampert et al., 2004
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 .
Is there some kind of error (cont.)?
• Is the resolution of seismic tomography adequate?
Yes (Della Mora et al., 2011).
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.
Large Igneous Provinces (LIPs)
Torsvik et al., 2006
LIP/LLSVP Correlation?
Torsvik et al., 2006
Hotspot/LLSVP Correlation?
Torsvik et al., 2006
The “Hindsight Heresy”
• Constructing a hypothesis after looking at a data set, then
using the same data set to test the hypothesis.
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
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
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(!).
Large Igneous Provinces (LIPs)
Torsvik et al., 2006
Systematic Errors?
• Age of crust:
Sandwell et al., 2005
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.
THE END