Esci 203, Earth Structure and Deformation Heat flow and faulting (2) John Townend EQC Fellow in...
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Transcript of Esci 203, Earth Structure and Deformation Heat flow and faulting (2) John Townend EQC Fellow in...
Esci 203, Earth Structure and Deformation
Heat flow and faulting (2)
John Townend
EQC Fellow in Seismic Studies
Outline
• Recap on the last lecture– Conductive heat flow and the heat flow equation– Measuring surface heat flow– The brittle-ductile transition
• Characteristic times
• Interpreting heat flow maps
• Inferring paleoclimates from heat flow data
• Shear heating and the San Andreas paradox
Shallow geotherm
Photo: D.L. Homer
Temp. gradient: 63 ± 2°C km–1
Heat flow: 158 ± 69 mW m–2
Characteristic time/length scales
• When’s breakfast?
L
Characteristic time, t = L2/
So, if the rock is 20 cm thick, and =10–6 m2 s–1, then it takes
~40,000 s (or ~11 hours) for substantial heat to be
conducted through the slab
Age of oceanic lithosphere
Muller et al., 2008. Geo3
Ocean depth is related to the age of oceanic lithosphere, which cools and sinks as it propagates from a MOR
dkm ~ 2.6 + 0.36tMyr
Why?• We can relate water depth and age (√t) using
models of cooling– Half-space model (lithospheric thickness defined
by temperature)– Plate model (lithospheric thickness specified)
• The models differ in their boundary conditions
Thermal structure Density structure Elevation below sea-level
Assuming
isostatic eqbm.
Kelvin’s model of the Earth
• Assumptions:– The Earth is flat– The Earth’s surface
temperature has always been 0°C
– The Earth’s interior temperature was initially 4000°C
• Answer:– 20–400 Ma, with a
final preference for ~24 Ma
020406080
100120140
0 100 200 300 400 500
Time since the Earth formed (Ma)
Geo
ther
mal
gra
die
nt
(°C
km
–1)
It never rains but it pours...
• “[T]he inexorable physicist [has] remorselessly struck slice after slice for his allowance of geological time.”
— Sir Archibald Geikie, 1892
• “[T]he geologist who ten years ago was embarrassed by the shortness of time allowed to him for the evolution of the earth’s crust is now still more embarrassed by the superabundance with which he is confronted.”
— Arthur Holmes, Nature, 1913
Blackwell, D. D., and Richards, M. 2004. Geothermal Map of North America. American Assoc. Petroleum Geologist (AAPG), 1 sheet, scale 1:6,500,000.
Australian heat flow provinces
Beardsmore and Cull, 2001
Heat flow from seismic data
Paleoclimate research
• Changes in temperature at the ground surface propagate downwards over time
• If that’s the case, then we should be able to deduce what ground surface temperature changes have occurred in the past using borehole measurements of temperature vs. depth
• This is called an inverse problem
Background
Temperature fluctuations of longer period propagate to
greater depth
The depth at which the amplitude of the perturbation
is 0.37the value at the surface is known as the “skin
depth”
If P is the period of the perturbation, then the skin
depth d is
P
d ~Pollack and Huang, 2008
Borehole temperature profiles
The task is to relate T(z,t=0) to T(z=0,t)Pollack and Huang, 2008
Undisturbed background geotherms
Perturbed shallow sections
A global compilationP
olla
ck a
nd H
uang
, 20
08
Analysis of global borehole
measurements yields a record that can be
used to extend instrumental records
back in time
Shear heating
• Just like when you rub your hands together, rocks sliding past each other along a fault are frictionally heated
• The amount of heat generated depends on:– How fast the fault is slipping
– How much frictional stress is resisting slip
Do big faults generate much
heat?• The San Andreas slips at
20–30 mm/yr and we’d expect it to generate substantial shear heating ... unless the frictional stresses were low
• So, can we measure a temperature anomaly across the San Andreas fault?
The source of the controversy
Fulton et al., GRL, 2004
Is there some heating we’re
missing?Perhaps shallow groundwater flow
washes out the shear heating signal
But, most plausible groundwater
scenarios mean that we should see shear heating if it’s there
Fulton et al., GRL, 2004
The San Andreas Fault Observatory at Depth
(SAFOD)
IODP Expedition 343Japan Trench Fast Drilling Project (JFAST)
Science team• 28 scientists from 10 countries (Japan, US, UK, Canada,
Germany, France, Italy, China, India, NZ)• Geologists, geophysicists, geochemists, one microbiologist
Suggested reading material
• Fowler (2004)– Chapter 7, particularly §7.1, (7.2–7.3), 7.5.1, 7.8
• Mussett and Khan– Chapter 17, particularly §17.1, 17.2, 17.4
• Beardsmore and Cull (2001)– Any or all of chapters 1–3
• Turcotte and Schubert (1982)– Section 4.1
Not on
reserve (see me)